A TEXT-BOOK OF ENTOMOLOGY


[Illustration]




                                   A
                        TEXT-BOOK OF ENTOMOLOGY
                               INCLUDING
         THE ANATOMY, PHYSIOLOGY, EMBRYOLOGY AND METAMORPHOSES
                                   OF
                                INSECTS
      _FOR USE IN AGRICULTURAL AND TECHNICAL SCHOOLS AND COLLEGES_
                 AS WELL AS BY THE WORKING ENTOMOLOGIST


                                   BY

                    ALPHEUS S. PACKARD, M.D., PH.D.

 PROFESSOR OF ZOÖLOGY AND GEOLOGY, BROWN UNIVERSITY AUTHOR OF “GUIDE TO
          STUDY OF INSECTS,” “ENTOMOLOGY FOR BEGINNERS,” ETC.


                                New York
                         THE MACMILLAN COMPANY
                     LONDON: MACMILLAN & CO., LTD.
                                  1898

                         _All rights reserved_




                            COPYRIGHT, 1898,
                       BY THE MACMILLAN COMPANY.


                             Norwood Press
                  J. S. Cushing & Co.—Berwick & Smith
                          Norwood Mass. U.S.A.




                                PREFACE


[Illustration]

In preparing this book the author had in mind the wants both of the
student and the teacher. For the student’s use the more difficult
portions, particularly that on the embryology, may be omitted. The work
has grown in part out of the writer’s experience in class work.

In instructing small classes in the anatomy and metamorphoses of
insects, it was strongly felt that the mere dissection and drawing of a
few types, comprising some of our common insects, were by no means
sufficient for broad, thorough work. Plainly enough the laboratory work
is all important, being rigidly disciplinary in its methods, and
affording the foundation for any farther work. But to this should be
added frequent explanations or formal lectures, and the student should
be required to do collateral reading in some general work on structural
and developmental entomology. With this aim in view, the present work
has been prepared.

It might be said in explanation of the plan of this book, that the
students having previously taken a lecture course in the zoölogy of the
invertebrates, were first instructed in the facts and conclusions
bearing on the relations of insects to other Arthropoda, and more
especially the anatomy of Peripatus, of the Myriopoda, and of
Scolopendrella. Then the structure of Campodea, Machilis, and Lepisma
was described, after which a few types of winged insects, beginning with
the locust and ending with the bee, were drawn and dissected; the nymph
of the locust, and the larva and pupa of a moth and of a wasp and bee
being drawn and examined. Had time permitted, an outline of the
embryology and of the internal changes in flies during their
metamorphoses would have been added.

This book gives, of course with much greater fulness and detail for
reference and collateral reading, what we roughly outlined in our class
work. The aim has been to afford a broad foundation for future more
special work by any one who may want to carry on the study of some group
of insects, or to extend in any special direction our present knowledge
of insect morphology and growth.

Many of our entomologists begin their studies without any previous
knowledge of the structure of animals, taking it up as an amusement.
They may be mere collectors and satisfied simply to know the name of
their captures, but it is hoped that with this book in their hands they
may be led to desire farther information regarding what has already been
done on the structure and mode of growth of the common insects. For
practical details as to how to dissect, to make microscopic slides, and
to mount and preserve insects generally, they are referred to the
author’s “Entomology for Beginners.”

It may also be acknowledged that even in our best and latest general
treatises on zoölogy, or comparative anatomy, or morphology, the portion
related to insects is scarcely so thoroughly done as those parts devoted
to other phyla, that of Lang, however, his invaluable Comparative
Anatomy, being an exception. On this account, therefore, it is hoped
that this hiatus in our literature may be in a degree filled.

The author has made free use of the excellent article “Insecta” of
Newport, of Lang’s comprehensive summary in his most useful Text-book of
Comparative Anatomy, of Graber’s excellent Die Insecten, of Miall and
Denny’s The Structure and Life-History of the Cockroach, and of Sharp’s
Insecta. Kolbe’s Einführung has been most helpful. But besides these
helps, liberal use has been made of the very numerous memoirs and
monographic articles which adorn our entomological literature. The
account of the embryology of insects is based on Korschelt and Heider’s
elaborate work, Lehrbuch der Vergleichenden Entwicklungsgeschichte der
Wirbellosen Thiere, the illustrations of this portion being mainly taken
from it, through the Messrs. Swan Sonnenschein & Co., London.

Professor H. S. Pratt has kindly read over the manuscript and also the
proofs of the portion on embryology and metamorphoses, and the author is
happy to acknowledge the essential service he has rendered.

The bibliographical lists are arranged by dates, so as to give an idea
of the historical development of each subject. The aim has been to make
these lists tolerably complete and to include the earliest, almost
forgotten works and articles as well as the most recent.

Much care has been taken to give due credit either to the original
sources from which the illustrations are copied, or to the artist; about
ninety of the simpler figures were drawn by the author, many of them for
this work. For the use of certain figures acknowledgments are due to the
Boston Society of Natural History, to the Division of Entomology, U. S.
Department of Agriculture, through the kind offices of Mr. L. O. Howard,
and to the Illinois State Laboratory of Natural History, through
Professor S. A. Forbes and Mr. C. A. Hart. Professor W. M. Wheeler, of
the University of Chicago, has kindly loaned for reproduction several of
his original drawings published in the Journal of Morphology. A number
are reproduced from figures in the reports of the United States
Entomological Commission.

  PROVIDENCE, R. I.,
    March 4, 1898.




                           TABLE OF CONTENTS


[Illustration]

                    PART I. MORPHOLOGY AND PHYSIOLOGY

                                                                    PAGE
 POSITION OF INSECTS IN THE ANIMAL KINGDOM                             1

 RELATIONS OF INSECTS TO OTHER ARTHROPODA                              2

     The Crustacea                                                     4
     The Merostomata                                                   5
     The Trilobita                                                     5
     The Arachnida                                                     6
     Relations of Peripatus to insects                                 9
     Relation of Myriopods to insects                                 11
     Relations of the Symphyla to insects                             18
     Diagnostic or essential characters of Symphyla                   22

 INSECTA (HEXAPODA)                                                   26

     Diagnostic characters of insects                                 26


                           1. EXTERNAL ANATOMY

     _a._ Regions of the body                                         27

     _b._ The integument (exoskeleton)                                28

         Chitin                                                       29

     _c._ Mechanical origin and structure of the segments (somites,
       arthromeres, etc.)                                             30

     _d._ Mechanical origin of the limbs and of their jointed
       structure                                                      35

 THE HEAD AND ITS APPENDAGES                                          42

     _a._ The head                                                    42

         The labrum                                                   42
         The epipharynx and labrum-epipharynx                         43
         Attachment of the head to the trunk                          46
         The basal or gular region of the head                        46
         The occiput                                                  48
         The tentorium                                                49
         Number of segments in the head                               50
         The composition of the head in the Hymenoptera               55

     _b._ Appendages of the head                                      57

         The antennæ                                                  57
         The mandibles                                                59
         The first maxillæ                                            62
         The second maxillæ                                           68
         The hypopharynx                                              70
         Does the hypopharynx represent a distinct segment?           82

 THE THORAX AND ITS APPENDAGES                                        86

     _a._ The thorax: its external anatomy                            86

         The patagia                                                  89
         The tegulæ                                                   89
         The apodemes                                                 92
         The acetabula                                                94

     _b._ The legs: their structure and functions                     95

         Tenent hairs                                                 99
         Why do insects have but six legs?                           100
         Loss of limbs by disuse                                     101

     _c._ Locomotion (walking, climbing, and swimming)               103

         Mechanics of walking                                        103
         Locomotion on smooth surfaces                               111
         Climbing                                                    116
         The mode of swimming of insects                             116

     _d._ The wings and their structure                              120

         The veins                                                   121
         The squamæ                                                  123
         The halteres                                                124
         The thyridium                                               124
         The tegmina and hemelytra                                   124
         The elytra                                                  124

     _e._ Development and mode of origin of the wings                126

         Embryonic development of the wings                          126
         Evagination of the wing outside of the body                 132
         Extension of the wing; drawing out of the tracheoles        133

     _f._ The primitive origin of the wings                          137

         The development and structure of the tracheæ and veins of
           the wing                                                  144

     _g._ Mechanism of flight                                        148

         Theory of insect flight                                     150
         Graber’s views as to the mechanism of the wings, flight,
           etc.                                                      153

 THE ABDOMEN AND ITS APPENDAGES                                      162

     The median segment                                              163
     The cercopoda                                                   164
     The ovipositor and sting                                        167
     The styles and genital claspers (Rhabdopoda)                    176
     Velum penis                                                     181
     The suranal plate                                               181
     The podical plates or paranal lobes                             182
     The infra-anal lobe                                             183
     The egg-guide                                                   183

 THE ARMATURE OF INSECTS: SETÆ, HAIRS, SCALES, TUBERCLES, ETC.       187

     The cuticula                                                    187
     Setæ                                                            188
     Glandular hairs and spines                                      190
     Scales                                                          193
     Development of the scales                                       195
     Spinules, hair-scales, hair-fields, and androconia              197

 THE COLORS OF INSECTS                                               201

     Optical colors                                                  201
     Natural colors                                                  203
     Chemical and physical nature of the pigment                     206
     Ontogenetic and phylogenetic development of colors              207


                           2. INTERNAL ANATOMY

 THE MUSCULAR SYSTEM                                                 211

     Musculature of a caterpillar                                    213
     Musculature of a beetle                                         213
     Minute structure of the muscles                                 215
     Muscular power of insects                                       217

 THE NERVOUS SYSTEM                                                  222

     _a._ The nervous system as a whole                              222

     _b._ The brain                                                  226

         The optic or procerebral segment                            231
         Procerebral lobes                                           232
         The mushroom or stalked bodies                              233
         Structure of the mushroom bodies                            234
         The central body                                            237
         The antennal or olfactory lobes (Deutocerebrum)             237
         The œsophageal lobes (Tritocerebrum)                        237

     _c._ Histological elements of the brain                         238

     _d._ The visceral (sympathetic or stomatogastric) system        238

     _e._ The supraspinal cord                                       240

     _f._ Modifications of the brain in different orders of insects  240

     _g._ Functions of the nerve-centres and nerves                  243

 THE SENSORY ORGANS                                                  249

     _a._ The eyes and insect vision                                 249

         The simple or single-lensed eye (ocellus)                   249
         The compound or facetted eye (ommateum)                     250
         The facet or cornea                                         250
         The crystalline lens or cone                                251
         The pigment                                                 253
         The basilar membrane                                        253
         The optic tract                                             253
         Origin of the facetted eye                                  255
         Mode of vision by single eyes or ocelli                     255
         Mode of vision by facetted eyes                             256
         The principal use of the facetted eye to perceive the
           movements of animals                                      259
         How far can insects see?                                    260
         Relation of sight to the color of eyes                      260
         The color sense of insects                                  260

     _b._ The organs of smell                                        264

         Historical sketch of our knowledge of the organs of smell   264
         Physiological experiments                                   268
         Relation of insects to smelling substances before and
           after the loss of their antennæ                           269
         Experiments on the use of the antennæ in seeking for food   270
         Experiments testing the influence of the antennæ of the
           males in seeking the females                              270
         Structure of the organs of smell in insects                 271

     _c._ The organs of taste                                        281

         Structure of the taste organs                               282
         Distribution in different orders of insects                 282
         Experimental proof                                          286

     _d._ The organs of hearing                                      287

         The ears or tympanal and chordotonal sense-organs of
           Orthoptera and other insects                              288
         Antennal auditory hairs                                     292
         Special sense-organs in the wings and halteres              293

     _e._ The sounds of insects                                      293

 THE DIGESTIVE CANAL AND ITS APPENDAGES                              297

     _a._ The digestive canal                                        302

         The œsophagus                                               303
         The crop or ingluvies                                       303
         The “sucking stomach” or food-reservoir                     305
         The fore-stomach or proventriculus                          306
         The œsophageal valve                                        311
         Proventricular valvule                                      313
         The peritrophic membrane                                    313
         The mid-intestine                                           314
         Histology of the mid-intestine                              316
         The hind-intestine                                          316
         Large intestine                                             316
         The ileum                                                   317
         The gastro-ileal folds                                      317
         The colon                                                   317
         The rectum                                                  318
         The vent (anus)                                             319
         Histology of the digestive canal                            320

     _b._ Digestion in insects                                       324

         The mechanism of secretion                                  326
         Absorbent cells                                             328

 THE GLANDULAR AND EXCRETORY APPENDAGES OF THE DIGESTIVE CANAL       331

     _a._ The salivary glands                                        331

     _b._ The silk or spinning glands, and the spinning apparatus    339

         The process of spinning                                     340
         How the thread is drawn out                                 343
         Appendages of the silk-gland (Filippi’s glands)             345

     _c._ The cæcal appendages                                       347

     _d._ The excretory system (urinary or Malpighian tubes)         348

         Primitive number of tubes                                   353

     _e._ Poison-glands                                              357

     _f._ Adhesive or cement-glands                                  360

     _g._ The wax-glands                                             361

     _h._ “Honey-dew” or wax-glands of Aphids                        364

     _i._ Dermal glands in general                                   365

 DEFENSIVE OR REPUGNATORIAL SCENT-GLANDS                             368

     Eversible coxal glands                                          369
     Fœtid glands of Orthoptera                                      369
     Anal glands of beetles                                          372
     The blood as a repellent fluid                                  374
     Eversible glands of caddis-worms and caterpillars               375
     The osmeterium in Papilio larvæ                                 377
     Dorsal and lateral eversible metameric sacs in other larvæ      377
     Distribution of repugnatorial or alluring scent-glands in
       insects                                                       382

 THE ALLURING OR SCENT-GLANDS                                        391

 THE ORGANS OF CIRCULATION                                           397

     _a._ The heart                                                  397

         The propulsatory apparatus                                  401
         The supraspinal vessel                                      403
         The aorta                                                   404
         The pericardial cells                                       405
         Pulsatile organs of the legs                                405

     _b._ The blood                                                  407

         The leucocytes                                              407

     _c._ The circulation of the blood                               409

         Effects of poisons on the pulsations                        412

 THE BLOOD TISSUE                                                    419

     _a._ The fat-body                                               419

     _b._ The pericardial fat-body or pericardial cells              420

         Leucocytes or phagocytes in connection with the
           pericardial cells                                         421

     _c._ The œnocytes                                               423

     _d._ The phosphorescent organs                                  424

         Physiology of the phosphorescence                           426

 THE RESPIRATORY SYSTEM                                              430

     _a._ The tracheæ                                                431

         Distribution of the tracheæ                                 432

     _b._ The spiracles or stigmata                                  437

         The position and number of pairs of stigmata                439
         The closing apparatus of the stigma                         441

     _c._ Morphology and homologies of the tracheal system           442

     _d._ The spiral threads or tænidia                              444

     _e._ Origin of the tracheæ and of the “spiral thread”           447

         Internal, hair-like bodies                                  451

     _f._ The mechanism of respiration and the respiratory
       movements of insects                                          451

     _g._ The air-sacs                                               456

         The use of the air-sacs                                     457

     _h._ The closed or partly closed tracheal system                459

     _i._ The rectal, tracheal gills, and rectal respiration of
       larval Odonata and other insects                              463

     _j._ Tracheal gills of the larvæ of insects                     466

         Blood-gills                                                 475

     _k._ Tracheal gills of adult insects                            476

 THE ORGANS OF REPRODUCTION                                          485

     _a._ The male organs of reproduction                            494

         The testes                                                  495
         The seminal ducts                                           496
         The ejaculatory duct                                        497
         The accessory glands                                        497
         The spermatozoa                                             497
         Formation of the spermatozoön                               498

     _b._ The female organs of reproduction                          500

         The ovaries and the ovarian tubes                           500
         Origin of incipient eggs in the germ of the testes          504
         The bursa copulatrix                                        505
         The spermatheca                                             506
         The colleterial glands                                      506
         The vagina or uterus                                        507
         Signs of copulation in insects                              507


                     PART II. EMBRYOLOGY OF INSECTS

     _a._ The egg                                                    515

         Mode of deposition                                          518
         Vitality of eggs                                            520
         Appearance and structure of the ripe egg                    520
         The egg-shell and yolk-membrane                             520
         The micropyle                                               522
         Internal structure of the egg                               524

     _b._ Maturation or ripening of the egg                          525

     _c._ Fertilization of the egg                                   525

     _d._ Division and formation of the blastoderm                   526

     _e._ Formation of the first rudiments of the embryo and of the
       embryonic membranes                                           531

         Formation of the embryonic membranes                        532
         The gastrula stage                                          535
         Division of the embryo or primitive band into
           body-segments                                             536
         Differences between the invaginated and overgrown
           primitive band                                            538
         Revolution of the embryo where the primitive band is
           invaginated                                               540

     _f._ Formation of the external form of the body                 542

         Origin of the body-segments                                 542
         The procephalic lobes                                       544
         Fore-intestine (stomodæum) and hind-intestine
           (proctodæum), labrum                                      547
         Completion of the head                                      548

     _g._ The appendages                                             548

         The cephalic appendages                                     548
         The thoracic appendages                                     550
         The abdominal appendages                                    550
         Appendages of the first abdominal segment (pleuropodia)     551
         Are the abdominal legs of Lepidoptera and phytophagous
           Hymenoptera true limbs?                                   552
         The tracheæ                                                 553

     _h._ Nervous system                                             554

         Completion of the definite form of the body                 555

     _i._ Dorsal closure and involution of the embryonic membranes   556

     _j._ Formation of the germ-layers                               558

     _k._ Farther development of the mesoderm; formation of the
       body-cavity                                                   563

     _l._ Formation of organs                                        566

         The nervous system                                          566
         Development of the brain                                    567
         Development of the eyes                                     567
         Intestinal canal and glands                                 569
         The salivary glands                                         570
         The urinary tubes                                           572
         The heart                                                   572
         The blood-corpuscles                                        574
         Musculature; connective tissue; fat-body                    574
         The reproductive organs                                     575
         Development of the male germinal glands                     579

     _m._ Length of embryonic life                                   582

     _n._ The process of hatching                                    583

         The hatching spines                                         585


                 PART III. THE METAMORPHOSES OF INSECTS

     _a._ The nymph as distinguished from the larval stage           593

     _b._ Stages or stadia of metamorphosis                          594

     _c._ Ametabolous and metabolous stages                          594

 THE LARVA                                                           599

     _a._ The Campodea-form type of larva                            600

     _b._ The eruciform type of larva                                602

     _c._ Growth and increase in size of the larva                   608

     _d._ The process of moulting                                    609

         The number of moults in insects of different orders         615
         Reproduction of lost limbs                                  619
         Formation of the cocoon                                     619
         Sanitary conditions observed by the honey-bee larva, and
           admission of air within the cocoon                        623

 THE PUPA STATE                                                      625

     _a._ The pupa considered in reference to its adaptation to its
       surroundings and its relation to phylogeny                    631

     _b._ Mode of escape of the pupa from its cocoon                 632

     _c._ The cremaster                                              636

         Mode of formation of the cremaster and suspension of the
           chrysalis in butterflies                                  637

 FORMATION OF THE PUPA AND IMAGO IN THE HOLOMETABOLOUS INSECTS (THE
   DIPTERA EXCEPTED)                                                 640

     _a._ The Lepidoptera                                            642

         The changes in the head and mouth-parts                     646
         The change in the internal organs                           647
         The wings                                                   654
         Development of the feet and of the cephalic appendages      654
         Embryonic cells and the phagocytes                          655
         Formation of the femur and of the tibia; transformation of
           the tarsus                                                656
         The antennæ                                                 657
         Maxillæ and labial palpi                                    658
         Process of pupation                                         660

     _b._ The Hymenoptera                                            661

         Ocular or oculo-cephalic buds                               665
         The antennal buds                                           665
         The buds of the buccal appendages                           665
         The buds of the ovipositor                                  665

 DEVELOPMENT OF THE IMAGO IN THE DIPTERA                             666

     _a._ Development of the outer body-form                         668

         Formation of the imago in Corethra                          668
         Formation of the imago in Culex                             670
         Formation of the imago in Chironomus                        671
         Formation of the imago in Muscidæ                           673

     _b._ Development of the internal organs of the imago            678

         The hypodermis                                              678
         The muscles                                                 680
         The digestive canal                                         681
         The tracheal system                                         683
         The nervous system                                          684
         The fat-body                                                685
         Definitive fate of the leucocytes                           685
         The post-embryonic changes and imaginal buds in the
           Pupipara (Melophagus)                                     686

     _c._ General summary                                            687

 HYPERMETAMORPHISM                                                   688

 SUMMARY OF THE FACTS AND SUGGESTIONS AS TO THE CAUSES OF
   METAMORPHISM                                                      705

     Theoretical conclusions; causes of metamorphosis                708




                        TEXT-BOOK OF ENTOMOLOGY


[Illustration]




                   PART I.—MORPHOLOGY AND PHYSIOLOGY

[Illustration]


               POSITION OF INSECTS IN THE ANIMAL KINGDOM


Although the insects form but a single class of the animal kingdom, they
are yet so numerous in orders, families, genera, and species, their
habits and transformations are so full of instruction to the biologist,
and they affect human interests in such a variety of ways, that they
have always attracted more attention from students than any other class
of animals, the number of entomologists greatly surpassing that of
ornithologists, ichthyologists, or the special students of any other
class, while the literature has assumed immense proportions.

Insects form about four-fifths of the animal kingdom. There are about
250,000 species already named and contained in our museums, while the
number of living and fossil species in all is estimated to amount to
between one and two millions.

In their structure insects are perhaps more complicated than any other
animals. This is partly due to the serial arrangement of the segments
and the consequent segmental repetition of organs, especially of the
external appendages, and of the muscles, the tracheæ, and the nerves.
The brain is nearly or quite as complicated as that of the higher
vertebrates, while the sense-organs, especially those of touch, sight,
and smell are, as a rule, far more numerous and only less complex than
those of vertebrates. Moreover, in their psychical development, certain
insects are equal, or even superior, to any other animals, except birds
and mammals.

The animal kingdom is primarily divided into two grand divisions, the
one-celled (_Protozoa_) and many-celled animals (_Metazoa_). In the
latter group the cells and tissues forming the body are arranged in
three fundamental cell-layers; viz. the _ectoderm_ or outer layer, the
_mesoderm_, and _endoderm_. The series of branches, or phyla, comprised
under the term Metazoa are the Porifera, Cœlenterata, Vermes,
Echinodermata, Mollusca, Arthropoda, and Vertebrata. Their approximate
relationships may be provisionally expressed by the following


   TABULAR VIEW OF THE EIGHT BRANCHES OR PHYLA OF THE ANIMAL KINGDOM.

 VIII. _Vertebrata._
 Ascidians and Fishes
       to Man.
     |
     |         VII. _Arthropoda._
     |  Trilobites, Crustacea, Arachnida,
     |           Insects, etc.
     |                  |
     |                  |       VI. _Mollusca._
     |                  |   Clams, Snails, Cuttles.
     |                  |          |
     |                  |          |           V. _Echinodermata._
     |                  |          | Crinoids, Star-fish, Sea-urchins, etc.
     |                  |          |                     |
     |                  +----------+----------+----------+
     |                                        |
     +---------------------------+------------+
                                 |
                           IV. _Vermes._
       Flat and Round Worms, Polyzoa, Brachiopods, Annelids.
                                 |
   III. _Cœlenterata._           |           II. _Porifera._
    Hydra, Jelly-fishes.         |               Sponges.
              |                  |                  |
              +------------------+------------------+
                                 |
                              METAZOA.
               Many-celled animals with 3 cell-layers.
                                 |
                            I. PROTOZOA.
                       Single-celled animals.




                RELATIONS OF INSECTS TO OTHER ARTHROPODA


The insects by general consent stand at the head of the Arthropoda.
Their bodies are quite as much complicated or specialized, and indeed,
when we consider the winged forms, more so, than any other class of the
branch, and besides this they have wings, fitting them for an aërial
life. It is with little doubt that to their power of flight, and thus of
escaping the attacks of their creeping arthropod enemies, insects owe,
so to speak, their success in life; _i.e._ their numerical superiority
in individuals, species, and genera. It is also apparently their power
of moving or swimming swiftly from one place to another which has led to
the numerical superiority in species of fishes to other Vertebrata.
Among terrestrial vertebrates, the birds, by virtue of their ability to
fly, greatly surpass in number of species the reptiles and mammals.

The Arthropoda are in general characterized by having the body composed
of segments (somites or arthromeres) bearing jointed appendages. They
differ from the worms in having segmented appendages, _i.e._ antennæ,
jaws, and legs, instead of the soft unjointed outgrowths of the annelid
worms. Moreover, their bodies are composed of a more or less definite
number of segments or rings, grouped either into a head-thorax
(cephalothorax) and hind-body, as in Crustacea, or into a head
differentiated from the rest of the body (trunk), the latter not being
divided into a distinct thorax and abdomen, as in Myriopoda; or into
three usually quite distinct regions—the head, thorax, and hind-body or
abdomen, as in insects. In certain aberrant, modified forms, as the
Tardigrada, or the Pantopoda, and the mites, the body is not
differentiated into such definite regions.

In their internal organs arthropods agree in their general relations
with the higher worms, hence most zoölogists agree that they have
directly originated from the annelid worms.

The position and general shape of the digestive canal, of the nervous
and circulatory systems, are the same in Arthropoda as in annelid
(oligochete) worms, so much so that it is generally thought that the
Arthropoda are the direct descendants of the worms. It is becoming
evident, however, that there was no common ancestor of the Arthropoda as
a whole, and that the group is a polyphyletic one. Hence, though a
convenient group, it is a somewhat artificial one, and may eventually be
dismembered into at least three or four phyla or branches.

The following diagram may serve to show in a tentative way the relations
of the classes of Arthropoda to each other, and also may be regarded as
a provisional genealogical tree of the branch.

                                    9. _Insecta._
                                          |                  7. _Chilopoda._
                          4. _Arachnida._ |                           |
                                 |        | 6. _Diplopoda._           |
                                 |        |        |                  |
         3. _Merostomata._       |  8. _Symphyla._ |                  |
                 |               |        |        |                  |
 1. _Crustacea._ |               |        |        |6_a_. _Pauropoda._|
        |        |2. _Trilobita._|        |        |       |          |
        |        |       |       |        |        +-------+          |
        |        |       |       |        |        |                  |
        |        +-------+-------+        +--------+------------------+
        |                |                |
        |                |                +--------+
        |                |                         |
        |                |                  5. _Peripatus._
        |                |4_a_. _Pantopoda._         |
        |                |       |                   |
        |                |       |4_b_. _Tardigrada._|
        |                |       |        |          |
        +----------------+-------+--------+----------+
                         |
               _Different Annelida._
                   Trochosphæra.

We will now rapidly review the leading features of the classes of
Arthropoda.

=The Crustacea.=—These Arthropoda are in many most important
characteristics unlike the insects; they have two pairs of antennæ, five
pairs of buccal appendages, and they are branchiate Arthropoda. They
have evidently originated entirely independently, and by a direct line
of descent from some unknown annelid ancestor which was either a
many-segmented worm, with parapodia, or the two groups together with the
Rotifera may have originated from a common appendigerous Trochosphæra.
Their segments in the higher forms are definite in number (23 or 24) and
arranged into two regions, a head-thorax (cephalothorax) and hind-body
(abdomen). Nearly all the segments, both of the cephalothorax and
abdomen, bear a pair of jointed limbs, and to them at their base are, in
the higher forms, appended the gills (branchiæ). The limbs are in the
more specialized forms (shrimps and crabs) differentiated into
eye-stalks, two pairs of antennæ, a pair of palpus-bearing jaws
(mandibles), two pairs of maxillæ and three pairs of maxillipeds; these
appendages being biramose, and the latter bearing gills attached to
their basal joints. The legs are further differentiated into ambulatory
thoracic legs and into swimming or abdominal legs, and in the latter the
first pair of the male is modified into copulatory organs (gonopoda).
The male and female reproductive organs as a rule are in separate
individuals, hermaphrodites being very unusual, and the glands may be
paired or single. The sexual outlets are generally paired, and, as in
the male lobster and other Macrura, open in the basal joint of the last
pair of legs, and in the female in the third from the last; while
originally in all Crustacea the sexual organs were most probably paired
(Fig. 3, _B_).

They are, except a few land Isopoda, aquatic, mostly marine, and when
they have a metamorphosis, pass through a six-legged larval stage,
called the Nauplius, the shrimps and crabs passing through an additional
stage, the Zoëa. Crustacea also differ much from insects in the highly
modified nature of the nephridia, which are usually represented by the
green gland of the lobster, or the shell-glands of the Phyllopoda, which
open out in one of the head-segments; also in the possession of a pair
of large digestive glands, the so-called liver.

Intermediate in some respects between the Crustacea and insects, but
more primitive, in respect to what are perhaps the most weighty
characters, than the Crustacea, are the Trilobita, the Merostomata
(Limulus), and, finally, the Arachnida, these being allied groups. In
the Trilobita and Merostomata (Limulus), the head-appendages are more
like feet than jaws, while they have in most respects a similar mode of
embryonic development, the larval forms being also similar.

[Illustration:

  FIG. 1.—Restoration of under side of a trilobite (_Triarthrus becki_),
    the trunk limbs bearing small triangular respiratory lobes or
    gills.—After Beecher.
]

=The Merostomata.=—The only living form, Limulus, is undoubtedly a very
primitive type, as the genital glands and ducts are double, opening wide
apart on the basal pair of abdominal legs (Fig. 3). Moreover, their
head-appendages, which are single, with spines on the basal joint, are
very primitive and morphologically nearer in shape to those of the worms
(Syllidæ, etc.) than even those of the Crustacea. Besides, their four
pairs of coxal glands, with an external opening at the base of the fifth
pair of head-appendages, and which probably are modified nephridia
(Crustacea having but a single pair in any one form, either opening out
on the second antennal, green gland, or second maxillary, shell-gland,
segment), indicate a closer approximation to the polynephrous worms.
Limulus has other archaic features, especially as regards the structure
of the simple and compound eyes and the simple nature of the brain.

=The Trilobita.=—These archaic forms are still more generalized and
primitive than the Merostomata and Crustacea, and probably were the
first Arthropoda to be evolved from some unknown annelid worm. They had
jointed biramose limbs of nearly uniform shape and size on each segment
of the body, which were not, as in Crustacea, differentiated into
antennæ, jaws (mandibles), maxillæ, maxillipeds, and two kinds of legs
(thoracic and abdominal), showing that they are a much more primitive
type, and nearer to the annelids than any other Arthropoda. Their gills,
as shown by the researches of Walcott and of Beecher, were attached to
nearly if not every pair of limbs behind the antennæ (Figs. 1, 2). The
fact that in Trilobita the first pair of limbs is antenniform does not
prove that they are Crustacea, since Eurypterus has a similar pair of
appendages.

[Illustration:

  FIG. 2.—Restored section of Calymene: _C_, carapace; _en_, endopodite;
    _en′_, exopodite; with the gills on the epipodal or respiratory part
    of the appendage.—After Walcott.
]

The limbs in trilobites, as well as the abdominal ones of merostomes,
and all those of Crustacea, except the first antennæ, are biramose,
consisting of an outer (exopodite) and an inner division (endopodite).
In this respect the terrestrial air-breathing tracheate forms,
Arachnida, Myriopoda, and Insecta, differ from the branchiate forms, as
their legs are single or undivided, being adapted for supporting the
body during locomotion upon the solid earth. It is to be observed that
when, as in Limulus, the body is supported by cephalic ambulatory limbs,
they are single, while the abdominal limbs, used as they are in
swimming, are biramose, much as in Crustacea.

=The Arachnida.=—The scorpions and spiders are much less closely allied
to the myriopods and insects than formerly supposed. Their embryology
shows that they have descended from forms related to Limulus, possibly
having had an origin in common with that animal, or having, as some
authors claim, directly diverged from some primitive eurypteroid
merostome. But they differ in essential respects, and not only in the
nature and grouping of their appendages; the first pair instead of
antenniform being like mandibles, and the second pair like the maxillæ,
with the palps, of insects, the four succeeding segments (thoracic)
bearing each a pair of legs. They also have a brain quite unlike that of
Limulus, the nervous cord behind the brain, however, being somewhat
similar, though that of Limulus differs in being enveloped by an
arterial coat. Arachnida respire by tracheæ, besides book-lungs, which,
however, are possibly derivatives of the book-gills of Limulus, while
they perform the office of excretion by means of the malpighian tubes,
and like Limulus possess two large digestive glands (“liver”). Their
embryos have, on at least six abdominal segments, rudiments of limbs,
three pairs of which form the spinnerets, showing their origin from
Limulus-like or eurypteroid forms; their coxal glands are retained from
their eurypteroid ancestors. The Arachnida probably descended from
marine merostomes, and not from an independent annelid ancestry, hence
we have represented them in the diagram on p. 3 as branching off from
the merostomatous phylum, rather than from an independent one.

[Illustration:

  FIG. 3.—Paired genital openings of different classes of arthropods.
    _A_, the most primitive, of _Limulus polyphemus_: _gen. p_,
    generative papillæ; _d_, duct; _vd_, vas deferens; _t_, tendinous
    stigmata; _stig_, stigmata; _e_, external branchial muscle; _ant_,
    anterior lamellar muscle.—After Benham, with a few changes. _B_,
    lobster (_Homarus vulgaris_), ♀: _oe_, genital aperture on 3d pair
    of legs; _ov_, ovary; _u_, unpaired portion of the same; _od_,
    oviduct. _C_, ♀, scorpion: _ov_, ovary, with a single external
    opening. _D_, ♂: _t_, testis; _vd_, vasa deferentia; _sb_, seminal
    vesicle; _a_, glandular appendage; _p_, penis.—After Blanchard. _E_,
    a myriopod (_Glomeris marginata_, ♀): _os_, ovarian sac, laid open;
    _od_, paired oviducts. _F_, ♂: _t_, testis; _gvd_, common vas
    deferens; _pa_, paired ducts.—After Favre, from Lang. _G_, _Lepisma
    saccharina_, young ♂: _vd_, vas deferens, _ed_, ejaculatory duct;
    _ga_, external appendages.—After Nassonow. _H_, Ephemera, ♂, showing
    the double outlets.—After Palmén.
]

The characters in which arachnids approach insects, such as tracheæ and
malpighian tubes (none occur, as a rule, in marine or branchiate
arthropods), may be comparatively recent structures acquired during a
change from a marine to a terrestrial life, and not primitive heirlooms.

Arachnida also show their later origin than merostomes by the fact that
their sexual glands are in most cases single, and though with rare
exceptions the ducts are paired, these finally unite and open externally
by a common single genital aperture in the median line of the body, at
the base of the abdomen (Fig. 3, _C_, _D_). In this respect Limulus,
with its pair of genital male or female openings, situated each at the
end of a papilla, placed widely apart at the base of the first abdominal
limbs, is decidedly more archaic. Unlike Crustacea and insects,
Arachnida do not, except in the mites (Acarina), which is a very much
modified group, undergo a metamorphosis.

We see, then, that the insects, with the Myriopoda, are somewhat
isolated from the other Arthropoda. The Myriopoda have a single pair of
antennæ, and as they have other characters in common with insects, Lang
has united the two groups in a single class _Antennata_; but, as we
shall see, this seems somewhat premature and unnecessary. Yet the two
groups have perhaps had a common parentage, and may prove to belong to a
distinct, common phylum.

Not only by their structure and embryology, as well as their
metamorphosis, do the myriopods and insects stand apart from the
Arachnida and other arthropods, but it seems probable that they have had
a different ancestry, the arthropods being apparently polyphyletic.

There are two animals which appear to connect the insects with the
worms, and which indicate a separate line of descent from the worms
independent of that of the other classes. These are the singular
Peripatus, which serves as a connecting link between arthropods and
worms, and Scolopendrella (Symphyla). These two animals are guide-posts,
pointing out, though vaguely to be sure, the way probably trod by the
forms, now extinct, which led up to the insects.

=Relations of Peripatus to Insects.=—We will first recount the
characteristics of this monotypic class. Peripatus (Fig. 4) stands
alone, with no forms intermediate between itself and the worms on the
one hand, and the true Arthropoda on the other. Originally supposed to
be a worm, it is now referred to a class by itself, the Malacopoda of
Blainville, or Protracheata of Haeckel. It lives in the tropics, in damp
places under decaying wood. In general appearance it somewhat resembles
a caterpillar, but the head is soft and worm-like, though it bears a
pair of antenna-like tentacles. It may be said rather to superficially
resemble a leech with clawed legs, the skin and its wrinkles being like
those of a leech. There is a pair of horny jaws in the mouth, but these
are more like the pharyngeal teeth of worms than the jaws of arthropods.
The numerous legs end each in a pair of claws. The ladder-like nervous
system is unlike that of annelid worms or arthropods, but rather recalls
that of certain molluscs (Chiton, etc.), as well as that of certain flat
and nemertine worms. Its annelid features are the large number of
segmentally arranged true nephridia, and the nature of the integument.
Its arthropodan features, which appear to take it out of the group of
worms, are the presence of tracheæ, of true salivary and slime glands,
of a pair of coxal glands (Fig. 4, _C_, _cd_) as well as the claws at
the end of the legs. The tracheæ, which are by no means the only
arthropodan features, are evidently modified dermal glands. The heart is
arthropodan, being a dorsal tube lying in a pericardial sinus, with many
openings. This assemblage of characters is not to be found in any marine
or terrestrial worm.

The tracheæ (Fig. 4, _D_, _tr_) are unbranched fine tubes, without a
“spiral thread,” and are arranged in tufts, in _P. edwardsii_ opening by
simple orifices or pores (“stigmata”) scattered irregularly over the
surface of the body; but in another species (_P. capensis_) some of the
stigmata are arranged more definitely in longitudinal rows,—on each side
two, one dorsally and one ventrally. “The stigmata in a longitudinal row
are, however, more numerous than the pairs of legs.” (Lang.)

The salivary glands, opening by a short common duct into the under side
of the mouth, in the same general position as in insects, are evidently,
as the embryology of the animal proves, transformed nephridia, and being
of the arthropodan type explain the origin and morphology of those of
insects. It is so with the slime glands; these, with the coxal glands,
being transformed and very large dermal glands. Those of insects arose
in the same manner, and are evidently their homologues, while those of
Peripatus were probably originally derived from the setiparous glands in
the appendages (parapodia) of annelid worms.

[Illustration:

  FIG. 4.—_A_, _Peripatus novæ zealandiæ_.—After Sedgwick, from Lang.
    _B_, _Peripatus capensis_, side view, enlarged about twice the
    natural size.—After Moseley, from Balfour. _C_, Anatomy of
    _Peripatus capensis_. The enteric canal behind the pharynx has been
    removed. _g_, brain; _a_, antenna; _op_, oral or slime papillæ;
    _sd_, slime gland; _sr_, slime reservoir, which at the same time
    acts as a duct to the gland; _so_{4}_, _so_{5}_, _so_{6}_, _so_{9}_,
    nephridia of the 4th, 5th, 6th, and 9th pairs of limbs; _cd_,
    elongated coxal gland of the last pair of feet; _go_, genital
    aperture; _an_, anus; _ph_, pharynx; _n_, longitudinal trunk of the
    nervous system.—After Balfour, from Lang. _D_, Portion of the body
    of _Peripatus capensis_ opened to show the scattered tufts of
    tracheæ (_tr_); _v_, _v_, ventral nerve cords.—After Moseley.
]

The genital glands and ducts are paired, but it is to be observed that
the outlets are single and situated at the end of the body. In the male
the ejaculatory duct is single; in its base a spermatophore is formed.
It will be seen, then, that Peripatus is not only a composite type, and
a connecting link between worms and tracheate arthropods, but that it
may reasonably be regarded, if not itself the ancestor, as resembling
the probable progenitor of myriopods and insects, though of course there
is a very wide gap between Peripatus and the other antennate,
air-breathing Arthropoda.

[Illustration:

  FIG. 4.—_E_, _Peripatus edwardsii_, head from the under side: _a_,
    base of antenna; _op_, oral papilla; the figure also shows the
    papillæ around the mouth, and the four jaws.—After Balfour, from
    Lang. _F_, Anterior end of _Peripatus capensis_, ventral side, laid
    open: _a_, antenna; _z_, tongue; _k_, jaw; _sd_, salivary gland;
    _gs_, union of the two salivary glands; _ph_, pharynx; _œ_,
    œsophagus; _l_, lip papillæ around the mouth; _op_, oral or slime
    papilla; _sld_, duct or reservoir of the slime gland.—After Balfour,
    from Lang.
]

=Relation of Myriopods to Insects.=—The Myriopoda are the nearest allies
of the insects. They have a distinct head, with one pair of antennæ. The
eyes are simple, with the exception of a single genus (Cermatia), in
which they are aggregated or compound. The trunk or body behind the head
is, as a rule, long and slender, and composed of a large but variable
number of segments, of equal size and shape, bearing jointed legs, which
invariably end in a single claw.

The mouth-parts of the myriopods are so different in shape and general
function from those of insects, that this character, together with the
equally segmented nature of the portion of the body behind the head (the
trunk), forbids our merging them, as some have been inclined to do, with
the insects. There are two sub-classes of myriopods, differing in such
important respects that by Pocock[1] and by Kingsley they are regarded
as independent classes, each equivalent to the insects.

Of these the most primitive are the Diplopoda (Chilognatha), represented
by the galley-worms (Julus, etc.).

[Illustration:

  FIG. 5.—Mandible of Julus: _l_, lacinia; _g_, galea; _p_, dens
    mandibularis; _ma_, “mala”; _lt_, lamina tritoria; _st_, stipes;
    _c_, cardo; _m_, muscle.—After Latzel.
]

In the typical Diplopoda the head consists of three segments, a preoral
or antennal, and two postoral, there being two pairs of jaw-like
appendages, which, though in a broad morphological sense homologues of
the mandibles and first maxillæ of insects, are quite unlike them in
details.

[Illustration:

  FIG. 6.—Under lip or deutomala of _Scoterpes copei_: _hyp_, hypostoma
    or mentum; _lam. lab_, lamina labialis; _stip. e_, stipes exterior;
    with the malella exterior (_mal. e_) and malella interior (_mal.
    i_); the stipes interior, with the malulella; and the labiella
    (hypopharynx of Vom Rath) with its stilus (_stil._).
]

As we have previously stated,[2] the so-called “mandibles” of diplopods
are entirely different from those of insects, since they appear to be 2–
or 3–jointed, the terminal joint being 2–lobed, thus resembling the
maxillæ rather than the mandibles of insects, which consist of but a
single piece or joint, probably the homologue of the galea or molar
joint of the diplopod protomala. The mandible of the Julidæ (Fig. 5,
_Julus molybdinus_), Lysiopetalidæ, and Polydesmidæ consists of three
joints; viz. a basal piece or cardo, a stipes, and the _mala
mandibularis_, which supports two lobes analogous to the galea and
lacinia of the maxilla of an insect. There is an approach, as we shall
see, in the mandible of Copris, to that of the Julidæ, but in insects in
general the lacinia is wanting, and the jaw consists of but a single
piece.

The deutomalæ (gnathochilarium), or second pair of diplopod jaws, are
analogous to the labium or second maxillæ of insects, forming a
flattened, plate-like under-lip, constituting the floor of the mouth
(Fig. 6). This pair of appendages needs farther study, especially in the
late embryo, before it can be fully understood. So far as known, judging
by Metschnikoff’s work on the embryology of the diplopods, these
myriopods seem to have in the embryo but two pairs of post-antennal
mouth-parts, which he designated as the “mandibles” and “labium.”
Meinert, however, regards as a third pair of mouth-parts or “labium”
what in our Fig. 7 is called the internal stipes (_stip. i._), behind
which is a triangular plate, _lamina labialis_ (_lam. lab_), which he
regards as the sternite of the same segment.

[Illustration:

  FIG. 7.—Deutomala of Julus, the lettering as in Fig. 6.
]

[Illustration:

  FIG. 8.—Head of _Scolopendra_, seen from beneath, showing the
    “mandible” (protomala) with its cardo (_card._) and stipes (_st._),
    also the labrum and epilabrum.
]

The hypopharynx, our “labiella,” (Fig. 6), with the supporting rods or
_stili linguales_ (_sti. l_), of Meinert, are of nearly the same shape
as in some insects.

Of the clypeus of insects there is apparently no homologue in myriopods,
though in certain diplopods there is an interantennal clypeal region.
The labium of insects is represented by a short, broad piece, which,
however, unlike that of insects, is immovable, and is flanked by a
separate piece called the _epilabrum_ (Fig. 8). Vom Rath has observed an
epipharynx, which has the same general relations as in insects.

[Illustration:

  FIG. 9.—Larva of Julus: _a_, the 3d abdominal segment, with the new
    limbs just budding out; _b_, new segments arising between the
    penultimate and the last segment.—After Newport.
]

The embryology of myriopods is in many respects like that of insects.
The larva of diplopods hatches with but few segments, and with but three
pairs of limbs; but these are not, as in insects, appended to
consecutive segments, but in one species the third, and in another,
_Julus multistriatus?_ (Fig. 10), the second, segment from the head is
footless, while Vom Rath represents the first segment of an European
Blaniulus as footless, the feet being situated consecutively on segments
2 to 4. The new segments arise at “the growing point” situated between
the last and penultimate segment, growing out in groups of sixes
(Newport) or in our _Julus multistriatus?_ in fives (Fig. 10). In adult
life diplopods (Julus) have a single pair of limbs on the three first
segments, or those corresponding to the thoracic segments of insects,
the succeeding segments having two pairs to each segment.

[Illustration:

  FIG. 10.—Freshly hatched larva of _Julus multistriatus?_ 3 mm. long:
    _a_, 5 pairs of rudimentary legs, one pair to a segment.
]

  Sinclair (Heathcote) regards each double segment in the diplopods as
  not two original segments fused together, nor a single segment
  bearing two pairs of legs, but as “two complete segments perfect in
  all particulars, but united by a large dorsal plate which was
  originally two plates which have been fused together.” (Myriopods,
  1895, p. 71.) That the segments were primitively separate is shown,
  he adds, by the double nature of the circulatory system, the nerve
  cord, and the first traces of segmentation in the mesoblast. Kenyon
  believes that from the conditions in pauropods, Lithobius, etc.,
  there are indications of alternate plates (not segments) having
  disappeared, and of the remaining plates overgrowing the segments
  behind them, so as to give rise to the anomalous double segments.[3]

[Illustration:

  FIG. 11.—Sixth pair of legs of _Polyzonium germanicum_, ♀: _cs_,
    ventral sacs; _cox_, coxa; _st_, sternal plate; _sp_,
    spiracle.—After Haase.
]

Diplopods are also provided with eversible coxal sacs, in position like
those of Symphyla and Synaptera; Meinert, Latzel, and also Haase having
detected them in several species of Chordeumidæ, Lysiopetalidæ, and
Polyzonidæ (Fig. 11). In _Lysiopetalum anceps_ these blood-gills occur
in both sexes between the coxæ of the third to sixteenth pair of limbs.
In the Diplopods the blood-gills appear to be more or less permanently
everted, while in Scolopendrella they are usually retracted within the
body (Fig. 15, _cg_).

Diplopods also differ externally from insects in the genital armature, a
complicated apparatus of male claspers and hooks apparently arising from
the sternum of the sixth segment and being the modified seventh pair of
legs. In myriopods there are no pleural pieces or “pleurites,” so
characteristic of winged insects.

Perhaps the most fundamental difference between diplopods and insects is
the fact that the paired genital openings of the former are situated not
far behind the head between the second and third pair of legs. Both the
oviducts and male ejaculatory ducts are paired, with separate openings.
The genital glands lie beneath, while in chilopods they lie above the
intestine; this, as Korschelt and Heider state, being a more primitive
relation, since in Peripatus they also lie above the digestive canal.

The nervous system of diplopods is not only remarkable for the lack of
the tendency towards a fusion of the ganglia observable in insects, but
for the fact that the double segments are each provided with two
ganglia. The brain also is very small in proportion to the ventral cord,
the nervous system being in its general appearance somewhat as in
caterpillars.

The arrangement of the tracheæ and stigmata is much as in insects, but
in the Diplopoda the tracheary system is more primitive than in
chilopods, a pair of stigmata and a pair of tracheal bundles occurring
in each segment, while the bundles are not connected by anastomosing
branches, branched tracheæ only occurring in the Glomeridæ. The tracheæ
themselves are without spiral threads (tænidia). It is noteworthy that
the tracheæ arise much later than in insects, not appearing until the
animal is hatched; in this respect the myriopods approximate Peripatus.

In the Chilopoda also the parts of the head, except the epicranium, are
not homologous with those of insects, neither are the mouth-parts, of
which there are five pairs.

The structure of the head of centipedes is shown in part in Fig. 12,
compare also Fig. 8. It will be seen that it differs much from that of
the diplopods, though the mandibles (protomalæ) are homologous; they are
divided into a cardo and stipes, thus being at least two-jointed.

The second pair of postoral appendages is in centipedes very different
from the gnathochilarium of diplopods. As seen in Fig. 12 2, they are
separate, cylindrical, fleshy, five-jointed appendages, the maxillary
appendages of Newport, which are “connected transversely at their base
with a pair of soft appendages” (_c_), the lingua of Newport. The third
and fourth pair are foot-jaws, and we have called them _malipedes_, as
they have of course no homology with the maxillipedes of Crustacea. The
second pair of these malipedes, forming the last pair of
mouth-appendages, is the poison-fangs (4), which are intermediate
between the malipedes and the feet; Meinert does not allow that these
are mouth-appendages.

[Illustration:

  FIG. 12.—Structure of a chilopod. _A_, _Lithobius americanus_, natural
    size. _B_, under side of head and first two body-segments and legs,
    enlarged: _ant_, antenna; 1, jaws; 2, first accessory jaw; _c_,
    lingua; 3, second accessory jaw and palpus; 4, poison-jaw. (Kingsley
    del.) _C_, side view of head (after Newport): _ep_, epicranium; _l_,
    frontal plate; _sc_, scute; 1, first leg; _sp_, spiracle.
]

The embryology of Geophilus by Metschnikoff shows plainly the four pairs
of post-antennal appendages. The embryo Geophilus is hatched in the form
of the adult, having, unlike the diplopods, no metamorphosis, its
embryological history being condensed or abbreviated. But in examining
Metschnikoff’s figures certain primitive diplopod features are revealed.
The body of the embryo shortly before hatching is cylindrical; the
sternal region is much narrower than in the adult, hence the insertions
of the feet are nearer together, while _the first six pairs of
appendages begin to grow out before the hinder ones_. Thus the first six
pairs of appendages of the embryo Geophilus correspond to the antennæ,
two pairs of jaws, and three pairs of legs of the larval Julus. These
features appear to indicate that the chilopods may be an offshoot from
the diplopod stem. The acquisition of a second pair of legs to a segment
in diplopods, as in the phyllopod Crustacea, is clearly enough a
secondary character, as shown by the figures of Newport in his memoir on
the development of the Myriopoda (Pl. IV.). Thus the tendency in the
Myriopoda, both diplopods and chilopods, is towards the multiplication
of segments and the elongation of the body, while in insects the
polypodous embryo has the three terminal segments of the abdomen well
formed, these being, however, before hatching, partly atrophied, so that
the body of insects after birth tends to become shortened or condensed.
This indicates the descent of insects from ancestors with elongated
polypodous hind-bodies like Scolopendrella. Korschelt and Heider suggest
that the stem-form of myriopods was a homonomously jointed form like
Peripatus, consisting of a rather large number of segments, but we
might, with Haase, consider that the great number of segments which we
now find indicates a late acquisition of this form.

The genital opening in chilopods is single, and situated in the
penultimate segment of the body, as in insects. While recognizing the
close relationship of the Myriopoda with the insects, it still seems
advisable not to unite them into a single group (as Oudemans, Lang, and
others would do), but to regard them as forming an equivalent class. On
the other hand, when we take into account the form and structure of the
head, antennæ, and especially the shape of the first pair of
mouth-appendages, being at least two-jointed in both groups, we think
these characters, with the homonomously segmented body behind the head,
outweigh the difference in the position of the genital outlet, important
as that may seem. It should also be taken into account that while
insects are derived from polypodous ancestors, no one supposes, with the
exception of one or two authors, that these ancestors are the Myriopoda,
the latter having evidently descended from a six-legged ancestor, quite
different from that of the Campodea ancestor of insects.[4]

In regard to the sexual openings of worms, though their position is in
general in the anterior part of the body, it is still very variable,
though, in general, paired. In the oligochete worms the genital zone,
with the external openings, is formed by the segments lying between the
9th and 14th rings, though in some the genital organs are situated still
nearer the head. The myriopods, which evolved from the worms earlier
than insects, appear to have in their most primitive forms (the
Diplopoda) retained this vermian position of the genital outlets. In the
later forms, the chilopods, the genital openings have been carried back
to near the end of the body, as in insects. From observations made by
three different observers on the freshly hatched larva of the Julidæ, it
appears that the ancestral diplopods were six-footed, or oligopod, the
larva of Pauropus (Fig. 13) approaching nearest to our idea of the
ancestral myriopod, which might provisionally be named Protopauropus.

=Relations of the Symphyla to Insects.=—Opinions respecting the position
of the Symphyla, represented by Scolopendrella (Fig. 14), are very
discordant. By most writers since Newport, Scolopendrella has been
placed among the myriopods. The first author, however, to examine its
internal anatomy was Menge (1851), who discovered among other structures
(tracheæ, etc.) the silk-glands situated in the last two segments, and
which open at the end of each cercus. He regarded the form as “the type
of a genus or family intermediate between the hexapod Lepismidæ and the
Scolopendridæ.”

[Illustration:

  FIG. 13.—_Pauropus huxleyi_, much enlarged. _A_, enlarged view of
    head, antennæ, and first pair of legs (original). _B_, young.—After
    Lubbock. _C_, longitudinal section of _Pauropus huxleyi_, ♂: _a_,
    brain; _b_, salivary gland; _k_, mid-intestine; _g_, rectum; _h_,
    ventral nerve-cord; _c_, bud-like remnants of coxæ; _d_, penis; _e_,
    vesicula seminalis; _f_, ductus glandularis; _i_^1, divisions of
    testes.—After Kenyon.
]

In 1873[5] the writer referred to this form as follows: “It may be
regarded as a connecting link between the Thysanura and Myriopoda, and
shows the intimate relation of the myriopods and the hexapods, perhaps
not sufficiently appreciated by many zoölogists.”

In 1880 Ryder regarded it as “the last survival of the form from which
insects may be supposed to have descended,” and referred it to “the new
ordinal group Symphyla, in reference to the singular combination of
myriopodous, insectean, and thysanurous characters which it
presents.[6]”

[Illustration:

  FIG. 14.—_Scolopendrella immaculata_, from above,—after Lang; also
    from beneath, the genital opening on the 4th trunk-segment: _sac_,
    eversible or coxal sac; _an_, anus; _c_, cereopod; _v_, vestigial
    leg.—After Haase, from Peytoureau. _A_ _B_ _C_, head and buccal
    appendages of _Scolopendrella immaculata_: _A_, head seen from
    above; _cl_, clypeus. _B_, head from beneath; _l_, first pair of
    legs; _mx_, 1st maxilla; _mx_^1, 2d maxilla; _t_, “labial plates” of
    Latzel, labium of Muhr. _C_, 1st maxilla; _l_, lacinia; _g_, galea;
    _p_, rudiment of the palpus.—After Latzel. _D_, end of the body:
    _p__{11}, eleventh, _p__{12}, twelfth undeveloped pair of legs;
    _p__{13}, modified, vestigial legs, bearing tactile organs (_so_);
    _sg_, cercopod, with duct of spinning gland, _dg_; _cd_, eversible
    or coxal gland; _h_{8}s_, coxal spur of the 11th pair of legs.—After
    Latzel from Lang.
]

Wood-Mason considered it to be a myriopod, and “the descendant of a
group of myriopods from which the Campodeæ, Thysanura, and Collembola
may have sprung.” We are indebted to Grassi for the first extended work
on the morphology of Scolopendrella (1885). In 1886 he added to our
knowledge facts regarding the internal anatomy, and gives a detailed
comparison with the Thysanura, besides pointing out the resemblances of
Scolopendrella to Pauropus, diplopods, chilopods, as well as Peripatus.

[Illustration:

  FIG. 15.—Section of _Scolopendrella immaculata_: _œ_, œsophagus; _oe.
    v_, œsophageal valve entering the mid-intestine (“stomach”); _i_,
    intestine; _r_, rectum; _br_, brain; _ns_, abdominal chain of
    ganglia; _ovd_, oviduct; _ov_, ovary; _s. gl_, silk-gland, and _op_,
    its outer opening in cercus, _ur. t_, urinary tube; _cg_, coxal
    glands or blood-gills.—Author _del._
]

In 1888 Grassi expressed his view as to the position of the Symphyla,
stating that it should not be included in the Thysanura, since it
evidently has myriopod characters; these being the supraspinal vessel,
the ventral position of the genital glands; the situation of the genital
opening in the fourth segment of the trunk, its ganglionic chain being
like that of diplopods, its having limbs on all the segments, etc. On
the other hand, Grassi has with much detail indicated the points of
resemblance to the Thysanura. The principal ones are the thin
integument, the want of sympathetic ganglia, the presence of a pair of
cephalic stigmata, like that said to occur in certain Collembola, and in
the embryo of Apis; two endoskeletal processes situated near the ventral
fascia of the head; the epicranial suture also occurring in Thysanura,
Collembola, Orthoptera, and other winged insects, and being absent in
diplopods and chilopods. He also adds that the digestive canal both in
Symphyla and Thysanura is divided into three portions; the malpighian
tubes in Thysanura present very different conditions (there being none
in Japyx), among which may be comprised those of Scolopendrella. In both
groups there is a single pair of salivary glands. The cellular
epithelium of the mid-intestine of Scolopendrella is of a single form as
in Campodea and Japyx. The fat-body, dorsal vessel, with its valves and
ostia, are alike in the two groups, as are the appendages of the end of
the abdomen, the anal cerci (cercopoda) of Scolopendrella being the
homologues of the multiarticulate appendages of Lepisma, etc., and of
the forceps of Japyx. In those of Scolopendrella, we have found the
large duct leading from the voluminous silk-gland, a single large sac
extending forwards into the third segment from the end of the body (Fig.
15, _s. gl_). Other points of resemblance, all of which he enumerates,
are the slight differences in the number of trunk-segments, the presence
in the two groups of the abdominal “false-legs” (parapodia), the dorsal
plate, and the mouth-parts. As regards the latter, Grassi affirms that
there is a perfect parallelism between those of Scolopendrella and
Thysanura. To this point we will return again in treating more
especially of those of the Symphyla. Finally, Grassi concludes that
there is “a great resemblance between the Thysanura and Scolopendrella.”
He, however, believed that the Symphyla are the forerunners of the
myriopods, and not of the insects, his genealogical tree representing
the symphylan and thysanuran phyla as originating from the same point,
this point also being, rather strangely, the point of origin of the
arachnidan phylum.

Haase (1889) regarded Scolopendrella as a myriopod, and Pocock (1893)
assigned the Symphyla to an independent class, regarding Scolopendrella
as “the living form that comes nearest to the hypothetical ancestor of
the two great divisions of tracheates.” Schmidt’s work (1895) on the
morphology of this genus is more extended and richly illustrated than
Grassi’s, his method of research being more modern. He also regards this
form as one of the lower myriopods.

In conclusion, it appears to us that, on the whole, if we throw out the
single characteristic of the anteriorly situated genital opening, the
ovarian tubes being directed toward the end of the body (Fig. 15, _ovd_,
_ov_), there is not sufficient reason for placing the Symphyla among the
Myriopoda, either below or near the diplopods. This is the only valid
reason for not regarding Scolopendrella as the representative of a group
from which the insects have descended, and which partly fills the wide
abyss between Peripatus and insects. With the view of Pocock, that both
insects and myriopods have descended from Scolopendrella, we do not
agree, because this form has so many insectean features, and a single
unpaired genital opening. For the same reason we should not agree with
Schmidt in interpolating the Symphyla between the Pauropoda and
Diplopoda. In these last two progoneate groups the genital openings are
paired, hence they are much more primitive types than Scolopendrella, in
which there is but a single opening. It seems most probable that the
Symphyla, though progoneate, are more recent forms than the progoneate
myriopods, which have retained the primitive feature of double sexual
outlets. It is more probable that the Symphyla were the descendants of
these polypodous forms. Certainly Scolopendrella is the only extant
arthropod which, with the sole exception of the anteriorly situated
genital opening, fulfils the conditions required of an ancestor of
Thysanura, and through them of the winged insects. No one has been so
bold as to suggest the derivation of insects from either diplopods or
chilopods, while their origin from a form similar to Scolopendrella
seems not improbable. Yet Uzel has very recently discovered that
Campodea develops in some respects like Geophilus, the primitive band
sinking in its middle into the yolk, with other features as in
chilopods.[7] The retention of a double sexual opening in the diplopods
is paralleled by the case of Limulus with its double or paired sexual
outlets, opening in a pair of papillæ, as compared with what are
regarded as the generalized or more primitive Crustacea, which have an
unpaired sexual opening.

The following summary of the structural features of the Symphyla, as
represented by Scolopendrella, is based mainly on the works of Grassi,
Haase, and Schmidt, with observations of my own.

=Diagnostic or essential characters of Symphyla.=—_Head shaped as in
Thysanura (Cinura), with the Y-shaped tergal suture, which occurs
commonly in insects (Thysanura, Collembola, Dermaptera, Orthoptera,
Platyptera, Neuroptera, etc.), but is wanting in Myriopoda (Diplopoda
and Chilopoda); antennæ[8] unlike those of Myriopoda in being very long,
slender, and moniliform. Clypeus distinct. Labrum emarginate, with six
converging teeth. Mandibles 2–jointed, consisting of a vestigial stipes
and distal or molar joint, the latter with eight teeth. First maxillæ
with an outer and inner mala situated on a well-developed stipes; with a
minute, 1–jointed palpus. Second pair of maxillæ: each forming two
oblong flat pieces, median sutures distinct, with no palpi; these pieces
are toothed in front, and appear to be homologous with the two median
pieces of the gnathochilarium of Diplopoda. Hypopharynx? Epipharynx?_

_Trunk with from fifteen to sixteen dorsal, more or less free subequal
scutes, the first the smallest. Pedigerous segments twelve; also twelve
pairs of 5–jointed legs, which are of nearly equal length, the first
pair 4–, the others 5–jointed, all ending in two claws, as in Synaptera
and winged insects. A pair of 1–jointed anal cerci homologous with those
of Thysanura and Orthoptera, into each of which opens a large abdominal
silk-gland. Abdominal segments with movable styles or “pseudopods”
(“Parapodia” of Latzel and of Schmidt), like those of Campodea and
Machilis, and situated on the base of the coxal joint in front of the
ventral sac. Within the body near the base of each abdominal style is an
eversible coxal sac or blood-gill (Fig. 15, cg). The single genital
opening is on the fourth trunk-segment in both sexes (Fig. 15, indicated
by the arrow). The malpighian tubes (ur. t) are two in number, opening
into the digestive canal at the anterior end of the hind intestine; they
extend in front to the third or second segment from the head. They are
broad and straight at their origin, becoming towards the end very
slender and convoluted._

_The three divisions of the digestive tract are as in insects, the
epithelium of the mid-gut being histologically as in Campodea and Japyx;
rectal glands are present. A pair of very large salivary glands are
situated in the first to the fourth trunk-segments, consisting of a
glandular portion with its duct, which unite into a common duct opening
on the under side of the head, probably in the labium._

_But a single pair of stigmata is present, and these are situated in the
front of the head, beneath the insertion of the antennæ and within the
stipes of the mandibles; the tracheæ are very fine, without spiral
threads (tænidia), and mostly contained within the head, two fine
branches extending on each side into the second trunk-segment._

_After birth the body increases in length by the addition of new
segments at the growing point._

In respect to the nervous system, there are no diagnostic characters;
there are, however, not as many as two pairs of ganglia to a segment.
The brain is well developed, sending a pair of slender nerves to the
small eyes. The ganglia of the segment bearing the first pair of legs is
fused with the subœsophageal ganglion. Grassi was unable to detect a
true sympathetic system, but he suspects the existence of a very small
frontal ganglion.

The slender dorsal vessel, provided with ostia and valvules, pulsates
along the entire length of the trunk; an aorta passes into the head.

The internal genital organs of both sexes are paired, and extend along
the greater part of the trunk; in either sex they may be compared to two
long, slender, straight cords extending from the fourth to the tenth
pair of legs. The two oviducts do not unite before reaching the sexual
opening (Fig. 15, _ovd_).

The male sexual organs are more complicated than the feminine. The
paired testicular tubes lie in trunk-segments 6 to 12, on each side, and
partly under the intestinal canal, communicating with each other by a
cross-anastomosis situated under the intestine, and which, like the
testes, is filled with sperm. Of the paired seminal ducts (vas deferens)
in trunk-segment 4, each unites again into a thick tube, sending a blind
tube forward into the third segment. Under the place of union of the two
vasa deferentia arise the paired ductus ejaculatorii, which open beneath
in the uterus masculinus. The anterior blind ends of the vasa deferentia
form a sort of small paired vesiculæ seminales in which a great quantity
of ripe sperm is stored. The uterus masculinus is in its structure
homologous with the evaginable penis of Pauropus, Polyxenus, and some
diplopods, and the sexual opening has without doubt become secondarily
unpaired. The sexual opening is rather long and is closed by two
longitudinal folds. “In several respects the male sexual organs of
Scolopendrella are like those of Pauropus; in the last-named form we
have indeed an unpaired testis, but also in Scolopendrella we see the
beginning of such a singleness; namely, the presence of an anastomosis
uniting the two tubes, their communication by means of a transverse
connecting canal and a glandular structure in the epithelium forming
them. The male sexual organs of Pauropus differ only through a still
greater complication.” (Schmidt.)

Scolopendrella in habits resembles chilopods, being found in company
with Geophilus burrowing deep in light sand under leaves, or living at
the surface of the ground under sticks or stones. It is very agile in
its movements, and is probably carnivorous. It was considered by Haase
to be eyeless, but the presence of two ocelli has been demonstrated both
by Grassi and by Schmidt. Whether the pigment and corneous facet are
present is not certain. The embryology is entirely unknown (although
Henshaw reports finding a hexapodous young one), and it need not be said
that a knowledge of it is a very great desideratum. It is most probable
that the young is hexapodous, since the first pair of limbs are
4–jointed, all the rest 5–jointed; while Newport, and also Ryder,
observed specimens with nine, ten, eleven, and twelve pairs, and
Wood-Mason confirms their observations, “which prove that a pair of legs
is added at each moult,” and he concludes that the addition of new
segments “therefore takes place in this animal by the intercalation of
two at each moult between the antepenultimate and penultimate sterna, as
in the Chilognatha, and as also in some of the Chilopoda.”

There is but one family, Scolopendrellidæ, and a single genus,
Scolopendrella, which seems to be, like other archaic types,
cosmopolitan in its distribution.

  Our commonest species is _S. immaculata_ Newport, which occurs from
  Massachusetts to Cordova, Mexico, and in Europe from England to the
  Mediterranean and Russia; Mr. O. F. Cook tells me he has found a
  species in Liberia, West Africa. The other species are _S.
  notacantha_ Gervais, Europe and Eastern United States; _S. nivea_
  Scopoli (_S. gratiæ_ Ryder), Europe and United States; _S. latipes_
  Scudder, Massachusetts.




                      LITERATURE ON SCOLOPENDRELLA


  =Newport, George.= Monograph of the class Myriopoda, order Chilopoda.
    (Trans. Linn. Soc. xix, pp. 349–439, 1 Pl., 1845.)

  =Menge, A.= Myriapoden der Umgegend von Danzig. (Neuste Schriften der
    naturforsch. Gesell. Danzig. iv, 1851.)

  =Ryder, John H.= Scolopendrella as the type of a new order of
    articulates (Symphyla). (Amer. Nat., May, 1880, xiv, pp. 375, 376.)

  —— The structure, affinities, and species of Scolopendrella. (Proc.
    Acad. Nat. Soc. Phil., pp. 79–86, 1881, 2 Figs.)

  =Packard, A. S.= Scolopendrella and its position in nature. (Amer.
    Nat., 1881, pp. 698–704, Fig.)

  =Muhr, Jos.= Die Mundtheile von Scolopendrella und Polyzonium. Prag,
    1882, 1 Pl.

  =Mason, J. Wood.= Morphological notes bearing on the origin of
    insects. (Trans. Ent. Soc. London, 1879, pp. 145–167, Figs.)

  —— Notes on the structure, post-embryonic development, and systematic
    position of Scolopendrella. (Ann. and Mag. Nat. Hist., July, 1883,
    pp. 53–63.)

  =Latzel, Robert.= Die Myriapoden der osterreichisch-ungarischen
    Monarchie, ii, Wien, 1884, pp. 1–39, Pls.

  =Haase, Erich.= Die Abdominalanhänge der Insekten mit Berücksichtigung
    der Myriapoden. (Morph. Jahrbuch, xv, pp. 331–435, 2 Pls., 1889.)

  =Schmidt, Peter.= Beiträge zur Kenntnis der niederen Myriapoden.
    (Zeits. f. wissen. Zool., lix, pp. 436–510, 2 Pls., 1895.)




                           INSECTA (HEXAPODA)


We are now prepared to discuss the fundamental or essential characters
of the insects, including the wingless subclass (Synaptera), and the
winged (Pterygota).

=Diagnostic characters of insects.=—_Body consisting of not more than
twenty-one segments, which are usually heteronomous or of unequal size
and shape, arranged in three usually well-defined regions_; i.e. _a
head, thorax, and hind-body or abdomen. Head small and flattened or
rounded, composed of not less than six segments, and bearing, besides
the eyes, at least four pairs of jointed appendages_; i.e. _one pair of
antennæ, and three pairs of masticatory appendages, the distal or molar
portion of which is primarily divided into three divisions, supported on
a stipes and cardo, and in certain orders modified into piercing or
sucking structures. The head is composed of an epicranium, bearing a
distinct clypeus and labrum, with the epipharynx. Mandibles 1–jointed,
without a palpus and very generally with no, or uncertain, traces of a
lacinia and a stipes. Two pairs of maxillæ; the first pair separate,
usually 3–lobed, comprising a lacinia, galea, and palpifer, with a
palpus which is never more than 6–jointed. The second pair united to
form the labium or under lip, composed of two laciniæ fused together; in
the generalized forms with a rudimentary galea; bearing a pair of palpi,
never more than 4–jointed; with paraglossæ sometimes present._

(_A third pair of mouth-appendages situated between the antennæ and
mandibles in the embryo of Anurida, and Apis, and adult Campodea._)

_The epipharynx forming the roof of the mouth, and bearing gustatory
organs. Hypopharynx usually well developed, lying on the under side of
the mouth, just above the labium, and receiving the end of the salivary
duct._

_Eyes of two kinds: a pair of compound, and from two to three simple
eyes (ocelli)._

_The thorax consisting of three segments, the two latter segments in the
winged orders highly differentiated into numerous tergal and lateral
pieces and a single sternum; in the Synaptera the segments are
undivided. (In the higher Hymenoptera the basal abdominal segment
coalesced with the thorax.) Three pairs of legs, each foot ending in a
pair of claws. Two pairs of wings (except in the Synaptera), a pair to
each of the two hinder thoracic segments; the wings occasionally reduced
or wanting in certain adaptive forms, which, however, had winged
ancestors._

_Abdomen consisting at the most of from ten to twelve segments. No
functional abdominal legs except in the Thysanura, and in the larvæ of
Lepidoptera. A pair of 1– or many-jointed cercopods on the tenth
segment; and in certain forms a pair of styles on the ninth segment. In
certain orders an ovipositor or sting formed of three pairs of styliform
processes; in Collembola a single pair of processes forming the elater._

_The genital openings opisthogoneate, usually single, but paired in
Thysanura (Lepisma), Dermaptera, and Plectoptera (Ephemeridæ)._

_The digestive canal in the winged orders is highly differentiated, the
fore-intestine being divided into an œsophagus and proventriculus, the
hind-intestine into an ileum, colon, and rectum, with rectal glands._

_The nervous system consists of a well-developed brain, in the more
specialized orders highly complicated; no more than thirteen pairs of
ganglia, which may be more or less fused in the more specialized orders.
Three frontal ganglia, and a well-developed, sympathetic system
present._

_Stigmata confined (except possibly in Sminthurus) to the thorax and
abdomen, not more than ten pairs in all, and usually but nine pairs.
Tracheal system as a rule highly differentiated; invariably with
tænidia._

_Dorsal vessel with ostia and valvules; no arteries except the cephalic
aorta; no veins. After birth there is in the more specialized pterygote
orders a reduction in the number of terminal segments of the abdomen._

_Development either direct (Synaptera), or with an incomplete (with
nymph and winged or imaginal stages), or complete metamorphosis; in the
latter case with a larval, pupal, and imago stage._

The insects may be divided into two sub-classes,—the _Synaptera_, and
the winged orders, _Pterygota_, of Gegenbaur (1877), since the
differences between the two groups appear on the whole to be of more
than ordinal rank.




                          1. EXTERNAL ANATOMY


                      _a._ The regions of the body

The insects differ from other arthropods in that the body is divided
into three distinct regions,—the head, thorax, and abdomen, the latter
regions in certain generalized forms not always very distinctly
differentiated. The body behind the head may also conveniently be called
the trunk, and the segments composing it the trunk-segments.

In insects the head is larger in proportion to the trunk than in other
classes, notably the Crustacea; the thorax is usually slightly or
somewhat larger than the head, while the hind-body or abdomen is much
the larger region, as it consists of ten to eleven, and perhaps in the
Dermaptera and Orthoptera twelve, segments, and contains the mid- and
hind-intestine, as well as the reproductive organs.

When we compare the body of an insect with that of a worm, in which the
rings are distinctly developed, we see that in insects ring distinctions
have given way to regional distinctions. The segments lose their
individuality. It is comparatively easy to trace the segments in the
hind-body of an insect, as in this region they are least modified; so
with the thorax; but in the head of the adult insect it is impossible to
discover the primitive segments, as they are fused together into a sort
of capsule, and have almost entirely lost their individuality.

In general it may be said that the head contains or bears the organs of
sense and of prehension and mastication of the food; the thorax the
organs of locomotion; and the abdomen those of reproduction.

When we compare the body of a wasp or bee with that of a worm, we see
that there is a decided transfer of parts headward; this process of
cephalization so marked in the Crustacea likewise obtains in insects.
Also the two hinder regions of the body are, in a much greater degree
than in worms, governed by the brain, the principal seat of the
intelligence, which, so to speak, dominates and unifies the functions of
the body, both digestive, locomotive, and reproductive, as also those of
the muscles moving the different segments and regions of the body. To a
large extent arthropodan morphology and class distinctions are based on
the regional arrangement of the somites themselves. Thus in the process
of grouping of the segments into the three regions, some increase in
size, while others undergo a greater or less degree of reduction; one
segment being developed at the expense of one or more adjoining ones.
This principle was first pointed out by Audouin, and is called Audouin’s
law. It is owing to the greater development of certain segments and the
reduction of others, both of the body-segments and of the segments of
the limbs, that we have the wonderful diversity of form in the species
and genera, and higher groups of insects, as well as those of other
arthropods.


                   _b._ The integument (exoskeleton)

The skin or integument of insects consists, primarily, as in worms and
all arthropods, of an epithelial layer of cells called the _hypodermis_.
This layer secretes the cuticle, which is of varying thickness and
flexibility, and is usually very dense, impermeable, and light, compared
with the crust of the Crustacea, where the cuticle becomes heavy and
solid by the deposition of the carbonate and phosphate of lime. This is
due to the presence of a substance called by Odier chitin.[9] The
cuticle is thin, delicate, and flexible between the joints; it is
likewise so in such diaphanous aquatic larvæ as that of Corethra, and in
the gills of aquatic insects, also in the walls of the tracheæ and of
the salivary ducts. The cuticle thus forms a more or less solid crust
which is broken into joints and pieces (sclerites), forming supports for
the attachments of the muscles and serving to protect the soft parts
within.

=Chitin.=—If we allow an insect to soak for a long time in acids, or
boil it in liquid potassa or caustic potash, the integument is not
affected. The muscles and the other soft parts are dissolved, leaving
the cuticle clear and transparent. This insolubility of the cuticle is
due to the presence of chitin, the insoluble residue left after such
treatment. It also resists boiling in acids, in any alkalies, alcohol or
ether. The chemical formula is C_{15}H_{26}N_{2}O_{10}.[10]

  “Chitin forms less than one-half by weight of the integument, but it
  is so coherent and uniformly distributed that when isolated by
  chemical reagents, and even when cautiously calcined, it retains its
  original organized form. The color which it frequently exhibits is
  not due to any essential ingredient; it may be diminished or even
  destroyed by various bleaching processes.” (Miall and Denny.)

  “The chemical stability of chitin is so remarkable that we might
  expect it to accumulate like the inorganic constituents of animal
  skeletons, and form permanent deposits. Schlossberger (Ann. d. chem.
  u. pharm., bd. 98) has, however, shown that it changes slowly under
  the action of water. Chitin kept for a year under water partially
  dissolved, turned into a slimy mass, and gave off a peculiar smell.
  This looks as if it were liable to putrefaction. The minute
  proportion of nitrogen in its composition may explain the complete
  disappearance of chitin in nature.” (Miall and Denny, The Cockroach,
  p. 29.)

  Chitin, or a substance closely similar to it, occurs in worms and in
  their tubes, especially in the pharyngeal teeth of annelids and in
  their setæ. The shell of Lingula and the pen of cuttle-fish contain
  true chitin (Krukenberg). The integument of Limulus, of trilobites,
  and of Arachnida, as well as Myriopoda, appears to consist of
  chitin.[11]

The chitin is rapidly deposited at the end of embryonic life, also
during the larval and pupal stages. As is well known, insects after
moulting are white, but in a few hours turn dark, and those which live
in total darkness are white, showing that light has a direct effect in
causing the dark color of the integument.

Moseley analyzed one pound weight of Blatta, and found plenty of iron
with a remarkable quantity of manganese.

Schneider regarded chitin as a hardening of the protoplasm rather than a
secretion, and the cuticle is looked upon as an exudation. It is
structureless, not consisting of cells, and consists of fine irregular
laminæ. “A cross-section of the chitinous layer or ‘cuticle’ examined
with a high power shows extremely close and fine lines perpendicular to
the laminæ.” In the cockroach the free surface of the cuticle is divided
into polygonal, raised spaces or areas which correspond each to a
chitinous cell of the hypodermis. (Miall and Denny.)

Numerous pore-canals pass through the cuticle of all the external parts
of the body. The larger canals nearly always form the way for the
passage of secretions from dermal cells, or connect with the cavities of
hairs or setæ; when very fine and not connected with hairs or scales,
they are either empty or filled with air, and may possibly serve for
respiration.

Vosseler distinguishes in the cuticle two layers of different physical
and chemical characters. Besides the external chitinous layer there is
an inner layer which entirely agrees with cellulose. (Zool.
Centralblatt, ii, 1895, p. 117.)

The reparative nature of chitin is seen in the fact that Verhoeff finds
that a wound on an adult Carabus, and presumably on other insects, is
speedily closed, not merely by a clot of blood, but by a new growth of
chitin.


     _c._ Mechanical origin and structure of the segments (somites,
                    arthromeres, metameres, zonites)

The segments are merely thickenings of the skin connected by folds or
duplications of the integument, and not actually separate or individual
rings or segments. This is shown by longitudinal (sagittal) sections
through the body, and also by soaking or boiling the entire insect in
caustic potash, when it is seen that the integument is continuous and
not actually subdivided into separate somites or arthromeres, since they
are seen to be connected by a thin intersegmental membrane (Fig. 16).
But this segmentation or metamerism of the integument is, however, the
external indication of the segmentation of the arthropodan body most
probably inherited from the worms, being a disposition of the soft parts
which is characteristic of the vermian type. This segmentation of the
integument is correlated with the serial repetition of the ganglia of
the nervous system, of the ostia of the dorsal vessel, the primitive
disposition of the segmental and reproductive organs, of the soft,
muscular dissepiments which correspond to the suture between the
segments, and with the metameric arrangement of the muscles controlling
the movements of the segments on each other, and which internal
segmentation or metamerism is indicated very early in embryonic life by
the mesoblastic somites.

[Illustration:

  FIG. 16.—Diagram of the anterior part of an insect, showing the
    membranous intersegmental folds, _g_.—After Graber.
]

In the unjointed worms, as Graber states, the body forms a single but
flexible lever. In the earthworm the muscular tube or body-wall is
enclosed by a stiffer cuticle, divided into segments; hence the worm can
move in all required directions, but only by sections, as seen in Fig.
16, which represents the thickened integument divided into segments, and
folded inward between each segment, this thin portion of the skin being
the intersegmental fold. Each segment corresponds to a special zone of
the subdivided muscular tube (_m_), the fascia extending longitudinally.
The figure shows the mode of attachment of the fascia of the muscle-tube
to the segment. The anterior edge is inserted on the stiff, unyielding,
inner surface of each segment: the hinder edge of the muscle is attached
to the thin, flexible, intersegmental fold, which thus acts as a tendon
on which the muscle can exert its force. (Graber.)

[Illustration:

  FIG. 17.—Diagram of the integument and arrangement of the segmental
    muscles: _A_, relaxed; _m_, muscle; _g_, membranous articulation;
    _r_, chitinous ring. _B_, the same contracted on both sides. _C_, on
    one side.—After Graber.
]

“Fig. 17 makes this still clearer. The muscles (_m_) extend between two
segments immediately succeeding each other. Supposing the anterior one
(_A_) to be stationary, what do we then see when the muscle contracts?
Does it also become shorter? The intersegmental fold is drawn forwards,
and hence the entire hinder segment moves forward and is shoved into the
front one, and so on with the others, as at _B_. Afterwards, if the
strain of the muscle is relieved by the diminishing action of the
tensely stretched, intersegmental membrane, it again returns to a state
of rest.” (Graber.)

[Illustration:

  FIG. 18.—Diagrams to demonstrate the mechanism of the motion of the
    segmented body in the Arthropoda: One larger segment (_cf_) and 4
    smaller. The exoskeleton is indicated by black lines, the
    interarticular membranes by dotted lines. The hinges between
    consecutive segments are marked _at_, tergal (dorsal) skeleton; _s_,
    sternal (ventral) skeleton; _d_, dorsal longitudinal muscles =
    extensors (and flexors in an upward direction); _v_, ventral
    longitudinal muscles = flexors. In _B_, the row of segments is
    stretched; in _A_, by the contraction of the muscles (_d_) bent
    upward; in _C_, downward; _tg_, tergal; _sg_, sternal interarticular
    membranes.—After Lang.
]

While we look upon the dermal tube of worms as a single but flexible
lever, the body of the arthropods, as Graber states, is a linear system
of stiff levers. We have here a series of stiff, solid rings, or hooks,
united by the intersegmental membrane into a whole. When the muscles,
extending from one ring to the next behind contract, and so on through
the entire series, the rings approximate each other.

The ectoskeletal segments bend to one side by the contraction of the
muscles on one side, the point of the outer segmental fold opposite the
fixed point becoming converted into the turning-point (_C_).

The usual result of the arrangement of the locomotive system is the
simple curving of the body (_C_), and then the alternate bending of the
body to right and left, which produces the serpentine movements
characteristic of the earthworms, the centipede, and many insect larvæ.
The most striking example of the wonderful variety of movements which
can be made by an insect are those of the Syrphus larva. When feeding
amid a herd of aphides, it is seen to now raise the front part of the
body erect and stiff, then to bend it down, or rapidly turn it to either
side, or move it in a complete circle. (Graber, pp. 23–26.)

The arrangement and mode of working of the muscles, says Lang, is
illustrated by Fig. 18, which shows us five segments, one larger (_ct_)
and four smaller, in vertical projection. The thicker portion of the
integument is marked by strong outlines, the delicate and flexible
interarticular membranes (_tg_, _sg_) in dotted lines. The hinges
between two consecutive segments are marked _a_. A dorsal muscle (_d_)
is attached to the larger segment (_ct_), and runs through the smaller
segments, being inserted in the dorsal portion of the crust (_t_) of
each by means of a bundle of fibres. A ventral muscle (_v_) does the
same on the sternal side (_s_).

“The skeletal segments,” adds Lang, “may be compared to a double-armed
lever, whose fulcrum lies in the hinges. If the dorsal muscle contracts,
it draws the dorsal arm of the lever (the tergal portion of the
skeleton) in the direction of the pull towards the larger segments; the
tergal interarticular membranes become folded, the ventral stretched,
and the four segments bend upward (Fig. 18, _A_). If the ventral muscle
contracts, while at the same time the dorsal slackens, the row of
segments will be bent downwards (Fig. 18, _C_).”

L. B. Sharp suggests, that in the Crustacea the rings formed by “the
regularity and stress of muscular action” would be hardened by the
deposition of lime at the most prominent portion, _i.e._ between what we
have called the intersegmental folds. (American Naturalist, 1893, p.
89.) Cope also states that “with the beginning of induration of the
integument, segmentation would immediately appear, for the movements of
the body and limbs would interrupt the deposit at such points as would
experience the greatest flexure. The muscular system would initiate the
process, since flexure depends on its contractions, and its presence in
animals prior to the induration of the integuments in the order of
phylogeny, furnishes the conditions required.” (The Primary Factors of
Organic Evolution, p. 268, 1895.)

It is apparent that the jointed or metameric structure of the bodies of
insects and other arthropods is an inheritance from the segmented worms.
In the worms the body is a continuous dermo-muscular tube, while in
arthropods this tube is divided into regions, and the cuticle is thicker
and more resistant. To go back to the incipient stages in the process of
segmentation of the body, we conceive that the worms probably arose from
a creeping gastrula-like form, the gastræa. The act of creeping
gradually induced an elongated shape of the body. The movement of such
an organism in a forward direction would gradually evolve a fore and
aft, dorsal and ventral, and bilateral symmetry. As soon as this was
attained, as the effect of creeping over rough irregular surfaces there
would result mechanical lateral strains intermittently acting during the
serpentine movements of the worm. The integument would, we can readily
suppose, tend to bend or yield, or become permanently wrinkled, at more
or less regular intervals. The arrangement of the muscles would
gradually conform to this habit of creeping, and finally the nervous
system and other organs more directly connected with the creeping
movements of the organism would tend to be correlated in their
arrangement with that of the segments. In this way the homonomous
segments of the annelid body probably became developed, and their
relations and shapes were eventually fixed by inheritance. After this
stage was reached, and limbs began to appear, the segments would tend to
become heteronomous, and to be grouped into regions.

[Illustration:

  FIG. 19.—_Dujardinia rotifera_, with jointed tentacles and caudal
    appendages.—With some changes, after Quatrefages.
]

The origin of the joints or segments in the limbs of arthropods was
probably due to the mechanical strains to which what were at first soft
fleshy outgrowths along the sides of the body became subjected. Indeed,
certain annelid worms of the family Syllidæ have segmented tentacles and
parapodia, as in Dujardinia (Fig. 19). We do not know enough about the
habits of these worms to understand how this metamerism may have arisen,
but it is possibly due to the act of pushing or repeated efforts to
support the body while creeping over the bottom among broken shells,
over coarse gravel, or among seaweeds.

It is obvious, however, that the jointed structure of the limbs of
arthropods, if we are to attempt any explanation at all of the origin of
such structure, was primarily due mainly to lateral strains and impacts
resulting from the primitive endeavors of the ancestral arthropods to
raise and to support the body while thus raised, and then to push or
drag it forward by means of the soft, partially jointed, lateral limbs
which were armed with bristles, hooks, or finally claws.

On the other hand, by adaptation, or as the result of parasitism and
consequent lack of active motion, the original number of segments may by
disuse be diminished. Thus in adult wasps and bees, the last three or
four abdominal segments may be nearly lost, though the larval number is
ten. During metamorphosis the body is made over, and the number, shape,
and structure of the segments greatly modified. In the female of the
Stylopidæ the thorax loses all traces of segments, and is fused with the
head, and the abdominal segments are faintly marked, losing their
chitin.

While the maxillæ have several joints, the mandibles are 1–jointed, but
there are traces of two joints in Campodea, certain beetles, etc. In the
antenna there is a great elasticity in respect to the number of joints,
which vary from one or two to a hundred or more. It is likewise so in
the thoracic legs, where the number of tarsal joints varies from one to
five; also in the cercopoda, the number of joints varying from one or
two to twelve or more.


   _d._ Mechanical origin of the limbs and of their jointed structure

We have already hinted at the mode of origin of the limbs of arthropods.
Like the body or trunk, the limbs are chitinous dermo-muscular tubes,
with a dense solid cuticle, and internal muscles, and were it not for
their division at more or less regular intervals into segments, forming
distinct sets of levers, set up by the strains in these tubular
supports, there would be no power of varied motion.

Even certain worms, as already stated, have their tentacles and
parapodia, or certain appendages of their parapodia, more or less
jointed, but there are no indications of claws or of any other hard
chitinous armature at the extremity, and the skin is thin and soft.

In the most simple though not the most primitive arthropods, such as the
Tardigrades, whose body is not segmented, there are four pairs of short
unjointed legs, ending each in two claws, which have probably arisen in
response to the stimulus of pushing or dragging efforts.

The legs of Peripatus are unjointed, and have a thin cuticle, but end in
a pair of claws, which have evidently arisen as a supporting armature,
the result of the act of moving or pulling the body over the uneven
surface of the ground.

[Illustration:

  FIG. 20.—A prothoracic leg of Chironomus larva; and pupa.
]

[Illustration:

  FIG. 21.—_A_, larva of _Ephydra californica_: _a_, _b_, _c_, pupa.
]

There is good reason to suppose that such limbs arose from dynamical
causes, similar to those exciting the formation of secondary adaptations
such as are to be seen in the prop or supporting legs of certain
dipterous larvæ, as the single pair of Chironomus (Fig. 20) and
Simulium, or the series of unjointed soft tubercles of Ephydra (Fig.
21), etc., which are armed with hooks and claws, and are thus adapted
for dragging the insect through or over vegetation or along the ground.

Now by frequent continuous use of such unjointed structures, the cuticle
would tend to become hard, owing to the deposit of a greater amount of
chitin between the folds of the skin, until finally the body being
elongated and homonomously segmented, the movements of walking or
running would be regular and even, and we would have homonomously
jointed legs like those of the trilobites, or of the most generalized
Crustacea and of Myriopoda.

In the most primitive arthropods,—and such we take it were on the whole
the trilobites, rather than the Crustacea,—the limbs were of nearly the
same shape, being long and slender and evenly jointed from and including
the antennæ, to the last pair of limbs of the abdominal region. In these
forms there appear to be, so far as we now know, no differentiation into
mandibles, maxillæ, maxillipedes, and thoracic legs, or into gonopoda.
The same lack of diversity of structure and function of the
head-appendages has survived, with little change, in Limulus. In the
trilobites (Fig. 1) none of the limbs have yet been found to end in
claws or forceps; being in this respect nearly as primitive as in the
worms. Secondary adaptations have arisen in Limulus, the cephalic
appendages being forcipated, adapted as supports to the body and for
pushing it onward through the sand or mud, while the abdominal legs are
broad and flat, adapted for swimming and bearing the broad gill-leaves.

It is thus quite evident that we have three stages in the evolution of
the arthropodan limb; _i.e._ 1, the syllid stage, of simple, jointed,
soft, yielding appendages not used as true supports (Fig. 19); 2, the
trilobite stage, where they are more solid, evenly jointed, but not
ending in claws; and by their comparatively great numbers (as in the
trilobite, Triarthrus) fully supporting the body on the bottom of the
sea. In Limulus they are much fewer in number, thicker, and acting as
firm supports, the cephalic limbs of use in creeping, and ending in
solid claws. 3, The third stage is the long slender swimming
head-appendages of the nauplius stage of Crustacea.

As regards the evolution of limbs of terrestrial arthropods, we have the
following stages: 1, the soft unjointed limbs of Tardigrades, ending in
two claws, and those of Peripatus, and the pseudo- or prop-legs of
certain dipterous larvæ; 2, finally the evolution of the long, solid,
jointed limbs of Pauropus and other primitive myriopods, the legs
forming solid, firm supports elevating the body, and enabling the insect
to drag itself over the ground or to walk or run. When the body is
elongated and many-segmented, the legs are necessarily numerous; but
when it is short, the legs become few in number, _i.e._ six, in the
hexapodous young of myriopods and in insects, or eight in Arachnida.
Whenever the legs are used for walking, _i.e._ to raise and support the
body, they end in a solid point or in a pair of forceps or claws. On the
other hand, as in phyllopods, where the legs are used mainly for
swimming, they are unarmed and are soft and membranous, or, as in the
limbs of the nauplius or zoëa stage of crustaceans, end in a simple soft
point, which often bears tactile setæ.

The tarsal joints are more numerous in order to give greater flexibility
to the limb in seizing and grasping objects, both to drag the body
forwards and to support it.

Unlike those of the Crustacea, the limbs of insects are not primitively
biramose, but single, the three-lobed first maxillæ, and secondarily
bilobed second maxillæ being the result of adaptation. Embryology on the
whole proves the truth of this assumption; the maxillæ of both pairs are
at first single buds, afterwards becoming lobed. All the appendages of
the body, including the ovipositor or sting, are modified limbs, as
shown by their embryological development.

It is noticeable that in the crab, where the body is raised by the limbs
above the bottom, it is much shorter and more cephalized than in the
shrimps. Also in the simply walking and running spiders, the hind-body
is shorter than in scorpions, while in the running and flying insects,
such as the Cicindelidæ, and in the swiftly flying flies and bees, there
is a tendency to a shortening of the body, especially of the abdomen.
The long body of the dragon-fly is an impediment to flight, but
compensated for by the action of the large wings.

The arthropodan limb is a compound leverage system. It is, says Graber,
a lateral outgrowth of the trunk, which repeats in miniature that of the
main trunk, its single series of joints or segments forming a jointed
dermo-muscular tube. Yet the lateral appendages of an insect differ from
the main trunk in two ways: (1) they taper to the end which bears the
two claws, and (2) their segments are in the living animal arranged not
in a straight line, but at different angles to each other. The basal
joint turning on the trunk acts as the first of a whole series of
levers. The second joint, however, is connected with the musculature of
the first or basal joint, and thus each succeeding joint is moved on the
one preceding. Each lever, from the first to the last, is both an active
and a passive instrument. (Graber.)

While, however, as Graber states, the limbs possess their own sets of
muscles and can move by the turning of the basal joint, the labor is
very much facilitated, as is readily seen, by the trunk, though the
latter has to a great extent delegated its locomotive function to the
appendages, which again divide its labor among the separate joints.

Graber then calls attention to the analogy of the mechanics of
locomotion of insects to those of vertebrates. An insect’s and a
vertebrate’s legs are constructed on the same general mechanical
principles, the limbs of each forming a series of levers.

[Illustration:

  FIG. 22.—Diagram of the knee-joint of a vertebrate (_A_) and an
    insect’s limb (_B_): _a_, upper; _b_, lower, shank, united at _A_ by
    a capsular joint, at _B_ by a folding joint; _d_, extensor or
    lifting muscle; _d_^1, flexor or lowering muscle of the lower joint.
    The dotted line indicates in _A_ the contour of the leg.—After
    Graber.
]

Fig. 22, _A_, represents diagrammatically the knee joint of a
vertebrate, and _B_ that of an insect; _a_, the femur or thigh, and _b_,
the tibia or shank. In the vertebrate the internally situated bones are
brought into close union and bend by means of a hinge-joint; so also in
the chitinous-skinned insect.

The stiff dermal tube of the insect acts as a lever by means of the thin
intersegmental membrane (_c_) pushed in or telescoped in to the thigh
joint, a special joint-capsule being superfluous. The muscles are in
general the same in both types; they form a circle. In both the shank is
extended by the contraction of the upper muscles (_d_) and is bent by
the contraction of the lower (_d^1_). The intersegmental membrane of the
insect’s limb is in a degree a two-armed lever, whose pivot (_f_) lies
in the middle. The internal invagination of the intersegmental fold
(_B_, _g-h_) affords the necessary support to the muscles acting like
the tendon in the vertebrate. (Graber.)

[Illustration:

  FIG. 23.—Primitive band or germ of a Sphinx moth, with the segments
    indicated, and their rudimentary appendages: _c_, upper lip; _at_,
    antennæ; _md_, mandibles; _mx_, _mx′_, first and second maxillæ;
    _l_, _l′_, _l″_, legs; _al_, abdominal legs.—After Kowalevsky.
]

Graber also calls attention to the fact that this insect limb differs in
one important respect from that of land vertebrates. The leverage system
in the last is divided at the end into five parallel divisions or
digits. In arthropods, on the contrary, all the joints succeed one
another in a linear series.

In insects, as well as in other arthropods, modifications of the limbs
usually take the form of a simple reduction in the number of segments.
Thus while the normal number of tarsal joints is five, we have trimerous
and dimerous Coleoptera, and in certain Scarabæidæ the anterior tarsi
are lost.

Savigny was the first, in 1816, in his great work, “Théorie des organes
de la bouche des Crustacés et des Insectes,” to demonstrate that not
only were the buccal appendages of biting insects homologous with those
of bugs, moths, flies, etc., but that they were homologous with the
thoracic legs, and that thus a unity of structure prevails throughout
the appendages of the body of all arthropods. Oken also observed that
“the maxillæ are only repeated feet.”

What was modestly put forth as a theory by the French morphologist has
been abundantly proved by the embryology of insects of different orders
to be a fact. As shown in Fig. 23 the antennæ and buccal appendages
arise as paired tubercles exactly as the thoracic legs. The abdominal
region also bears similar embryonic or temporary limbs, all of which in
those insects without an ovipositor disappear, except the cercopoda,
after birth.




                   LITERATURE ON THE EXTERNAL ANATOMY


                                General

  =Swammerdam, Johann.= Biblia naturæ. (In Dutch, German, and English.)
    1737–1738, fol., London, 1758, Pls.

  =Réaumur, Réné Antoine Ferchault, de.= Mémoires pour servir à
    l’histoire des insectes. i-iv, 4º, Paris, 1734–1742.

  =Lyonet, Pieter.= Traité anatomique de la chenille, qui ronge le bois
    de saule, etc. 4º, pp. xxii, 616. À la Haye, 1732. Tab. 18.

  —— Recherches sur l’anatomie et les metamorphoses de differentes
    espèces d’insectes. Ouvrage posthume, publié par M. W. de Haan. pp.
    580, tab. 54, 1832.

  =Latreille, Pierre André.= Des rapports généraux de l’organization
    extérieure des animaux invertébres articulés, et comparaison des
    Annelides avec les Myriapodes. (Mémoires du Mus. d’Hist. Nat., 1820,
    vi, pp. 116–144.)

  —— De quelques appendices particuliers du thorax de divers insectes.
    (Mémoires du Mus. d’Hist. Nat., 1821, vii, pp. 1–21).

  —— Observations nouvelles sur l’organization extérieure et générale
    des animaux articulés et à pieds articulés, et application de ces
    connoissances à la nomenclature des principales parties des mêmes
    animaux. (Mèmoires du Mus. d’Hist. Nat., viii, 1822, pp. 169–202.)

  =Cuvier, George Leopold Christian Dagobert.= Rapport sur les
    recherches anatomiques sur le thorax des animaux articulés et celui
    des insectes en particulier par M. V. Audouin. 4º, pp. 15, tab. 1,
    Paris, 1823.

  =Audouin, Jean Victor.= Recherches anatomiques sur le thorax des
    animaux articulés et celui des insectes hexapodes en particulier.
    (Annales des Sciences naturelles, i, pp. 97–135, 416–432, 1824.)

  =Kirby, William, and William Spence.= Introduction to entomology.
    i-iv, 1816–1828, London.

  =Straus-Durckheim, Hercule.= Considérations générales sur l’anatomie
    comparée des animaux articulés. Paris, 1828, atlas of 19 plates.

  =MacLeay, William Sharp.= Explanation of the comparative anatomy of
    the thorax in winged insects, with a review of the present state of
    the nomenclature of its parts. (Zoöl. Journal, v, pp. 145–179, 1830,
    2 Pls.)

  =Burmeister, Hermann.= A manual of entomology. Trans. by W. E.
    Shuckard, 8º, pp. 654, London, 1836, 32 Pl.

  =Westwood, John Obadiah.= An introduction to the modern classification
    of insects, i, ii, 8º, pp. 462, 587, 158, 1 Pl. and 133 blocks of
    figs., 1839–1840.

  =Newport, George.= Art. Insecta in Todd’s Cyclopædia of Anatomy and
    Phys. ii, pp. 853–994, 1839, Figs. 329–439.

  =Erichson, Wilhelm Ferdinand.= Entomographien. Berlin, 1840.

  =Brullé, Auguste.= Recherches sur les transformations des appendices
    dans les articulés. (Annales des Sciences nat. Sér. 3, ii, pp.
    271–374, tab. 1, 1844.)

  =Winslow, A. P.: son.= Om byggnaden af thorax hos Insekterna.
    Helsingborg, 1862, 1 Pl, pp. 24.

  =Packard, Alpheus Spring.= Guide to the study of insects. 1869.

  —— Systematic position of the Orthoptera in relation to other insects.
    (Third report U. S. Ent. Commission, pp. 286–345, 1883, Pls.
    xxiii-lxi.)

  =Graber, Vitus.= Die Insekten. 12º, pp. 403, 603, München, 1877, many
    Figs.

  =Huxley, Thomas Henry.= A manual of the anatomy of invertebrated
    animals. 12º, pp. 397–451, Figs., London, 1877.

  =Hammond, Arthur.= Thorax of the blow-fly. (Journ. Linn. Soc., London,
    xv. Zoöl., 1880, pp. 31.)

  =Brauer, Friedrich.= Ueber das Segment médiaire Latreille’s. (Sitzb.
    d. k. Akad. d. Wissensch. Wien, 1882, pp. 218–241, 3 tab.)

  —— Systematisch-zoologische Studien. (Ibid., 1885, pp. 237–413.)

  =Gosch, C. C. A.= On Latreille’s theory of “Le Segment médiaire.”
    (Nat. Tidsskrift (3), xiii, pp. 475–531, 1883.)

  =Miall, L. C., and Denny, Alfred.= The structure and life-history of
    the cockroach (_Periplaneta orientalis_). An introduction to the
    study of insects. 8º, pp. 224, London, 1886.

  =Cheshire, Frank R.= Bees and bee-keeping. i, Scientific, London,
    1886, Pls. and Figs.

  =Lang, Arnold.= Text-book of comparative anatomy. i, pp. 426–508,
    1891, many Figs.

  =Kolbe, H. J.= Einführung in die Kenntniss der Insekten. 8º, pp. 709,
    324 figs., Berlin, 1893.

  =Sharp, David.= The Cambridge natural history. Insecta, i, 8º, pp.
    83–584, 1895, Figs. 47–371.

Also the works of Bos, Chabrier, Cholodkowsky, Comstock, Dewitz, Eaton,
Erichson, Gerstaecker, Girard, Grassi, Hagen, Haase, Kellogg, Knoch,
Lacordaire, Latreille, Leuckart, Lendenfeld, Lowne, Lubbock, Mayer,
Meinert, F. Müller, Osten-Sacken, Pagenstecher, Reinhard, Schaum,
Schiödte, Scudder, J. B. Smith, Spinola, Stein, Weismann, Wood-Mason.




                      THE HEAD AND ITS APPENDAGES


                             _a._ The head

[Illustration:

  FIG. 24.—Presumed larva of Nemoptera (_Necrophilus arenarius_),
    Pyramids of Egypt.—After Roux, from Sharp.
]

While the head is originally composed of probably not less than six
segments, these are in the adult insect fused together into a capsule or
hard chitinous box, the _epicranium_, with no distinct traces of the
primitive segments. The head contains the brain and accessory ganglia,
the mouth or buccal cavity, also the air-sacs in many winged forms, and
gives support to the external organs of sense, the antennæ, and to the
buccal appendages, the larger part of the interior being filled with the
muscles moving these structures. The solid walls of the head serve as a
lever or support for the attachment of these muscles, especially those
of the mandibles. Thus there is a correlation between the large size of
the mandibles of the soldier white ants and ants, the head being
correspondingly large to accommodate the great mandibular muscles. The
other extreme is seen in the larva of Necrophilus (Fig. 24), with its
long slender neck and diminutive head.

=The clypeus.=—This is that part of the head situated in front of the
epicranium, and anterior to the eyes, forming the roof of the posterior
part of the mouth, and is, as embryology shows, probably a tergal
sclerite. It varies greatly in shape and size in the different orders of
insects. It is often divided into two parts, the _clypeus posterior_ and
_clypeus anterior_, or which may be designated as the _post-_ and
_ante-clypeus_ (Figs. 29, _B_).

=The labrum.=—The “upper lip” or labrum is an unpaired flap-like piece
hinged to the front edge of the clypeus, and may be seen to move up and
down when the insect moves its mandibles. It forms the roof of the
anterior part of the mouth (Figs. 69, 74), and its inner side is lined
with a soft membrane, usually provided with hairs and sense-papillæ or
cups, forming the epipharynx.

The labrum is more or less deeply bilobed, especially in caterpillars
and in adult Staphylinidæ, and has been thought by some writers
(Kowalevsky, Carrière, and also Chatin) to represent a pair of
appendages, but Heymons (1895) refutes this view, stating as his reason
that the labrum arises between the two halves of the nervous system
(protocerebrum), while all the true appendages arise on each side of the
nervous system. (See also Fig. 34.)

[Illustration:

  FIG. 25.—Front view of the head of _C. spretus_: _E_, epicranium; _C_,
    clypeus; _L_, labrum; _O_, _o_, ocelli; _e_, eye; _a_, antenna;
    _md_, mandible; _mx_, portion of maxilla uncovered by the labrum;
    _p_, maxillary palpus; _p′_, labial palpus.
]

In the fleas (Siphonaptera) both the clypeus and labrum are wanting.

While it apparently forms an anterior specialized portion of the
procephalic lobes, Viallanes regarded it as belonging to the third, or
his tritocerebral, segment, since the labral nerves arise from the
tritocerebral ganglia. But since in all the early as well as late stages
of embryonic life it appears to be situated in front of the mouth, it
would seem to belong to the first segment.

In the embryo of Blatta it first appears as a thick crescentic fold
being slightly divided anterior to the mouth, and in Doryphora it
appears as a heart-shaped or deeply bilobed prominence situated in front
of the mouth (Wheeler).

=The epipharynx and labrum-epipharynx.=—The epipharynx is the under
surface or pharyngeal lining of the clypeus and labrum, forming the
membranous roof of the mouth. As it contains the organs of taste and has
been generally overlooked by entomologists, we may dwell at some length
on its structure in different orders.

Réaumur was, so far as we have been able to ascertain, the first author
to describe and figure the epipharynx, which he observed in the honey
bee and bumble bee, and called _la langue_, remarking that it closes the
opening into the œsophagus, and that it is applied against the palate.
According to Kirby and Spence, De Geer described the epipharynx of the
wasp; and Latreille referred to it, calling it the _sous labre_.

The name _epipharynx_ was bestowed upon this organ by Savigny, who thus
speaks of that of the bees: “Ce pharynx est, à la vérité, non seulement
caché par la lèvre supérieure, mais encore exactement recouvert par un
organe particulier que Réaumur a déjà décrit. C’est une sorte
d’appendice membraneux qui est reçu entre les deux branches des
mâchoires. Cette partie ayant pour base le bord supérieur du pharynx,
peut prendre le nom d’_épipharynx_ ou d’_épiglosse_.”

He also describes that of Diptera. What Walter has lately proved to be
the epipharynx of Lepidoptera was regarded by Savigny and all subsequent
writers as the labrum.

The latest account of the function of this organ is that by Cheshire,
who states that the tube made by the maxillæ and labial palpi cannot act
as a suction pipe, because it is open above. “This opening is closed by
the front extension of the epipharynx, which closes down to the maxillæ,
fitting exactly into the space they leave uncovered, and thus the tube
is completed from their termination to the œsophagus.”

[Illustration:

  FIG. 26.—Epipharynx of _Phaneroptera angustifolia_: _cl_, clypeus;
    _lbr. e_, labrum-epipharynx; _t_ _c_, taste cups, both on the
    clypeal and on the labral regions.
]

[Illustration:

  FIG. 27.—Epipharynx of _Hadenœcus subterraneus_, cave cricket.
]

It is singular that this organ is not mentioned in Burmeister’s Manual
of entomology, in Lacordaire’s Introduction à l’entomologie, or by
Newport in his admirable article _Insecta_ in Todd’s Cyclopedia of
anatomy. Neither has Straus-Durckheim referred to or figured it in his
great work on the anatomy of _Melolontha vulgaris_.

In their excellent work on the cockroach, Miall and Denny state that
“The epipharynx, which is a prominent part in Coleoptera and Diptera, is
not recognizable in Orthoptera” (p. 45). We have, however, found it to
be always present in this order (Figs. 26, 27).

We are not aware that any modern writers have described or referred to
the epipharynx of the mandibulate orders of insects. Although Dr. G.
Joseph speaks of finding taste-organs on the palate of almost every
order of insects, especially plant-feeding forms, we are unable to find
any specific references, his detailed observations being apparently
unpublished.

The epipharynx is so intimately associated with the elongated labium of
certain Diptera, that, with Dr. Dimmock, we may refer to the double
organ as the labrum-epipharynx; and where, as in the lepidopterous
_Micropteryx semipurpurella_, described and figured by Walter, and the
Panorpidæ (Panorpa and Boreus), the labrum seems pieced out with a thin,
pale membranous fold which appears to be an extension of the epipharynx,
building up the dorsal end of the labrum, this term is a convenient one
to use.

In the lower orders of truly mandibulate insects, from the Thysanura to
the Coleoptera, excluding those which suck in liquid food, such as the
Diptera, Lepidoptera, and Hymenoptera, and the Mecoptera (Panorpidæ)
with their elongated head and feeble, small mandibles, the epipharynx
forms a simple membranous palatal lining of the clypeus and labrum. In
such insects there is no soft projecting or pendant portion, fitted to
close the throat or to complete a partially tubular arrangement of the
first and second maxillæ.

In all the mandibulate insects, then, the epipharynx forms simply the
under surface or pharyngeal lining of the clypeus and labrum, the
surface being uniformly moderately convex, and corresponding in extent
to that of the clypeus and labrum, posteriorly merging into the palatal
wall of the pharynx; the armature of peculiar gathering-hairs sometimes
spreading over its base, being continuous with those lining the mouth
and beginning of the œsophagus. The suture separating the labrum from
the clypeus does not involve the epipharynx, though since certain
gustatory fields lie under the front edge of the clypeus, as well as
labrum, one may in describing them refer to certain fields or groups of
cups or pits as occupying a labral or clypeal region or position.

The lack of traces of a suture in the epipharynx corresponding to the
labral suture above, suggests that the labrum does not represent a pair
of coalesced appendages, and that it, with the clypeus, simply forms the
solid cuticular roof of the mouth.

The only soft structures seen between the epipharynx and labrum, besides
the nerves of special sense, are the elevator muscles of the labrum, and
two tracheæ, one on each side.

The structure and armature of the epipharyngeal surface even besides the
taste-pits, taste-cups and rods, is very varied, the setæ assuming very
different shapes. There seem to be two primary forms of setæ, (1) the
normal forms which arise from a definite cell; and (2) soft, flattened,
often hooked hairs which are cylindrical towards the end, but arise from
a broad triangular base, without any cell-wall. These are like the
“gathering hairs” of Cheshire, situated on the bees’ and wasps’ tongue;
they also line the walls of the pharynx and extend toward the œsophagus.
They are also similar to the “hooked hairs” of Will. The first kind, or
normal setæ, are either simply defensive, often guarding the sense-cups
or sensory fields on which the sense-cups are situated, or they have a
nerve extending to them and are simply tactile in function.

The surface of the epipharynx, then, appears to be highly sensitive, and
to afford the principal seat of the gustatory organs, which are
described under the head of organs of taste.




                      LITERATURE ON THE EPIPHARYNX


  =Réaumur.= Mémoires pour servir à l’histoire des insectes, v, 1740, p.
    318, Pl. 28, Figs. 4, 7, 8, 9, 10, 11 l.

  =Kirby and Spence.= Intr. to entomology, iii, 1828, p. 457.

  =De Geer.= ii, 1778; v, 26, Fig. 11, M.

  =Kirby and Spence.= Pl. xii, Fig. 2 K.

  =Latreille.= Organisation extérieure des insectes, p. 184. (Quoted
    from Kirby and Spence.)

  =Savigny.= Mémoires sur les animaux sans vertèbres. Partie I^{re},
    1816, p. 12.

  =Walter, Alfred.= Beiträge zur Morphologie der Schmetterlinge. Erster
    Theil. Zur Morphologie des Schmetterlingsmundtheile. (Jena. Zeits.,
    xviii, 1885, p. 752.)

  =Cheshire, F. R.= Bees and bee-keeping, i, London, 1886, p. 93.

  =Joseph, Gustav.= Zur Morphologie des Geschmacksorganes bei Inseckten.
    (Amtlicher Bericht der 50 Versammlung deutscher Naturforscher u.
    Artzte in München. 1877, pp. 227, 228.)

  =Dimmock, George.= The anatomy of the mouth-parts and of the sucking
    apparatus of some Diptera, 1881. (Also in Psyche, iii, pp. 231–241,
    Pl. 1, 1882.)

  =Packard, A. S.= On the epipharynx of the Panorpidæ. (Psyche, 1889, v,
    pp. 159–164.)

  —— Notes on the epipharynx and the epipharyngeal organs of taste in
    mandibulate insects. (Psyche, v, pp. 193–199, 222–228, 1889.)

=Attachment of the head to the trunk.=—The head is either firmly
supported by the broad prothoracic segment in Orthoptera, many beetles,
etc., into which it is more or less retracted, or it is free and
attached by a slender neck, easily turning on the trunk, as in
dragon-flies, flies, etc. In some insects there are several chitinous
plates, situated on an island in the membrane on the under side of the
neck; these are the “cervical sclerites” of Sharp, occurring “in
Hymenoptera, in many Coleoptera, and in Blattidæ.”

=The basal or gular region of the head.=—At the hinder part of the head
is the opening (occipital foramen) into the trunk. The cheek (gena) is
the side of the head, and to its inner wall is attached the mandibular
muscle; it thus forms the region behind the eye and over the base of the
mandibles. In the Termitidæ, where the head is broad and flat, it forms
a distinct piece on the under side of the head bounding the gulo-mental
region (Fig. 28). In the Neuroptera (Corydalus, Fig. 29, and Mantispa,
Fig. 30) it is less definitely outlined.

[Illustration:

  FIG. 29.—Head of _Corydalus cornutus_, ♂: _A_, from above. _B_, from
    beneath. _C_, from the side. _a. cly_, clypeus anterior; _p. cly_,
    clypeus posterior; _lbr_, labrum; _md_, mandible; _mx_, base of
    first maxilla; _mp_, its palpus; _m_, mentum; _sm_, submentum;
    _plpr_, palpifer; _lig_, fused second maxillæ; _ant_, antenna;
    _occ_, occiput.
]

[Illustration:

  FIG. 28.—Head of _Termopsis angusticollis_, seen from beneath, showing
    the gena and gula: _m_, mentum; _sm_, submentum; _labr_, under side
    of the labrum; _x_, hypopharyngeal chitinous support.
]

All the gulo-mental region of the head appears to represent the base of
the second maxillæ, and the question hence arises whether the submentum
is not the homologue of the cardines of the first maxillæ fused, and the
mentum that of the stipites of the latter also fused together. If this
should prove to be the case, the homologies between the two pairs of
maxillæ will be still closer than before supposed. Where the gula is
differentiated, this represents the basal piece of the second maxillæ.
In Figs. 28, 29, 30, and 31, these three pieces are clearly shown to
belong to the second maxillary segment. It is evident that these pieces
or sclerites belong to the second maxillary or labial segment of the
head, as does the occiput, which may represent the tergo-pleural portion
of the segment. Miall and Denny also regarded the submentum as the basal
piece of the second maxillæ.

[Illustration:

  FIG. 30.—Head of _Mantispa brunnea_, under side: _e_, eye; other
    lettering as in Fig. 29.
]

[Illustration:

  FIG. 31.—Head of _Limnephilus pudicus_, under side: _e_, eye; _l_,
    ligula; _p_, palpifer; _lp_, labial palpi.
]

=The occiput= (Fig. 29, _B_, _C_), as stated beyond, is very rarely
present as a separate piece; in the adult insect we have only observed
it in Corydalus. The occipital region may be designated as that part of
the head adjoining and containing the occipital foramen. Newport
considers the occiput as that portion of the base of the head “which is
articulated with the anterior margin of the prothorax. It is perforated
by a large foramen, through which the organs of the head are connected
with those of the trunk. It is very distinct in Hydroüs and most
Coleoptera, and in some, the Staphylinidæ, Carabidæ, and Silphidæ is
constricted and extended backwards so as to form a complete neck.” (See
also p. 51.)

[Illustration:

  FIG. 32.—Interior and upper and under surface of the head of _Hydroüs
    piceus_: _d_, clypeus; _e_, labrum; _g_, maxilla; _h_, its palpus;
    _i_, labium; _k_, labial palpus; _p_, sutura epicranii; _q_,
    cotyloid cavity; _r_, torulus; _s_, _v_, laminæ squamosa; _t_,
    laminaæ posteriores; _u_, tentorium; _w_, laminæ orbitales; _x_, os
    transversum; _y_, articulating cavity for the mandible; _z_, os
    hypopharyngeum.—After Newport.
]

=The tentorium.=—The walls of the head are supported or braced within by
two beams or endosternites passing inwards, and forming a solid
chitinous process or loop which extends in the cockroach downwards and
forwards from the lower edge of the occipital foramen. “In front it
gives off two long crura or props, which pass to the ginglymus, and are
reflected thence upon the inner surface of the clypeus, ascending as
high as the antennary socket, round which they form a kind of rim.”
(Miall and Denny.) The œsophagus passes upwards between its anterior
crura, the long flexor of the mandible lies on each side of the central
plate; the supraœsophageal ganglion rests on the plate above, and the
subœsophageal ganglion lies below it, the nerve cords which unite the
two passing through the circular aperture. (Miall and Denny.) In
Coleoptera (Hydroüs) it protects the nervous cord which passes under it.
(Newport, Fig. 32, _u_.)

[Illustration:

  FIG. 33.—Posterior view of head of Anabrus; _t_, tentorium. Joutel
    _del._
]

In Anabrus the tentorium is V-shaped, the two arms originating on each
side of the base of the clypeus next to the base of each mandible the
origin being indicated by two small foramina partly concealed externally
and passing inwards and backwards and uniting just before reaching the
posterior edge of the large occipital foramen (Fig. 33).

  Palmén regards the tentorium as representing a pair of tracheæ (with
  the cephalic spiracles) which have become modified for supports or
  for muscular attachment, since he finds that in Ephemera the
  tentorium breaks across the middle during exuviation, each half
  being drawn out of the head like the chitinous lining of a tracheal
  tube. This view is supported by Wheeler, who has shown that the
  tentorium of Doryphora originates from five pairs of invaginations
  of the longitudinal commissures, and which are anterior to those of
  the second maxillary segment. “These invaginations grow inwards as
  slender tubes, which anastomose in some places. Their lumina are
  ultimately filled with chitin.” (Jour. Morph., iii, p. 368.)

  This view has also been held by Carrière and Cholodkowsky, but
  Heymons concludes from his embryological studies on Forficula and
  Blattidæ (1895) that it is unfounded. That this is probably the case
  is proved by the fact that the apodemes of the thoracic region are
  evidently not modified tracheæ, since the stigmata and tracheæ are
  present.

=Number of segments in the head.=—While it is taken for granted by many
entomologists that the head of insects represents a single segment,
despite the circumstance that it bears four pairs of appendages, the
more careful, philosophical observers have recognized the fact that it
is composed of more than a single segment. Burmeister recognized only
two segments in the head; Carus and Audouin recognized three; Macleay
and Newman four; Straus-Durckheim even so many as seven. Huxley supposed
that there are five segments bearing appendages, remarking, “if the eyes
be taken to represent the appendages of another somite, the insect head
will contain six somites.” (Manual of Anat. Invert. Animals, p. 398.)

These discordant views were based on the examination of the head in
adult insects; but if we confine ourselves to the imago alone, it is
impossible to arrive at a solution of the problem.

Newport took a step in the right direction when he wrote: “It is only by
comparing the distinctly indicated parts of the head in the perfect
insect with similar ones in the larva that we can hope to ascertain the
exact number of segments of which it is composed.” He then states that
in the head of _Hydroüs piceus_ are the remains of four segments, though
still in the next paragraph, when speaking of the head as a whole, he
considers it as the first segment, “while,” he adds, “the aggregation of
segments of which it is composed we shall designate individually
_subsegments_.”

That the head of insects is composed of four segments was shown on
embryological grounds by the writer (1871) and afterwards by Graber
(1879). The antennæ and mouth-parts are outgrowths budding out from the
four primitive segments of the head; the antennæ grow out from the under
side of the procephalic lobes, and these should therefore receive the
name of antennal lobes. In like manner the mandibles and first and
second maxillæ arise respectively from the three succeeding segments.

[Illustration:

  FIG. 34.—Embryo of _Anurida maritima_: _tc. ap_, minute temporary
    appendage of the tritocerebral segment, the premandibular appendage;
    _at_, antenna; _md_, mandible; _mx_^1, first maxilla; _mx_^2, second
    maxilla; _p_^1–_p_^3, thoracic; _ap_^1, _ap_^2, abdominal
    appendages; _an_, anus—After Wheeler.
]

While the postoral segments and their appendages are readily seen to be
four in number, the question arises as to whether the eyes represent the
appendages of one or more preoral segments. In this case embryology thus
far has not afforded clear, indubitable evidence. We are therefore
obliged to rely on the number of neuromeres, or primitive ganglia. In
the postoral region of the head, as also in the trunk, a pair of
neuromeres correspond to each segment. (See also under Nervous System,
and under Embryology.) We therefore turn to the primitive number of
neuromeres constituting the procephalic lobes or brain.

From the researches of Patten, Viallanes, and of Wheeler, especially of
Viallanes, it appears that the brain or supraœsophageal ganglion is
divided into three primitive segments. (See Nervous System, Brain.) The
antennæ are innervated from the middle division or deutocerebrum. Hence
the ocular segment, _i.e._ that bearing the compound and simple eyes, is
supposed to represent the first segment of the head. This, however, does
not involve the conclusion that the eyes are the homologues of the
limbs, however it may be in the Crustacea.

The second head-segment is the antennal, the antennæ being the first
pair of true jointed appendages.

The third segment of the head is very obscurely indicated, and the facts
in proof of its existence are scanty and need farther elucidation.

Viallanes’ tritocerebral lobes or division of the brain is situated in a
segment found by Wheeler to be intercalated between the antennal and
mandibular segments. He also detected in _Anurida maritima_, the
rudiments of a pair of appendages, smaller than those next to it, and
which soon disappear (Fig. 34, _tc. ap_). He calls this segment the
intercalary.[12] Heymons (1895) designates it as the “Vorkiefersegment,”
and it may thus be termed the premandibular segment.

[Illustration:

  FIG. 35.—Head of embryo of honey bee: _B_, a little later stage than
    _A_. _pr.m_, premandibular segment; _cl_, clypeus; _ant_, antenna;
    _md_, mandible; _mx_, first maxilla; _mx′_, second maxilla; _sp_,
    spiracle.—After Bütschli.
]

As early as 1870 Bütschli observed in the embryo of the honey bee the
rudiments of what appeared to be a pair of appendages between the
antennæ and mandibles, but, judging by his figures, nearer to and more
like the mandibles than the rudimentary antennæ (Fig. 35); they seemed
to him “almost like a pair of inner antennæ.”

“I find,” he says, “in no other insects any indication of this peculiar
appendage, which at the time of its greatest development attains a
larger size than the antennæ, and which, afterwards becoming less
distinct, forms by fusion with that on the other side a sort of larval
lower lip. That this appendage does not belong to the category of
segmental appendages is indicated by the site of its origin on the upper
side of the primitive band.” (Zeitschr. wissen. Zool., xx, p. 538.)

Grassi has also observed it in Apis, and regards it as the germ of a
first, but deciduous, pair of jaws. In the embryo of Hylotoma Graber
(Figs. 134, 135) found what he calls three pairs of “preantennal
projections,” one of which he thinks corresponds to the “inner antennæ”
of Bütschli. This subject needs further investigation.

It thus appears that the procephalic lobes of the embryo of insects,
with the rudiments of the antennæ, constitute the primitive head, and
perhaps correspond to the annelidan head, while gradually the antennal
appendages were in the phylogenetic development of the class fused with
the two segments of the primary head. That the second maxillary segment,
the occiput, was the last to be added, and at first somewhat
corresponded in position to the poison-fangs of centipedes (Chilopods),
is shown by our observations on the embryology of Æschna (Fig. 36).

[Illustration:

  FIG. 36.—Æschna nearly ready to hatch: 4, labium, between _T_ and _e_
    the occipital tergite; 5–7, legs.
]

[Illustration:

  FIG. 37.—Head of embryo Nematus, showing the labial segment: _occ_,
    forming the occiput; _cl_, clypeus; _lb_, labrum; _md_, mandible;
    _mdm_, muscle of same; _mx_, maxilla; _mx′_, second maxilla
    (labium); _oe_, œsophagus.
]

The mandibular segment appears to form a large part of the post-antennal
region of the epicranium on account of the great mandibular muscle which
arises from so large an area of the anterior region of the head (Fig.
37).

Judging from the embryo of Nematus (Fig. 37), the first maxillary
segment is tergally aborted, there being no tergo-pleural portion
left.[13]

The second maxillary segment tergally appears to be represented by the
occipital region of the head.

All the gular region, including the submentum and mentum, probably
represents the base of the labium or second maxillæ.[14] The so-called
“occiput” forms the base of the head of Corydalus, a neuropterous
insect, which, however, is more distinct in the larva. In most other
adult insects the occiput is either obsolete or fused with the hinder
part of the epicranium. We have traced the history of this piece
(sclerite) in the embryo of Æschna, a dragon-fly, and have found that it
represents the tergal portion of the sixth or labial segment. In our
memoir on the development of this dragon-fly, Pl. 2, Fig. 9, the head of
the embryo is seen to be divided into two regions, the anterior, formed
of the antennal, mandibular, and first maxillary segments, and the
posterior, formed of the sixth or labial segment. This postoral segment
at first appears to be one of the thoracic segments, but is afterwards
added to the head, though not until after birth, as it is still separate
in the freshly hatched nymph (Fig. 4; see also Kolbe, p. 132, Fig. 59,
_sq._ 5). A. Brandt’s figure of _Calopteryx virgo_ (Pl. 2, Fig. 19)
represents an embryo of a stage similar to ours, in which the postoral
or sixth (labial) segment is quite separate from the rest of the head.
The accompanying figure, copied from our memoir, also shows in a saw-fly
larva (_Nematus ventricosus_) the relations of the labial or sixth
segment to the rest of the head. The suture between the labial segment
and the preoral part of the head disappears in adult life. From this
sketch it would seem that the back part of the head, _i.e._ of the
epicranium, may be made up in part of the tergite or pleurites of the
mandibular segment, since the mandibular muscles are inserted on the
roof of the head behind the eyes. It is this labial segment which in
Corydalus evidently forms the occiput, and of which in most other
insects there is no trace in larval or adult life, unless we except
certain Orthoptera (Locusta), and the larva of the Dyticidæ.

The following table is designed to show the number and succession of the
segments of the head, with their respective segments.

 TABULAR VIEW OF THE SEGMENTS, PIECES (SCLERITES), AND APPENDAGES OF THE
                                  HEAD

 ═══════════════════════════════╤═══════════════════╤═══════════════════
         NAME OF SEGMENT        │ PIECES OR REGIONS │ APPENDAGES, ETC.
                                │OF THE HEAD-CAPSULE│
 ───────────┬───────────────────┼───────────────────┼───────────────────
            │1. Ocellar         │Epicranium,        │Compound and simple
 _Preoral_, │  (Protocerebral). │  anterior region  │  eyes (Ocelli).
   in early │                   │  with the clypeus │
   embryo.  │                   │  labrum, and      │
            │                   │  epipharynx.      │
            │                   │                   │
 _Postoral_,│2. Antennal        │Epicranium,        │Antennæ.
   in early │  (Deutocerebral). │  including the    │
   embryo.  │                   │  antennal sockets.│
            │                   │                   │
            │3. Premandibular,  │Wanting in         │Premandibular
            │  or intercalary   │  postembryonic    │  appendages (in
            │  (Tritocerebral). │  life, except in  │  Campodea).
            │                   │  Campodea.        │
            │                   │                   │
            │4. Mandibular.     │Epicranium behind  │Mandibles.
            │                   │  the antennæ,     │
            │                   │  genæ.            │
            │                   │                   │
            │5. 1st Maxillary.  │Epicranium, hinder │1st Maxillæ.
            │                   │  edge? Tentorium. │
            │                   │                   │
            │6. 2d Maxillary, or│Occiput.           │2d Maxillæ or
            │  labial.          │                   │  Labium.
            │                   │                   │  Post-gula, gula,
            │                   │                   │  submentum,
            │                   │                   │  mentum,
            │                   │                   │  hypopharynx
            │                   │                   │  (lingua, ligula),
            │                   │                   │  paraglossæ,
            │                   │                   │  spinneret.
 ───────────┴───────────────────┴───────────────────┴───────────────────

[Illustration:

  FIG. 38.—Larva (_a_) of a chalcid, about to pupate, with the head,
    including the eyes and three ocelli, in the prothoracic segment:
    _b_, _c_, pupa.
]

=The composition of the head in the Hymenoptera.=—Ratzeburg stated in
1832 that the head in the adult Hymenoptera (Cynips, Hemiteles, and
Formica) does not correspond to that of the larva, but is derived from
the head and the first thoracic segment of the larva. Westwood and also
Goureau made less complete but similar observations, though Westwood
afterwards changed his opinion, and the same view was maintained by
Reinhard. Our own observations (as seen in Fig. 38) led us to suppose
that this was a mistaken view; that the larval head, being too small to
contain that of the semipupa, was simply pushed forward, as in
caterpillars. Bugnion, however, reaffirms it in such a detailed way that
we reproduce his account. He maintains that the views of Ratzeburg are
exact and easy to verify in the chalcid genus Encyrtus, except, however,
that which concerns the ventral part and the posterior border of the
prothoracic segment.

As the time of transformation approaches, the head of the larva, he
says, is depressed and soon concealed under the edge of the prothoracic
segment; the latter elongates, becomes thicker and more convex, and
within can be seen the two oculo-cephalic imaginal buds. The head of the
perfect insect is derived not only from the head of the larva, but also
from the portion of the prothoracic segment which is occupied by the
buds, _i.e._ almost its entire dorsolateral face. But the hinder and
ventral part of this segment (which contains the imaginal buds of the
first pair of legs) takes no part in the formation of the head; these
parts, according to Bugnion, towards the end of the larval period
detaching themselves so as to become fused with the thorax and
constitute the pronotum and the prosternum.

[Illustration:

  FIG. 39.—Anterior half of larva of Encyrtus, ventral face, showing the
    upper (wing) and lower (leg) thoracic imaginal buds: _b_, mouth;
    _ch_, chitinous arch; _gl_, silk gland; _g_, brain; _n_, nervous
    cord; _a_^1, bud of fore, _a_^2, bud of hind, wing; _p_^1–_p_^3,
    buds of legs; _st_^1–_st_^3, stigmata.
]

[Illustration:

  FIG. 40.—Anterior part of Encyrtus larva, 1.2 mm. in length; dorsal
    face; the cellular masses beginning to form the buds of the wings,
    eyes, and antennæ: _o_, eye bud; _e_, stomach.
]

[Illustration:

  FIG. 41.—Older Encyrtus larva, lateral view, showing the buds of the
    antennæ (_f_), legs, and wings; _oe_, œsophagus; _q_^1, _q_^2,
    _q_^3, buds of the genital armature; _x_, rudiment of the sexual
    gland (ovary or testis); _u_, urinary tube; _i_, intestine (rectum);
    _a_, anus.
]

[Illustration:

  FIG. 42.—A still older larva, ready to transform. The imaginal buds of
    the antennæ, eyes, wings, and legs have become elongated; lettering
    as in Fig. 41.—This and Figs. 39–41 after Bugnion.
]

  This mode of formation of the head may be observed still more easily
  in Rhodites, Hemiteles, and Microgaster, from the fact that their
  oculo-cephalic buds are much more precocious, and that the eyes are
  charged with pigment at a period when the insect still preserves its
  larval form.

  “... I believe that this mode of formation of the head occurs in all
  Hymenoptera with apodous larvæ, in this sense; that a more or less
  considerable part of the first thoracic segment is always soldered
  to the head of the larva to constitute the head of the perfect
  insect. The arrangement of the nervous system is naturally in accord
  with this peculiarity of development, and the cephalic ganglia of
  the larva to which the ocular blastems later adapt themselves, are
  found not in the head, but in the succeeding segment (Figs. 39, 40,
  41).

  “Relying on these facts, I maintain that the encroachment of the
  head on the prothorax is a consequence of the preponderance in size
  of the brain, and indicates the superiority of the Hymenoptera over
  other insects....”

  That the pronotum is derived from the larval prothoracic segment is
  proved by the fact that the first pair of stigmata becomes what
  authors call the “prothoracic” stigmata of the perfect insect. But
  Bugnion thinks that the projection which carries it, and which he
  calls the shoulder (Figs. 41 and 42), belongs to the mesonotum.


                      _b._ Appendages of the head

=The antennæ.=—These are organs of tactile sense, but also bear
olfactory, and in some cases auditory organs; they are usually inserted
between or in front of the eyes, and moved by two small muscles at the
base, within the head. In the more generalized insects the antennæ are
simple, many-jointed appendages, the joints being equal in size and
shape. The antennæ articulate with the head by a ball and socket joint,
the part on which it moves being called the _torulus_ (Fig. 32, _r_). In
the more specialized forms it is divided into the _scape_, the
_pedicel_, and a _flagellum_ (or _clavola_); but usually, as in ants,
wasps, and bees, there are two parts, the basal three-jointed one being
the scape, and the distal one, the usually long filiform flagellum. The
antennæ, especially the flagellum, vary greatly in form in insects of
different families and orders, this variation being the result of
adaptation to their peculiar surroundings and habits. The number of
antennal joints may be one (Articerus, a clavigerid beetle), or two in
Paussus and in _Adranes cœcus_ (Fig. 43^{12}), where they are short and
club-shaped; in flies (Muscidæ, etc.), they are very short and with few
joints, and when at rest lying in a cavity adapted for their reception.
In the lamellicorn beetles the flagellum is divided into several leaves,
and this condition may be approached in the serrate or flabellicorn
antennæ of other beetles. In Lepidoptera, and in certain saw-flies and
beetles, they are either pectinate or bipectinate, being in one case at
least, that of the Australian Hepialid (_Abantiades argenteus_),
tripectinate (Fig. 44), and in the dipterous (Tachinid) genus Talarocera
the third joint is bipectinate (Fig. 45). In Xenos and in Parnus they
may be deeply forked, while in Otiocerus, two long processes arise from
the base, giving it a trifid shape. In dragon-flies and cicadæ, they are
minute and hair-like, though jointed, while in the larvæ of many
metabolous insects they are reduced to minute three-jointed tubercles.
In aquatic beetles, bugs, etc., the antennæ are short, and often, when
at rest, bent close to the body, as long antennæ would impede their
progress.

[Illustration:

  FIG. 43.—Different forms of antennæ of beetles: 1, serrate; 2,
    pectinate; 3, capitate (and also geniculate); 4–7, clavate; 8, 9,
    lamellate; 10, serrate (Dorcatoma); 11, irregular (Gyrinus); 12,
    two-jointed antenna of _Adranes cæcus_.—After LeConte. _a_, first
    joint of flagellum of antenna of _Troctes silvarum_; _b_, of _T.
    divinatorius_.—After Kolbe.
]

[Illustration:

  FIG. 44.—Tripectinate antenna of an Australian moth.
]

While usually more or less sensorial in function, Graber states that the
longicorn beetles in walking along a slender twig use their antennæ as a
rope-dancer does his balancing pole.

[Illustration:

  FIG. 45.—Antenna of _Talarocera nigripennis_, ♂.—After Williston.
]

Recent examination of the sense-organs in the antennæ of an ant, wasp,
or bee enables us, he says, to realize what wonderful organs the antennæ
are. In such insects we have a rod-like tube which can be folded up or
extended out into space, containing the antennal nerve, which arises
directly from the brain and sends a branch to each of the thousands of
olfactory pits or pegs which stud its surface. The antenna is thus a
wonderfully complex organ, and the insect must be far more sensitive to
movements of the air, to odors, wave-sounds, and light-waves, than any
of the vertebrate animals.

That ants appear to communicate with each other, apparently talking with
their antennæ, shows the highly sensitive nature of these appendages.
“The honey-bee when constructing its cells ascertains their proper
direction and size by means of the extremities of these organs.”
(Newport.)

How dependent insects are upon their antennæ is seen when we cut them
off. The insect is at once seriously affected, its central nervous
system receiving a great shock, while it gives no such sign of distress
and loss of mental power when we remove the palpi or legs. On depriving
a bee of its antennæ, it falls helpless and partially paralyzed to the
earth, is unable at first to walk, but on partly recovering the use of
its limbs, it still has lost the power of coördinating its movements,
nor can it sting; in a few minutes, however, it becomes able to feebly
walk a few steps, but it remains over an hour nearly motionless. Other
insects after similar treatment are not so deeply affected, though bees,
wasps, ants, moths, certain beetles, and dragon-flies are at first more
or less stunned and confused.

The antennæ afford salient secondary sexual differences, as seen in the
broadly pectinated antennæ of male bombycine moths, certain saw-flies
(Lophyrus), and many other insects.

The mouth-parts, buccal appendages, or trophi, comprise, besides the
labrum, the mandibles and maxillæ.

=The mandibles.=—These are true jaws, adapted for cutting, tearing, or
crushing the food, or for defence, while in the bees they are used as
tools for modelling in wax, and in Cetonia, etc., as a brush for
collecting pollen. They are usually opposed to each other at the tips,
but in many carnivorous forms their tips cross each other like shears.
They are situated below the clypeus on each side, and are hinged to the
head by a true ginglymus articulation, consisting of two condyles or
tubercles to which muscles are attached, the principal ones being the
flexor and great extensor (Fig. 48). They are solid, chitinous, of
varied shapes, and in the form of the teeth those of the same pair
differ somewhat from each other (Fig. 46 _A_). In the pollen-eating
beetles (Cetoniæ) and in the dung-beetles (Aphodius, etc.) the edge is
soft and flexible. In the males of Lucanus, etc. (Fig. 47), and of
Corydalus (Fig. 29), they are of colossal size, and are large and
sabre-shaped in the larvæ of water-beetles, ant-lions, Chrysopa, etc.
where they are perforated at the tips, through which the blood of their
prey is sucked.

While the mandibles are generally regarded as composed of a single
piece, in Campodea and Machilis there appears to be an additional basal
piece apparently corresponding to the stipes of the first maxilla, and
separated by a faint suture from the molar or distal joint. In Campodea
there is a minute movable appendage figured both by Meinert and by
Nassonow, which appears to represent the lacinia of the maxilla (Fig.
48). Wood-Mason has observed in the mandibles of the embryo of a
Javanese cockroach, _Blatta (Panesthia) javanica_, indications of “the
same number of joints as in that of chilognathous myriopods, or one less
than in that of Machilis.” Also he adds: “In both ‘larvæ’ and adults of
_Panesthia javanica_ a faint groove crosses the ‘back’ of the mandible
at the base. This groove appears to be the remains of the joint between
the third and apical segments of the formerly 4–segmented mandibles.”

[Illustration:

  FIG. 46.—Various forms of mandibles. _A_, right and left of Termopsis.
    _A′_, showing at the shaded portion the “molar” of Smith. _B_,
    _Termes flavipes_, soldier; _md_, its mandible. _C_, Panorpa.
]

[Illustration:

  FIG. 47.—_Chiasognathus grantii_, reduced. Male.—After Darwin.
]

[Illustration:

  FIG. 48.—Mandible of Campodea: _l_, prostheca or lacinia; _g_, galea;
    _f_, _f_, flexor muscles; _e_, extensor; _r_, _r_, retractor; _rt_,
    muscle retaining the mandible in its place.—After Meinert. _A_,
    extremity of the same.—After Nassonow.
]

[Illustration:

  FIG. 49.—Mandible of _Passalus cornutus_ with the prostheca (_l_):
    _A_, that of a Nicaraguan species; _a_, inside, _b_, outside view,
    with the muscle.
]

He also refers to the _prostheca_ of Kirby and Spence (Fig. 49), which
he thinks appears to be a mandibular lacinia homologous with it in
Staphylinidæ and other beetles (J. B. Smith also considers it as
“homologous to the lacinia of the maxilla”), and on examining it in _P.
cornutus_ and a Nicaragua species (Fig. 49), we adopt his view, since we
have found that it is freely movable and attached by a tendon and muscle
to the galea. In the rove beetles (Goërius, Staphylinus, etc.) and in
the subaquatic Heteroceridæ, instead of a molar process, is a membranous
setose appendage not unlike the coxal appendages of Scolopendrella,
movably articulated to the jaw, which he thinks answers to the molar
branch of the jaws in Blatta and Machilis. “It has its homologue in the
diminutive Trichopterygidæ in the firmly chitinized quadrant-shaped
second mandibular joint, which is used in a peculiar manner in crushing
the food”; also in the movable tooth of the Passalidæ, and in the
membranous inner lobe of the mandibles of the goliath-beetles, etc.

J. B. Smith has clearly shown that the mandibles are compound in certain
of the lamellicorns. In _Copris carolina_ (Fig. 50), he says, the small
membranous mandibles are divided into a basal piece (basalis), the
homologue of the stipes in the maxilla; another of the basal pieces he
calls the molar, and this is the equivalent of the subgalea, while a
third sclerite, only observed in Copris, is the _conjunctivus_, the
lacinia (prostheca) being well developed. Smith therefore concludes
“that the structure of the mandible is fundamentally the same as that of
the labium and maxilla, and that we have an equally complex organ in
point of origin. Its usual function, however, demands a powerful and
solid structure, and the sclerites are in most instances as thoroughly
chitinized and so closely united to the others that practically there is
only a single piece, in which the homology is obscured.” (Trans. Amer.
Ent. Soc., xix, pp. 84, 85. 1892.) From the studies of Smith and our
observations on Staphylinus, Passalus, Phanæus, etc. (Fig. 50, _A_, _B_)
we fully agree with the view that the mandibles are primarily 3–lobed
appendages like the maxillæ. Nymphal Ephemerids have a lacinia-like
process. (Heymons.)

[Illustration:

  FIG. 50.—Mandible of _Copris carolina_.—After Smith. _A_′_C.
    anaglypticus_. _A_ (figure to right), do. of _Leistotrophus
    cingulatus_; _B_, of _Phanæus carnifex_; _g′_, end of galea,—_g_,
    enlarged; _c_, conjunctivus. _C_, of _Meloë angusticollis_: _l_,
    lacinia; _a_, lacinia enlarged.
]

Mandibles are wanting in the adults of the more specialized Lepidoptera,
being vestigial in the most generalized forms (certain Tineina and
Crambus), but well developed in that very primitive moth, Eriocephala
(Fig. 51). They are also completely atrophied in the adult Trichoptera,
though very large and functional in the pupa of these insects (Fig. 52),
as also in the pupa of Micropteryx (Fig. 53). They are also wanting in
the imago of male Diptera and in the females of all flies except
Culicidæ and Tabanidæ.

They are said by Dr. Horn to be absent in the adult _Platypsyllus
castoris_, though well developed in the larva; and functional mandibles
are lacking in the Hemiptera.

=The first maxillæ.=—These highly differentiated appendages are inserted
on the sides of the head just behind the mandibles and the mouth, and
are divided into three lobes, or divisions, which are supported upon
two, and sometimes three basal pieces, _i.e._ the basal joint or
_cardo_, the second joint or _stipes_, with the _palpifer_, the latter
present in Termitidæ (Fig. 54, _plpgr_), but not always separately
developed (Fig. 55). The cardo varies in shape, but is more or less
triangular and is usually wedged in between the submentum and mandible.
It is succeeded by the stipes, which usually forms the support for the
three lobes of the maxilla, and is more or less square in shape.

[Illustration:

  FIG. 51.—Mandible of _Eriocephala calthella_: _a_, _a′_, inner and
    outer articulation; _s_, cavity of the joint (acetabulum); _A_, end
    seen from one side of the cutting edge.—After Walter.
]

[Illustration:

  FIG. 52.—_A_, Pupa of _Phryganea pilosa_.—After Pictet. _B_, mandibles
    of pupa of _Molanna angustata_.—After Sharp.
]

[Illustration:

  FIG. 53.—Pupa of _Micropteryx purpuriella_, front view: _md_,
    mandibles; _mx.p_, maxillary palpus, end drawn separately; _mx.’p_,
    labial palpi; _lb_, labrum; _A_, another view from a cast skin.
]

The three distal divisions of the maxilla are called, respectively,
beginning with the innermost, the _lacinia_, _galea_, and _palpifer_,
the latter being a lobe or segment bearing the _palpus_. The lacinia is
more or less jaw-like and armed on the inner edge with either flexible
or stiff bristles, spines, or teeth, which are very variable in shape
and are of use as stiff brushes in pollen-eating beetles, etc. The galea
is either single-jointed and helmet-shaped or subspatulate, as in most
Orthoptera, or 2–jointed in Gryllotalpa, or lacinia-like in Myrmeleon
(Fig. 55, _C_); or, in the Carabidæ (Fig. 56) and Cicindelidæ, it is
2–jointed and in form and function like a palpus.

[Illustration:

  FIG. 54.—_A_, maxilla of _Termopsis angusticollis_. _B_, _Termes
    flavipes_: _c_, cardo; _sti_, stipes; _plpgr_, palpiger; _palp_,
    palpus; _lac_, lacinia; _g_, _gal_, galea.
]

[Illustration:

  FIG. 55.—_A_, maxilla of _Mantispa brunnea_. _B_, _Ascalaphus
    longicornis_. _C_, _Myrmeleon diversum_. Lettering as in Fig. 54.
]

[Illustration:

  FIG. 56.—Maxilla of a carabid, _Anophthalmus tellkampfii_: _l_,
    lacinia; _g_, 2–jointed galea; _p_, palpus; _st_, stipes; _c_,
    cardo.
]

[Illustration:

  FIG. 57.—Maxilla of _Nemognatha_, ♀, from Montana. _A_, base of
    maxilla enlarged to show the taste-papillæ (_tp_) and cups (_tc_),
    on the galea (_ga_). _B_, part of end of galea to show the imperfect
    segments and taste-organs: _n_, nerve; a ganglionated nerve supplies
    each taste-papilla or cup; _l_, lacinia; _p_, palpifer; _s_,
    subgalea.
]

[Illustration:

  FIG. 58.—Maxilla of Panorpa.
]

[Illustration:

  FIG. 59.—Maxilla of _Limnephilus pudicus_: _mx_, stipes; _lac_, galea.
]

The palpus is in general antenniform and is composed of from 1 to 6
joints, being usually 4– or 5–jointed, and is much longer than the
galea. In the maxilla of the beetle Nemognatha (Fig. 57), the galea is
greatly elongated, the two together forming an imperfect tube or
proboscis and reminding one of the tongue of a moth, while the lacinia
is reduced. In the Mecoptera the lacinia and galea are closely similar
(Fig. 58); in the Trichoptera only one of the lobes is present (Fig.
59), while in the Lepidoptera the galea unites with its mate to form the
so-called tongue (Fig. 60). The maxilla of the male of _Tegeticula
yuccasella_ is normal, though the galeæ are separate; but in the female,
what Smith regards as the palpifer (the “tentacle” of Riley) is
remarkably developed, being nearly as long as the galea (Fig. 61) and
armed with stout setæ, the pair of processes being adapted for holding a
large mass of pollen under the head.

[Illustration:

  FIG. 60.—Tongue of _Aletia xylina_, with the end magnified.—Pergande
    _del._, from Riley. _A_, much reduced maxilla (_mx_) of _Paleacrita
    vernata_; _mx.p_, palpus.
]

[Illustration:

  FIG. 61.—_A_, maxilla of _Tegeticula yuccasella_, ♂: _g_, galea. _B_,
    ♀: _pl_, enormously developed palpifer; _mx.p_, palpus; _c_, cardo;
    _st_, stipes; _sty_, stylus.
]

In coleopterous larvæ the maxillæ are 2–lobed (Fig. 62), the galea being
undifferentiated, but in those of saw-flies the galea is present (Fig.
63, _gal_).

[Illustration:

  FIG. 62.—Larva of _Rhagium lineatum_: _lat_, lateral view of head and
    thoracic segments; _mx_, first maxilla; _ml_, undifferentiated
    lacinia and galea; _v_, under side of head and pro- and meso-
    thoracic segments; _v.m.s._, one of the middle ventral segments,
    magnified six times; _mx′_, 2d maxilla.
]

It now seems most probable that in the first maxillæ we have the primary
form of buccal appendage of insects, the appendage being composed of
three basal pieces with three variously modified distal lobes or
divisions; and that the mandibles and second maxillæ are modifications
of this type.

How wonderfully the maxillæ of the Lepidoptera are modified, and the
peculiar shapes assumed in the Diptera, Hymenoptera, and other groups,
will be stated in the accounts of those orders, but it is well to recall
the fact that in the most primitive and generalized moth, Eriocephala,
the lacinia is well developed (Fig. 64).

As Newport remarks, the office of the maxillæ in the mandibulate insects
is of a twofold kind; since they are adapted not only for seizing and
retaining the food in the mouth, but also as accessory jaws, since they
aid the mandibles in comminuting it before it is passed on to the
pharynx and swallowed. Hence, as the food varies so much in nature and
situation, it will be readily seen that the maxillæ, especially their
distal parts, vary correspondingly. Thus far no close observations on
the exact use of the first and second maxillæ have been published.

The palpi also are not only organs of touch, but in some cases act as
hands and also bear minute sense-organs, the function of which is
unknown, but would appear to be usually that of smell.

[Illustration:

  FIG. 63.—_Selandria_ larva, common on _Carya porcina_, with details of
    mouth-parts: _leg_, leg; _mx_, maxilla; _gal_, galea; _lac_,
    lacinia.
]

=The second maxillæ.=—The “under-lip” or labium of insects is formed by
the fusion at the basal portion of what in the embryo are separate
appendages, and which arise in the same manner as the first maxillæ.
They are invariably solidly united, no cases of partial or incomplete
fusion being known. The so-called labium is situated in front of the
gula or gular region, and is bounded on each side by the gena, or cheek.
As already observed, the second maxillæ appear to be the appendages of
the last or occipital segment of the head.

[Illustration:

  FIG. 64.—Maxilla of _Eriocephala calthella_: _l_, lacinia; _g_, galea;
    _mx.p_, maxillary palpus; _st_, stipes; _c_, cardo.—After Walter.
]

The second maxillæ are very much differentiated and vary greatly in the
different orders, being especially modified in the haustellate or
suctorial orders, notably the Hymenoptera and Diptera. In the
mandibulate orders, particularly the Orthoptera, where they are most
generalized and primitive in shape and structure, they consist of the
following parts: the _gula_ (a _postgula_ is present in Dermaptera),
_submentum_ (lora of Cheshire, i, p. 91), _mentum_, _palpifer_, the
latter bearing the _palpi_; the _lingua_ (_ligula_) and _paraglossæ_,
while the hypopharynx or lingua is situated on the upper side. The
labial palpi are of the same general shape as those of the first
maxillæ, but shorter, with very rarely more than three joints, though in
Pteronarcys there are four. Leon has detected vestigial labial palpi in
several Hemiptera (Fig. 73). As to the exact nature and limits of the
gula, we are not certain; it is not always present, and may be only a
differentiation of the submentum, or the latter piece may be regarded as
a part of the gula.

We are disposed to consider the second maxillæ as morphologically nearly
the exact equivalents of the first pair of maxillæ, and if we adopt this
view it will greatly simplify our conception of the real nature of this
complicated organ. The object of the fusion of the basal portion appears
to be to form an under-lip, in order both to prevent the food from
falling backwards out of the mouth, and, with the aid of the first pair
of maxillæ, to pass it forward to be crushed between the mandibles, the
two sets of appendages acting somewhat as the tongue of vertebrates to
carry and arrange or press the morsels of food between the teeth or
cutting edges of the mandibles.

The spines often present on the free inner edges of the first and second
maxillæ (Figs. 54, 62) form rude combs which seem to clean the antennæ,
etc., often aiding the tibial combs in this operation.

The submentum and mentum, or the mentum when no submentum is
differentiated (with the gula, when present), appear to be collectively
homologous with the cardines of the first pair of maxillæ, together with
the palpifers and the stipites.[15] These pieces are more or less
square, and have a slightly marked median suture in Termitidæ, the sign
of primitive fusion or coalescence.

The most primitive form of the second maxillæ occurs in the Orthoptera
and in the Termitidæ. The palpifer is either single (Periplaneta,
Diapheromera, Gryllidæ) or double (_Blatta orientalis_, Locustidæ). In
Prisopus the single piece in front of the palpifer is in other forms
divided, each half (Blatta, Locustidæ, Acrydidæ) bearing the two
“paraglossæ,” which appendages in reality are the homologues of the
lacinia and galea of the first maxillæ.[16] In the Termitidæ (Fig. 65)
the lingua is not differentiated from the palpifer, and the two
paraglossæ (or the lamina externa and interna of some authors) with the
palpus are easily seen to be the homologues of the three lobes of the
first maxillæ. In the Perlidæ (Pteronarcys, Fig. 66) the palpifer is
divided, while the four paraglossæ arise, as in Prisopus and
Anisomorpha, from an undivided piece, the lingua not being visible from
without. In the Neuroptera the lingua or ligula is a large, broad,
single lobe, without “paraglossæ,” and the palpifer is either single
(Myrmeleon, Fig. 67), or divided (Mantispa, Fig. 68). In Corydalus (Fig.
29) the palpifer forms a single piece, and the lingua is undivided,
though lobed on the free edge.

[Illustration:

  FIG. 65.—Second maxillæ of _Termopsis angusticollis_: _li_, the
    homologue of the lacinia; _le_, galea.
]

In the metabolic orders above the Neuroptera the lingua is variously
modified, or specialized, with no vestiges of the lacinia or galea,
except in that very primitive moth, Eriocephala, in which Walter found a
minute free galea, _me_, and an inner lobe (Figs. 76, 77), the lacinia.

[Illustration:

  FIG. 66.—Second maxillæ of _Pteronarcys californica_.
]

[Illustration:

  FIG. 67.—Second maxillæ of _Myrmeleon diversum_.
]

[Illustration:

  FIG. 68.—Second maxillæ of _Mantispa brunnea_.
]

=The hypopharynx.=—While in its most generalized condition, as in
Synaptera, Dermaptera, Orthoptera, and Neuroptera, this anterior median
fold or outgrowth of the labium forming the floor of the mouth may
retain the designation of “tongue,” lingua, or ligula; in its more
specialized form, particularly when used as a piercing or lapping organ,
the use of the name _hypopharynx_ seems most desirable. And this is
especially the case since, like the epipharynx, it is morphologically a
median structure, and while the epipharynx forms the soft, sensitive
roof of the mouth, or pharynx; its opposite, the hypopharynx, rises as a
fold from the floor of the mouth, forming in its most generalized
condition a specialized fold of the buccal integument. In certain cases,
as in the honey-bee, the very long slender “tongue” or hypopharynx is
evidently, as in the case of the epipharynx, a highly sensitive armature
of the mouth.

In all insects this organ—whether forming a soft, tongue-like, anterior
portion or fold of the labium, and “continuous with the lower wall of
the pharynx,” or a hard, piercing, awl-like appendage (fleas and flies),
or a long, slender, hairy or setose, trough-like structure like the
“tongue” of the honey-bee—has a definite location at the end and on the
upper side of the labium, and serves to receive at its base the external
opening of the salivary duct.

The hypopharynx, as well shown in its lingua condition in Orthoptera, is
continuous with and forms the anterior part or fold of the base of the
coalesced second maxillæ. It does not seem to be paired, or to represent
a pair of appendages.

Opinion regarding the homology of this unpaired piercing organ is by no
means settled, and while there is a general agreement as to the nature
of the paired mouth-parts, recent observers differ very much as to the
morphology of the organ in question.

It is the langue or _lingua_ of Savigny (1816), the _ligula_ of Kirby
and Spence (1828), the _langue ou languette_ (_lancette médiane du
suçoir_) of Dugès (1832), the _lingua_ of Westwood (Class, ins., ii, p.
489, 1840), “the unpaired median piercing organ” (“the analogon of the
epipharynx of Diptera”) of Karsten (1864), the “tongue” of Taschenberg
(1880).

The name _hypopharynx_ was first proposed by Savigny in 1816, who, after
naming the membranous plate which has for its base the upper side of the
pharynx, the _epipharynx_, remarks: “Dans quelques genres, notamment
dans les Eucères, le bord inférieur de ce même pharynx donne naissance à
un autre appendice plus solide que le précédent, et qui s’emboîte avec
lui. Je donnerai à ce dernier le nom de _langue_ ou d’_hypopharynx_.
Voilà donc la bouche des Hyménoptères composée de quatre organes
impaires, sans y comprendre la ganache ou le menton; savoir, la lèvre
supérieure, l’épipharynx, l’hypopharynx, et la lèvre inférieure, et de
deux organes paires, les mandibules et les mâchoires.”

As stated by Dimmock: “The hypopharynx is usually present in Diptera
(according to Menzbier absent in Sargus), and contains a tube, opening
by a channel on its upper surface; this channel extends back, more or
less, from the tip, and is the outlet for the salivary secretion. The
tip of the hypopharynx may be naked and used as a lance (Hæmatopota,
according to Menzbier), or may be hairy (Musca). The upper side of the
base of the hypopharynx is continuous with the lower wall of the
pharynx; its under surface may entirely coalesce with the labium (Culex,
male), may join the labium more or less, anterior to the month (Musca),
or, if either mandibles or maxillæ are present, its base may join them
(Culex, female).” (p. 43.)

[Illustration:

  FIG. 69.—Section of head of _Machilis maritima_: _hyp_, hypopharynx;
    _lbr_, labrum; _t_, tentorium; _ph_, room in which the mandibles
    move on each other; _p_, paraglossa; _mx_, labium; _sd_, salivary
    duct; _s.gl_, salivary gland. _oe_, œsophagus.—After Oudemans.
]

We will now briefly describe the lingua, first of the mandibulate or
biting insects, and then its specialized form, the hypopharynx of the
haustellate and lapping insects.

The lingua (hypopharynx) exists in perhaps its most generalized
condition in the Thysanura (Fig. 69), where it forms a soft projection,
having the same relations as in Anabrus and other Orthoptera.[17]

In the cockroach (Fig. 70), as stated by Miall and Denny, the lingua is
a chitinous fold of the oral integument situated in front of the labium,
and lying in the cavity of the mouth. The common duct of the salivary
glands enters the lingua, and opens on its hinder surface. The lingua is
supported by a chitinous skeleton (Figs. 70, _B_; 82, _shp_). “The thin
chitinous surface of the lingua is hairy, like other parts of the mouth,
and stiffened by special chitinous rods or bands.” (Miall and Denny.)

[Illustration:

  FIG. 70.—Hypopharynx of _Periplaneta orientalis_; the arrow points out
    of the opening of the salivary duct: _A_, origin of salivary duct.
    _B_, side view. _C_, front view.—After Miall and Denny.
]

In the Acrydiidæ (_Melanoplus femur-rubrum_) the tongue is a large,
membranous, partly hollow expansion of the base of the labium. It may be
exposed by depressing the end of the labium, when the opening of the
salivary duct may be seen at the bottom or end of the space or gap
between the hinder base of the tongue, and the inner anterior base of
the labium, as shown by the arrows in Fig. 70. It is somewhat pyriform,
slightly keeled above, and bearing fine stiff bristles, which, as they
point more or less inwards, probably aid in retaining the food within
the mouth. The base of the tongue is narrow, and extends back to near
the pharynx, there being on the floor of the mouth, behind the tongue,
two oblique, slight ridges, covered with stiff, golden-yellow hairs,
like those on the tongue. The opening of the salivary duct is situated
on the under or hinder side of the hypopharynx, between it and the base
of the labium, the base of the former being cleft; the hollow thus
formed is situated over the opening, and forms the salivary receptacle.

[Illustration:

  FIG. 71.—Section through the anterior part of the head of Anabrus (the
    mandibles removed), showing the relations of the hypopharynx (_hyp_)
    to the opening of the salivary duct (_sd_): _g_, galea; _l_,
    lacinia; _mt_, mentum; _oe_, œsophagus; _lbr_, labrum; _cl_,
    clypeus.
]

In the Locustidæ (Anabrus, Fig. 71) the tongue (hypopharynx) is a broad,
somewhat flattened lobe arising from the upper part of the base of the
mentum and behind the palpifer. This lobe is cavernous underneath, the
hollow being the salivary receptacle (_sr_); the latter is situated over
the opening of the salivary duct, which is placed between the base of
both the hypopharynx and the labium. The salivary fluid apparently has
to pass up and around on each side of the hypopharynx in order to mix
with the food.

These relations in the Orthoptera are also the same in the Perlidæ,
where the hypopharynx is well developed, forming an unusually large
tongue-like mass, nearly filling the buccal cavity.

[Illustration:

  FIG. 72.—Lingua of a May-fly, _Heptagenia longicauda_, ×16: _m_,
    central; _l_, lateral pieces.—After Vayssière from Sharp.
]

In the Odonata the lingua is a small, rounded lobe, as also in the
Ephemeridæ; in the nymph, however, of Heptagenia (Fig. 72) it is highly
developed, according to Vayssière, who seems inclined to regard it as
representing a pair of appendages. The tongue in Hemiptera is said by
Léon to be present in _Benacus griseus_ (Say) and to correspond to the
subgalea of Brullé or hypodactyle of Audouin (Fig. 73), but this appears
to correspond to the labium proper, rather than a true lingua, the
latter not being differentiated in this order. In the Coleoptera the
lingua is rather small. In beetles, as Anopthalmus (Fig. 74), it forms a
setose lobe; and a well-developed nerve, the lingual nerve, passes to
it, dividing at the end into several branches (_n-l_). In Sialis the
lingua is short, much less developed than usual, being rounded, and
bears on the edge what appear to be numerous taste-hairs, like those on
the ends of the maxillary and labial palpi.

[Illustration:

  FIG. 73.—_A_, labium of _Zaitha anura_. _B_, of _Z. margineguttata_.
    _C_, of _Gerris najas_: _mt_, mentum; _lp_, labial palpi; _sg_,
    subgalea; _l_, lacinia (= intermaxillare and præmaxillare of
    Brullé); _g_, galea.—After Léon.
]

In the adult Panorpidæ the lingua is a minute, simple lobe.

[Illustration:

  FIG. 74.—Section through head of a carabid, _Anopthalmus telkampfii_:
    _br_, brain; _f. g_, frontal ganglion; _soe_, subœsophageal
    ganglion; _co_, commissure; _n. l_, nerve sending branches to the
    lingua (_l_); _mn_, maxillary nerve; _mx_, 1st maxilla; _mm_,
    maxillary muscle; _mx′_, 2d maxilla; _mt_, muscle of mentum; _le_,
    elevator muscle of the œsophagus; _l_ of the clypeus, and a third
    beyond raising the labrum (_lbr_); _eph_, epipharynx; _g_, _g_,
    salivary glands above; _g_^2, lingual gland below the œsophagus
    (_oe_); _m_, mouth; _pv_, proventriculus; _md_, mandible. _A_,
    section passing through lingual gland (_g_^2).
]

In the larval Trichoptera the spinneret is well developed, and in
structure substantially like that of caterpillars, and it is plainly the
homologue of the hypopharynx, receiving as it does the end of the
silk-duct.

In the adult Trichoptera the hypopharynx is a very large, tongue-like,
fleshy outgrowth, and is, both in situation and structure, since it
contains the opening of the silk-duct, exactly homologous with the
hypopharynx of insects of other orders, being somewhat intermediate
between the fleshy tongue or lingua of the mandibulate insects,
especially the Neuroptera, and the hypopharynx of the bees (Fig. 86).
Lucas describes and figures it under the name of “haustellum,” but does
not homologize it with the hypopharynx. The caddis-flies have been
observed to drink water and take in both fluid and fine particles of
solid food, and to use the haustellum for this purpose, the end being
provided with minute sense-organs like those on the first maxillary
lacinia, and possibly of a gustatory nature.

[Illustration:

  FIG. 75.—Head of _Anabolia furcata_: _A_, front view, showing the
    labrum removed. _B_, side view; _ant_, antenna; _oc_, ocellus; _ol_,
    labrum; _gh_, articulatory process; _cmx__{1}, cardo; _stmx__{1},
    stipes; _lemx__{1}, outer lobe (galea); _ptmx__{1}, palpus of 1st
    maxilla; _pl_, palpus of 2d maxilla; _ha_, haustellum; _so_,
    gustatory pits; _spr_, opening of salivary duct; _chsp_, chitinous
    hook of the clasp; _spr_, furrow or gutter of the haustellum.—After
    Lucas.
]

[Illustration:

  FIG. 76.—Hypopharynx of _Eriocephala calthella_: _lig_, ligula, its
    membranous hinder edge; _lig′_, anterior horny edge of the
    ligula-tube opening outwards; _hp_, contour of the hypopharynx;
    _mi_, mala interior (lacinia); _me_, mala exterior (galea), of
    second maxilla; _mx′ p_, labial palpus.—After Walter.
]

The spinneret of the larvæ of Lepidoptera is evidently the homologue of
the hypopharynx of insects of other orders. It will be seen that the
homology of the different parts is identical, the common duct of the
silk-glands opening at the end of the hypopharynx, which here forms a
complete tube or proboscis extending beyond the end of the labium, in
adaptation to its use as a spinning organ.

Walter refers to Burgess’s discovery of a hypopharynx in _Danais
archippus_, remarking that this organ in the adult Eriocephalidæ (Fig.
76) exhibits a great similarity to the relations observable in the lower
insects, adding:—

[Illustration:

  FIG. 77.— Labium of _Micropteryx anderschella_ seen from within (the
    labial palpi (_mx.′ p_) removed to their basal joint). Lettering as
    in Fig. 76.—After Walter.
]

“The furrow is here within coalesced with the inner side of the labium,
and though I see in the entire structure of the head the inner edge of
the ligula tube extended under the epipharynx as far as the mandible, I
must also accept the fact that here also the hypopharynx extends to the
mouth-opening as in all other sucking insects with a well-developed
under-lip, viz. the Diptera and Hymenoptera.”

He has also discovered in Micropteryx a paired structure which he
regards as the hypopharynx (Fig. 77). As he states:

[Illustration:

  FIG. 78.—Hypopharynx (_hph_) of Danais: _cl_, clypeus; _sd_, salivary
    duct; _m_, labial palp muscles; _fm_, frontal muscle; _ph_, pharynx;
    _cor_, cornea.—After Burgess.
]

“A portion of the inner surface of the tube-like ligula is covered by a
furrow-like band which, close to the inner side, is coalesced with it,
and in position, shape, as well as its appendages or teeth on the edge,
may be regarded as nothing else than the hypopharynx.”

A hypopharynx is also present in the highest Lepidoptera, Burgess having
detected it in _Danais archippus_. He states that the hypopharynx forms
the floor of the pharyngeal cavity; “it is convex on each side of a
median furrow (Fig. 78, _hph_) and somewhat resembles in shape the human
breast. The convex areas are dotted over with little papillæ, which
possibly may be taste-organs.”

As a piercing organ the hypopharynx reaches its greatest development in
the Siphonaptera and Diptera, where the chitinous parts are greatly
hypertrophied, the fleshy tongue-like portion so developed in the
mandibulate orders being greatly reduced. The chitinous parts are alike
on each side of the median organ, being bilaterally symmetrical.

[Illustration:

  FIG. 79.—_A_, hypopharynx of _Pulex canis_: _x_, basal portion
    situated within the head; _s. d_, common duct of the four
    bladder-shaped salivary glands; _s. d′_, opening of the tubular
    salivary glands into the throat. _B_, end of the hypopharynx,
    showing the gutter-like structure and teeth at the end.—After
    Landois.
]

[Illustration:

  FIG. 80.—Beak of Vermipsylla: _hyp_, hypopharynx.—After Wagner.
]

In the fleas the hypopharynx is a large, slender, unpaired, long,
chitinous trough, as long as the mandibles, and toothed at the end.
Figures 79 and 80 show its relations to the other parts of the mouth; in
Fig. 79, _x_, is seen where the salivary duct opens into the pharynx.
Although this organ is not unanimously referred to the hypopharynx, yet
from the description of Landois and others, it is evident that this
structure does not correspond to the labrum or epipharynx, but belongs
to or arises from the floor of the mouth, and, being in close relation
to the labium, and also receiving the salivary duct, must be a true
hypopharynx.

In the Diptera the hypopharynx reaches its highest development as a
large, stout, awl-like structure.

Meinert, in his detailed and elaborately illustrated work, Trophi
Dipterorum (1881), has made an advance on our knowledge of the
hypopharynx and its homologies, both by his evidently faithful
descriptions and dissections, and by his admirably clear figures.

[Illustration:

  FIG. 81.—_Culex pipiens_, section of head: _oe_ œsophagus; _sm_, upper
    muscle, _lm_, lower muscle of the œsophagus; _ph_, pharynx; _rm_,
    retractor muscle of the receptacle (_r_) of the salivary duct
    (_s.d_); _lbr_, labrum; _ep_, left style of the epipharynx; _f_,
    part of front of head.—After Meinert.
]

[Illustration:

  FIG. 82.—Pharynx and hypopharynx of _Simulium fuscipes_: _lph_, lower
    lamina of the pharynx; _p_, the salivary duct (_s.d_) perforating
    the pharynx; _o_, orifice of the duct; _shp_, styles of the
    hypopharynx; _mph_, membranous edge of the hypopharynx; _m_,
    protractor muscle of the pharynx; _gp_, gustatory papillæ.—After
    Meinert.
]

  “The hypopharynx, a continuation of the lower edge (_lamina_) of the
  pharynx, most generally free, more or less produced, acute
  anteriorly, forms with the labrum the tube of the pump (_antliæ_).
  (The hypopharynx when obsolete, or coalesced with the canal of the
  proboscis, is the _theca_; in such a case the siphon or tube is
  formed by the theca and labrum.) Meanwhile the hypopharynx, the
  largest of all the trophi (_omnium trophorum maximus_), constitutes
  the chief piercing organ (_telum_) of Diptera. The hypopharynx is
  moved by protractor, most generally quite or very powerful, and by
  retractor muscles.

  “The efferent duct of the thoracic salivary glands (_ductus
  salivalis_) perforates the hypopharynx, more or less near the base,
  that the saliva may be ejected through the canal into the wound, or
  that it may be conducted along the labella. Very rarely the salivary
  duct, perforating the hypopharynx, is continued in the shape of a
  free, very slender tube.

  “The salivary duct behind the base of the hypopharynx forms the
  receptacle or _receptaculum_, provided with retractor and levator
  muscles.”

[Illustration:

  FIG. 83.—Labrum-epipharynx (_lbr_ and _eph_) and hypopharynx (_hyp_)
    of _Tabanus brominus_: _oe_, posterior cylindrical portion of the
    œsophagus; _a_, anterior swollen portion of the same; _ph_, pharynx;
    _ph.m_, pharyngeal muscle; _p.ph_, protractor muscle of the pharynx;
    _r.oe_, retractor muscle of the œsophagus; _r.ph_, retractor muscle
    of the pharynx; _f.oe_, flexor muscle of the pharynx; _t.oe_,
    twisting muscle of the œsophagus; _s.r_, receptacle of the salivary
    duct; _l_, its elevator muscle; _s_, its retractor muscle; _cl_,
    clypeus.—After Meinert.
]

[Illustration:

  FIG. 84.—Œsophagus (_oe_), pharynx (_ph_) with epipharynx and labrum
    (_lbr_) of _Asilus atricapillus_: _m_, _ph_, pharyngeal muscle;
    _sr_, salivary receptacle; _t_, twisting; _r_, _l′r_, retractor
    muscles; other lettering as in Fig. 83.—After Meinert.
]

It has been carefully studied by Meinert in a species of Culex (Fig.
81), Simulium (Fig. 82), Tabanus (Fig. 83), and in Asilus (Fig. 84),
where it is seen to attain enormous proportions. In the Hymenoptera,
this organ in its most specialized condition is a trough-like rod,
adapted for lapping nectar (Fig. 85, 86, _hyp_). The tongue or
hypopharynx of the honey-bee has been elaborately described by Cheshire
in his Bees and Bee Keeping.[18] He calls it the tongue or ligula. It is
situated in a tube formed by the maxillæ and labial palpi, and can be
partially retracted into the mentum. He states that it can move up and
down in the tube thus formed, and then describes it as covered by a
hairy sheath, its great elasticity being due to a rod running through
its centre enabling it to be used as a lapping tongue. The sheath

  “passes round the tongue to the back, where its edges do not meet,
  but are continuous with a very thin plaited membrane (_G_, _pm_)
  covered with minute hairs. This membrane, after passing towards the
  sides of the tongue, returns to the angle of the nucleus, or rod,
  over the under surface of which it is probably continued. The rod
  passes through the tongue from end to end, gradually tapering
  towards its extremity, and is best studied in the queen, where I
  trace many nerve threads and cells. It is undoubtedly endowed with
  voluntary movement, and must be partly muscular, although I have
  failed completely in getting any evidence of striation. The rod on
  the underside has a gutter, or trough-like hollow (_cd_, the central
  duct) which is formed into a pseudotube (false tube) by
  intercrossing of black hairs. It will also be seen that, by the
  posterior meeting of the sheath, the space between the folded
  membrane (_G_, _sd_) becomes two pseudotubes of larger size, which I
  shall call the side ducts.

[Illustration:

  FIG. 85.—Head of honey bee, worker: _a_, antenna; _g_, epipharynx;
    _m_, mandible; _mx_, maxilla; _mxp_, maxillary palpus; _pg_,
    paraglossa; _lp_, labial palpus; _l_, hypopharynx; _b_, its
    spoon.—After Cheshire; from Bull. Div. Ent. U. S. Dept. Agr.
]

  “These central and side ducts run down to that part of the tongue
  where the spoon, or bouton (_K_, Fig. 86) is placed. This is
  provided with very delicate split hairs (_b_, Fig. 86) capable of
  brushing up the most minute quantity of nectar, which by capillarity
  is at once transferred by the gathering hairs (which are here
  numerous, long, and thin) to two side groove-like forms at the back
  of the bouton, and which are really the opened-out extremity of the
  centre and side ducts, assuming, immediately above the bouton, the
  form seen in _F_, Fig. 86. The central duct, which is only from
  1⁄600 inch to 1⁄1000 inch in diameter, because of its smaller size,
  and so greater capillary attraction, receives the nectar, if
  insufficient in quantity to fill the side ducts. But good
  honey-yielding plants would bring both centre and side ducts into
  requisition. The nectar is sucked up until it reaches the paraglossæ
  (_pa_, _B_, Fig. 86), which are plate-like in front, but membranous
  extensions, like small aprons, behind; and by these the nectar
  reaches the front of the tongue, to be swallowed as before
  described.”

[Illustration:

  FIG. 86.—Tongue or ligula of the honey bee: _A_, under side of the
    tongue; _lp_, labial palpi; _r_, _r_, rod; _p_, pouch; _sh_, sheath;
    _gh_, gathering hairs; _b_, bouton or spoon. _B_, under lip or
    labium, with appendages, partly dissected; _l_, lora or submentum;
    _a_, _a_, retractor linguæ longus; _sd_, salivary duct; _rb_ and
    _b_, retractor linguæ biceps; _mx_, maxillæ; _lp_, labial palpi;
    _pa_, paraglossa; _gr_, feeding groove; _sh_, sheath of ligula. _C_,
    _D_, _E_, sections of ligula; _hp_, hyaline plate of maxilla; _h_,
    hairs acting as stops; _mx_, maxilla; _lp_, labial palpi; _sd_, side
    duct. _F_, cross-section of extremity of tongue near the “spoon”;
    _th_, tactile hairs; _r_, rod; _n_, nucleus; _gh_, gathering hairs.
    _G_, cross-section of tongue without gathering hairs, × 400 times;
    _sh_, sheath; _b_, blood space; _t_, trachea: _ng_, gustatory nerve;
    _cd_, central duct; _sd_, lateral duct; _pm_, plaited membrane. _H_,
    same as _G_, but magnified two hundred times, and with _pm_, plaited
    membrane, turned outwards; _h_, closing hairs; _lp_, labial palpi;
    _b_, blood; _n_, nucleus; _r_, rod; _h_, closing hairs. _I_, small
    portion of the sheath; lettering as before. _K_, extremity of the
    tongue, with spoon; _b_, branching hairs for gathering.—After
    Cheshire.
]

Cheshire then settles the question which has been in dispute since the
time of Swammerdam, whether the bee’s tongue is solid or tubular. He
agrees with Wolff that the duct is a trough and not a tube, and proves
it by a satisfactory experiment. He remarks:

[Illustration:

  FIG. 87.—Longitudinal section through the head of the honey bee, ♀,
    just outside of right antenna: _ant_, antenna with three muscles
    attached to _mes_, mesocephalic pillar; _cl_, clypeus; _lbr_,
    labrum; 1, chyle-gland (system no. 1, of Siebold); _o_, opening of
    the same; _oc_, ocellus; _br_, brain; _n_, neck; _th_, thorax; _oe_,
    œsophagus; _s.d_^2, _s.d_^3, common salivary ducts of systems 2 and
    3; _v_, salivary valve; _c_, cardo; _ph_, pharynx; _mx′_, labium;
    _mx.′p_, labial palpi; _mt_, mentum; _mx_, maxilla; _hyp_,
    hypopharynx; _s_, bouton.—After Cheshire.
]

  “Bees have the power, by driving blood into the tongue, of forcing
  the rod out from the sheath, and distending the wrinkled membrane so
  that in section it appears as at _H_, Fig. 86, the membrane assuming
  the form of a pouch, given in full length at _A_. It will be seen at
  once that this disposition of parts abolishes the side ducts, but
  brings the central duct to the external surface. The object of this
  curious capability on the part of the bee is, in my opinion, to
  permit of cleaning away any pollen grains, or other impediment that
  may collect in the side ducts. The membrane is greasy in nature, and
  substances or fluids can be removed from it as easily as water from
  polished metal. If, now, the sides of a needle, previously dipped
  into clove oil in which rosanilin (magenta) has been dissolved, so
  as to stain it strongly red, be touched on the centre of the rod,
  the oil immediately enters, and passes rapidly upwards and
  downwards, filling the trough.”

  =Does the hypopharynx represent a distinct segment?=—The facts which
  suggest that the hypopharynx may possibly represent a highly
  modified pair of appendages, arising from a distinct intermaxillary
  segment, are these: Heymons plainly shows that, in the embryo of
  Lepisma, the hypopharynx originates as a transverse segment-like
  fold in front of the 2d maxillary segment, and larger than it, and
  though he does not mention it in his text, it appears like the
  rudiment of a distinct segment; the hypopharynx of Ephemeridæ;
  arises and remains separate in the nymph from the labium (see
  Heymons’ Fig. 29, and there are two lateral projections; see also
  Fig. 72, and Vayssiere’s view that it may represent a pair of
  appendages; Kolbe also regards it as representing a third pair of
  maxillæ, his endolabium, p. 213). Though what is called an unpaired
  organ, it is composed of, or supported by, two bilaterally
  symmetrical styles, both in Myriopods (Fig. 6, labiella, _stil_) and
  in insects (Fig. 77, etc.). On the other hand, in the embryo of
  pterygote insects, an intermaxillary segment has not been yet
  detected.




           LITERATURE OF THE MOUTH-PARTS OR BUCCAL APPENDAGES


                              _a._ General

  =Savigny, Jules-César.= Mémoires sur les animaux sans vertèbres.
    1^{re} Part. Description et classification des animaux invertébrés
    et articulés, etc. Fasc. 1^{re}. Mém. 1–2. Théorie des organes de la
    bouche des crustacés et des insectes. 12 Pl., Paris, 1816, pp.
    1–117.

  =Gerstfeld, Georg.= Ueber die Mundteile der saugenden Insekten.
    Dorpat, 1853.

  =Olfers, Ernestus V.= Annotationes ad anatomiam Podurarum. Berolini,
    1862, 4 Pls.

  =Gerstaecker, Carl Eduard Adolph.= Zur Morphologie der Orthoptera
    amphibiotica. (Festschrift zur Feier des hundertjährigen Bestehens
    der Gesellschaft naturf. Freunde zu Berlin. 4º, 1873, pp. 39–59, 1
    Taf.)

  =Muhr, Joseph.= Die Mundteile der Orthoptera. Ein Beitrag zur
    vergleichenden Anatomie. (Jahrbuch “Lotos.” Prag, 1877, pp. 40–71, 8
    Taf.)

  =Burgess, Edward.= The anatomy of the head and the structure of the
    maxilla in the Psocidæ. (Proc. Boston Soc. Nat. Hist., xix, 1878,
    pp. 291–296, 1 Pl.)

  =Meinert, Fr.= Sur la conformation de la tête et sur l’interpretation
    des organes buccaux chez les insectes, ainsi que sur la systématique
    de cette ordre. (Ent. Tidsskr., 1. Arg., 1880, pp. 147–150.)

  —— Tungens udskydelighed hos Steninerne, en slaegt af Staphylinernes
    familie. (Vidensk. meddel. fra den naturh. Foren, 1884–1886, pp.
    180–207, 2 Pls. Also Zool. Anzeiger, 1887, pp. 136–139.)

  =Müller, A.= Vergleichend-anatomische Darstellung der Mundteile der
    Insekten. Villach, 1881, 3 Taf.

  =Kraepelin, Karl.= Ueber die Mundwerkzeuge der saugenden Insekten.
    (Zool. Anzeiger, 1882, pp. 574–579.)

  =Dewitz, H.= Ueber die führung an den Körperanhangen der Insekten.
    (Berlin. Zeitschr. xxvi., 1882, pp. 51–68, Figs.)

  =Wolter, Max.= Die Mundbildung der Orthopteren mit specieller
    Berücksichtigung der Ephemeriden. 4 Taf. Greifswald, 1883.

  =Oudemans, J. T.= Beiträge zur Kenntniss der Thysanura und Collembola.
    (Bijdragen tot de Dierkunde, pp. 149–226. Amsterdam, 1888, 3 Taf.)

  =Smith, John B.= An essay on the development of the mouth-parts of
    certain insects. (Trans. Amer. Philosophical Soc., xix, pp. 175–198,
    3 Pls.)

      Also articles by Chatin, McLachlan, Riley, Wood-Mason.


                     _b._ Thysanoptera (Physapoda)

=Jordan, Karl.= Anatomie und biologie der Physapoda. (Zeitschr. f.
    wissens. Zool., xlvii, pp. 541–620, 3 Taf. 1888.)

=Garman, H.= The mouth-parts of the Thysanoptera. (Bull. Essex Inst.,
    xxii, 4 pp., Fig. 1890.)

—— The asymmetry of the mouth-parts of Thysanoptera. (Amer. Naturalist,
    July, 1896, pp. 591–593, Fig.)

=Bohls, J.= Die Mundwerkzeuge der Physapoden. Dissertation Göttingen,
    1891, pp. 1–36.

=Uzel, Heinrich.= Monographie der Ordnung Thysanoptera. Königgrätz,
    1895, pp. 472, 10 Taf., 9 Figs.


                             _c._ Hemiptera

=Léon, N.= Beiträge zur Kenntniss der Mundteile der Hemipteren. Jena,
    1887, pp. 47, 1 Taf.

—— Labialtaster bei Hemipteren. (Zool. Anzeiger, pp. 145–147, 1892, 1
    Fig.)

—— Beiträge zur Kenntniss des Labiums der Hydrocoren. (Zool. Anzeiger,
    März 29, 1897, pp. 73–77, Figs. 1–5.)

=Geise, O.= Mundteile der Rhynchoten. (Archiv f. Naturgesch., xlix,
    1883, pp. 315–373, 1 Taf.)

=Wedde, Hermann.= Beiträge zur Kenntniss des Rhynchotenrüssels. (Archiv
    f. Naturgesch., li Jahrg., 1 Bd., 1885, pp. 113–148, 2 Taf.)

=Smith, John B.= The structure of the hemipterous mouth. (Science, April
    1, 1892, pp. 189–190, Figs. 1–5.)


                            _d._ Coleoptera

=Smith, John B.= The mouth-parts of _Copris carolina_; with notes on the
    homologies of the mandibles. (Trans. Amer. Ent. Soc., xix, April,
    1892, pp. 83–87, 2 Pls.)


                            _e._ Lepidoptera

=Kirbach, P.= Ueber die Mundwerkzeuge der Schmetterlinge. (Zool.
    Anzeiger, vi Jahrg., 1883, pp. 553–558, 2 Figs.)

—— Ueber die Mundwerkzeuge der Schmetterlinge. (Archiv f.
    Naturgeschichte, 1884, pp. 78–119, 2 Taf.)

=Walter, Alfred.= Palpus maxillaris Lepidopterorum. (Jenaische Zeitschr.
    f. Naturwiss, xviii, 1884, pp. 121–173, Taf.)

—— Beiträge zur Morphologie der Lepidoptera. I, Mundteile. (Jenaische
    Zeitschr. f. Naturwiss, xviii, 1885, pp. 751–807, 2 Taf.)

=Breitenbach, W.= Vorläufige Mitteilung über einige neue Untersuchungen
    an Schmetterlingsrüsseln. (Archiv f. mikroskop. Anatomie, xiv, 1877,
    pp. 308–317, 1 Taf.)

—— Untersuchungen an Schmetterlingsrüsseln. (Ibid., xv, 1878, pp. 8–29,
    1 Taf.)

—— Ueber Schmetterlingsrüssel. (Entomolog. Nachr. 5 Jahrg., 1879, pp.
    237–243, 1 Taf.)

—— Der Schmetterlingsrüssel. (Jenaische Zeitschr. f. Naturwiss, 1881.)


                           _f._ Siphonaptera

=Kräpelin, K.= Ueber die systematische Stellung der Puliciden.
    (Festschrift z. 50 jahr. Jubil. d. Realgymnas. Iohanneum, Hamburg,
    pp. 17, 1 Taf. 1884.)

=Kellogg, V. L.= The mouth-parts of the Lepidoptera. (Amer. Nat., xxix,
    1895, pp. 546–556, 1 Pl. and Fig.)


                              _g._ Diptera

=Menzbier, Michael Alexander.= Ueber das Kopfskelett und die
    Mundwerkzeuge der Zweiflügler. (Bull. Soc. Imp. Natur. de Moscou,
    lv, 1880, pp. 8–71, 2 Taf.)

=Dimmock, George.= The anatomy of the mouth-parts and of the sucking
    apparatus of some Diptera. Boston, 1881, pp. 48, 4 Pls.

=Meinert, F.= Fluernes Munddele. Trophi Dipterorum. Kjöbenhavn, 1881, 6
    Pls.

—— Die Mundteile der Dipteren. (Zool. Anz. 1882, pp. 570–574, 599–603.)

=Becher, E.= Zur Kenntniss der Mundteile der Dipteren. (Denkschr. Akad.
    d. Wissensch. Wien., xlv, 1882, pp. 123–162, 4 Taf.)

=Hansen, H. J.= Fabrica oris dipterorum: Dipterernes mund: anatomisk og
    systematisk henseende. 1 Tabanidae, Bombyliidae, Asilidae, Thereva,
    Mydas, Apiocera. (Naturhist. Tidsskrift, 1883, xiv, pp. 1–186, Taf.
    1–5.)

=Kräpelin, Karl.= Zur Anatomie und Physiologie des Rüssels von Musca.
    (Zeitschr. f. wissensch. Zool., xxxix, 1883, pp. 683–719, 2 Taf.)

=McCloskie, George.= Kraepelin’s Proboscis of the house-fly. (American
    Naturalist, xviii, 1884, pp. 1234–1244, Figs.)

=Langhoffer, August.= Beiträge zur Kenntniss der Mundtheile der
    Dipteren. Jena, 1888, pp. 1–32.

=Smith, John B.= A contribution toward a knowledge of the mouth-parts of
    the Diptera. (Trans. Amer. Ent. Soc., xvii, Nov. 1890, pp. 319–339,
    Figs. 1–22.)


                            _h._ Hymenoptera

=Briant, Travers J.= On the anatomy and functions of the tongue of the
    honey-bee (worker). (Journ. Linn. Soc., London, xvii, 1884, pp.
    408–416, 2 Pls.)

=Breithaupt, P. F.= Ueber die Anatomie und die Funktionen der
    Bienenzunge. (Archiv f. Naturgesch., Jahrg. lii, 1886, pp. 47–112, 2
    Taf.)


                           _i._ Larval stages

=Brauer, F.= Die Zweiflügler des kaiserlichen Museums zu Wien. III,
    Systematische Studien auf Grundlage der Dipterenlarven nebst einer
    Zusammenstellung von Beispielen aus der Litteratur uber dieselben
    und Beschreibung neuer Formen. (Denkschr. math.-naturwiss. Cl. k.
    Akad. Wiss. Wien, 1883, xlvii, pp. 100, 5 Taf.)

=Dewitz, H.= Ueber die Führung an den Körperanhangen der Insekten
    speziell betrachtet an der Legescheide der Acridier, dem Stachel der
    Meliponem und den Mundteilen der Larve von Myrmeleon, nebst
    Beschreibung dieser Organe. (Berliner ent. Zeitschr., xxvi, 1882,
    pp. 51–68.)

—— Die Mundteile der Larve von Myrmeleon. (Sitzungsber. d. Ges.
    naturforsch. Freunde zu Berlin, 1881, pp. 163–166.)

=Redtenbacher, Josef.= Uebersicht der Myrmeleonidenlarven. (Denkschrift,
    math.-naturwiss. Cl. k. Akad. Wiss. Wien, 1884, xlviii, pp. 335–368,
    7 Taf.)

=Schiödte, J. G.= De metamorphosi Eleutheratorum. Bidrag til insekternes
    udviklingshistorie. (Kroyer’s Naturhist. Tidsskrift. Kjöbenhavn. 12
    Teile mit 88 Taf., 1862–1883.)


                         _j._ Embryonic stages

=Heymons, Richard.= Grundzüge der Entwicklung und des Körpersbaues von
    Odonaten und Ephemeridem (Anhang zu den Abhandl. K. Akad. d.
    Wissens. Berlin, 1896, p. 22, 2 Taf. See Figs. 5, 29.)

—— Entwicklungsgeschichtliche Untersuchungen an _Lepisma saccharina_ L.
    (Zeitschr. f. Wissens. Zoologie, lxii, 1897, p. 595, 2 Taf. See Fig.
    10.)




                     THE THORAX AND ITS APPENDAGES


                 _a._ The thorax; its external anatomy

The middle region of the body is called the thorax, and in general
consists of three segments, which are respectively named the
_prothorax_, _mesothorax_, and _metathorax_ (Figs. 88, 89, 98).

[Illustration:

  FIG. 88.—External anatomy of _Melanoplus spretus_, the head and thorax
    disjointed.
]

The thorax contains the muscles of flight and those of the legs, besides
the fore intestine (œsophagus and proventriculus), as well as, in the
winged insects, the salivary glands.

In the more generalized orders, notably the Orthoptera, the three
segments are distinct and readily identified.

[Illustration:

  FIG. 89.—Locust, Melanoplus, side view, with the thorax separated from
    the head and abdomen, and divided into its three segments.
]

Each segment consists of the _tergum_, _pleurum_, and _sternum_. In the
prothorax these pieces are not subdivided, except the pleural; in such
case the tergum is called the _pronotum_. The prothorax is very large in
the Orthoptera and other generalized forms, as also in the Coleoptera,
but small and reduced in the Diptera and Hymenoptera. In the winged
forms the tergum of the mesothorax is differentiated into four pieces or
plates (sclerites). These pieces were named by Audouin, passing from
before backwards, the _præscutum_, _scutum_, _scutellum_, and
_postscutellum_. In the nymph stage and in the wingless adults of
insects such as the Mallophaga, the true lice, the wingless Diptera,
ants, etc., these parts by disuse and loss of the wings are not
differentiated. It is therefore apparent that their development depends
on that of the muscles of flight, of which they form the base of
attachment. The scutum is invariably present, as is the scutellum. The
former in nearly all insects constitutes the larger part of the tergum,
while the latter is, as its name implies, the small shield-shaped piece
directly behind the scutum.

[Illustration:

  FIG. 90.—Thorax of _Telea polyphemus_, side view, pronotum not
    represented: _em_, epimerum of prothorax, the narrow piece above
    being the prothoracic episternum; _ms_, mesoscutum; _scm_,
    mesoscutellum; _ms″_, metascutum; _scm‴_, metascutellum; _pt_, a
    supplementary piece near the insertion of tegulæ; _w_, pieces
    situated at the insertion of the wings, and surrounded by membrane;
    _epm″_, episternum of the mesothorax; _em″_, epimerum of the same;
    _epm‴_, episternum of the metathorax; _em‴_ epimerum of the same,
    divided into two pieces; _c′_, _c″_, _c‴_, coxæ; _te′_, _te″_,
    _te‴_, trochantines; _tr_, _tr_, _tr_, trochanters. _A_, tergal view
    of the mesothorax of the same; _prm_, præscutum; _ms_, scutum;
    _scm_, scutellum; _ptm_, postscutellum; _t_, tegula.
]

The præscutum and postscutellum are usually minute and crowded down out
of sight between the opposing segments. As seen in Fig. 90, the
præscutum of most moths (Telea) is a small rounded piece, bent
vertically down so as not to be seen from above. In Polystœchotes and
also in Hepialus the præscutum is large, well-developed, triangular, and
wedged in between the two halves of the scutum. The postscutellum is
still smaller, usually forming a transverse ridge, and is rarely used in
taxonomy.

[Illustration:

  FIG. 91.—Thorax of the house-fly: _prn_, pronotum; _prsc_, præscutum;
    _sc′_, mesoscutum; _sct′_, mesoscutellum; _psct′_, postscutellum;
    _al_, insertion of squama, extending to the insertion of the wings,
    which have been removed; _msphr_, mesophragma; _h_, balancer
    (halter); _pt_, tegula; _mtn_, metanotum; _epis_, _epis′_, _epis″_,
    episternum of pro-, meso-, and metathorax; _epm′_, _epm″_, meso- and
    meta-epimerum; _st′_, _st″_, meso- and metasternum; _cx′_, _cx″_,
    _cx‴_, coxæ; _tr′_, _tr″_, _tr‴_, trochanters of the three pairs of
    legs; _sp′_, _sp″_, _sp‴_, _sp‴′_, _sp‴″_, first to fifth spiracles;
    _tg′_, _tg″_, tergites of first and second abdominal segments; _u′_,
    _u″_, urites.
]

The metathorax is usually smaller and shorter than the mesothorax, being
proportioned to the size of the wings. In certain Neuroptera and in
Hepialidæ and some tineoid moths, where the hind wings are nearly as
large as those of the anterior pair, the metathorax is more than half or
nearly two-thirds as large as the mesothorax. In Hepialidæ the præscutum
is large and distinct, while the scutum is divided into two widely
separated pieces. The postscutellum is nearly or quite obsolete.

The pleurum in each of the three thoracic segments is divided into two
pieces; the one in front is called the _episternum_, since it rests upon
the sternum; the other is the _epimerum_. To these pieces, with the
sternum in part, the legs are articulated (Fig. 89).

Between the episterna is situated the breastplate or _sternum_, which is
very large in the more primitive forms, as the Orthoptera, and is small
in the Diptera and Hymenoptera.

[Illustration:

  FIG. 92.—Prothorax of _Geometra papilionaria_: _n_, notum; _p_,
    pleura; _st_, sternum; _pt_, patagia; _m_, membrane; _f_, femur;
    _h_, a hook bent backwards and beneath, and connecting the pro- with
    the mesothorax.—After Cholodkowsky.
]

The episterna and epimera are in certain groups, Neuroptera, etc.,
further subdivided each into two pieces (Fig. 102). The smaller pieces,
hinging upon each other and forming the attachments of the muscles of
flight, differ much in shape and size in insects of different orders.
The difference in shape and degree of differentiation of these parts of
the thorax is mentioned and illustrated under each order, and reference
to the figures will obviate pages of tedious description. A glance,
however, at the thorax of a moth, fly, or bee, where these numerous
pieces are agglutinated into a globular mass, will show that the
spherical shape of the thorax in these insects is due to the enlargement
of one part at the expense of another; the prothoracic and metathoracic
segments being more or less atrophied, while the mesothorax is greatly
enlarged to support the powerful muscles of flight, the fore wings being
much larger than those appended to the metathorax. In the Diptera, whose
hinder pair of wings are reduced to the condition of halteres, the
reduction of the metathorax as well as prothorax is especially marked
(Fig. 91).

=The patagia.=—On each side of the pronotum of Lepidoptera are two
transversely oval, movable, concavo-convex, erectile plates, called
_patagia_ (Fig. 92). On cutting those of a dry Catocala in two, they
will be seen to be hollow. Cholodkowsky[19] states that they are filled
with blood and tracheal branches; and he went so far as to regard them
as rudimentary prothoracic wings, in which view he was corrected by
Haase,[20] who compares them with the tegulæ, regarding them also as
secondary or accessory structures.

=The tegulæ.=—On the mesothorax are the _tegulæ_ of Kirby (_pterygodes_
of Latreille, _paraptera_ of McLeay, _hypoptère_ or _squamule_), which
cover the base of the fore wings, and are especially developed in the
Lepidoptera (Fig. 90, _A_, _t_) and in certain Hymenoptera (Fig. 95,
_c_).

The external opening of the spiracles just under the fore wings, is
situated in a little plate called by Audouin the _peritreme_.

[Illustration:

  FIG. 93.—Transformation of the bumble bee, Bombus, showing the
    transfer of the 1st abdominal larval segment (_c_) to the thorax,
    forming the propodeum of the pupa (_D_) and imago; _n_, spiracle of
    the propodeum. _A_, larva; _a_, head; _b_, 1st thoracic; _c_, 1st
    abdominal segment. _B_, semipupa; _g_, antenna; _h_, maxillæ; _i_,
    1st; _j_, 2d leg; _k_, mesoscutum; _l_, mesoscutellum; _m_,
    metathorax; _d_, urite (sternite of abdomen); _e_, pleurite; _f_,
    tergite; _o_, ovipositor; _r_, lingua; _q_, maxilla.
]

In the higher or aculeate Hymenoptera, besides the three segments
normally composing the thorax, the basal abdominal segment is during the
change from the larva to the pupa transferred to this region, making
four segments. This first abdominal is called “the median segment”
(Figs. 93–95). In such a case the term _alitrunk_ has been applied to
this region, _i.e._ the thorax, as thus constituted. Latreille wrongly
stated that in the Diptera the first abdominal segment also entered into
the composition of the thorax; but Brauer has fully disproved that view,
as may be seen by an examination of his sketches which we have copied
(Fig. 94).

[Illustration:

  FIG. 94.—7, 8, thorax of _Tipula gigantea_; 9, of Leptis; 10, thorax
    of _Tabanus bromius_ after the removal of the abdomen, in order to
    bring into view the inner mesophragma (_f_), and to show the
    extension of the metathorax _g_ and _g′_; _tr_, trochanter; 11, hind
    end of the mesothorax, the entire metathorax, and the 1st and 2d
    abdominal segments of _Volucella zonaria_, seen from the side. The
    internal mesophragma (_f_), and the position of the muscle inserted
    in it, are indicated by the two lines _M_. _p_, Callus postalaris;
    _pr_ (_pz_ in 8), callus præalaris Osten Sacken (= “patagium” of
    some authors); _g_, metanotum; _g′_, metepimerum, “segment médiaire”
    of Latreille (wrongly considered by him to be the 1st abdominal
    segment); 4, metasternum (hypopleura of Osten Sacken); 5 (?
    “episternum of metathorax” (Brauer) = metapleura of Osten Sacken);
    6, and also _H_, halter; _st_^1, mesothoracic stigma; _st_^2,
    metathoracic stigma; _st_^3, first abdominal stigma; γ,
    dorsopleural; δ, sternopleural; ε, mesopleural sutures; _h_, 1st,
    _i_, 2d, abdominal segment; _al_, wing; _alul_, alula. 12, the head
    and the three thoracic rings, and the 1st abdominal segment of
    _Ephemera vulgata_, the connecting membranes are in white: _a_,
    prothorax; _b_, præscutum; _c_, scutum; _d_, scutellum; _e_,
    postscutellum; _ps_, postscutellum of mesothorax.—After Brauer.
]

[Illustration:

  FIG. 95.—Alitrunk of _Sphex chrysis_: _A_, dorsal aspect; _a_,
    pronotum; _b_, mesonotum; _c_, tegula; _d_, base of fore,—_e_, of
    hind, wing; _f_, _g_, divisions of metanotum; _h_, median (true
    first abdominal) segment; _i_, its spiracle; _k_, second abdominal
    segment, usually called the petiole or first abdominal segment. _B_,
    posterior aspect of the median segment; _a_, upper part; _b_,
    superior,—_c_, inferior, abdominal foramen; _d_, ventral plate of
    median segment; _e_, coxa.—After Sharp.
]

The sternum is in rare cases subdivided into two halves, as in the meso-
and metathorax of the cockroach; in Forficula the prosternum is divided
into four pieces besides the sternum proper (Fig. 96); and in Embia,
also, the sternites, according to Sharp, are complex.

[Illustration:

  FIG. 96.—Sternal view of pro-, meso-, and metathorax of _Forficula
    tæniata_: _pst_, præsternum, divided into 4 pieces; _st_, pro-,
    _st′_, meso-, _st″_, metasternum; _cx_, coxa; _not_, notum.
]

[Illustration:

  FIG. 97.—_A_, under surface of prothorax, or prosternum, of _Dyticus
    circumflexis_: 2._g_, prosternum; 2._f_, episternum; 2._h_,
    epimerum; 2._s_, antefurca or entothorax.
]

[Illustration:

  FIG. 98.—Meso- (_G__{2}) and metathoracic ganglia (_G__{1}), with the
    apodemes of Gryllotalpa.—After Graber.
]

[Illustration:

  FIG. 99.—Parts of the mesothorax of Dyticus: _A_, mesosternum; 3._a_,
    præscutum; 3._b_, scutum; 3._c_, scutellum; 3._d_, postscutellum;
    3._e_, parapteron; 3._g_, mesosternum; 3._f_, episternum; 3._h_,
    epimerum; 3._s_, medifurca or entothorax.
]

[Illustration:

  FIG. 100.—Parts of the metathorax of Dyticus: _A_, metasternum; 4._a_,
    præscutum; 4._b_, scutum; 4._c_, scutellum; 4._d_, postscutellum;
    4._e_, parapteron; 4._f_, episternum; 4._g_, metasternum; 4._h_,
    epimerum; 4._s_, postfurca.—This and Figs. 97 and 99 from Audouin,
    after Newport.
]

=The apodemes.=—The thorax is supported within by beam-like processes,
or _apodemes_, which pass inward and also form attachments for the
muscles. Those passing up from the sternum form the _entothorax_ of
Audouin, and the process of each thoracic segment is called respectively
the _antefurca_, _medifurca_, and _postfurca_. In the Orthoptera
(Caloptenus and Anabrus), the antefurca is large, thin, flattened,
directed forward, and bounds each side of the prothoracic ganglion. In
the Coleoptera two plates (Fig. 97, 2._s_) arise from the inside of the
sternum and “form a collar or leave a circular hole between them for the
passage of the nervous cord” (Newport). The medifurca is a pair of flat
processes which diverge and bridge the commissure, while the postfurca
is situated under the commissure. In beetles (Dyticus) Newport states
that it is expanded into two broad plates, to which the muscles of the
posterior legs are attached. Graber also notices in the mole cricket
between the apodemes of the meso- and metathorax, a flattened spine
(Fig. 98, _do_) with two perforations through which pass the commissures
connecting the ganglia. Besides these processes there are large, thin,
longitudinal partitions passing down from the tergum (or dorsum), called
_phragmas_; they are most developed in those insects which fly best,
_i.e._ in Coleoptera (Figs. 97–101), Lepidoptera, Diptera, and
Hymenoptera, none being developed in the prothorax. (The term _phragma_
has also been applied to a partition formed by the inflexed hinder edge
of this segment, and is present only in those insects in which the
prothorax is movable.—Century Dictionary.) All these ingrowths may be in
general termed _apodemes_. There are similar structures in Crustacea and
also in Limulus; but Sharp restricts this term to minute projections in
beetles (Goliathus) situated at the sides of the thorax near the wings.
(Insecta, p. 103, Fig. 57.) The internal processes arising from the
sternal region have been called _endosternites_.

[Illustration:

  FIG. 101.—Internal skeleton of _Lucanus cervus_, ♂, head: _A_,
    antenna; _f_, mandible; _d_, mentum; 2, 4, tendons of mandible; _f_,
    _u_, _t_, parts of the tentorium; 3 _e_, labial muscles. Thorax: 2,
    prothorax; 3, 4, meso- and metathorax fused solidly together; 3 _r_,
    acetabulum of prothorax, into which the coxa is inserted; 2 _s_,
    sternum; 3_t_, acetabulum of mesothorax, 4_r_, of metathorax; 3 _s_,
    mesothoracic sternum fused with that of the metathorax (4_g_); 4
    _s_, apodeme.—After Newport.
]

=The acetabula.=—These are the cavities in which the legs are inserted.
They are situated on each side of the posterior part of the sternum, in
each of the thoracic segments. They are, in general, formed by an
approximation of the sternum and epimerum, and sometimes, also, of the
episternum, as in Dyticus (Fig. 97, _A_). This consolidation of parts,
says Newport, gives an amazing increase of strength to the segments, and
is one of the circumstances which enables the insect to exert an
astonishing degree of muscular power.

   TABULAR VIEW OF THE SEGMENTS, PIECES, AND APPENDAGES OF THE THORAX

 ═══════════════════════╤═══════════════════════╤═══════════════════════
     NAME OF SEGMENT    │  PIECES (SCLERITES)   │      APPENDAGES
 ───────────────────────┼───────────────────────┼───────────────────────
 1. Prothorax           │Pronotum, sometimes    │
                        │  differentiated into  │
                        │Scutum                 │1st pair of legs
                        │Scutellum              │Patagia
                        │Episternum             │
                        │Epimerum               │
                        │Sternum                │
                        │Antefurca              │
                        │                       │
 2. Mesothorax          │Præscutum              │
                        │Scutum                 │2d pair of legs
                        │Scutellum              │1st pair of wings
                        │Proscutellum           │Tegulæ
                        │Episternum             │Squamæ (Alulæ)
                        │Epimerum               │Peritreme
                        │Sternum                │
                        │Mesofurca              │
                        │Mesophragma            │
                        │Apodemes               │
                        │                       │
 3. Metathorax          │Præscutum              │
                        │Scutum                 │3d pair of legs
                        │Scutellum              │2d pair of wings
                        │Postscutellum          │(Halteres of Diptera)
                        │Episternum             │
                        │Epimerum               │
                        │Sternum                │
                        │Postfurca              │
                        │Metaphragma            │
                        │Apodemes               │
 ───────────────────────┴───────────────────────┴───────────────────────

[Illustration:

  FIG. 102.—External anatomy of the trunk of _Hydröus piceus_: _A_,
    sternal—_B_, tergal aspect; 2, pronotum; 2 _a_, prosternum; 2 _f_,
    episternum; 3 _a_, præscutum; 3 _b_, scutum; 3 _c_, scutellum; 3
    _d_, postscutellum; 3 _g_, mesosternum; 3 _h_, episternum; 3 _f_,
    epimerum; 3 _i_, crest of the mesosternum; 3 _a_, parapteron; 3 _k_,
    coxa; 4 _a_, metapræscutum; 4 _b_, metascutum; 4 _c_, metascutellum;
    4 _d_, postscutellum; 4 _e_, tegula; 4 _f_, episternum; 4 _h_,
    epimerum; 4 _g_, metasternum; 4 _i_, crest of metasternum; 4 _k_ and
    _l_, coxa; 4 _m_, trochanter; _n_, femur; _o_, tibia; _p_, tarsus;
    _q_, unguis; 7–11, abdominal segments.—After Newport.
]


              _b._ The legs: their structure and functions

The mode of insertion of the legs to the thorax is seen in Figs. 90, 97,
101, and 103. They are articulated to the episternum, epimerum, and
sternum, taken together, and consist of five segments. The basal segment
or joint is the _coxa_, situated between the episternum and trochanter.
The coxa usually has a posterior subdivision or projection, the
_trochantine_; sometimes, as in Mantispa (Fig. 103), the trochantine is
obsolete. We had previously supposed that the trochantine was a separate
joint, but now doubt whether it represents a distinct segment of the
leg, and regard it as only a subdivision of the coxa. It is attached to
the epimerum, and is best developed in Panorpidæ, Trichoptera, and
Lepidoptera. In the Thysanura the trochantine is wanting, and in the
cockroach it merely forms a subdivision of the coxa, its use being to
support the latter. The second segment is the trochanter, a more or less
short spherical joint on which the leg proper turns; in the parasitic
groups (Ichneumonidæ, etc., Fig. 104) it is usually divided into two
pieces, though there are some exceptions. The trochanter is succeeded by
the _femur_, _tibia_, and _tarsus_, the latter consisting of from one to
five segments, the normal number being five. Tuffen West believed that
the pulvillus is the homologue of an additional tarsal joint, “a sixth
tarsal joint.” The last tarsal segment ends in a pair of freely movable
claws (ungues), which are modified setæ; between the claws is a
cushion-like pad or adhesive lobe, called the _empodium_ or _pulvillus_
(Fig. 105, also variously called _arolium_, _palmula_, _plantula_,
_onychium_, its appendage being called _paronychium_ and also
_pseudonychium_). It is cleft or bilobate in many flies, but in Sargus
trilobate. All these parts vary greatly in shape and relative size in
insects of different groups, especially Trichoptera, Lepidoptera,
Diptera, and Hymenoptera. In certain flies (_e.g._ Leptogaster) the
empodium is wanting (Kolbe). By some writers the middle lobe is called
the empodium and the two others pulvilli.

[Illustration:

  FIG. 103.—Side view of meso- and metathorax of _Mantispa brunnea_,
    showing the upper and lower divisions of the epimerum (_s. em′_, _s.
    em″_, _i. em′_, _i. em″_); _s. epis_, _i. epis″_, the same of the
    episternum.
]

[Illustration:

  FIG. 104.—Divided (ditrochous) trochanter of an ichneumon: _cx_, coxa;
    _tr_, the two divisions of the trochanter; _f_, femur.—After Sharp.
]

The fore legs are usually directed forward to drag the body along, while
the middle and hind legs are directed outward and backward to push the
body onwards. While arachnids walk on the tip ends of their feet,
myriopods, Thysanura, and all larval insects walk on the ends of the
claws, but insects generally, especially the adults, are, so to speak,
plantigrade, since they walk on all the tarsal joints. In the aquatic
forms the middle and hind tarsi are more or less flattened, oar-like,
and edged with setæ. In leaping insects, as the locusts and
grasshoppers, and certain chrysomelids, the hind femora are greatly
swollen owing to the development of the muscles within. The tibia,
besides bearing large, lateral, external spines, occasionally bears at
the end one or more spines or spurs called _calcaria_. The fore tibia
also in ants, etc., bear tactile hairs, and chordotonal organs, as well
as other isolated sense-organs (Janet), and, in grasshoppers, ears.

In the Carabidæ the legs are provided with combs for cleaning the
antennæ (Fig. 107), and in the bees and ants these cleansing organs are
more specialized, the pectinated spine (_calcar_) being opposed by a
tarsal comb (Fig. 106, _d_; for the wax-pincers of bees, see _g_). In
general the insects use their more or less spiny legs for cleansing the
head, antennæ, palpi, wings, etc., and the adaptations for that end are
the bristles or spinules on the legs, especially the tibiæ.

[Illustration:

  FIG. 105.—Foot of honey-bee, with the pulvillus in use: _A_, under
    view of foot; _t_, _t_, 3d–5th tarsal joints; _a n_, unguis; _f h_,
    tactile hairs; _p v_, pulvillus; _cr_, curved rod. _B_, side view of
    foot. _C_, central part of sole; _pd_, pad; _cr_, curved rod; _pv_,
    pulvillus unopened.—After Cheshire.
]

[Illustration:

  FIG. 106.—Modifications of the legs of different bees. _A_, Apis: _a_,
    wax-pincer and outer view of hind leg; _b_, inner aspect of
    wax-pincer and leg, with the nine pollen-brushes or rows of hairs;
    _c_, compound hairs holding grains of pollen; _d_, anterior leg,
    showing antenna-cleaner; _e_, spur on tibia of middle leg. _B_,
    Melipona: _f_, peculiar group of spines at apex of tibia of hind
    leg; _g_, inner aspect of wax-pincer and first tarsal joint. _C_,
    Bombus: _h_, wax-pincer; _i_, inner view of the same and first
    tarsal joint, all enlarged.—From _Insect Life_, U. S. Div. Ent.
]

Osten Sacken states that among Diptera the aerial forms (Bombylidæ,
etc.) with their large eyes or holoptic heads, which carry with them the
power of hovering or poising, have weak legs, principally fit for
alighting. On the other hand, the pedestrian or walking Diptera
(Asilidæ, etc.) “use the legs not for alighting only, but for running,
and all kinds of other work, seizing their prey, carrying it, climbing,
digging, etc.; their legs are provided not only with spines and
bristles, but with still other appendages, which may be useful, or only
ornamental, as secondary sexual characters.”

[Illustration:

  FIG. 107.—End of tibia and tarsal joints of Anophthalmus; _c_, comb.
]

=Tenent hairs.=—Projecting from the lower surface of the empodium are
the numerous “tenent hairs,” or holding hairs, which are modified
glandular setæ swollen at the end and which give out a minute quantity
of a clear adhesive fluid (Figs. 108, 109, 130, 134). In larval insects,
and the adults of certain beetles, Coccidæ, Aphidæ, and Collembola,
which have no empodium, there are one or more of these tenent hairs
present. They enable the insect to adhere to smooth surfaces.

[Illustration:

  FIG. 108.—Transverse section through a tarsal joint of Telephorus, a
    beetle: _ch_, cuticula of the upper side; _m_, its matrix; _ch′_,
    the sole; _m′_, its matrix; _h_, adhesive hair; _h′_, tactile hair,
    supplied with a nerve (_n′_), and arising from a main nerve (_n_);
    _n″_, ganglion of a tactile hair; _t_, section of main trachea, from
    which arises a branch (_t′_); _dr_, glands which open into the
    adhesive hairs, and form the sticky secretion; _e_, chitinous
    thickening; _s_, sinew; _b_, membrane dividing the hollow space of
    the tarsal joint into compartments. See p. 111.—After Dewitz.
]

Striking sexual secondary characters appear in the fore legs of the male
Hydrophilus, the insect, as Tuffen West observes, walking on the end of
the tibia alone and dragging the tarsus after it. The last tarsal joint
is enlarged into the form of an irregular hollow shield. The most
completely suctorial feet of insects are those of the anterior pair of
Dyticus (Fig. 132). The under side of the three basal joints is fused
together and enlarged into a single broad and nearly circular shield,
which is convex above and fringed with fine branching hairs, and covered
beneath with suckers, of which two are exceptionally large; by this
apparatus of suckers the male is enabled to adhere to the back of its
mate during copulation. The line branching hairs around the edge prevent
the water from penetrating and thus destroying the vacuum, “while if the
female struggle out of the water, by retaining the fluid for some time
around the sucker, they will in like manner under these altered
conditions equally tend to preserve the effectual contact.” (Tuffen
West.)

[Illustration:

  FIG. 109.—Cross-section through tarsus of a locust: _ch_, cuticula of
    upper side,—_ch′_, _ch″_, _ch‴_, of sole; _ch_, tubulated layer;
    _ch″_, lamellate layer; _ch‴_, inner projections of _ch″_. Other
    lettering as in Fig. 101. See p. 113.—After Dewitz.
]

In the saw-flies (Uroceridæ and Tenthredinidæ) and other insects, there
are small membranous oval cushions (_arolia_, Figs. 109 and 131) beneath
each or nearly each tarsal joint.

  The triunguline larvæ of the Meloidæ are so called from apparently
  having three ungues, but in reality there is only a single claw,
  with a claw-like bristle on each side.

  =Why do insects have but six legs?=—Embryology shows that the
  ancestors of insects were polypodous, and the question arises to
  what cause is due the process of elimination of legs in the
  ancestors of existing insects, so that at present there are no
  functional legs on the abdomen, these being invariably restricted
  (except in caterpillars) to the thorax, and the number never being
  more than six. It is evident that the number of six legs was fixed
  by heredity in the Thysanura, before the appearance of winged
  insects. We had thought that this restriction of legs to the thorax
  was in part due to the fact that this is the centre of gravity, and
  also because abdominal legs are not necessary in locomotion, since
  the fore legs are used in dragging the insect forwards, while the
  two hinder pairs support and push the body on. Synchronously with
  this elimination by disuse of the abdominal legs, the body became
  shortened, and subdivided into three regions. On the other hand, as
  in caterpillars, with their long bodies, the abdominal legs of the
  embryo persist; or if it be granted that the prop-legs are secondary
  structures, then they were developed in larval life to prop up and
  move the abdominal region.

  The constancy of the number of six legs is explained by Dahl as
  being in relation to their function as climbing organs. One leg, he
  says, will almost always be perpendicular to the plane when the
  animal is moving up a vertical surface; and, on the other hand, we
  know that three is the smallest number with which stable equilibrium
  is possible; an insect must therefore have twice this number, and
  the great numerical superiority of the class may be associated with
  this mechanical advantage. (This numerical superiority of insects,
  however, seems to us to be rather due to the acquisition of wings,
  as we have already stated on pages 2 and 120.)

=Loss of limbs by disuse.=—Not only are one or both claws of a single
pair, or those of all the feet atrophied by disuse, but this process of
reduction may extend to the entire limb.

  In a few insects one of the claws of each foot is atrophied, as in
  the feet of the Pediculidæ, of many Mallophaga, all of the Coccidæ,
  in Bittacus, Hybusa (Orthoptera), several beetles of the family
  Pselaphidæ, and a weevil (Brachybamus). Hoplia, etc., bear but a
  single claw on the hind feet, while the allied Gymnoloma has only a
  single claw on all the feet. Cybister has in general a single
  immovable claw on the hind feet, but _Cybister scutellaris_ has,
  according to Sharp, on the same feet an outer small and movable
  claw. In the water bugs, Belostoma, etc., the fore feet end in a
  single claw, while in others (Corisa) both claws are wanting on the
  fore feet. Corisa also has no claws on the hind feet; Notonecta has
  two claws on the anterior four feet, but none on the hind pair. In
  Diplonychus, however, there are two small claws present. (Kolbe.)

[Illustration:

  FIG. 110.—Last tarsal joint of _Melolontha vulgaris_, drawn as if
    transparent to show the inner mechanism: _un_, claws; _str_,
    extensor plate; _s_, tendon of the flexor muscle; _vb_, elastic
    membrane between the extensor plate and the sliding surface _u_;
    _krh_, process of the ungual joint; _emp_, extensor spine, and _th_,
    its two tactile hairs.—After Ockler, from Kolbe.
]

Among the Scarabæidæ, the individuals of both sexes of the fossorial
genus Ateuchus (_A. sacer_) and eight other genera, among them
_Deltochilum gibbosum_ of the United States, have no tarsi on the
anterior feet in either sex. The American genera Phanæus (Fig. 111),
Gromphas, and Streblopus have no tarsal joints in the male, but they are
present in the female, though much reduced in size, and also wanting,
Kolbe states, in many species of Phanæus. The peculiar genus
Stenosternus not only lacks the anterior feet, but also those of the
second and third pair of legs are each reduced to a vestige in the shape
of a simple, spur-like, clawless joint. The ungual joint is wanting in
the weevil Anoplus, and becomes small and not easily seen in four other
genera.

  Ryder states that the evidence that the absence of fore tarsi in
  Ateuchus is due to the inheritance of their loss by mutilation is
  uncertain. Dr. Horn suggests that cases like Ateuchus and
  Deltochilum, etc., “might be used as an evidence of the persistence
  of a character gradually acquired through repeated mutilation, that
  is, a loss of the tarsus by the digging which these insects
  perform.” On the other hand, the numerous species of Phanæus do
  quite as much digging, and the anterior tarsi of the male only are
  wanting. “It is true,” he adds, “that many females are seen which
  have lost their anterior tarsi by digging; have, in fact, worn them
  off; but in recently developed specimens the front tarsi are always
  absent in the males and present in the females. If repeated
  mutilation has resulted in the entire disappearance of the tarsi in
  one fossorial insect, it is reasonable to infer that the same
  results should follow in a related insect in both sexes, if at all,
  and not in the male only. It is evident that some other cause than
  inherited mutilation must be sought for to explain the loss of the
  tarsi in these insects.” (Proc. Amer. Phil. Soc., Philadelphia,
  1889, pp. 529, 542.)

[Illustration:

  FIG. 111.—Fore tibia of _Phanæus carnifex_, ♂, showing no trace of the
    tarsus.
]

[Illustration:

  FIG. 112.—Fore leg of the mole-cricket: _A_, outer, _B_, inner,
    aspect; _e_, ear-slit.—After Sharp.
]

The loss of tarsi may be due to disuse rather than to the inheritance of
mutilations. Judging by the enlarged fore tibiæ, which seem admirably
adapted for digging, it would appear as if tarsi, even more or less
reduced, would be in the way, and thus would be useless to the beetles
in digging. Careful observations on the habits of these beetles might
throw light on this point. It may be added that the fore tarsi in the
more fossorial Carabidæ, such as Clivina and Scarites, as well as those
of the larva of Cicada and those of the mole crickets (Fig. 112), are
more or less reduced; there is a hypertrophy of the tibiæ and their
spines. The shape of the tibia in these insects, which are flattened
with several broad triangular spines, bears a strong resemblance to the
nails or claws of the fossorial limbs of those mammals which dig in hard
soil, such as the armadillo, manis, aardvark, and Echidna. The principle
of modification by disuse is well illustrated in the following cases.

In many butterflies the fore legs are small and shortened, and of little
use, and held pressed against the breast. In the Lycænidæ the fore tarsi
are without claws; in Erycinidæ and Libytheidæ the fore legs of the
males are shortened, but completely developed in the females, while in
the Nymphalidæ the fore legs in both sexes are shortened, consisting in
the males of one or two joints, the claws being absent in the females.
Among moths loss of the fore tarsi is less frequent. J. B. Smith[21]
notices the lack of the fore tarsi in the male of a deltoid, _Litognatha
nubilifasciata_ (Fig. 113), while the hind feet of _Hepialus hectus_ are
shortened. In an aphid (_Mastopoda pteridis_, Esl.) all the tarsi are
reduced to a single vestigial joint (Fig. 114).

[Illustration:

  FIG. 113.—Leg of Litognatha: _cx_, coxa; _f_, femur; _t_, tibia; _ep_,
    its epiphysis, and _sh_, its shield-like process. The tarsus
    entirely wanting.—After Smith.
]

Entirely legless adult insects are rare, and the loss is clearly seen to
be an adaptation due to disuse; such are the females of the Psychidæ,
the females of several genera of Coccidæ (Mytilaspis, etc.), and the
females of the Stylopidæ.

Apodous larval insects are common, and the loss of legs is plainly seen
to be a secondary adaptive feature, since there are annectant forms with
one or two pairs of thoracic legs. All dipterous and siphonapterous
larvæ, those of all the Hymenoptera except the saw-flies, a few
lepidopterous larvæ, some coleopterous, as those of the Rhyncophora,
Buprestidæ, Eucnemidæ, and other families, and many Cerambycidæ are
without any legs. In _Eupsalis minuta_, belonging to the Brenthidæ, the
thoracic legs are minute.

The legs of larvæ end in a single claw, upon the tips of which the
insect stands in walking.


           _c._ Locomotion (walking, climbing, and swimming)

=Mechanics of walking.=—To Graber we owe the best exposition of the
mechanics of walking in insects.

  “The first segment of the insect leg,” he says, “upon which the
  weight of the body rests first of all, is the coxa. Its method of
  articulation is very different from that of the other joints. The
  enarthrosis affords the most extensive play, particularly in the
  Hymenoptera and Diptera.”

  In the former the development of their social conditions is very
  closely connected with the freest possible use of the legs, which
  serve as hands. In the beetles, however, which are very compactly
  built, there exists a solid articulation whereby the entire hip
  rests in a tent-like excavation of the thorax, and can only be
  turned round a single axis, as may be seen in Fig. 115, where _c_
  represents the imaginary revolving axis and _d_ the coxa. In the
  case we are supposing, therefore, only a backward and forward
  movement of the coxa is possible, the extent of the play of which
  depends on the size of the coxal pan, as well as certain groin or
  bar-like structures which limit further rotation. In the very
  dissimilar arrangement which draws in the fore, middle, and hind
  legs toward the body it is self-evident that their extent of action
  is also different. This arrangement seems to be most yielding on the
  fore legs, where the hips, to confine ourselves to the stag-beetles,
  can be turned backward and forward 60° from the middle or normal
  position, and therefore describe on the whole a curve of 120°. The
  angle of turning on the middle leg hardly exceeds a legitimate
  limit, yet a forward as well as a backward rotation takes place. The
  former is entirely wanting in the hind hips; they can only be moved
  backward.

[Illustration:

  FIG. 114.—Leg of an Aphid, with the tarsus (_t_) much reduced: 1, 2,
    3, legs of 1st, 2d, and 3d pairs.
]

  The number and strength of the muscles on which the rotation of the
  hips depends, correspond with these varying movements of the
  individual legs. Thus, according to Straus Durckheim, the fore coxa
  of many beetles possesses five separate muscles and four forward and
  one backward roll; the middle coxa a like number of muscles but only
  two forward rolls, while the hind hips succeed in accomplishing each
  of the motions named with a single muscle.

  One can best see how these muscles undertake their work, and above
  all how they are situated, if he lays bare the prothorax of the stag
  beetle (Fig. 116). Here may be seen first the thick muscle which
  turns to the front the rotating axis in its cylindrical pan, and
  thus helps to extend the leg, while two other tendons, which take
  the opposite direction, are fitted for reflex movements.

[Illustration:

  FIG. 115.—Mechanics of an insect’s leg: _d_, coxa,—_c_, axis of
    revolution; _a_ and _b_, the coxal muscles; _e_, trochanter muscle
    (elevator of the femur); _f_, extensor,—_g_, flexor, of the tibia
    (_pn_); _n_, tibial spine; _h_, flexor.—_i_, extensor, of the
    foot; _k_, extensor,—_l_, flexor, of the claw; _po_, place of
    flexure of the tibia; _p^1q_, leg after being turned back by the
    coxa.—_p^1r_, by the simultaneous flexure of the tibia. The
    resulting motion of the end of the tibia, through the simultaneous
    movement (_no_) and revolution (_nq_), indicates the curve
    _nr_.—After Graber.
]

  In Fig. 115 the muscles mentioned above, and their modes of working,
  may be distinguished by the arrows _a_ and _b_.

  In order to simplify matters, we will imagine the second component
  part of the normal insect leg, _i.e._ the trochanter (Figs. 116,
  117, _r_), as grown together with the third lever, _i.e._ the femur,
  as the movement of both parts mostly takes place uniformly.

[Illustration:

  FIG. 116.—Section of the fore leg of a stag-beetle, showing the
    muscles: _S_, extensor,—_B_, flexor, of the leg; _s_,
    extensor,—_b_, flexor, of the femur; _o_, femur; _u_, tibia; _f_,
    tarsus; _k_, claw; 109, _s_, extensor,—_b_, flexor, of the
    femoro-tibial joint, both enlarged.—After Graber.
]

  The pulling of the small trochanter muscle works against the weight
  of the body when this is carried over on to the trochanter by means
  of the coxa, as seen at the arrow _e_ in Fig. 115. It may be
  designated as the femoral lever.

  The plane of direction in which the femur, as seen by the rotation
  just mentioned, is moved, exactly coincides in insects with that of
  the tibia and the foot, while all can be simultaneously raised or
  dropped, or, as the case may be, stretched out or retracted.
  Therein, therefore, lies an essential difference from the fully
  developed extremities of vertebrates among which, even on the lever
  arms which are stationary at the end, an extensive turning is
  possible.

  The muscles which move the tibia, and indirectly the femur, also
  consist of an extensor muscle which is situated in the upper side of
  the femur (Fig. 116, _s_, Fig. 115, _f_), and of a flexor (Fig. 116,
  _b_, Fig. 115, _g_), which lies under the former.

  The stilt-like spines on the point (Figs. 115 and 118, _L_{3}n_) on
  which this segment is directly supported are important parts of the
  tibia. (Graber.)

[Illustration:

  FIG. 117.—Left fore leg of a cerambycid beetle: _h_, coxa; _r_,
    trochanter; _o_, femur; _u_, tibia; _f_, tarsus; _k_, claw.—After
    Graber.
]

Considering the respective positions of the individual levers of the leg
and the nature of the materials of which they are made, the legs of
insects may be likened, as Graber states, to elastic bows, which, when
pressed down together from above, their own indwelling elasticity is
able to raise again and thus keep the body upright.

This is very plainly shown in certain stilt-legged bark-beetles, in
which, as in a rubber doll, as soon as the body is pressed down on the
ground, the organs of motion extend again without the intervention of
muscles; indeed this experiment succeeds even with dead, but not yet
wholly stiff, insects.

Graber then turns to the analysis of the movements of insect legs when
in motion, and the mode of walking of these insects in general. This
subject had been but slightly investigated until Graber made a series of
observations and experiments, of which we can give only the most
important results.

The locomotion of insects is an extremely complicated subject.

  Let us consider, Graber says, first, a running or carabid beetle,
  when walking merely with the fore and hind legs. The former will be
  bent forward and the latter backward.

  “Let us begin with the left fore leg (Fig. 118, _L_{1}_). Let the
  same be extended and fixed on the ground by means of its sharp claws
  and its pointed heel. Now what happens when the tibial flexors draw
  together? As the foot, and therefore the tibia also, have a firm
  position, then the contraction of the muscles named must cause the
  femur to approach the tibia, whereby the whole body is drawn along
  with it. This individual act of motion may be well studied in
  grasshoppers when they are climbing on a twig by stretching out
  their long fore leg directly forward, and then drawing up the body
  through the shortening of the tibial flexors until the middle leg
  also reaches the branch.

  “But while the fore legs advance the body by drawing the free lever
  to the fixed leg-segment, the hind legs do this in exactly the
  opposite way. The hind leg, namely, seeks to stretch out the tibia,
  and thus to increase the angle of the knee (_R_{3})_, thereby giving
  a push on the ground, by means of which the body is shoved forward a
  bit.

  “Though it might be supposed that the feet would remain stationary
  during the extension or retraction of the limbs, this never occurs
  in actual walking. Not merely the upper, but also the lower, thigh
  is either drawn in or stretched out, as the case may be. The latter
  then describes a straight line with its point during this scraping
  or scratching motion (Fig. 115, _no_), which is obviously the chord
  to that quadrant which would be drawn by the tibia or foot in a
  yielding medium, as water, for instance. But even this motion
  results extremely rarely, and never in actual walking. If we fix our
  eye anew upon the fore leg at the very moment when it is again
  retracted, after the resultant ‘fixing,’ we shall then observe that
  the hip also is simultaneously turned backward in a definite angle.
  The tibia would describe the arc _nq_ (Fig. 115) by means of the
  latter alone.

  “This plane, in conjunction with the rectilinear ‘movement’ (_no_)
  obtained by the retraction of the tibia, produces a path (_nr_), and
  this is what is actually described by a painted foot upon a properly
  prepared surface, as a sheet of paper;[22] supposing, however, that
  the body in the meantime is not moved forward by other forces. In
  the last case, and this indeed always takes place in running, the
  trunk is moved a bit forward, together with the leg which is just
  describing its curve with a rapidity corresponding to the momentum
  obtained; the result of this is that the curve of the foot from its
  beginning (_n_) to its end (_a_) bends round close to itself, just
  as a man who, when on board a ship in motion, walks across it
  diagonally, and yet on the whole moves forward, because his line of
  march, uniting with that of the ship, results in a change of
  position in space.

  “The case is the same in the middle and hind legs, which must make a
  double course also, yet in such a way that the straight line is
  drawn, not during the retraction, but during the extension; during
  which, however, quite as in the fore leg, the members mentioned
  (_R_{3}_) gradually approach the body.

  “When the legs have reached the maximum of their retraction, or of
  their extension, as the case may be, and therefore the end of their
  active course for that time, then begins the opposite or backward
  movement; that is, the fore legs are again extended, while their
  levers draw the remaining legs together again.

[Illustration:

  FIG. 118.—A Carabus beetle in the act of walking or running: three
    legs (_L_{1}_, _R_{2}_, _L_{3}_) are directed forward, while the
    others (_R_{1}_, _L_{2}_, _R_{3}_), which are directed backward
    toward the tail, have ended their activity; _ab_, _cd_, and _ef_
    are curves described by the end of the tibiæ, and passing back to
    the end of the body; _bh_, _di_, and _fg_ are curves described by
    the same legs during their passive change of position.—After
    Graber.
]

  “At the same time, as we may see by the uniting leg, the limb is
  either a little raised, that there may be no unnecessary friction,
  or it remains during the passive step also, with its means of
  locomotion in slight contact with the ground.

  “The curve of two steps, as inscribed by the end of the tibia of the
  left fore leg of a stag-beetle, affords an instructive summary of
  the conditions of which we have been speaking (Fig. 121, _B_). We
  see two curves. The thick one (_ab_), directed toward the axis of
  the body, corresponds to the effective act of a single walking
  function, which brings the body a bit forward; the thinner, on the
  other hand, or we might say the hair line (_bc_), which, however, is
  but rarely made quite clearly, is produced by the ineffectual
  backward movement, by which the insect again approaches its working
  posture (_c_). It is at first placed at some distance from the body,
  in order that (like _c_ also) it may draw near to the body again;
  but in such a way, naturally, that it coincides with the
  starting-point of the following active curve (_cd_). It is evident
  that even the passive curve is not the imprint of the movement
  accomplished exclusively by the leg, for this latter, while
  struggling to reach its resting-place, is really involuntarily
  carried forward with the rest of the body.

  “The scroll-like lines drawn by the swimming beetle (Dyticus), with
  the large, sharp points of its hind tibia, are also very instructive
  (Fig. 119, _A_).

[Illustration:

  FIG. 119.—_A_, trail curves described by the tibial spines of the
    right and left hind limb of Dyticus. _B_, the same made by the
    right hind leg (_r_{3}_) alone. Natural size.—After Graber.
]

[Illustration:

  FIG. 120.—The same by the two hind legs of Melolontha: _a_, the
    active and thickened section of the curve. Natural size.
]

[Illustration:

  FIG. 121.—_A_, track curves of two of the tibial spines of the left,
    middle legs of a stag-beetle. Natural size. _B_, the same
    enlarged; _fg_, the longitudinal axis of the trunk; _cd_ and _ab_,
    the active curve passing inward,—_bc_ and _de_, the passive going
    outward. _C_, two curves described by the left hind legs; in this
    case, the curves are not inwards or backwards, but partly directly
    inward (_b_), and in part obliquely forwards (_a_).
]

  “The diversions and modifications in the course of the active step,
  as furnished by the moving factor of the remaining legs, are already
  clearly illustrated by the curves shown by the joints of the hind
  tibia of a May-beetle (Fig. 120) and a stag-beetle (Fig. 121, _c_).
  The actual faint line in this case does not run from the front
  toward the back, as would correspond to the active leg-motion, but
  either directly inward (Fig. 121, _cb_), or even somewhat to the
  front. In the May-beetles, and even more in the running
  garden-beetle, the curves of the hind legs present themselves as
  screw-like lines (Fig. 122, _l_{3}_), while the scrawling of the
  remaining members (_l_{1}_, _l_{2}_) is much simpler.

  “Inasmuch as we now have a cursory knowledge of the movements made
  by each individual leg for itself,—movements, however, which plainly
  occur very differently according to the structure of these
  appendages,—the question now is of the combined play, the total
  effect of all the legs taken together, and therefore of the walk and
  measure of the united work of the foot.

  “In opposition to the caterpillars and many other crawling animals
  which extend their legs in pairs and really swing them by the
  worm-like mode of contraction of the dermo-muscular tube, the legs
  of fully grown insects are moved in the contrary direction and in no
  sense in pairs, but alternately—or, more strictly speaking, in a
  diagonal direction.

  “For an examination of the gait of insects, we choose, for obvious
  reasons, those which have very long legs and which at the same time
  are slow walkers.

  “Insects may be called ‘double-three-footed,’ from the manner in
  which they alternately place their legs. There are always three legs
  set in motion at the same time, or nearly so, while in the meantime
  the remaining legs support the body, after which they change places.

[Illustration:

  FIG. 122.—The same by the left fore (_l_{1}_), middle (_l_{2}_), and
    hind, leg (_l_{3}_) of a Carabus. Natural size.
]

[Illustration:

  FIG. 123.-Tracks of a _Blaps mortisaga_ marked by the differently
    painted tibial points: ●, tracks of fore, —○, middle, —/, hind
    leg. Natural size.
]

[Illustration:

  FIG. 124.—Tracks of _Necrophorus vespilio_. Natural size.
]

  “To be more exact, it is usually thus: At first (Fig. 118) the left
  fore leg (_L_{1}_) steps out, then follows the right middle leg
  (_R_{2}_), and the left hind leg (_L_{3}_). Then while the left fore
  leg begins to retract and thus make the backward movement, the right
  fore leg is extended, whereupon the left middle leg and the right
  hind leg are raised in the same order as the first three feet.”

  Graber[23] painted the feet of beetles and let them run over paper,
  and goes on to say:

  “Let us first pursue the tracks of the Blaps, for example (Fig.
  123). Let the insect begin its motion. The left fore leg stands at
  _a_, the right middle leg at β, and the left hind leg at _c_. The
  corresponding number of the other set of three feet at α, _b_, γ. At
  the first step the three feet first mentioned advance to _a′_β′_c′_,
  the second set on the other hand to α′_b′_γ′. Thereby the tracks
  made by the successive steps fall quite, or almost quite, on each
  other, as appear also in the tracks of a burying beetle (Fig. 124).

  “As the fore legs are directed forward and the hind legs backward,
  while the middle legs are placed obliquely, the reason of the more
  marked impressions of the latter is evident.

  “The highest testimony to the precise exactitude and accuracy of the
  walking mechanism of insects is furnished by the fact that in most
  insects, and particularly in those most fleet of foot, which,
  whether they are running away or chasing their prey, must be able to
  rely entirely upon their means of locomotion;—the fact, we say, that
  whether they desire to move slowly or more quickly, the distances of
  the steps, measured by the length as well as by the cross-direction,
  hardly differ a hair’s breadth from one another, and this is also
  the case when the tarsi are cut off and the insects are obliged to
  run on the points of their heels (tibiæ).

  “Thence, inasmuch as the trunk of insects is carried by two legs and
  by one on each side alternately, it may surely be concluded _a
  priori_ that when walking it is inclined now to the right and now to
  the left, and that the track, too, which is left behind by a precise
  point of the leg, can in no wise be a straight line; and in reality
  this is not the case.

  “A plainly marked regular curve, which approaches a sinuous line, as
  seen in Fig. 125, is often obtained by painting many insects, for
  example Trichodes, Meloë, etc., which, when running, either bring
  the end of their hind body near to the ground or into contact with
  it.

[Illustration:

  FIG. 125.—Tracks of Trichodes; the middle sinuous line is made by
    the tip of the abdomen. Natural size.
]

[Illustration:

  FIG. 126.—Tracks of another insect which, in running, can only use
    three legs (_r_{1}_, _l_{4}_, _r_{3}_) which become indicated
    differently from normal conditions. Natural size.
]

[Illustration:

  FIG. 127.—The same of an insect crossing over a surface inclined 30°
    from the horizon, whereby the placing of the feet becomes changed.
    Natural size.—This and Figs. 120–126 after Graber.
]

  “The locomotive machine of insects may be called, to a certain
  extent, a double set of three feet each, as most insects, and
  particularly those provided with a broad trunk, are able to balance
  themselves with one of these two sets of feet, and indeed when
  walking, as well as when standing still, can move about even better
  with one set of these feet than with four legs. In the latter case,
  that is, if one cuts off a pair of legs from an insect, the trunk
  can balance itself only with extreme difficulty, and there is
  therefore little prospect that insects will ever become four-footed.

  “But if one compels insects to run on three legs, he will thus make
  the interesting discovery that to make up the deficiency they place
  the remaining feet and bring them to the ground somewhat differently
  than when the second set of feet is active. Figs. 124 and 126 may be
  compared for this purpose. The former shows the footprints of a
  burying beetle running with all six legs, the latter the track of
  the same insect, which, however, has at its disposal only the right
  fore leg, the left middle leg, and the right hind leg. One may
  plainly see here that the track of the hind leg on the right side
  (_r_{3}_) approaches the track of the middle leg on the left side,
  and then further, that the _right fore leg_ (_r_{1}_) _steps out
  more to the right to make up for the deficiency of the middle leg_.

  “A similar adaptation of the position of the legs, which is entirely
  dependent on the choice of the insect, may also be observed there,
  if one compels insects which are not provided with corresponding
  adhesive lobes to run away over crooked surfaces. Fig. 123 shows the
  footprints of a Blaps when running upon a horizontal plane. Fig.
  127, on the contrary, shows the tracks of the legs when going
  diagonally over a gradually inclined surface. Here, also, the insect
  holds on with his fore and middle legs (_r_{1}_, _r_{2}_) stretched
  upward, whereby also the impressions on both sides come to lie
  farther apart than in the normal mode of walking.

  “It will not surprise the reader who is familiar with the gait of
  crabs, to hear that many insects also understand the laudable art of
  going backward, wherein the hind legs simply change places with the
  fore legs.

  “The jumping motion of insects may be best studied in grasshoppers.
  When these insects are preparing for a jump, they stretch out the
  upper thigh horizontally, clap the tibiæ together, and also retract
  the foot-segment. After a slight pause for rest, during which they
  are getting ready for the jump, they then jerk the tibiæ suddenly
  backward and against the ground with all their strength by means of
  the extensor muscles.”

The correctness of Graber’s views has been confirmed by Marey by
instantaneous photographs (Figs. 128, 129).

=Locomotion on smooth surfaces.=—How flies and other insects are able to
walk up, or run with the body inverted, on hard surfaces has been lately
discovered by Dewitz, Dahl, and others. All authors are agreed that this
power is due to the presence of the specialized empodium of each tarsus.

Dewitz confirmed the opinion of Blackwell, that a glutinous liquid is
exuded from the apices of the tenent hairs which fringe the empodium. By
fastening insects feet uppermost on the under side of a covering glass
which projects from a glass slide, the hairs which clothe the empodia of
the foot of a fly (_Musca erythrocephala_) may be seen to be tipped with
drops of transparent liquid. On the leg being drawn back from the glass,
a transparent thread is drawn out, and drops are found to be left on the
glass. In cases where these hairs are wanting, as in the Hemiptera, the
adhesive fluid exudes directly from pores in the foot. In the beetles
(_Telephorus dispar_) and other insects the tenent hairs on the foot end
in sharp points, below which are placed the openings of the canals. The
glands, Dewitz states, are chiefly flask-shaped and unicellular,
situated in the hypodermis of the chitinous coat; each gland opening
into one of the hairs (Fig. 108); they are each invested by a
structureless tunica propria, and contain granular protoplasm, a nucleus
placed at the inner side, and a vesicle, prolonged into a tube which,
traversing the neck of the gland, is attached to the root of the hair;
the vesicle receiving the secretion. Each gland is connected with a fine
nerve-twig, and secretion is probably voluntary. Among the tenent hairs
of the empodium are others which must be supplied with a nerve, forming
tactile hairs, as they each proceed from a unicellular ganglion (Fig.
108, _n″_). The secretion is forced out of the gland by the contraction
of the protoplasm, Dewitz having seen the secretion driven out from the
internal vesicle into its neck.

[Illustration:

  FIG. 128.—The walk of an orthopterous insect: series to be followed
    from right to left.—After Marey.
]

[Illustration:

  FIG. 129.—Beetle walking: series to be followed from left to
    right.—After Marey.
]

[Illustration:

  FIG. 130.—_A_, end of an adhesive hair of a weevil (Eupolus): _i′_,
    canal: _i‴_, its external opening at the end of the hair. _B_, end
    of a similar hair of Telephorus with drops of the secretion.—After
    Dewitz.
]

  In the spherical last tarsal joint of Orthoptera (Fig. 109), which
  is without these tenent hairs, nearly all the cells of the
  hypodermis are converted into unicellular glands, each of which
  sends out a long, fine, chitinous tubule, which is connected with
  its fellows by very fine hairs and is continuous with the chitinous
  coat of the foot and opens through it. The sole of the foot is
  elastic and adapts itself to minute inequalities of surfaces, while
  the anterior of each tarsal joint is almost entirely occupied by an
  enlargement of the trachea, which acts on the elastic sole like an
  air chamber, rendering it tense and at the same time pliant. Dewitz
  adds that the apparatus situated on the front legs of the male of
  _Stenobothrus sibiricus_ (Fig. 131) must have the function of
  causing the legs to adhere closely to the female by the excretion of
  an adhesive material. The hairs of the anterior tarsi of male Carabi
  also appear to possess the power of adhesion. In the house-fly the
  empodia seem to be only called into action when the insect has to
  walk on vertical smooth surfaces, as at other times they hang
  loosely down.

  Burmeister observed the use of a glutinous secretion for walking in
  dipterous larvæ, and Dewitz found that the larva of a Musca used for
  this purpose a liquid ejected from the mouth. The larvæ of another
  fly (_Leucopis puncticornis_) perform their loop-like walk by
  emitting a fluid from both mouth and anus. A Cecidomyia larva is
  able to leap by fixing its anterior end by means of an adhesive
  fluid. The larva of the leaf-beetle, Galeruca, moves by drawing up
  its hinder end, fixing it thus, and carrying the anterior part of
  the body forward with its feet until fully extended, when it breaks
  the glutinous adhesion. The abdominal legs of some saw-fly larvæ
  have the same power.

Dahl could not detect in the foot of the hornet (_Vespa crabro_) any
space which could be considered as a vacuum.

[Illustration:

  FIG. 131.—_Stenobothrus sibiricus_ pairing: _A_. the ♂, fore tarsus
    (_t_) greatly enlarged; _ar_, arolia; _p_, pulvillus.—After
    Pagenstecher.
]

Simmermacher states that in most cases of climbing beetles the tubular
tenent hairs pour out a secretion (Figs. 133, 134), “and it is probable
that we have here to do with the phenomena not of actual attachment by,
as it were, gluing, but of adhesion; the orifice of the tubes is divided
obliquely, and the tubes are, at this point, extremely delicate and
flexible, so as to adhere by their lower surface; in this adhesion they
are aided by the secreted fluid.” In the case of the Diptera he does not
accept the theory by which the movement of the fly along smooth surfaces
is ascribed to an alternate fixation and separation, but believes in a
process of adhesion, aided by a secretion, as in many Coleoptera. (In
the Cerambycidæ there is no secretion, and the tubules are merely
sucking organs, like those observed in the male Silphidæ.) “The
attaching lobes, closely beset with chitinous hairs, are enabled, in
consequence of the pressure of the foot, to completely lie along any
smooth surface; this expels the air beneath the lobes, which are then
acted on by the pressure of the outer air.” (Journ. Roy. Micr. Soc.,
1884, p. 736.) Another writer (Rombouts) thinks this power is due to
capillary adhesion.

[Illustration:

  FIG. 132.—Fore leg of ♂ Dyticus, under side, with sucker, formed of 3
    enlarged tarsal joints: with a small cupule highly magnified. ×
    120.—After Miall.
]

The action of the pulvillus and claws when at rest or in use by the
honey-bee is well shown by Cheshire (Fig. 135, _B_). In ascending a
rough surface, “the points of the claws catch (as at _B_) and the
pulvillus is saved from any contact, but if the surface be smooth, so
that the claws get no grip, they slide back and are drawn beneath the
foot (as at _A_), which change of position applies the pulvillus, so
that it immediately clings. It is the character of the surface, then,
and not the will of the bee, that determines whether claw or pulvillus
shall be used in sustaining it. But another contrivance, equally
beautiful, remains to be noticed. The pulvillus is carried folded in the
middle (as at _C_, Fig. 105), but opens out when applied to a surface;
for it has at its upper part an elastic and curved rod (_cr_, Figs. 105
and 135), which straightens as the pulvillus is pressed down; _C_ and
_D_, Fig. 135, making this clear. The flattened-out pulvillus thus holds
strongly while pulled, by the weight of the bee, along the surface, to
which it adheres, but comes up at once if lifted and rolled off from its
opposite sides, just as we should pull a wet postage stamp from an
envelope. The bee, then, is held securely till it attempts to lift the
leg, when it is freed at once; and, by this exquisite yet simple plan,
it can fix and release each foot at least twenty times per second.”
(Bees and Bee-keeping, p. 127.)

[Illustration:

  FIG. 133.—Cross-section through a tarsal joint of fore leg of Dyticus,
    ♂, showing the stalked chitinous suckers (_s_), with a marginal
    bristle on each side: _t_, trachea; _a_, an isolated tubule or
    sucker of Loricera,—_b_, of Chlænius,—_c_, of Cicindela; _d_, two
    views of one of _Necrophorus germanicus_, ♂.
]

[Illustration:

  FIG. 134.—Section through the tarsus of a Staphylinid beetle; the
    glandular or tenent hairs arising from chitinous processes. _A_,
    section through the tarsal joint of the pine weevil, _Hylobius
    abietis_, showing the crowded, bulbous, glandular, or tenent hairs
    arising from unicellular glands.—This and Fig. 133 after
    Simmermacher.
]

Ockler divides the normal two-clawed foot into three subtypes: (1) with
an unpaired median empodium; (2) with two outer lateral adhesive lobes;
(3) with two adhesive lobes below the claws; the latter is the chief
type and forms either a climbing or a clasping foot. The amount of
movement possessed by the claws is limited, and what there is, is
effected by means of an elastic membrane and the extensor plate (Fig.
110). The “extensor sole” which is always present in insects with an
unpaired median fixing or adhesive organ (empodium) is to be regarded as
a modification of the extensor seta. The extensor plate is peculiar to
an insect’s foot. Ockler states that the so-called “pressure plate” of
Dahl is only a movably articulated, skeletal, supporting plate for the
median fixing lobule.

[Illustration:

  FIG. 135.—Honey-bee’s foot in the act of climbing, showing the
    automatic action of the pulvillus, × 30: _A_, position of foot in
    climbing on a slippery surface, or glass; _pv_, pulvillus; _fh_,
    tactile hairs; _un_, unguis; _t_, last tarsal joint. _B_, position
    of foot in climbing rough surface. _C_, section of pulvillus just
    touching flat surface; _cr_, curved rod. _D_, the same applied to
    the surface.—After Cheshire.
]

=Climbing.=—In certain respects the power of climbing supplies the want
of wings, and even exists often in house-flies among which there is
shown a many-sided motion that is quite unheard of in other groups of
insects.

The best climbers are obviously those insects which live on trees and
bushes, as, for example, longicorn beetles and grasshoppers. These may
be accurately called the monkeys of the insect kind, even if their
movements take place less gracefully, and indeed rather stiffly and
woodenly. We already know what are the proper climbing organs; that is,
the sharp easily movable claws on the foot. With the help of these claws
certain insects, May-beetles for example, can hang upon one another like
a chain; indeed, bees and ants in this manner bind themselves together
into living garlands and bridges. There are still added to the chitinous
hooks flaps and balls of a sticky nature, by help of which likewise the
insects glue themselves together. To facilitate the spanning of still
thicker twigs, the climbing foot of insects has a greater movability
even than when it only serves as a sole. (Graber.)

=The mode of swimming of insects.=—To study the swimming movements of
insects, let us examine a Dyticus. It will appear, as Graber states, to
be wonderfully adapted to its element.

  “The body resembles a boat. There is nowhere a projecting point or a
  sharp corner which would offer unnecessary resistance to motion;
  bulging out in the middle and pointed at the end, it cuts through
  the resistance of the water like a wedge. The movable parts, the
  oars, seem to be as well fitted for their purpose as the burden to
  be moved by them. That the hind legs must bear the brunt of this
  follows from their position exactly in the middle of the body, where
  it is widest. In other insects also these legs are used for the same
  purpose as soon as the insects are put in the water. But the
  swimming legs of water-beetles are oars of quite peculiar
  construction. _They are not turned about in the coxæ, as are other
  legs, but at the foot-joint._ The coxa, namely, has grown entirely
  together with the thoracic partition. The muscles we have mentioned,
  exceeding in strength all the soft parts taken together, take hold
  directly of the large wing-shaped tendons of the upper thigh, and
  extend and retract the leg in one of the planes lying close to the
  abdominal partition. The foot forms the oar, however. It is very
  much lengthened and still more widened, and can be turned and bent
  in by separate muscles in such a way that in the passive movement,
  that is, the retraction, the narrow edge is turned to the fore, and
  therefore to the medium to be dislodged; however, as soon as the
  active push is to be performed and the leg is extended with greater
  force, it cuts down through the water with its whole width. These
  effective oar-blades are still considerably enlarged by the hairs
  arising on the side of the foot, which spread out at the decisive
  moment.

  “Every one knows that the oar-blades of swimming beetles always go
  up and down simultaneously and in regular time. On the other hand,
  as soon as one puts a Dyticus on the dry land, _i.e._ on an
  unyielding medium, it uses its hind legs entirely after the manner
  of other land insects; that is, they are drawn in and extended again
  _alternately_, as takes place clearly enough from the footsteps in
  Fig. 119, _A_. We learn from this that water insects have not yet,
  from want of practice, forgotten the mode of walking of land
  insects.

  “The forcing up of the water as a propelling power is added to the
  repulsion produced by the strong strokes of the oars. If the beetle
  stood up horizontally in the water, he would be lifted up.

  “As the trunk, however, assumes an oblique position when the insect
  wishes to swim, one can then imagine the driving up of the water as
  being divided into two forces, one of which drives the body forward
  in a horizontal direction, while the other, that is, the vertical
  component, is supplied by the moving of the oars. The swimming
  insect is thus, as it were, a snake flying in the water.

  “The long streamer-like hind legs of many water-bugs, for example
  Notonecta, approach more nearly our artificial oars. These legs are
  turned out from the bottom.

  “There is no doubt but that the legs of insects, as regards the
  many-sidedness and exactitude of their locomotive actions, place the
  similar contrivances of other animals far in the shade. We shall be
  forced to admire these ingenious levers still more, however, when we
  take into consideration their energy and strength. That the force
  with which the locomotive muscles of insects is drawn together is
  enormous compared with that of vertebrates, we may learn if we try
  to subdue the rhythmical movements of the thorax of a large
  butterfly by the pressure of our finger or to open against the
  insect’s will the closed jumping leg of a grasshopper, or the
  fossorial shovel of a mole-cricket.”




                      LITERATURE ON LEGS AND FEET


  =MacLeay, W. S.= On the structure of the tarsus in the tetramerous
    Coleoptera of the French entomologists. (Trans. Linn. Soc. London,
    xv, 1825, pp. 63–73.)

  =Speyer, O.= Untersuchung der Beine der Schmetterlinge. (Isis, 1843,
    pp. 161–207, 243–264.)

  =Pokorsky Joravko, A. von.= Quelques remarques sur le dernier article
    du tarse des Hyménoptères. (Bull. Soc. imp. Natur. Moscou, 1844,
    xvii, pp. 140–159. Ref. in Isis, 1848, v, p. 347.)

  =Rossmassler, E. A.= Das Bein der Insekten. (Aus der Heimath, 1860, 3
    kap., pp. 327–334, Fig.)

  =West, Tuffen.= The foot of the fly; its structure and action;
    elucidated by comparison with the feet of other insects, etc. Part
    I. (Trans. Linn. Soc. London, xxiii, 1861, pp. 393–421, 1 Pl.)

  =Sundevall, C.= On insektenas extremiteter samt deras hufoud och
    munddelar. (Kongl. Vetenskaps Akad. Handlingar. iii, Nr. 9, 1861.)

  =Lindemann, C.= Notizen zur Lehre vom ausseren Skelete der Insekten
    (Gelenke und Muskeln der Füsse). 1 Taf. (Bull. Soc. imp. d. Natur.
    Moscou, xxxvii, 1864, pp. 426–432.)

  =Liebe, O.= Die Gelenke der Insekten. Chemnitz, 1873. 4º. 1 Taf.

  =Canestrini, J.= Ueber ein sonderbares Organ der Hymenopteren. (Zool.
    Anzeiger, 1880, pp. 421, 422.)

  =Dahl, F.= Beiträge zur Kenntnis des Baues und der Funktionen der
    Insektenbeine. (Archiv f. Naturgesch. 1 Jahrg., 1884, pp. 146–193, 3
    Taf. Sep., 48 pp. Vorlauf. Mitteil, in Zool. Anz., 1884, pp. 38–41.)

  =Langer, K.= Ueber den Gelenkbau bei den Arthrozoen. Vierter Beitrag
    zur vergleichenden Anatomie und Mechanik der Gelenke. (Denkschriften
    der Akad. d. Wissensch. Wien, xviii, Bd. Physikal.-mathem. Classe,
    pp. 99–140. 3 Taf.)

  =Graber, Vitus.= Ueber die Mechanik des Insektenkörpers. (Biolog.
    Centralbl., iv, 1884, pp. 560–570.)

  —— Die ausseren mechanischen Werkzeuge der Tiere, ii Teil. Wirbellose
    Tiere, 1886, pp. 175–182, 208–210.

  =Dewitz, H.= Ueber die Fortbewegung der Tiere an senkrechten glatten
    Flächen vermittelst eines Sekretes. 3 Taf. (Pflüger’s Archiv f. d.
    ges. Physiologie, xxxiii, 1884, pp. 440–481.)

  =Ockler, A.= Das Krallenglied am Insektenfuss. (Archiv f. Naturgesch.,
    1890, pp. 221–262, 2 Taf.)




                LITERATURE OF LOCOMOTION (WALKING, ETC.)


  =Carlet, G.= Sur le mode de locomotion des chenilles. (Compt, rend.
    Acad. Paris, 1888, cvii, pp. 131–134. Naturwiss. Rundschau, iii
    Jahrg., 1888, No. 42, p. 543.)

  —— De la marche d’un insecte rendu tetrapode par la suppression d’une
    paire de pattes. (Ibid., pp. 565, 566.)

  —— Sur la locomotion des insectes et des arachnides. (Ibid., 1879, T.
    89, pp. 1124, 1125.)

  —— Ueber den Gang eines vierfüssig gemachten Insekts. (Naturwiss.
    Rundschau, viii Jahrg., 1888, pp. 666–667; Compt. rend. 1888, cvii.)

  =Demoor, J.= Recherches sur la marche des insectes et des arachnides.
    Étude experimentale d’Anatomie et de Physiologie comparées. (Archiv
    de Biologie, Liège, 1880, 42 pp. 3 Pls.)

  —— Ueber das Gehen der Arthropoden mit Berücksichtigung der
    Schwankungen des Körpers. (Compt. rend. Acad. d. Sc. Paris, 1890,
    cxi, pp. 839–840.)

  =Osten-Sacken, C. R. von.= Ueber das Betragen des kalifornischen
    flügellosen Bittacus (apterus McLachl.). (Wiener Ent. Zeit., 1882,
    pp. 123.)

  =Dixon, H. H.= Preliminary note on the walking of some of the
    Arthropoda. (Proc. R. Dublin Soc. vii, pp. 574–578, 1892. Also
    Nature, 1897.)

      Also the works of Graber, Marey, Cheshire, etc.




                LITERATURE OF WALKING ON SMOOTH SURFACES


  =Blackwell, J.= Remarks on the pulvilli of insects. (Trans. Linn. Soc.
    London, xvi, 1831, pp. 487–492, 767–770.)

  =Lowne, B. T.= On the so-called suckers of Dytiscus and the pulvilli
    of insects. (Trans. Roy. Micr. Soc., pp. 267–271, 1871, 1 Pl.)

  =West, Tuffen.= On certain appendages to the feet of insects
    subservient to holding or climbing. (Journ. of the Proceed. Linn.
    Soc. London, Zoölogy, vi, 1862, pp. 26–88.)

  =Dewitz, H.= Ueber die Fortbewegung der Tiere an senkrechten, glatten
    Flächen vermittelst eines Sekrets. (Pflüger’s Archiv f. d. ges.
    Physiologie, xxxiii, 1884, pp. 440–481. 3 Taf. Also Zool. Anzeiger,
    1884, pp. 400–405.)

  —— Wie ist es den Stubenfliegen und anderen Insekten möglich, an
    senkrechten Glaswanden emporzulaufen. (Sitzungsb. Ges. naturf.
    Freunde zu Berlin, 1882, pp. 5–7.)

  —— Weitere Mitteilungen über den Klettern der Insekten (Ibid., 1882,
    pp. 109–113).

  —— Die Befestigung durch einen klebenden schleim beim springen gegen
    senkrechte Flächen. (Zool. Anzeiger, 1883, pp. 273, 274.)

  —— Ueber die Wirkung der Haftlappchen toter Fliegen. (Ent. Nachr., x
    Jahrg., 1884, pp. 286, 287.)

  —— Weitere Mitteilungen über das Klettern der Insekten an glatten
    senkrechten Flächen. (Zoolog. Anzeiger, 1885. viii Jahrg., pp.
    157–159.)

  —— Richtigstellung der behauptungen des Herrn F. Dahl. (Archiv f.
    mikroskop. Anat., 1885, xxvi, pp. 125–128.)

  =Rombouts, J. E.= Ueber die Fortbewegung der Fliegen an glatten
    Flächen. (Zool. Anzeiger, 1884, pp. 619–623.)

  —— De la faculté qu’out les mouches de se mouvoir sur le verre et sur
    les autres corps polis. (Archiv Museum Teyler (2), 4 Part, pp. 16.
    Fig.)

  =Simmermacher, G.= Untersuchungen über Haftapparate an Tarsalgliedern
    von Insekten. (Zeitschr. f. wissensch. Zool. xl, 1884, pp. 481–556.
    3 Taf., 2 Figs. Also Zoolog. Anzeiger, vii Jahrg., 1884, pp.
    225–228.)

  —— Antwort an Herrn Dr. H. Dewitz. (Ibid., pp. 513–517.)

  =Dahl, F.= Die Fussdrüsen der Insekten. (Archiv f. mikroskop. Anat.,
    1885, xxv, pp. 236–263. 2 Taf. See also p. 118.)

  =Emery, C.= Fortbewegung von Tieren an senkrechten und überhangenden
    glatten Flächen. (Biolog. Centralbl., 1884, 4 Bd., pp. 438–443.)

  =Léon, N.= Disposition anatomique des organes de succion chez les
    Hydrocores et les Géocores. (Bull. Soc. des Medec. et Natur, de
    Jassy., 1888.)


                   _d._ The wings and their structure

The insects differ from all other animals except birds in possessing
wings, and as we at the outset have claimed, it is evidently owing to
them that insects are numerically so superior to any other class of
animals, since their power of flight enables them to live in the air out
of reach of many of their enemies, the greatest destruction to insect
life occurring in the wingless larval and pupal stages.

The presence of wings has exerted a profound influence on the shape and
structure of the body, and it is apparently due to their existence that
the body is so distinctly triregional, since this feature is least
marked in the synapterous insects. The wings are thin, broad leaf-like
folds of the integument, attached to the thorax and moved by powerful
muscles which occupy the greater part of the thoracic cavity. The two
pairs of wings are outgrowths of the middle and hinder part of the
thorax, the anterior pair being attached to the mesothoracic and the
hinder pair to the metathoracic segment. The larger pair is developed
from the middle segment of the thorax. The differentiation of the
tergites into scutum, scutellum, etc., is the result of the appearance
of wings, because these sclerites are more or less reduced or effaced in
wingless insects, such as apterous Orthoptera and moths, ants, etc.

The size of the hinder thoracic segments is closely related to that of
the wings they bear. In those Orthoptera which have hind wings larger
than those of the fore pair, the metathorax is larger than the
mesothorax. In such Neuroptera as have the hind wings nearly or quite as
large as the anterior pair, or in the Trichoptera and in the Hepialidæ,
the metathorax is nearly as large as the mesothorax, while in Coleoptera
the metathorax is as large and often much larger. In the Ephemeridæ,
Diptera, and Hymenoptera, which have either only rudimentary (halteres)
or small hind wings, the metathorax is correspondingly reduced in size.

The wings morphologically, as their development shows, are simple
sac-like outgrowths of the integument, _i.e._ of the free hinder edge of
the tergal plates, their place of origin being apparently above the
upper edge of the epimera or pleural sclerites. Calvert[24] however,
regards the upper lamina of the wing as tergal, and the lower, pleural.

The wings in most insects are attached to the thorax by a membrane
containing several little plates of chitin called by Audouin
articulatory epidemes.

The wings, then, are simple, very thin chitinous lamellate expansions of
the integument, which are supported and strengthened by an internal
framework of hollow chitinous tubes.

=The veins.=—The so-called “veins” or “nervures,” which are situated
between the upper and under layers of the wing are so disposed as to
give the greatest lightness and strength to the wings. Hagen has shown
that in the freshly formed wings these two layers can be separated, when
it can be seen that the veins pass through each layer.

These veins are in reality quite complex, consisting of a minute central
trachea enclosed within a larger tube which at the instant the insect
emerges from the nymph, or pupa, as the case may be, is filled with
blood (Fig. 136). Since these tubes at first contain blood, which has
been observed to circulate through them, and since the heart can be most
easily injected through them, they may more properly be called veins
than nervures. The shape and venation of the wings afford excellent
ordinal as well as family and generic characters, while they also enable
the systematist to exactly locate the spots and other markings of the
wings. The spaces enclosed by the veins and their cross-branches are
called cells, and their shape often affords valuable generic and
specific characters.

[Illustration:

  FIG. 136.—Cross-section of wing of Pronuba.—After Spuler.
]

[Illustration:

  FIG. 137.—Cross-section of wing of Pieris: _s_, insertions of
    scales.-After Spuler.
]

The structure of a complete vein is described by Spuler. In a
cross-section of a noctuid moth (_Triphæna pronuba_, Fig. 136) the
chitinous walls are seen to consist of two layers, an outer (U) and
inner (_c_), the latter of which takes a stain and lies next to the
hypodermis (_hy_). In the cavity of the vein is the trachea (_tr_),
which shows more or less distinctly the so-called spiral thread; within
the cavity are also Semper’s “rib” (_r_) and blood-corpuscles (_bc_),
which proves that the blood circulates in the veins of the completely
formed wing, though this does not apply to all Lepidoptera with hard
mature wings. We have been able to observe the same structure in
sections of the wing of Zygæna.

A cross-section of a vein of _Pieris brassicæ_ shows that the large
trachea is first formed, and that it extends along the track between the
protoplasmic threads connecting the two hypodermal layers.

The main tracheæ throw off on both sides a number of secondary branches
showing at their end a cell with an intracellular tracheal structure;
these accessory tracheæ afterwards branch out. The accessory or
transverse tracheæ often disappear, though in some moths they remain
permanently. Fig. 137 _tr_{2}_ represents these secondary veins in the
edge of the fore wing of _Laverna vanella_, arising from a main trachea
(_tr_) passing through vein I (_v_), two of the twigs extending to the
centre, showing that the latter has no homology with a true vein. Only
rarely and in strongly developed thick folds are the transverse tracheæ
provided with a chitinous thickening, as for example in _Cossus
ligniperda_. Since from such accessory tracheæ the transverse veins in
lepidopterous wings are developed, we can recognize in them the
homologies of the net-veins in reticulated venations. There is no
sharply defined difference between reticulated and non-reticulated
venations; no genetic difference exists between the two kinds of
venation, since there occur true Blattidæ both with and without a
reticulated venation (Spuler).

In the fore wings of Odonata, Psocina, Mantispidæ, and most Hymenoptera
is an usually opaque colored area between the costal edge and the median
vein, called the _pterostigma_.

In shape the wings are either triangular or linear oval, and at the
front edge the main veins are closer together than elsewhere, thus
strengthening the wings and affording the greatest resistance to the air
in making the downward stroke during flight. It is noticeable that when
the veins are in part aborted from partial disuse of the wings, they
disappear first from the hinder and middle edge, those on the costal
region persisting. This is seen in the wings of Embiidæ (Oligotoma),
Cynipidæ, Proctotrupidæ, Chalcids, ants, etc.

The front edge of the wing is called the costal, its termination in the
outer angle of the wing is called the apex; the outer edge (termen) is
situated between the apex and the inner or anal angle, between which and
the base of the wing is the inner or internal edge.

While in Orthoptera, dragon-flies, Termitidæ, and Neuroptera the wings
are not attached to each other, in many Lepidoptera they are loosely
connected by the loop and frenulum, or in Hymenoptera by a series of
strong hooks. These hooks are arranged, says Newport, “in a slightly
twisted or spiral direction along the margin of the wing, so as to
resemble a screw, and when the wings are expanded attach themselves to a
little fold on the posterior margin of the anterior wing, along which
they play very freely when the wings are in motion, slipping to and fro
like the rings on the rod of a window curtain.”

At the base of the hind wings of Trichoptera and in the lepidopterous
Micropteryx there is an angular fold (_jugum_) at the base of each wing
(Fig. 138); that of the anterior wings is retained in Eriocephala and
Hepialidæ.

[Illustration:

  FIG. 138.—Venation of fore and hind wings of _Micropteryx purpurella_:
    _j_, jugum, on each wing; _d_, discal vein; the Roman numerals
    indicate veins I.-VIII. and their branches.
]

In the wings of Orthoptera as well as other insects, the fore wings,
especially, are divided into three well-marked areas, the costal,
median, and internal; of these the median area is the largest, and in
grasshoppers and crickets is more or less modified to form the musical
apparatus, consisting of the drum-like resonant area, with the file or
bow.

=The squamæ.=—In the calyptrate Muscidæ, a large scale-like membranous
broad orbicular whitish process is situated beneath the base of the
wing, above the halter; (Fig. 94, 10 _sq._) it is either small or
wanting in the acalyptrate muscids. Kirby and Spence state that when the
insect is at rest the two divisions of this double lobe are folded over
each other, but are extended during flight. Their exact use is unknown.
Kolbe, following other German authors, considers the term _squama_ as
applicable to the whole structure, restricting the term _alula_ to the
other lobe-like division.

  More recently (1890 and 1897) Osten-Sacken recommends “_squamæ_; in
  the plural, as a designation for both of these organs taken
  together; _squama_, in the singular, would mean the posterior squama
  alone, and _antisquama_ the anterior squama alone;” the strip of
  membrane running in some cases between them, or connecting the
  squama with the scutellum, should be called the _post-alar
  membrane_. By a mistake Loew, and others following him, used the
  word _tegula_ for _squama_, but this term should be restricted to
  the sclerite of the mesothorax previously so designated (Fig. 90,
  _A_, _t_). The squama or its two subdivisions has also by various
  authors been termed alula, calypta, squamula, lobulus, axillary
  lobe, aileron, cuilleron, schuppen, and scale. (Berlin Ent.
  Zeitschrift, xli, 1896, pp. 285–288, 328, 338.)

=The halteres.=—In the Diptera the hind wings are modified to form the
_halteres_ or balancers, which are present in all the species, even in
Nycteribia, but are absent in Braula.

  Meinert finds structures in the Lepidoptera which he considers as
  the homologues of the halteres of Diptera. “In the Noctuidæ,” he
  remarks, “I find arising from the fourth thoracic segment (segment
  médiaire), but covered by hair, an organ like the halter of
  Diptera.” (Ent. Tidskrift., i, 1880, p. 168.) He gives no details.

In the Stylopidæ, on the contrary, the fore wings are reduced to little
narrow pads, while the hind wings are of great size.

The _thyridium_ is a whitish spot marking a break in the cubital vein of
the fore wing of Trichoptera; these minute thyridia occur in the fore
wings of the saw-flies; there is also an intercostal thyridium on the
costal part of the wings of Dermaptera.

The fore wings of Orthoptera are thicker than the hinder ones, and serve
to protect the hind-body when the wings are folded; they are sometimes
called _tegmina_. It is noteworthy, that, according to Scudder, in all
the paleozoic cockroaches the fore wings (tegmina) were as distinctly
veined as the hinder pair, “and could not in any sense be called
coriaceous.” (Pretertiary Insects of N. A., p. 39.) Scudder also
observes that in the paleozoic insects as a rule the fore and hind wings
were similar in shape and venation, “heterogeneity making its appearance
in mesozoic times.” In the heteropterous Hemiptera, also, the basal half
of the fore wings is thick and coriaceous or parchment-like, and also
protects the body when they are folded; these wings are called
_hemelytra_. In the Dermaptera the small short fore wings are thickened
and elytriform.

=The elytra.=—This thickening of the fore wings is carried out to its
fullest extent in the fore wings of beetles, where they form the
sheaths, shards, or _elytra_, under which the hind wings are folded. The
indexed costal edge is called the _epipleurum_, being wide in the
Tenebrionidæ. During flight “the elytra are opened so as to form an
angle with the body and admit of the free play of the wings” (Kirby and
Spence). In the running beetles (Carabidæ), also in the weevils and in
many Ptinidae, the hind wings are wanting, through disuse, and often the
elytra are firmly united, forming a single hard shell or case. The
firmness of the elytra is due both to the thickness of the chitinous
deposit and to the presence of minute chitinous rods or pillars
connecting the upper and lower chitinous surfaces.

[Illustration:

  FIG. 139.—Longitudinal section through the edge of the elytrum of
    _Lina ænea_: _gl_, glands; _r_, reservoir; _fb_, fat-body; _m_,
    matrix; _u_, upper,—_l_, lower, lamella.—After Hoffbauer.
]

Hoffbauer finds that in the elytra of beetles of different families the
venation characteristic of the hind wings is wanting, the main tracheæ
being irregular or arranged in closely parallel longitudinal lines, and
nerve-fibres pass along near them, sense-organs being also present. The
fat-bodies in the cavity of the elytra, which is lined with a matrix
layer, besides nerves, tracheæ, and blood, contain secretory vesicles
filled with uric-acid concretions such as occur in the fat-body of
Lampyris. There are also a great many glands varying much in structure
and position, such occurring also in the pronotum (Fig. 139).

  Meinert considers the elytra of Coleoptera to be the homologues of
  the tegulæ of Lepidoptera and of Hymenoptera. He also calls
  attention to the alula observed in Dyticus, situated at the base of
  the elytra, but which is totally covered by the latter. The alulæ of
  these beetles he regards as the homologues of the anterior wings of
  Hymenoptera and Diptera. No details are given in support of these
  views. (Ent. Tidskrift, i, 1880, p. 168.)

  Hoffbauer (1892) also has suggested that the elytra are not the
  homologues of the fore wings of other insects, but of the tegulæ.

  Kolbe describes the alula of Dyticus as a delicate, membranous lobe
  at the base of the elytra, but not visible when they are closed: its
  fringed edge in Dyticus is bordered by a thickening forming a tube
  which contains a fluid. The alula is united with the inner basal
  portion and articulation of the wing-cover, forming a continuation
  of them. Dufour considered that the humming noise made by these
  beetles is produced by the alulets.

  Hoffbauer finds no structural resemblances in the alulæ of Dyticus
  to the elytra. He does not find “the least trace of veins.” They are
  more like appendages of the elytra. Lacordaire considered that their
  function is to prevent the disarticulation of the elytra, but
  Hoffbauer thinks that they serve as contrivances to retain the air
  which the beetle carries down with it under the surface, since he
  almost always found a bubble of air concealed under it; besides,
  their folded and fringed edge seems especially fitted for taking in
  and retaining air. Hoffbauer then describes the tegulæ of the hornet
  and finds them to be, not as Cholodkowsky states, hard, solid,
  chitinous plates, but hollow. They are inserted immediately over the
  base or insertion of the fore wings, being articulated by a
  hinge-joint, the upper lamella extending into a cavity of the side
  of the mesothorax, and connected by a hinge-like, articulating
  membrane with the lower projection of the bag or cavity. The lower
  lamella becomes thinner towards the place of insertion, is slightly
  folded, and merges without any articulation into the thin, thoracic
  wall at a point situated over the insertion of the fore wing. The
  tegulæ also differ from the wings in having no muscles to move them,
  the actual movements being of a passive nature, and due to the
  upward and downward strokes of the wings.

  Comstock adopts Meinert’s view that the elytra are not true fore
  wings, but gives no reasons. (Manual, p. 495.)

  Dr. Sharp,[25] however, after examining Dyticus and Cybister,
  affirms that this structure is only a part of the elytron, to which
  it is extensively attached, and that it corresponds with the angle
  at the base of the wing seen in so many insects that fold their
  front wings against the body. He does not think that the alula
  affords any support to the view that the elytra of beetles
  correspond with the tegulæ of Hymenoptera rather than with the fore
  wings.

  That the elytra are modified paraptera (tegulæ) is negatived by the
  fact that the latter have no muscles, and that the elytra contain
  tracheæ whose irregular arrangement may be part of the modified
  degenerate structure of the elytra. Kolbe finds evidences of veins.
  The question may also be settled by an examination of the structure
  of the pupal wings. A study of a series of sections of both pairs of
  wings of the pupa of Doryphora and of a Clytus convinces us that the
  elytra are the homologues of the fore wings of other insects.


            _e._ Development and mode of origin of the wings

=Embryonic development of the wings.=—The wings of insects are
essentially simple dorsal outgrowths of the integument, being
evaginations of the hypodermis. They begin to form in the embryo before
hatching, first appearing as folds, buds, or evaginations, of the
hypodermis, which lie in pouches, called peripodal cavities. They are
not visible externally until rather late in larval life, after the
insect, such as a grasshopper, has moulted twice or more times; while in
holometabolous insects they are not seen externally until the pupa state
is attained.

The subject of their origin is in a less satisfactory state than
desirable from the fact that at the outset the development of the wings
of the most generalized insects, such as Orthoptera, Termes, etc., was
not first examined, that of the most highly modified of any insects,
_i.e._ the Muscidæ, having actually been first studied.

In the course of his embryological studies on the Muscidæ (_Musca
comitoria_ and _Sarcophaga carnaria_) Weismann (1864) in examining the
larvæ of these flies just before pupation, found that the wings, as well
as the legs and mouth-appendages, developed from microscopic masses of
indifferent cells, which he called “imaginal discs.” From the six
imaginal discs or buds in the lower part of the thorax arise the legs,
while from four dorsal discs, two in the meso- and two in the
metathoracic segment, arise the fore and hind wings (Fig. 141.) These
imaginal buds, as we prefer to call these germs, usually appear at the
close of embryonic life, being found in freshly hatched larvæ.

[Illustration:

  FIG. 140.—Imaginal buds in Musca,—_A_, in Corethra,—_B_, in
    Melophagus,—_C_, in embryo of Melophagus; dorsal view of the head;
    _b_, bud; _p_, peripodal membrane; _c_, cord; _hy_, hypodermis;
    _cl_, cuticula; _st_, stomodæum; _v_, ventral cephalic, behind are
    the two dorsal cephalic buds.—After Pratt.
]

As first observed by Weismann, the buds are, like those of the
appendages, simply attached to tracheæ and sometimes to nerves, in the
former case appearing as minute folds or swellings of the peritoneal
membrane of certain of the tracheæ. In Volucella the imaginal buds were,
however, found by Künckel d’Herculais to be in union with the
hypodermis. Dewitz detected a delicate thread-like stalk connecting the
peripodal membrane with the hypodermis, and Van Rees has since proved in
Musca, and Pratt in Melophagus, the connection of the imaginal buds with
the hypodermis (Fig. 140). These tracheal enlargements increase in size,
and become differentiated into a solid mass which corresponds to the
upper part of the mesothorax, while a tongue-shaped continuation becomes
the rudiment of the wing. During larval life the rudiments of the wings
crumple, thus forming a cavity. While the larva is transforming into the
pupa, the sheath or peripodal membranes of the rudimentary wings are
drawn back, the blood presses in, and thus the wings are everted out of
the peripodal cavities.

  Due credit, however, should be given to Herold, as the pioneer in
  these studies, who first described in his excellent work on the
  development of _Pieris brassicæ_ (1815) the wing-germs in the
  caterpillar after the third moult. This discovery has been
  overlooked by recent writers, with the exception of Gonin, whose
  statement of Herold’s views we have verified. Herold states that the
  germs of the wings appear on the inside of the second and third
  thoracic segments, and are recognized by their attachment to the
  “protoplasmic network” (_schleimnetz_), which we take to be the
  hypodermis, the net-like appearance of this structure being due to
  the cell-walls of the elements of the hypodermal membrane. These
  germs are, says Herold, also distinguished from the flakes of the
  fat-body by their regular symmetrical form. Fine tracheæ are
  attached to the wing-germs, in the same way as to the flakes of the
  fat-body. It thus appears that Herold in a vague way attributes the
  origin of these wing-germs, and also the germs of the leg, to the
  hypodermis, since his schleimnetz is the membrane which builds up
  the new skin. Herold also studied the later development of the
  wings, and discovered the mode of origin of the veins, and in a
  vague way traced the origin of the scales and hairs of the body, as
  well as that of the colors of the butterfly.

  Herold also says that as the caterpillar grows larger, and also the
  wing-germs, “the larval skin in the region under which they lie
  hidden is spotted and swollen,” and he adds in a footnote: “This is
  the case with all smooth caterpillars marked with bright colors. In
  dark and hairy caterpillars the swelling of the skin through the
  growth of the underlying wing-germs is less distinct or not visible
  at all” (pp. 29, 30).

  It should be added that Malpighi, Swammerdam, and also Réaumur had
  detected the rudiments of the wings in the caterpillar just before
  pupation under the old larval skin. Lyonet (1760) also describes and
  figures the four wing-germs situated in the second and third
  thoracic segments, but was uncertain as to their nature. Each of
  these masses, he says, is “situated in the fatty body without being
  united to it, and is attached to the skin in a deep fold which it
  makes there.” He could throw no certain light on their nature, but
  says: “their number and situation leads to the supposition that they
  may be the rudiments of the wings of the moth” (pp. 449, 450).

During the transformation into the pupa the imaginal buds unite and grow
out or extend along their edges, while the enveloping membrane
disappears. The rudimentary wings are now like little sacs, and soon
show a fusion of the two wing-membranes or laminæ with the veins, while
the tracheæ disappear, the places occupied by the tracheæ becoming the
veins. “Very early, as soon as the scales are indicated, begin in a very
peculiar way the fusion of the wing-laminæ. There occur openings in the
hypodermis into which the cells extend longitudinally and then laterally
give way to each other. Hence no complete opening is found, but the
epithelium appears by sections through a straight line sharply bordered
along the wingcavity. It is a continuous membrane formed of plasma which
I will call the ground membrane of the epithelium. Through this ground
membrane pass blood-corpuscles as well as blood-lymph.” (Schaeffer.)

[Illustration:

  FIG. 141.—Anterior part of young larva of _Simulium sericea_, showing
    the thoracic imaginal buds: _p_, prothoracic bud (only one not
    embryonic); _w_, _w′_, fore and hind wing-buds; _l_, _l′_, _l″_,
    leg-buds; _n_, nervous system; _br_, brain; _e_, eye; _sd_, salivary
    duct; _p_, prothoracic foot.—After Weismann.
]

Afterwards (1866) Weismann studied the development of the wings in
_Corethra plumicornis_, which is a much more primitive and generalized
form than Musca, and in which the process of development of the wings is
much simpler, and, as since discovered, more as in other holometabolous
insects. He also examined those of Simulium (Fig. 141).

  In Corethra, after the fourth and last larval moulting, there arises
  at first by evagination and afterwards by invagination a cup-shaped
  depression on each side in the upper part of the mesothoracic
  segment within which the rudiment of the wings lies like a plug. The
  wings without other change simply increase in size until, in the
  transformation into the pupa by the withdrawal of the hypodermis,
  the wings project out and become filled with blood, the tracheæ now
  being wholly wanting, and other tissues being sparingly present.

[Illustration:

  FIG. 142.—Section through thorax of a Tineid larva on sycamore,
    passing through the 1st pair of wings (_w_): _ht_, heart; _i_,
    œsophagus; _s_, salivary gland: _ut_, urinary tube; _nc_, nervous
    cord; _m_, recti muscles; a part of the fat body overlies the heart.
    _A_, right wing-germ enlarged.
]

  These observations on two widely separate groups of Diptera were
  confirmed by Landois, and afterwards by Pancritius, for the
  Lepidoptera, by Ganin for the Hymenoptera, by Dewitz for Hymenoptera
  (ants) and Trichoptera; also for the Neuroptera by Pancritius. In
  the ant-lion (_Myrmeleon formicarius_) Pancritius found no rudiments
  of the wings in larvæ a year old, but they were detected in the
  second year of larval life, and do not differ much histologically or
  in shape from those of Lepidoptera. In the Coleoptera and
  Hymenoptera the imaginal buds appear rather late in larval life, yet
  their structure is like that of Lepidoptera. In Cimbex the rudiments
  of the wings are not found in the young larva, but are seen in the
  semipupa, which stage lasts over six weeks.

[Illustration:

  FIG. 143.—Section of the same specimen as in Fig. 142, but cut through
    the second pair of wings (_w_): _i_, mid-intestine; _h_, heart;
    _fb_, fat-body; _l_, leg; _n_, nervous cord.
]

The general relation of the rudiments (imaginal buds) of the wings of a
tineid moth to the rest of the body near the end of larval life may be
seen in Figs. 142, 143 (Tinea?), the sections not, however, showing
their connection with the hypodermis, which has been torn away during
the process of cutting. That the wing is but a fold of the hypodermis is
well seen in Fig. 144, of Datana, which represents a much later stage of
development than in Figs. 142 and 143, the larva just entering on the
semipupa stage.

In caterpillars of stage I, 3 to 4 mm. in length, Gonin found the
wing-germs as in Fig. 145, _A_ being a thickening of the hypodermis,
with the embryonic cells, _i.e._ of Verson, on the convex border. The
two leaves, or sides of the wing, begin to differentiate in stage II
(_C_, _D_), and in stage III the envelope is formed (_E_), while the
tracheæ begin to proliferate, and the capillary tracheæ or tracheoles at
this time arise (Fig. 145, _tc_). The wall of the principal trachea
appears to be resolved into filaments, and all the secondary branches
assume the appearance of bundles of twine. Landois regarded them as the
product of a transformation of the nuclei, but Gonin thinks they arise
from the entire cells, stating that from each cell arises a ball
(peloton) of small twisted tubes.

[Illustration:

  FIG. 144.—Section through mesothoracic segment of _Datana ministra_,
    passing through the wings (_w_): _c_, cuticula; _hyp_, hypodermis:
    _ap_, apodeme; _dm_, dorsal longitudinal.—_vm_, ventral
    longitudinal. muscles; _dmt_, depressor muscle of tergum; _t_,
    trachea; _n_, nerve cords; _i_, intestine; _u_, urinary tubes; _l_,
    insertion of legs.
]

As the large branches penetrate into the wing, the balls (pelotons) of
fine tracheal threads tend to unroll, and each of the new ramifications
of the secondary tracheal system is accompanied in its course by a
bundle of capillary tubes. This secondary system of wing-tracheæ, then,
arises from the mother trachea at the end of the third stage, when we
find already formed the chitinous tunic, which will persist through the
fourth stage up to pupation. It differs from the tracheoles in not
communicating with the air-passage; it possesses no spiral membrane at
the origin, and takes no part in respiration.

Gonin thus sums up the nature of the two tracheal systems in the
rudimentary wing, which he calls the provisional and permanent systems.
“The first, appearing in the second stage of the larva, comprises all
the capillary tubes, and arising from numerous branches passes off from
the lateral trunk of the thorax before reaching the wing; the second is
formed a little later by the direct ramification of the principal
branch.

“These two systems are absolutely independent of each other within the
wing. Their existence is simultaneous but not conjoint. One is
functionally active after the third moult; the other waits the final
transformation before becoming active.”

[Illustration:

  FIG. 145.—_A_, section of wing-bud of larva of _Pieris brassicæ_ of
    stage I, in front of the invagination pit. _B_, section passing
    through the invagination pit. _C_, section of same in stage II,
    through the invagination pit;—_D_, behind it, making the bud appear
    independent of the thoracic wall. _E_, wing-bud at the beginning of
    the 3d larval stage, section passing almost through the pedicel or
    hypodermic insertion, the traces of which appear at _hi_; _h_,
    hypodermis; _t_ or _tr_, trachea; _i_, opening of invagination;
    _ec_, embryonic cells; _l_, external layer or envelope; _in_,
    internal wall of the wing; _ex_, external wall; _s_, cell of a
    tactile hair; _tc_, capillary tubes; _c_, cavity of
    invagination.—After Gonin.
]

=Evagination of the wing outside of the body.=—We have seen that the
alary germs arise as invaginations of the hypodermis; we will now, with
the aid of Gonin’s account, briefly describe, so far as is known, the
mode of evagination of the wings. During the fourth and last stage of
the caterpillar of Pieris, the wings grow very rapidly, and undergo
important changes.

Six or seven days after the last larval moult the chitinous wall is
formed, the wing remaining transparent. It grows rapidly and its lower
edge extends near the legs. It is now much crumpled on the edge, owing
to its rapid growth within the limits of its own segment. Partly from
being somewhat retracted, and partly owing to the irregularity of its
surface, the wing gradually separates from its envelope, and the cavity
of invagination (Fig. 145, _c_) becomes more like a distinct or real
space. The outer opening of the alary sac enlarges quite plainly, though
without reaching the level of the edge of the wing.

This condition of things does not still exactly explain how the wing
passes to the outside of the body. Gonin compares these conditions to
those exhibited by a series of sections of the larva, made forty-eight
hours later, on a caterpillar which had just spun its girdle of silk. At
this time the wings have become entirely external, but, says Gonin, we
do not see the why or the how. The partition of the sac has disappeared,
and with it the cavity and the leaf of the envelope.

  It appears probable that the partition has been destroyed, because
  the space between the two teguments is strewn with numerous bits,
  many of which adhere to the chitinous integument, while others are
  scattered along the edges of the wings, in their folds, or between
  the wings and the wall of the thorax.

  Another series of sections showed that the exit of the fore wings
  had been accomplished, while the hinder pair was undergoing the
  process of eversion. In this case the partition showed signs of
  degeneration: deformation of the nuclei, indistinct cellular limits,
  pigmentation, granular leucocytes, and fatty globules.

  After the destruction of the partition, what remains of the layer of
  the envelope is destined to make a part of the thoracic wall and
  undergoes for this purpose a superficial desquamation. The layer of
  flattened cells is removed and replaced by a firmer epithelium like
  that covering the other regions. It is this renewed hypodermis which
  conceals the wing within, serves to separate it from the cavity of
  the body, and gives the illusion of a complete change in its
  situation. Other changes occur, all forming a complete regeneration,
  but which does not accord with the description of Van Rees for the
  Muscidæ. Finally, Gonin concludes that the débris scattered about
  the wing comes from the two layers of the partition of the sac, from
  the flattened hypodermis of the renewed envelope, from the chitinous
  cuticle of the wing, and from the inner surface of the chitinous
  integument.

  He thinks that the metamorphosis of Pieris is intermediate between
  the two types of Corethra and of Musca, established by Weismann, as
  follows:

  =Corethra.=—The wing is formed in a simple depression of the
  hypodermic wall. No destruction.

  =Pieris.=—The rudiment is concealed in a sac attached to the
  hypodermis by a short pedicel. Destruction of the partition and its
  replacement by a part of the thoracic wall by means of the imaginal
  epithelium.

  =Musca.=—The pedicel is represented by a cord of variable length,
  whose cavity may be obliterated (Van Rees). The imaginal hypodermis
  is substituted for the larval hypodermis, which has completely
  disappeared, either by desquamation (Viallanes), or by histolytic
  resorption (Van Rees).

=Extension of the wing; drawing out of the tracheoles.=—When it is
disengaged from the cavity, the wing greatly elongates and the creases
on its surface are smoothed out; the blood penetrates between the two
walls, and the cellular fibres, before relaxed and sinuous, are now
firmly extended.

Of the two tracheal systems, the large branches are sinuous, and they
are rendered more distinct by the presence of a spiral membrane; but the
two tunics are not separated as in the other tracheæ of the thorax;
moreover, the mouth choked up with débris does not yet communicate with
that of the principal trunk. The bundles of tracheoles on their part
form straight lines, as if the folds of the organ had had no influence
on them. As they have remained bound together, apart from the chitinous
membrane of the tracheal trunk, they become drawn out with this
membrane, at the time of exuviation, _i.e._ of pupation, and are drawn
out of the neighboring spiracle.

[Illustration:

  FIG. 146.—Full-grown larva of _Pieris brassicæ_, opened along the
    dorsal line: _d_, digestive canal; _s_, silk-gland; _g_, brain; _st
    I_, prothoracic stigma; _st IV_, 1st abdominal stigma; _a_, _a′_,
    germs (buds) of fore and hind wings; _p_, bud of prothoracic
    segment;—those of the third pair are concealed under the
    silk-glands; _I–III_, thoracic rings.—After Gonin.
]

  “This is a very curious phenomenon, which can be verified
  experimentally: if we cut off the wing, while sparing the larval
  integument around the thoracic spiracles, we preserve the two
  tracheal systems; the same operation performed after complete
  removal of the larval skin does not give the secondary tracheal
  system.” (Gonin.) Deceived by the appearance of the tracheoles while
  still undeveloped, Landois and Pancritius, who have not mentioned
  the drawing out of the capillaries of the larva, affirm that they
  are destroyed by resorption in the chrysalis.

  “The study of the tracheæ is closely connected with that of the
  veins (nervures). It is well to guard against the error of Verson,
  who mistakes for these last the large tracheal branches of the wing.
  This confusion is easily explained; it proves that Verson had, with
  us, recognized that the secondary system is, in the larva, exempt
  from all respiratory function. Landois thought that the pupal period
  was the time of formation of the veins. It seems to me probable that
  they are derived from the sheath of the peritracheal spaces.”
  (Gonin, pp. 30–33.)

[Illustration:

  FIG. 147.—Left anterior wing of a larva 3 days before pupation. The
    posterior part is rolled up: _st_, prothoracic stigma; _tr. i._,
    internal tracheal trunk; _tr. e._, _tr. e.′_, external tracheal
    trunk; _p_, cavity of a thoracic leg, with the imaginal bud
    _b_.—After Gonin.
]

The appearance of the wing-germs in the fully grown caterpillar, as
revealed by simple dissection, is shown at Fig. 146; Fig. 147 represents
a wing of a larva three days before pupation, with the germ of a
thoracic leg.

[Illustration:

  FIG. 148.—Graber’s diagrams for explaining the origin and primary
    invagination of the hypodermis to form the germs of the leg (_b_),
    and wings (_f_, _A-C_), and afterwards their evagination _D_, so
    that they lie on the outside of the body. _E_, stage _B_, showing
    the hypodermal cavities (_f_) and stalks connecting the germs with
    the hypodermis (_z_).—After Graber.
]

[Illustration:

  FIG. 149.—Section lengthwise through the left wing of mature larva in
    _Pieris rapæ_: _t_, trachea; _hyp_, hypodermis; _c_, cuticula.—After
    Mayer.
]

A. G. Mayer has examined the late development of the wings in _Pieris
rapæ_. Fig. 149 represents a frontal section through the left wing of a
mature larva and shows the rudiment of the wing, lying in its hypodermal
pocket or peripodal cavity. How the trachea passes into the rudimentary
wing, and eventually becomes divided into the branches, around which the
main veins afterwards form, is seen in Figs. 144, 147, 159.

The histological condition of the wing at this time is represented by
Fig. 151, the spindle-like hypodermal cells forming the two walls being
separated by the ground-membrane of Semper.

“While in the pupa state,” says Mayer, “the wing-membrane is thrown into
a very regular series of closely compressed folds, a single scale being
inserted upon the crest of each fold. When the butterfly issues from the
chrysalis, these folds in the pupal wings flatten out, and it is this
flattening which causes the expansion of the wings.... It is evident
that the wings after emergence undergo a great stretching and
flattening. The mechanics of the operation appears to be as follows. The
hæmolymph, or blood, within the wings is under considerable pressure,
and this pressure would naturally tend to enlarge the freshly emerged
wing into a balloon-shaped bag; but the hypodermal fibres (_h_) hold the
upper and lower walls of the wing-membrane closely together, and so,
instead of becoming a swollen bag, the wing becomes a thin flat one. And
thus it is that the little thick corrugated sac-like wings of the
freshly emerged insect become the large, thin, flat wings of the
imago.... The area of the wing of the imago of _Danais plexippus_ is 8.6
times that of the pupa. Now, as the wing of the young pupa has about 60
times the area of the wing in the mature larva, it is evident that in
passing from the larval state to maturity the area of the wings
increases more than 500 times.”

[Illustration:

  FIG. 150.—Diagrammatic reproduction of Fig. 149 showing the wing-germ
    in its peripodal cavity (_p_): _h’drm_, hypodermis; _tr_, trachea;
    _cta_, cuticula; _a_, anterior end.—After Mayer.
]

[Illustration:

  FIG. 151.—Section of the wing-germ, the upper and lower sides
    connected by spindle-like hypodermic cells (_h_), forming the rods
    of the adult wing; _mbr_, ground-membrane of Semper.—After Mayer.
]


                 _f._ The primitive origin of the wings

Farther observations are needed to connect the mode of formation of the
wings in the holometabolous insects with the more primitive mode of
origin seen in the hemimetabolous orders, but the former mode is
evidently inherited from the latter. Pancritius remarks that the
development of the rudiments of the wing in a hypodermal cavity is in
the holometabolic insects to be regarded as a later inherited character,
the external conditions causing it being unknown.

Fritz Müller was the first to investigate the mode of development of the
wings of the hemimetabolic insects, examining the young nymphs of
Termites. He regards the wings as evaginations of the hypodermis, which
externally appear as thoracic scale-like projections, into which enter
rather late in nymphal life tracheæ which correspond to the veins which
afterward arise.

[Illustration:

  FIG. 152.—Rudimentary wing of young nymph of Blatta, with the five
    principal veins developed.
]

The primitive mode of origin of the wings may, therefore, be best
understood by observing the early stages of those insects, such as the
Orthoptera and Hemiptera, which have an incomplete metamorphosis. If the
student will examine the nymphs of any locust in their successive
stages, he will see that the wings arise as simple expansions downward
and backward of the lateral edges of the meso- and metanotum. In the
second nymphal stage this change begins to take place, but it does not
become marked until the succeeding stage, when the indications of veins
begin to appear, and the lobe-like expansion of the notum is plainly
enough a rudimentary wing.

Graber[26] thus describes the mode of development of the wings in the
nymph of the cockroach:

  “If one is looking only at the exterior of the process, he will
  perceive sooner or later on the sides of the meso- and metathorax
  pouch-like sacs, which increase in extent with the dorsal integument
  and at the same time are more and more separated from the body.
  These wing-covers either keep the same position as in the
  flat-bodied Blattidæ, or in insects with bodies more compressed the
  first rudiments hang down over the sides of the thorax. As soon as
  they have exceeded a certain length, these wing-covers are laid over
  on the back. However, if we study the process of development of the
  wings with a microscope, by means of sections made obliquely through
  the thorax, the process appears still more simple. The chief force
  of all evolution is and remains the power of growth in a definite
  direction. In regard to the skin this growth is possible in insects
  only in this way; namely, that the outer layer of cells is increased
  by the folds which are forced into the superficial chitinous skin.
  These folds naturally grow from one moult to another in proportion
  to the multiplication of the cells, and are not smoothed out until
  after the moulting, when the outer resistance is overcome.

[Illustration:

  FIG. 153.—Partial metamorphosis of _Melanoplus femur-rubrum_,
    showing the five nymph stages, and the gradual growth of the
    wings, which are first visible externally in 3, 3_b_,
    3_c_.—Emerton _del._
]

  “As, however, the first wing-layers depend upon the wrinkling of the
  general integument of the body through the increase in the upper
  layer, the further growth of the wings depends in the later stages
  upon the wrinkling of the epidermis of the wing-membrane even, which
  fact we also observe under the microscope when the new wings drawn
  forth from the old covers appear at first to be quite creased
  together. These wing-like wrinkles in the skin are not empty
  pouches, but contain tissues and organs within, which are connected
  with the skin, as the fat of the body, the network of tracheæ,
  muscles, etc. Alongside the tracheæ, running through the former
  wing-pouches and accompanied by the nerves, there are canals through
  which the blood flows in and out.

[Illustration:

  FIG. 154.—Stages in the growth of the wings of the nymph of _Termes
    flavipes_: _A_, young; _a_, a wing enlarged. _B_, older nymph;
    _b_, fore wing; _n_, a vein. _C_, wings more advanced;—_D_,
    mature.
]

[Illustration:

  FIG. 155.—Wings of nymph of Psocus.
]

  “After the last moult, however, when the supply of moisture is very
  much reduced in the wing-pouches, which are contracted at the
  bottom, their two layers become closely united, and afterward grow
  into one single, solid wing-membrane.

  “These thick-walled blood-tubes arising above and beneath the upper
  and lower membrane of the wing are the veins of the wings; the
  development of the creased wings in the pupa of butterflies is
  exactly like that of cockroaches and bugs. The difference is only
  that the folds of integument furnishing the wings with an ample
  store of material for their construction reach in a relatively
  shorter time, that is the space of time between two moults, the same
  extent that they would otherwise attain only in the course of
  several periods of growth in the ametabolous insects.”

[Illustration:

  FIG. 156.—Nymph of _Aphrophora permutata_, with enlarged view of the
    wings and the veins: _pro_, pronotum; _sc_, mesoscutum; 1_ab_, 1st
    abdominal segment.
]

Ignorant of Graber’s paper, we had arrived at the same result, after an
examination of the early nymph-stages of the cockroach, as well as the
locusts, Termites, and various Hemiptera. In all these forms it is
plainly to be seen that the wings are simply expansions, either
horizontal or partly vertical (where, as in locusts, etc., the body is
compressed, and the meso- and metanota are rounded downwards), of the
hinder and outer edge of the meso- and metanotum. As will be seen by
reference to the accompanying figures, the wings are notal (tergal)
outgrowths from the dorsal arch of the two hinder segments of the
thorax. At first, as seen in the young pupal cockroach (Fig. 152) and
locust (Fig. 153, also Figs. 154 and 156) the rudiments of the wings are
continuous with the notum. Late in nymphal life a suture and a
hinge-joint appear at the base of the wing, and thus there is some
movement of the wing upon the notum; finally, the tracheæ are well
developed in the wings, and numerous small sclerites are differentiated
at the base of the wing, to which the special muscles of flight are
attached, and thus the wings, after the last nymphal moult, have the
power of flapping, and of sustaining the insect in the air; they thus
become true organs of flight.

It is to be observed, then, that the wings in all hemimetabolous insects
are outgrowths from the notum, and not from the flanks or pleurum of the
thorax. There is, then, no structure in any other part of the body with
which they are homologous.

[Illustration:

  FIG. 157.—Development of wings of Trichoptera: _A_, portion of
    body-wall of young larva of Trichostegia; _ch_, cuticula, forming at
    _r_ a projection into the hypodermis, _m_; _r_, and _d_, forming
    thus the first rudiment of the wing. _B_, the parts in a larva of
    nearly full size; _a_, _c_, _d_, _b_, the well-developed hypodermis
    of the wing-germ separated into two parts by _r_, the penetrating
    extension of the cuticula; _v_, mesoderm, _C_, wing-pad of another
    Phryganeid freed from its case at its change to the pupa: _b_, _d_,
    outer layer of the hypodermis (_m_) of the body-wall; _v_, inner
    layer within nuclei.—After Dewitz, from Sharp.
]

The same may be said of the true Neuroptera, Trichoptera (Fig. 157), the
Coleoptera, and the Diptera, Lepidoptera, and Hymenoptera. As we have
observed in the house fly,[27] the wings are evidently outgrowths of the
meso- and metanotum; we have also observed this to be most probably the
case in the Lepidoptera, from observations on a Tortrix in different
stages of metamorphosis. It is also the case with the Hymenoptera, as we
have observed in bees and wasps;[28] and in these forms, and probably
all Hymenoptera, the wings are outgrowths of the scutal region of the
notum.

With these facts before us we may speculate as to the probable origin of
the wings of insects. The views held by some are those of Gegenbaur,
also adopted by Lubbock, and originally by myself.[29] According to
Gegenbaur:

  “The wings must be regarded as homologous with the lamellar tracheal
  gills, for they do not only agree with them in origin, but also in
  their connection with the body, and in structure. In being limited
  to the second and third thoracic segments they point to a reduction
  in the number of the tracheal gills. It is quite clear that we must
  suppose that the wings did not arise as such, but were developed
  from organs which had another function, such as the tracheal gills;
  I mean to say that such a supposition is necessary, for we cannot
  imagine that the wings functioned as such in the lower stages of
  their development, and that they could have been developed by having
  such a function.”

[Illustration:

  FIG. 158.—Changes in external form of the young larva of _Calotermes
    rugosus_, showing, in _A_ and _B_, the mode of origin of the
    wing-pads: _A_, newly hatched, with 9 antennal joints, × 8. _B_,
    older larva, with 10 joints, × 8. _C_, next stage, with 11 joints, ×
    8. _D_, larva, with twelve joints; the position of the parts of the
    alimentary canal are shown: _v_, crop; _m_, stomach; _b_, “paunch”;
    _e_, intestine; _r_, heart, × 16⁄3.—After Fritz Müller, from Sharp.
]

If we examine the tracheal gills of the smaller dragon-fly (Agrion), or
the May-flies, or Sialidæ, or Perlidæ, or Phryganeidæ, we see that they
are developed in a very arbitrary way, either at the end of the abdomen,
or on the sternum, or from the pleurum; moreover, in structure they
invariably have but a single trachea, from which minute twigs branch
out;[30] in the wings there are five or six main tracheæ, which give
rise to the veins. Thus, in themselves, irrespective of their position,
they are not the homologues of the gills. The latter are only developed
in the aquatic representatives of the Neuroptera and Pseudoneuroptera,
and are evidently adaptive, secondary, temporary organs, and are in no
sense ancestral, primitive structures from which the wings were
developed. There is no good reason to suppose that the aquatic Odonata
or Ephemerids or Neuroptera were not descendants of terrestrial forms.

To these results we had arrived by a review of the above-mentioned
facts, before meeting with Fritz Müller’s opinions, derived from a study
of the development of the wings of Calotermes (Fig. 158). Müller[31]
states that “(1) The wings of insects have not originated from ‘tracheal
gills.’ The wing-shaped continuations of the youngest larvæ are in fact
the only parts in which air tubes are completely wanting, while tracheæ
are richly developed in all other parts of the body.[32] (2) The wings
of insects have arisen from lateral continuations of the dorsal plates
of the body-segments with which they are connected.”

  Now, speculating on the primary origin of wings, we need not suppose
  that they originated in any aquatic form, but in some ancestral land
  insect related to existing cockroaches and Termes. We may imagine
  that the tergites (or notum) of the two hinder segments of the
  thorax grew out laterally in some leaping and running insect; that
  the expansion became of use in aiding to support the body in its
  longer leaps, somewhat as the lateral expansions of the body aid the
  flying squirrel or certain lizards in supporting the body during
  their leaps. By natural selection these structures would be
  transmitted in an improved condition until they became flexible,
  _i.e._ attached by a rude hinge-joint to the tergal plates of the
  meso- and metathorax. Then by continued use and attempts at flight
  they would grow larger, until they would become permanent organs,
  though still rudimentary, as in many existing Orthoptera, such as
  certain Blattariæ and Pezotettix. By this time a fold or hinge
  having been established, small chitinous pieces enclosed in membrane
  would appear, until we should have a hinge flexible enough to allow
  the wing to be folded on the back, and also to have a flapping
  motion. A stray tracheal twig would naturally press or grow into the
  base of the new structure. After the trachea running towards the
  base of the wing had begun to send off branches into the rudimentary
  structure, the number and direction of the future veins would become
  determined on simple mechanical principles. The rudimentary
  structures beating the air would need to be strengthened on the
  front or costal edge. Here, then, would be developed the larger
  number of main veins, two or three close together, and parallel.
  These would be the costal, subcostal, and median veins. They would
  throw out branches to strengthen the costal edge, while the branches
  sent out to the outer and hinder edges of the wings might be less
  numerous and farther apart. The net-veined wings of Orthoptera and
  Pseudoneuroptera, as compared with the wings of Hymenoptera, show
  that the wings of net-veined insects were largely used for
  respiration as well as for flight, while in beetles and bees the
  leading function is flight, that of respiration being quite
  subordinate. The blood would then supply the parts, and thus
  respiration or aëration of the blood would be demanded. As soon as
  such expansions would be of even slight use to the insect as
  breathing organs, the question as to their permanency would be
  settled. Organs so useful both for flight and aëration of the blood
  would be still further developed, until they would become permanent
  structures, genuine wings. They would thus be readily transmitted,
  and being of more use in adult life during the season of
  reproduction, they would be still further developed, and thus those
  insects which could fly the best, _i.e._ which had the strongest
  wings, would be most successful in the struggle for existence. Thus
  also, not being so much needed in larval life before the
  reproductive organs are developed, they would not be transmitted
  except in a very rudimentary way, as perhaps masses of internal
  indifferent cells (imaginal discs), to the larva, being the rather
  destined to develop late in larval and in pupal life. Thus the
  development of the wings and of the generative organs would go hand
  in hand, and become organs of adult life.[33]

=The development and structure of the tracheæ and veins of the
wing.=—The so-called veins (“nervures”) originate from fine tracheal
twigs which pass into the imaginal discs. A single longitudinal trachea
grows down into the wing-germ (Fig. 147), this branch arising through
simple budding of the large body-trachea passing under the rudiment of
the wing.

[Illustration:

  FIG. 159.—Germ of a hind wing detached from its insertion, and
    examined in glycerine: _i_, pedicel of insertion to the hypodermis;
    _tr_, trachea; _b_, semicircular pad; _e_, enveloping membrane; _c_,
    bundle of capillary tracheoles; the large tracheæ of the wing not
    visible; they follow the course of the bundles of tracheoles.—After
    Gonin.
]

Gonin states that before the tracheæ reach the wing they divide into a
great number of capillary tubes united into bundles and often tangled.
This mass of tracheæ does not penetrate into the wing-germ by one of its
free ends, but spreading over about a third of the surface of the wing,
separates into a dozen bundles which spread out fan-like in the interior
of the wing. (Fig. 159). These ramifications, as seen under the
microscope, are very irregular; they form here and there knots and
anastomoses. They end abruptly in tufts at a little distance from the
edge of the wing. A raised semicircular ridge (_b_) surrounds the base
of the wing, and within this the capillaries are formed, while on the
other side they are covered by a cellular layer.

  Landois, he says, noticed neither the pedicel of the insertion of
  the wing (_i_) nor the ridge (_b_). Herold only states that the
  tracheæ pass like roots into the wing. Landois believed that they
  formed an integral part of it. Dewitz and Pancritius used sections
  to determine their situation.

Fig. 160 will illustrate Landois’ views as to the origin of the tracheæ
and veins. _A_ represents the germ of a hind wing attached to a trachea;
_c_ the elongated cells, in which, as seen at _B_, _c_, a fine tangled
tracheal thread (_t_) appears, seen to be magnified at _C_. The cell
walls break down, and the threads become those which pass through the
centre of the veins.

[Illustration:

  FIG. 160.—Origin of the wings and their veins.—After Landois.
]

[Illustration:

  FIG. 161.-Section of the “rib” of a vein: _c_, cord; _b_, twig.—After
    Schaeffer.
]

=The wing-rods.=—Semper discovered in transverse sections of the wings,
what he called _Flügelrippen_; one such rib accompanying the trachea in
each vein. He did not discover its origin, and his description of it is
said to be somewhat erroneous. Schaeffer has recently examined the
structure, remarking: “I have surely observed the connection of this
cellular tube with the tracheæ. It is found in the base of the wing
where the lumen of the tracheæ is much widened. I only describe the
fully formed rib (_rippe_). In a cross-section it forms a usually
cylindrical tube which is covered by a very thin chitinous intima which
bears delicate twigs (Fig. 161). These twigs are analogous to the
thickened ridge of the tracheal intima. I can see no connection between
the branches of the different twigs. Through the ribs (_rippen_) extend
a central cord (_c_) which shows in longitudinal section a clear
longitudinal streaking. Semper regarded it as a nerve. But the
connection of the tube with the trachea contradicts this view. I can
only regard the cord as a separation-product of the cells of the walls.”

[Illustration:

  FIG. 162.—Parts of a vein of the cockroach, showing the nerve (_n_) by
    the side of the trachea (_tr_); _c_, blood-corpuscles.—After
    Moseley.
]

=Other histological elements.=—These are the blood-lymph, corpuscles,
blood-building masses, and nerves. Schaeffer states that in the immature
pupal wings we find besides the large tracheæ, which are more or less
branched, and in the wing-veins at a later period, blood-corpuscles
which are more or less gorged with nutritive material, and also the
“balls of granules” of Weismann, which are perhaps the “single fat-body
cells” detected by Semper. Schaeffer also states that into the
hypodermal fold of the rudiments of the wings pass peculiar formations
of the fat-body and tracheal system, and connected with the fat-body are
masses of small cells which by Schaeffer are regarded as blood-building
masses.

Fine nerves have also been detected within the veins, Moseley stating
that a nerve-fibre accompanies the trachea in all the larger veins in
the insects he has examined (Fig. 162), while it is present in
Melolontha, where the trachea is absent.




                        LITERATURE ON THE WINGS


  =Jurine, L.= Nouvelle méthode de classer les Hyménoptères et les
    Dipterès. Genève, 1807, 4º pp. 319, 14 Pls.

  —— Observations sur les ailes des Hyménoptères. (Mém. acad. Turin,
    1820, xxiv, pp. 177–214.)

  =Latreille, P. A.= De la formation des ailes des Insectes. (Mém. sur
    divers sujets de l’histoire naturelle des Insectes, etc. Paris,
    1819. Fasc. 8.)

  —— De quelques appendices particuliers du thorax de divers Insectes.
    (Mém. du Mus. d’Hist. nat., 1821, vii, pp. 1–21, 354–363.)

  =Chabrier, J.= Essai sur le vol des insectes. (Mém. du Mus. d’Hist.
    nat., 1820, vi, pp. 410–476; 1821, vii, pp. 297–372; 1822, viii, pp.
    47–99, 349–403.) Separate, pp. 328, 13 Pls.

  =Burmeister, Hermann.= Handbuch der entomologie, i, 1832, pp. 96–106,
    263–267, 494–505.

  —— Untersuchungen über die Flügeltypen der Coleopteren. (Abhandl. d.
    naturf. Ges. Halle, 1854, ii, pp. 125–140, 1 Taf.)

  =Romand, B. E. de.= Tableau de l’aile supérieure des Hyménoptères, 1
    Pl. Paris, 1839. (Revue Zool., ii, pp. 339; Bericht von Erichson für
    1839, pp. 54–56.)

  =Lefebure, A.= Communication verbale sur la ptérologie des
    Lépidoptères. (Annal. Soc. Ent. France, 1842, i, pp. 5–35, 3 Pls.
    Also Revue Zool. Paris, 1842, pp. 52–58, 1 Pl.)

  =Deschamps, B.= Recherches microscopiques sur l’organisation des
    élytres des Coléoptères. (Ann. sc. nat., sér. 3, iii, 1845, pp.
    354–363.)

  =Heer, Oswald.= Die Insektenfauna der Tertiärgebilde von Oeningen und
    Radaboj., 1847, 1. Teil, pp. 75–94.

  =Newman, E.= Memorandum on the wing-rays of insects. (Trans. Ent. Soc.
    London, ser. 2, iii, 1855, pp. 225–231.)

  =Westwood, J. O.= Notes on the wing-veins of insects. (Trans. Ent.
    Soc. London, ser. 2, iv, 1857, pp. 60–64.)

  =Loew, H.= Die Schwinger der Dipteren. (Berlin, Entom. Zeitschr.,
    1858, pp. 225–230.)

  =Saussure, H. de.= Études sur l’aile des Orthoptères. (Ann. scienc.
    nat., 5 sér. x, p. 161.)

  =Schiner, J. R.= Ueber das Flügelgeäder der Dipteren. (Verhdl. k. k.
    Zool.-bot., Ges. Wien, 1864, pp. 193–200, 1 Taf.)

  =Hagen, H. A.= Ueber rationelle Benennung des Geäders in den Flügeln
    der Insekten. (Stettin. Ent. Zeitung, 1870, xxxi, pp. 316–320, 1
    Taf.)

  —— Kurze Bemerkungen über das Flügelgeäder der Insekten. (Wiener
    Entom. Zeit., 1886, v, pp. 311, 312.)

  =Plateau, F.= Qu’est-ce que l’aile d’un insecte? (Stett. Ent. Zeit.
    Jahrg. 32, 1871, pp. 33–42, 1 Taf. Journal d. Zool., ii, 1873, pp.
    126–137.)

  =Moseley, H. N.= On the circulation in the wing of _Blatta orientalis_
    and other insects, etc. (Quart. Journ. Micr. Sc. 1871, xi, pp.
    389–395, 1 Pl.)

  =Roger, Otto.= Das Flügelgeäder der Kafer. Erlangen, 1875, 90 p.

  =Rade, E.= Die westfalischen Donacien und ihre nachsten Verwandten. 3
    Taf. (Vierter Jahresber. d. Westfal Prov.-Vereins f. Wiss. u. Kunst,
    1876, pp. 52–87; Flügel, pp. 61–68.)

  =Katter, F.= Ueber Inseckten, speziell Schmetterlingsflügel. (Entom.
    Nachr., iv, 1878, pp. 279–281, 293–298, 304–309, 321–323.)

  =Hofmann, Georg v.= Ueber die morphologische Deutung der
    Insektenflügel. (Jahresber. d. akad.-naturwiss. Vereins, Graz, v
    Jahrg., 1879, pp. 63–68.)

  =Kolbe, H. J.= Das Flügelgeäder der Psociden und seine systematische
    Bedeutung. (Stettin. Entom. Zeitung, 1880, pp. 179–186, 1 Taf.)

  —— Die Zwischenraume zwischen den Punktstreifen der
    punktiertgestreiften Flügeldecken der Coleoptera als rudimentare
    Rippen aufgefasst. (Jahresber. zool. Sektion d. Westfal. Prov.-Ver.
    f. Wiss. u. Kunst. Münster, 1886, pp. 57–59, 1 Taf.)

  =Lee, A. Bolles.= Les balanciers des Diptères, leurs organes
    sensifères et leurs histologie. (Recueil Zool. Suisse, i, 1885, pp.
    363–392, 1 Pl.)

  =Poppius, Alfred.= Ueber das Flügelgeäder der finnischen
    Dendrometriden. 1 Taf. (Berl. Entom. Zeitschr., 1888, pp. 17–28.)

  =Comstock, J. H.= On the homologies of the wing-veins of insects.
    (American Naturalist, xxi, 1887, pp. 932–934.)

  =Brauer, F.= Ansichten über die paläozoischen Insekten und deren
    Deutung. (Annal. d. k. k. naturhist. Mus. Wien, Bd. i, 1886, pp.
    86–126, 2 Taf.)

  =Brauer, F., und J. Redtenbacher.= Ein Beitrag zur Entwicklung des
    Flügelgeäders der Insekten. (Zool. Anz. 1888, pp. 443–447.)

  =Redtenbacher, J.= Vergleichende Studien über das Flügelgeäder der
    Insekten. 12 Taf. (Annalen d. k. k. naturhist. Hofmuseums zu Wien,
    1886, i, pp. 153–231.)

  =Schoch, G.= Miscellanea entomologica. I. Das Geäder des
    Insektenflügels; II. Prolegomena zur Fauna dipterorum Helvetiae,
    Wissenschaftl. (Beilage z. Programm d. Kantonsschule Zurich, 1889,
    4º, 40 p.)

  =Bondsdorff, A.= von. Ueber die Ableitung der Skulpturverhältnisse bei
    den Deckflügeln der Coleopteren. (Zool. Anz., 1890, xiii Jahrg., pp.
    342–346.)

  =Spuler, Arnold.= Zur Phylogenie und Ontogenie des Flügelgeäders der
    Schmetterlinge. (Zeitschr. wissens. Zool., liii, 597–646, 2 Taf.,
    1892.)

  Also the writings of Adolph, Bugnion, Calvert, Comstock, Diez, Giraud,
    Gonin, Graber, Kellogg, Packard, Pratt, Scudder, Walsh.


                        _g._ Mechanism of flight

=Marey’s views on the flight of insects.=—As we owe more to Marey than
to any one else for what exact knowledge we have of the theory of flight
of insects, the following account is condensed from his work entitled
“Movement.” The exceedingly complicated movements of the wings would
lead us, he says, to suppose that there exists in insects a very complex
set of muscles of flight, but in reality, he claims, there are only the
two elevator and depressor muscles of each wing.[34] And Marey says that
when we examine more closely the mechanical conditions of the flight of
insects, we see that an upward and downward motion given by the muscles
is sufficient to produce all these successive acts, so well coordinated
with each other; the resistance of the air effecting all the other
movements. He also refers to the experiments of Giraud which prove that
the insect needs for flight a rigid main-rib and a flexible membrane.

[Illustration:

  FIG. 163.—The two upper lines are produced by the contacts of a
    drone’s wing on a smoked cylinder. In the middle are recorded the
    vibrations of a tuning-fork (250 vibrations per second) for
    comparison with the frequency of the wing movements. Below are seen
    the movements of the wing of a bee.—After Marey.
]

If we take off the wing of an insect, and holding it by the small joint
which connects it with the thorax, expose it to a current of air, we see
that the plane of the wing is inclined more and more as it is subjected
to a more powerful impulse of the wind. The anterior nervure resists,
but the membranous portion which is prolonged behind bends on account of
its greater pliancy.

The wings of insects may be regarded simply as vibrating wires, and
hence the frequency of their movements can be calculated by the note
produced. Their movements can be recorded directly on a revolving
cylinder, previously blackened with smoke, the slightest touch of the
tip of the wing removing the black and exposing the white paper beneath;
Fig. 163 was obtained in this way. By this method it was calculated that
in the common fly the wings made 330 strokes per second, the bee 190,
the Macroglossus 72, the dragon-fly 28, and the butterfly (_Pieris
rapæ_) 9. Thus the smaller the species, the more rapid are the movements
of the wings.

[Illustration:

  FIG. 164.—Appearance of a wasp flying in the sun: the extremity of the
    wing is gilded.—After Marey.
]

The path or trajectory made by the tip of the wing is like a figure 8.
Marey obtained this by fastening a spangle of gold-leaf to the extremity
of a wasp’s wing. The insect was then seized with a pair of forceps and
held in the sun in front of a dark background, the luminous trajectory
shaping itself in the form of a lemniscate (Fig. 164).

  To determine with accuracy the direction taken by the wing at
  different stages of the trajectory, a small piece of capillary glass
  tubing was blackened in the smoke of a candle, so that the slightest
  touch on the glass was sufficient to remove the black coating and
  show the direction of movement in each limb of the lemniscate. This
  experiment was arranged as shown in Fig. 165. Different points on
  the path of movement were tested by the smoked rod, and from the
  track along which the black had been removed the direction of
  movement was deduced. This direction is represented in the figure by
  means of arrows.

[Illustration:

  FIG. 165.—Experiment to test the direction of movement of an insect’s
    wing: _a_, _a′_, _b_, _b′_, different positions of the smoked rod.
]

=Theory of insect flight.=—“The theory of insect flight,” says Marey,
“may be completely explained from the preceding experiments. The wing,
in its to-and-fro movement, is bent in various directions by the
resistance of the air. Its action is always that of an inclined plane
striking against a fluid and utilizing that part of the resistance which
is favorable to its onward progression.

“This mechanism is the same as that of a waterman’s scull, which as it
moves backwards and forwards is obliquely inclined in opposite
directions, each time communicating an impulse to the boat.”

The mechanism in the case of the insect’s wing is far simpler, however,
than in the process of sculling, since “the flexible membrane which
constitutes the anterior part of the wing presents a rigid border, which
enables the wing to incline itself at the most favorable angle.”

“The muscles only maintain the to-and-fro movement, the resistance of
the air does the rest, namely, effects those changes in surface
obliquity which determine the formation of an 8–shaped trajectory by the
extremity of the wing.”

[Illustration:

  FIG. 166.—Bee flying about in the chamber of the apparatus.—After
    Marey.
]

  Lendenfeld has applied photography to determine the position of the
  wings of a dragon-fly, and Marey has carried chronophotography
  farther to indicate the normal trajectory of the wing, and to show
  the position in flight. Fig. 166 shows a bee in various phases of
  flight. “The insect sometimes assumes almost a horizontal position,
  in which case the lower part of its body is much nearer the
  object-glass than is its head, and yet both extremities are equally
  well defined in the photograph. The successive images are separated
  by an interval of 1⁄20 of a second (a long time when compared to the
  total time occupied by a complete wing movement, _i.e._ 1⁄190 of a
  second). And hence it is useless to attempt to gain a knowledge of
  the successive phases of movement by examining the successive
  photographs of a consecutive series representing an insect in
  flight. Nevertheless an examination of isolated images affords
  information of extreme interest with regard to the mechanism of
  flight.

  “We have seen that owing to the resistance of the air the expanse of
  wing is distorted in various directions by atmospheric resistance.
  Now, as the oscillations during flight are executed in a horizontal
  plane, the obliquity of the wing-surface ought to diminish the
  apparent breadth of the wing. This appearance can be seen in Fig.
  167. There is here a comparison between two Tipulæ: the one in the
  act of flight, the other perfectly motionless and resting against
  the glass window.

[Illustration:

  FIG. 167.—Illustration to show two Tipulæ, one of them remaining
    motionless on the glass, and the other moving its limbs in
    different directions, and setting its body at various
    inclinations: the illustration only represents a small part of a
    long series.—After Marey.
]

  “The motionless insect maintains its wings in a position of vertical
  extension; the plane is therefore at right angles to the axis of the
  object-glass. The breadth of the wing can be seen in its entirety;
  the nervures can be counted, and the rounding off of the extremities
  of the wings is perfectly obvious. On the other hand, the flying
  insect moves its wings in a horizontal direction, and owing to the
  resistance of the air the expanse of the wings is obliquely
  disposed, and only the projection of its surface can be seen in the
  photograph. This is why the extremity of the wings appears as if it
  were pointed, while the other parts look much narrower than normal.
  The extent of the obliquity can be measured from the apparent
  alteration in width, for the projection of this plane with the
  vertical is the sine of the angle. From this it may be gathered that
  the right wing (Fig. 168, third image) was inclined at an angle of
  about 50° with the vertical, say 40° with the horizontal. This
  inclination necessarily varies at different points of the trajectory
  and must augment with the rapidity of movement; the obliquity
  reaching its maximum in those portions of the wings which move with
  the greatest velocity, namely, towards the extremities. The result
  is that the wing becomes twisted at certain periods of the
  movement.” (See the fourth image in Fig. 168.) The position of the
  balancers seems to vary according to that of the wings. (Marey’s
  Movement, pp. 253–257.)

[Illustration:

  FIG. 168.—Tipula in the act of flying, showing the various attitudes
    of the wings and the position of the balancers.
]

=Graber’s views as to the mechanism of the wings, flight, etc.=—Although
in reality insects possess but four wings, nature, says Graber,
evidently endeavors to make them dipteral. This end is attained in a
twofold manner. In the butterflies, bees, and cicadas, the four wings
never act independently of each other, as two individual pairs, but they
are always joined to a single flying plate by means of peculiar hooks,
rows of claws, grooved clamps, and similar contrivances proceeding from
the modified edges of the wings; indeed, this connection is usually
carried so far that the hind wings are entirely taken in tow by the
front, and consequently possess a relatively weak mechanism of motion.
The other mode of wing reduction consists in the fact that one pair is
thrown entirely out of employment. We observe this for instance in bugs,
beetles, grasshoppers, etc.

  In the meantime, then, we may not trust to appearances. As their
  development indeed teaches us, the wings as well as the additional
  members must be regarded as actual evaginations of the common
  sockets of the body, and in order especially to refute the prevalent
  opinion that these wing-membranes are void of sensation, it should
  be remembered that Leydig has proved the existence, as well as one
  can be convinced by experiment, of a nerve-end apparatus in certain
  basal or radical veins of the wing-membrane, which is very extensive
  and complicated, and therefore indicates the performance of an
  important function, perhaps of a kind of balancing sense, and also
  that these same insect wings, with their delicate membrane, are very
  easily affected by different outside agents, as, for instance,
  warmth, currents of air, etc.

Usually in their inactive or passive state the wings are held off
horizontally from the body during flight, and are laid upon the back
again when the insect alights; but an exception occurs in most
butterflies and Neuroptera, among which the wing-joint allows only one
movement round the oblique and long axis of the wings. From this cause,
too, the insects just mentioned can unfold their wings suddenly.

[Illustration:

  FIG. 169.—Anterior part of a Cicada for demonstrating the mechanism of
    the articulation of the fore wing: _a_, articular head; _b_,
    articular pan, frog, or cotyla; _g_, elastic band; _c_, _d_, _e_,
    system of elastic rods; _r__{1}, _r__{2}, 1st and 2d abdominal
    segments. _HF_, hind wings.—After Graber.
]

The transition of the wings from the active to the resting condition
seems to be by way of a purely passive process, which, therefore,
usually gives no trouble to the insect. The wing being extended by the
tractive power of the muscles, flies back, when this ceases, to its
former or resting posture by means of its natural elasticity, like a
spiral spring disturbed from its balance. The structure of this spring
joint is very different, however.

  It usually consists (Fig. 169) of two parts. The wing can move
  itself up and down in a vertical plane by means of the forward
  joint, and at the same time can rotate somewhat round its long axis,
  because the chitinous part mentioned above is ground off after the
  fashion of a mandrel.

  The hinder joint, at a greater distance from the body, virtually
  consists of a rounded piece (_a_) capitate towards the outside, and
  of a prettily hollowed socket (_b_) formed by the union of the thick
  ribs of the hind wings, which slides round the head joint when the
  wings snap back upon the back. The mechanism which causes this
  turning is, however, of a somewhat complicated nature. The most
  instrumental part of it is the powerful elastic band (_g_) which is
  stretched over from the hinder edge of the mesothorax (_R_{2}_)
  towards that of the wings. This membrane is extended by the
  expansion of the wings, and draws them towards the body as soon as
  the contraction of the muscles relaxes. This closing band of the
  wings is assisted by a leverage system consisting of three little
  chitinous rods (_c_, _d_, _e_), which at its joining presses inwards
  on the body on one side, and on the hinder edge and head-joint of
  the wing on the other.

  We must, however, lay great stress on a few more kinds of wing
  support.

[Illustration:

  FIG. 170.—Mesothoracic skeleton of a stag beetle: _schi_, scutellum,
    on each side of which is the articulation of the fore wing (_V_),
    consisting of two small styliform processes (_v_, _h_) of the base
    of the wing; _za_, tooth which fits into the cavity of the
    wing-lock (_gr_); _l_, edge of the right wing, passing into the
    corresponding groove (_fa_) of the left; _Di_, diaphragm for the
    attachment of the tergal muscle of the metasternum; _Di_{1}_ (not
    explained by author); _Ka_, acetabulum of the coxa (_Hü_); _Se_,
    chitinous process for the attachment of the coxal muscle; _Fe_,
    femur; _Sch_, tibia; _B_{2}_, sternum.—After Graber.
]

  The wing-cases of beetles at their return from flight are joined
  together like the shells of a mussel on the inside as well as to the
  wedge-shaped plate (Fig. 170, _schi_) between their bases. There is
  even a kind of clasp at hand for this purpose. The base of the wing,
  that is, bears a pair of tooth-like projections (_za_), which fit
  into the corresponding hollows of the little plate.

  The commissure arising from the joining of the inner edges is
  characteristic. Usually the wings on both sides interlock by means
  of a groove, as in stag-beetles, but sometimes even, as in Chlamys,
  after the manner of two cog-wheels, so that we have here also an
  imitation of the two most prevalent methods which the cabinet-maker
  uses in joining boards together.

  The act of folding the broad hind wings among beetles is not less
  significant than the arrangement of the fore wing. If we forcibly
  spread out the former in a beetle which has just been killed and
  then leave it to its own resources again, we observe the following
  result: According to its peculiar mode of joining, the costal vein
  on the fore edge approaches the mid or discoidal vein of the basal
  half as well as the distal half of the wing, whence arises a
  longitudinal fold which curves in underneath. Then the distal half
  snaps under like the blade of a pocket knife and lies on the plane
  of the costal edge of the wing, while it also draws after it the
  neighboring wing-area. The soft hinder-edge portion turns in
  simultaneously when this wing-area remains fixed to the body while
  the costal portion is moving towards the middle line of the body.

  The wing-membranes of almost all insects have, moreover, the
  capability of folding themselves somewhat, and this power of
  extending or contracting the wing-membrane at will is of great
  importance in flight.

  Yes, but how is the folded wing spread out again? The fact may be
  shown more simply and easily than one might suppose, and may be most
  plainly demonstrated even to a larger public by making an artificial
  wing exactly after the pattern of the natural one, in which bits of
  whalebone may take the place of veins and a piece of india rubber
  the membrane spread out between them. The reader will be patient
  while we just explain to him the act of unfolding of the membranous
  wing of the beetle. The actual impulse for this unfolding is due to
  the flexor muscles which pull on, and at the same time somewhat
  raise the vein on the costal edge. By this means the membranous fold
  lying directly behind the costal vein is first spread out. But since
  this fold is connected with the longitudinal fold of the distal end
  of the wing which closes like a blade, the wing-area last mentioned
  which is attached to the middle fold of the wing by the elastic
  spring-like diagonal vein becomes stretched out. The hinder rayed
  portion adjacent to the body is, on the other hand, simply drawn
  along when the wing stands off from the body.

  In order to properly grasp the mechanism of the insect wing we must
  again examine its mode of articulation to the body somewhat more
  accurately.

[Illustration:

  FIG. 171.—Longitudinal section through a Tipula: _a_, mouth; _an_,
    antenna; _k_{3}_, maxillary palpus; _ol_, labrum; _oG_, brain;
    _uG_, subœsophageal ganglion; _BG_, thoracic ganglion; _schl_,
    œsophagus; _mD_, digestive canal; _Ov_, ovary; _vF_, fore wing;
    _sch_, halter; _lm_, longitudinal—_b-r_, lateral muscles.—After
    Graber.
]

  If we select the halteres of a garden gnat (Tipula) at the moment of
  extension, we shall find them to be formed almost exactly after the
  pattern of our oars, since the oblong oar-blade passes into a
  longitudinal handle. The pedicel of the balancer is formed by the
  thick longitudinal primary veins of the wing-membrane. This pedicel
  (Fig. 171) is implanted in the side of the thorax in such a manner
  that the wing may be compared to the top of a ninepin. One may
  think, and on the whole it is actually the fact, that the stiff
  pedicel of the wing is inserted in the thoracic wall, and that a
  short portion of it (Fig. 172), projects into the cavity of the
  thorax. It is true there is no actual hole to be found in the
  thoracic wall, as the intermediate space between the base or pedicel
  of the wing and the aperture in the thorax is lined with a thin
  yielding membrane, on which the wing is suspended as on an
  axle-tree. According to this, therefore, the insect wing, as well as
  any other appendage of arthropods, acts as a lever with two arms.
  The reader can then conjecture what may be the further mechanism of
  the wing machine. We only need now two muscles diametrically opposed
  to each other and seizing on the power arm of the wing, one of which
  pulls down the short wing arm, thereby raising the oar, while the
  other pulls up the power arm. And indeed the raising of the wing
  follows in the manner indicated, since a muscle (_hi_) is attached
  to the end of the wing-handle (_a_) which projects freely into the
  breast cavity by the contraction of which the power arm is drawn
  down.

[Illustration:

  FIG. 172.—Scheme of the flying apparatus of an insect: _mnl_,
    thoracic walls; _ab_, wings; _c_, pivot; _d_, point of insertion
    of the depressor muscle of the wing (_kd_);—_a_, that of the
    elevator of the wing (_ai_); _rs_, muscle for expanding,—_ml_, for
    contracting, the walls of the thorax.—After Graber.
]

[Illustration:

  FIG. 173.—Muscles of the fore wing of a dragon-fly (_an_, _ax_),
    exposed by removing the thoracic walls: _h_{1}_, _h_{2}_,
    elevators,—_s_{1}_-_s_{5}_, depressors, of the wings (_s_{1}_,
    _s_{2}_, rotators).—After Graber.
]

  On the other hand, we have been entirely mistaken in reference to
  the mechanism which lowers the wings. The muscle concerned, that is
  _kd_, is not at all the antagonist of the elevator muscle of the
  wing, since it is placed close by this latter, but nearer to the
  thoracic wall. But then, how does it come to be the counterpart of
  its neighbor? In fact, the lever of the wing is situated in the
  projecting piece alone. The extensor muscle of the wing does not
  pull on the power arm, but on the resistant arm on the other side of
  the fulcrum (_c_). The illustration shows, however, how such a case
  is possible. The membrane of the joint fastening the wing-stalk to
  the thorax is turned up outwards below the stalk like a pouch. The
  tendon of the flexor of the wing passes through this pouch to its
  point of attachment (_c_) lying on the other side of the fulcrum
  (_d_). Thus it is very simply explained how two muscles which act in
  the same direction can nevertheless have an entirely contrary
  working power.

  This is in a way the bare physical scheme of the flying machine by
  the help of which we shall more easily become acquainted with its
  further details.

  Dragon-flies are unquestionably the most suitable objects for the
  study of the muscles pulling directly on the wing itself. If the
  lateral thoracic wall (Fig. 173) be removed or the thorax opened
  lengthwise there appears a whole storehouse of muscular cords which
  are spread out in an oblique direction between the base of the wing
  and the side of the thoracic plate. There is first to be
  ascertained, by the experiment of pulling the individual muscles in
  the line with a pincers, which ones serve for the lifting and which
  for the lowering of the wings. In dragon-flies the muscles are
  arranged in two rows and in such a way that the flexors or
  depressors (_s_, 1 bis) cling directly to the thoracic wall (compare
  also the muscle _dk_ in Fig. 172 and _se_ in Fig. 174), while the
  raiser or extensor (_h_ 1, to _h_ 2, Fig. 172, _hi_ and Fig. 174
  _he_) lie farther in. The form of the wing-muscles is sometimes
  cylindrical, sometimes like a prism, or even ribbon-like. However,
  the contracted bundles of fibres do not come directly upon the
  joint-process we have described, but pass over often indeed at a
  very considerable distance from them, into peculiar chitinous
  tendons. These have the form of a cap-like plate, often serrate on
  the edge, which is prolonged into a thread, which should be
  considered as the direct continuation of the base of the wings. The
  wings, therefore, sink down into the thoracic cavity as if they were
  a row of cords ending in handles where the strain of the muscles is
  applied.

[Illustration:

  FIG. 174.—Transverse section through the thorax of a locust
    (Stenobothrus): _b_{1}_, leg; _h_, heart; _ga_, ventral cord;
    _se_, depressor,—_he_, elevator, of the wing (_fl_); _b-r_,
    lateral muscles which expand the thoracic walls;—_lm_,
    longitudinal muscles which contract them; _shm_, _uhm_, muscles to
    the legs; _bg_, apodemes.—After Graber.
]

[Illustration:

  FIG. 175.—Inner view of a portion of the left side of body of
    _Libellula depressa_, showing a part of the mechanism of flight,
    viz., some of the chitinous ridges at base of the upper wing, and
    some of the insertions of the tendons of muscles: _A_, line of
    section through the base of the upper wing, the wing being
    supposed to be directed backwards. _C_, upper portion of mechanism
    of the lower wing; _b_, lever extending between the pieces
    connected with the two wings.—After von Lendenfeld, from Sharp.
]

  As may be seen in Fig. 173, the contractile section of several of
  the muscles of the wing (_s_{5}_) is extraordinarily reduced, while
  its thread-like tendon is proportionately longer. This gradation
  being almost like that of the pipes of an organ in the length of the
  wing-muscles, as may so easily be observed in the large
  dragon-flies, plainly indicates that the strain of the individual
  muscles is quite different in strength, since, as the phenomenon of
  flight demands it, the different parts of the base of the wing
  become respectively relaxed in very dissimilar measure.

  We have thus far discussed only the elevator and depressor muscles.
  Other groups (_s^1s^3_) are yet to be added, however, crossing under
  the first at acute angles, which when pulling the wing sidewise,
  bring about in union with the other muscles a screw-like turning of
  the wings.

  While in dragon-flies all the muscles which are principally
  influential in moving the wing are directly attached to it, and thus
  evidently assert their strength most advantageously, the case is
  essentially different with all other insects. Here, as has already
  been superficially mentioned above, the entire set of muscles
  affecting the wing is analyzed into two parts of which the smaller
  only is usually directly joined to the wings, while the movement is
  indirectly influenced by the remainder (Graber).

  In the dragon-fly the two wings are “brought into correlative action
  by means of a lever of unusual length existing amongst the chitinous
  pieces in the body wall at the base of the wings (Fig. 175, _b_).
  The wing-muscles are large; according to von Lendenfeld there are
  three elevator, five depressor, and one abductor muscles to each
  wing. He describes the wing-movements as the results of the
  correlative action of numerous muscles and ligaments, and of a great
  number of chitinous pieces connected in a jointed manner” (Sharp).

  If again we take the longitudinal section of the thoracic cavity of
  gnats in Fig. 171, we shall perceive a compactly closed system of
  muscular bars intersecting each other almost at right angles and
  interlaced with a tangled mass of tracheæ, some of which muscles
  extend (_lm_) longitudinally, that is from the front to the back,
  while others (_b-r_) stretch out in a vertical direction, that is
  between the plates of the abdomen and back.

  In order that we may more easily comprehend this important muscular
  apparatus we will illustrate the thoracic cavity of insects by an
  elastic steel ring (Fig. 172), to which we may affix artificial
  wings. If this ring be pressed together from above downward, along
  the line _rs_, thus imitating the pulling of the vertical or lateral
  thoracic muscles, then the wings on both sides spring up. This is to
  be explained by the fact that through this manipulation a pressure
  is exerted on the lifting power arm of the wings. If, on the other
  hand, the ring be compressed on the sides (_ml_), which is the same
  thing as if the longitudinal muscles contracted the thorax from
  before backward, and thus arched it more, then the wings are
  lowered.

  Agrioninæ, according to Kolbe, can fly with the fore pair of wings
  or with the hind pair almost as well as with both pairs together.
  Also the wings of these insects can be cut off before the middle of
  their length without injuring their power of flight. Butterflies,
  Catocalæ, and Bombycidæ fly after the removal of the hind wings.
  Also the balancers of the Diptera must be useful in flying, since
  their removal lessens the power of flight.

  Chabrier regarded the under sides of the shell-like extended
  wing-covers of the beetles as wind-catchers, which, seized by wind
  currents, carry the insect through the air. We may also consider the
  wing-covers as regulators of the centre of gravity of flight.

  The observations of insects made by Poujade (Ann. Soc. Ent., France,
  1887, p. 197) during flight teaches us, says Kolbe, that in respect
  to the movement during flight of both pairs of wings, they may be
  divided into two categories:—

  1. Into those where both pairs of wings (together), either united,
  and also when separated from each other, perform flight. Such are
  the Libellulidæ, Perlidæ, Sialidæ, Hemerobiidæ, Mymeleonidæ,
  Acridiidæ, Locustidæ, Blattidæ, Termitidæ, etc.

  2. Into those whose fore and hind wings act together like one wing,
  since they are connected by hooks (hamuli), as in certain
  Hymenoptera, or are attached in other ways. Here belong Hymenoptera,
  Lepidoptera, Trichoptera, Cicadidæ, Psocidæ, etc.

  The musculature of the mesothorax and metathorax is similar in those
  insects both of whose pairs of wings are like each other, and act
  independently during flight, viz. in the Libellulidæ. On the other
  hand, in the second category, where the fore and hind wings act as a
  single pair and the fore wings are mostly larger than the hinder
  (except in most of the Trichoptera), the musculature of the
  mesothorax is more developed than that of the metathorax.

  To neither category belong the beetles, whose wing-covers are
  peculiar organs of flight, and not for direct use, and the Diptera,
  which possess but a single pair of wings. In the beetles the hind
  wings, in the Diptera the fore wings, serve especially as organs of
  flight. It may be observed that the Diptera are the best fliers, and
  that those insects which use both pairs of wings as a single pair
  fly better than those insects whose two pairs of wings work
  independently of each other. An exception are the swift-flying
  Libellulidæ, whose specially formed muscles of flight explain their
  unusual capabilities for flying (Kolbe).




                          LITERATURE ON FLIGHT


  =Marey, E. J.= La machine animale. Locomotion terrestre et aërienne.
    Paris, 1874.

  —— Mémoire sur le vol des insectes et des oiseaux. (Annal. Scienc.
    natur., 5 sér., Zool. xii, 1869, pp. 49–150; 5 sér., Zool. xv, 1872,
    42 Figs.)

  —— Note sur le vol des insectes. (Compt. rend. et Mém. Soc. d. Biol.
    Paris, 4 sér., v, 1869, C. R. pp. 136–139.)

  —— Recherches sur le mécanisme du vol des insectes. (Journal de
    l’Anatomie et de la Physiologie, 6 Année, 1869, pp. 19–36, 337–348.)

  —— Animal mechanism. New York, 1879, pp. 180–209.

  —— Movement. New York, 1895, pp. 239–274.

  =Hartings.= Ueber den Flug. (Niederland. Archiv f. Zoologie, iv,
    Leiden, 1877–78.)

  =Lucy.= Le vol des oiseaux, chauvesouris et insectes. Paris.

  =Tatin, V.= Expériences physiologiques et synthétiques sur le
    mécanisme du vol. (Ecole prat. d. haut. étud. Physiol. expérim.
    Trav. du laborat. de Marey, 1877, pp. 293–302.)

  —— Expériences sur le vol mécanique. (Ibid., 1876, pp. 87–108.)

  =Bellesme, Jousset de.= Recherches expérimentales sur les fonctions du
    balancier chez les insectes Diptères, Paris, 1878, 96 pp., Figs.

  —— Sur une fonction de direction dans le vol des insectes. (Compt.
    rend., lxxxix, 1879, pp. 980–983.)

  =Pettigrew, J. Bell.= On the mechanical appliances by which flight is
    attained in the animal kingdom. (Trans. Linn. Soc., 1868, xxvi, Pt.
    I, pp. 197–277, 4 Pl.)

  —— On the physiology of wings. (Trans. Roy. Soc. Edinburgh, 1871,
    xxvi, pp. 321–446.)

  =Krarup-hansen, C. J. L.= Beitrag zu einer Theorie des Fluges der
    Vogel, Insekten und Fledermause. (Copenhagen u. Leipzig, Fritsch,
    1869, 48 pp.)

  =Lendenfeld, R. V.= Der Flug der Libellen. (Sitzungsber. d. kais.
    Akad. d. Wiss. Wien, lxxxiii, 1881, pp. 289–376, 7 Taf.; Zool. Anz.,
    1880, p. 82.)

  =Girard, M.= Note sur diverses expériences relatives à la fonction des
    ailes chez les insectes. (Ann. Soc. Ent. France, 4 sér., ii, 1862,
    pp. 154–162.)

  =Mühlhäuser, F. A.= Ueber das Fliegen der Insekten. (22. bis 24.,
    Jahresb. d. Pollichia, Dürkheim, 1866, pp. 37–42.)

  =Plateau, Félix.= Recherches expérimentales sur la position du centre
    de gravité chez les insectes. (Archiv d. Scienc. phys. et natur. d.
    Genève, Nouv. période, xliii, 1872, pp. 5–37.)

  =Plateau, Félix.= Ueber die Lage des Schwerpunktes bei den Insekten.
    Auszug. (Naturforscher v. Sklarek, v. Jahrg., 1872, pp. 112–113.)

  —— Recherches physico-chimiques sur les articulés aquatiques. (Bull.
    d. l’Acad. Roy. Belg., xxxiv, 1872, pp. 1–50, Fig.)

  —— Qu’est-ce que l’aile d’un insecte? (Stett. Ent. Zeit., 1871, pp.
    33–42, Pl.)

  —— L’aile des insectes. (Journ. d. Zool., ii, 1873, pp. 126–137.)

  =Perez, J.= Sur les causes de bourdonnement chez les insectes.
    (Comptes rend., lxxxvii, p. 535, Paris, 1878.)

  =Strasser, Hans.= Mechanik des Fluges. (Archiv f. Anat. u. Phys.,
    1878, p. 310–350, 1 Taf.)

  —— Ueber die Grundbedingungen der aktiven Locomotion. (Abhandl. d.
    naturf. Gesellsch., Halle, 1880, xv, pp. 121–196, Figs.)

  =Moleyre, L.= Recherches sur les organes du vol chez les insectes de
    l’ordre des Hemiptères. (Compt. rend. de l’Acad. d. Scienc. de
    Paris, 1882, xcv, pp. 349–352.)

  =Amans, P.= Essai sur le vol des insectes. (Revue d. Sc. Nat.
    Montpellier, 3 sér., ii, 1883, pp. 469–490, 2 Pl.; iii, 1884, pp.
    121–139, 3 Pl.)

  —— Étude de l’organe du vol chez les Hyménoptères. (Ibid., iii, pp.
    485–522, 2 Pl.)

  —— Comparaisons des organes du vol dans la série animale. Des organes
    du vol chez les insectes. (Annal. d. Scienc. nat. Zool., 6 sér.,
    xix, pp. 1–222, 8 Pl.)

  =Mullenhoff, K.= Die Grosse der Flugflächen. (Pflüger’s Archiv f. d.
    ges. Physiologie, 1884, xxxv, pp. 407–453.)

  —— Die Ortsbewegungen der Tiere. (Wissensch. Beil. z. Programm d.
    Andreas-Realgymnas. Berlin, 1885, 19 pp.)

  =Poujade, G. A.= Note sur les attitudes des insectes pendant le vol.
    (Ann. Soc. Ent. France, 1884, 6 sér., iv, pp. 197–200, 1 Pl.)

  =Krancher, O.= Die Töne der Flügelschwingungen unserer Honigbiene.
    (Deutscher Bienenfreund, 1882, 18. Jahrg., pp. 197–204.)

  =Landois, H.= Ueber das Flugvermögen der Insekten. (Natur. und
    Offenbarung, vi, 1860, pp. 529–540.)

  =Ungern-Sternberg, von.= Betrachtungen über die Gesetze des
    Fluges. (Zeitschr. d. Deutschen Vereins z. Förderung d.
    Luftschiffahrt-Naturwissensch. Wochenschrift v. Potonie, iv,
    1889, p. 158.)

  =Baudelot, E.= Du mecanisme suivant lequel s’effectue chez les
    Coléoptères le retract des ailes inférieures sous les élytres au
    moment du passage a l’état de repos. (Bull. Soc. d. Scienc. nat.,
    Strasbourg, 1 Année, 1868, pp. 137–138.)

  =Ris, Fr.= Die schweizerischen Libellen. Schaffhausen, 1885. (Beiheft
    der Mitteil. d. Schweiz. Ent. Ges., vii, pp. 35–84.)




                     THE ABDOMEN AND ITS APPENDAGES


[Illustration:

  FIG. 176.—Abdomen of _Termes flavipes_: 1–10, the ten tergites; 1–9,
    the nine urites; _c_, cercopod.
]

[Illustration:

  FIG. 177.—End of abdomen of _Panorpa debilis_ drawn out, the chitinous
    pieces shaded: _L_, lateral, _D_, dorsal view; _c_, jointed
    cercopoda.—Gissler _del._
]

In the abdomen the segments are more equally developed than elsewhere,
retaining the simple annular shape of embryonic life, and from their
generalized nature their number can be readily distinguished (Fig. 176).
The tergal and sternal pieces of each segment are of nearly the same
size, the tergal often overlapping the sternal (though in the Coleoptera
the sternites are larger than the tergites), while there are no pleural
pieces, the lateral region being membranous when visible and bearing the
stigmata (Fig. 177, _L_). In the terminal segments beyond the genital
outlet, however, there is a reduction in and loss of segments,
especially in the adults of the metabolous orders, notably the Panorpidæ
(Fig. 177), Diptera, and aculeate Hymenoptera; in the Chrysididæ only
three or four being usually visible, the distal segments being reduced
and telescoped inward.

The typical number of abdominal segments (uromeres), _i.e._ that
occurring in each order of insects, is ten; and in certain families of
Orthoptera, eleven. In the embryos, however, of the most generalized
winged orders, Orthoptera (Fig. 199), Dermaptera, and Odonata, eleven
can be seen, while Heymons has recently detected twelve in blattid and
Forficula embryos, and he claims that in the nymphs of certain Odonata
there are twelve segments, the twelfth being represented by the anal or
lateral plates. It thus appears that even in the embryo condition of the
more generalized winged insects, the number of uromeres is slightly
variable.

  We have designated the abdomen as the _urosome_; the abdominal
  segments of insects and other Arthropods as _uromeres_, and the
  sternal sclerites as _urosternites_, farther condensed into
  _urites_. (See Third Report U. S. Entomological Commission, 1883,
  pp. 307, 324, 435, etc.)

[Illustration:

  FIG. 178.—Nymph of the pear tree Psylla, with its glandular
    hairs.—After Slingerland. Bull. Div. Ent. U. S. Dep. Agr.
]

The reduction takes place at the end of the abdomen, and is usually
correlated with the presence or absence of the ovipositor. In the more
generalized insects, as the cockroaches, the tenth segment is, in the
female, completely aborted, the ventral plate being atrophied, while the
dorsal plate is fused during embryonic life, as Cholodkowsky has shown,
with the ninth tergite, thus forming the suranal plate.

  In the advanced nymph of Psylla the hinder segments of the abdomen
  appear to be fused together, the traces of segmentation being
  obliterated, though the segments are free in the first stage and in
  the imago (Fig. 178). It thus recalls the abdomen of spiders, of
  Limulus, and the pygidium of trilobites.

=The median segment.=—There has been in the past much discussion as to
the nature of the first abdominal segment, which, in those Hymenoptera
exclusive of the phytophagous families, forms a part of the thorax, so
that the latter in reality consists of four segments, what appearing to
be the first abdominal segment being in reality the second.

  Latreille and also Audouin considered it as the basal segment of the
  abdomen, the former calling it the “segment médiaire,” while Newman
  termed it the “propodeum.” This view was afterward held by Newport,
  Schiödte, Reinhard, and by the writer, as well as Osten-Sacken,
  Brauer, and others. The first author to attempt to prove this by a
  study of the transformations was Newport in 1839 (article
  “Insecta”). He states that while the body of the larva is in general
  composed of thirteen distinct segments, counting the head as the
  first, “the second, third, fourth, and, as we shall hereafter see,
  in part also the fifth, together form the thorax of the future
  imago” (p. 870). Although at first inclined to Audouin’s opinion, he
  does not appear to fully accept it, yet farther on (p. 921) he
  concludes that in the Hymenoptera the “fifth” segment (first
  abdominal) is not in reality a part of the true thorax, “but is
  sometimes connected more or less with that region, or with the
  abdomen, being intermediate between the two. Hence we have ventured
  to designate it the _thoracico-abdominal_ segment.” Had he
  considered the higher Hymenoptera alone, he would undoubtedly have
  adopted Latreille’s view, but he saw that in the saw-flies and
  Lepidoptera the first abdominal segment is not entirely united with
  the thorax, being still connected with the abdomen as well as the
  thorax. Reinhard in 1865 reaffirmed Latreille’s view. In 1866 we
  stated from observations on the larvæ made three years earlier, that
  during the semipupa stage of Bombus the entire first abdominal
  segment is “transferred from the abdomen to the thorax with which it
  is intimately united in the Hymenoptera,” and we added that we
  deemed this to be “the most essential zoölogical character
  separating the Hymenoptera from all other insects.” (See Fig. 93,
  showing the gradual transfer and fusion of this segment with the
  thorax.) In the saw-flies the fusion is incomplete, as also in the
  Lepidoptera, while in the Diptera and all other orders the thorax
  consists of but three segments. (See also pp. 90–92.)

[Illustration:

  FIG. 179.—Abdomen of _Machilis maritima_, ♀, seen from beneath: the
    left half of the 8th ventral plate removed; I-IX, abdominal
    segments; _c_, cercopoda; _cb_, coxal glands; _hs_, coxal stylets;
    _lr_, ovipositor.—After Oudemans, from Lang.
]

=The cercopoda.=—We have applied this name to the pair of anal cerci
appended to the tenth abdominal segment, and which are generally
regarded as true abdominal legs. As is now well known, the embryos of
insects of different orders have numerous temporary pairs of abdominal
appendages which arise in the same manner, have the same embryonic
structure, and are placed in a position homologous with those of the
thorax. In the embryo of Œcanthus rudimentary legs appear, as shown by
Ayers, on the first to tenth abdominal segment, the last or tenth pair
becoming the cercopoda; and similar rudimentary appendages have been
detected in the embryos of Coleoptera, Lepidoptera, and Hymenoptera
(Apidæ). Cholodkowsky has observed eleven pairs of abdominal appendages
in Phyllodromia.

They are very long and multiarticulate in the Thysanura (Fig. 179). In
the Dermaptera they are not jointed and are forcep-like. It should also
be observed that in the larva or Sisyra (Fig. 181) there are seven pairs
of 5–jointed abdominal appendages, though these may be secondary
structures or tracheal gills. In the Perlidæ and the Plectoptera
(Ephemeridæ), they are very long, sometimes over twice as long as the
body, and composed of upward of 55 joints; they also occur in the
Panorpidæ (Fig. 177). In the dragon-flies the cerci are large, but not
articulated, and serve as claspers or are leaf-like[35] (Fig. 180). In a
few Coleoptera, as the palm-weevil (_Rhynchophorus phœnicis_), Cerambyx,
Drilus, etc., the so-called ovipositor ends in a hairy, 1–jointed,
palpiform cercus. Short 25–jointed cercopoda are present in Termitidæ,
and 2–jointed ones in Embiidæ.

[Illustration:

  FIG. 180.—End of abdomen of _Æschna heros_, ♀: _ur_, urosternite;
    _or_, outer, _ir_, inner styles of the ovipositor; 11, 11th
    abdominal segment; _c_, cercopod.
]

[Illustration:

  FIG. 181.—Larva of Sisyra, from beneath. _B_, an abdominal
    appendage.—After Westwood, from Sharp.
]

[Illustration:

  FIG. 182.—Cercopoda (P) of Mantis.—After Lacaze-Duthiers.
]

The anal cerci are present in the Orthoptera and, when multiarticulate,
function as abdominal antennæ. They are longest in the Mantidæ (Fig.
182); they also occur in the larva of the saw-fly, Lyda (Fig. 183). Dr.
A. Dohrn has stated that the cerci of Gryllotalpa are true sensory
organs, and we have called those of the cockroach abdominal antennæ,
having detected about ninety sacs on the upper side of each joint of the
stylets, which are supposed to be olfactory in nature, and which are
larger and more numerous than similar sacs or pits in the antennæ of the
same insect.[36] From his experiments upon decapitated cockroaches,
Graber concluded that these cerci were organs of smell.

[Illustration:

  FIG. 183.—Lyda larva: _a_, head; _b_, end of body seen from above;
    _c_, from side, with cercopod.
]

  Haase regarded these appendages, from their late development and
  frequent reduction, as old inherited appendages which are
  approaching atrophy through disuse.

  Cholodkowsky states that Tridactylus, a form allied to Gryllotalpa,
  bears on the tenth abdominal segment two pairs of cerci (ventral and
  dorsal), and that the ventral pair may correspond to the atrophied
  appendages of the tenth embryonic segment of Phyllodromia, with
  which afterward the eleventh segment becomes fused.

  The cercopods are not necessarily confined to the eleventh or to the
  tenth segment, for when there are only nine segments, with the
  vestige of a tenth, as in Xiphidium, they arise from the ninth
  uromere, and in the more modern cockroaches, as Panesthia, in which
  there are but seven entire segments, they are appended to the last
  or eighth uromere.

[Illustration:

  FIG. 184.—Anabrus, ♀, side-view, dissected; showing the relative size
    of the ovipositor: _c_, the minute cercopod.—Kingsley _del._
]

As to the homology and continuity of these cercopods with the ventral
outgrowths of the embryo, several embryologists, notably Wheeler, are
emphatic in regarding them as such. It thus appears that either the
embryonic appendages of the seventh or eighth, ninth or tenth uromere
may persist, and form the cercopoda of the adult.

=The ovipositor.=—The end of the oviduct is guarded by three pairs of
chitinous, unjointed styles closely fitted together, forming a strong,
powerful apparatus for boring into the ground or into leaves, stems of
plants, the bodies of insects, or even into solid wood, so that the eggs
may be deposited in a place of safety. In the ants, wasps, and bees the
ovipositor also functions as a sting, which is further provided with a
poison-sac.

Morphologically, the ovipositor is composed of three pairs of unjointed
styles (_rhabdites_ of Lacaze-Duthiers, _gonapophyses_ of Huxley), which
are closely appressed to or sheathed within each other, the eggs passing
out from the end of the oviduct, which lies, as Dewitz states, between
the two styles of the lowest or innermost pair, and under the cross-bars
or at the base of the stylets mentioned; the styles or blades spreading
apart to allow of the passage of the egg.

[Illustration:

  FIG. 185.—Saw of Hylotoma: _a_, lateral scale; _i_, saw; _f_, gorget;
    7_t_, 7th tergite; 6_s_, 6th sternite; _ov_, oviduct; _in_,
    intestine.—After Lacaze-Duthiers.
]

The ovipositor is best developed in the Thysanura (Fig. 179, Campodea
excepted), in Orthoptera (Fig. 184), in the Odonata, Hemiptera, certain
Physapoda, Rhaphiidæ, and in the phytophagous Hymenoptera, where it is
curiously modified to form a rather complicated saw for cutting slits in
wood or leaves (Fig. 185). It is wanting or quite imperfect in
Coleoptera, Diptera, and Lepidoptera.

Morphologically, the ovipositor appears to be formed out of the
abdominal appendages of the seventh, eighth, and ninth segments of the
female, which, instead of disappearing in the orders first mentioned,
persist as permanent styles.

Wheeler asserts from his study of the embryonic development of Xiphidium
“there can be no doubt concerning the direct continuity of the embryonic
appendages with the gonapophyses.” He goes on to say:—

  “One embryo, which had just completed katatrepis, still showed
  traces of all the abdominal appendages. The pairs on the eighth,
  ninth, and tenth segments were somewhat enlarged. In immediately
  succeeding stages the appendages of the second to sixth segments
  disappear; the pair on the seventh disappear somewhat later. Up to
  the time of hatching the gonapophyses could be continuously traced,
  since in Xiphidium there is no flexure of the abdomen, as in other
  forms, to obscure the ventral view of the terminal segments. From
  the time of hatching Dewitz has traced the development of the
  ovipositor in another locustid (_Locusta viridissima_), so that now
  we have the complete history of the organ.”

Heymons, however, is inclined to believe that they are simply hypodermal
outgrowths.

[Illustration:

  FIG. 186.—Ideal plan of the structure of the ovipositor to illustrate
    Lacaze-Duthiers’ view: _b_, 8th tergite; _c_, epimerum; _a′_, _a_,
    two pieces forming the outer pair of rhabdites; _i_, the 2d pair, or
    stylets; and _f_, the inner pair, or sting; _d_, support of sting;
    _e_, piece supporting the stylet; _R_, anus; _o_, outlet of oviduct.
    The 7th, 8th, and 9th sternites are aborted.—After Lacaze-Duthiers.
]

The first to study the morphology of the ovipositor was Lacaze-Duthiers,
who referred their origin to the partially atrophied dorsal or ventral
sclerites of one of the last abdominal segments; a view accepted by
Gerstaecker[37] (Figs. 186, 187). The present writer (1866), however,
showed that the sting of Bombus was not formed of the reduced pieces of
the segments themselves, but arose from special outgrowths on the
ventral side of the eighth and ninth abdominal segments. These
appendages he did not at first regard as the homologues of the limbs,
until in 1871, after studying the origin of the spring of the Podurans
(Isotoma), he found that it was a true jointed appendage and therefore a
homologue of a pair of the styles forming the ovipositor of the winged
insects, and that the three pairs of styles of the latter were
homologues of the thoracic legs and cephalic appendages. The view was
stated in the Guide to the Study of Insects. (See also Amer. Nat.,
March, 1871, p. 6.) Kraepelin also affirms that the styles of the
ovipositor are segmental appendages and homologues of the antennæ, wings
(_sic_), and legs.

[Illustration:

  FIG. 187.—1, abdomen of Cynips, showing the great dorsal segment, the
    peduncle, and the position of the ovipositor within; 2, the entire
    ovipositor; _a_, lateral scale; _a′_, its valve; _b_, anal scale;
    _b′_, stylet; _c_, support of the stylet; _e_, base or support of
    sting (_fi_); 3, profile showing the relation of the genital
    armature to the rest of the abdomen, the 6th sternite having been
    drawn to show its full size; 4, anal scale (_b_) and stylet; _e_,
    _i_, supports and body of the stylet; _c_, piece uniting the two
    scales; 5, lateral scale (_a_), and _a_′ sheath; _d_, support of the
    sting (_f_); 6, transverse section of the body through the sting
    (diagrammatic); _R_, internal armature; _o_, oviduct; _a_, lateral
    scale; _a′_ its valve; _e_, support of the stylets (_i_); _b_, anal
    scale; _c_, piece uniting two scales; _f_, sting; _d_, its support;
    7, a second section simpler and more theoretical than the first; 8,
    diagrammatic, all the elements of the sting have been reduced to
    pieces of the same form.—After Lacaze-Duthiers.
]

An objection to this view is the fact that the posterior pairs of styles
appear to arise both from one and the same segment,—the ninth. Dewitz
questions whether the four appendages of the ninth segment represent two
pairs of limbs, or one pair split into two branches, and prefers the
latter view, but leaves it as a point to be settled by future
investigations. As will be seen below, both Kraepelin and Bugnion
observed a pair of rudiments to each of the three penultimate segments,
those of the middle pair splitting in two. Wheeler maintains,
erroneously we think, that the inner of the two pairs on the ninth
segment represents the tenth pair of abdominal appendages; but in
reality this latter pair become the cercopods. That there are probably
originally in insects of all the orders provided with an ovipositor
three distinct pairs of appendages, one to each segment, is proved, or
at least strongly suggested, by Ganin’s researches on the three pairs of
abdominal imaginal discs of the third larva of Platygaster and Polynema
(Fig. 188), which are transformed into the ovipositor. He remarks that
these imaginal discs have the same origin and pass through the same
changes as those in front, _i.e._ those destined to form the thoracic
legs. Dewitz has shown that the germs of the ovipositor of the honey-bee
arise as buds on the two segments before the last (Fig. 189).

[Illustration:

  FIG. 188.—Third larva of Polynema: _at_, antenna; _fl_, imaginal buds
    of the wing; _l_, of the legs; _tg_, buds of the middle pair of
    stylets of the ovipositor; _fk_, fat-body; _eg_, ear-like
    process.—After Ganin.
]

[Illustration:

  FIG. 189.—Imaginal buds and papillæ of the ovipositor of the honey-bee
    attached to tracheæ; at different stages: _b′_, 1st; _b″_, 2d or
    middle; and _c′_, 3d pair of papillæ.—After Dewitz.
]

Kraepelin also detected in the larva of the honey-bee a pair of what he
regarded as genuine imaginal buds on abdominal segments eight, nine, and
ten; the buds on the tenth segment are divided each in two; of these
four appendages the two median ones form the barbed sting (_gorgeret_ or
_stachelrinne_), and the two lateral stylets, the valves
(_stachelscheiden_). The two buds of the ninth segment give rise to the
vagina and to the oviducts, and these unite secondarily with the
posterior end of the ovaries. The genital appendages of the male
correspond to those of the female, and arise from four imaginal buds
situated on the under side of the tenth abdominal segment.

[Illustration:

  FIG. 190.—1, sting and poison sac of the honey-bee: _GD_, poison
    gland; _Gb_, poison reservoir; _D_, accessory gland; _sh_, sheathing
    style or sting-“feeler”; _Str_ sting; _Ba_, sheath; _Q_, quadrate
    plate; _O_, oblong piece; _W_, angular piece; _B_, base of the sting
    and stylets; _Stb′_, _Stb″_, the two barbed stylets or darts. 2,
    sting seen from the ventral face: lettering as in the other
    figure.—After Kraepelin, from Perrier.
]

In the ants, according to Dewitz, the genital armature is derived from
imaginal buds situated on the under side of the seventh, eighth, and
ninth abdominal segments. Bugnion has observed the formation of six
imaginal buds of the genital armature in the larva of a chalcid
(Encyrtus, Figs. 41, 42, 191, _q^1_, _q^2_, _q^3_), the transformation
of the central part of these structures into small digitiform pads, then
the division of the two intermediate buds into four (?) (Fig. 191, _B_,
_q^2_), but was unable to trace their farther development.

  The subject still needs farther investigation, since certain
  observers, as Haase, and, more recently, Heymons, do not believe
  that they are homologues of the legs, but integumental structures,
  though of somewhat higher value than the style of the base of the
  legs of Scolopendrella and Thysanura; but it is to be observed that
  as yet we know but little of the embryological history of these
  styles.

  Those authors who have examined the elements of the ovipositor, and
  regard them as homologues (_homodynamous_) of the limbs, are
  Weismann (1866), Ganin (1869), Packard (1871), Ouljanin (1872),
  Kraepelin, Kowalevsky (1873), Dewitz (1875), Huxley (1877),
  Cholodkowsky, Bugnion (1891), and Wheeler (1892).

As shown, then, by our observations and those of Dewitz (Figs. 189 and
192), the rudiments of the ovipositor consist of three pairs of
tubercles, arising, as Kraepelin and also Bugnion (Fig. 191) have shown,
from three pairs of imaginal discs, situated respectively on the
seventh, eighth, and ninth uromeres, or at least on the three
penultimate segments of the abdomen. With the growth of the semipupa,
the end of the abdomen decreases in size, and is gradually incurved
toward the base (Fig. 193), and the three pairs of appendages approach
each other so closely that the two outer ones completely ensheath the
inner pair, until a complete extensible tube is formed, which, by the
changes in form of the muscles within, is gradually withdrawn entirely
within the body.

[Illustration:

  FIG. 191.—_A_, end of larva of Encyrtus of 2d stage, showing the three
    pairs of imaginal buds of the ovipositor _q^1_, _q^2_, _q^3_. _B_,
    the same in an older larva ready to transform; _i_, intestine; _x_,
    genital gland; _a_, anus.—After Bugnion.
]

  An excellent account of the honey-bee’s sting is given by Cheshire
  (Figs. 194, 195). The outermost of the three pairs of stylets
  forming the apparatus is the two thick, hairy “palpi” or feelers
  (_P_), these being freer from the sting proper than in the
  ovipositor of Orthoptera. The sting itself is composed of the two
  inner pairs of stylets; one of these pairs is united to form the
  sheath (_sh_), while the other pair form the two barbed darts. The
  sheath has three uses: first, to open the wound; second, to act as
  an intermediate conduit for the poison; and third, to hold in
  accurate position the long barbed darts. The sheath does not enclose
  the darts as a scabbard, but is cleft down the side presented in
  Fig. 194, which is below when the sting points backward. But, says
  Cheshire, as the darts move up and down, they would immediately slip
  from their position, unless prevented by a mechanical device,
  exhibited by _B_ and _C_, giving in cross-section sheath and darts
  near the end, and at the middle of the former. “The darts (_d_) are
  each grooved through their entire length, while upon the sheath
  (_sh_) are fixed two guide rails, each like a prolonged dovetail,
  which, fitted into the groove, permits of no other movement than
  that directly up and down.” The darts are terminated by ten barbs of
  ugly form (_D_, Fig. 194), and much larger than those of the sheath,
  and as soon as the latter has established a hold, first one dart and
  then the other is driven forward by successive blows. These in turn
  are followed by the sheath, when the darts again more deeply plunge,
  until the murderous little tool is buried to the hilt. But these
  movements are the result of a muscular apparatus yet to be examined,
  and which has been dissected away to bring the rigid pieces into
  view. The dovetail guides of the sheath are continued far above its
  bulbous portion, as we see by _E_, Fig. 195; and along with these
  the darts are also prolonged upward, still held to the guides by the
  grooved arrangement before explained; but both guides and darts, in
  the upper part of their length, curve from each other somewhat like
  the arms of a Y, to the points _c_, _c′_ (_A_, Fig. 194), where the
  darts make attachment to two levers (_i_, _i′_). The levers (_k_,
  _l_ and _k′_, _l′_) are provided with broad muscles, which terminate
  by attachment to the lower segments of the abdomen. These, by
  contraction, revolve the levers aforesaid round the points _f_,
  _f′_, so that, without relative movement of rod and groove, the
  points _c_, _c′_ approach each other. The arms of the Y straighten
  and shorten, so that the sheath and darts are driven from their
  hiding-place together and the thrust is made by which the sheath
  produces its incision and fixture. The sides being symmetrical, we
  may, for simplicity’s sake, concentrate our attention on one, say
  the left in the figure. A muscular contraction of a broad strap
  joining _k_ and _d_ (the dart protractor) now revolves _k_ on _l_,
  so that _a_ is raised, by which clearly _c_ is made to approach _d_;
  _i.e._ the dart is sent forward, so that the barbs extend beyond the
  sheath and deepen the puncture. The other dart, and then the sheath,
  follow, in a sequence already explained, and which _G_, Fig. 195, is
  intended to make intelligible, _a_ representing the entrance of the
  sheath, _b_ the advance of the barbs, and _c_ the sheath in its
  second position. The barb retractor muscle is attached to the outer
  side of _i_, and by it _a_ is depressed and the barbs lifted. These
  movements, following one another with remarkable rapidity, are
  entirely reflex, and may be continued long after the sting has been
  torn, as is usual, from the insect. By taking a piece of
  wash-leather, placing it over the end of the finger, and applying it
  to a bee held by the wings, we may get the fullest opportunity of
  observing the sting movements, which the microscope will show to be
  kept up by continued impulses from the fifth abdominal ganglion and
  its multitudinous nerves (_n_, Fig. 194, _A_), which penetrate every
  part of the sting mechanism, and may be traced even into the darts.
  These facts, together with the explanation at page 49, will show why
  an abdomen separated many hours may be able to sting severely, as I
  have more than once experienced.

[Illustration:

  FIG. 192.—Base of the ovipositor of _Locusta viridissima_ seen from
    beneath: _c′_, sheath, or outer and lower pair of stylets turned to
    one side to show the others; _b′_, upper and inner pair; _b″_, third
    or innermost, smallest pair of stylets. _A_, the same on one side,
    in section. The shaded parts show the muscular attachments. The
    muscles which extend the apparatus and are attached to ν, δ, and η,
    as also the membranes which unite the pieces from η; to γ with each
    other and the body, are removed, so that only the chitinous parts
    remain.—After Dewitz.
]

[Illustration:

  FIG. 193.—Development of the sting in Bombus: _A_, _a_, 1st pair on
    8th sternite; _b_, 2d inner pair forming the darts; _c_, outer pair.
    _B_-_E_, more advanced stages. _F_, _x_, _y_, _z_, three pairs of
    tubercles, the germs of the male organs.
]

[Illustration:

  FIG. 194.—Sting of bee × 30 times: _A_, sting separated from its
    muscles; _ps_, poison sac; _pg_, poison gland; _5th g_, 5th
    abdominal ganglion; _n_, _n_, nerves; _e_, external thin membrane
    joining sting to last abdominal segment; _i_, _k_, _l_, and _i′_,
    _k′_, _l′_, levers to move the darts; _sh_, sheath; _v_, vulva; _p_,
    sting-palpus or feeler, with tactile hairs and nerves. _B_ and _C_,
    sections through the darts and sheath, × 300 times: _sh_, sheath;
    _d_, darts; _b_, barbs; _p_, poison-channel. _D_, end of a dart, ×
    200: _o_, _o_, openings for poison to escape into the wound.—After
    Cheshire.
]

[Illustration:

  FIG. 195.—Details of sting of bee: _E_, darts, sheath, and valves;
    _pb_, poison-bag duct; _fo_, fork; _s_, slide piece; _va_, valve;
    _b_, barbs. _F_, terminal abdominal segments; _w_, worker’s sting;
    _q_, queen’s sting; _r_, _r′_, anal plate; _G_, sting entering skin;
    _sh_, sheath; _a_, _b_, _c_, positions in first, second, and third
    thrusts with the sting. _H_, portion of poison gland, × 300; _cn_,
    cell nucleus; _n_, nerve; _g_, ganglionic cell. _I_, portion of the
    poison gland, cells removed; _cd_, central duct; _d_, individual
    small ducts; _pr_, tunica propria. _K_, gland of _Formica rufa_;
    _cd_, central duct; _d_, small ducts; _sc_, secreting cells. _L_,
    valve and support; _t_, trachea; _va_, valve; _tr_, truss or
    valve-prop.—After Cheshire.
]

[Illustration:

  FIG. 196.—_A_, rudimentary ovipositor of nymph of Æschna. _B_, the
    corresponding ♂ structures; _a_, enlarged. _C_, ovipositor of nymph
    of Agrion; _d_, gill.
]

The male genital armature in the bees is originally composed of three
pairs of tubercles, homologous with those of the female, all originally
arising from three abdominal segments, two afterward being anterior, and
the third pair nearer the base of the abdomen.

The ovipositor of the dragon-flies (Odonata) is essentially like that of
the Orthoptera and Hymenoptera. Thus in Æschna (Fig. 196), Agrion (Fig.
196, _C_), and also in Cicada it consists of a pair of closely appressed
ensiform processes which grow out from under the posterior edge of the
eighth uromere and are embraced between two pairs of thin lamelliform
pieces of similar form and structure.

=The styles and genital claspers= (_Rhabdopoda_).—Other appendages of
the end of the abdomen of pterygote insects, and generally, if not
always, arising from the ninth segment, are the clasping organs, or
_rhabdopoda_ as we may call them, of Ephemeridæ (Fig. 197), Neuroptera
(Corydalus [Fig. 198], Myrmeleon, Rhaphidia), Trichoptera, Lepidoptera,
Diptera, and certain phytophagous Hymenoptera. They do not appear to
occur in insects which are provided with an ovipositor. In Thysanura the
styles are present on segments 1–9 (Fig. 179). Those of the male
Ephemeridæ, of which there are two pairs arising from the ninth segment,
are remarkable, since they are jointed, and they serve to represent or
may be the homologues of two of the pairs of stylets composing the
ovipositor of insects of other orders. The lower pair (Fig. 197, _rh_)
are either 2–, 3–, or 4–jointed (in Oniscigaster 5–jointed), while those
of the upper pair are 2–jointed (_rh′_). These rhabdopods in the
ephemerids are evidently very primitive structures, since they approach
nearest in shape and in being jointed to the abdominal legs of
Scolopendrella and the Myriopoda. The styles of the Orthoptera are
survivals of the embryonic appendages of the ninth segment (Wheeler,
etc.). In Mantis they are seen to have the same relations as the cerci,
as shown by Heymons (Fig. 200).

[Illustration:

  FIG. 197.—Abdomen of _Ephemera_ (Leptophlebia) _cupida_, ♂: _c_, base
    of cercopoda; _rh_, outer 3–jointed claspers or rhabdopods; _rh′_,
    inner pair. _A_, side view.
]

[Illustration:

  FIG. 198.—End of abdomen of _Corydalus cornutus_, ♂: _vh_, rhabdopod;
    _c_, cercopod.
]

In the Phasmidæ, in Anabrus, and in the Odonata the cercopods, which are
not jointed, are converted into claspers, and in the Odonata the
claspers are spiny within, so as to give a firmer hold. The suranal
plate is apparently so modified as to aid in grasping the female. In
nearly all the Trichoptera there are, besides the suranal plate, which
is sometimes forked (Nosopus), a pair of superior and of inferior
claspers, and in certain genera (Ascalaphomerus, Macronema, Rhyacophila,
Hydropsyche, Amphipsyche, Smicridea, and Ganonema) the lower pair are
2–jointed like those of Ephemeridæ. The number of abdominal segments in
the adult Trichoptera is nine, and McLachlan states that the genital
armature consists of three pairs of appendages, _i.e._ a superior,
inferior, and intermediate pair, besides the suranal plate (vestige of a
tenth segment) and the penis. Judging by his figures, these three pairs
of appendages arise from the last or ninth uromere, and the upper pair
seem to be the homologue of the cercopoda of ephemerids. It needs still
to be ascertained whether the intermediate pair is a separate set, or
merely subdivisions of the upper or lower, and whether one of the latter
may not arise from the penultimate segment, because we should not expect
that the last segment should bear more than one pair of appendages, as
we find to be the case in arthropods in general, and in the Neuroptera,
from which the Trichoptera may have originated.

[Illustration:

  FIG. 199.—End of abdomen of embryo of Mantis: _r_, rhabdopod; _c_,
    cercopod; _sp_, suranal plate; _st_, stigma on 8th segment.—After
    Heymons.
]

[Illustration:

  FIG. 200.—End of abdomen of _Periplaneta americana_, ♂, side view:
    _c_, cercopod; _st_, stilus; _p_, penis; _t_, titillator; _d_,
    “bird’s head” (clasper?); _i_, “oblong plate”; IX-XI, terminal
    segments; X, suranal plate; XI′, 11th sternite.—After Peytoureau.
]

In most larvæ of the Trichoptera, especially the Rhyacophilidæ and
Hydropsychidæ, the last abdominal segment bears a pair of 2–jointed legs
(cercopoda), ending in either one or two claws, which under various
forms, sometimes forming long processes, persist in the pupa; and there
appears to be a suranal plate, the vestige of the tenth uromere. In the
pupa, judging by Klapálek’s figure of Leptocerus (24_{8,9}, 25), a pair
of lateral spines arise which may in the imago form one of the pairs of
appendages or styles. In the pupa of _Œcetis furva_ his figure 28_{9}
shows two pairs of 1–jointed appendages arising from the last segment;
whether the long dorsal or upper styles arise from the vestige of a more
distal segment is not distinctly shown in Klapálek’s sketch. The origin
of these elements of the genital armature evidently needs further study.

Whether the abdominal legs or so-called false or prop-legs of
lepidopterous larvæ are genuine legs, homologous with those of the
thorax and with the cephalic appendages, or whether they are secondary
adaptive structures, is a matter still under discussion. That, however,
they are true legs is shown by the embryology of the Lepidoptera, where
there is a pair to each abdominal segment. It may also be asked whether
the anal legs of lepidopterous larvæ are not the homologues of the
2–jointed anal appendages of caddis-worms.

[Illustration:

  FIG. 201.—_Eriocephala calthella_, ♂, side view: _t_, palpiform
    suranal plate; _cl_, claspers; _s_, inferior claspers; _mxp_,
    maxillary palpi; _cx_. coxa; _tr_, trochanter; _sc_, scutum; _sc′_,
    scutellum.
]

In Lepidoptera, notably the male of the very generalized _Eriocephala
calthella_ (Fig. 201), besides the broad unjointed claspers, which are
curved upward and provided with a brush of stiff hooked setæ (this upper
pair being perhaps modified cercopods), there is an accessory lower
slenderer pair, while the suranal plate (_t_) is palpiform or clavate
and also adapted to aid in the action of the claspers. The examination
of the cercopods and rhabdopods in the Trichoptera and in a generalized
lepidopterous form like this enables one to understand the morphology of
the genital armature, since it consists, besides the suranal plate,
which is often deeply forked (in Sphingidæ, Smith), of a pair of
modified hook-like cercopoda, and in some cases (Eriocephala) of an
additional pair of claspers which may be the homologues of the ephemerid
rhabdopods. A pair of hooks, often strong and claw-like (_harpes_), are
situated, one near the base on the inside of each clasper; they are
especially developed in the Noctuidæ (Smith), and appear to be present
in certain Trichoptera, but this remains to be proved. This complicated
apparatus of claspers and hooks is utilized by those insects which pair
while on the wing, and is wanting in such forms as Coleoptera and
Hemiptera. Besides the forceps of Panorpa, there are two pairs of
slender filiform appendages which need farther examination. In the
Diptera, especially Tipulidæ, there is a pair of 2–jointed appendages or
forceps, as in Limnophila (Osten Sacken). The male genital armature of
Diptera appears to be on the same general plan as in Lepidoptera, but
more complicated.

[Illustration:

  FIG. 202.—Male organs of generation of Athalia.—After Newport.
]

Notice should also be taken of the paired uncinate hooks which are
modifications of the penis-sheath of the male of cockroaches
(Phyllodromia), which Haase states appear to originate on the tenth
ventral plate, and which probably “serve to open and dilate the vagina
of the female, especially as a perforated penis, which is highly
developed in Machilis, seems to be wanting in the Blattidæ.” (Haase.)

=The penis.=—This is a single or double median style-like structure
either hollow and perforated, or solid, very variable in shape,
receiving the end of the ejaculatory duct. It is usually enclosed
between two lateral plates, the homologues perhaps of the inner pair of
sheaths of the ovipositor. In the Coleoptera, as in Carabidæ and
Melolonthidæ, the penis is a long chitinous tube, “retractile within the
abdomen on the under surface as far as the anterior segments.”
(Newport.) In the Hymenoptera, of which that of the saw-flies is a type,
Newport states that it “consists of a short valvular projectile organ,
covered externally by two pointed horny plates (_i_) clothed with soft
hairs.” Above these are two other irregular double-jointed plates (Fig.
202, _l_) surrounded at their base by a chitinous ring (_k_); they are
edged with prehensile hooked spines (_i_). Between these in the middle
line are two elongated muscular parts (_m_) which enclose the penis
(_h_), and which, like those in beetles, perhaps aid in dilating the
vulva of the female.

An examination of Figs. 203–207 will aid in understanding the various
modifications in beetles, etc., of this organ.

A general study of the anatomy and homologies of the male genital
armature, from a developmental point of view, together with a comparison
of them with the corresponding female organs, is still needed.

[Illustration:

  FIG. 203.—End of abdomen situated under the anal lobes of _Hydrophilus
    piceus_, drawn out, seen from the ventral side: 6, sternal region of
    6th segment; 7, 8, 9, segments telescoped, when retracted, in 6th
    segment; _zw_, membrane connecting 6th and 7th segments; _G_,
    intromittent apparatus; _vl_, external lobes; _vlu_, inner lobes;
    _pn_, penis.
]

[Illustration:

  FIG. 204.—The same as in Fig. 203, seen from the side: 6, the free 6th
    segment; 7–10, the four last, when at rest, retracted and telescoped
    within the 6th segment, with the copulatory apparatus (_g_); _vl_,
    outer, _vlu_, inner lobe; 10_s_, tergite of 10th segment; 10_i_,
    sternite of the same; _an_, anal opening.
]

[Illustration:

  FIG. 205.—Terminal parts of the male copulatory apparatus of
    _Hydrophilus piceus_, torn apart: _vlu_, the two inner lodes; _pn_,
    penis; _x_, membrane torn from under side of penis; _ej_,
    ejaculatory duct; _os_, its opening on the under side of the penis,
    directly under its tip. The muscles, tracheæ, and nerves are not
    drawn.
]

=Velum penis.=—In the locusts (Acrydiidæ) the penis is concealed by a
convex plate, flap, or hood, free anteriorly and attached posteriorly
and on the sides to the ridge forming the upper edge of the tenth
sternite. When about to unite sexually, the tip of the abdomen is
depressed, the hood is drawn backward, uncovering the chitinous penis.

=The suranal plate.=—This is a triangular, often thick, solid plate or
area, the remnant of the tergum of the last, usually tenth, segment of
the abdomen, the supra-anal or suranal plate, or anal operculum (_lamina
supra-analis_) of Haase. In most lepidopterous larvæ this plate is well
marked; in those of the Platypteridæ it is remarkably elongated, forming
an approach to a flagellum-like terrifying appendage, and in that of
_Aglia tau_ it forms a long, prominent, sharp spine. In the cockroach,
both Cholodkowsky and Haase maintain that the tenth abdominal segment is
suppressed in the female, the tergal portion being fused with the
suranal plate (the latter in this case, as we understand it, being the
remnant of the eleventh segment of the embryo). As to the nature of the
middle jointed caudal appendage in Thysanura and May-flies Heymons has
satisfactorily shown that it is a hypertrophied portion of the suranal
plate, being in Lepisma but a filamental elongation of the small
eleventh abdominal tergite.

[Illustration:

  FIG. 206.—Copulatory organ of a weevil, _Rhychophorus phœnicis_, seen
    from above. _A_, _vl_, the lobes united into a capsule; _pp_, torn
    membrane which connects the capsule with the 9th abdominal segment;
    _ej_, ejaculatory duct. _B_, the same seen from the side; _mu_, end
    of the muscle of the penis. _C_, the same as _B_, without the
    capsule; _os_, opening of the ejaculatory duct (_ej_). Other letters
    as in _A_.
]

[Illustration:

  FIG. 207.—_A_, penis (_pn_) of _Carabus hortensis_: _bl_, wrinkled
    membranous vesicle; _vlu_, the valves; _g_, part of 9th segment.
    _B_, end of penis of the same, enlarged; _os_, cleft-like opening;
    also a wrinkled vesicle, as at _bl_.—This and Figs. 203–205 after
    Kolbe.
]

At the base of the suranal plate of locusts (Acrydiidæ) is the suranal
fork or suranal furcula (_furcula supra-analis_, as we have called it)
(Fig. 88, 89, _f_).

=The podical plates or paranal lobes.=—In the cockroach and other
insects, also in the nymphs of Odonata, the anus is bounded on each side
by a more or less triangular plate, the two valves being noticeable in
lepidopterous larvæ. They are the _valvulæ_ of Burmeister, and podical
plates of Huxley, who also regarded them as the tergites of an eleventh
abdominal segment;[38] and the subanal laminæ of Heymons. They are
wanting in Ephemeridæ.

=The infra-anal lobe.=—Our attention was first called to this lobe or
flap, while examining some geometrid larvæ. It is a thick, conical,
fleshy lobe, often ending in a hard, chitinous point, and situated
directly beneath the vent. Its use is evidently to aid in tossing the
pellets of excrement away so as to prevent their contact with the body.
The end may be sharp and hard or bear a bristle. Whether this lobe is
the modified ventral plate of the ninth urite, we will not undertake at
present to say.

=The egg-guide.=—In the Acrydiidæ the external opening of the oviduct is
bounded on the ventral side by a movable, triangular, acute flap, the
egg-guide (Fig. 88, _B_, _eg_). Whether this occurs in other orders
needs to be ascertained.




              LITERATURE ON THE ABDOMEN AND ITS APPENDAGES


            _a._ General (including the cerci, stili, etc.)

  =Cornelius, C.= Beiträge zur naheren Kenntnis von _Palingenia
    longicauda_. (Programm d. Real- u. Gewerbeschule zu Elberfeld, 1848,
    pp. 1–38, 4 Taf.)

  =Schiödte, J. G.= Bemerkungen über Myrmecophilen. Ueber den Bau des
    hinterleibes bei einigen Käfergattungen. (Germar’s Zeits. f. Ent.,
    1844.)

  —— De metamorphosi Eleutheratorum observationes. (Naturhist. Tidsskr.
    i-xiii, 1861–1883.)

  =Meinert, F.= Anatomia Forficularum. Anatomisk undersogelse af de
    Danske Orentviste, i. (Naturhist. Tidsskr. 3 raekke, ii, 1863, pp.
    427–482, 1 Pl.)

  —— Om dobbelte Saedgange hos insecter. (Naturhist. Tidsskrift, 3
    raekke, v, 1868.)

  =Tullberg, Tycho.= Sveriges Podurider. (Svenska Vetensk.-Akad. Handl.,
    1872, x, Nr. 10, 12 Pls.)

  =Davis, H.= Notes on the pygidia and cerci of insects. (Journ. R.
    Micr. Soc., 1879, ii.)

  =Westhoff, F.= Ueber den Bau des Hypopygiums der Gattung Tipula Meig.
    (Münster, 1882, pp. 1–62, 6 Taf.)

  =Saunders, Edward.= Further notes on the terminal segments of aculeate
    Hymenoptera. (Trans. Entom. Soc. London, 1884, pp. 251–267, 1 Pl.)

  =Palmén, J. A.= Ueber paarige Ausführungsgange der Geschlechtsorgane
    bei Insekten. (Helsingfors, 1884, 5 Taf.)

  =Haase, E.= Die Abdominalanhänge der Insekten mit Berücksichtigung der
    Myriopoden. (Morpholog. Jahrbuch, 1889, xv, pp. 331–435, 2 Taf.)

  —— Abdominalanhänge bei Hexapoden. (Sitzungsber. d. Gesellsch.
    naturforsch. Freunde, 1889, pp. 19–29.)

  =Wheeler, William M.= On the appendages of the first abdominal segment
    of embryo insects. (Trans. Wis. Acad. Sc., viii, 1890, pp. 87–140, 3
    Pls.)

  =Janet, Charles.= Études sur les fourmis, 5^e note, sur la morphologie
    du squelette des segments post-thoraciques chez les Myrmicides.
    (Mém. Soc. Acad, de l’Oise, xv, pp. 591–611, Figs. 1–5, 1894.)

  =Heymons, Richard.= Die Segmentirung des Insektenkörpers. (Anh. Abh.
    Akad. Berlin, Phys. Abh., pp. 39, Taf., 1895. See also Die
    Embryonalent, von Dermapteren und Orthopteren, under Embryology.)

  —— Grundzüge der Entwicklung und des Körperbaues von Odonaten und
    Ephemeriden. (Abhandl. k. Preuss. Akad. d. Wissens. Berlin, 1896, 2
    Taf., pp. 1–66.)

  —— Zur Morphologie der Abdominalanhänge bei den Insecten. (Morphol.
    Jahrb., iv, 1896, pp. 178–203, 2 Taf.)

  =Peytoureau, S. A.= Contribution à l’étude de la morphologie de
    l’armure génital des insectes. (Bordeaux, 1895, pp. 248, 22 Pls., 43
    Figs. in text.)

  =Verhoeff, C.= Cerci und styli der Tracheaten. (Ent. Nachr., xxi, pp.
    166–168, 1895.)

  Also the writings of Oudemans, Packard, etc.


                          _b._ The ovipositor

  =Lacaze-Duthiers, Henri.= Recherches sur l’armure génitale femelle des
    insectes. (Ann. d. sc. natur., 1849, xii, pp. 353–374, 1 Pl.; 1850,
    xiv, pp. 17–52, 1 Pl. (Hyménoptères); 1852, xvii, pp. 206–251, 1 Pl.
    (Orthoptères); 1853, xix, pp. 25–88, 4 Pl. (Neuroptères, Thysanures,
    Coléoptères, Diptères); pp. 203–237 (Lépidoptères, Aphaniptères en
    general. Also separate.))

  =Sollmann, A.= Der Bienenstachel. (Zeitschr. f. Wissens. Zool., xiii,
    1863, pp. 528–540, 1 Taf.)

  =Fengger, H.= Anatomie und Physiologie des Giftapparates bei den
    Hymenopteren. (Archiv f. Naturgesch., 1863, pp. 139–178, 1 Taf.)

  =Eaton, A. E.= Remarks upon the homologies of the ovipositor. (Trans.
    Ent. Soc. London, 1868, pp. 141–144.)

  =Packard, A. S.= On the structure of the ovipositor and homologous
    parts in the male insects. (Proc. Boston Soc. Nat. Hist., xi, 1868,
    pp. 393–399, Figs. 1–11.)

  =Lambrecht, A.= Samtliche Teile des Stechapparates in Bienenkörper und
    ihre Verwendung zu technischen und vitalen Zwecken. (Bienemwirtsch.
    Centralbl., 7 Jahrg., 1871, pp. 5–11.)

  =Kraepelin, K.= Untersuchungen über den Bau, Mechanismus und die
    Entwicklung des Stachels der bienenartigen Tiere. (Zeitschr. f.
    wiss. Zoologie, xxiii, 1873.)

  =Dewitz, H.= Vergleichende Untersuchungen über Bau und Entwicklung des
    Stachels der Honigbiene und der Legescheide der grünen Heuschrecke.
    (Königsberg, 1874.)

  —— Ueber Bau und Entwicklung des Stachels und der Legescheide einiger
    Hymenopteren und der grünen Heuschrecke. (Zeitschr. f. wissen.
    Zoologie, xxv, 1874, pp. 174–200, 2 Taf.)

  —— Ueber Bau und Entwicklung des Stachels der Ameisen. (Zeitschr. f.
    wiss. Zoologie, 1877, xxviii, pp. 527–556, 1 Taf.)

  =Adler, H.= Legeapparat und Eierlegen der Gallwespen. (Deutsche Entom.
    Zeitschr., 1877, xxi Jahrg., pp. 305–332, 1 Taf.)

  =Cholodkowsky, N.= Ueber den Hummelstachel und seine Bedeutung für die
    Systematik. (Zool. Anzeiger, vii, 1884, pp. 312–316.)

  =Ihering, H. von.= Der Stachel der Meliponen. (Ent. Nachr., 1886, xii,
    Jahrg., pp. 177–188, 1 Taf.)

  =Meinert, F.= Bidrag til de danske Myrers Naturhistorie. (Kjöbenhavn,
    1890, 68 s.u. Danske Vidensk. Selsk. Skrifter, 5 Raek, v, 3 Pls.)

  =Beyer, O. W.= Der Giftapparat von _Formica rufa_ ein reduziertes
    Organ. (Jena. Zeitschr. Naturw., 1890, xxv, pp. 26–112, 2 Taf.)

  =Carlet, G.= Mémoire sur le venin et l’aiguillon de l’abeille. (Ann.
    d. sc. nat. Zool., 7 sér., ix, 1890, pp. 1–17, 1 Pl.)

  =Künckel d’Herculais, J.= Méchanisme physiologique de la ponte chez
    les insectes orthoptères de la famille des Acridides.—Rôle de l’air
    comme agent mécanique et fonctions multiples de l’armure génitale.
    (Compt. Rend., cxix, pp. 244–247, 1894.)

  Also the writings of Verhoeff, Heymons.


                   _c._ The external genital armature

  =Klug, Johann C. F.= Versuch einer Berichtigung der Fabriciusschen
    Gattungen Scolia u. Tiphia. (Ueber u. Mohr Beiträge zur Naturkunde,
    i, pp. 8–40, 1805, 1 Taf.)

  =Audouin, J. V.=, and =Lachat=. Observations sur les organes
    copulateurs males des Bourdons. (Annal. général, d. sc. phys., 1821,
    viii, pp. 285–289.)

  =Audouin, J. V.= Lettre sur la génération des insectes. (Ann. des sc.
    nat., sér. 1, ii, 1824.)

  =Suckow, F. W.= L. Geschlechtsorgane der Insekten. (Heusinger,
    Zeitschr. organ. Physik., 1828, ii, pp. 231–264, 1 Taf.)

  =Rathke, H.= De Libellularum partibus genitalibus. Regiomonti, 1832,
    pp. 6 + 38, 3 Pls.

  =Siebold, C. Th. E. von.= Ueber die Fortpflanzungsweise der
    Libellulinen. (Germar’s Zeitschr. f. Ent., 1840, ii, pp. 421–438.)

  =Selys-Longchamps, E. de.= Monographie des Libellulidees d’Europe. 4
    Pls., 1840.

  =Bassi, C. A.= Studi sulle funzioni degli organi genitali degl’
    insetti da lui osservati piu specialmente nella _Bombyx mori_. (Atti
    della 5 Riun. d. Scienz. Ital. Lucca, 1844, pp. 39–94.)

  =Stein, F.= Vergleichende Anatomie und Physiologie der Insekten, i.
    Die weiblich. Geschlechtsorgane der Käfer. Berlin, 1847, 9 Taf.

  =Ormancey, P.= Recherches sur l’étui penial considéré comme limite de
    l’espèce dans les coléoptères. (Ann. sc. nat., 1849, 3 sér., Zool.,
    xii, pp. 227–242.)

  =Roussel, C.= Recherches sur les organes génitaux des insectes
    coléoptères de la famille des Scarabéides. (Compt. rend. Acad. d.
    sc. Paris, 1860, l, pp. 158–161.)

  =MacLachlan, R.= A monographic revision and synopsis of the
    Trichoptera of the European fauna. London, 1874–80, 59 Pls.

  —— On the sexual apparatus of the male Acentropus. (Trans. Ent. Soc.
    London, 1872, pp. 157–162.)

  =Thomson, C. G.= Nagra anmarkningar ofver arterna af slagtet Carabus.
    (Thomson’s Opuscula Entomologica, vii, 1857, pp. 615–729, 1 Pl.)

  =Dufour, L.= Sur l’appareil génital male du _Coræbus bifasciatus_.
    (Thomson, Archiv ent., 1857, i, pp. 378–381.)

  =White, F. Buchanan.= On the male genital armature in the Rhopalocera.
    (Trans. Linn. Soc., ser. 1, Zool., i, pp. 357–369, 1876, 3 Pls.)

  =Graber, V.= Die Aehnlichkeit im Baue der ausseren weiblichen
    Geschlechtsorgane bei den Lokustiden und Akridiern auf Grund ihrer
    Entwicklungsgeschichte. (Sitzber. k. Akad. d. Wissensch. Wien.,
    1870, lxi, pp. 1–20, 1 Taf.)

  =Scudder, Samuel H., and Edward Burgess.= On asymmetry in the
    appendages of hexapod insects, especially as illustrated in the
    lepidopterous genus Nisoniades. (Proc. Boston Soc. Nat. Hist., 1871,
    xiii, pp. 282–306.)

  =Chadima, J.= Ueber die Homologie zwischen den männlichen und
    weiblichen ausseren Sexualorganen der Orthoptera Saltatoria Latr.
    (Mitteil. d. naturwiss. Vereins f. Steiermark, 1872, pp. 25–33, 1
    Taf.)

  =Hagens, von.= Ueber die Genitalien der männlichen Bienen, besonders
    der Gattung Sphecodes. (Berlin Ent. Zeitschr., 1874, pp. 25–43.)

  —— Ueber die männlichen Genitalien der Bienengattung Sphecodes.
    (Deutschen Entom. Zeitschr., 1882, pp. 209–228, 2 Taf.)

  =Lindenman, C.= Vergleichend-anatomische Untersuchung ueber das
    männliche Begattungsglied der Borkenkäfer. (Bull. Soc. Imp. d.
    Natural. Moscou, 1875–77.)

  =Forel, A.= Der Giftapparat und die Analdrüsen der Ameisen. (Zeitschr.
    f. wiss. Zoologie, xxx, suppl., 1878.)

  =Kraatz, G.= Ueber die Wichtigkeit der Untersuchung des
    männlichen Begattungsgliedes der Käfer für die Systematik und
    Artunterscheidung. (Deutschen Entom. Zeitschr., 1881, xxv, pp.
    113–126.)

  —— Ueber das männliche Begattungsglied der europaischen Cetoniiden und
    seine Verwendbarkeit für deren scharfe spezifische Unterscheidung.
    (Ibid., pp. 129–149.)

  =Gosse, Ph. H.= On the clasping-organs ancillary to generation in
    certain groups of the Lepidoptera. (Trans. Linn. Soc., 1882, Ser. 2,
    Zool., ii, pp. 265–345, 8 Pls.)

  —— The prehensors of male butterflies of the genera Ornithoptera and
    Papilio. (Proc. Roy. Soc. London, 1881, xxxiii, pp. 23–27.)

  =Radoszkowski, O.= Revision des armures copulatrices des males du
    genre Bombus. (Bull. Soc. Natur. Moscou, 1884, xlix, pp. 51–92, 4
    Pls.)

  —— Revision des armures copulatrices des males de la tribu
    Philérémides. (Ibid., 1885, lxi, pp. 359–370, 2 Pls.)

  —— Revision des armures copulatrices des males de la famille des
    Mutillidæ. (Horæ Soc. Ent. Ross., 1885, xix, pp. 3–49, 9 Pls.)

  —— Revision des armures copulatrices des males de la tribu des
    Chrysides. (Horæ Soc. Ent. Ross, xxiii, 1890, pp. 3–40, 6 Pls.)

  =Hofmann, O.= Beiträge zur Kenntnis der Butaliden. (Stett. Ent. Zeit.,
    1888, pp. 335–347, 1 Taf.)

  =Driedzichi, H.= Revue des espèces européennes du genre Phronia Winn.
    (Horæ Soc. Ent. Ross., 1889, xxiii, pp. 404–532, 10 Pls.)

  =Sharp, David.= On the structure of the terminal segment in some male
    Hemiptera. (Trans. Ent. Soc. London, 1890, pp. 399–427, 3 Pls.)

  =Escherich, K.= Die biologische bedeutung der Genitalanhänge der
    Insekten. (Verhandl. d. zool. bot. Ges. Wien., 1892.)

  —— Anatomische Studien über das männliche Genitalsystem der
    Coleopteren. (Zeits. f. wissens. Zool., lvii, pp. 620–641, 1 Taf., 3
    figs.)

  =Verhoeff, C.= Zur vergleichenden Morphologie der “Abdominalanhänge”
    der Coleopteren. (Ent. Nachr., xx, pp. 93–96, 1894. Compare also O.
    Schwarz and J. Weise’s criticisms in D. Ent. Zeit., pp. 153–157;
    also pp. 101–109, 155–157. Zool. Anzeiger, pp. 100–106, 1894.)

  —— Vergleichende—morphologische Untersuchungen ueber das Abdomen
    der Endomychiden, etc., und über die Musculature des
    Copulationsapparates von Triplax. (Archiv f. Naturg., lxi, pp.
    213–287, 2 Taf., 1895.)

  —— Vergleichende Untersuchungen über die abdominal Segmente der
    weiblichen Hemiptera-Heteroptera und Homoptera. (Verh. Nat. Ver.
    Bonn, l, pp. 307–374, 1894.)

  =Verhoeff, C.= Beiträge zur vergleichenden Morphologie des Abdomens
    der Coccinelliden, etc. (Archiv f. Naturg., lxi, pp. 1–80, 6 figs.,
    1895.)

  =Boas, J. E. V.= Organe copulateur et accouplement der hanneton.
    (Oversigt over det K. Danske Vidensk. Selskab Forhand, 1892,
    Copenhagen, 1893, 1 Pl., pp. 239–261.)

  =Pérez, J.= De l’organe copulateur mâle des Hyménoptères et de sa
    valeur taxonomique. (Ann. Soc. Ent. France, lxiii, pp. 74–81, Figs.,
    1894.)

  =Goddard, Martha Freeman.= On the second abdominal segment in a few
    Libellulidæ. (Proc. Amer. Philos. Soc., xxxv, pp. 205–212, January
    11, 1897, 2 Pls.)

  Also the writings of Eaton, Emery, Fischer, Forel, Géhin, Godart,
    Hagen, Joly, Koletani, Loew, Meinert, Mik, Nicolet, Osten Sacken,
    Pictet, Roussel, Schaeffer (1754), Schaum, Schenk, J. B. Smith,
    Thompson, Buchanan-White, Brunner von Wattle-Wyll, Weise, Wyenbergh.

  The subject of copulation has been treated by Hoffer, Hartig,
    Schiedeknecht, Verhoeff, etc.




     THE ARMATURE OF INSECTS: SETÆ, HAIRS, SCALES, TUBERCLES, ETC.


[Illustration:

  FIG. 208.—Larva of _Dryocampa rubicunda_, stage II.—Bridgham _del._
]

=The cuticula.=—The integument is externally either smooth and shining
or variously punctured, granulated, tuberculated, striated, or hairy. In
certain orders the skin is clothed with flattened setæ or scales, while
many forms, as some caterpillars (Figs. 208, 209), beetles (Fig. 210),
etc., are protected by spines, horns, etc., these in adult insects often
forming secondary sexual characters, usually being more developed in the
males than in the females.

[Illustration:

  FIG. 209.—Larva of _Hyperchiria io_, on hatching.
]

  The cuticula is not always smooth, but is often finely granulated or
  even minutely spinulated. On the abdominal segments of Anabrus, as
  observed by Minot, the cuticula is armed with microscopic conical
  nodules scattered irregularly over it. They do not correspond, he
  says, in any way to hairs; for they do not rest over pores, nor did
  he see any specially modified cells underlying them. “As far as I
  have observed, they are mere local irregularities, each nodule being
  apparently supported by some four or six unmodified epidermal
  cells.” Minot adds that the whole of the cuticula, except the cones
  just described and the hairs, is divided into numerous minute
  fields, each of which corresponds to a single cell of the underlying
  hypodermis. Each field is bounded by a distinct polygonal outline,
  and its surface is either covered by a large number of extremely
  minute projecting points, as on the dorsal arch of the segment, or
  is smooth, as upon the articular membrane and ventral arch. Upon the
  sides of the dorsal arch and upon the spiracular membrane each field
  has a projecting spine or sometimes two or even three. (See also pp.
  28, 30.)

[Illustration:

  FIG. 210.—_Phanæus pegasus_, ♂, from Mexico.—After Graber.
]

[Illustration:

  FIG. 211.—Section of integument of _Datana ministra_: _c_, cuticula;
    _hyp_, hypodermis; _p_, outer pigmented nodulated layer.
]

  The cuticle of lepidopterous larvæ has also been described and
  figured by Minot. In the caterpillars of different groups
  investigated by him, the cuticle was found to be rough with
  microscopic teeth or spinules, erect or flattened and scale-like,
  and either densely crowded or scattered, and affording excellent
  generic and specific characters. In the slug-worms (Limacodids) we
  have observed that the cuticula is unusually rough, especially on
  the spiniferous tubercle of Empretia, Parasa, etc. (Fig. 213, _c_).
  The skin of the body between the tubercles is seen to be finely
  shagreened, due to the presence of fine teeth, which are more or
  less curved and bent, these teeth arising from a very finely
  granulated surface (_d_). The cuticle of neuropterous,
  trichopterous, and tenthredinid larvæ will probably afford similar
  cases. The integument of the larva of Datana is, on the black bands,
  rough and nodulated, the irregular nodules being filled with a black
  pigment, and forming a layer (_p_) external to the true cuticula
  (Fig. 211).

[Illustration:

  FIG. 212.—Hairs of Datana: _f_, formative hair-cell; _c_, cuticula;
    _p_, pigmented layer; _hy_, hypodermis.
]

The integument of many insects contains fine canals passing through the
chitinous layers and opening externally in minute pores. Certain of the
pore-canals communicate with hollow setæ which sit directly over the
pores; other pores form the external openings of dermal glands, but in
many cases they are empty or only filled with air, and do not have any
hairs connected with them. Each of these pores communicates with a
hair-forming hypodermal cell, called by Graber a _trichogen_.

=Setæ= (“hairs” and bristles).—The setæ of insects are, as in worms,
processes of the cuticle originating from certain of the hypodermal
cells. They arise either from a ring-like pit, or from a minute
tubercle, and are usually situated at the outlet of a pore-canal, which
connects with an underlying cell of the hypodermis (Fig. 212). They are,
then, bristle or hair-like processes arising from the hypodermis. Where
the hairs or setæ are rubbed off, their site is indicated by a minute
ring like a follicle in the chitinous integument. The cuticular hair,
says Leydig, is in its first condition the secretion of the cellular
element of the skin, and a thread-like continuation of the cell-body may
rise up through the pore-canal into the centre of the hair, remaining
there permanently.

While the setæ are usually simple, they are often branched, plumose, or
spinulose, as in larval Hemerobiidæ, Anthrenus, and Dermestes, the larvæ
of certain coccinellid beetles, notably Epilachna, and of Cassida, the
larvæ of arctians, etc., and in bees (Anthophila, Megachile, Osmia,
Colletes, Apis, etc.).

The use of these spinulose, plumose, and twisted hairs in the bees is
clearly shown by J. B. Smith, who states that as these insects walk over
flowers, the pollen grains adhere to the vestiture, “and this also
accounts for the fact, probably noticed by every observant fruit-grower,
that bees frequently bury themselves completely in the blossoms, or roll
over every part of them. Such insects are after pollen, not honey, and
by so rolling about, the pollen grains are brought into contact with and
adhere to the surface of the insect.” The syrphid flies also pollenize
flowers, the pollenizing of chrysanthemums being effected, as Smith
states, by _Eristalis tenax_, and he adds that the body vestiture of the
syrphids “is often composed of spurred and branched hairs.” (For
reference to gathering hairs, see p. 45.)

[Illustration:

  FIG. 213.—Cuticular spinules of larva of Adoneta: _a_, _b_, _c_, _d_,
    different forms; _e_, _e′_, caltrops.
]

Certain remarkable spines occur in limacodid larvæ, notably Empretia and
Adoneta. These we have called caltrops spines, from their resemblance to
the caltrops formerly used in repelling the attacks of cavalry. They are
largely concerned in producing the poisonous and irritating effects
resulting from contact with the caterpillars of these moths, and are
situated in scattered groups near the end of the tubercles. A group of
three is represented at Fig. 213, _e_. They are not firmly embedded in
the cuticle, but on the contrary appear to become very easily loosened
and detached, and they probably, when brought into contact with the skin
of any aggressor, burrow underneath, and are probably in part the cause
of the continual itching and annoyance occasioned by these creatures. It
will be seen by reference to Fig. 213, _e′_, that the body of the spine
is spherical, with one large, elongated, conical spine arising from it,
the spherical base being beset with a number of minute, somewhat obtuse
spinules.

[Illustration:

  FIG. 214.—Glandular hairs of caterpillars. _A_, _Dasylophia anguina_:
    _a_, of body; _b_, of head; _c_, of prothoracic shield. _B_,
    _Ceratosia tricolor_: _a_, on body; _b_, on abdominal legs. _C_,
    _Schizura ipomeæ_: _a_, from third thoracic segment; _b_, from larva
    stage II; _c_, simple setæ from minute warts.
]

=Glandular hairs and spines.=—In some insects occur fine, minute, hollow
setæ from which exude, perhaps through pore-canals of extreme fineness,
droplets of a clear watery or plasma-like sticky fluid. The club-shaped
tenent hairs of the feet of Collembola, and the hairs fringing the feet
of Diptera, are modified glandular hairs. Here they serve to give out a
sticky fluid enabling the insect to walk on smooth surfaces; they end in
a vesicle-like bulbous expansion, which may contain numerous
pore-canals. Those of caterpillars were first noticed by Zeller, and
Dimmock has particularly described those of the larvæ of Pterophoridæ.
They are either club-shaped, or variously forked at the end (Fig. 214,
_B_, _a_). They are usually replaced after the first larval moult by
ordinary, simple, solid, pointed setæ, and their use in caterpillars is
as yet unknown. Whether these hairs, as seems most probable, arise from
a specialized glandular hypodermal cell, or not, has not yet been
discovered.

[Illustration:

  FIG. 215.—_A_, group of setæ arising from a subdorsal tubercle: _cut_,
    the cuticle; _hy_, the hypodermis; _sc_, the enlarged and
    specialized cells of the hypodermis which secrete the spines
    themselves; _pglc_, the nuclei which secrete the venomous fluid
    which fills the cavity of the seta (_s_), seen at _p_ in a broken
    spine. _B_, a short entire, and a long broken seta (_s-p_); _pgle_,
    four poison cells; _p_, the poison in the hollow of the spine.
]

These temporary fine glandular hairs are probably the homologues of the
larger true glandular bristles and spines of the later stages of certain
lepidopterous larvæ, which are brightly colored and lead an exposed
life, living through a large part of the summer. In these structures the
bristles or spines are hollow, filled with a poisonous secretion formed
in a single large, or several smaller specialized hypodermal cells
situated under the base of the spine. In the venomous spines of _Lagoa
crispata_ the poisonous fluid in the larger spines (Figs. 215, _C_, 216,
_b_) is secreted in several large cells situated at the base of the
spine, and this is the usual form. In the finer spines of a large
tubercle (Figs. 215, _A_, 216) there appears to be a differentiation of
the hypodermal cells into two kinds, the large, basal deep-seated,
setigenous cells (216, _sc_) and the poison-secreting nuclei (216,
_pglc_) situated nearer the base of the setæ. The spines being filled
with poison and breaking into bits in the skin of the hands or neck,
cause great irritation and smarting. These nettling or poisonous hairs
or spines are especially venomous in the larva of Orgyia, _Empretia
stimulea_, _Hyperchiria io_, the larvæ of the saturnians (Fig. 217) and
lasiocampids, etc. They rarely occur in insects of other orders, though
the skin of Telephorus is said by Leydig to bear glandular hairs.

[Illustration:

  FIG. 216.—Section of a subdorsal tubercle from a larva in stage 1:
    _sc_, the setigenous cells, one for each seta; _pglc_, nuclei by
    which the poison is secreted; _s_, seta; _p_, poison in middle of a
    broken spine; _cut_, cuticle; _sd_, _tub_, spinulated surface of the
    subdorsal tubercle.
]

  Leydig states that in the stout bristles of Saturnia there is, as in
  the integument of the body, a homogeneous cuticula, under which is
  the cellular matrix (hypodermis), and the clear contents
  (hyaloplasma) are secreted from the blood. The cell-structure of the
  hairs consist, as in the cells of the body, of spongioplasma and
  hyaloplasma. Leydig has observed the droplets of the secretion of
  the caterpillar of _Saturnia carpini_ oozing through distinctly
  observable pores, and states that there are similar openings in the
  hairs and scales. Dewitz found easily observable openings at the end
  of the hair of a large exotic weevil (Fig. 130).

  The advanced nymph of Psylla is also armed with clavate glandular
  hairs (Fig. 178).

[Illustration:

  FIG. 217.—Armature of last four segments of _Callosamia promethea_:
    _a_, a dorsal seta; _b_, one showing the poison (_p_) within.
]

  The tubercles are outgrowths of the body-walls; they are either
  smooth, warty, or spiny, as in many caterpillars. While the armature
  of insects is of little morphological importance, it is evidently of
  great biological importance, the welfare or even the life of the
  insect depending upon it; and it varies in each species of insect,
  especially in Diptera, where the position of even a single seta
  characterizes the species.

[Illustration:

  FIG. 218.—Section through an antennal pectination of _Saturnia
    carpini_: _a_, hypodermis, formative cells of the hairs (_c_); _d_,
    cuticula; _e_, trachea.—After Semper.
]

[Illustration:

  FIG. 219.—Flattened hairs from the lateral tufts of larva of
    _Gastropacha americana_: _A_, three from the lateral tuft of
    _Heteropacha rileyana_.
]

The mode of development of the hairs was first described by Semper. In
the pectination of the antenna of _Saturnia carpini_ he observed that
the hairs arise, like the scales of the wings, from large round
formative-cells lying in the cavity, which send out through the
hypodermis and cuticle a long slender process which finally becomes the
hair (Fig. 218).

Tactile hairs are those setæ arising over nerve cells or nerve
terminations and will be discussed under the organs of sense.

[Illustration:

  FIG. 220.—The same in _G. quercifolia_: _a_, a small hair ending in
    two minute processes.
]

=Scales.=—In very rare cases the hairs of caterpillars (Fig. 219) are
flattened and scale-like, and this passage in the same insect of
cylindrical hairs into flattened scale-like ones, shows that the scales
are only modified hairs. Also, as we shall see farther on, Semper has
proved that their mode of origin is identical. While true scales are
characteristic of Synaptera (Thysanura and Colembola), as well as
Lepidoptera and Trichoptera, they also occur in the Psocidæ
(Amphientomum), in many Coleoptera (Curculionidæ, Cleridæ, Ptinidæ,
Dermestidæ, Byrrhidæ, Scarabæidæ, Elateridæ, and Cerambycidæ), and in
the Culicidæ, and a few other Diptera, though they are especially
characteristic of the Lepidoptera, not a species of this great order
being known to be entirely destitute of them.

[Illustration:

  FIG. 221.—Flattened and spinulated hairs of tufts of larva of
    _Acronycta hastulifera_.
]

[Illustration:

  FIG. 222.—Scales from dorsal tuft, on second thoracic segment of larva
    of _Gastropacha quercifolia_.
]

The scales vary much in shape, but are more or less tile-like, attached
to the surface of the body or wing by a short slender pedicel, and are
more loosely connected with the integument than the hairs, which are
thicker at the base or insertion than beyond.

The markings of the scales, both of Synaptera and Lepidoptera, are very
elaborate, consisting of raised lines, ridges, or striæ with transverse
ridges between. “The striæ of the transparent scales of Micropteryx are
from about 500 to 300 to the millimetre, varying in different species.
The opaque scales of Morpho, which show metallic reflections, have about
1400 striæ to the millimetre.” (Kellogg.)

The primary use of scales, as observed by Kellogg, is to protect the
body, as seen in Synaptera and Lepidoptera. A nearly as important use is
the production of colors and patterns of colors and markings, while in
certain butterflies certain scales function as the external openings of
dermal scent-glands, and they afford in some cases (as first claimed by
Kettelhoit in 1860) generic and specific characters. Spuler has shown
that the scales are strengthened by internal chitinous pillars. Burgess
has observed in the scales of _Danais plexippus_ that the under surface
of the scales is usually smooth, or provided with few and poorly
developed ridges, and this has been confirmed by Spuler and by Mayer
(Fig. 226).

In the irised and metallic scales the ridges, says Spuler, are not
divided into teeth, and they converge at the base to the pedicel and
also toward the end of the scale (Micropteryx), or end in a single
process beyond the middle (the brass-colored scales of _Plusia
chrysitis_).

The arrangement of the scales on the wings is, in the generalized moths,
irregular; in the more specialized forms they are arranged in bands
forming groups, and in the most specialized Lepidoptera they are more
thickly crowded, overlapping each other and inserted in regular rows
crossing the wings, these rows either uniting with each other or running
parallel. (Spuler.) The scattered irregular arrangement seen in
Micropteryx is also characteristic of the Trichoptera and of
Amphientomum.

[Illustration:

  FIG. 223.—Portion of a longitudinal section through one of the young
    pupal wings of a summer pupa of _Vanessa antiopa_: _s_, young scale;
    _leu. cy._, leucocyte; _mbr. pr._, ground membrane; _prc_,
    hypodermis-cells.
]

[Illustration:

  FIG. 224.—Portion of a longitudinal section through one wall only of
    the pupal wing of a specimen slightly older than that of Fig. 223;
    _s_, older scale.
]

=Development of the scales.=—The mode of origin of the scales was first
worked out by Semper in 1886, who stated that in the wing of the pupal
Sphinx and Saturnia they are seen, in sections, to arise from large
roundish cells just under the hypodermis and which have a projection
which passes out between the hypodermis (his “epidermis”) cells,
expanding into a more or less spherical vesicle, the latter being the
first indication of the future scale. He observed that the scales are
not all formed at once, but arise one after another, so that on one and
the same wing the scales are in different stages of development.

[Illustration:

  FIG. 225.—Portion of a longitudinal section through a pupal wing about
    eight days before emergence: _s_, formative scale-cell; upper _s_, a
    scale.
]

More recently Schaeffer has stated that the scales and also the hairs
are evaginations of greatly enlarged hypodermis cells, and still more
complete evidence has been afforded by A. G. Mayer (1896). In the wings
of Lepidoptera, about three weeks before the imago emerges, certain of
the hypodermis cells, which occur at regular intervals, begin to
increase in size and to project slightly above the level of the
hypodermis; these are Semper’s “formative cells,” and are destined to
secrete the scales. They increase in length, and appear as in Fig. 223.
In the next stage observed, the projections are much longer (Fig. 224).
The hypodermis is now thrown up into a regular series of ridges, which
run across the wing. Each ridge, says Mayer, corresponds in position
with a row of formative cells, and each furrow with the interval between
two adjacent rows. The scales always project from the tops of these
ridges. The ground or basal membrane has not participated in this
folding, and the deep processes of the hypodermis (_prc_) that once
extended to this membrane have largely disappeared. Figure 225
represents a more advanced stage almost eight days before the emergence
of the imago.

The scales are originally filled with protoplasm, which gradually
withdraws, leaving behind it little chitinous bars or pillars which
serve to bind together the upper and lower surfaces of the scales, and
finally the scales become “merely little flattened hollow sacs
containing only air.” As Mayer shows (Figs. 226, 227), from the study of
scales examined four days before emergence of the butterfly (Danais),
“the striations upon the upper surface of the scale are due to a series
of parallel longitudinal ridges,” while the under side is usually
smooth.

The mode of insertion is seen in Fig. 227. The narrow cylindrical
pedicel of the scale is merely, according to Semper, inserted into a
minute close-fitting socket, which perforates the wing-membrane, and not
into a tube, as Landois supposed. Spuler describes a sort of double sac
structure or follicle (_Schuppenbalg)_ which receives the hollow pedicel
of the scale. This was originally (1860) observed by F. J. Carl Mayer,
but more fully examined by Spuler (Fig. 228) though not detected by A.
G. Mayer.

[Illustration:

  FIG. 226.—Portion of a cross-section through the pupal wing of _Danais
    plexippus_, about six days before emergence: _sg_, scale; _cta.al_,
    wing-membrane; _cl.frm_, formative cell of the scale; _mbr.pr_,
    ground-membrane; _fbr.h′drm_, hypodermal fibres of pupal wings. _A_,
    portion of a longitudinal section through the pupal wing, eight or
    nine days before emergence; _prc_, processes of young hypodermis
    scales.—This and Figs. 223–225 after Mayer.
]

=Spinules, hair-scales, hair-fields, and androconia.=—Besides the
scales, fine spinules occur on the thickened veins of the wings of the
Blattidæ, where they resemble fir-cones; also in the Perlidæ, in the
Trichoptera, and in the more generalized Lepidoptera (Micropterygidæ and
Hepialidæ), occur, as indicated by Spuler, delicate chitinous hollow
spinules scarcely one-tenth as long as, and more numerous than, the
scales, which sometimes form what he calls “Haftfelds,” or holding
areas. These spinules have also been noticed by Kellogg, and by myself
in Micropteryx; Kellogg, and also Spuler, have observed them in certain
Trichoptera (Hydropsyche). These also occur on the veins, and detached
ones near large one-jointed hairs, or hair-scales, said by Kellogg to be
striated. Kellogg has detected these scale-hairs, as he calls them, in
Panorpa.

[Illustration:

  FIG. 227.—View looking down upon the upper (_i.e._ exposed) surface of
    one of the large scales situated on the veins of _Danais plexippus_,
    about four days before emergence: _clm_, chitinous pillars found in
    scales. _A_, _a_ smaller scale, _a_, _a′_, sections of the scales.
    _B_, leucocyte found in the larger scale.—After Mayer.
]

[Illustration:

  FIG. 228.—Scale-follicles: _A_, of a scale of _Galleria mellonella_:
    _r_, neck-ring. _B_, the same of _Polyommatus phlæas_. _C_, the same
    of a hair on inner edge of hind wing of _Lycæna alexis_ ♀.—After
    Spuler.
]

[Illustration:

  FIG. 229.—_A_, portion of wing of a caddis-fly (Mystacides). _B_,
    enlarged, showing the androconia and hair-scales. _C_, a separate
    androconium.—After Kellogg.
]

The “hair-scales” of the phylogenetically older Trichoptera correspond
to certain scales of Lepidoptera, especially the Psychidæ (Spuler),
variously called “plumules” (Deschamps), “battledore scales,” also
certain minute cylindrical hairs. To these scent-scales is applied the
term _androconia_. They are found, almost without exception, on the
upper side of the fore wings, occurring in limited areas, such as the
discal spots, or on folds of the wings. Fritz Müller has shown that they
function as scent-scales, and are confined to the males. Kellogg has
detected androconia-like scales on the wings of a caddis-fly,
_Mystacides punctata_ (Fig. 229).

[Illustration:

  FIG. 280.—Cross-section of androconia surface on wing of _Thecla
    calanus_; _a_, androconia; _gl_, gland of base; _s_, ordinary
    scales; _w_, wing in section.—After Thomas.
]

Thomas has proved by sections of the wing of Danais, etc., that the
androconia arise from glands situated in a fold of the wing (Fig. 230),
and he states that the material elaborated by the local glands, and
distributed upon the surface of the wing by the androconia, is that
which gives to many of the Lepidoptera their characteristic odor. On
comparing these “glands,” it is evident that they are groups of
specialized formative cells of Semper (trichogens), which secrete an
odorous fluid, issuing perhaps from extremely fine pore-canals at the
ends of the androconia. They thus correspond to the glandular hairs,
poison-hairs, and spines of caterpillars, the formative cells of which
contain either a clear lymph or poison.




                               LITERATURE


      _a._ Hairs, bristles, cleaning spines, calcaria, combs, etc.

  =Leydig, Franz.= Zum feineren Bau der Arthropoden. (Müller’s Archiv f.
    Anat. und Phys., 1855, pp. 376–480.)

  =Fobel, Auguste.= Les fourmis de la Suisse. Bâle, 1874.

  =Saunders, Edward.= Remarks on the hairs of some of our British
    Hymenoptera. (Trans. Ent. Soc. London, 1878, pp. 169–171.)

  =Perez, J.= Notes d’apiculture. (Bull. Soc. d’Apic. de la Gironde,
    Bordeaux, 1882.)

  =Osten Sacken, C. R. von.= An essay on comparative chætotaxy, or the
    arrangement of characteristic bristles of Diptera. (Trans. Ent. Soc.
    London, 1884, pp. 497–517.)

  —— Preliminary notice of a subdivision of the suborder Orthorrhapha
    Brachycera (Diptera) on chætotactic principles. (Berlin Ent.
    Zeitschr., 1896, pp. 365–373.)

  =Janet, Charles.= Études sur les fourmis. 8^e Note. Sur l’organe de
    nettoyage tibio-tarsien de _Myrmica rubra_. (Ann. Soc. Ent. France,
    1895, pp. 691–704, 6 Figs.)

  See also J. B. Smith’s Economic Entomology, 1896, hairs of bees. Also
    the writings of De Geer, Huber, Fenger, Mayr, Forel, Canestrini, and
    Berlese (1880); Dahl, Cheshire, etc.


              _b._ Glandular and poisonous setæ and spines

  =Ratzeburg, J. Th. Ch.= Ueber entomologische Krankheiten. (Stettin
    Ent. Zeit., 1846, vii, pp. 35–41.)

  =Zeller, P. C.= Revision der Pterophoriden. (Linnæa Ent., vi, pp.
    319–416, 1852, at p. 356 speaks of “Drüsenhärchen.”)

  =Dimmock, George.= On some glands which open externally on insects.
    (Psyche, iii, pp. 387–401, 1882.)

  =Goossens, Th.= Des chenilles urticants. (Ann. Soc. Ent. France, 1881,
    pp. 231–236.) Des chenilles vésicants. (Ibid., 1886, pp. 461–464.)

  =Packard, A. S.= Notes on some points in the external structure and
    phylogeny of insects. (Proc. Boston Soc. Nat. Hist., xxv, 1890, pp.
    83–114, 2 Pls.)

  —— A study of the transformations and anatomy of _Lagoa crispata_, a
    bombycine moth. (Proc. Amer. Phil. Soc., xxxii, pp. 275–292, 7 Pls.,
    1894.)

  =Holmgren, Emil.= Studier öfner hudens och de körtelartade hudorganens
    morfologi hos Skandinaviska macrolepidopterlarver. (K. Svenska
    Vetenskaps-Akad. Handl., xxvii, pp. 1–83, Stockholm, 1895, 9 Pls.)

  Also the writings of Leydig, Keller, Bach, Karsten, Scribner, Riley,
    etc.

  (See also Literature of repugnatorial glands.)


                            _c._ Androconia

  =Deschamps, Bernard.= Récherches microscopiques sur l’organisation des
    ailes der Lépidoptères. (Ann. des Sc. nat. [?], iii, pp. 111–157,
    1835.)

  =Waufor, T. W.= On certain butterfly scales characteristic of sex.
    (London, 1867–68.)

  =McIntire, S. J.= Notes on the minute structure of the scales of
    certain insects. (London, 1871.)

  =Anthony, J.= The markings on the battledore scales of some of the
    Lepidoptera. (London, 1872.)

  =Scudder, S. H.= Antigeny or sexual dimorphism in butterflies. (Proc.
    Amer. Acad. Arts and Sc., xii, 1877, pp. 150–158.) Also Butterflies,
    etc. (New York, 1881, pp. 192–206, figs.)

  =Müller, Fritz.= A prega costal das Hesperideas. (Archivas do Museo
    nac. do Rio de Janeiro, iii, pp. 41–50, 2 Pls., 1878.)

  =Thomas, M. B.= The androconia of Lepidoptera. (Amer. Nat., xxvii, pp.
    1018–1021, 2 Pls., 1893.)


                              _d._ Scales

  =Leydig, Franz.= Zum feineren Bau der Arthropoden. (Archiv f. Anat.
    und Phys., 1855, pp. 376–480, 1 Taf.)

  =Semper, Carl.= Beobachtungen über die Bildung der Flügel, Schuppen,
    und Haare bei den Lepidopteren. (Zeitschrift f. wissensch. Zoologie,
    1857, pp. 326–339, 1 Taf.)

  =Mayer, F. T. Karl.= Ueber den Staub der Schmetterlingsflügel.
    (Allgem. mediz. Centralzeitung, 1860, pp. 772–774.)

  =Landois, H.= Beiträge zur Entwicklungsgeschichte der
    Schmetterlingsflügel in der Raupe und Puppe. (Zeitschr. f.
    wissensch. Zoologie, xxi, 1871, pp. 305–316, 1 Taf.)

  =Weismann, August.= Ueber Duftschuppen. (Zool. Anzeiger, i, 1878, pp.
    98–99.)

  =Dimmock, George.= Scales of Coleoptera. (Psyche, iv, pp. 1–11, 23–27,
    43–47, 63–71, 1883.)

  =Schaeffer, Cäsar.= Beiträge zur Histologie der Insekten. (Zool.
    Jahrbücher, Abth. f. Anat. u. Ontog., iii, pp. 611–652, 2 Pls.,
    1889.)

  =Kellogg, Vernon L.= The taxonomic value of the scales of the
    Lepidoptera. (Kansas Univ. Quart., iii, pp. 45–89, figs. 1–17, 9
    Taf., 1894.)

  =Mayer, Alfred G.= The development of the wing-scales and their
    pigment in butterflies and moths. (Bull. Mus. Comp. Zool., xxix.,
    1896, pp. 209–236, 7 Pls.)

  =Spuler, Arnold.= Beiträge zur Kenntniss des feineren Baues und der
    Phylogenie der Flügeltedeckung der Schmetterlings. (Zool. Jahrb.
    Abth. f. Anat. u. Ontog., viii, pp. 520–543, 1 Taf., 1895.)

  —— Ueber das Vorhandensein von Schuppenbalg bei den Schmetterlingen.
    (Biol. Centralblatt, xvi, Sept. 15, 1896, pp. 677–679, 3 figs.)




                         THE COLORS OF INSECTS


The colors and bright markings of insects, especially those of
butterflies, render them the most brilliant and beautiful creatures in
existence, rivalling and even excelling the gay hues of our most
splendidly colored birds. The subject has been but recently taken up and
is in a somewhat crude condition, but the leading features have been
roughly sketched out by the work of a few observers from a physical,
chemical, and biological point of view.

The colors of insects, as of all other animals, are primarily due to the
action of light and air; other factors are, as Hagen observes, heat and
cold, moisture and dryness, as recently shown by the experiments on
butterflies by Dorfmeister, Weismann, W. H. Edwards, and later
observers. They have their seat in the integument. Hagen divides colors
into optical and natural.

=Optical colors.=—“These,” says Hagen, “are produced by the interference
of light, and are by no means rare among insects, but they are solely
optical phenomena. Colors by the interference of light are produced in
two different ways: either by thin superposed lamellæ, or by many very
fine lines or small impressions in very close juxtaposition.

“1. There must be present at least two superposed lamellæ to produce
colors by interference. The naked wings of Diptera, of dragon-flies, and
of certain Neuroptera often show beautiful interference colors. The
wings of Chrysopa and Agrion show interference colors only for a certain
time, viz., as long as the membranes of the wings are soft and not
firmly glued together. Afterwards such wings become simply hyaline.

  “The scales of Entimus and other Curculionidæ are well known for
  their brilliancy, and it is interesting to remark that when dry
  scales are examined with the microscope, many are found partly
  injured, which give in different places different colors, according
  to the number of layers which remain. The elytra of some
  Chrysomelina and other beetles with iridescent colors probably
  belong to the same category.

“2. When there are scales with many fine lines or small impressions
close to each other, we have the second mode of producing colors.

“The fine longitudinal and transversal lines of lepidopterous scales
seem to serve admirably well to produce the brilliant effect of
color-changing butterflies. But there must be something more present, as
most of the scales of Lepidoptera are provided with similarly fine
lines, and only comparatively few species change colors. I remark
purposely that the lines in the color-changing scales are not in nearer
juxtaposition.” (Hagen.)

  “The colors of butterflies change mostly from purple to blue,
  sometimes to yellow. The splendid violet color at the end of the
  wings of _Callosune ione_ is brought out by a combination of the
  natural with interference colors. Originally the scales are colored
  lake-red; but a blue interference color is mixed with it; hence the
  violet hue results. The blue tones, _i.e._ the splendid varying blue
  of the Morpho butterflies, Schatz claims, owe their hue less to the
  interference of light than to a clouded layer of scales situated
  over the dark ground, through which the light becomes reflected on
  the same. The scales of the Morphids are in reality brown, as we see
  by transmitted light; moreover, only the upper side of the scales
  sends off blue reflections—the under side is simply brown. But the
  blue scales of Urvilliana are also shining blue beneath; by
  transmitted light they appear as if clear yellow. The smaragd-green
  scales of Priamus show by transmitted light a bright red-orange, and
  the orange-yellow of Crœsus a deep grass-green.” (Schatz in Kolbe.)

  “Krukenberg presumes the golden-green color of _Carabus auratus_ to
  be an interference color. It is not changed by the interference of
  light, nor was he able to extract from the elytra any green pigment
  with ether, benzol, carbon of sulphur, chloroform, or alcohol, even
  after having previously submitted the elytra to the influence of
  muriatic acid or ammonia. Chlorophyll is not present, whether free
  or combined with an acid.” (Hagen.)

  Leydig has shown that the interference colors of the hairs of
  certain worms (Aphrodite and Eunice) may be produced by very small
  impressions in juxtaposition, which bring about the same effect as
  striæ. Such an arrangement occurs on the feathers of birds, _i.e._
  on the necks of pigeons and elsewhere, and Hagen suggests that this
  kind of interference colors occurs more frequently among insects
  than is commonly known. At least the limbs of certain forms appear
  yellow, but when held in a certain position change to brown or
  blackish. “I know of no other explanation of this not uncommon fact
  on the legs of Diptera, of Hymenoptera, and of Phryganidæ.”
  Interference colors, he adds, may occur in the same place together
  with natural colors. “The mirror spots of _Saturnia pernyi_ show
  besides the interference colors a white substance in the cells of
  the matrix, which Leydig believes to be guanin. But this fact is
  denied by Krukenberg for the same species and also for _Attacus
  mylitta_ and _Plusia chrysitis_.”

=Natural colors.=—These are divided by Hagen into _dermal_ (cuticular)
and _hypodermal_. The dermal colors are due to pigment deposited in the
form of very small nuclei in the cuticula. Hagen considers them as
“produced mostly by oxidation or carbonization, in consequence of a
chemical process originating and accompanying the development and the
transformations of insects.”

  “To a certain extent the dermal colors may have been derived from
  hypodermal colors, as the cuticula is secreted by the hypodermis,
  and the colors may have been changed by oxidation and air-tight
  seclusion. The cuticula is in certain cases entirely colorless,—so
  in the green caterpillar of _Sphinx ocellata_; but the intensely red
  and black spots of the caterpillar of _Papilio machaon_ belong to
  the cuticula, and only the main yellow color of the body to the
  hypodermis.” (Leydig, Histiol., p. 114.)

“The dermal colors are red, brown, black, and all intermediate shades,
and all metallic colors, blue, green, bronze, copper, silver, and gold.
The dermal colors are easily to be recognized as such, because they are
persistent, never becoming obliterated or changed after death.” (Hagen.)

  Minot and Burgess refer to the cuticular colors of the cotton-worm
  (Aletia), the dark brown color belonging to the cuticula or crust.
  “Upon the outside of the crust is a very thin but distinct layer,
  which in certain parts rises up into a great number of minute,
  pointed spines that look like so many dots in a surface view. Each
  spine is pigmented diffusely, and together they produce the brown
  markings. The spines are clustered in little groups, one group over
  each underlying hypodermal cell.” (U. S. Ent. Comm., 4th Report, p.
  46.) Minot also shows that in caterpillars generally a part of the
  coloration is caused by pigmentation of the cuticula.

  In a dull-colored insect, such as the Mormon cricket (Anabrus), the
  coloration, as Minot states, depends principally upon the pigment of
  the hypodermis shining through the cuticula. “Most of the cells
  contain dull, reddish-brown granules, but scattered in among them
  are patches of cells bright green in color. I have observed no cells
  intermediate in color; on the contrary, the passage is abrupt, a
  brown or red cell lying next a green one. Indeed, I have never seen
  any microscopic object more bizarre than a piece of the epidermis of
  Anabrus spread out and viewed from the surface.” (2d Report U. S.
  Ent. Comm., p. 189.)

The pigment may extend through the entire cuticula, but it is usually
confined to the outermost layers, and occurs there in union with a
peculiar modelling of the upper surface into microscopic figures which
are of interest not only from their delicacy, but because they vary with
each species. (See p. 184.)

The hypodermal colors, situated in the hypodermis, are, according to
Hagen, the result of a chemical process, generating color out of
substances contained in the body. They are easily recognized, since they
fade, change, and disappear after death. But where these colors are
preserved after death and enclosed in air-tight sacs, as in the elytra
and scales and hairs of the body, they persist, though, as we well know,
they may fade after exposure to light.

The hypodermal colors are mostly brighter and lighter than the dermal
ones, being light blue or green in different shades, yellow to orange,
and the numerous shades of these colors combined with white;
exceptionally they are metallic, as in Cassida, and are then obliterated
after death.

  “The fact that such metallic colors can be retained in dead
  specimens by putting a drop of glycerine under the elytra, leads us
  to conclude that those colors are based upon fat substances. The
  hypodermal colors are never glossy, as far as I know; the dermal
  colors frequently.

  “As the wings, elytra, and hairs all possess a cuticula, dermal
  colors are frequently to be found, together with hypodermal ones,
  chiefly in metallic colors. In the same place both colors may be
  present, or one of them alone. So we find hypodermal colors in the
  elytra of Lampyridæ. In the elytra of the Cicindelidæ the main
  metallic color is dermal, the white lines or spots are hypodermal,
  by which arrangement the variability in size and shape of those
  spots is explained.

  “There occur in a number of insects external colors, that is, colors
  upon the cuticula, which I consider to be in fact displaced
  hypodermal colors: the mealy pale blue or white upon the abdomen of
  some Odonata, the white on many Hemiptera, the pale gray on the
  elytra and on the thorax of the Goliath beetle, and the yellowish
  powder on Lixus. Some of these colors dissolve easily by ether or
  melt in heat, and some of them are a kind of wax. I believe that
  those colors are produced in the hypodermis, and are exuded through
  the pore-canals.” (Hagen.)

The white colors are simply for the most part due to the inclusion of
air in scales. The white mother-of-pearl spots of Argynnis are produced
by a system of fine transverse pore-canals filled with air; in
Hydrometra the white ventral marks have the same origin. (Leydig.)

The further statements and criticisms of Hagen regarding the relation of
color to mimicry, sexual selection, and the origin of patterns are of
much weight and will be referred to under those heads. Indeed, these
subjects cannot well be discussed without reference to the fundamental
facts stated in the masterly papers of Leydig and of Hagen, and much of
the theorizing of these latter days is ill-founded, because the colors
of insects and animals are attributed to natural selection, when they
seem really the result of the action of the primary factors of organic
evolution, such as changes of light, heat, cold, and chemical processes
dependent on the former.

As to the chemical nature of color, Hagen, after quoting the results of
Krukenberg and others, thinks that the colors of insects are chemically
produced by a combination of fats or fat-acids with other acids or
alkalis under the influence of air, light, and heat. He concludes:—

1. That some colors of insects can be changed or obliterated by acids.

2. That two natural colors, madder-lake and indigo, can be produced
artificially by the influence of acid on fat-bodies.

3. As protein bodies in insects are changed into fat-bodies, and may be
changed by acids contained in insects into fat-acids, the formation of
colors in the same manner seems probable.

4. That colors can be changed by different temperatures.

5. That the pattern is originated probably by a combination of oxygen
with the integument.

6. That mimicry of the hypodermal colors may be effected by a kind of
photographic process.

7. Finally, color and pattern are produced by physiological processes in
the interior of the bodies of insects.

  Krukenberg concludes that change of color (in perfectly developed
  insects) is a consequence of the change of food, and can be
  explained by the alteration of the pigment through heat and light.
  His experiments were made in order to ascertain the cause of the
  turning of green grasshoppers in autumn into yellow and pink. He
  tried to answer two questions: First, does the pigment of
  grasshoppers originate directly out of the food, and does it consist
  of pure chlorophyll or of a substance containing chlorophyll, or is
  it to be accepted as a peculiar product of the organism? Second, is
  the color the consequence of only one pigment, or of several?
  Special analysis proves that the green color has no connection with
  chlorophyll. He concludes: “It is evident that the green color of
  the grasshopper is the consequence of several different pigments
  which can be separated by a chemical process.” Krukenberg believes
  that light has a marked influence on the color of insects and that
  light turns to red or pink the insects which were green during the
  summer. It would seem, however, more probable that cold was the
  agent, the change being due to the colder autumn weather.

  Here we might refer to the results of the studies of Buckton and
  Sorby, on the changes in color of Aphides:—

  “1. The purple coloring matter appears to be a quasi-living
  principle, and not a product of a subsequent chemical oxidizing
  process. Mounted in balsam or other preserving fluids, the darker
  species stain the fluid a fine violet.

  “2. As autumn approaches and cold weather reduces the activity of
  the Aphides, the lively greens and yellows commonly become converted
  into ferruginous red, and even dark brown, which last hue in reality
  partakes more or less of intense violet or purple. These changes
  have some analogy with the brilliant hues assumed by maple and other
  leaves during the process of slow decay.

  “3. Aqueous solutions of crushed dark brown and yellow-green
  varieties of Aphides originate different colors with acids and
  alkalies.

  “4. In the generality of cases coloring-matters, such as indigo,
  Indian yellow, madder-lake, and the like, do not separately exist in
  the substance of vegetables, but the pigments are disengaged through
  fermentation or oxygenation. Again, alizarin itself is reddish
  yellow, but alkaline solutions strike it a rich violet just as we
  find them to act towards the substance which Mr. Sorby calls
  aphidilutein.

  “5. Mr. Sorby’s four stages of the changes effected by the oxidation
  of aphideine produce four different substances.”

=Chemical and physical nature of the pigment.=—Researches in this
difficult field of inquiry have been made by Landois (1864), Sorby
(1871), Meldola (1871), by Krukenberg (1884), and more recently by
Coste, Urech, Hopkins, and Mayer, and the subject is of fundamental
importance in dealing with mimicry and protective coloration, the
primary causes of which appear to be due to the action of physical and
chemical agents.

Over twenty years ago Meldola observed that the yellow pigment of the
sulphur-yellow butterfly (_Gonopteryx rhamni_) was soluble in water, and
showed that its aqueous solution had an acid reaction.

  Besides the yellow uranidin found by Krukenberg in different beetles
  and lepidopterous pupæ, still other coloring-matters, which are very
  constant in different species are readily recognized by the
  spectroscope. “Thus there appear in the brownish yellow lymph of
  _Attacus pernyi_, _Callosamia promethea_ and _Telea polyphemus_,
  after saponification of the precipitated soap readily effected by
  ether, or incompletely or not removed by benzine, a chlorophane-like
  lipochrome; and in the yellowish green lymph of _Saturnia pyri_ and
  of _Platysamia cecropia_ besides this pigment still another whose
  spectrum shows a broad band on D, but which disappears with the
  addition of acetic acid or ammonia, as also after a long heating of
  the lymph up to 66° C.”

Coste, and more especially Urech, have shown that many of the pigments
may be dissolved out of the scales by means of chemical reagents, giving
colored solutions, and leaving the scales white or colorless. They have
also shown that some of these pigments may be changed in color by the
action of reagents, and then restored to their original color by other
reagents. They have proved that reds, yellows, browns, and blacks are
always due to pigments, and in a few cases greens, blues, violets,
purples, and whites, and not, as is usually the case, to structural
conditions, such as striæ on the scales (Mayer). They confined
themselves solely to the chemical side of the problem, not considering
the structure of the scales themselves.

Urech has also discovered a beautiful smaragd-green coloring-matter in
the wings (not in the scales) of the pupa of _Pieris brassicæ_. It is
not chlorophyll, and Urech suggests that it may be either the germinal
substance of the pigments of the scales or its bearer. It is not the
pigment of the blood.

Urech has also demonstrated that in many Lepidoptera the color of the
urine which is voided upon emergence from the chrysalis is similar to
the principal color of the scales.

  Hopkins has worked on the pigments within the scales of butterflies.
  The yellow pigment in _Gonopteryx rhamni_ is a derivation of uric
  acid, and he calls it lepidotic acid. Its aqueous solution is
  strongly acid to litmus, and must be bad-tasting to birds.

  Hopkins has dissolved the red pigment from the border of the hind
  wing of _Delias eucharis_, an Indian butterfly, in pure water,
  finding as the result a yellow solution; but if the solution be
  evaporated to dryness, the solid residue of pigment is red once
  more. He has obtained from this pigment of _eucharis_ a silver
  compound which contains a percentage of metals exactly equal to that
  from the pigment of _G. rhamni_. (Nature, April 2, 1892.)

  “The scales of the wings of the white butterflies (Pieridæ) are also
  shown by Hopkins to contain uric acid, this substance practically
  acting as a white pigment in these insects. A yellow pigment, widely
  distributed in the same family, is shown to be a derivative of uric
  acid, and its artificial production as a by-product of the
  hydrolysis of uric acid is demonstrated. That this yellow pigment is
  an ordinary excretory product of the butterfly is indicated by the
  fact that an identical substance is voided from the rectum on
  emergence from the pupa. These excretory pigments, which have
  well-marked reactions, are apparently confined to the Pieridæ, and
  are not found in other Rhopalocera. This fact shows that when a
  Pierid mimics an insect belonging to another group, the pigments of
  the mimicked and mimicking insects, respectively, are chemically
  quite distinct. Other pigments existing, not in the scales, but
  between the wing-membranes, are shown to be of use for ornament.”
  (Proc. Royal Soc., London, 1894.)

  Griffiths (1892) claims that the green pigment found in several
  species of Papilio, Hesperia, and Limenitis, also in Noctuidæ,
  Geometridæ, and Sphingidæ likewise consists of a derivative of uric
  acid, which he calls lepidopteric acid. By prolonged boiling in HCl
  it is converted into uric acid.

  Spuler, however, finds that green does not depend on pigmentation,
  but is an optical color. As remarked by Spuler, either the chitin of
  the scales itself is colored reddish (yellow grayish), or the
  pigment is secreted in the nuclei.

A. G. Mayer believes that the pigments of the scales are derived from
the hæmolymph or blood of the pupa, for the following reasons: (1) He is
unable to find anything but blood within the scales during the time when
the pigment is formed. (2) In Lepidoptera generally the first color to
appear upon the pupal wings is a dull ochre-yellow, or drab, and this is
also the color assumed by the blood when it is removed from the pupa and
exposed to the air. (3) He has succeeded by artificial means in
manufacturing several pigments from the blood which are similar in color
to various markings upon the wing of the imago; chemical reagents have
the same effect upon these manufactured pigments that they do upon the
similarly colored pigments of the wings. “It should be here noted,” he
says, “that in 1866 Landois pointed out the fact that the color of the
dried blood of many caterpillars is similar to the ground color of the
wings of the mature insect.”

=Ontogenetic and phylogenetic development of colors.=—The colors of the
wings of Lepidoptera, as is well known, are acquired at the end of the
pupal state. The order of development of the colors in the pupal wings
has been observed by Schaeffer, Van Bemmelen, Urech, Haase, Dixey,
Spuler, and A. G. Mayer. The immature wings are at first transparent and
full of protoplasm. The transparent condition of the wings corresponds
to the period before the scales are formed, and when they are full of
protoplasm; they then become whitish as the scales develop; the latter
are at first filled with protoplasm, and afterwards turn whitish, being
little hollow sacks filled with air. After the protoplasm has completely
withdrawn from the scales, the blood of the pupa enters them, and then
the coloring-matter forms. (Mayer.) He adds that “about twenty-four
hours after the appearance of the dull yellow suffusion the mature
colors begin to show themselves. They arise, faint at first, in places
near the centre of the wings, and are distinguished by the fact that
they first appear upon areas between the nervures, never upon the
nervures themselves. Indeed, the last place to acquire the mature
coloration are the outer and costal edges of the wings, and the
nervures.”

The faint color of the scales gradually increases in intensity. “For
example, if a scale be destined to become black, it first becomes pale
grayish brown, and this color gradually deepens into black.”

Urech states that in _Vanessa io_ first a white, and in _V. urticæ_ a
pale reddish hue, are spread over the entire wings, and then
successively arise other colors in the following order: yellow, yellow
to brown, red, brown and black.

Spuler, however, claims that the differentiation of colors and markings
do not follow one another, but arise simultaneously, and that his view
is confirmed by Fischer. This may be the case with the highly
specialized and diversely marked butterflies, but certainly taking the
Lepidoptera as a whole the yellows and drabs must have been the
primitive hues, the other colors being gradually added in the later more
specialized forms.

It is noticeable that the most generalized moths, such as the species of
Micropteryx, Tinea, Psychidæ, Hepialidæ (in general), etc., are dull
brown or yellow-drab without bars, stripes, or spots of bright hues.
These shades prevail in others of the more primitive Lepidoptera, such
as many bombycine moths, and they even appear to a slight extent in
certain caddis-flies. The authors mentioned, especially Mayer, whom we
quote, claim that “dull ochre-yellows and drabs are, phylogenetically
speaking, the oldest pigmental colors in the Lepidoptera; for these are
the colors that are assumed by the hæmolymph upon mere exposure to the
air. The more brilliant pigmental colors, such as bright yellow, reds,
greens, etc., are derived by more complex chemical processes. We find
that dull ochre-yellow and drabs are at the present day the prevalent
colors among the less differentiated nocturnal moths. The diurnal forms
of Lepidoptera have almost a monopoly of the brilliant colorations, but
even in these diurnal forms one finds that dull yellow or drab colors
are still quite common upon those parts of their wings that are hidden
from view.”

The more primitive moths being more or less uniformly yellowish or drab,
the next step was the formation of bars, stripes, finally spots, and
eyed spots, these markings in the later forms appearing simultaneously
in one and the same species of certain highly specialized moths and
butterflies. All that has been said will prepare the reader for the
consideration of the subject of insect coloration. The origin of such
markings has been discussed by Weismann, Eimer, Haase, Dixey, Fischer,
and others.




                               LITERATURE


  =Heer, O.= Einfluss des Alpenklimas auf die Farbe der Insecten.
    (Froebel u. Heer, Mitth. aus dem Gebiete der theoret. Erdkunde,
    1836, i, pp. 161–170.)

  =Goureau.= Mémoire sur l’irisation des ailes des insectes. (Ann. Soc.
    Ent. France, 2 sér., i, 1848, pp. 201–215.)

  =Laboulbène, A., et M. Follin.= Note sur la matière pulvérulente qui
    recouvre la surface du corps des Lixus et de quelques autres
    insectes. (Ann. Soc. Ent. de France, 1848, vi, pp. 301–305, Fig.)

  =Coquerel, Ch.= Note sur la prétendue poussière cryptogamique qui
    recouvre le corps de certains insectes. (Ann. Soc. Ent. France,
    1850, viii, pp. 13–15.)

  =Brauer, F.= Beobachtungen in Bezug auf den Farbenwechsel bei
    _Chrysopa vulgaris_. (Verhandl. k. k. zool.-botan. Gesellsch. Wien.,
    1852, pp. 12–14.)

  =Prittwitz, O. F. W. v.= Bemerkungen über die geographische
    Farbenverteilung unter den Lepidopteren. (Stett. Ent. Zeit., 1855,
    xvi, pp. 175–185.)

  =Latham, A. G.= The causes of the metallic lustre of the scales on the
    wings of certain moths. (Proc. Lit. and Phil. Soc. Manchester, iii,
    1864, pp. 198–199. Quart. Journ. Micr. Sc., new ser., iv, 1864, pp.
    48–49.)

  =Sorby, H. C.= On the coloring matter of some Aphides. (Quart. Journ.
    Micr. Sc., new ser., xi, 1871, pp. 352–361.)

  =Leydig, Franz.= Bemerkungen über Farben der Hautdecke und Nerven der
    Drüsen bei Insekten. (Archiv f. mikr. Anatomie, xii, 1876, pp.
    536–550, 1 Taf.)

  =Weismann, A.= Studien zur Descendenz-Theorie, ii, 1876.

  =Hemmerling, Hermann.= Ueber die Hautfarbe der Insecten. (Bonn, 1878,
    p. 27.)

  =Buckton, C. B.= Monograph of the British Aphides. (London, 1879, ii,
    p. 167.)

  =Cameron, P.= Notes on the coloration and development of insects.
    (Trans. Ent. Soc. London, 1880, pp. 69–79.)

  =Hagen, Hermann A.= On the color and pattern of insects. (Proc. Amer.
    Acad. Arts and Sc., 1882, pp. 234–267.)

  =Poulton, Edward Bagnall.= The essential nature of the colouring of
    phytophagous larvæ (and their pupæ), etc. (Proc. Roy. Soc. London,
    xxxviii, pp. 269–315, 1884–1885.)

  —— An inquiry into the cause and extent of a special colour-relation
    between certain exposed lepidopterous pupæ and the surfaces which
    immediately surround them. (Phil. Trans. Roy. Soc. London, clxxviii,
    pp. 311–441, 1 Pl., 1887.)

  =Krukenberg, C. Fr. W.= Grundzüge einer vergleichenden Physiologie der
    Farbstoffe und der Farben. (Heidelberg, 1884, pp. 102.)

  =McMunn, C. A.= Krukenberg’s chromatological speculation. (Nature,
    xxxi, p. 217, 1885.)

  =Müller, Fritz, and Dr. H. A. Hagen.= The color and pattern of
    insects. (Kosmos, xiii, 1886, pp. 466–469.)

  =Slater, J. W.= On the presence of tannin in insects and its influence
    on their colors. (Trans. Ent. Soc. London, 1887, iii, Proceed., pp.
    32–34.)

  =Bemmelen, J. F. van.= Ueber die Entwicklung der Farben und Adern auf
    den Schmetterlingsflügeln. (Tijdschrift der nederland. Dierkundige
    Vereeniging, ser. 2, pp. 235–247, 1889.)

  =Hopkins, F. G.= Uric acid derivatives functioning as pigments in
    butterflies. (Proc. Chem. Soc. London, 1889, p. 117; also Nature,
    xl, p. 335.)

  —— Pigment in yellow butterflies. (Nature, xlv, p. 197, 1891.)

  —— The pigments of the Pieridæ. (Proc. Roy. Soc. London, lvii, No.
    340, pp. 5, 6, 1894. Phil. Trans. Roy. Soc. London, clxxxvi, pp.
    661–682, 1896.)

  =Coste, F. H. P.= Contributions to the chemistry of insect colors.
    (The Entomologist, xxiii, 1890; xxiv, 1891, pp. 9–15, etc. Nature,
    xlv., pp. 513–517, 541–542, 605.)

  =Urech, F.= Beobachtungen über die verschiedenen Schuppenfarben und
    die zeitliche Succession ihres Auftretens. (Zool. Anzeiger, xiv, pp.
    466–473, 1891; Ibid., August 1, 1892.)

  —— Beiträge zur Kenntniss der Farbe von Insektenschuppen. (Zeits. f.
    Wissens. Zool., lvii, pp. 306–384, 1893.)

  =Griffiths, A. B.= Recherches sur les couleurs de quelques insectes.
    (C. R. Acad. Sc. Paris, cxv, pp. 958, 959.)

  =Mayer, Alfred Goldsborough.= On the color and color-patterns of moths
    and butterflies. (Proc. Bost. Soc. Nat. Hist., xxvii., March, 1897,
    pp. 243–330, 10 Pls. See also p. 201 under Mayer.)

  Also the writings of Bates, Beddard, Belt, Butler, Darwin, Dimmock,
    Dixey, Eimer, Haase, Higgins, Müller, Poulton, Seitz, Wallace,
    Weismann.




                          2. INTERNAL ANATOMY




                          THE MUSCULAR SYSTEM


In its general arrangement the muscular system of insects corresponds to
the segmented structure of the body. Of the muscles belonging to a
single segment, some extend from the front edge of one segment to that
of the next behind it, and others to the hinder edge; there are also
sets of dorsal and ventral muscles passing in an oblique or vertical
course (Figs. 16–18). As Lang observes, “the greater part of the muscles
of the body can be traced back to a paired system of dorsal and ventral
intersegmental longitudinal muscles.” The muscular system is simplest in
larval insects, such as caterpillars, where the musculature is serially
repeated in each segment.

In the larva of Cossus Lyonet found on one side of the body 217 dorsal,
154 lateral, 369 ventral, and in the thoracic legs 63, or 803 muscles in
all. “Adding to this number the 12 small muscles of the second segment,
and 8 others of the third, which he did not describe, there would be for
all the muscles on one side of the caterpillar 823. This would make for
the entire body 1646, without counting a small single muscle which
occurs in the subdivision of the last segment,” and also those of the
internal organs as well as those of the head, so that the total number
probably amounts to about 2000, not 3000, as usually stated in the
books. Lubbock admits that Lyonet was right in his mode of estimating
the number. In the larva of _Pygærci bucephala_ he found that “the large
muscles scarcely vary at all,” though certain smaller ones are very
variable. Lubbock observed that certain of the longitudinal muscles in
the caterpillar of Diloba split up into numerous, not less than ten,
separate fascicles. “This separation of the fibres composing a muscle
into separate fascicles is carried on to a much greater extent in the
larvæ of Coleoptera. Of course in the imago the number of thoracic
muscles is greatly increased, or at least in Dyticus and the
wood-feeding Lamellicorns, which alone I have examined. In these two
groups each of the larger muscles is represented by at least twenty
separate fascicles, which makes it far more difficult to distinguish the
arrangement of the muscles.”

The muscles are whitish or colorless and transparent, those in the
thorax being yellowish or pale brown; and of a soft, almost gelatinous
consistence. In form they are simply flat and thin, straight, band-like,
or in rare cases pyramidal, barrel or feather shaped. They act variously
as rotators, elevators, depressors, retractors, protractors, flexors,
and extensors.

[Illustration:

  FIG. 231.—Diagram of the muscles and nerves of the ventral surface of
    the segments in the larva of _Sphinx ligustri_: _A_, _A_, recti
    muscles; 1, 2, ventral recti muscles (1, recti majores; 2, recti
    minores); 3, ridge giving origin to recti muscles of one segment,
    and insertion to the same of the adjoining segment; 4, ridge for
    attachment of muscle; 5, retractor ventriculi, connecting the
    mid-intestine with the outer integument of the body. _B_, 6, first
    oblique,—7, second oblique,—9, 10, third oblique, muscles; 11,
    fourth oblique,—12. third rectus,—13, fifth oblique,—14,
    triangularis, muscle; 15, transversus medius; 16, transverse ridge;
    17, transversi abdominales; 18, abdominales anteriores; 19, 20,
    abdominales laterales, some (20) longer than others; 21, obliquus
    posterior; 22, postero-laterales obliqui; 23, transversus lateralis;
    24, second transversus lateralis; 25, retractor spiraculi, or
    constrictor of the spiracles, attached by a long tendon (26); 27,
    retractor valvulæ.

  Nerves: _a_, ganglion,—_c_, transverse nerves, of which _p_ is the
    first, _q_ the second, _r_ the third,
  and _s_ the fourth branch; _t_, the main trunk, which crosses the
    great longitudinal trachea, receives a
  filament from the transverse nerve (_n_), and divides into two
    branches (_t_);—some of these branches
  form a small plexus (_u_); the nerve _t_ divides in two divisions (_p_
    and _v_). The second division ends
  in _w_ and _x_; the branch _q_ divides into _y_ and _z_. For other
    explanations, see Newport, art. _Insecta_.—After
  Newport.
]

[Illustration:

  FIG. 232.—Musculature of the European cockchafer, _Melolontha
    vulgaris_: _a_, _a_, levatores capitis; _b_, depressores capitis;
    _c_, rotatores capitis; _d_, depressors externi; _e_, retractor or
    flexor of the jugular plate; _f_, oblique extensor of the jugular
    plate; _g_, the other retractor of the jugular plate; _h_, retractor
    prothoracis superior; _i_, inferior retractor, the proper depressor
    of the prothorax; _k_, elevator prothoracis; _l_, one of the
    rotatores prothoracis; _m_, _n_, _o_, flexors of the coxa; _x_,
    great depressor muscle of the wing; _y_, _y_, elevators and
    protractors attached to the metaphragma and base of the postfurca;
    _z_, second flexor of hind leg; _a_, _a_, extensors of hind leg;
    _c_, _c_, dorsal recti of abdomen. _Q_, ejaculatory duct; _R_,
    penis; _S_, its prepuce. _M_, rectum.—After Straus-Durckheim, from
    Newport.
]

  Our knowledge of the muscular system of insects is still very
  imperfect. To work it out thoroughly one should begin first with
  that of Scolopendrella, then some generalized synapterous form, as
  Japyx or Lepisma, then passing to that of a caterpillar, and ending
  with some of the more highly specialized forms, such as a beetle,
  etc. Thus far our knowledge is confined to that of the caterpillars
  (Lyonet, Newport, and Lubbock) and the beetle (Straus-Durckheim) and
  ants (Forel, Lubbock, and Janet).

  =Musculature of a caterpillar.=—Newport’s account of that of the
  larva of _Sphinx ligustri_ is the most useful (Fig. 231). The
  muscles here present, he says, great uniformity of size and
  distribution in every segment, the motions of each of these
  divisions of the body being almost precisely similar, especially in
  the 4th to 9th trunk segments. In these segments the first layer
  seen on removing the fat and viscera are the flat straight recti
  muscles. They are the most powerful of all the trunk muscles, and
  are those which are most concerned in shortening the body, in
  effecting the duplicature of the external teguments during the
  changes of the insect, and which during the larval state mainly
  assist in locomotion. There are four sets, two dorsal and two
  ventral (Fig. 231, _A_, _A_). Without entering into farther details,
  the reader is referred to the works of Newport and to Fig. 231.

  =Musculature of a beetle.=—The best general account of the
  musculature of a perfect insect is that of Straus-Durckheim in his
  famous work on the Melolontha. We will copy the summary of Newport,
  who adopted the nomenclature applied to these parts by Burmeister:—

  “The muscles that connect the head with the thorax are contained
  within the prothorax (Fig. 232, 2), and are of three kinds,
  extensors, flexors, and retractors. The extensors, _levatores
  capitis_ (_a_, _a_), consist of two pairs, one of which arises from
  the middle line of the pronotum, and diverging laterally from its
  fellow of the opposite side, passes directly forwards, and is
  inserted by a narrow tendon into the anterior superior margin of the
  occipital foramen. The other arises further back from the
  prophragma. It is a long, narrow muscle that passes directly
  forwards through the prothorax, and is inserted by a tendon near the
  superior median line of the foramen; so that, while this muscle and
  its fellow of the opposite side elevate the head almost in a
  straight line, the one first described, when acting alone or singly,
  draws the head a little on one side; but when the whole of these
  muscles act in unison, they simply elevate the head upon the
  prothorax. The depressors or flexors, _depressores capitis_ (_b_),
  are exceedingly short muscles, which arise from the jugular plate,
  or, when that part does not exist, from the border of the
  prosternum, and are attached to the inferior margin of the occipital
  foramen. They simply flex the head on the prothorax. The lateral
  flexors, _depressores externi_ (_d_), are two little muscles that
  arise from the same point as the preceding, and are attached to the
  lateral inferior margin of the occipital foramen. The rotatory
  muscles, _rotatores capitis_ (_c_), are two flat muscles like the
  elevators, which arise, one at the side of the antefurca and the
  other from the posterior jugular plate, and passing upwards and
  outwards are attached to the lateral margin of the occipital
  foramen. The _retractor_ or flexor of the jugular plate is a small
  muscle (_e_) that arises from the margin of the antefurca, and
  passing directly forwards is inserted by a small tendon into the
  middle of the jugular piece. The _oblique extensor_ of the jugular
  plate is a long, slender muscle (_f_) that arises from the external
  margin of the pronotum, and passing obliquely downwards and forwards
  traverses the prothorax and is inserted by a narrow tendon to the
  jugular plate immediately before the retractor. The other retractor
  (_g_) arises from the anterior superior boundary of the pronotum,
  and passing downwards is inserted into the jugular plate between the
  larger levator and _flexor capitis_.

  “The muscles proper to the prothorax consist of four pairs, by which
  it is united to the succeeding segments. The first of these, the
  superior retractor, _retractor prothoracis superior_ (_h_), arises
  by a broad, fleshy head from the anterior external margin of the
  pronotum, and passing directly backwards is inserted by a tendon
  into the prophragma, a little on one side of the median line. The
  next muscle of importance, the inferior retractor (_i_), arises from
  the anterior border of the medifurca, and is united to the posterior
  of the antefurca, thus forming with that muscle part of the great
  recti of the larva. This muscle must be considered as the proper
  depressor of the prothorax. The _elevator prothoracis_ (_k_) is
  narrow, pyramidal, and arises fleshy from the lateral surface of the
  prophragma. It passes downwards and is attached by a narrow tendon
  to the superior portion of the antefurca. The _rotatores
  prothoracis_ are the largest of all the muscles of this segment.
  They arise, one on each side (_l_), by a narrow head from the
  posterior part of the pronotum, and passing beneath the prophragma
  are considerably enlarged and attached to the tegument between the
  two segments, and also to the anterior portion of the mesothorax.
  The remaining muscle proper to the prothorax is the closer of the
  spiracle, an exceedingly small muscle not shown in the drawing.

  “The other muscles of this segment are those of the legs, which are
  of considerable size. There are three distinct flexors of the coxa
  (_m_, _n_, _o_). The first of these arises from the superior lateral
  border of the pronotum, the second from the superior posterior
  border, the third from the sides of the prothorax, and the fourth a
  little nearer posteriorly, and the whole of them are attached by
  narrow tendons to the sides of the coxa. But there is only one
  extensor muscle to this part. In like manner, the extensor of the
  trochanter is formed of three portions (Fig. 233, _a_, _b_, _c_);
  but there is only one flexor (_d_), and one abductor (_e_). In the
  femur, there is one extensor (_f_),—a long penniform muscle that
  occupies the superior part of the thigh, and is attached by a tendon
  to the anterior-posterior margin of the joint formed by the end of
  the tibia. There is also but one flexor (_g_) in the femur, which,
  like the preceding muscle, is penniform, and occupies the inferior
  portion of the femur, and its tendon is attached to the inferior
  border of the tibia. In the tibia itself there is also one flexor
  and one extensor. The _flexor_ (_i_) occupies the superior portion
  of the limb, and ends in a long tendon (_l_) that passes directly
  through the joints of the tarsus, on their inferior surface, and is
  attached to the inferior margin of the claw (_g_). The _extensor_
  (_h_) occupies the inferior portion of the tibia, and is shorter
  than the preceding muscle, like which it ends in a long tendon that
  is attached to the upper margin of the claw. Besides these muscles,
  which are common to the joints of the tarsus, there are two others
  belonging to the claw, situated in the last joint. The first of
  these, the _extensor_ (_m_), is short, and occupies the superior
  portion of the last phalanx of the tarsus, and the other, the
  _flexor_ (_n_), is a much longer penniform muscle, which occupies
  nearly the whole of the upper and under surface of the posterior
  part of the phalanx, and is attached, like the long flexor of the
  tarsus, to the inferior part of the claw.”

[Illustration:

  FIG. 233.—Muscles of the fore leg of _Melolontha vulgaris_: _a_,
    _b_, _c_, three divisions of the extensor of the trochanter; _d_,
    flexor,—_e_, abductor, of the trochanter; _f_, extensor of the
    femur; _g_, flexor of the femur; _h_, extensor of the tibia; _i_,
    flexor of the tibia; _l_, tendon attached to the lower edge of the
    claw (_g_); _m_, extensor,—_n_, flexor, of the claw.—After
    Straus-Durckheim, from Newport.
]

  These are the muscles of the prothorax, and its organs of
  locomotion. The reader is referred for a further account of the
  muscles of the hinder thoracic and of the abdominal segments to
  Straus-Durckheim’s original work.

=Minute structure of the muscles.=—The muscular fibres of insects are
striated (Figs. 235–238), even those of the alimentary canal; the only
notable exception being the alary muscles of the pericardial septum,
while Lowne states that certain of the thoracic muscles of the blow-fly
are not striated (Miall and Denny).

[Illustration:

  FIG. 234.-Section through the prothorax of _Diapheromera femoratum_:
    _prov_, proventriculus; _tr_, trachea; _n. c_, nervous cord; _s.
    gl_, salivary gland; _hyp_, hypodermis; _ur. t_, urinary tube; _ht_,
    heart; _m_, _m″_, _m‴_, muscles for lowering and raising the tergum;
    _m′_, another muscle, its use unknown.
]

[Illustration:

  FIG. 235.—Striated muscular fibre of Hydrophilus: _A_ and _B_, two
    fibrillæ in a state of extension; _a_, thick disk; _b_, thin disk;
    _c_, intermediate space. _C_, _D_, portion of the same fibrillæ seen
    by moving the objective farther away and using a small diaphragm;
    _n_, thick; _c_, thin disk. × 2000 diam.—After Ranvier, from
    Perrier. _E_ after Gehuchten, from Lang.
]

In describing the minute structure of the muscles of ants, wasps, and
bees, C. Janet states that each consists of a group of fibres diverging
from a tendon, which is an integumentary invagination (Fig. 236). Each
fibre may be regarded as a multinucleate cell; the sarcolemma represents
the cell-membrane. It forms a resistant and extremely elastic tube. The
longitudinal (Fig. 236, _E_) and radiating filaments or reticulum
(spongioplasm of Gehuchten) lie in a nutritive filling substance (the
hyaloplasm of Gehuchten). The radiating filaments are formed of an
exceedingly elastic substance, and serve to sustain the longitudinal
filaments, to transmit the nervous stimulus to them, and to bring them
back into position after contraction. Janet’s account agrees on the
whole with that of Gehuchten.

[Illustration:

  FIG. 236.—Preparations from the adductor muscle of the mandible of
    _Vespa crabro_, worker, fixed by heat and alcohol several hours
    after leaving its cell. _A_ to _E_ × 425; _F_ × 212: _A_, terminal
    cupule of the tendon of a fibre. _B_, _C_, union of the fibres with
    their tendon. _D_, branch of the tendon of a muscle sending out
    tendons of some of the fibres; this branch is accompanied with
    numerous nervous ramifications (_N_). _E_, fragment of a nerve which
    furnishes the ramifications of Fig. _D_. _F_, fragment of the tendon
    of the adductor muscle of the mandible; at the left are seen the
    terminal cupules of the fibres (_td, c_); on the right, on the body
    of the tendons, some sessile cupules, each of which forms the
    attachment of a fibre; _td, b_, tendons of the fibres.—After Janet.
]

The muscles of flight are said to be penetrated by fine tracheal
branches, probably to supply a greater amount of oxygen, as the most
energetic movements of the insect are made in moving the wings during
flight; while the other muscles of the body are only surrounded by the
air-tubes. (Sharp.)

Without entering into tedious details, the reader is referred to figures
or references to the more important systems of muscles, such as those of
the legs and other appendages, of the wings, of respiration, etc., to
the sections treating of those organs or functions; also to Figs. 16,
17, 18, 22, 48, 74, 81, 83, 84, 115, 116, 172, 173, 174, etc.

=Muscular power of insects.=—The most detailed and careful experiments
are those of Plateau. His experiments prove that even the weakest
insects pull at least five times their own weight; many of them,
however, get the better of a burden twelve to twenty fold as heavy as
themselves, while a strong man or a draught horse, for example, is not
even able to pull a burden which is equal to the weight of his body.
Plateau came to the following results as to the relation of the weight
of the body to the load drawn (1 and 2 are to be compared with each
other, 1 being the larger, and 2 the smaller insect; it will be seen
that the smaller insect is the stronger).

[Illustration:

  FIG. 237.—_Vespa crabro_, worker, fixed by heat and alcohol some hours
    after leaving its cell. _A_ × 425; _B_ to _D_ × 850 times: _A_,
    muscular fibre of the motor muscles of the mandibles treated, for
    ten minutes, by 1 per cent potassium to bring out the reticulum; the
    nodes of union of the rayed filaments with the longitudinal
    filaments are indicated by distinct granulations (_l.d_), and these
    longitudinal filaments present accessory thickenings (_d.a_); T,
    trachea; N, junction of a nervous filament with the muscular fibres.
    _B_, fibre of the same muscle, not treated with potassium, stained
    by hæmatoxylin; _C_, transverse section of a disk at the level of a
    layer of rayed filaments; _Sarc_, sarcolemma. _D_, transverse
    section of a disk at the level of the rods; _nuc_, nucleus.—After
    Janet.
]

                   1. _Carabus auratus_ 17.4.
                   2. _Nebria brevicollis_ 25.3.
                   1. _Cetonia aurata_ 15.
                   2. _Trichius fasciatus_ 41.3.
                   1. _Melolontha vulgaris_ 14.3.
                   2. _Anomala frischii_ 24.3.
                   1. _Oryctes nasicornis_ 4.7.
                   2. _Geotrupes stercorarius_ 9.8.
                   3. _Onthophagus nuchicornis_ 14.4.
                   1. _Necrophorus vespillo_ 15.1.
                   2. _Silpha livida_ 24.4.
                   1. _Ocypus morio_ 17.
                   2. _Quedeus fulgidus_ 29.6.
                   1. _Donacia nymphææ_, 42.7.
                   2. _Crioceris merdigera_ 39.2.
                   1. _Bombus terrestris_ 16.1.
                   2. _Bombus rupestris_ 14.5.
                   3. _Apis mellifica_ 20.2.

As regards the pushing power, the relation of the load to the size of
the body in different large beetles, gave the following figures:—

                    _Oryctes nasicornis_ 3.2.
                    _Geotrupes stercorarius_ 28.4.
                    _Onthophagus nuchicornis_ 92.9.

The leaping force of locusts was found by Straus-Dürckheim to be in
_Œdipoda grossa_ as 1.6, in _Œ. parallela_ as 3.3 of their weight.

[Illustration:

  FIG. 238.—_Vespa crabro_, fixed and stained as in the subjects of the
    other figures. _I_, _N_, _P_ × 1700; _H_, _J_, _M_ × 850; the others
    × 425 times: _A-C_, motor muscles of the antennal scape. _D-P_,
    motor muscles of the 3d coxa. _A_, _B_, the two ends, in very
    different states of contraction, of the same fibre; on one side the
    transverse striæ are near together, on the other very far apart.
    _C_, a crushed and split fibre showing a fibrous appearance, owing
    to the rupture of the radiated filaments, and the separation of the
    longitudinal filaments. _D_, muscular disk seen in section, with two
    rows of nuclei. _E_, a muscular fibre with three rows of nuclei.
    _F_, a nucleus, accompanied with coagulated protoplasm, oozing from
    a previous break of the muscular fibre. _G_, nerve-terminations very
    near each other on the same muscular fibre. _H_, longitudinal
    filaments, evenly covered with the coagulated substance, and
    forming, throughout the mass of the fibre, continuous filaments.
    _I_, filaments widely separated. _J_, longitudinal filaments showing
    the beginning of one of the transverse breaks which isolate some of
    the disks. _K_, oblique view of a disk obtained by such a break, and
    of a fibre in circular section, with an axial row of nuclei; this
    piece comprises three stages of radiated filaments. _L_, muscular
    fibre with a row of nuclei; at the lower part, the nuclei have
    issued from a longitudinal fissure in the fibre, and have remained
    attached in a chain. _M_, edge of fibre in which there is quite a
    large, clear space between the sarcolemma and the rods. _N_, passage
    of the trachea, with the spiral thread, into three capillaries with
    a smooth cuticula. _O_, elliptical disk from a fibre, with two rows
    of nuclei, and showing a layer of radiated filaments. _P_, fragment
    (highly magnified) of the edge of a disk seen in section.—After
    Janet.
]

A humble bee (_Bombus terrestris_) can carry while flying a load 0.63 of
its own weight, and a honey bee 0.78; here, as usual, the smaller insect
is the stronger.[39]




                       LITERATURE ON THE MUSCLES


                              _a_. General

  =Lyonet, P.= Traité anatomique de la chenille. La Haye, 1762.

  =Cornalia, E.= Monographia del Bombyce del gelso. (Mem. R. Instituto
    Lombardo Sc. Lett. ed Arte, 1856.)

  =Basch, S.= Skelett und Muskeln des Kopfes von Termes. (Zeitschr. f.
    wissens. Zool., xv, 1865, pp. 55–75, 1 Taf.)

  =Lubbock, John.= Arrangement of the cutaneous muscles of the larva of
    _Pygæra bucephala_. London, 1858. 2 Pls.

  —— On some points in the anatomy of ants. (Month. Micr. Journ., xviii,
    pp. 121–142, 1877, 4 Pls.)

  —— On the anatomy of ants. (Trans. Linn. Soc., Ser. 2; Zool., ii,
    1879, pp. 141–154, 2 Pls.)

  =Poletajeff, N.= Du développement des muscles d’ailes chez les
    Odonates. (Horæ Soc. Ent. Ross., xvi, 1879, pp. 10–37, 5 Pls.)

  —— Die Flugmuskeln der Lepidopteren und Libelluliden. (Zool. Anzeiger,
    1880, pp. 212, 213.)

  —— Ueber die Flugmuskeln der Rhopaloceren. (Arbeiten d. Russ. Ent.
    Ges., 1881, xiii, p. 9, 1 Taf., in Russian.)

  =Lendenfeld, R. von.= Der Flug der Libellen. (Sitzb. k. Akad.
    Wissens., 1 Abth. Wien, 1881, lxxxiii, pp. 289–376, 7 Taf.)

  =Luks, Constantine.= Ueber die Brustmuskulature der Insekten. (Jena.
    Zeitschr. f. Naturwissen., xvi, N. Folge IX, 1883, pp. 520–552, 2
    Taf.)

  =Carlet, G.= Sur les muscles de l’abdomen de l’abeille. (Comptes
    rend., 1884, xcviii, pp. 758, 759.)

  =Janet, Charles.= Sur les muscles des fourmis, des guêpes et des
    abeilles. (Comptes rend., cxxi, p. 610, 1 Fig., 1895.)

  Also the writings of Straus-Durckheim, Newport, Graber, Burgess,
    Leydig, Dahl, Ockler, Dogiel, Dimmock, Kraepelin, Becher, Langer,
    Kolbe.


                             _b_. Histology

  =Aubert, H.= Ueber die eigenthümliche Struktur der Thoraxmuskeln der
    Insekten. (Zeitschr. f. wissens. Zool., iv, 1853, pp. 388–399, 1
    Taf.)

  =Verson, E.= Zur Insertionsweise der Muskeln. (Sitzsb. Akad. d. wiss.
    math. naturw. Cl. Wien., lvii, 1 Abth., pp. 63–66, 1868.)

  =Künckel d’Herculais.= Sur le développement des fibres musculaires
    striées chez les insectes. (Compt. rend. de l’Acad. Sc. Paris, lxxv,
    1872.)

  =Grunmach, Emil.= Ueber die Structur der quergestreiften Muskelfaser
    bei den Insekten. Berlin, 1872. pp. 47.

  =Fredericq, L.= Note sur la contraction des muscles striés de
    l’Hydrophile. (Bull. Acad. Roy. Belgique, xli, p. 583, 2 Pls.)

  =Gehuchten, A. van.= Étude sur la structure intime de la cellule
    musculaire striée. (La Cellule, ii, pp. 289, 293–453, 1886, 6 Pls.)

  =Janet, Charles.= Études sur les fourmis, les guêpes et les abeilles.
    12^e note. (Structure des membranes articulaires des tendons et des
    muscles, Limoges, 1895, pp. 25, 11 Figs.)

  Also the writings of Burmeister, Chabrier, Leydig, Meckel, Lebert,
    Wagner, Wagener, Amici, Krause, Heppner, Retzius, Rollet, G. Elias
    Müller, F. Merkel, Hensen, Kölliker, Dogiel, Dönitz, Hagen,
    Vosseler, Bütschli u. Schewiakoff, Lowne, Ciaccio, Biedermann,
    Cohnbeim, Brücke, Haycraft, Melland, Bowman.


                     _c_. Muscular power of insects

  =Plateau, Félix.= Sur la force musculaire des insectes. (Bull. Acad.
    Roy. Belgique, 2 Sér. xx, 1865, pp. 732–757; xxii, 1866, pp.
    283–308.)

  —— Recherches sur la force absolue des muscles des invertébrés. 1884.

  =Radan, R.= La force musculaire des insectes. (Revue de deux mondes, 2
    Sér., lxiv, 1866, pp. 770–777.)

  =Bibiakoff, Paul von.= Zur Muskelkraft der Insekten. (Natur, xvii,
    1868, p. 399.)

  =Delbœuf.= Nains et géants, Étude comparative de la force des petits
    et des grands animaux. Bruxelles. (Also in Kosmos, xiii, 1883, pp.
    58–62.)

  =Camerano.= Mem. Acc. Torino (2), xliii, 1893, p. 229.

  Also Newport, Art. Insecta, p. 76. Kirby and Spence, Burmeister,
    Graber, Kolbe, pp. 375, 376.




                           THE NERVOUS SYSTEM


                   _a._ The nervous system as a whole

[Illustration:

  FIG. 239.—Central nervous system of _Machilis maritima_: _au_, eye;
    _lo_, optic tract; _g_, brain; _an_, antennal nerve; _oe_, œsophagus
    passing between the œsophageal commissures; _usg_, infraœsophageal
    ganglion; I-III, thoracic ganglia; 1–8, abdominal ganglia, the last
    (_Sabc_) consisting of three fused ganglia; _s_, sympathetic nervous
    system of the ventral cord.—After Oudemans, from Lang.
]

The nervous system of insects consists of a double series or chain of
ganglia connected by nervous cords or commissures. The first of these is
the brain or supraœsophageal ganglion; it is situated in the upper part
of the head, above the gullet or œsophagus, while the rest of the
system, called the ventral cord, lies on the floor of the body, under
the digestive canal.

A ganglion or nerve-centre consists of a mass of ganglion-cells, from
each of which a process or fibre passes off, uniting with others to form
a nerve; by means of these nerves the ganglia are connected with other
ganglia, and with the sensory cells and muscle-fibres. The ganglia may
be simple, and arranged in pairs, corresponding to each segment of the
body, or they may be compound, the result of the fusion of several pairs
of ganglia, which in the early stages of the embryo are separate. Thus
the brain of insects is a compound ganglion, or ganglionic mass.

The nerves are of two kinds: 1. _Sensory_, which transmit sensations
from the peripheral sense-cells to the ganglion, or brain; 2. _Motor_,
which send stimuli from the brain or any other ganglion to the muscles.

Of ganglion cells, some are tactile, and others give rise to nerves of
special sense, being distributed to the eyes, or to the organs of
hearing, smell, taste, or touch.

[Illustration:

  FIG. 240.—Nervous system of _Melanoplus spretus_: _sp_,
    supraœsophageal ganglion, sending off the large optic nerve (_op_)
    to the eyes, and an ocellar nerve to each ocellus (the dotted line
    _oc_ stops short of the left ocellus); _if_, infraœsophageal
    ganglion; 1, 2, 3, thoracic ganglia; 1–5, five abdominal ganglia
    (the fifth the largest, and sending branches to the ovipositor,
    etc.) The sympathetic nerve and ganglia are represented by the two
    main nerves which arise from the medio-cephalic (_as_) resting on
    and above the œsophagus, and two ganglia (_ps_) on the under side of
    the crop. From each of these ganglia, two nerves are sent under the
    crop, and a larger nerve on each side to as far as the stomachal
    cæca, ending the figure at the dotted line 2, near the second
    thoracic ganglion. _u_, a round, shining body, connected by a nerve
    with the medio-cephalic ganglion, its nature unknown.
]

[Illustration:

  FIG. 241.—Section through the head of Machilis, showing the brain
    (_br_), and subœsophageal ganglion (_soe. g_); _cl_, clypeus; _lbr_,
    labrum; _oc_, ocellus.
]

While the supraœsophageal ganglion, or “brain,” of the insect is much
more complex than any other ganglion, consisting more exclusively both
of sensory as well as motor ganglia and their nerves, it should be borne
in mind that the subœsophageal ganglion also receives nerves of special
sense, situated on the palpi and on the tongue, as in the bee and other
insects; hence this ganglion is probably complex, consisting of sensory
and motor cells. The third thoracic ganglion is also, without doubt, a
complex one, as in the locusts the auditory nerves pass into it from the
ears, which are situated at the base of the abdomen, while in the green
grasshoppers, such as the katydids and their allies, whose ears are
situated in their fore legs, the first thoracic ganglion is a complex
one. In the cockroach and in Leptis (Chrysopila), a common fly, the
caudal appendages bear what are probably olfactory organs, and as these
parts are undoubtedly supplied from the last abdominal ganglion, this is
probably composed of sensory and motor ganglia; so that we have in the
ganglionated cord of insects a series of brains, as it were, running
from head to tail, and thus in a still stronger sense than in
vertebrates the entire nervous system, and not the brain alone, is the
organ of the mind of insects.

The simplest, most primitive form of the nervous system of insects is
seen in that of the Thysanura. That of Campodea has not yet been fully
examined, but in that of the more complicated genus, Machilis (Fig.
239), we see that there is a pair of ganglia to nearly each segment,
while the brain (Fig. 241) is composed of three lobes, viz. the optic,
the cerebral (Fig. 239, _g_), behind which is the antennal lobe, from
which the antennal nerve takes its origin. Behind the opening for the
throat (_oe_) is situated the first ganglion of the ventral cord, the
subœsophageal ganglion, which gives rise to the nerves supplying the
jaws and other mouth-parts.

[Illustration:

  FIG. 242, _A-D._—The nervous systems of 4 genera of Diptera, to
    demonstrate their various degrees of fusion of ganglia: _A_,
    non-concentrated more primitive nervous system of _Chironomus
    plumosus_, with 3 thoracic and 6 abdominal ganglionic masses. _B_,
    nervous system of _Empis stercorea_, with 2 thoracic and 5 abdominal
    ganglionic masses. _C_, nervous system of _Tabanus bovinus_, with 1
    thoracic ganglionic mass, and the abdominal ganglia closely
    approximated. _D_, highly modified nervous system of _Sarcophaga
    carnaria_, in which all the ganglia of the ventral cord behind the
    subœsophageal ganglion are fused into a single ganglionic
    mass.—After Brandt, from Lang.
]

In the Collembola, which are retrograde Thysanura, there are from one
(Smynthurus), to three or four ventral ganglia.

In the winged insects, where the ganglia are more or less fused, the
fusion taking place in the head and at the end of the abdomen; there are
in the more simple and generalized forms, such as Ephemera, the
grasshopper, locusts (Fig. 240), etc., thirteen ganglia besides the two
pairs of compound ganglia in the head, three pairs of thoracic ganglia,
and usually from five to eight pairs of ganglia in the abdomen.

[Illustration:

  FIG. 243.—Nervous system of the May beetle, _Lachnosterna fusca_:
    _w^1_, nerve to 1st,—_w^2_, nerve to 2d, pair of wings; _ig_,
    infraœsophageal ganglion.
]

[Illustration:

  FIG. 244.—The same of the stag-beetle, _Lucanus dama_, where there are
    3 thoracic, and 3 separate abdominal ganglia.
]

In certain winged insects the process of fusion or degeneration is
carried to such an extreme that there are either no abdominal ganglia
(Fig. 242, _D_), or their vestiges are situated in the thorax and
partially fused with the thoracic ones, as in the May beetle, in which
the prothoracic pair of ganglia is separate, while the two other
thoracic ganglia are fused with the abdominal, the latter being situated
in the thorax; this fusion is carried to a further extent than in any
other Coleoptera yet examined. In many Diptera and Hemiptera the
abdominal ganglia are either absent or the vestiges are fused with the
thoracic ganglia.

Rhizotrogus, which is allied to our May beetle, as also Hydrometra and
the Stylopidæ are said to lack the subœsophageal ganglion (Brandt).

In numerous Coleoptera (Acilius, Gyrinus, Necrophorus, Melolontha,
Bostrichus, Rhynchænus); in many Diptera (Culex, Tipula, Asilus,
Xylophaga, and Phora); and in the higher Hymenoptera (Crabronidæ,
Vespidæ, and Apidæ), as well as in many Lepidoptera (Vanessa, Argynnis,
and Pontia), two of the thoracic ganglia are fused together, while all
three are partially fused into a single mass in many brachycerous
Diptera (Conops, Syrphus, Pangonia, and the Muscidæ); in certain
Hemiptera (Pentatoma, Nepa, and Acanthia); also in a beetle (_Serica
brunnea_). Sometimes the subœsophageal ganglion is fused with the first
thoracic, as in Acanthia, Nepa, and Notonecta. The greatest amount of
variation is seen in the number of abdominal ganglia, all being fused
into a single one or from one to eight. The fusion is usually greatest
where the abdomen is shortened, due to the partial atrophy and
modification of the terminal segments which bear the ovipositor, where
present, and the genital armature.

There is only one pair of abdominal ganglia in Gyrinus and in certain
flies (Conops, Trypeta, Ortalis, and Phora); two in Rhynchænus, a
weevil, and in the flies, Syrphus and Volucella; three in Crabro and
Eucera; four in Sargus, Stratiomys and in butterflies, five in the
beetle, Silpha, and in the fly, Sciara, and the moth, Hepialus.

The nervous system in the larvæ of the metabolous orders is not
concentrated, though in that of the neuropterous Myrmeleo it has
undergone fusion from adaptation to the short compressed form of this
insect.


                             _b._ The brain

The brain of insects appears to be nearly, if not quite, as complex as
that of the lower vertebrates. As in the latter, the pair of
supraœsophageal ganglia, or brain, is the principal seat of the senses,
the chief organ of the insect’s mind.

It is composed of a larger number of pairs of primitive ganglia than any
of the succeeding nerve-centres, and is, structurally, entirely
different from and far more complicated than the other ganglia of the
nervous system. It possesses a central body in each hemisphere, a
“mushroom body,” optic lobes and optic ganglia and olfactory lobe, with
their connecting and commissural nerve-fibres, and a number of other
parts not found in the other ganglia.

In the succeeding ganglia the lobes are in general motor; the fibres
composing the œsophageal commissures, and which arise from the
œsophageal commissural lobes, extend not only to the subœsophageal
ganglion, but pass along through the succeeding ganglia to the last pair
of abdominal nerve-centres.[40] Since, then, there is a direct
continuity in the fibres forming the two main longitudinal commissures
of the nervous cord, and which originate in the brain, it seems to
follow that the movements of the body are in large part directed or
coördinated by the brain.[41] Still, however, a second brain, so to
speak, is found in the third thoracic ganglion of the locust, which
receives the auditory nerves from the ears situated in the base of the
abdomen; or in the first thoracic ganglion of the green grasshoppers
(katydids, etc.), whose ears are situated in their fore legs; while even
the last pair of abdominal ganglia in the cockroach and mole cricket,
is, so to speak, a secondary brain, since it distributes sensory nerves
to the caudal stylets, which are provided with organs probably olfactory
in nature.

It is impossible to understand the morphology of the brain unless we
examine the mode of origin of the nervous system in the early life of
the embryo. The head of an embryo insect consists of six segments,
_i.e._ the ocular, antennal, premandibular, mandibular, and the 1st and
2d maxillary segments, so named from the appendages they bear. Of these
the first three in the larva and adult are preoral, and the last three
are postoral. The antennal segment was probably either postoral in the
progenitors of insects, or the antennæ were inserted on the side of the
mouth, the latter finally moving back.[42]

The nervous system in the early embryonic condition, as shown by Wheeler
(Fig. 245), at first consists of nineteen pairs of primitive ganglia,
called _neuromeres_. Those of the head, which later in embryonic life
fuse together to form the brain, are the first three, corresponding to
the _protocerebrum_, _deutocerebrum_, and _tritocerebrum_ of Viallanes.
The first pair of primitive ganglia, and which is situated in front of
the mouth, is divided into three lobes.

[Illustration:

  FIG. 245, _A-D_.—Diagrams of four consecutive stages in the
    development of the brain and nerve-chain of the embryo of Xiphidium:
    I, cephalic,—II, thoracic,—III, abdominal, region; _st_, stomodæum
    or primitive mouth; _an_, anus; _e_, optic plate; _pc(og)_, 1st
    protocerebral lobe, or optic ganglion; _pc^2_, _pc^3_, 2d and 3d
    protocerebral lobes; _dc_, deutocerebrum; _tc_, tritocerebrum; 1–16,
    the 16 postoral ganglia; _po. c_, postoral commissure; _fp_, furcal
    pit; _ac_, anterior,—_pc_, posterior, ganglionic commissure; _ag_,
    anterior,—_pg_, posterior,—_cg_, central,—_lg_, lateral
    gangliomeres.—After Wheeler.
]

The first or outermost lobe, according to Wheeler, forms the optic
ganglion of the larva and imago, while the second and third lobes.
(_pc^2_, _pc^3_) ultimately form the bulk of the brain proper, or the
protocerebral lobes. The second (primitively postoral) brain-segment or
pair of ganglia gives origin to the antennæ, while the third brain, or
premandibular (intercalary) segment, gives origin to a temporary
embryonic pair of appendages found in Anurida and Campodea (the
premandibular ganglia), and also to the nerves supplying the labrum.
These three pairs of ganglia later on in embryonic life become preoral,
the mouth moving backwards. The three pairs of primitive ganglia,
behind, _i.e._ the mandibular and 1st and 2d maxillary ganglia, become
fused together to form the subœsophageal ganglion, and which in larval
and adult life is postoral.

If the tongue (ligula, or hypopharynx) represents a distinct pair of
appendages, then there are seven segments in the head.

[Illustration:

  FIG. 246.—Section through head of a carabid, _Anopthalmus telkampfii_:
    _br_, brain; _fg_, frontal ganglion; _soe_, subœsophageal ganglion;
    _co_, commissure; _n_. _l_, nerve sending branches to the lingua
    (_l_); _mn_, maxillary nerve; _mx_, 1st maxilla; _mm_, maxillary
    muscle; _mx′_ 2d maxilla; _mt_, muscle of mentum; _le_, elevator
    muscle of the œsophagus; _l′_ of the clypeus, and a third beyond
    raising the labrum (_lbr_); _eph_, epipharynx; _g. g′_, salivary
    glands above; _g^2_, lingual gland below the œsophagus (_œ_); _m_,
    mouth; _pv_, proventriculus; _md_, mandible.
]

The brain, then, supplies nerves to the compound and simple eyes, and to
the antennæ, and gives origin to the sympathetic nerves; it is thus the
seat of the senses, also of the insect’s mind, and coördinates the
general movements of the body.

[Illustration:

  FIG. 247.—Median longitudinal section through the head of _Blatta
    orientalis_. The nervous system of the head is drawn entire. _hyp_,
    hypopharynx; _os_. oral cavity; _lbr_, upper lip; _gf_, frontal
    ganglion; _g_, brain; _na_, root of the antennal nerve; _no_, root
    of the optic nerve; _ga_, anterior,—_gp_, posterior ganglion of the
    paired visceral nervous system; _œ_, œsophagus; _c_, œsophageal
    commissure; _usg_, infraœsophageal ganglia; _cc_, longitudinal
    commissure between this and the first thoracic ganglion; _sg_,
    common duct of the salivary glands; _lb_, labium (2d maxillæ); _nr_,
    recurrent nerve; _d_, nerve uniting the frontal ganglion with the
    œsophageal commissure; _e_, nerve from this commissure to the
    labrum; _f_, nerve from the infraœsophageal ganglion to the
    mandible, —_g_, to the 1st maxillæ, —_h_, to the lower lip (2d
    maxillæ).—After Hofer, from Lang.
]

[Illustration:

  FIG. 248.—1, front view of the brain of _Melanoplus femur-rubrum_:
    _opt. gang_, optic ganglion; _oc_, ocelli and nerves leading to them
    from the two hemispheres, each ocellar nerve arising from the region
    containing the calices; _m. oc_, median ocellar nerve; _opt. l_,
    optic lobe sending off the optic nerve to the optic ganglion; _ant.
    l_, antennal or olfactory lobe; _ant. n_, antennal nerve; _f. g_,
    frontal ganglion of sympathetic nerve; _lbr. n_, nerve to labrum;
    _x_, cross-nerve or commissure between the two hemispheres; _œ. c_,
    œsophageal commissure to subœsophageal ganglion. 2, side view of the
    brain and subœsophageal ganglion (lettering of brain as in 1): _s.
    g_, stomatogastric or sympathetic nerve; _a. s. g_, anterior, and
    _p. s. g_, posterior, sympathetic ganglia; _g^2_, subœsophageal
    ganglion; _md_, nerve to mandible; _mx_, maxillary nerve; _ln_,
    labial nerve; _nl_, unknown nerve,—perhaps salivary. 3, interior
    view of the right half of the head, showing the brain in its natural
    position: _an_, antenna; _cl_, clypeus; _lbr_, labrum; _m_,
    mouth-cavity; _md_, mandible; _t_, tongue; _œ_, œsophagus; _c_,
    crop; _en_, right half of the endocranium or =X=-shaped bone,
    through the anterior angle of which the œsophagus passes, while the
    great mandibular muscles play in the lateral angles. The moon-shaped
    edge is that made by the knife passing through the centre of the
    =X=. 4, view of brain from above (letters as before). 5,
    subœsophageal ganglion from above: _t. c_, commissure to the
    succeeding thoracic ganglion (other letters as before). Fig. 3 is
    enlarged 8 times; all the rest 25 times.—Drawn from original
    dissections, by Mr. Edward Burgess, for the Second Report of the U.
    S. Entomological Commission.
]

The pair of subœsophageal ganglia distributes nerves to the mandibles,
to the 1st and 2d maxillæ, and to the salivary glands (Fig. 248).

Its general shape and relations to the walls and to the outer organs of
the head is seen in Figs. 247, 248. In all the winged insects
(Pterygota) its plane is situated more or less at right angles to the
horizontal plane of the ventral cord. On the dorsal and anterior sides
are situated the ocular lobes, and below these the antennal lobes.

Viallanes first, independently of embryonic data, divided the brain of
adult insects into three regions or segments; _i.e._ the
“_protocerebron_,” “_deutocerebron_” and “_tritocerebron_,” which he
afterwards found to correspond with the three primitive elements
(neuromeres) of the brain and with the segments of the head of the
embryo.

The brain of the locusts (Melanoplus and Œdipoda) being best known will
serve as the basis of the following description, taken mainly from
Viallanes, with minor changes in the name of the three segments, and
other modifications.

=I.= =The optic or procerebral segment= is composed of a median portion,
_i.e._ two fused procerebral lobes (median protocerebrum), and of two
lateral masses, the optic ganglia (_protocerebrum_), and comprises the
following regions fused together and forming the median procerebral mass
(Viallanes):—

     1. Procerebral lobes.
     2. Optic ganglia.
     3. Layer of postretinal fibres.
     4. Ganglionic plate. (_Periopticon_ of Hickson.)
     5. External chiasma.
     6. External medullary mass. (_Epiopticon_ of Hickson.)
     7. Internal chiasma.
     8. Internal medullary mass. (_Opticon_ of Hickson.)
     9. Optic ganglia and nerves.
     10. Pedunculated or stalked body. (Mushroom body of Dujardin.)
     11. Bridge of the procerebral lobes.
     12. Central body.

[Illustration:

  FIG. 249.—Diagram of an insect’s brain: _cc_, central body; _cg_,
    ganglionic cells; _che_, external, _chi_, internal chiasma; _cœ_,
    œsophageal commissure; _cp_, mushroom body; _ctc_, tritocerebral
    commissure; _fpr_, postretinal fibres; _goc_, ocellar ganglion;
    _goc_^1, œsophageal ganglion, the dotted ring the œsophagus; _gv^1_,
    _gc^2_, _gv^3_, 1st, 2d, 3d, unpaired visceral ganglion; _gvl_,
    lateral visceral ganglion; _ld_, dorsal lobe of the deutocerebrum;
    _lg_, ganglionic plate; _lo_, olfactory lobe; _lpc_, protocerebral
    lobe; _me_, external, _mi_, internal medullary mass; _na_, olfactory
    or antennal nerve; _nl_, nerve to labrum; _no_, ocular nerve; _nt_,
    tegumentary nerve; _œ_, œsophagus; _plp_, bridge of the
    protocerebral lobes; _rvd_, visceral root arising from the
    deutocerebrum; _rvt_, visceral root arising from the tritocerebrum;
    _tr_, tritocerebrum; _to_, optic nerve or tract.—After Viallanes.
]

=Optic ganglia.=—Each of the two optic ganglia is formed of a series of
three ganglionic masses situated between the compound eyes and the
median procerebral mass, _i.e._ the ganglionic plate (Fig. 249, _lg_),
the external medullary mass (_me_), and the internal medullary mass
(_mi_).

The postretinal fibres (_fpr_) arising from the facets or single eyes of
the compound eye (ommatidia) pass into the ganglionic plate (_lg_),
which is united within by the chiasmatic fibres (_che_, external
chiasma) of the external medullary mass (_me_). The last is attached to
the internal medullary mass (_mi_) by fibres (_chi_), some of which are
chiasmatic, and others direct. Finally, the internal medullary mass
connects with the median part of the protocerebrum by direct fibres
forming the optic nerve or tract (_to_).

=Procerebral lobes.=—The median procerebral lobes are fused together on
the median line, forming a single central mass. From each side or lobe
arises the mushroom or stalked body. In the middle of the mass is the
central body, and directly in front is the procerebral bridge (_plp_).
The latter is a band uniting the two halves of the brain.

The procerebral lobes also give origin to the nerves to the ocelli
(_no_).

[Illustration:

  FIG. 250.—Transverse section through the brain of the locust (Œdipoda
    and Caloptenus): _c′_, lower part of the wall of _c_, calyx;—_st_,
    stalk of the same; _bpcl_, bridge of the protocerebral lobes; _mo_,
    nerve of median ocellus; _ch_, transverse fascia of the
    optico-olfactory chiasma; _fcb_, fibrous region of the central body;
    _lcb_, tubercle of the central body; _fch_, descending fascia of the
    optico-olfactory chiasma; _choo_, superior fascia of the
    optico-olfactory chiasma; _pt_, protocerebral lobes; _ld_, dorsal
    lobe of the deutocerebrum; _lt_, tritocerebral lobe; _gcld_, _gc_,
    ganglion cells.—After Viallanes.
]

=The mushroom or stalked bodies.=—These remarkable organs were first
discovered by Dujardin, who compared them to mushrooms, and observed
that they were more highly developed in ants, wasps, and bees than in
the lower insects, and thus inferred that the higher intelligence of
these insects was in direct relation to the development of these bodies.
We will call them the _mushroom bodies_.

These two bodies consist of a rounded lobular mass (the trabecula) of
the procerebral lobe, from which arises a double stalk (Fig. 253), the
larger called the _cauliculus_, the smaller the _peduncle_ (or pedicel);
these support the cap or _calyx_. The calices of the bee were compared
by Dujardin to a pair of disks on each side of the brain as seen from
above, “each disk being folded together and bent downwards before and
behind, its border being thickened, and the inner portion radiated.” In
the locust there are but two divisions of the calyx; in the cockroach,
ants, wasps, and bees, four.

The shape and relation of the mushroom bodies are represented in Figs.
252 and 253. The bodies are connected by commissural fibres, and are
connected with the optic ganglion of the same side, and with the central
body; while they are connected with the antennal lobes by the
optico-olfactory chiasma.

[Illustration:

  FIG. 251.—Sagittal section through the brain of the locust: _l. oc.
    n_, lateral ocellus nerve; _a. t_, anterior tubercle of the mushroom
    body; _i. t_, internal tubercle of the mushroom body; _c. l_,
    cerebral lobes; _l. l_, lateral lobe of the middle protocerebrum;
    _com_, commissural cord; _c. mol_, central mass of the olfactory
    lobe; _ac. an. l_, fibres uniting the median lobe of the middle
    protocerebrum with dorsal lobes of the deutocerebrum; _gc. trit. l_,
    ganglionated cortex of the tritocerebral lobe; _c. an. l_, cortex of
    antennal (olfactory) lobe; _lab. fr_, labrofrontal nerve; _oe. com_,
    œsophageal commissure; _tr. com_, transverse commissure of
    œsophageal ring; other letters as in Fig. 250.—After Viallanes.
]

The stalked bodies are enveloped by the cortical layers of
ganglion-cells, those filling the hollow of the calyx having little or
no protoplasm around the nucleus.

  =Structure of the mushroom bodies.=—By staining the brain of the
  honey bee with bichromate of silver, Kenyon has worked out the
  structure of the mushroom bodies, with their cells. The cup-shaped
  bodies or calyces are composed of fibrillar substance
  (_punktsubstanz_). Each of these cups, he says, is “filled to
  overflowing with cells having large nuclei and very little
  cytoplasm.” From the under surface of each of these cups there
  descends into the general fibrillar substance of the brain “a column
  of fibrillar substance, which unites with its fellow of the same
  side to send a large branch obliquely downward to the median line of
  the brain, and an equally large or larger branch straight forwards
  to the anterior cerebral surface.”

  The cells of the mushroom bodies, observes Kenyon, “stand out in
  sharp contrast to all other nerve cells known, though they recall to
  some extent the cells of Purkinje in the higher mammals. Each of the
  cells contained within the fibrillar cup sends a nerve-process into
  the latter, where it breaks up into a profusely arborescent system
  of branchlets, which often appear with fine, short, lateral
  processes, such as are characteristic of the dendrites of some
  mammalian nerve-cells.” Just before entering the fibrillar
  substance, a fine branch is given off that travels along the inner
  surface of the cup along with others of the same nature, forming a
  small bundle to the stalk of the mushroom body, down which it
  continues until it reaches the origin of the anterior and the inner
  roots above mentioned. “Here it branches, one branch continuing
  straight on to the end of the anterior root, while the other passes
  to the end of the inner root. Throughout its whole course the fibre
  and its two branches are very fine. Nearly the whole stalk and
  nearly the whole of each root is made up of these straight, parallel
  fibres coming from the cells within the cup of the mushroom bodies.
  What other fibres there are enter these bodies from the side, and
  branch between the straight fibres very much as the dendrites of the
  cells of Purkinje branch among the parallel fine fibres from the
  cells of the granular layer in the mammalian cerebellum. These
  fibres are of the nature of association fibres.”

  Viallanes showed that from the olfactory or antennal lobes, as well
  as from the optic ganglia, there are tracts of fibres which finally
  enter the cups of the mushroom bodies, and Kenyon has confirmed this
  observation. Kenyon has also, by the Golgi method, detected another
  tract, before unknown, “passing down the hinder side of the brain,
  from the cups to the region above the œsophagus, where it bends
  forward and comes in contact with fibres from the ventral cord,
  which exists, although Binet was unable to discover any growth of
  fibres connecting the cord with the brain.

  “The fibres entering the cups from the antennal lobe, the optic
  ganglia, and the ventral region, spread out and branch among the
  arborescent endings of the mushroom-body cells. The fibres branching
  among the parallel fibres of the roots and the stalk lead off to
  lower parts of the brain, connecting with efferent or motor-fibres,
  or with secondary association fibres, that in their turn make such
  connections. This portion of the circuit has not been perfectly made
  out, though there seems to be sufficient data to warrant the
  assumption just made.

[Illustration:

  FIG. 252.—Section 17, showing the central body (_centr. b_) and
    mushroom body, optic and antennal lobes (_a. l_), and procerebral
    lobes (_pc. l_); _o. cal_, outer division of the calyx; _op. n_,
    optic nerve; _trab_, trabeculum; _tc. n_, transverse nerve.
]

  “Such fibres existing as described, there is then a complete circuit
  for sensory stimuli from the various parts of the body to the cells
  of the mushroom bodies. The dendritic or arborescent branches of
  these cells take them up and pass them on out along the parallel
  fibres or neurites in the roots of the mushroom bodies as motor or
  other efferent impulses.

  “This, however, is not all. For there are numerous fibres evident in
  my preparations, the full courses of which I have not been thus far
  able to determine, but which are so situated as to warrant the
  inference that they may act as association fibres between the
  afferent fibres from the antennæ, optic ganglia, and ventral system,
  and the efferent fibres. There is then a possibility of a stimulus
  entering the brain and passing out as a motor impulse without going
  into the circuit of the fibres of the mushroom bodies; or, in other
  words, a possibility of what may be compared to reflex action in
  higher animals.”

[Illustration:

  FIG. 253.—Enlarged view of the trabeculum (the dotted lines _tcn_ and
    _obt. n_ pass through it) and its nerves, of the mushroom body,—its
    calices and stalk, and the origin of the optic nerve × 225
    diameters: _atn_, ascending trabecular nerve; _obt. n_, oblique
    trabecular nerve; _tcn_, transverse nerve; _lat. n_, lateral nerve;
    _cent. n_, central nerve.
]

The mushroom bodies have not yet been found to be present in the
Synaptera, but occur in the larvæ, at least of those of most metamorphic
insects (Lepidoptera and Hymenoptera), though not yet found in the larvæ
of Diptera. The writer has found these bodies in the nymphs of the
locust (_Melanoplus spretus_), but not in the embryo just before
hatching. They occur in the third larval or nymph stage of this insect.
It is evident that by the end of the first larval stage the brain
attains the development seen in the third larval state of the two-banded
species (_C. bivittatus_).

[Illustration:

  FIG. 254.—Section through the brain of _Caloptenus bivittatus_ in the
    third larval stage, showing the two hemispheres or sides of the
    brain, and the ocelli and ocellar nerves, which are seen to arise
    from the top of the hemispheres directly over the calices (compare
    Fig. 251): _o. cal_, outer division of calyx of left mushroom body.
]

The result of our studies on the brain of the embryo locust was that
from the embryonic cerebral lobes are eventually developed the central
body and the two mushroom bodies. Fig. 254 shows the early condition of
the mushroom bodies and their undoubted origin from the cerebral
ganglia. Hence these bodies appear to be differentiations of the
cerebral ganglia or lobes, having no connection with the optic or
antennal lobes.

=The central body= (Fig. 252, _centr. b_).—This is the only single or
unpaired organ in the brain. Dietl characterizes it as a median
commissural system. Viallanes describes it as formed entirely of a very
fine and close fibrillar web, like a thick hemispherical skull-cap,
situated on the median line and united with the cerebral lobes. “It is
like a central post towards which converge fibres passing from all
points of the brain; being bound to the cerebral lobes, to the stalked
bodies, to the optic ganglia, and to the olfactory lobes by distinct
fibrous bundles.”

=The antennal or olfactory lobes (Deutocerebrum).=—This portion of the
brain consists of two hemispherical lobes, highly differentiated for
special sensorial perceptions, and connected by a slightly
differentiated medullary mass, the dorsal lobe (Figs. 248, 249 _lo_),
from which arise the motor fibres and those of general sensibility. The
antennal lobes are in part attached to the optic ganglia, and partly to
the stalked body on the same side, by the optic olfactory chiasma (Fig.
250 _fch_, _choo_), a system of fibres partially intercrossed on the
median line.

=The œsophageal lobes (Tritocerebrum)= (Figs. 249, 250).—From this
region the labrum and viscera are innervated, the nerves to the latter
being called the visceral, sympathetic, or stomatogastric system. As
Viallanes remarks, though plainly situated in front of the mouth, they
are in fact post-œsophageal centres. The two lobes are situated far
apart, and are connected by a bundle of fibres passing behind the
œsophagus, called the transverse commissure of the œsophageal ring
(Lienard). The œsophageal ganglia, besides giving rise to the labral
nerves, also give origin to the root of the frontal ganglion.


                _c._ Histological elements of the brain

The brain and other ganglia are composed of two kinds of tissue.

1. The outer slightly darker, usually pale grayish white portion
consists of cortical or ganglion-cells differing in size. This portion
is stained red by carmine, the cells composing it readily taking the
stain.

The large ganglion cells (represented in Figs. 252 and 253) are oval,
and send off usually a single nerve-fibre; they have a thin fibrous
cell-wall, and the contents are finely granular. The nucleus is very
large, often one-half the diameter of the entire cell, and is composed
of large round refractive granules, usually concealing the nucleolus.

2. The medullary or inner part of the brain consists of matter which
remains white or unstained after the preparation has remained thoroughly
exposed to the action of the carmine. It consists of minute granules and
interlacing fibres. The latter often forms a fine irregular network
inclosing masses of finely granulated nerve matter.

  This is called by Dietl “marksubstanz.” Leydig, in his Vom Bau des
  thierischen Körpers, p. 89, thus refers to it:—

  “In the brain and ventral ganglia of the leech, of insects, and
  in the brain of the gastropods (Schnecken) I observe that the
  stalks (stiele) of the ganglion-cells in nowise immediately
  arise as nerve-fibres, but are planted in a molecular mass or
  _punktsubstanz_, situated in the centre of the ganglion, and
  merged with this substance. It follows, from what I have seen,
  that there is no doubt that _the origin of the nerve-fibres
  first takes place from this central punktsubstanz_.”

  “This relation is the rule. But there also occur in the
  nerve-centres of the invertebrates single, definitely situated
  ganglion-cells, whose continuations become nerve-fibres without the
  intervention of a superadded _punktsubstanz_.” We may, with Kenyon,
  call it the fibrillar substance.

  Leydig subsequently (p. 91) further describes this fibrillar
  substance, stating that the granules composing it form a reticulated
  mass of fibrillæ, or, in other words, a tangled web of very fine
  fibres:—

  “We at present consider that by the passage of the continuation of
  the ganglion-cells into the _punktsubstanz_ this continuation
  becomes lost in the fine threads, and on the other side of the
  _punktsubstanz_ the similar fibrillar substance forms the origin of
  the axis-cylinders arranged parallel to one another; so it is quite
  certain _that the single axis-cylinder derives its fibrillar
  substance as a mixture from the most diverse ganglion-cells_.”


        _d._ The visceral (sympathetic or stomatogastric) system

This system in insects is composed (1) of a series of three unpaired
ganglia (Fig. 249, _gv^1_, _gv^2_, _gv^3_), situated over the
dorso-median line of the œsophagus, and connected by a median nervous
cord or recurrent nerve (_nr_, _vagus_ of Newport). The first of these
ganglia is the frontal ganglion, which is connected with the œsophageal
ganglia by a pair of roots (_rvt_), which have an origin primitively
common with that of the labral nerves (Fig. 248, _fg_ and _lbr_).

[Illustration:

  FIG. 255.—Anterior portion of the paired and unpaired visceral nervous
    system of _Blatta orientalis_ seen from above. The outlines of the
    brain (_g_) and the roots of the antennal nerve (_na_), which cover
    a portion of the sympathetic nervous system, are given by dotted
    lines. Lettering as in Fig. 247. _nsd_, nerve to salivary gland. The
    nervus recurrens (_nr_) enters an unpaired stomach ganglion farther
    back.—After Hofer, from Lang.
]

2. Of two pairs of lateral ganglia (Fig. 255, _ga_, _gp_) situated two
on each side of the œsophagus. They are connected both with the antennal
lobes by a nerve (_rvd_), and to the chain of unpaired ganglia by a
special connective. The first pair of these ganglia sends nerves to the
heart and aorta; the second pair to the tracheæ of the head.

The unpaired median or recurrent nerve (_nr_) extends back from under
the brain along the upper side of the œsophagus, and (in Blatta), behind
the origin of the nerves to the salivary glands, enters an unpaired
ganglion, called the stomachic ganglion (_ganglion ventriculare_),
situated in front of the proventriculus. The number of these stomachic
ganglia varies in different orders of insects.

  In Blatta, Küpffer and also Hofer have shown (Fig. 255) (Müller,
  Brandt, _ex_ Kolbe) that the nerve to each salivary gland arises
  from three different centres: the anterior end situated under the
  œsophagus is innervated by the paired visceral nerves from the
  hinder paired ganglia; the remaining part by nerves arising from
  each side of the recurrent nerve; and thirdly by a pair of nerves
  arising from the subœsophageal ganglion which accompanies the common
  salivary duct, and ends in branches which partly innervate the
  salivary glands and in part their muscles.

Hofer considers that the function of this complex system of paired and
unpaired ganglia, with their nerves, is a double one, viz. serving both
as a centre for the peristaltic action of the œsophagus, and as
innervating the salivary glands.

Besides these a second portion of the visceral system arises from the
thoracic and abdominal ventral cord. It may be seen in the simplest
condition yet known in the nervous system of Machilis (Fig. 239 _s_). It
consists of a fine, slender nerve, which extends along the surface of
the ventral chain of ganglia, and sending off a pair of branches
(accessory transverse nerves) in front of each ganglion. These accessory
nerves receive nerve-twigs from the upper cord of the ventral chain,
dilating near their origins into a minute elongated ganglion, and then
passing partly outwards to the branches of the tracheæ and the muscles
of the spiracles, uniting in the middle line of each segment of the body
behind the head, _i.e._ of those segments containing a pair of ganglia.


                       _e._ The supraspinal cord

In the adult Lepidoptera has been detected, continuous with and on the
upper side of the abdominal portions of the ventral cord, a longitudinal
cord of connective tissue forming a white or yellowish band, and which
seems to be an outgrowth of the dorsal portion of the neurilemma of the
ventral cord. Muscles pass from it to the neighboring ventral portions
of the integument. Its use is unknown, and attention was first called to
it by Treviranus, who called it “an unknown ventral vessel”
(_Bauchgefäss_). Afterwards it was re-discovered by Newport, who
described it as “a distinct vascular canal.” But Burger has proved by
cross-sections that it is not tubular, but a comparatively solid cord
composed, however, of loose connective tissue. Newport found it in the
larva of _Sphinx ligustri_, but Cattie states that it is not present in
that of _Acherontia atropos_. It has not yet been observed in insects of
other orders, but its homologue exists in the scorpion and in the
centipede, and it may prove to correspond with the far more complete
arterial coat which, with the exception of the brain, envelops the
nervous system of Limulus.


     _f._ Modifications of the brain in different orders of insects

There are different grades of cerebral development in insects, and
Viallanes claimed that it was no exaggeration to say that the brain of
the locust (Melanoplus) differs as much from that of the wasp as that of
the frog differs from that of man. He insists that the physiological
conditions which determine the anatomical modifications of the brain are
correlated with 1, the food; 2, the perfection of the senses; and 3,
with the perfection of the psychic faculties. For example, in those
which feed on solid food and whose œsophagus is large (Orthoptera and
Coleoptera), the connectives are elongated, the subœsophageal commissure
free in all its extent, and the tritocerebrum is situated quite far from
the preceding segment of the brain.

On the other hand, in insects which feed on fluid food (Hymenoptera,
Lepidoptera, Diptera, Hemiptera), the œsophagus is slender and the
nervous centres which surround them are very much condensed; the
connectives are short, and the tritocerebrum is closely fused, partly to
a portion of the antennal lobes (deutocerebrum) and partly to the
mandibular ganglion.

As regards the perfection of the senses, where, as in dragon-flies, the
eyes are very large, the optic ganglia are correspondingly so, and in
the same insects the antennæ being very small, the antennal lobes are
almost rudimentary. The ants exhibit inverse conditions; in their brain
the antennal lobes are well developed, while the optic ganglia are
reduced, and where, as in Typhlopone, the eyes are wanting, they are
completely atrophied.

[Illustration:

  FIG. 256.—Head of _Anophthalmus tellkampfii_, showing the brain,—the
    optic ganglia, nerves, and eyes totally atrophied.
]

[Illustration:

  FIG. 257.—Head of another Carabid, with the brain and eyes normal:
    _op_, optic ganglion; _pcl_, brain.
]

In certain cave insects where the eyes are wanting, the optic ganglia
are also absent. In the eyeless cave species of Anophthalmus the optic
ganglia and nerves are entirely atrophied, as they are in Adelops,
which, however, has vestiges of the facets (ommatidia). Fig. 257
represents the brain of _Chlænius pennsylvanicus_, a Carabid beetle,
with its eyes and optic ganglia (_op_) which may be compared with
Anopthalmus, in which these parts are totally atrophied.

Dujardin claimed that the degree of complication of the stalked body of
the Hymenoptera was in direct relation with their mental powers. This
has been proved by Forel, who has shown that in the honey bee and ants
the mushroom bodies are much more developed in the workers than in the
males or females and Viallanes adds that these bodies are almost
rudimentary in the dragon-flies, whose eyes are so large; while on the
contrary in the blind ants (Typhlopone), these bodies are as perfect and
voluminous as in the ants with eyes.

[Illustration:

  FIG. 258.—Diagrammatic outlines of sections of the upper part of the
    brain of a cockroach. Only one side of the brain is here
    represented. The numbers indicate the position in the series of 34
    sections into which this brain was cut. _mb_, mushroom bodies, with
    their cellular covering (_c_) and their stems (_st_); _a_, anterior
    nervous mass; _m_, median nervous mass.—After Newton.
]

Within the limits of the same order the stalked bodies are most perfect
in the most intelligent forms. Thus in the Orthoptera, says Viallanes,
the Blattæ, Forficulæ, and the crickets, the mushroom bodies are more
perfect than in the locusts, which have simpler herbivorous habits. This
perfection of the mushroom bodies is seen not only in the increase in
size, but also in the complication of its structures. Thus in the groups
with lower instincts (Tabanus, Æschna) the stalk does not end in a calyx
projecting from the surface of the brain, but its end, simply truncated,
is indicated externally only by an accumulation of the ganglionic nuclei
which cover it.[43]

In types which Viallanes regards as more advanced, _i.e._ Œdipoda and
Melanoplus, the end of the stalk projects and is folded into a calyx.

The brain of the cockroach (Periplaneta, Fig. 258) is a step higher than
that of the locusts, each calyx being divided into two adjacent calices,
although the cockroaches are an older and more generalized type than
locusts.

The stalked bodies of cockroaches are thus complex, like those of the
higher Hymenoptera, the calices in Xylocopa, Bombus, and Apis being
double and so large as to cover almost the entire surface of the brain.

Finally, in what Viallanes regards as the most perfect type (Vespa), the
sides of the calices are folded and become sinuous, so as to increase
the surface, thus assuming an appearance which, he claims, strongly
recalls that of the convolutions of the brain of the mammals.

  Cheshire also calls attention to a progression in the size of these
  appendages, as well as in mental powers as we rise from the
  cockchafer (_Melolontha vulgaris_) to the cricket, up to the
  ichneumon, then to the carpenter bee, and finally to the social hive
  bee, “where the pedunculated bodies form the ⅕ part of the volume of
  the cerebral mass, and the 1⁄870 of the volume of the entire
  creature, while in the cockchafer they are less than 1⁄2300 the
  part. The size of the brain is also a gauge of intelligence. In the
  worker bee the brain is 1⁄174 of the body; in the red ant, 1⁄296; in
  the Melolontha, 1⁄3500; in the Dyticus beetle, 1⁄4400.” (Bees and
  bee-keeping, p. 54.)


             _g._ Functions of the nerve-centres and nerves

As we have seen, the central seat of the functions of the nervous system
is not the brain alone (supraœsophageal ganglion), but each ganglion is
more or less the seat of vital movements, those of the abdomen being
each a distinct motor and respiratory centre. The two halves of a
ganglion are independent of each other.

According to Faivre, the brain is the seat of the will and of the power
of coördinating the movements of the body, while the infraœsophageal
ganglion is the seat of the motive power and also of the will.

The physiological experiments of Binet, which are in the line of those
of Faivre, but more thorough, demonstrate that an insect may live for
months without a brain, if the subœsophageal ganglion is left intact,
just as a vertebrate may exist without its cerebrum. As Kenyon says:
“Faivre long ago showed that the subœsophageal ganglion is the seat of
the power of coördination of the muscular movements of the body. Binet
has shown that the brain is the seat of the power directing these
movements. ‘A debrained hexapod will eat when food is placed beneath its
palpi, but it cannot go to its food even though the latter be but a very
small space removed from its course or position. Whether the insect
would be able to do so if the mushroom bodies only were destroyed, and
the antennal lobes, optic lobes, and the rest of the brain were left
intact, is a question that yet remains to be answered’” (Kenyon).

In insects which are beheaded, however readily they respond to
stimulation of the nerves, they are almost completely wanting in will
power. Yet insects which have been decapitated can still walk and fly.
Hymenoptera will live one or two days after decapitation, beetles from
one to three days, and moths (Agrotis) will show signs of life five days
after the loss of their head.

That the loss of will power is gradual was proved by decapitating
_Polistes pallipes_. A day after the operation she was standing on her
legs and opening and closing her wings; 41 hours after the operation she
was still alive, moving her legs, and thrusting out her sting when
irritated. _Ichneumon otiosus_, after the removal of its head, remained
very lively, and cleaned its wings and legs, the power of coördination
in its wings and legs remaining. A horse-fly, a day after decapitation,
was lively and flew about in a natural manner.[44]

When the abdomen is cut off, respiration in that region is not at first
interrupted. The seat of respiratory movements was referred by Faivre to
the hinder thoracic ganglion, but Plateau says that this view must be
entirely abandoned, remarking: “All carefully performed experiments on
the nervous system of Arthropoda have shown that each ganglion of the
ventral chain is a motor centre, and in insects a respiratory centre,
for the somite to which it belongs” (Miall and Denny’s The Cockroach, p.
164).

The last pair of abdominal ganglia serve as the nervous centre of the
nerves sent to the genital organs.

The recurrent or stomatogastric nerve, which, through the medium of the
frontal ganglion, regulates digestion, has only a slight degree of
sensibility; the insect remains quiet even when a powerful allurement is
presented to the digestive tract (Kolbe).

Faivre states that the destruction of the frontal ganglion, or a section
of the commissures connecting it with the brain, puts an end to
swallowing movements; on the other hand, stimulation results in
energetic movements of this nature.

Yersin, by cutting through the commissure in different places, and thus
isolating the ganglia of the nervous cord of _Gryllus campestris_,
arrived at the following results:—

1. The section of a nerve near its origin rendered the organ supplied by
this nerve incapable of performing its functions.

2. If the connectives between two ganglia, _i.e._ the second and third
thoracic ganglia, are cut through, the fore as well as hinder parts of
the body retain their power of motion and sensation; but a stimulus
applied to the anterior part of the body does not pass to the hinder
portion.

3. Insects with an incomplete metamorphosis after section of the
connectives are not in every case unable to moult and to farther
develop.

4. If only one of the two connectives be cut through, the appendages of
the side cut through which take their origin between the place injured
and the hinder end of the body, often lose sensation and freedom of
motion, or the power of coördination of movements becomes irregular.
Sometimes this is shown by an unsteadiness in the gait, so that the
insect walks around in a circle; after a while these irregularities
cease, and the movements of the limbs on the injured side are only
slightly restrained. By a section of both connectives in any one place
the power of coördination of movements is not injured.

5. The section of the connectives appear to have no influence on
nutrition, but affects reproduction, the attempt at fertilization on the
part of the male producing no result, and the impregnated female laying
no eggs.

6. Injury to the brain, or to the subœsophageal, or one of the thoracic
ganglia, is followed by a momentary enfeeblement of the ganglion
affected. Afterwards there results a convulsive trembling, which either
pervades the whole body or only the appendages innervated by the injured
ganglion.

7. As a result of an injury to the brain there is such a lack of
steadiness in the movements that the insect walks or flies in a circle;
for instance, a fly or dragon-fly thus injured in flying describes a
circle or spiral. Steiner, in making this experiment, observed that the
insect circled on its uninjured side. The brain is thus a motor centre.

8. By injuring a thoracic ganglion, one or all the organs which receive
nerves from the ganglion are momentarily weakened. Afterwards the
functions become restored. Sometimes, however, the insect walks in a
circle. Faivre observed that after the destruction of the metathoracic
ganglion of _Dyticus marginalis_ the hind wings and hind legs were
partially paralyzed (Kolbe, _ex_ Yersin).




                    LITERATURE ON THE NERVOUS SYSTEM


                              _a._ General

  =Newport, George.= On the nervous system of the _Sphinx ligustri_ L.,
    and on the changes which it undergoes during a part of the
    metamorphoses of the insect. (Phil. Trans. Roy. Soc., London, 1832,
    pp. 383–398; 1834, pp. 389–423, Pls.)

  =Helmholtz, H. L. F.= De fabrica systematis nervosi evertebratorum.
    Diss. in aug. Berolini, 1842.

  =Blanchard, E.= Recherches anatomiques et zoologiques sur le système
    nerveux des animaux sans vertèbres. Du système nerveux des insectes.
    (Annales des Sciences nat., Sér. 3, v, 1846, pp. 273–379, 8 Pls.)

  —— Du système nerveux chez les invertèbres dans ses rapports avec la
    classification de ces animaux. Paris, 1849.

  —— in Cuvier’s Règne animal. (Edition accompagnée de planches gravées.
    Insectes. Pl. 3, 3_a_, and 4.)

  =Leidy, Joseph.= History and anatomy of the hemipterous genus
    Belostoma. (Memoirs Amer. Acad. Arts and Sc., N. S. iv, 1849, pp.
    57–67, 1 Pl.)

  =Scheiber, S. H.= Vergleichende Anatomie und Physiologie der
    Œstridenlarven. (Sitzungsb. k. Akad. wiss. Wien. Math.-Naturwiss.
    Cl., xli, 1860, pp. 439–496; xlv, 1862, pp. 7–68; 5 Taf.)

  =Tullberg, Tycho.= Sveriges Podurider. (K. Svenska vet. Akad. Handl.
    x, 1872, pp. 1–70, 12 Taf.)

  =Berlese, A.= Osservazione sulla anatomia descrittiva del _Gryllus
    campestris_ L. (Atti della soc. Veneto-Trentina, 1880, vii, pp.
    200–299.)

  =Baudelot, E.= Contributions à la physiologie der système nerveux des
    insectes. (Revue d. sc. nat., i, pp. 269–280, 1872.)

  =Studer, Th.= Ueber Nervenendigung bei Insekten. Kleine Beiträge zur
    Histologie der Insekten. (Mitt. Naturf. Ges., Bern, 1874, pp.
    97–104, 1 Taf.)

  =Brandt, E.= Recherches anatomiques et morphologiques sur le système
    nerveux des insectes Hyménoptères. (Compt. rendus de l’Acad. Sc.,
    Paris, 1875.)

  —— Ueber das Nervensystem der Apiden. (Sitzungsb. d. naturf. Ges., in
    Petersbourg, vii, 1876.)

  —— Ueber das Nervensystem der Schmetterlingsraupen. (Verhandl. der
    Russ. Ent. Gesellsch., x, 1877. Also 16 other articles with plates,
    in Horæ Soc. Ent. Ross., 1878–1882.)

  =Mark, E. L.= The nervous system of Phylloxera. (Psyche, ii, pp.
    201–207, 1879.)

  =Riley, Charles Valentine.= The nervous system and salivary glands of
    Phylloxera. (Psyche, ii, pp. 225, 226, 1879.)

  =Cholodkowsky, N.= Zur Frage über den Baue und über die Innervation
    der Speicheldrüsen der Blattiden. (Horæ Soc. Ent. Ross., 1881, xvi,
    pp. 6–9, 2 Taf.)

  =Liénard, V.= Constitution de l’anneau œsophagien. (Archives de
    Biologie, i, pp. 381–391, 1880, 1 Taf.)

  =Michaels, H.= Nervensystem von _Oryctes nasicornis_ im Larven-,
    Puppen-, und Käferzustande. (Zeits. f. wissens. Zool., xxxiv, 1880,
    pp. 641–702, 4 Taf.)

  =Rossi, A.= Sul modo di terminare dei nervi nei muscoli dell’ organo
    sonoro della Cicala commune (_Cicada plebeja_). (Mem. accad. sc.
    Bologna, 1880, 4 Ser., i, pp. 661–665.)

  =Foettinger, A.= Sur le termination des nerfs dans les muscles des
    insectes. (Archiv de Biologie, i, 1880.)

  =Binet.= Contribution à l’étude der system nerveux sous intestinal des
    insectes. (Journ. l’anat. et phys., xxx, pp. 449–580, 1894.)

  =Paulowa.= Zum Bau des Eingeweide Nervensystems der Insekten. (Zool.
    Anzeiger., xviii, Feb. 25, 1895, pp. 85–87.)

  Also the writings of Lyonet, Cuvier, Rolando, Straus-Durckheim,
    Leydig, Newport, Graber, Viallanes, Grassi, Oudemans.


                             _b._ The brain

  =Dujardin, F.= Mémoires sur le système nerveux des insectes. (Annales
    des Sciences nat, Sér. 3, 1850, xiv, pp. 195–206, Pl. 1, 1850.)

  =Rabl-Rückhard.= Studien über Insectengehirne. (Archiv für Anatomie,
    Physiologie, etc., herausg. von Reichert u. R. du Bois-Raymond,
    1876, p. 480, Taf. i.)

  =Dietl, M. J.= Die Organization des Arthropodengehirns. (Zeitschr.
    wissens. Zool., xxvii, 1876, p. 488, Taf. xxxvi.-xxxviii.)

  =Flogel, T. H. L.= Ueber den einheitlichen Bau des Gehirns in den
    verschiedenen Insectenordnungen. (Zeitschr. wissens. Zool., xxx,
    Suppl., 1878, p. 556, Taf. xxiii, xxiv.)

  =Newton, E. T.= On a new method of constructing models of the brains
    of insects, etc. (Journ. Quekett Microscopical Club, pp. 150–158,
    1879.)

  —— On the brain of the cockroach, _Blatta orientalis_. (Quart. Journ.
    Microscopical Science, July, 1879, p. 340, Pl. xv, xvi.)

  =Packard, A. S.= The brain of the locust. (Chapter xi, Second Report
    of the U. S. Entomological Commission, pp. 223–242, Pls. ix-xv,
    1880.)

  =Cuccati, Giovanni.= Sulla stuttura del ganglio sopraesofageo di
    alcuni ortotteri. (Acrydium lineola, Locusta viridissima, Locusta
    (species?), Gryllotalpa vulgaris, Bologna, 1887, 4º, pp. 1–27, Pl.
    i-iv.)

  —— Intorno alla struttura del cervello della Sonomya erythrocephala,
    nota preventiva. Bologna, 1887.

  —— Ueber die Organization des Gehirns des Sonomya erythrocephala.
    (Zeitschr. f. wissens. Zool., 1888, xlvi, pp. 240–269, 2 Taf.)

  =Viallanes, H.= Études histologiques et organologiques sur les centres
    nerveux et les organes des sens des animaux articulés.

  1. Mémoire. Le ganglion optique de la langouste (_Palinurus
    vulgaris_). (Annal. d. Sc. Nat. Zool., 1884, 6^e Sér., xvii, Art. 3,
    pp. 1–74, 5 Pls.)

  2. Mémoire. Le ganglion optique de la Libellule (_Æschna
    maculatissima_). (Ibid., 1885, 6^e Sér., xviii, Art. 4, pp. 1–34, 3
    Pls.)

  3. Mémoire. Le ganglion optique de quelques larves de Diptères
    (_Musca_, _Eristalis_, _Stratiomys_). (Ibid., 1886, 6^e Sér., xix,
    Art. M. 4, pp. 34, 2 Pls.)

  4. Mémoire. Le cerveau de la guêpe (_Vespa crabro et vulgaris_).
    (Ibid., 1887, 7^e Sér., ii, pp. 5–100, 6 Pls.)

  5. Mémoire. 1. Le cerveau du criquet (_Œdipoda cœrulescens_ et
    _Caloptenus italicus_). 2. Comparaison du cerveau des Crustacés et
    des Insectes. 3. Le cerveau et la morphologie du squelette
    céphalique. (Ibid., 1888, 7^e Sér., iv, pp. 1–120, 6 Pls.)

—— Sur la structure interne du ganglion optique de quelques larves de
Diptères. (Bull. Soc. Phil., Paris, 1885, 7^e Sér., ix, pp. 75–78.)

—— La structure du cerveau des Hyménoptères. (Bull. Soc. Philomat.,
Paris, 1886, 7^e Sér., x, pp. 82, 83.)

—— La structure du cerveau des Orthoptères. (Bull. Soc. Philomat.,
Paris, 1886, 7^e Sér., xi, pp. 119–126.)

—— Sur la morphologie comparée du cerveau des Insectes et des Crustacés.
(Compt. rend. Acad. Sc. Paris, 1887, civ, pp. 444–447.)

=Kenyon, F. C.= The meaning and structure of the so-called “mushroom
bodies” of the hexapod brain. (Amer. Naturalist, xxx, 1896, pp. 643–650,
1 fig.)

—— The brain of the bee. (Journ. Comp. Neurology, vi, fasc. 3, 1896, pp.
133–210.)

—— The optic lobes of the bee’s brain in the light of recent
neurological methods. (Amer. Nat., xxxi, 1897, pp. 369–376, 1 Pl.)

With the embryological works of Graber, Heider, Korscheldt, Patten,
Wheeler, etc.


                  _c._ Histology of the nervous System

  =Helmholtz.= De fabrica systematis nervosi evertebratorum. Diss.
    Berolini, 1842.

  =Remak.= Ueber d. Inhalt d. Nervenprimitivröhren. (Archiv f. Anat. u.
    Phys., 1843.)

  =Leydig.= Lehrbuch der Histologie der Menschen und der Thiere. 1857.

  —— Vom Bau des thierischen Körpers. i. 1864.

  —— Tafeln zur vergleichenden Anatomie. i. Tübingen, 1864.

  —— Zelle und Gewebe, neue Beiträge zur Histologie des Tier-Körpers.
    Bonn, 1885, pp. 219, 6 Taf.

  =Walter.= Mikroscopische Studien über das Centralnervensystem
    wirbelloser Thiere. 1863.

  =Dietl, M. J.= Die Gewebselemente des Centralnervensystems bei
    wirbellosen Thieren. (Aus den Berichten des naturw.-medic. Vereins
    in Innsbruck.) Innsbruck, 1878.

  =Berger.= Untersuchungen über den Bau des Gehirns und der Retina der
    Arthropoden. (Arbeiten des zool. Instituts zu Wien, Heft 2, p. 173,
    1878.)

  —— Nachtrag zu den Untersuchungen über den Bau des Gehirns und der
    Retina der Arthropoden. (Ibid., Heft 3.)

  =Viallanes, H.= Recherches sur l’histologie des insectes, etc. Paris,
    1882. (Annales des Sciences nat., pp. 1–348, Pls. 1–18.)

  —— Sur la structure de la substance ponctuée des insectes. Paris,
    1885.

  =Haller, B.= Ueber die sogenannte Leydig’sche Punktsubstantz im
    Centralnervensystem. (Morp. Jahrb., xi, 1886.)

  =Nansen, F.= The structure and combination of the histological
    elements of the central nervous system. (Bergen’s Museum
    Aarsberetning for 1886. Bergen, 1887.)

  Also the writings of Benedicenti, Holmgren.




                           THE SENSORY ORGANS


                    _a._ The eyes and insect vision

[Illustration:

  FIG. 259.—Different forms of compound eyes. _A_, a bug (Pyrrhocoris).
    _B_, worker bee. _C_, drone. _D_, male Bibio, a holoptic
    insect.—From Judeich and Nitsche.
]

Of the eyes of insects there are two kinds, the simple and the compound.
Of the former there are usually three, arranged in a triangle near the
top of the head, between the compound eyes (Fig. 259, _B_). The compound
or facetted eyes, which are usually round and prominent, differ much in
size and in the number of facets.

  The number of facets varies from 12 in Lepisma,—though in a
  Brazilian beetle (Lathridius) there are only seven unequal
  facets,—to 50 in the ant, and up to 4000 in the house-fly, 12,000 in
  _Acherontia atropos_, 17,000 in Papilio, 20,000 in the dragon-fly
  (Æschna), 25,000 in a beetle (Mordella), while in _Sphinx
  convolvuli_, the number reaches 27,000. The size of the facets seems
  to bear some relation to that of the insect, but even in the
  smallest species none have been observed less than 1⁄2000 of an inch
  in diameter. Day-flying Lepidoptera have smaller facets than moths
  (Lubbock).

[Illustration:

  FIG. 260.—Section through the ocellus of a young Dyticus larva: _ct_,
    cuticula; _l_, corneal lens; _gh_, cells of the vitreous body, being
    modified hypodermal cells (_hy_); _st_, rods; _re_, retinal cells;
    _no_, optic nerve.—After Grenacher, from Lang.
]

=The simple, or single-lensed eye (ocellus).=—Morphologically the simple
eye is a modified portion of the ectoderm, the pigment enclosing the
retinal cells arising from specialized hypodermal cells, and covered by
a specialized transparent portion of the cuticula, forming the corneal
lens. The apparatus is supplied with a nerve, the fibres of which end in
a rod or solid nerve-ending, as in other sensory organs.

As seen in the ocellus of Dyticus (Fig. 260), under the corneal lens the
hypodermis forms a sort of pit, and the cells are modified to form the
vitreous body (vitrella) and retina. Each retinal cell (_re_) is
connected with a fibre from the optic nerve, contains pigment, and ends
in a rod directed outwards towards the lens. The cells at the end of the
pit or depression are, next to the lens, without pigment, and, growing
in between the retina and the lens, fill it up, and thus form a sort of
vitreous body.

  The ocellus appears to be a direct heirloom from the eyes of worms,
  while the many-facetted compound eye of the crustaceans and of
  insects is peculiar to these classes. The compound eye of the
  myriopod Scutigera differs structurally in many respects from the
  compound eye of insects, and that of Limulus still more so.

  It should be observed that in the young nymph of Ephemera, as well
  as in the semipupa of Bombus, each of the three ocelli are situated
  on separate sclerites. In Bombus the anterior ocellus has a double
  shape, being broad, transversely ovate, and not round like the two
  others, as if resulting from the fusion of what were originally two
  distinct ocelli.

  The ocelli are not infrequently wanting, as in adult Dermaptera, in
  the Locustidæ, and in certain Hemiptera (Hydrocora). In Lepidoptera
  there are but two ocelli; in geometrid moths they are often
  atrophied, and they are absent in butterflies (except Pamphila).

=The compound or facetted eye (ommateum).=—The facetted arthropod eye is
wonderfully complex and most delicately organized, being far more so
than that of vertebrates or molluscs. The simplest or most primitive
facetted eye appears to be that of Lepisma. As stated by Watase, the
compound eye of arthropods is morphologically “a collection of
ectodermic pits whose outer open ends face towards the sources of light,
and whose inner ends are connected with the central nervous system by
the optic nerve fibres.”

The facetted eye is composed of numerous simple eyes called _ommatidia_,
each of which is complicated in structure. The elements which make up an
_ommatidium_ are the following: (1) The facet or cornea, which is a
specialized portion of the cuticula; and (2), the crystalline lens or
cone; (3), the nerve-ending or _retinula_, which is formed out of the
retinula cells and the _rhabdom_ or rod lying in its axis; and (4) of
the pigment enclosing the lens and rod; the last three elements are
derived from the hypodermis. The single eyes are separated from each
other by pigment cells.

=The facet or cornea.=—This is biconvex, clear, transparent, usually
hexagonal in outline, and refracts the light. The corneal lenses are
cast in moulting.

  The corneal lenses are circular in most cases where they are very
  convex, as in Lathridius and Batocera. The hexagonal ones are very
  irregular. When they are very convex the eye has a granular
  appearance, but when not greater than the convexity of the eye
  itself, the eye appears perfectly smooth (Bolbocerus, etc.). The
  facets in the lower part of the eye of Dineutes are a trifle larger
  than in the upper part (about nine to ten). In many insects the
  reverse is the case, the upper facets being larger than the lower, a
  notable instance being Anax. The intervening lines between the
  facets are often beset with hairs, sometimes very long and dense, as
  in the drone bee and Trichophthalmus; and the modifications of the
  hairs into scales which takes place on the body occurs on the eyes
  also, the scales on the eyes of some beetles of the family Colydiidæ
  being very large, arranged in lines over the eyes like tombstones
  (Trachypholis).[45]

[Illustration:

  FIG. 261.—Section through the eye of a fly (_Musca vomitoria_): _c_,
    cornea, or facet; _pc_, pseudocone; _r_, retinula; _Rh_, rhabdom;
    _pg^1_, _pg^2_, _pg^3_, pigment cells; _b.m_, basilar membrane; _T_,
    _Tt_{1}_, _Tt_{2}_, trachea; _tv_, tracheal vesicle; _t.a_, terminal
    anastomosis; _op_, opticon; _c.op_, epiopticon; _p.op_, periopticon;
    _n.c_, nuclei; _n.c.s_, nerve-cell sheath; _N.f_, decussating
    nerve-fibres.—After Hickson, from Lubbock.
]

=The crystalline lens or cone.=—Behind or within the facets is a layer
composed of the cones, behind which are the layers of retinulæ and
rhabdoms, and which correspond to the layer of rods and cones, but not
the retina as a whole, of vertebrate animals.

The crystalline lens is, when present, usually more or less conical, and
consists of four or more hypodermis-cells.

The cones are of various shapes and sizes in insects of different
groups, or are entirely wanting, and Grenacher has divided the eyes of
insects into _eucone_, _pseudocone_, and _acone_. As the pseudocone
seems, however, to be rather a modification of the eucone eye, the
following division may be made:—

1. _Eucone eyes_, comprising those with a well-developed cone. They
occur in Lepisma, Blatta (Fig. 262), and other Orthoptera, in
Neuroptera, in Cicadidæ, in those Coleoptera with five tarsal joints, in
the dipterous genus Corethra, and in the Lepidoptera and Hymenoptera
(Fig. 263).

[Illustration:

  FIG. 262.—Ommatidium of cockroach (Periplaneta): _lf_, cornea; _kk_,
    crystalline cone; _pg′_ pigment cell; _rl_, retinula; _rm_,
    rhabdom.—After Grenacher, from Lubbock.
]

[Illustration:

  FIG. 263.—Two separate elements of the eucone eye of a bee; _Lf_,
    cornea; _n_, nucleus of Semper; _Kk_, crystalline cone; _Pg_,
    pigment cells; _Rl_, retinula; _Rm_, rhabdom.—After Grenacher, from
    Lubbock.
]

[Illustration:

  FIG. 264.—Three ommatidia of a pseudocone eye, diagrammatic: _A_, a
    separate ommatidium of _Musca vomitoria_, semi-diagrammatic: _c_,
    cornea; _p.c_, pseudocone; _pg′_, pigmented cells surrounding the
    pseudocone; _p.g_{2}_, additional pigment cells; _p.g_{3}_, basal
    pigment cells; _n.p.c_, nuclei of pseudocone; _r_, retinulæ; _n.r_,
    _n.r′_, nucleus of retinulæ; _R_, rhabdom; _b.m_, basal membrane;
    _t.a_, terminal anastomosis sending nerve-fibrils to the retinulæ.
    _B_, section through a retinula and rhabdom near the basal membrane,
    the six retinulæ (_r_) fused into a tube ensheathing the rhabdom
    (_R_).—After Hickson.
]

_a._ Pseudocone eyes; in which, instead of the crystalline lens or cone,
there are four cells filled with a transparent fluid medium, and a
smaller protoplasmic portion containing a nucleus (Muscidæ, Fig. 264,
_pc_). Hickson states that the difference between the eucone and
pseudocone eyes lies in the fact that in the pseudocone eye “the
refracting body formed by the cone-cell lies behind the nuclei,” and in
the eucone eye in front of it.

2. _Acone eyes_, where the cone or refracting body is wanting, but is
represented by the four primitive cone-cells. Acone eyes occur in
Forficulidæ, Hemiptera (except Cicadidæ), the nematocerous Diptera
(Tipula, etc.), and those Coleoptera which have less than five tarsal
joints.

=The retinula and rod.=—The retinula is morphologically a nerve-end
cell, situated at the end of a nerve-fibril arising from the optic
nerve. The elements of the retinula of Musca are six in number and
surround the _rhabdom_ (Fig. 264), which consists of a bundle of six
long, delicate chitinous rods, more or less firmly united together (Fig.
264, _R_).

The six elements of the retinula of Musca are in their outer or distal
portion free from one another, but towards their base are fused into a
sheath (Fig. 264, _r_). They are true nerve-end cells, as shown by
Müller and by Max Schultze, their views having been confirmed by
Grenacher and by Hickson. The relations of the nerves to the rods after
passing through the basal membrane is seen in Fig. 266.

=The pigment.=—The cones or pseudocones are mostly buried in pigment, as
well as the rods; and the pigment forms two layers. The outer of the two
layers is called the iris pigment (Fig. 265, _e_, _iris tapetum_), and
the inner (_f_) the retinal pigment.

Between the ommatidia internally there occur, according to Hickson,
pigment cells (Fig. 264, _p.g_{3}_), each of which stands on the basilar
membrane and sends a fine process outwards towards the internal process
of the external pigment-cell (_p.g_{2}_). A long, slender tracheal
vesicle also passes in between the retinulæ.

[Illustration:

  FIG. 265.—Two ommatidia from the eye of _Colymbetes fuscus_, × 160:
    _a_, cornea; _b_, cone; _c_, rhabdom; _d_, basal membrane, with
    nerve filaments below it: _e_, iris pigment; _f_, retina
    pigment.—After Exner, from Sharp.
]

=The basilar membrane.=—This is a thin fenestrate membrane (Fig. 261)
separating the cones and rods from the optic tract (Fig. 264, _b.m_). It
is perforated for the passage of tracheal diverticula and of the optic
nerve fibrils. It separates the dioptric or instrumental portion of the
eye from the percipient portion, _i.e._ the optic tract.

=The optic tract.=—This is the optic ganglion of earlier writers, and
appears to be the percipient portion of the eye, as opposed to the
dioptric portion. If the reader will examine Figs. 249 and 261, he will
see that it consists of three distinct ganglionic swellings, _i.e._ the
_opticon_, _epiopticon_, and _periopticon_, whose structure is very
complicated. In Musca (Fig. 261) the first ganglionic swelling (opticon)
is separated from the brain by a slight constriction, which Berger
regards as the homologue of the optic nerve of the other arthropods. It
consists of a very fine granular matrix traversed throughout by a fine
meshwork of minute fibrillæ, the neurospongium of Hickson. In the young
cockroach (Periplaneta) the optic nerve separating the cerebral ganglion
from the opticon is much longer in proportion than it is in the adult
blow-fly.

[Illustration:

  FIG. 266.—Periopticon and terminal anastomosis of Agrion, showing the
    character of the elements of the periopticon (_p.op_) and the
    structure of the terminal anastomosis (_t.a_). 1. The first layer of
    the terminal anastomosis, consisting of a plexus of fibrils and
    nerve-cells (_n.c_). 2. The second layer, in which the fibrils are
    collected together in bundles. 3. The final optic plexus and
    nerve-cells. 4. The layer in which the optic fibrils are collected
    in bundles to be distributed to the retinulæ (_r_); _b.m_, basal
    membrane.—After Hickson.
]

The second ganglionic swelling (epiopticon, Fig. 261, _c.op_) is
separated from the opticon by a tract of fine nerve-fibrils, which
partially decussate; at the decussation two or three larger nerve-cells
may be seen. It also contains a few scattered nerve-cells (_n.c_). The
third ganglionic swelling (periopticon, _p.op_) is separated from the
others by a bundle of long optic nerve-fibrils, which cross one another.
It is composed of a number of cylindrical masses of neurospongium
arranged side by side (Fig. 261, _p.op_). Between these elements of the
periopticon, which do not seem to bear any relation to the number of
ommatidia, a single nerve-cell is very frequently seen. The periopticon
does not occur in Periplaneta and Nepa (Hickson). The three optic
ganglia thus described, together with the cerebral ganglia, are
surrounded by a sheath of densely packed nerve-cells.

  Bearing in mind the fact that the retinulæ are the nerve-end cells
  of the fibres passing through the periopticon, it will be well to
  read the following account, by Hickson, of the terminal anastomosis
  of the optic fibrils in the periopticon of _Agrion bifurcatum_, and
  to examine his sketch (Fig. 266):

  “The terminal anastomosis of Agrion may be conveniently divided into
  four regions. First the region (1) lying nearest to the periopticon
  in which the nerve-cells are numerous, and the fibrils leaving the
  periopticon form a complicated plexus; the region (2) next to this,
  in which the fibrils have collected into bundles separated by spaces
  occupied by very thin-walled tracheæ in which there are no spiral
  markings, and lymph-spaces; next, the region (3) in which the
  fibrils form a final plexus, and in which there are again a
  considerable number of nerve-cells; and, lastly, the region (4) in
  which the fibrils are again collected into bundles, separated by
  spaces containing tracheæ, which perforate the basement membrane to
  supply the retinulæ.”

  It would seem as if the decussation of the optic nerve-fibrils were
  a matter of primary importance, as it so generally occurs, but in
  the young of that most generalized of all pterygote insects, the
  cockroach (Periplaneta), Hickson states that the optic nerve-fibrils
  which leave the periopticon pass without decussating to the
  ommateum, and in the adult there is only a partial decussation. In
  Nepa there is no decussation, but the anastomosis is complicated by
  the presence of looped and transverse anastomoses.

Looking at the eye as a whole, Hickson regards all the nerve structure
of the eye lying between the crystalline cone-layer and the true optic
nerve to be analogous with the retina of other animals. With Ciaccio,
Berger, and others, he does not regard the layer composed of the
retinulæ and rhabdoms as the equivalent of the retina of vertebrates,
etc.

=Origin of the facetted eye.=—The two kinds of eye, the simple and the
compound, are supposed to have been derived from a primitive type,
resembling the single eye (ommatidium) of the acone eye of Tipula. As
stated by Lang, “an increase of the elements of this primitive eye led
to the formation of the ocellus; an increase in number of the primitive
eyes, and their approximation, led to the formation of the compound
facet eye.” This view is suggested, he says, by the groups of closely
contiguous single eyes of the myriopods, considered in connection with
the compound eye of Scutigera. Grenacher looks upon simple (ocelli) and
compound eyes as “sisters,” not derived from one another, but from a
common parentage.

  Immature insects rarely possess compound eyes; they are only known
  to occur in the nymphs of Odonata and Ephemeridæ, and in the larvæ
  and pupa of Corethra.

=Mode of vision by single eyes or ocelli.=—In their simplest condition,
the eyes of worms and other of the lower invertebrates, probably only
enable those animals to distinguish light from darkness. The ocelli of
spiders and of many insects, however, probably enable them, as Lubbock
remarks, to see as our eyes do. The simple lens throws on the retina an
image, which is perceived by the fine terminations of the optic nerve.
The ocelli of different arthropods differ, however, very much in degree
of complexity.

Müller considered that the power of vision of ocelli “is probably
confined to the perception of very near objects.”

  “This may be inferred,” Müller states, “partly from their existing
  principally in larvæ and apterous insects, and partly from several
  observations which I have made relative to the position of these
  simple eyes. In the genus Empusa the head is so prolonged over the
  middle inferior eye that, in the locomotion of the animal, the
  nearest objects can only come within the range. In _Locusta
  cornuta_, also, the same eye lies beneath the prolongation of the
  head.... In the Orthoptera generally, also, the simple eyes are, in
  consequence of the depressed position of the head, directed
  downwards towards the surface upon which the insects are
  moving.”[46] Lowne considers that in the ocellus of Eristalis, the
  great convexity of the lens must give it a very short focus, and the
  comparatively small number of rods render the picture of even very
  near objects quite imperfect and practically useless for purposes of
  vision, and that the function of the ocelli is “the perception of
  the intensity and the direction of light, rather than of vision, in
  the ordinary acceptation of the term.”

  Réaumur, Marcel de Serres, Dugès, and Forel have shown by
  experiment, that in insects which possess both ocelli and compound
  eyes, the former may be covered over without materially affecting
  the movements of the animals, while if the facetted eyes are
  covered, they act as if in the dark (Lubbock).

  While Plateau regards the ocelli as of scarcely any use to the
  insect, and Forel claims that wasps, humble bees, ants, etc., walk
  or fly almost equally well without as with the aid of their ocelli,
  Lubbock demurs to this view, and says the same experiments of
  Forel’s might almost be quoted to prove the same with reference to
  the compound eyes. Indeed, the writer has observed that in caves,
  eyeless beetles apparently run about as freely and with as much
  purpose, as their eyed relatives in the open air.

  Plateau has recently shown that caterpillars which have ocelli alone
  are very short-sighted, not seeing objects at a distance beyond one
  or two centimetres, and it has been fully proved by Plateau and
  others, that spiders, with their well-formed ocelli, are myopic, and
  have little power of making out distinctly the shape of the objects
  they see.

  On the whole, we are rather inclined to agree with Lubbock and
  Forel, that the ocelli are useful in dark places and for near
  vision. They are, as Lubbock states, especially developed in
  insects, such as ants, bees, and wasps, which live partly in the
  open light and partly in the dark recesses of nests. Moreover, the
  night-flying moths nearly all possess ocelli, while with one known
  exception (Pamphila) they are wanting in butterflies.

  Finally, remarks Lubbock, “Whatever the special function of ocelli
  may be, it seems clear that they must see in the same manner as our
  eyes do—that is to say, the image must be reversed. On the other
  hand, in the case of compound eyes, it seems probable that the
  vision is direct, and the difficulty of accounting for the existence
  in the same animal of two such different kinds of eyes is certainly
  enhanced by the fact that, as it would seem, the image given by the
  medial eyes is reversed, while that of the lateral ones is direct”
  (p. 181).

=Mode of vision by facetted eyes.=—The complexity of the facetted eyes
of insects is amazing, and difficult to account for unless we accept the
mosaic theory of Müller, who maintained that the distinctness of the
image formed by such an eye will be greater in proportion to the number
of separate cones. His famous theory is thus stated: “An image formed by
several thousand separate points, of which each corresponds to a
distinct field of vision in the external world, will resemble a piece of
mosaic work, and a better idea cannot be conceived of the image of
external objects which will be depicted on the retina of beings endowed
with such organs of vision, than by comparing it with perfect work of
that kind.”

[Illustration:

  FIG. 267.—From Lubbock.
]

How vision is effected by a many-facetted eye is thus explained by
Lubbock: “Let a number of transparent tubes, or cones with opaque walls,
be ranged side by side in front of the retina, and separated from one
another by black pigment. In this case the only light which can reach
the optic nerve will be that which falls on any given tube in the
direction of its axis.” For instance, in Fig. 267, the light from _a_
will pass to _a′_, that from _b_ to _b′_, that from _c_ to _c′_, and so
on. The light from _c_, which falls on the other tubes, will not reach
the nerve, but will impinge on the sides and be absorbed by the pigment.
Thus, though the light from _c_ will illuminate the whole surface of the
eye, it will only affect the nerve at _c′_.

According to this view those rays of light only which pass directly
through the crystalline cones, or are reflected from their sides, can
reach the corresponding nerve-fibres. The others fall on, and are
absorbed by, the pigment which separates the different facets. Hence
each cone receives light only from a very small portion of the field of
vision, and the rays so received are collected into one spot of light.

It follows from this theory that the larger and more convex the eye, the
wider will be its field of vision, while the smaller and more numerous
are the facets, the more distinct will be the vision (Lubbock).

The theory is certainly supported by the shape and size and the immense
number of facets of the eye of the dragon-fly, which all concede to see
better, and at a longer range, than probably any other insect.

  Müller’s mosaic theory was generally received, until doubted and
  criticised by Gottsche (1852), Dor (1861), Plateau, and others. As
  Lubbock in his excellent summary states, Gottsche’s observation
  (previously made by Leeuwenhoek) that each separate cornea gives a
  separate and distinct image, was made on the eye of the blow-fly,
  which does not possess a true crystalline cone. Plateau’s objection
  loses its force, since he seems to have had in his mind, as Lubbock
  states, Gottsche’s, rather than Müller’s, theory.

  Müller’s theory is supported by Boll, Grenacher, Lubbock, Watase,
  and especially by Exner, who has given much attention to the subject
  of the vision of insects, and is the weightiest authority on the
  subject.

  Gottsche’s view that each of the facetted eyes makes a distinct
  image which partially overlaps and is combined with all the images
  made by the other facets, was shown by Grenacher to be untenable,
  after repeating Gottsche’s experiments with the eyes of moths, in
  which the crystalline cones are firm and attached to the cornea. He
  was thus able to remove the soft parts, and to look through the
  cones and the cornea. When the microscope was focussed at the inner
  end of the cone, a spot of light was visible, but no image. As the
  object-glass was moved forward, the image gradually came into view,
  and then disappeared again. Here, then, the image is formed in the
  interior of the cone itself.

  Exner attempted to make this experiment with the eye of Hydrophilus,
  but in that insect the crystalline cones always came away from the
  cornea. “He, however, calculated the focal length, refraction, etc.,
  of the cornea, and concluded that, even if, in spite of the
  crystalline cone, an image could be formed, it would fall much
  behind the retinula.”

  “In these cases, then,” adds Lubbock, “an image is out of the
  question. Moreover, as the cone tapers to a point, there would, in
  fact, be no room for an image, which must be received on an
  appropriate surface. In many insect eyes, indeed, as in those of the
  cockchafer, the crystalline cone is drawn out into a thread, which
  expands again before reaching the retinula. Such an arrangement
  seems fatal to any idea of an image.”

  Lubbock thus sums up the reasons which seem to favor Müller’s theory
  of mosaic vision, and to oppose Gottsche’s view: “(1) In certain
  cases, as in Hyperia, there are no lenses, and consequently there
  can be no image; (2) the image would generally be destroyed by the
  crystalline cone; (3) in some cases it would seem that the image
  would be formed completely behind the eye, while in others, again,
  it would be too near the cornea; (4) a pointed retina seems
  incompatible with a clear image; (5) any true projection of an image
  would in certain species be precluded by the presence of
  impenetrable pigment, which only leaves a minute central passage for
  the light-rays; (6) even the clearest image would be useless, from
  the absence of a suitable receptive surface, since both the small
  number and mode of combination of the elements composing that
  surface seem to preclude it from receiving more than a single
  impression; (7) no system of accommodation has yet been discovered;
  finally (8), a combination of many thousand relatively complete eyes
  seems quite useless and incomprehensible.”

  In his most recent work (1890) on the eyes of crustacea and insects,
  Exner states that the numerous simple eyes which make up the
  compound eye have each a cornea, but it is more or less flat, and
  the crystalline part of the eye has not the shape of a lens, but of
  a “lens cylinder,” that is, of a cylinder which is composed of
  sheets of transparent tissue, the refracting powers of which
  decrease toward the periphery of the cylinder. If an eye of this
  kind is removed and freed of the pigment which surrounds it, objects
  may be looked at through it from behind; but its field of vision is
  very small, and the direct images received from each separate eye
  are either produced close to one another on the retina (or rather
  the retinulæ of all the eyes) or superposed. In this last case no
  less than thirty separate images may be superposed, which is
  supposed to be of great use to night-flying insects. Exner claims
  that many other advantages result from the compound nature of an
  insect’s eye. Thus the mobile pigment, which corresponds to our
  iris, can take different positions, either between the separate eyes
  or behind the lens cylinders, in which case it acts as so many
  screens to intercept the over-abundance of light. Exner finds that
  with its compound eyes the common glow-worm (Lampyris) is capable of
  distinguishing large signboard letters at a distance of ten or more
  feet, as well as extremely fine lines engraved one-hundredth of an
  inch apart, if they are at a distance of less than half an inch from
  the eye. Exner substantiates the truth of the results of Plateau’s
  experiments, and claims that while the compound eye is inferior to
  the vertebrate eye for making out the forms of objects, it is
  superior to the latter in distinguishing the smallest movements of
  objects in the total field of vision.

  More recently Mallock has given some optical reasons to show that
  Müller’s view is the true one. He concludes, and thus agrees with
  Plateau, that insects do not see well, at any rate as regards their
  power of defining distant objects, and their behavior certainly
  favors this view. It might be asked, What advantage, then, have
  insects with compound eyes over those with simple eyes? Mallock
  answers, that the advantage over simple-eyed animals lies in the
  fact that there is hardly any practical limit to the nearness of the
  objects they can examine. “With the composite eye, indeed, the
  closer the object the better the sight, for the greater will be the
  number of lenses employed to produce the impression; whereas, in the
  simple eye the focal length of the lens limits the distance at which
  a distinct view can be obtained.” He gives a table containing
  measures of the diameters and angles between the axes of the lenses
  of various insect eyes, and states that the best of the eyes would
  give a picture about as good as if executed in rather coarse
  woodwork and viewed at a distance of a foot, “and although a distant
  landscape could only be indifferently represented on such a
  coarse-grained structure, it would do very well for things near
  enough to occupy a considerable part of the field of view.”

=The principal use of the facetted eye to perceive the movements of
animals.=—Plateau adopts Exner’s views as to the use of the facetted eye
in perceiving the movements of other animals. He therefore concludes
that insects and other arthropods with compound eyes do not distinguish
the form of objects; but with Exner he believes that their vision
consists mainly in the perception of moving bodies.

  Most animals seem but little impressed by the form of their enemies
  or of their victims, though their attention is immediately excited
  by the slightest displacement. Hunters, fishermen, and entomologists
  have made in confirmation of this view numerous and demonstrative
  observations.

  Though the production of an image in the facetted eye of the insect
  seems impossible, we can easily conceive, says Plateau, how it can
  ascertain the existence of a movement. Indeed, if a luminous object
  is placed before a compound eye, it will illuminate a whole group of
  simple eyes or facets; moreover, the centre of this group will be
  clearer than the rest. Every movement of the luminous body will
  displace the centre of clearness; some of the facets not illuminated
  will first receive the light, and others will reënter into the
  shade; some nervous terminations will be excited anew, while those
  which were so formerly will cease to be. Hence the facetted eyes are
  not complete visual organs, but mainly organs of orientation.

  Plateau experimented in the following way: In a darkened room, with
  two differently shaped but nearly equal light-openings, one square
  and open, the other subdivided into a number of small holes, and
  therefore of more difficult egress, he observed the choices of
  opening made by insects flying from the other end of the room.
  Careful practical provisions were made to eliminate error; the
  light-intensity of the two openings was as far as possible equalized
  or else noted, and no trees or other external objects were in view.
  The room was not darkened beyond the limit at which ordinary type
  ceases to be readable, otherwise the insects refused to fly (it is
  well known that during the passage of a thick cloud insects usually
  cease to fly). These observations were made on insects both with or
  without ocelli, in addition to the compound eyes, and with the same
  results.

  From repeated experiments on flies, bees, etc., butterflies and
  moths, dragon-flies and beetles, Plateau concludes that insects with
  compound eyes do not notice differences in form of openings in a
  half-darkened room, but fly with equal readiness to the apparently
  easy and apparently difficult way of escape; that they are attracted
  to the more intensely lighted opening, or to one with apparently
  greater surface; hence he concludes that they cannot distinguish the
  form of objects, at least only to a very slight extent, though they
  readily perceive objects in motion.

  One result of his experiments is that insects only utilize their
  eyes to choose between a _white_ luminous orifice in a dark chamber,
  or another orifice, or group of orifices, _equally white_. They are
  guided neither by odorous emanations nor by differences of color. He
  thinks that bees have as bad sight and act almost exactly as flies.

  From numerous experiments on Odonata, Coleoptera, Lepidoptera,
  Diptera, and Hymenoptera Plateau arrives provisionally at the
  following conclusions:

  1. Diurnal insects have need of a quick strong light, and cannot
  direct their movements in partial obscurity.

  2. Insects with compound eyes do not notice differences of form
  existing between two light orifices, and are deceived by an excess
  of luminous intensity as well as by the apparent excess of surface.
  In short, they do not distinguish the form of objects, or if they
  do, distinguish them very badly.

  Lubbock, however, does not fully accept Plateau’s experiments with
  the windows, and thinks they discern the form of bodies better than
  Plateau supposes.

  =How far can insects see?=—It is now supposed that no insects can
  perceive objects at a greater distance than about six feet. On an
  average Lepidoptera can see the movements of rather large bodies
  1.50 meters, but Hymenoptera only 58 cm., and Diptera 68 cm.; while
  the firefly (Lampyris) can see tolerably well the form of large
  objects at a distance of over two meters.

  Until further experiments are made, it seems probable, then, that
  few if any insects have acute sight, that they see objects best when
  moving, and on the whole—except dragon-flies and other predaceous,
  swiftly flying insects, such as certain flies, wasps, and bees,
  which have very large rounded eyes—insects are guided mainly rather
  by the sense of smell than of sight.

  =Relation of sight to the color of eyes.=—It appears from the
  observations of Girschner that those Diptera with eyes of a uniform
  color see better than those with brightly banded or spotted eyes.
  Thus those flies (Asilidæ, Empidæ, Leptidæ, Dolichopidæ) whose
  predaceous habits requires good or quick sight have uniformly dark
  eyes, as have also such flies as live constantly on the wing,
  _i.e._, the holoptic Bombyliidæ, Syrphidæ, Pipunculidæ, etc., whose
  eyes are also very large.

  Those flies whose larvæ are parasitic on other animals have eyes of
  a uniform color that they may readily detect the most suitable host
  for their young; such are the Bombyliidæ, Conopidæ, Pipunculidæ, and
  Tachinidæ.

  Certain flies which live in the clear sunlight, as many Dolichopidæ,
  some Bombyliidæ, and certain Tabanidæ (Tabanus, Chrysops,
  Hæmatopota), and which are often easily caught with the hand, have
  eyes spotted or banded with bright or metallic colors. This is also
  a sexual trait, as the males of some horse-flies visiting flowers
  have eyes of a single color, the spots and bands surviving only on
  the lower and hinder parts of the eye, while their voracious
  blood-sucking females have the entire eye spotted or banded (Kolbe).

  =The color-sense of insects.=—Insects, as Spengel first suggested,
  appear to be able to distinguish the color of objects. Lubbock has
  experimentally proved that bees, wasps, and ants have this power,
  blue being the favorite color of the honey-bee, and violet of ants,
  which are sensitive to ultra-violet rays.

  It is well known that butterflies will descend from a position high
  in the air, mistaking white bits of paper for white flowers; while,
  as we have observed, white butterflies (Pieris) prefer white
  flowers, and yellow butterflies (Colias) appear to alight on yellow
  flowers in preference to white ones.

  The late Mr. S. L. Elliott once informed us that on a red barn with
  white trimmings he observed that white moths (Spilosoma, Hyphantria,
  and _Acronycta oblinita_) rested on the white parts, while on the
  darker, reddish portions sat Catocalæ and other dark or reddish
  moths. Gross observed that house-flies would frequent a bluish green
  ring on the ceiling of his chamber; but if it were covered by white
  paper, the flies would leave the spot, though they would return as
  soon as the paper ring was removed (Kolbe). We have observed that
  house-flies prefer green paper to the yellowish wall of a kitchen,
  but were not attracted to sheets of a Prussian blue paper, attached
  to the same wall and ceiling.

  It is generally supposed that the shape and high colors of flowers
  attract insects; but Plateau has made a number of ingenious
  experiments which tend to disprove this view. He used in his
  investigations the dahlia, with its central head of flowerets, which
  contrast so strongly with the corolla. He finds (1) that insects
  frequent flowers which have not undergone any mutilation, but whose
  form and colors are hidden by green leaves. (2) Neither the shape
  nor lively colors of the central head (capitulum) seem to attract
  them. (3) The gayly colored peripheral flowerets of simple dahlias
  and, consequently, of the heads of other composite flowers, do not
  play the rôle of signals, such as has been attributed to them. (4)
  The insects are evidently guided by another sense than that of
  sight, and this sense is probably that of smell.




                   LITERATURE ON THE EYES AND VISION


                              _a_. General

  =Serres, Marcel de.= Mémoires sur les yeux composés et les yeux lisses
    des insectes. Montpellier, 1813.

  =Müller, Johannes.= Zur vergleichenden Physiologie des Gesichtssinnes
    der Menschen und der Tiere. 8 Taf. Leipzig, 1826.

  —— Ueber die Augen des Maikäfers. (Meckel’s Archiv f. Anat. u. Phys.,
    1829, pp. 177–181; Ann. d. Sc. nat., 1829, sér. 1, xviii, pp.
    108–112.)

  =Dujardin, F.= Sur les yeux simples ou stemmates des animaux
    articulés. (C. R. Acad. Sci., Paris, 1847, xxv, pp. 711–714.)

  =Gottsche, C. M.= Beitrag zur Anatomie und Physiologie des Auges der
    Krebse und Fliegen. (Müller’s Archiv für Anat. u. Phys., 1852, pp.
    483–492. Figs.)

  =Murray, Andrew.= On insect vision and blind insects. (Edinburgh New
    Phil. Jour., new ser. vi, 1857, pp. 120–138.)

  =Claparède, Édouard.= Zur Morphologie der zusammengesetzten Augen bei
    den Arthropoden. (Zeitschr. f. wissensch. Zool., 1859, x, pp.
    191–214, 3 Taf.)

  =Dor, H.= De la vision chez les Arthropodes. (Archives Sci. Phys, et
    Nat., 1861, xii, p. 22, 1 Pl.)

  =Landois, H.= Die Raupenaugen (Ocelli compositi mihi). (Zeitschr. f.
    wissensch. Zool., xvi, 1866, pp. 27–44, 1 Taf.)

  —— und =W. Thelen.= Zur Entwicklungsgeschichte der fasettierten Augen
    von _Tenebrio molitor L._ (Zeitschr. f. wissensch. Zool., xvii,
    1867, pp. 34–43, 1 Taf.)

  =Schultze, Max.= Untersuchungen über die zusammengesetzten Augen der
    Krebsen und Insecten. Bonn, 1868.

  =Schmidt, Oscar.= Die Form der Krystallkegel in Arthropodenauge.
    (Zeitschr. f. wissensch. Zool., xxx, Suppl., 1878, pp. 1–12, 1 Taf.)

  =Grenacher, H.= Untersuchungen ueber das Sehorgan der Arthropoden,
    insbesondere Spinnen, Insecten und Crustaceen. (Göttingen, 1879, 4º,
    pp. 1–188, 11 Taf.)

  =Reichenbach, H.= Wie die Insekten sehen. Fig. (Daheim, xvi Jahrg.,
    1880, pp. 284–286.)

  =Poletajew, N.= Ueber die Ozellen und ihr Sehvermögen bei den
    Phryganiden. (Horæ Soc. Ent. Ross., 1884, xviii, p. 23, 1 Taf. In
    Russian.)

  =Hickson, S. J.= The eye and optic tract of insects. (Quart. Journ.
    Micr. Sc., ser. 2, xxv, 1885, pp. 215–221, 3 Pls.)

  =Notthaft, Jul.= Ueber die Gesichtswahrnehmungen vermittelst des
    Fazettenauges. (Abhandl. Senckenberg. naturf. Ges., xii., 1880, pp.
    35–124, 5 Taf.)

  —— Die physiologische Bedeutung des fazettierten Insektenauges.
    (Kosmos, 1886, xviii, pp. 442–450, Fig.)

  =Mark, E. L.= Simple eyes in arthropods. (Bull. Mus. Comp. Zool.,
    1887, xiii, pp. 49–105, 5 Pls.)

  =Girschner, E.= Einiges über die Färbung der Dipterenaugen. (Berlin.
    Ent. Zeitschr., 1888, xxxi, pp. 155–162, 1 Taf.)

  =Graber, V.= Das unicorneale Tracheatenauge. (Archiv f. Mikroskop.
    Anat., xvii, 1879, pp. 58–93, 3 Taf.; Nachtrag, p. 94.)

  —— Fundamentalversuche über die Helligkeits- und Farbenempfindlichkeit
    augenloser und geblendeter Tiere. (Sitzgs.-Ber. Akad. Wissensch.,
    Wien, 1883, lxxxvii, pp. 201–236.)

  =Dahl, Fr.= Die Insekten können Formen unterscheiden. (Zool. Anz.,
    xii, 1889, pp. 243–247.)

  =Ciaccio, G. V.= Figure dichiarative della minuta fabbrica degli occhi
    de’ Ditteri. Bologna, 1884, 12 Taf., 30 pp.

  —— Della minuta fabbrica degli occhi de’ Ditteri. (Mem. Accad.
    Bologna, 1886, ser. 4, vi, pp. 605–660.)

  —— Sur la forme et la structure des facettes de la cornée et sur les
    milieux refringents des yeux composés des Muscidés. (Journ. Micr.,
    Paris, 1889, xiii Année, pp. 80–84.)

  =Carrière, J.= On the eyes of some invertebrata. (Quart. Journ. Micr.
    Sc. 1884, ser. 2, xxiv, pp. 673–681, 1 Pl.)

  —— Ueber die Arbeiten von Viallanes, Ciaccio und Hickson. (Biolog.
    Centralblatt, v, 1885, pp. 589–597.)

  —— Die Sehorgane der Tiere vergleichend anatomisch dargestellt.
    München u. Leipzig, 1885, 205 pp., 147 Figs., 1 Taf.

  —— Kurze Mitteilungen aus fortgesetzten Untersuchungen über die
    Sehorgane. (Zool. Anz., ix Jarhg., 1886, pp. 141–147, 479–481,
    496–500.)

  =Forel, A.= Les fourmis de la Suisse. (Neue Denkschriften der schweiz.
    naturforsch. Gesellsch. xxvi. 1874, pp. 480, 2 Pls.) Separate. pp.
    iv u. 457. Genève.

  —— Beitrag zur Kenntnis der Sinnesempfindungen der Insekten. (Mitteil.
    d. Münchener Ent. Vereins, ii Jahrg., 1878, pp. 1–21.)

  —— Sensations des insectes. (Recueil Zool. Suisse, iv, 1886 et 1887.)

  =Plateau, F.= L’instinct chez les insectes mis en défaut par les
    fleurs artificielles? (Assoc. française avancement des sciences.
    Congrès de Clermont. Ferrand, 1876.)

  =Plateau, F.= Recherches expérimentales sur la vision chez les
    insectes. Les insectes distinguent-ils la forme des objets? (Bull.
    Acad. Belg. 3 Sér. x, 1885, pp. 231–250.)

  —— Recherches expérimentales sur la vision chez les insectes.

  1. Part, _a_. Résumé des travaux effectués jusqu’en 1887 sur la
    structure et le fonctionnement des yeux simples. _b_. Vision chez
    les Myriapodes. (Ibid. Sér. 3, xiv, 1887, pp. 407–448, 1 Pl.)

  3. Part, _a_. Vision chez les chenilles, _b_. Rôle des ocelles
    frontaux chez les insectes parfaits. (Ibid. Sér. 3, xv, 1888, pp.
    28–91.)

  4. Part. Vision à l’aide des yeux composés. _a_. Résumé
    anatomo-physiologique. _b_. Expériences comparatives sur les
    insectes et sur les vertébrés. (Mém. cour. et autres Mém. Acad.
    Belg. 1888, xliii, pp. 1–91, 2 Pls.)

  5. Part, _a_. Perception des mouvements chez les insectes. _b_.
    Addition aux recherches sur le vol des insectes avenglés. _c_.
    Résumé général. (Bull. Acad. Belg. 1888, sér. 3, xvi, pp. 395–457, 1
    Pl.)

—— Recherches expérimentales sur la vision chez les Arthropodes, 2 Pls.
(Mém. couronn. et autres Mém. publ. p. l’Acad. Roy. d. Sciences, etc.,
de Belgique, xliii, Bruxelles, 1889.)

=Watase, S.= On the morphology of the compound eyes in the Arthropoda.
(Studies from biol. laborat. Johns-Hopkins Univ., 1890, pp. 287–334, 4
Pls.)

=Stefanowska, M.= La disposition histologique du pigment dans les yeux
des Arthropodes. (Recueil Zool. Suisse, 1890, pp. 151–200, 2 Pls.)

=Pankrath, O.= Das Auge der Raupen und Phryganiden larven. (Zeitschr. f.
wissensch. Zool., 1890, xlix, pp. 690–708, 2 Taf.)

=Lowne, B. Th.= On the modifications of the simple and compound eyes of
insects. (Philos. Trans. Roy. Soc., London, clxix, 1878, pp. 577–602, 3
Pls.)

—— On the structure and functions of the eyes of Arthropoda. (Proc. Roy.
Soc., London, 1883, xxxv, pp. 140–145.)

—— On the compound vision and the morphology of the eye in insects.
(Trans. Linn. Soc., London, 1884, ii, pp. 389–420, 4 Pls.)

—— On the structure of the retina of the blow-fly (_Calliphora
erythrocephala_). (Jour. Linn. Soc., London, 1890, xx, pp. 406–417, 1
Pl.)

=Patten, W.= Eyes of molluscs and arthropods. (Journal of Morphol.,
Boston, 1887, i, pp. 67–92, 1 Pl.; Mitteil. Zool. Stat. Neapel, vi,
1886, pp. 542–756, 5 Taf.)

—— Studies on the eyes of arthropods.—1. Development of the eyes of
Vespa, with observations on the ocelli of some insects. (Ibid., pp.
193–226, 1 Pl.)—2. Eyes of Acilius. (Ibid., 1888, ii., pp. 190–97, 7
Pls.)

—— On the eyes of molluscs and arthropods. (Zool. Anzeiger, 1887, x
Jahrg., pp. 256–261.)

—— Is the ommatidium a hair-bearing sense-bud? (Anatom. Anzeiger, 1890,
v, pp. 353–359, 4 Figs.)

=Exner, S.= Ueber das Sehen von Bewegungen und die Theorie des
zusammengesetzten Auges. (Sitzgsber. d. math. naturwiss. Cl. kais. Akad.
d. Wissens. Wien, lxxii Jahrg., 1875, 3 Abt. Physiologie, pp. 156–190, 1
Taf.)

—— Die Frage von der Funktionsweise der Fazettenauges. (Biolog.
Centralblatt, i, 1881, pp. 272–281.)

—— Das Netzhautbild des Insektenauges. (Sitzgsber. kais. Akad. d.
Wissensch. Wien, 1889, xcviii, 3 Abt., pp. 13–65, 2 Taf. u. 7 Figs.)

—— Durch Licht bedingte Verschiebungen des Pigmentes im Insektenauge und
deren physiologische Bedeutung. (Ibid., pp. 143–151, 1 Taf.)

=Exner, S.= Die Physiologie der fazettierten Augen von Krebsen und
Insekten, 7 Taf., 1, Lichtdruck u. 23 Holzschn. pp. 206. Wien, F.
Deuticke, 1891.

=Lubbock, John.= On the senses, instincts, and intelligence of animals,
with special reference to insects. London, 1888, pp. 292.

=Mallock, A.= Insect sight and the defining power of composite eyes.
(Proc. Roy. Soc., London, 1894, lv, pp. 85–90, 3 Figs.)


                          _b_. The color-sense

  =Nussli, J.= Ueber den Farbensinn der Bienen. (Schweiz. Bienenzeitung,
    N. F., ii Jahrg., 1879, pp. 238–240.)

  =Kramer.= Der Farbensinn der Bienen. (Ibid., iii Jahrg., 1880, pp.
    179–198.)

  =Gross, Wilhelm.= Ueber den Farbensinn der Tiere, insbesondere der
    Insekten. (Isis v. Russ., v Jahrg., 1880, pp. 292–294, 300–302,
    308–309.)

  =Lubbock, John.= Ants, bees, and wasps. London, 1882, pp. 448. Also On
    the senses, etc., of animals, 1889.

  =Graber, Vitus.= Grundlinien zur Erforschung des Helligkeits und
    Farbensinnes der Tiere. Prag u. Leipzig, 1884, pp. 322. (See also p.
    262.)

  =Forel, Auguste.= Les Fourmis perçoisent-elles l’ultra-violet avec
    leurs yeux ou avec leur peau? (Arch. Sci. Phys. Nat. Genève, 1886, 3
    sér., xvi, pp. 346–350.)

  Also the works of Darwin, Wallace, F. Müller, Grant Allen’s The Color
    Sense (1879), Beddard’s Animal Coloration, etc.


                        _b_. The organs of smell

The seat of the organs of smell is mainly in the antennæ, and they may
be regarded as the principal olfactory organs. For our present knowledge
of the anatomy and physiology of the olfactory organs of insects we are
mainly indebted to the recent investigations of Hauser and of Kraepelin.
The following historical and critical remarks are translated from
Kraepelin’s able treatise:

  =Historical sketch of our knowledge of the organs of smell.=—In the
  first half of the last century began the inquiries as to the seat of
  the sense of smell in the arthropods. Thus Réaumur, in his Mémoires
  (i, p. 283; ii, 224), expressed the view that in the antennæ was
  situated a special organ which might be an organ of smell.

  Lesser, Roesel, Lyonet, Bonnet, and others expressed the same
  opinion. Before this Sulzer suggested that an “unknown sense” might
  exist in the antennæ; others regarded the stigmata as organs of
  smell, as these were considered the natural passages for the
  olfactory currents. Duméril, in two special treatises as well as in
  his Considérations générales, sought to prove the theory as to the
  seat of the organs of smell in the stigmata.

  Against both of these leading views as to the seat of the sense of
  smell were expressed, in the last century, different opinions. Thus
  Comparetti thought that the sense of smell might be localized in
  very different points of the head, in the antennal club of
  lamellicorns, in the sucking-tube of Lepidoptera, in special frontal
  holes of flies and Orthoptera, etc., while Bonsdorf considered the
  palpi as organs of smell.

  Thus four different views, confused, were held at the opening of
  this century; the Hamburg zoölogist, M. C. S. Lehrman, in three
  different treatises, brought together all the hitherto known
  observations and arguments, treated them critically, and completed
  them by his own extended studies. Lehrman adopted the opinions of
  Reimarus, Baster, Duméril, and Schelver, that the stigmata presented
  the most convenient place for the site of the organs of smell.
  Cuvier followed throughout the lead of Lehrman, but Latreille
  returned to the view of the perception of smell by the antennæ,
  while Treviranus considered the mouth of arthropods as the probable
  site of the sense of smell, an opinion which, before his time,
  Huber, in his experiments on bees, had thought to be correct. Marcel
  de Serres (1811) returned again to the palpi, and asserted—at least
  in the Orthoptera—their functions to be olfactory, while Blainville,
  ten years later, again expressed anew the old opinion that the
  antennæ, or at least their terminations, were organs of smell. Up to
  that date there was an uncertainty as to the seat of the organs both
  of smell and hearing. Fabricius, indeed, had already, in 1783,
  thought he had found an organ of hearing at the base of the outer
  antenna. In 1826 J. Müller mentioned an already well-known organ in
  the abdomen of crickets as an organ of hearing. Müller, however, was
  doubtful, from the fact that the nerve passing to this organ arose,
  not from the brain, but from the third thoracic ganglion; but,
  notwithstanding, he remarks: “Perhaps we have not found the organ of
  hearing in insects because we sought for it in the head.” This
  discovery was afterwards considerably broadened and extended by
  Siebold’s work, for the views of these naturalists on the seat of
  both organs had a definite influence, especially in Germany. For
  awhile, indeed, Müller’s hypothesis stood in complete contradiction,
  so that during the following decennial was presented anew the
  picture of opposing observations and opinions as to the nature of
  the organs of smell. While Robineau-Desvoidy, at the end of the
  twentieth year, and also later, in different writings, strove
  energetically for the olfactory nature of the antennæ,
  Straus-Dürckheim held fast to the view that the tracheæ possessed
  the function under discussion. At the same period Kirby and Spence,
  in their valuable Introduction to Entomology, maintained that “two
  white cushions on the under side of the upper lip” in the mouth of
  biting insects formed a nose or “rhinarium” peculiar to insects.
  This opinion was afterwards adopted by Lacordaire (Introduction à
  Entomologie), and also by Oken in his Lehrbuch der Naturphilosophie,
  while Burmeister, rejecting all the views previously held, believed
  that insects might perhaps smell “with the inner upper surface of
  the skin.” Müller’s locust’s ear he regarded as a vocal organ.

  Besides these occasional expressions of opinion, the French
  literature of the thirtieth and fortieth years of this century
  recorded a long series of special works, with weighty experimental
  and physiological contents, on this subject. Thus Lefebre, in 1838,
  described the experiments which he made on bees, and which seemed to
  assign the seat of the sense of smell to the antennæ. Dugès reported
  similar researches on the Scolopendræ, and Pierret thought that the
  great development of the antennæ in the male Bombycidæ might be
  similarly interpreted. Driesch sought to give currency to the views
  of Bonsdorf, Lamarck, and Marcel de Serres, that the sense of smell
  was localized in the palpi, though Duponchel went back to the old
  assertion of æroscepsis of Lehrman, _i.e._ of the air-test through
  the antennæ, and Goureau again referred the seat of the sense of
  smell to the mouth. In England, Newport at this period put forth a
  work in which he considered the antennæ as organs of touch and
  hearing, and the palpi as organs of smell—a view which, as regards
  the antennæ, was opposed by Newman.

  Thus the contention as to the use of the antennæ and the seat of the
  organs of smell and hearing fluctuated from one side to the other,
  and when in 1844 Küster, by reason of his experiments on numerous
  insects, again claimed that “the antennæ are the smelling organs of
  insects,” he argued on a scientific basis; yet v. Siebold and
  Stannius (1848), in their valuable Lehrbuch der vergleichenden
  Anatomie (p. 581), remarked that “organs of smell have not yet with
  certainty been discovered in these animals.”

  The following decennial was of marked importance in the judgment of
  many disputed questions. Almost contemporaneously with Siebold and
  Stannius’ Lehrbuch appeared an opportune treatise by Erichson, in
  which this naturalist first brought forward certain anatomical data
  as to the structure of the antennæ of insects. In a great number of
  insects Erichson described on the upper surface of the antennæ
  peculiar minute pits, “pori,” which, according to him, were covered
  by a thin membrane, and to which he ascribed the perception of
  smell. A still more thorough work on this subject was published in
  the following year by Burmeister, who recognized in the pits of
  lamellicorns many small tubercles and hairs; and about the same time
  Slater, as also Pierret and Erichson before him had done, out of the
  differences of the antennal development in the males and females in
  flesh and plant-eating insects, brought together the proof of the
  olfactory function of the antennæ. But the most valuable work of
  this period is that of Perris, who, after a review of previous
  opinions, by exact observations and experiments, a model of their
  kind, sought to discover the seat of the sense of smell. He comes to
  the conclusion that the antennæ, and perhaps also the palpi, may
  claim this sense, and finds full confirmation of Dufour’s views, and
  adopts as new the physiological possibility expressed by Hill and
  Bonnet, that the antennæ might be the seat of both senses—those of
  smell and hearing.

  The beautiful works of Erichson, Burmeister, and Perris could not
  remain long unnoticed. In 1857 Hicks published complete researches
  on the peculiar nerve-endings which he had found in the antennæ,
  also in the halteres of flies and the wings of all the other groups
  of insects, and which he judged to be for the perception of smell.
  But Erichson’s and Burmeister’s “pori” were by Lespès, in 1858,
  explained to be so many auditory vesicles with otoliths. This view
  was refuted by Claparède and Claus without their deciding on any
  definite sense. Leydig first made a decided step in advance. In
  different writings this naturalist had busied himself with the
  integumental structures of arthropods, and declared Erichson’s view
  as to the olfactory nature of the antennal pits as the truest,
  before he, in his careful work on the olfactory and auditory organs
  of crabs and insects, had given excellent representations of the
  numerous anatomical details which he had selected from his extensive
  researches in all groups of arthropods. Besides the pits which were
  found to exist in Crustacea, Scolopendræ, beetles, Hymenoptera,
  Diptera, Orthoptera, Neuroptera, and Hemiptera, and which had only
  thus far been regarded as sense-organs, Leydig first calls attention
  to the widely distributed pegs and teeth, also considering them as
  sense-organs. “Olfactory teeth,” occurring as pale rods, perforated
  at the end, on the surface of the antennæ of Crustacea, Myriopoda,
  Hymenoptera, Lepidoptera, Coleoptera, are easily distinguished, and
  besides the “olfactory pegs” of the palpi, may be claimed as organs
  of smell. The nerve-end apparatus first discovered by Hicks in the
  halteres and wings, Leydig thinks should be ranked as organs of
  hearing.

  There was still some opposition to Leydig’s opinion that in the
  insects the sense of smell is localized in the antennæ (teeth and
  pits), and here the work of Hensen might be mentioned, which in 1860
  had a decided influence upon the conclusion of some inquiries.

  Thus Landois denied that the antennæ had the sense of smell, and
  declared that the pits in the antennæ of the stag beetle were
  auditory organs. So, also, Paasch rejected Leydig’s conclusion,
  while he sought to again reinstate the old opinion of Rosenthal as
  to the olfactory nature of the frontal cavity of the Diptera. In
  spite of the exact observations and interesting anatomical
  discoveries of Forel in ants, made in 1874, there appeared the great
  work of Wolff on the olfactory organs of bees, in which this
  observer, with much skill and acuteness, sought to give a basis for
  the hypothesis of Kirby and Spence that the seat of the sense of
  smell lay in the soft palatine skin of the labrum within the mouth
  (_i.e._ the epipharynx). Joseph, two years later, drew attention to
  the stigmata as olfactory organs, referring to the olfactory girdle,
  and Forel sought by an occasional criticism of Wolff’s conclusions
  to prove experimentally the olfactory function of the antenna; but
  Graber, in his widely read book on insects, defended the Wolffian
  “nose” in the most determined way, and denied to the antennæ their
  so often indicated faculty of smell. In 1879 Berté thought he had
  observed in the antenna of the flea a distinct auditory organ, and
  Lubbock considered the organs of Forel in the antennæ of ants as a
  “microscopic stethoscope.” In 1879 Graber described a new
  otocyst-like sense-organ in the antennæ of flies, which was
  accompanied by a complete list of all the conceivable forms of
  auditory organs in arthropods. In this work Graber described in
  Musca and other Diptera closed otocysts with otoliths and auditory
  hairs, as Lespès had previously done. But Paul Mayer, in two essays,
  refuted this view in a criticism of the opinion of Berté, referring
  the “otocysts with otoliths” to the well-known antennal pits into
  which tracheæ might pass. Mayer did not decide on the function of
  the hairs which extend to the bottom of the pits; while in the most
  recent research, that of Hauser, the author again energetically
  contended for the olfactory function of the antennæ. Both through
  physiological experiments and detailed anatomical investigations
  Hauser sought to prove his hypothesis, as Pierrot, Erichson, Slater,
  and others had done before him, besides working from an evolutional
  point of view. In a purely anatomical aspect, especially prominent
  are his discovery of the singularly formed nerve-rods in the pits
  and peg-like teeth of the Hymenoptera and their development, as well
  as the assertion that numerous hairs in the pits described by
  Leydig, Meyer, etc., should be considered as direct terminations of
  nervous fibres passing into the pits. In the pits he farther, with
  Erichson, notices a serous fluid, which may serve as a medium for
  the perception of smells. Among the latest articles on this subject
  are those of Künckel and Gazagnaire, which are entirely anatomical,
  while the latest treatise of Graber on the organs of hearing in
  insects opposes Hicks’s theory of the olfactory function of the
  nerve-end apparatus in the halteres, wings, etc., and argues for the
  auditory nature of these structures. Finally, according to Voges,
  the sense of smell is not localized, but spread over the whole body.

  My own observations on different groups of insects agree, in
  general, with those of Perris, Forel, and Hauser, without being in a
  position to confirm or deny the varying relations of the Hemiptera.
  That irritating odorous substances (chloroform, acetic acid) cause
  the limbs to move in sympathy with the stimulus, I have seen several
  times in Acanthosoma; still it may be a gustatory rather than
  olfactory stimulus.

  Turning now from speculation and simple observation to exact
  anatomical and histological data, the nerve-end apparatus seems to
  have a distinct reference to the perception of odors. It comprises a
  structure composed of nervous substances which are enclosed in a
  chitinous tube, and either only stand in relation to the surrounding
  bodies by the perforated point, or pass to the surface as free
  nerve-fibrillæ.

  In insects there is a remarkable and fundamental difference in the
  structures of the parts supposed to be the organs of smell. Erichson
  was acquainted only with the “pori” covered by a thin membrane; but
  Burmeister, in his careful work on the antennæ of the lamellicorns,
  distinguished pits at the bottom of which hairs rise from a cup-like
  tubercle, from those which were free from hairs. Leydig afterwards
  was the first to regard as olfactory organs the so-called pegs
  (_kegel_), a short, thick, hair-like structure distinctly perforated
  at the tip, which had already, by Lespès in Cercopis, etc., been
  described as a kind of tactile papilla. Other very peculiar
  olfactory organs of different form, Forel (Fourmis de la Suisse)
  discovered in the antennæ of ants, which Lubbock incorrectly
  associated with the nerve-end apparatus found by Hicks in other
  insects.

As the final result of his researches Kraepelin states that the great
variety of antennal structures previously described may be referred to a
single common fundamental type of a more or less developed free or
sunken hair-like body which stands in connection by means of a wide
pore-canal with a many-nucleated ganglion-cell. The latter sends only a
relatively slender nerve-fibre (axial cord) through the pore-canal into
the hair; but the same is enclosed by epithelial cells which surround
the pore-canal.

Hauser’s researches on the organs of smell in insects were so carefully
made and conclusive that our readers will, we feel sure, be glad to have
laid before them in detail the facts which prove so satisfactorily that
the antennæ of most insects are olfactory rather than auditory in their
functions.

=Physiological experiments.=—First of all one should observe as exactly
as possible the normal animal in its relation to certain odorous
substances, whose fumes possess no corrosive power or peculiarities
interfering with respiration; then remove the antennæ and try after
several days to ascertain what changes have taken place in the relation
of the animal to the substance. In order to come to no false results it
is often necessary to let the insects operated upon rest one or two
days, for immediately after the operation they are generally so restless
that a careful experiment is impossible.

The extirpation of the antennæ is borne by different insects in
different ways; many bear it very easily, and can live for months after
the operation, while others die in the course of a few days after the
loss of these appendages. The animals seem to be least injured if the
operation is performed at a time when they are hibernating. _Pyrrhocoris
apterus_, and many other insects, afforded a very striking proof of this
relation.

Experiments made by placing the antennæ in liquid paraffine so as to
cover them with a layer of paraffine, thus excluding the air, gave the
same result as if the antennæ had been removed.

The experiments may be divided, according to their object, into three
groups. Experiments of the first kind were made on insects in their
relation to strong-smelling substances, as turpentine, carbolic acid,
etc., before and after extirpation of the antennæ. The second group
embraces experiments on the relation of animals as regards their search
for food; and finally the third group embraces experiments on the
relation of the sexes relative to reproduction before and after the
extirpation of the antennæ.

=Relation of insects to smelling substances before and after the loss of
their antennæ.=—Taking a glass rod dipped in carbolic acid and holding
it within 10 cm. of _Philonthus œneus_, found under stones at the end of
February, it was seen to raise its head, turn it in different
directions, and to make lively movements with its antennæ. But scarcely
had Hauser placed the rod close to it when it started back as if
frightened, made a sudden turn, and rushed, extremely disturbed, in the
opposite direction. When he removed the glass rod, the creature busied
itself for some time with its antennæ, while it drew them, with the aid
of its fore limbs, through its mouth, although they had not come into
direct contact with the carbolic acid. There was the same reaction
against oil of turpentine, and it was still more violent against acetic
acid.

After having many times carefully tested the relations of the normal
animal to the substances mentioned, the antennæ were removed from the
socket-cavity.

On the second day after Hauser experimented with the insects, they
exhibited no reaction either against the carbolic acid, the oil of
turpentine, or even against the acetic acid, although he held the glass
rod which had been dipped into it for one or two minutes before and over
the head. The creatures remained completely quiet and immovable, at the
most slightly moving the palpi. They showed otherwise no change in their
mode of life and their demeanor; they ate with great eagerness flesh
which had been placed before them, or dead insects, and some were as
active as usual as late as May. These beetles had, as proved by the
experiments, lost the sense of smell alone; how far the sense of touch
was lost Hauser could not experimentally decide.

The same results followed experiments with species of the genus Ptinus,
Tenebrio, Ichneumon, Formica, Vespa, Tenthredo, Saturnia, Vanessa, and
Smerinthus; also many species of Diptera and Orthoptera, besides Julus
and Lithobius, while many larvæ reacted in the same manner.

  Less satisfactory were the experiments with Carabus, Melolontha, and
  Silpha; there is no doubt that the species of these genera, through
  the extirpation of their antennæ, become more or less injured as to
  the acuteness of their powers of smelling; but they never show
  themselves wholly unable to perceive strong-smelling substances.

  The allurement of the substance acts for a longer time on those
  deprived of their antennæ, then they become restless, then they
  wander away from the glass tube held before them; still all their
  movements are but slightly energetic, and the entire reaction is
  indeterminate and enfeebled.

  Experiments with the Hemiptera gave still more unfavorable results;
  after the loss of their antennæ they reacted to smells as eagerly as
  those did which were uninjured.

=Experiments on the use of the antennæ in seeking for food.=—Under this
head experiments were made with Silpha, Sarcophaga, Calliphora, and
Cynomyia.

Silpha and its larva were treated in the following manner: they were
placed in large boxes whose bottoms were covered with moss, etc.; in a
corner of the box was placed a bottle with a small opening, in which was
placed strong-smelling meat. So long as the beetles were in possession
of their antennæ they invariably after a while discovered the meat
exposed in the bottle, while after the loss of their antennæ they did
not come in contact with it.

In a similar way acted the species of Sarcophaga, Calliphora, and
Cynomyia. Hauser, in experimenting with these, placed a dish with a
large piece of decayed flesh on his writing-table. In a short time
specimens of the flies referred to entered through the open window of
the room. The oftener he drove them away from the meat would they swarm
thickly upon it. Then closing the window and catching all the flies, he
deprived them of their antennæ and again set them free. They flew about
the room, but none settled upon the flesh nor tried to approach it.
Where a fly had alighted on a curtain or other object, the decayed flesh
was placed under it so that the full force of the effluvium should pass
over it, but even then no fly would settle upon it.

=Experiments testing the influence of the antennæ of the males in
seeking the females.=—For this purpose Hauser chose those kinds in which
the male antennæ differ in secondary sexual characters from those of the
female, and in which it is known that they readily couple in
confinement, as _Saturnia pavonia_, _Ocneria dispar_, and _Melolontha
vulgaris_. The two first-named insects did not couple after the
extirpation of their antennæ. Of _Melolontha vulgaris_ twenty pairs were
placed in a moderately sized box. On the next morning twelve pairs of
them were found coupling. Hauser then, after removing the first lot,
placed a new set of thirty pairs in the same box, cut off all the
antennæ of the males and those of a number of females. On the following
morning only four pairs were found coupling, and at the end of three
days five others were observed sexually united.

  From these experiments Hauser inferred that those insects deprived
  of their antennæ were placed in the most favorable situation, such
  as they would not find in freedom; for the space in which the
  insects moved about was so limited that the males and females must
  of necessity meet. But at the same time the results of the
  experiments cannot absolutely be regarded as proving that the males,
  after the loss of their antennæ, were then not in condition to find
  the females, because in the case of the above-mentioned moths, under
  similar conditions, after the extirpation of the antennæ no sexual
  union took place. If, however, the experiments made do not all lead
  to the results desired, Hauser thinks that the results agree with
  those of his histological researches, that in the greater number of
  insects the sense of smell has its seat in the antennæ. His results
  also agree with those of Perris.

=Structure of the organs of smell in insects.=—The olfactory organs
consist, in insects,—_i.e._, all Orthoptera, Termitidæ, Psocidæ,
Diptera, and Hymenoptera, also in most Lepidoptera, Neuroptera, and
Coleoptera,—

1. Of a thick nerve arising from the brain, which passes into the
antennæ.

2. Of a sensitive apparatus at the end, which consists of staff-like
cells, which are modified hypodermis cells, with which the fibres of the
nerves connect.

3. Of a supporting and accessory apparatus, consisting of pits, or peg-
or tooth-like projections filled with a serous fluid, and which may be
regarded as invaginations and outgrowths of the epidermis.

Hauser adds a remark on the distribution of the pits and teeth in the
larvæ of insects, saying that his observations are incomplete, but that
it appears that in the larvæ the teeth are most generally distributed,
and that they occur not on the antennæ alone, but on the palpi; but in
very many larvæ neither pits nor teeth[47] occurred. In the Myriopoda
teeth-like projections occur on the ends of the antennæ. In Lithobius
they form very small, almost cylindrical, pale organs.

[Illustration:

  FIG. 268.—Olfactory organ of Caloptenus.
]

[Illustration:

  FIG. 269.—Olfactory pits of the antenna of Stenobothrus. This and Fig.
    268 after Hauser.
]

  LETTERING FOR FIGS. 268, 269, 273, 275, 276, 278–281.—_a_, _a_,
  circular thickening of the skin surrounding the opening of the
  olfactory pit; _ax_, thread-like continuation of the nerve-cell;
  _b_, vesicle-like bottom of the olfactory pit, through which the
  olfactory style passes; _br_ bristle in Fig. 283, stout, and
  protecting the olfactory pit; _bs_, bent bristle or seta; _ch_,
  chitinous integument of the antennæ; _d_, seen in section; _f_,
  invaginated pit; _Fv_, Forel’s flask-shaped organ; _Fvo_, its
  opening seen from the surface; _gl_, gland-like mass of cells;
  _hyc_, hypodermic cells; _i_, entrance into the canal belonging to
  the pit; _m_, olfactory membrane; _m′_, _m″_, _mc_, membrane-forming
  cell; _n_, nerve of special sense; _nc_, nucleus of the sense- or
  ganglion-cell; _o_, opening into the olfactory pit; _p_, olfactory
  pit; _cp_, compound pits; _pw_, wall of the pit; _s_, a large seta;
  _sc_, sense- or ganglion-cell; _st_, olfactory or sense-style,
  sometimes peg-shaped; _tb_, tactile bristle.

[Illustration:

  FIG. 270.—_A_, _b_, sense-organ on the abdominal appendages of a fly
    (Chrysopila); _c_, sense-organ on the terminal joint of palpus of
    Perla.
]

[Illustration:

  FIG. 271.—Longitudinal section of part of cercus of _Acheta
    domestica_: _ch_, cuticula; _hyp_, hypodermis; _n._ nerve; _h′^1_,
    integumental hairs, not sensory; _h^2_, ordinary hair; _h^3_,
    sensory hair; _h^4_, bladder-like hair; _sz_, sense-cell.—After Vom
    Rath, from Sharp.
]

In the course of a special description of these sense-organs in the
Orthoptera, Hauser describes at length those of _Œdipoda cœrulescens_
and _Caloptenus italicus_. On one antennal joint of Caloptenus (Fig.
268) was often counted 50 pits; on the anterior joints the number
diminishes to about 30. Hauser thinks that in all Orthoptera whose
antennæ are like those of Caloptenus occur similar pits, as he found
them in Stenobothrus (Fig. 269) as well as in Œdipoda. Gryllotalpa
possesses similar pits,—four to six on each antennal joint, making
between 300 and 400 pits on each antenna.[48] In _Mantis religiosa_ the
pits were not detected, but on each joint, except the eighth basal,
there are about 200 small, hollow, curved teeth with a fine opening in
front.

In the Neuroptera (Chrysopa) there occur on the antennæ, besides
numerous very long tactile bristles, small pale, transparent teeth. No
pits could be detected.

In the Hemiptera (two species of Pyrrhocoris only were examined) only
two kinds of tactile bristles occurred, but Hauser detected no pits,
though Lespès states that they are present.

[Illustration:

  FIG. 272.—Longitudinal section of apex of palpus of _Pieris brassicæ_:
    _sch_, scales; _ch_, cuticula; _hyp_, hypodermis; _n_, nerve; _sz_,
    sense-cells; _sh_, sense-hairs.—After Vom Rath, from Sharp.
]

Of the Diptera, Hauser examined more than 60 species. The pits in the
_Diptera brachycera_ (Muscidæ, etc.) are unexceptionally confined to the
third antennal joint. Their number varies extraordinarily in the
different species. _Helophilus florens_ has on each antennal disk only a
single pit, while _Echinomyia grossa_ possesses 200 of them. In flies of
certain families the pits are compound, and contain 10, 20, and often
100 olfactory hairs, partly arising from the coalescence of several
pits. Such pits are usually divided by lateral walls into several
chambers, whose connection is only indicated by their common outlet.
Simple olfactory pits with a single olfactory style were observed only
in the Tabanidæ, Asilidæ, Bombylidæ, Leptidæ, Dolichopidæ, Stratiomyidæ,
and Tipulidæ. In the last the compound forms do not occur at all, but in
the other families mentioned also occur compound pits, receiving from
two to ten nerve-terminations.

The antennal pits of flies are always sac-like invaginations of the
external chitinous integument, of manifold shapes, opening externally
and never closed by a membrane. The pits differ but slightly in the
different species, and that of _Cyrtoneura stabulans_ (Fig. 273) is
described at length as typical of those of brachycerous flies in
general.

The olfactory pits of the Tipulidæ seem to have a somewhat different
structure, since the external passage is closed. It is circular,
surrounded with a slight chitinous wall, and not covered with bristles.
Such pits in their external appearance are like those of the locust
(Caloptenus) and many Hymenoptera. They are situated usually on the
third antennal joint. _Pachyrhina pratensis_ L. has about 60 of them, as
have _Tipula oleracea_ L. and Ctenophora.

In the Lepidoptera, olfactory pits are much like those of flies. Hauser
describes in detail those of _Vanessa io_. Those of the moths were not
examined, but they can be readily and satisfactorily proved to be the
site of the olfactory sense.

[Illustration:

  FIG. 273.—Longitudinal section through the third antennal joint of a
    fly (_Cyrtoneura stabulans_), showing the compound pits from above
    and in section.—After Hauser.
]

[Illustration:

  FIG. 274.—Antenna of Adelops, showing the olfactory organs (_p_) in
    the five last joints.
]

Historical researches in respect to the Coleoptera generally gave a very
unfavorable result, contrary to Lespès’s views. That author states that
in the Carabidæ the pits are found on the four first joints, but Hauser
could discover them in none which he examined. Usually only tactile
bristles occur, so also in the Cerambycidæ, Curculionidæ, Chrysomelidæ,
and Cantharidæ. In a blind silphid beetle (_Adelops hirtus_) of Mammoth
Cave we have found well-marked olfactory organs (Fig. 274). Similar
organs occur in the antennæ of the Panorpidæ.

Olfactory pits, however, without doubt occur in Silpha, Necrophorus,
Staphylinus, Philonthus, and Tenebrio. The openings of the pits are
small and surrounded with a small chitinous ring; in Silpha,
Necrophorus, and Tenebrio they cannot easily be distinguished from the
insertion-cavities of the bristles, but in Philonthus and Staphylinus
they are less like them, being distinguished by their somewhat larger
size and their often more oval form. In _Philonthus æneus_ about 100
such small pits occur irregularly on the terminal joints; besides, in
this species on each side of the terminal joint is an apparatus which is
like the compound pit generally occurring in the Diptera.

[Illustration:

  FIG. 275.—Olfactory pits of the antenna of _Melolontha
    vulgaris_.—After Kraepelin.
]

[Illustration:

  FIG. 276.—Antennal pit of _Melolontha vulgaris_, seen in vertical
    section.—After Hauser.
]

Very remarkable pits occur in the antennal lamellæ of _Melolontha
vulgaris_ (Fig. 275) and other lamellicorns. On the outer surface of the
first and seventh (in the female the sixth) antennal leaf, as also on
the edges of the other leaves, only arise scattered bristles; on the
inner surface of the first and seventh leaves, as also on both surfaces
of the second to sixth leaves, are close rows of rather shallow
depressions of irregular form, some circular, others regularly
hexagonal. Their number is enormous: in the males 39,000, in the females
about 35,000, occur on each antenna.

[Illustration:

  FIG. 277.—Organ of smell of Anophthalmus.—After Hauser. _A_, _a_, _b_,
    the same in _A. tenuis_, _B_ in _A. tellkampfii_.
]

[Illustration:

  FIG. 278.—Section through antennal joint of _Vespa crabro_, showing
    the great number of olfactory pits, olfactory and tactile bristles.
    _A_, section through an olfactory pit of _Vespa crabro_.—After
    Hauser.
]

The antennal pits and teeth of _Dyticus marginalis_ are morphologically
and physiologically identical with those of bees and wasps. In
_Anophthalmus bilimekii_, Hauser found on the last antennal joints about
60 teeth, which essentially differ in form from those previously
described; they are very pale, transparent, cylindrical, elongated, and
bent elbow-shaped on the first third, so that the last two-thirds run
parallel with the antenna. The length of these remarkable teeth is 0.035
mm., their breadth 0.005 mm. He only found them in Anophthalmus, and in
no other species of Carabidæ; they must resemble the teeth described in
Chrysopa. Our species possesses similar processes (Fig. 277). Similar
teeth occur on the maxillary and labial palpi of beetles. _Dyticus
marginalis_ possesses at the end of each terminal palpal joint a group
of very small teeth, which were also detected in _Anophthalmus
bilimekii_, _Melolontha vulgaris_, etc. In _Carabus violascens_ were
detected on the maxillary palpi large, plainly microscopical, white
disks, which are surrounded with a great number of extremely small
teeth.

Whether the above-described organs on the palpi of beetles should be
considered as olfactory or gustatory in their nature can only be
determined by means of physiological experiments; they probably receive
taste-nerve terminations.

[Illustration:

  FIG. 279.—Olfactory pits of the antenna of _Vespa vulgaris_.—After
    Kraepelin.
]

The Hymenoptera furnished very good material for histological purposes,
so that Hauser could not only study the terminal apparatus of the
olfactory nerves in the perfect insect, but also in three different
stages of the pupa. These are described at length, as regards the
distribution of the pits and teeth, in _Vespa crabro_; each joint of the
antenna (flagellum) possesses between 1300 and 1400 pits, nearly 60
teeth, and about 70 tactile hairs; on the terminal joint there are more
than 200 teeth, so that each antenna has between 13,000 and 14,000
olfactory pits and about 700 teeth (Kegeln). Fig. 278 represents a
cross-section through the penultimate antennal joint of _Vespa crabro_;
we can see how thick are the series of openings on the surface of the
antennæ, and how regular is the distribution of the teeth.

The distribution of the olfactory pits and olfactory teeth is thus seen
to be very general; the deviations are so insignificant that there is no
reason for the establishment of more than one type.

Antennal pits with a small crevice-like opening occur in genera nearly
allied to Vespa and also in most Ichneumonidæ, Braconidæ, and Cynipidæ.
But the crevice-like openings in these families are considerably longer
and often are of a somewhat twisted shape. In all the species with
translucent antennæ we can recognize the inner mouth of the pit as a
round or nearly round disk situated usually under the middle of the
opening. The antennal pits of _Apis mellifica_, as well as those of
Bombus (Fig. 280) and allied genera, differ from those of the
Ichneumonidæ in being not like crevices, but circular openings.

[Illustration:

  FIG. 280.—Olfactory pits of the antenna of Bombus.—After Kraepelin.
]

[Illustration:

  FIG. 281.—Olfactory pits of the antenna of Formica: _Fv_, Hicks’
    “bottle,” Forel’s flask-shaped organ, _Fvo_, its opening.—After
    Kraepelin.
]

[Illustration:

  FIG. 282.—Supposed olfactory organs at end of antenna of Campodea:
    _A_, _C. staphylinus_. _B_, _C. cookei_, from Mammoth Cave.
]

[Illustration:

  FIG. 283.—Vertical section through a single olfactory pit in the
    antenna of the horse-fly (_Tabanus bovinus_). For lettering see p.
    272.—After Hauser.
]

The distribution of the olfactory peg or tooth-like projections seems to
be much more limited than that of the pits in the Ichneumonidæ. Hauser
could not find any. _Apis mellifica_ possesses on each antennal joint
only about twenty slender pale teeth, scarcely a third as many as in
_Vespa crabro_; on the other hand, Formica, of which genus several
species were examined, seems to have far more teeth than pits; they are
relatively long, pale, transparent, and somewhat clavate; they are not
unlike those of Chrysopa; on the terminal joint only occur the round
openings (_Fvo_), which lead into a bottle-shaped invagination of the
integument (_Fv_) and contain an olfactory style (Fig. 281). In the
Tenthredinidæ only teeth and no pits were to be detected. Sirex has on
the under side of the nine last joints of each antenna a group of from
200 to 300 small teeth, which resemble those of _Vespa crabro_; Lyda has
on the terminal joints about 100 teeth. We may add that supposed organs
of smell occur on the antennæ of Campodea (Fig. 282).

Kraepelin also thus briefly summarizes Hauser’s statements as to the
forms of the different organs of smell.

  The manifold nature of the antennal organs has, by Hauser, from
  thorough studies of the nerve-elements belonging to them, been not
  simplified but rendered more complicated. According to this
  naturalist we may distinguish the following forms which the
  olfactory organs may assume: 1. “Pale, tooth-like chitinous hairs on
  the outer surface of the antennæ, which are perforated at the end;
  nothing is known as to the relation of the nerve passing into it
  (Chrysopa, Anophthalmus). 2. In pit-like depressions of the antennæ
  arise _nerve-rods_ (without a chitinous case) which stand in direct
  relation with a ganglion-cell lying under it. These pits are either
  _simple_, viz. with only an ‘olfactory rod’ (Tabanus, Fig. 283, and
  other Diptera, Vanessa), or _compound_ (Muscidæ, and most other
  Diptera, and Philonthus). It is important that these pits are partly
  _open_ (in the above-named groups of insects), and partly _closed_
  and covered with a thin membrane, under whose concavity the
  olfactory rods end (Orthoptera, Melolontha, and other lamellicorns).
  3. Short, thick pits sunken slightly into the surface of the
  antennæ, and over this a chitinous peg perforated at the end, in
  whose base, from the interior, projects a very singular nerve-peg,
  which is situated over an olfactory ganglion-cell, and provided with
  a slender crown of little rods, and flanked on each side by a
  flagellum-cell (Hymenoptera). 4. Round or crevice-like pits covered
  over by a perforated chitinous membrane with nerve-rods like those
  in 3, but in place of the flagellum-cell with ‘membrane-forming’
  cells spread before it. Hauser finally mentions further differences
  in the ganglion-cells sent out into the nerve-end apparatus. These
  exhibit in Diptera and Melolontha only one nucleus, in Hymenoptera a
  single very large one (with many nucleoli) and three small ones, in
  Vanessa six, in Orthoptera a very large number of nuclei, etc.”




                   LITERATURE OF THE ORGANS OF SMELL


  =Réaumur, Réné Ant. de.= Mémoires pour servir à l’histoire des
    insectes. Paris, 1734–42. (i, 283; ii, 224).

  =Lesser, F. C.= Insecto-theologia, 1740, p. 262.

  =Roesel, A. J.= Insektenbelustigungen, 1746, ii, p. 51.

  =Reimarus, H. S.= Allgemeine Betrachtung ueber die Triebe der Thiere
    hauptsächlich ihre Kunsttriebe (Instinct). Hamburg, 1760, p. 355.

  =Sulzer, J. H.= Die Kenntzeichen der Insecten. Zürich, 1761.

  =Lyonet, P.= Traité anatomique de la chenille, 1762, pp. 42, 96, 195.

  =Comparetti, A.= De aure interna comparata. Patavii, 1769.

  =Bonnet, C.= Œuvres complètes, 1779–1783, ii, p. 36.

  —— Contemplation de la nature, Ch. iii, p. 18.

  =Scarpa, Ant.= Anatomicæ disquisitiones de auditu et olfactu. Ticini,
    1789.

  =Huber, F.= Nouvelles observations sur les abeilles, 1792, ii, p. 475.

  =Lehrmann, M. C. G.= De antennis insectorum. Londini, Hamburgi, 1799,
    p. 48, Diss. posterior; Hamburg and London, 1800, p. 80 (especial
    sense, ærocepsis).

  =Latreille, P. A.= Histoire naturelle des crustacés et des insectes,
    1806–1809, ii, 50.

  =Blainville, M. H. D.= Principes d’anatomie comparée, 1822, i, p. 339.

  =Dugès, A. L.= Traité de physiologie comparée, 1838, i, p. 161.

  =Newport, G.= On the use of the antennæ of insects. (Trans. Ent. Soc.,
    London, ii, 1840, pp. 229–248.)

  =Robineau-Desvoidy, A. J. B.= Sur l’usage réel des antennes chez les
    insectes. (Ann. Soc. Ent. France, 1842, xi Bull., pp. 23–27.)

  =Erichson, W. F.= De fabrica et usu antennarum in insectis. Berlin,
    1847, 1 Tab., p. 13.

  =Perris, E.= Mémoire sur le siège de l’odorat dans les articulés.
    (Ann. sc. nat., Sér. 3, 1850, xiv, pp. 159–178.)

  =Dufour, L.= Quelques mots sur l’organe de l’odorat et sur celui de
    l’ouie dans les insectes. (Actes d. l. Soc. Linn., Bordeaux, 1850,
    xvii, Ann. sc. nat., Sér. 3, Zool., xiv, 1850, pp. 179–184.)

  =Leydig, F.= Zum feineren Bau der Arthropoden. (Müller’s Archiv, 1855,
    pp. 376–480); Lehrbuch der Histologie, 1857, p. 220; Zur Anatomie
    der Insekten (Archiv für Anatomie, 1859, pp. 35–89 and 149–183).

  —— Ueber Geruchs- und Gehörorgane der Krebse und Insekten. (Archiv f.
    Anat. u. Phys., 1860.)

  —— Die Hautsinnesorgane der Arthropoden. (Zool. Anzeiger, 1886, pp.
    284–291, 308–314, 265–314.)

  =Landois, H.= Das Gehörorgan des Hirschkäfers. (Archiv f. mikrosp.
    Anat., 1868, iv, pp. 88–95.)

  =Troschel, H.= Ueber das Geruchsorgan der Gliedertiere. (Verhandl. d.
    naturhist. Vereins d. preuss. Rheinlande u. Westfal., xxvii Jahrg.,
    1870, pp. 160–161.)

  =Packard, A. S.= The caudal styles of insects sense-organs, _i.e._,
    abdominal antennæ. (Amer. Naturalist, 1870, pp. 620, 621. Also Proc.
    Bost. Soc. Nat. Hist., 1868, xi, p. 398.)

  =Paasch, A.= Von den Sinnesorganen der Insekten im Allgemeinen, von
    Gehörund Geruchsorganen im Besondern. (Archiv für Naturgesch., xxxix
    Jahrg., i, 1873, pp. 248–275.)

  =Chadima, Jos.= Ueber die von Leydig als Geruchsorgane bezeichneten
    Bildungen bei den Arthropoden. (Mitteil. d. naturwiss. Ver. f.
    Steiermark, 1873, pp. 36–44.)

  =Forel, A.= Les Fourmis de la Suisse. (Neue Denkschr. Allg. Schweiz.
    Gesellsch. f. d. ges. Naturw., xxvi, 1874, pp. 118, 144.)

  —— Études myrmécologiques en 1884, avec une description des organes
    sensoriels des antennes. (Bull. Soc. Vaud. sc. nat., 1885, Sér. 2,
    xx, pp. 316–380.)

  =Graber, V.= Die Insekten. München, 1877.

  —— Neue Versuche über die Functionen der Insektenfühler. (Biol.
    Centralb., vii, 1887, pp. 13–19.)

  =Trouvelot, L.= The use of the antennæ in insects. (Amer. Naturalist,
    xi, 1877, pp. 193–196.)

  =Mayer, P.= Sopra certi organi di senso nelle antenne dei Ditteri.
    (Atti R. Accad. d. Lincei, Roma, Ser. 3, Mem. Cl. sc. fis., mat. e
    natur., iii, 1879, p. 11.)

  =Hauser, Gustav.= Physiologische und histiologische Untersuchungen
    über das Geruchsorgan der Insekten. (Zeitschr. f. wissens. Zool.,
    xxxiv, 1880, pp. 367–403, 3 Taf.)

  =Kraepelin, Karl.= Ueber die Geruchsorgane der Gliedertiere
    (Oster-Programm der Realschule des Johanneums. Hamburg, 1883, pp.
    48, 3 Taf.)

  =Schiemenz, Paulus.= Ueber das Herkommen des Futtersaftes und die
    Speicheldrüsen der Bienen nebst einem Anhange über das Riechorgan.
    (Zeitschr. f. wissens. Zool., xxxviii, pp. 71–135, 3 Taf.)

  =Plateau, F.= Expériences sur le rôle des palpes chez les arthropodes
    maxillés, I., Palpes des insectes broyeurs. (Bull. Soc. Zool.
    France, x, 1885, pp. 67–90.)

  =Plateau, F.= Une expérience sur la fonction des antennes chez la
    Blatte (_Periplaneta orientalis_). (Comptes rend. Soc. Ent. de
    Belgique, 1886, pp. 118–122, 1 Fig.)

  =Lubbock, J.= On some points in the anatomy of ants. (Monthly Micr.
    Journ., 1887, pp. 121–142.)

  =Ruland, Franz.= Beiträge zur Kenntniss der antennalen Sinnesorgane
    der Insekten. Diss. Marburg, 1888, p. 31, 1 Taf.

  =Wasmann, E.= Die Fühler der Insekten. (Stimmen aus Maria-Laach.
    Freiburg i. B., 1891, p. 37.)

  =Sergi, G.= Ricerche su alcuni organi di senso nelle antenne delle
    formiche. (Riv. Filos. Sci. Milano, 1891, p. 10, 3 Figs.)

  =Nagel, Wilibald.= Die niederen Sinne der Insekten, 19 Figs.,
    Tübingen, 1892, pp. 67.

  With the writings of Baster, Lamarck, Cuvier, Treviranus, Oken,
    Lefebure, Duméril, Schelver, Bousdorf, Rosenthal, Burmeister,
    Slater, Balbiani, Marcel de Serres, Garnier, Berté, Porter, Sazepin,
    Reuter, Pierret, Duponchel, Driesch, Küster, Peckham, Lubbock, A.
    Dohrn, Lespès.


                        _c_. The organs of taste

The gustatory organs of insects are microscopic pits or setae, either
hair-like or resembling short pegs, which form the ends of ganglionated
nerves. They are difficult to distinguish morphologically from certain
olfactory structures, and it is owing to their position at or very near
the mouth that they are supposed to be gustatory in nature.

Meinert was the first (1860) to suggest that organs of taste occurred in
ants. He observed in the maxillæ, and tongue of these insects a series
of canals in the cuticula of these organs connected with ganglion-cells,
and through them with the nerves, and queried whether they were not
organs of taste. Forel afterwards (1874) confirmed these observations.
Wolff in an elaborate work (1875) described a group of minute pits (Fig.
284) at the base of the tongue of the honey bee, which he supposed to
possess the sense of smell, but Forel and also Lubbock attributed to
these sensory pits the function of taste. Ten years afterward Will
showed conclusively, both by anatomical studies and by experiments, that
Diptera and Hymenoptera possess gustatory organs. He, however, denied
that the organs of Wolff were gustatory, and maintained that organs of
smell were confined to the maxillæ, paraglossæ, and tongue. As we shall
see, however, what appear to be with little doubt taste-pits, with hairs
or pegs arising from them, are most numerous on the epipharynx of nearly
all insects, and situated at a point where they necessarily must come in
contact with the food as it enters the mouth and passes down the throat.

[Illustration:

  FIG. 284.—Taste-pits on the epipharynx (_C_) of the honey-bee: _B_,
    horny ridge; _R_, _R_, taste-pits; _L_, _A_, _A_, muscular fibres;
    _S_, _S′_, _a b c d e f_, section of skin of œsophagus.—After Wolff.
]

[Illustration:

  FIG. 285.—Tip of the proboscis of the honey bee, × 140: _L_, terminal
    button or ladle: _Gs_ taste-hairs; _Sh_, guard-hairs; _Hb_, hooked
    hairs.—After Will.
]

Kraepelin (1883) discovered taste-organs on the proboscis of the fly,
and taste-hairs at the end of the tongue of the humble-bee (Fig. 285),
and afterwards Lubbock critically discussed the subject, and concluded
that the organs of taste in insects are situated “either in the mouth
itself, or on the organs immediately surrounding it.”

=Structure of the taste-organs.=—The organs have been best studied by
Will, who, besides describing and figuring the chitinous structures,
such as the pits or cups, hairs and the pegs, showed that they were the
terminations of ganglionated nerves.

Figure 286 represents the taste-cups on the maxilla of a wasp, and Fig.
287 the taste-cone or peg projecting from the cup or pit. The cell out
of which the pit and projecting hair or peg are formed is a modified
hypodermis cell; and the seta is apparently a modification of a tactile
hair, situated at the end of a nerve, which just beneath the chitinous
structures passes into a ganglion-cell, which sends off a nerve-fibre to
the main nerve.

Will detected on the tongue of the yellow ant (_Lasius flavus_) from 20
to 24, and in Atta from 40 to 52, of these structures. The number of
pits on the maxillæ vary much, not always being the same on the two
sides of the same insect. We have observed these taste-cups in the honey
and humble bee, not only at the base of the second maxillæ (Fig. 288,
_g_), but also on the paraglossæ (_pg_).

  =Distribution in other orders of insects.=—The writer has detected
  these taste-cups in other orders than Diptera and Hymenoptera. They
  very generally occur in mandibulate insects on the more exposed
  surface of the epipharynx (compare pp. 43–46). We have not observed
  them in the Synaptera (Lepisma and Machilis).

  In the Dermaptera the taste-cups appear to be undeveloped in the
  nymph, while in the adult they are fewer in number than in any other
  pterygote order yet investigated.

  In a species of Forficula from Cordova, Mexico, the taste-pits are
  few in number, there being only about a dozen on each side in all;
  most of them being situated on the anterior half, and a few near the
  base. The taste-pits are provided each with a short fine seta, as
  usual arising from the centre.

[Illustration:

  FIG. 286.—Under side of left maxilla of Vespa: _Gm_, taste-cups;
    _Shm_, protecting hairs; _Tb_, tactile hairs; _Mt_, base of
    maxillary palpus.—After Will.
]

  In the order Platyptera (including Perla, Pteronarcys, Psocus,
  Termes, Eutermes, and Termopsis) we have been unable to detect any
  organs of taste.

[Illustration:

  FIG. 287.—Section through a taste-cup: _SK_, supporting cone; _N_,
    nerve; _SZ_, sense-cell.—After Will. This and Figs. 284–286 from
    Lubbock.
]

[Illustration:

  FIG. 288.—Tongue of worker honey-bee: _pg_, paraglossæ; _B_, the
    same enlarged, showing the taste-papillæ; _C_, _D_, base of a
    labial palpus (_mx.′p_) with the taste-papillæ; _E_, taste-cups on
    paraglossæ of Bombus; _F_, group of same on left, _G_, on right
    side, at base of labial palpus.
]

  In the Odonata, however, they are fairly well developed; in
  Calopteryx, about 50 taste-cups were discovered; in a species of
  Diplax about 28, there being a group of 14 at the base of the
  epipharynx on each side of the median line, while in _Æschna heros_
  there are two groups of from 25 to 30 taste-cups, situated as in the
  two aforenamed genera.

  In the Orthoptera the gustatory cups are numerous, well developed,
  and present in all the families except the Phasmidæ, where, however,
  they may yet be found to occur.

  In a large cockroach (Blabera) from Cuba they are well developed. On
  each side of the middle of the epipharynx is a curved row of stiff,
  defensive spines, and at the distal end of each row is a sensory
  field, containing 20 taste-cups on one side and 23 on the other.
  Near the front edge of the clypeal region are two more sensory
  fields, situated on each side of the median line, there being 35
  taste-cups in each field. The taste-cups in this form are rather
  smaller than usual in the order.

  In the Acrydiidæ they are more numerous than in the Blattidæ. For
  example, in _Camnula pellucida_, near what corresponds to the front
  edge of the clypeus are two gustatory fields, each bearing about 35
  taste-pits. Just in front, under the clypeo-labral suture, are two
  similar fields, each containing from 40 to 42 taste-pits. There are
  none in front of these. There are thus about 140–150 sense-cups in
  all.

  The members of the Locustidæ (Fig. 26) appear to be better provided
  with the organs of taste than any other Orthoptera, those of the
  katydid numbering from 170 to 180. There are from 50 to 60
  taste-cups in the front region; behind the middle a group of 25 on
  each side, and over an area corresponding to the base of the labrum
  and front edge of the clypeus is a sensory field with about 70
  taste-cups on each side. They are true cups or beaker-like papillæ,
  some with a fine, others with a short, stout, conical seta.

  The gustatory organs in the cave cricket (_Hadenœcus subterraneus_,
  Fig. 27), from Mammoth Cave, are highly developed, being rounded
  papillæ with the nucleus at the top or end. They are grouped on each
  side of the middle near the front edge, there being 25 on each side.
  An irregular row of these beaker-like organs extends along each
  side; some occur under the base of the labrum, but they are most
  numerous in a field corresponding to the front edge of the clypeus,
  there being 50 on each side, or 100 in all, where in Ceuthophilus
  there are only 9 or 10. It would thus appear as if the sense of
  taste were much more acute in the cave-dweller than in the
  out-of-doors form.

  In the Coleoptera taste-cups and setæ are very generally
  distributed, though we were unable to detect them in Dendroctonus or
  in _Lucanus dama_. As seen in Fig. 57, we have observed numerous
  taste-pegs along the maxilla of _Nemognatha lurida_, but otherwise
  taste-organs have only been detected in the epipharynx. They not
  only occur in the adult beetles, but we have found them in the larvæ
  of cerambycid, scarabæid, and other beetles. In the adults
  taste-cups appear to be about as well developed in the carnivorous
  forms (Carabidæ) as in the phytophagous or lignivorous groups.

  In _Chlænius tomentosus_ there are about half as many of these
  organs as in Harpalus, while in Calosoma there are 90 taste-cups, 45
  on each side, under the base of the labrum. The cups are
  papilliform, being rather high, with a seta arising from each.

  In the Cicindelidæ, the epipharynx bears a sensory field quite
  different from that of the Carabidæ. There are no normal taste-cups,
  except a few situated on two large, round, raised areas which are
  guarded in front by a few very long setæ. On the surface of each
  area are numerous very long setæ which may, if not tactile, have
  some other sense, as they arise from cup-like bases or cells. Those
  on the outside are like true taste-cups, with a bristle but little
  larger than normal in taste-cups generally. We are disposed to
  regard this sensory field as a highly specialized gustatory
  apparatus.

  In the Dyticidæ the taste-cups are nearly as described in the
  Carabidæ.

  The Staphylinidæ are not well provided with taste-organs. Under the
  clypeus of _Staphylinus violaceus_, on each side near the middle, is
  a bare rounded area, in which are situated 4 or 5 papilliform
  taste-cups, and at the base behind them is another linear group of
  about 7 slenderer, somewhat curved, taste-cups. In the Elateridæ
  these organs are scantily developed.

  In the Buprestidæ (_Buprestis maculiventris_ alone examined) no true
  taste-cups were detected. On the other hand, the Lampyridæ are well
  supplied with them. Under the clypeus is situated a sensory field
  bearing 26 taste-cups, which are rather smaller than usual. Over the
  epipharyngeal surface are scattered a few taste-cups, but they are
  small and perhaps not gustatory. Under the clypeus of _Lucidota
  punctata_ Lec. is a group of 12 taste-cups, and in the middle region
  of the epipharynx, situated in a field extending from near the base
  to near the front edge, are about 40 taste-cups, which, however, are
  not, as is usual, arranged on each side of the median line, but are
  scattered among the hairs of the pilose surface of the epipharynx.
  In the Cleridæ the taste-cups are few in number.

  In the great family of Scarabæidæ, the presence of gustatory organs
  is variable. None occur in _Lucanus dama_, though in the June beetle
  (_Lachnosterna fusca_ Fröhl.) they are abundantly developed. The
  epipharynx bears on each side outside of a spiny area a group of
  about 50 taste-cups, each bearing a long seta, those on the outside
  of the area passing into a few high, rather slender, papillæ,
  without a seta. On the under side of the clypeus is a median group
  of 10 taste-cups of singular form, the cups being large, with broad
  bases, which posteriorly bear three spines, of which the median one
  is the largest.

  Taste-cups occur without any known exception in the longicorn
  beetles. In _Leptura canadensis_ they are numerous; in _Euryptera
  lateralis_ they are abundant along and near the middle of the
  anterior half of the labral region, and in _Cyllene robiniæ_ Forst.
  (or _pictus_ Drury) they are more numerous than usual, extending in
  an unbroken sensory field from near the front margin of the clypeal
  region to near the front edge of the epipharynx. The cups vary much
  in size, some being one-half as large as others; and those on the
  sides of the sensory field bear short, and a few others rather long,
  bristles, showing that the taste-cups are modified tactile bristles.

  The Tenebrionidæ are fairly well endowed with taste-cups, their
  number in _Eleodes obsoleta_ Say amounting to 30 or 40.

  Those of the Meloidæ especially are unusual in size and number.

[Illustration:

  FIG. 289.—Epipharynx (_ep_) of Nemognatha: _cl_, clypeus; _gh_,
    gathering hairs; _tc_, triangular sensory field dotted with taste
    cups; _A_, the field enlarged.
]

  In _Nemognatha lurida_ Lec. (Fig. 289) the front edge of the
  epipharynx contains about 80 remarkably small taste-cups, arranged
  irregularly in a triangular sensory space, and not more than ¼ to ⅙,
  as large as those on the maxillæ of the same beetle. Unless the
  former structures are gustatory it is difficult to account for their
  presence here, and it will be observed that the taste-cups in
  Epicauta are unusually abundant. Thus in the middle and near the
  front of the epipharynx of the blister-beetle over 100 gustatory
  cups were counted. They are conical, papilliform, and truncated at
  the end as if open, the edge of the opening being ragged, though
  bearing no bristle, except in a few cases. Around the edge of the
  sinus, on the under side of the labrum, is a regular marginal row of
  large, longer, more distinctly chitinized taste-cups, whose walls
  are streaked up and down by chitinous thickenings. In _E. callosa_
  Lec. there are about 55 taste-cups under the labrum, besides about
  10 cells, which may be gustatory structures, situated on either side
  of a median setose ridge which passes back under the clypeal region.

  The taste-cups of the leaf-beetles are fairly numerous, judging from
  an examination of _Diabrotica vittata_. The surface of the
  epipharynx is pilose, but the median region is naked, and on the
  anterior half bears from 11 to 12 taste-cups, arranged each side of
  the median line in a rude Y. On each side, at the base of the labial
  region, are two sensitive fields, each bearing about 25 to 26
  taste-cups. More were seen under the clypeus.

  In the Neuroptera unmistakable taste-cups are not always present. In
  _Sialis infumata_ along the median line of the epipharynx and near
  the front are about 20 scattered gustatory pegs, which are minute,
  but longer and more acute than usual. In _Chauliodes maculatus_
  there are one or two taste-cups under the front edge of the clypeus;
  others are scattered along the middle from the base of the labrum to
  the front, but are not arranged in definite order. In _Corydalis
  cornutus_ no sense-cups, pits, or rods are present. In Chrysopa
  there are scattered cups armed with a short acute bristle, which are
  possibly gustatory in function. In _Myrmeleon diversum_ also the
  presence of sense-pits or of taste-cups is doubtful, though a group
  of about 12 pits on each side of the clypeal region of the
  epipharynx, and a few situated at the base of the labral region, may
  be endowed with the sense of taste. In _Mantispa brunnea_, however,
  along the middle of the epipharynx are scattered about 30
  unmistakable taste-cups, each bearing a short, fine hair.

  In the Mecoptera (_Panorpa debilis?_) taste-cups, giving rise to a
  minute hair, occur on the labium in two regions, and also on the
  maxillæ situated on the stipes near the base of the palpi, and on
  the lacinia and galea. They are also to be found on the maxillæ of
  _Boreus californicus_, but were not detected on the labium.

  They were first detected by Reuter in various microlepidoptera, and
  occur on the “basal spot” of the palpi of many butterflies. In a
  Tineid moth (_Coleophora coruscipennella_) we have detected what we
  suppose to be a group of four taste-pits on the inner side of the
  basal joint of the labial palpi.

=Experimental proof.=—No one, says Lubbock, who has ever watched a bee
or wasp can entertain the slightest doubt as to their possession of the
sense of taste. “Forel mixed morphine and strychnine with some honey,
which he offered to his ants. Their antennæ gave them no warning. The
smell of the honey attracted them, and they began to feed; but the
moment the honey touched their lips they perceived the fraud.”

Will at first fed wasps with sugar, so that they frequently visited it;
afterwards he substituted alum for the sugar. Eagerly flying to it, they
had scarcely touched it when they drew back from the distasteful
substance with the most comical gestures, and cleaned their tongues by
frequently running them in and out, repeatedly stroking them with their
fore feet. He noticed a great repugnance to quinine in nearly all the
insects experimented on. Bees and wasps were observed to have a more
delicate gustatory sense than flies, etc., which are more omnivorous in
their tastes.




                   LITERATURE ON THE ORGANS OF TASTE


  =Meinert, F.= Bidrag til de danske myrers naturhistorie. (Kgl. Dansk
    Vidensk. selsk. skrifter. Kjoebenhavn. Raekke 5 Naturvid. og math.
    Afd., v, 1861, pp. 273–340.)

  =Wolff, O. J. B.= Das Riechorgan der Biene. (Nova acta d. K.
    Leop.-Carol. Akad., xxxviii, 1875, pp. 1–251, 8 Taf.)

  =Joseph, G.= Zur Morphologie des Geschmacksorganes bei Insekten.
    (Amtlicher Bericht der 50. Versammlung deutscher Naturforscher und
    Aerzte in München, 1877, pp. 227–228.)

  =Künckel et Gazagnaire.= Du siège de la gustation chez les insectes
    Diptères. Constitution anatomique et physiologique de l’epipharynx
    et l’hypopharynx. (Comptes-rend. Acad. Sc., Paris, 1881, xcv, pp.
    347–350.)

  —— Recherches sur l’organisation et le développement des Diptères et
    en particulaire des Volucelles, i, 1875, ii, 1881, 26 Pls.

  =Kraepelin, K.= Zur Kenntniss der Anatomie und Physiologie des Rüssels
    von Musca. (Zeitschr. f. wissens. Zool., xxxix, 1883, pp. 683–719, 2
    Taf.)

  =Kirbach, P.= Ueber die Mundwerkzeuge der Schmetterlinge. (Zool.
    Anzeiger, 1883, pp. 553–558, 2 Figs.)

  =Will, F.= Das Geschmacksorgan der Insekten. (Zeitschr. f. wissens.
    Zool., 1885, xlii, pp. 674–707, 1 Taf.)

  =Gazagnaire, J.= Du siège de la gustation chez les insectes
    Coléoptères. (Comptes-rend. Acad. Sc., Paris, 1886, cii, pp.
    629–632; Ann. Soc. Ent. France, Sér. 6, Bull., pp. 79–80.)

  =Forel, A.= Expériences et remarques critiques sur les sensations des
    insectes. 2^{me} Part. (Recueil Zool. Suisse, iv, 1887, pp.
    161–240.)

  =Reuter, Enzio.= Ueber den “Basalfleck” auf den Palpen der
    Schmetterlinge. (Zool. Anzeiger, 1888, pp. 500–503.)

  —— Ueber die Palpen der Rhopaloceren, etc., 6 Taf. Helsingfors, 1896,
    pp. 1–578. (Acta Soc. Sc. Fennicæ, xxii, 1896.)

  =Packard, A. S.= On the occurrence of organs, probably of taste, in
    the epipharynx of the Mecoptera (Panorpa and Boreus). (Psyche, 1889,
    v, pp. 159–164.)

  —— Notes on the epipharynx, and the epipharyngeal organs of taste in
    mandibulate insects. (Psyche, 1889, v, pp. 193–199, 222–228.)

  Also Lubbock’s Senses, etc., of animals, and the writings of Briant,
    Breithaupt (titles on p. 85).


                       _d._ The organs of hearing

Although it has been denied by Forel that insects have the sense of
hearing, yet the majority of writers and experimenters agree that
insects are not deaf. On general grounds if, as we know, many insects
produce sounds, it must follow that they have ears to hear, for there is
every reason to suppose that the sounds thus made are, as in other
animals, either for attracting the sexes, for a means of communication,
or to express the emotions. We will begin by briefly describing the
structures now generally supposed to be auditory in function, and about
which there can be no reasonable doubt, and then consider the more
problematical organs, closing with an account of the extremely various
means of producing sounds and cries.

=The ears or tympanal and chordotonal sense-organs of Orthoptera and
other insects.=—The ears or tympana of locusts (Acrydiidæ) are situated
one on each side, on the basal joint of the abdomen, just behind the
first abdominal spiracle. That this is a true ear was first suggested by
J. Müller, and his opinion was confirmed by Siebold, Leydig, Hensen,
Graber, Schmidt, Lubbock, etc.[49]

[Illustration:

  FIG. 290.—Ear of a locust (_Caloptenus italicus_), seen from the inner
    side: _T_, tympanum; _TR_, its border; _o_, _u_, two horn-like
    processes; _bi_, pear-shaped vesicle; _n_, auditory nerve; _ga_,
    terminal ganglion; _st_, stigma; _m_, opening, and _m′_, closing,
    muscle of the same; _M_, tensor muscle of the tympanum
    membrane.—After Graber.
]

The apparatus consists of a tense membrane, the _tympanum_, surrounded
by a horny ring (Fig. 290). “On the internal surface of this membrane
are two horn-like processes (_o_, _u_), to which is attached an
extremely delicate vesicle (_bi_) filled with a transparent fluid, and
representing a membranous labyrinth. This vesicle is in connection with
an auditory nerve (_n_) which arises from the third thoracic ganglion,
forms a ganglion (_ga_) upon the tympanum, and terminates in the
immediate neighborhood of the labyrinth by a collection of cuneiform,
staff-like bodies, with very finely pointed extremities (primitive
nerve-fibres?), which are surrounded by loosely aggregated ganglionic
globules” (Siebold’s Anatomy of the Invertebrates).

[Illustration:

  FIG. 291.—Fore tibia of _Locusta viridissima_. _td_, cover of the
    drum; _tr_, fissure between the drum and its cover.—After Graber,
    from Lang.
]

In the green grasshoppers, katydids, and their allies, the ears are
situated on the fore tibiæ, where these organs can be found after a
careful search (Figs. 291, 292).

The presence of the structure is indicated by the oval disc, the drum,
which is a thin tense membrane covering the auditory apparatus of
nerves, ganglion cells, and auditory rods beneath.

  The tympana, or drums, are not present in all Locustidæ and
  Gryllidæ, and, as Lubbock states, it is an additional reason for
  regarding them as auditory organs, that in those species which
  possess no stridulating organs the tympana are also wanting. In many
  of the Locustidæ the tympana are covered or protected by a fold of
  the skin projecting over them. These covered ones are, Graber
  thinks, derived from the open ones.

[Illustration:

  FIG. 292.—_A_, fore tibia of a European grasshopper (Meconema),
    containing the ear: _Ty_, tympanum or outer membrane; _Tr_ 1, _Tr_
    2, tracheæ. _B_, diagrammatic cross-section through the tibia and
    ear of the same; _Ty_, tympanum; _Ct_, cuticula; _CM_, hypodermis:
    _A_, the auditory organ connecting with the tympanum; _B_,
    supra-tympanal auditory organ; _GZ_, the ganglion-cell belonging
    to them; _Hst_, the auditory rod connecting with the
    ganglion-cells.—After Graber, from Judeich and Nitsche.
]

On examining the apparatus within the leg under the drum, it is seen to
consist of the trachea, the auditory vesicles and rods, ganglion cells,
and acoustic nerve. The trachea is greatly modified (Fig. 292, _Tr_ 1).
On passing into the tibia the trachea enlarges and divides into two
branches, which reunite lower down. The spiracles supplying the air to
this enlarged trachea are considerably enlarged, while in the dumb
species it is of the normal size. The enlarged trachea passes close to
the tympanum, which thus has air on both sides of it: the open air on
the outer, the air of the trachea on its inner surface. In fact, as
Lubbock states, “the trachea acts like the Eustachian tube in our own
ear; it maintains an equilibrium of pressure on each side of the
tympanum, and enables it freely to transmit the atmospheric vibrations.”

[Illustration:

  FIG. 293.—The auditory apparatus in the tibia of a grasshopper,
    showing the tympanal nerve-endings in situ: _EBI_, terminal vesicles
    of Siebold’s organ; _SN_, nerve of the organ of Siebold; _Gr_, group
    of vesicles of same; _SO_, nerve-endings of the same; _vT_, front
    tympanum; _vTr_, front branch of the trachea; _hT_, hinder tympanum;
    _hTr_, hinder branch of the trachea; _Sp_, space between the
    tracheæ; _go_, supra tympanal ganglion; _rN_, connecting
    nerve-fibrils between the ganglion cells and the terminal vesicles;
    _R_, upper, _n-S_, lower, root of the transparent covering membrane.
    (Other lettering not explained by author.)—After Graber.
]

[Illustration:

  FIG. 294.—Auditory rod of _Gryllus viridissimus_: _fd_, auditory rod;
    _ko_, terminal piece.—After Graber, from Lubbock.
]

  These tracheæ, says Graber, though formed on a similar plan, present
  many variations, corresponding to those of the tympana, and showing
  that the tympana and the tracheæ stand in intimate connection with
  one another. For instance, in those species where the tympana are
  equal, the tracheæ are so likewise; in Gryllotalpa, where the front
  tympanum only is developed, though both tracheal branches are
  present, the front one is much larger than the other; and where
  there is no tympanum, the trachea remains comparatively small, and
  even in some cases undivided (Lubbock, _ex_ Graber).

The acoustic nerve, which next to the optic is the thickest in the body,
divides soon after entering the tibia into two branches, one almost
immediately forming a ganglion, the supra-tympanal ganglion, the other
passing down to the tympanum, where it expands into an elongated flat
ganglion, the organ of Siebold (Fig. 293), and closely applied to the
anterior tracheæ.

At the upper part of the ganglion is a group terminating below in a
single row of vesicles, the first few of which are approximately equal,
but which subsequently diminish regularly in size. Each of these
vesicles is connected with the nerve by a fibril (Fig. 293, _vN_), and
contains an auditory rod (Fig. 294). They are said by Graber to be
brightly refractive, hollow (thus differing from the retinal rods, which
are solid), and terminate in a separate end-piece (_ko_). The rods were
first discovered by Siebold, and, as Lubbock remarks, may be regarded as
specially characteristic of the acoustic organs of insects.

[Illustration:

  FIG. 295.—Chordotonal organ in nymph of a white ant.—After Müller,
    from Sharp.
]

[Illustration:

  FIG. 296.—Right half of 8th body-segment of _Corethra plumicornis_:
    _g_, ganglion of ventral cord; _lm_, longitudinal muscle; _cn_,
    chordotonal nerve; _cl_, chordotonal ligament; _cg_, chordotonal
    ganglion; _cs_, rod of chordotonal organ; _cst_, terminal cord;
    _tb_, tactile setæ; _hn_, out-going fibres of the integumental
    nerves.—After Graber, from Lang.
]

  As will be seen in Fig. 293, at the upper part of the tibial organ
  of Ephippigera there is a group of cells, and below them a single
  row of cells gradually diminishing in size from above downwards.
  “One cannot but ask oneself,” says Lubbock, “whether the gradually
  diminishing size of the cells in the organ of Siebold may not have
  reference to the perception of different notes, as is the case with
  the series of diminishing arches in the organ of Corti of our own
  ears.”

  These organs were supposed to be restricted to the Orthoptera, but
  in 1877 Lubbock discovered what seems to resemble the supra-tympanal
  auditory organ of Orthoptera in the tibia of the yellow ant (_Lasius
  flavus_). Graber confirmed Lubbock’s account, and also discovered
  these organs in the tibia of a Perlid (_Isopteryx apicalis_), and
  Fritz Müller has detected them in the fore tibiæ of the nymph of
  _Calotermes rugosus_ (Fig. 295). To these structures Graber gave the
  name of chordotonal organs.

  He has also detected these organs in all the legs of other insects
  (Trichoptera, Pediculidæ), and auditory rods have been discovered in
  the antennæ of Dyticus and of Telephorus by Hicks, Leydig, and
  Graber. Graber classifies the chordotonal organs into truncal and
  membral. In Coleoptera and Trichoptera they may occur on several
  joints of the leg; others are more localized,—thus he distinguishes
  femoral (Pediculidæ), tibial (Orthoptera, Perlidæ, Formicidæ), and
  tarsal organs (Coleoptera).

  A type of chordotonal organ, observed in the body-segments of the
  larvæ of several insects by Leydig, Weismann, Graber, Grobben, and
  Bolles Lee, is to be seen in the transparent larva of Corethra (Fig.
  296), where the auditory organ extends to the skin. It contains at
  the point _cs_ two or three auditory rods. In the opposite direction
  a fine ligament (_cl_) passes from _cg_ to the skin; in this way the
  auditory organ is suspended in a certain state of tension, and is
  favorably situated to receive even very fine vibrations. A similar
  apparatus has been detected in the larva of Ptychoptera.

=Antennal auditory hairs.=—It is not at all improbable that the antennæ
of different insects contain auditory as well as olfactory structures.
Lubbock has suggested that the singular organs which have only been
found in the antennæ of ants and certain bees, and to which he gives the
name of “Hicks’ bottles” (Fig. 281), may act as microscopic
stethoscopes, while Leydig also regards them as chordotonal organs.

That, however, some of the antennal hairs of the mosquito, as first
suggested by Johnson and afterwards proved experimentally by Mayer, are
auditory, seems well established. Fastening a male mosquito down on a
glass slide, Mayer then sounded a series of tuning-forks. With an Ut_{4}
fork of 512 vibrations per second, some of the hairs were seen to
vibrate vigorously, while others remained comparatively at rest. The
lower (Ut_{3}) and higher (Ut_{5}) harmonics of Ut_{4} also caused more
vibration than any intermediate notes. These hairs, then, are specially
tuned so as to respond to vibrations numbering 512 per second. Other
hairs vibrated to other notes, extending through the middle and next
higher octave of the piano.

Mayer then made large wooden models of these hairs, the one
corresponding to the Ut_{3} hair being about a metre in length, and on
counting the number of vibrations they made when they were clamped at
one end and then drawn on one side, he found that it “coincided with the
ratio existing between the numbers of vibrations of the forks to which
covibrated the fibrils,” or hairs. It should be observed that the song
of the female mosquito corresponds nearly to this note, and would
consequently set the hairs in vibration. Mayer observed that the song of
the female vibrates the hairs of one of the antennæ more forcibly than
those of the other. Those auditory hairs are most affected which are at
right angles to the direction from which the sound comes. Hence from the
position of the antennæ and the hairs a sound will be loudest or most
intense if it is directly in front of the head. If, then, the song of
the female affects one antenna more than another, the male turns his
head until the two antennæ are equally affected, and is thus able to fly
straight towards the female. From his experiments Mayer found that the
male can thus guide himself to within 5° of the direction of the female.
Hence he concludes that “these insects must have the faculty of the
perception of the direction of sound more highly developed than in any
other class of animals.” (Also see Child’s work.)

  =Special sense-organs in the wings and halteres.=—Organs of a
  special sense, which Hicks supposed to be those of smell, were found
  by him near or at the base of the wings of Diptera, Coleoptera, and
  less perfect ones in Lepidoptera, Neuroptera, and Orthoptera, with a
  trace of them in Hemiptera; but these were considered by Leydig to
  be auditory organs, since he found the nerves to end in club-shaped
  rods, like those of Orthoptera.

  Hicks found, as to the halteres and their sense-organs, that the
  nerve in the halter is the largest in the insect, except the optic
  nerve; and that at the base of the halteres is a number of vesicles
  arranged in four groups, to each of which the nerve sends a branch.
  Afterwards Bolles Lee discovered that the vesicles, undoubtedly
  perforated, contain a minute hair, those of the upper groups being
  protected by hoods of chitin. He regarded them as olfactory organs,
  while Lubbock seems inclined to consider them as auditory
  structures. Graber also regards the vesicles of Hicks as chordotonal
  organs.

  In his elaborate account of the balancers, Weinland concludes that
  the organs of sense of varying structure occurring at the base of
  these appendages allow the perception of movements which the
  halteres perform and which enable the fly to steer or direct its
  course. The halteres can thus cause differences in the direction of
  the flight of a fly in the vertical plane. If the balancers act
  unequally, there is a change in direction.


                       _e._ The sounds of insects

Insects have no true voice; but sounds of different intensity, shrill
cries, and other noises are produced mechanically by insects, either
being love-songs to attract the sexes, to give signals, to communicate
intelligence, or perhaps to express the emotions. The loud, shrill cry
of the Cicada, or chirp of the cricket, is evidently a love-call, and
results in the mating of individuals of separate broods more or less
widely scattered, thus preventing too close interbreeding.

The simplest means of making a noise is that of the death-watch
(Anobium), which strikes or taps on the wall with its head or abdomen.
Longicorn beetles make a sharp sound by the friction of the
mesoscutellum against the edge of the prothoracic cavity, the head being
alternately raised and lowered, Burying-beetles (Necrophorus) rub the
abdomen against the hinder edges of the elytra. Weevils make a loud
noise by rapidly rubbing the tips of the abdomen on the ends of the
elytra.

  Landois offers the following summary of the kinds of noises produced
  by beetles:

    1. Tapping sounds (Bostrycinæ, Anobium).

    2. Grating sounds (Elateridæ).

    3. Friction without special rasping organs (_Euchirus
         longimanus_).

    4. Rasping sounds produced by friction:

        _a._ Rubbing of the pronotum on the mesonotum (Cerambycidæ
         except Spondyli and Prionus).

       _b._ Friction of the prosternum on the mesosternum (_Omaloplia
         brunnea_).

       _c._ Elytra with a rasp at the end (Curculionidæ, Dyticidæ,
         Pelobius).

        _d._ With a coxal rasp (Geotrupes, Ceratophyus). The male of
         Ateuchus stridulates to encourage the female in her work, and
         from distress when she is removed. (Darwin.)

       _e._ Friction of the edge of the elytra against the femur
         (_Chiasognathus grantii_).

       _f._ Pygidium with two rasps in the middle (Crioceris, Lema,
         Copris, Oryctes, Necrophorus, Tenebrionidæ).

        _g._ Abdomen with a grating ridge and four grating plates
         (Trox).

       _h._ Abdomen with two toothed ridges rubbing on a rasp on edge
         of wing-cover (Elaphus, Blethisa, Cychrus).

       _i._ Rubbing the elytra on a rasp on the hind wings (_Pelobius
         hermanni_).

        _j._ Friction of the wing against the abdominal segments
         (_Melolontha fullo_).

  Mutilla makes a rather sharp noise by rubbing one abdominal segment
  against another. Ants (Ponera) have a stridulating apparatus, and
  other genera numerous (20) ridges between the segments.

  Even certain moths and butterflies emit a rasping or crackling
  noise. The death’s-head moth and other sphinges cause it by rubbing
  the palpi against the base of the proboscis. These and certain
  butterflies are provided with parallel ridges forming a rasp on the
  “basal spot” of the inner side of the basal joint of each palpus
  (Reuter). A South American butterfly (_Ageronia feronia_) can be
  heard for several yards as it flies with a crackling sound. Hampson
  finds that the cause of the clicking sound is due to a pair of
  strong chitinous hooks attached to the thorax, against which play
  the spatulate ends of a pair of hooks attached to the fore wings. An
  Australian moth (Hecatesia) flies with a whizzing sound; Vanessa is
  said to be sonorous.

  The males of Orthoptera produce their shrill cries or chirping
  noises, 1, by rubbing the thighs against the sides of the body
  (Acrydiidæ); 2, by the friction of the base of the fore wings on
  each other (Locustidæ); 3, by rubbing the base of the upper on the
  base of the hinder or under pair (Gryllidæ), in the two last there
  being a shrilling apparatus consisting of a file on the hind wings,
  which rubs on a resonant surface on the fore wings. The females are
  not invariably dumb, both sexes of the European Ephippigera being
  able to faintly stridulate. Corixa also produces shrill chirping
  notes. (Carpenter.)

Certain insects also hum, and have what may perhaps be called a voice.
The cockchafer, besides humming with the wings, produces a sound almost
like a voice. In the large trachea, just behind each spiracle, is a
chitinous process, which is thrown into vibrations by the air during
respiration, and thus produces a humming noise. (Lubbock.) Such is also
the case with flies, the mosquito, dragon-flies, and bees. In flies and
dragon-flies the “voice” is caused by the air issuing from the thoracic
spiracles; while in the humble-bee the abdominal spiracles are also
musical. The sound made by the spiracles bears no relation to that
caused by the wings. Landois tells us that the wing-tone of the
honey-bee is A′; its voice, however, is an octave higher, and often goes
to B″ and C″.

The sounds produced by the wings are constant in each species, except
where, as in Bombus, there are individuals of different sizes; in these
the larger ones generally give a higher note. Thus the comparatively
small male of _Bombus terrestris_ hums on A′, while the large female
hums an entire octave higher.

From the note produced the rapidity of the vibrations can be calculated.
For example, the house-fly, which produces the sound of F, vibrates its
wings 21,120 times in a minute, or 335 times in a second; and the bee,
which makes a sound of A′, as many as 26,400 times, or 440 times in a
second. On the contrary, a tired bee hums on E′, and therefore,
according to theory, vibrates its wings only 330 times in a second.
Marey has confirmed these numbers graphically, and found by experiment
that the fly actually makes 330 strokes in a second. (Lubbock.)

A different kind of musical apparatus is that of the cicada, which has
been elaborately described by Graber. The shrill, piercing notes issue
from a pair of organs on the under side of the base of the abdomen of
the male, these acting somewhat as two kettle-drums, the membrane
covering the depressions being rapidly vibrated.




                  LITERATURE ON THE ORGANS OF HEARING


                        _a._ The auditory organs

  =Siebold, C. Th. E. von.= Ueber das Stimm- und Gehörorgan der
    Orthopteren. (Archiv f. Naturgesch., 1844, x, pp. 52–81.)

  =Johnston, Christopher.= Auditory apparatus of the culex mosquito.
    (Quart. Journ. Micr. Soc., 1855, iii, pp. 97–102, 1 Fig.)

  =Hicks, Braxton.= On a new organ in insects. (Journ. Linn. Soc. Zool.,
    London, 1857, pp. 130–140, 1 Pl.)

  —— Further remarks on the organ found on the bases of the halteres and
    wings of insects. (Trans. Linn. Soc., London, 1857, xxii, pp.
    141–145, 2 Pls.)

  =Hensen, V.= Ueber das Gehörorgan von Locusta. (Zeitschr. f. wissens.
    Zool., xvi, 1866, pp. 190–207.)

  =Graber, V.= Bemerkungen über die Gehör- und Stimmorgane der
    Heuschrecken und Cicaden (Wiener Sitzungsber. Math.-natur-wiss. Cl.,
    lxvi, 1 Abt., 1872, pp. 205–213, 2 Figs.)

  —— Die tympanalen Sinnesapparate, der Orthopteren. (Denkschr. d. k.
    Akad. d. wissens. Wien, xxxvi, 1876, 2 Abt., pp. 1–140, 10 Taf.)

  —— Die abdominalen Tympanalorgane der Cicaden und Gryllodeen. (Ibid.,
    1870, xxxvi, pp. 273–290, 2 Taf.)

  —— Ueber neue, otocystenartige Sinnesorgane der Insekten. (Archiv f.
    mikroskop. Anat., 1878, pp. 35–57, 2 Taf.)

  —— Die chordotonalen Sinnesorgane und das Gehör der Insekten. (Archiv
    f. mikroskop. Anat., 1882, xx, pp. 506–640; 1883, xxi, pp. 65–145,
    Taf.)

  =Mayer, Alfred Marshall.= Researches in Acoustics No. 5. 3.
    Experiments on the supposed auditory apparatus of the culex
    mosquito. (Amer. Jour. Sc. and Arts, Ser. 3, viii, 1874, pp. 81–103;
    also Amer. Naturalist, viii, pp. 577–592.)

  =Schmidt, Oscar.= Die Gehörorgane der Heuschrecken. (Archiv f.
    mikroskop. Anat., xi, 1875, pp. 195–215, 3 Taf.)

  =Ranke, J.= Beiträge zu der Lehre von den Uebergangssinnesorganen, das
    Gehörorgan der Acridier und das Sehorgan der Hirudineen. (Zeitschr.
    f. wissens. Zool., xxv, 1875, pp. 143–164, 1 Taf.)

  =Lee, A. Bolles.= Les balanciers des Diptères, leurs organes
    sensifères et leur histologie. (Recueil Zool. Suisse, ii, 1885, pp.
    363–392, 1 Pl.)

  —— Bemerkungen über der feineren Bau der Chordotonalorgane. (Archiv f.
    mikroskop. Anat., 1883, xxiii, pp. 133–140, 1 Taf.)

  —— Les organes chordotonaux des Diptères et la méthode du chlorure
    d’or. (Observations critiques.) (Recueil Zool. Suisse, 1884, ii, pp.
    685–689, 1 Pl.)

  =Weinland, E.= Ueber die Schwinger (Halteren) der Dipteren (Zeitschr.
    f. wissens. Zool., 1890, li, pp. 55–166, 5 Taf.)

  =Adelung, N. v.= Beiträge zur Kenntnis des tibialen Gehörapparates der
    Locustiden. Inaug. Diss., Leipzig, 1892, 2 Taf.

  =Child, Ch. M.= Ein bisher wenig beachtetes antennales Sinnesorgan der
    Insekten, mit besonderer Berücksichtigung der Culiciden und
    Chironomiden. (Zeitschr. f. wissens. Zool., lviii, 1894, pp.
    475–528, 2 Taf.; also, Zool. Anzeiger, xvii Jahrg., pp. 35–38, and
    in Annals and Mag. Nat. Hist., 1894 (6), xiii, pp. 372–374).

  Also the writings of J. Müller, Kirby and Spence, Burmeister, Gilbert
    White, Westwood, Guilding, Meinert, Paasch, Leydig, Viallanes,
    Minot, Forel, Mayer, Darwin (Descent of Man, i, ch. x.), F. Müller,
    Lubbock (Senses of animals), Westring, Köppen, Bates, Vom Rath,
    Peckham, Jourdan, Nagel, etc.


                    _b._ The sounds made by insects

  =Scudder, S. H.= Notes on the stridulation of grasshoppers. (Proc.
    Bost. Soc. Nat. Hist., xi, 1868, pp. 306–313 and 316.)

  —— The songs of the grasshoppers. (Amer. Naturalist, ii, 1868, pp.
    113–120, 5 Figs.)

  =Riley, C. V.= The song notes of the periodical Cicada. (Proc. Amer.
    Assoc. Adv. Science, xxxiv, 1885, pp. 330–332; also in Kansas City
    Rev., October, 1885, pp. 173–175.)

  =Swinton, A. H.= (Ent. Month. Mag., 1877.) Sound produced in Ageronia
    by a modification of the hook and bristle of the wings.

  =Hampson, G. F.= On stridulation in certain Lepidoptera, etc. (Proc.
    Zool. Soc. London, 1892, ii, pp. 188–193, Fig; also Psyche, vi, p.
    491, 1 Fig.)

  With the writings of Landois, Lubbock, Graber, Kolbe, Carpenter (Nat.
    Science), Bruyant, and others.




                 THE DIGESTIVE CANAL AND ITS APPENDAGES


[Illustration:

  FIG. 297.—Transverse section through an abdominal segment of larva of
    _Megalopyge crispata_, showing the relations of the digestive canal
    to the other organs: _int_, hind-intestine, with its mucous or
    epithelial layer (_ep_), and _ml_ its outer or muscular layer; _ng_,
    ventral ganglion; _ht_, heart; _mp_, urinary tubes; _f_, fat-body;
    _sc_, thickened portion of the hypodermis (_hy_) containing the
    setigenous cells; _m_, muscles; _m′_, a pair of retractor muscles
    inserted near the base of the lateral glandular process (_lgp_);
    _cut_, cuticula; _l_, legs. Also compare Figs. 142–144 and 234.
]

[Illustration:

  FIG. 298.
  The alimentary or digestive canal of insects is a more or less
  straight tube, which connects the mouth and anus, the latter
    invariably
  situated in the last segment of the body, under the last
  tergite or suranal plate. It lies directly over the ventral nervous
  cord and under the dorsal vessel, passing through the middle of the
  body (Fig. 297). It is loosely held in place by delicate retractor
  muscles (_retractores ventriculi_, found by Lyonet in the larvæ of
  Lepidoptera, and occurring in those of Diptera), but is principally
  supported by exceedingly numerous branches of the main tracheæ.
  FIG. 298.—Internal anatomy of _Melanoplus femur-rubrum_: _at_, antenna
    and nerve leading
  to it from the “brain” or supraœsophageal ganglion (_sp_); _oc_,
    ocelli, anterior and vertical ones,
  with ocellar nerves leading to them from the brain; _œ_, œsophagus;
    _m_, mouth; _lb_, labium or under
  lip; _if_, infraœsophageal ganglion, sending three pairs of nerves to
    the mandibles, maxillæ, and
  labium respectively (not clearly shown in the engraving); _sm_,
    sympathetic or vagus nerve, starting
  from a ganglion resting above the œsophagus, and connecting with
    another ganglion (_sg_) near the
  hinder end of the crop; _sal_, salivary glands (the termination of the
    salivary duct not clearly
  shown by the engraver); _nv_, nervous cord and ganglia; _ov_, ovary;
    _ur_, origin of urinary tubes;
  ovt, oviduct; _sb_, sebaceous gland; _bc_, bursa copulatrix; _ovt_,
    site of opening of the oviduct (the
  left oviduct cut away); 1–10, abdominal segments. The other organs
    labelled in full.—Drawn from
  his original dissections by Mr. Edward Burgess.
]

[Illustration:

  FIG. 299.—Digestive canal of _Anabrus_: _m_, mouth: _œ_, œsophagus;
    _sm_, the sympathetic nerve passing along the crop; _t_, tongue;
    _fg_, frontal ganglion; _br_, brain, the nervous cord passing
    backward from it; _sr_, salivary reservoir; _sg_, salivary gland;
    _pv_, proventriculus; _ur_, origin of urinary tubes; _sb_, sebaceous
    gland; 1–10, the ten abdominal segments.—Burgess _del._
]

It is in the higher adult insects differentiated into the mouth and
_pharynx_, the _œsophagus_ or gullet, supplementary to which is the
_crop_ (_ingluvies_) or “_sucking stomach_” of Lepidoptera, Diptera, and
Hymenoptera; the _proventriculus_ or gizzard; the _ventriculus_,
“chyle-stomach,” or, more properly, mid-intestine, and the
hind-intestine, which is divided into the _ileum_, or short intestine,
the long intestine, often slender and coiled, with the colon and the
rectum. Morphologically, however, the digestive or enteric canal is
divided into three primary divisions, which are indicated in the embryo
insect; _i.e._, the _fore-intestine_ (_stomodæum_ of the embryo),
_mid-intestine_ or “chyle-stomach,” and _hind-intestine_ or _proctodæum_
(Fig. 300). The three primary regions, with their differentiations, may
be tabulated thus:—

 _Fore-intestine_ (Stomodæum).  Mouth and pharynx.
                                Pumping apparatus of Hemiptera,
                                  Lepidoptera, and Diptera.
                                Œsophagus.
                                Crop or ingluvies, food reservoir, or
                                  “sucking stomach.”
                                Proventriculus.

 _Mid-intestine_ (Mesenteron).  Mid-intestine, “chylific stomach,” or
                                  ventriculus (with cœcal glands).

 _Hind-intestine_ (Proctodæum). Ileum, or short intestine (with the
                                  urinary tubes).
                                Long intestine.
                                Colon.
                                Rectum (with rectal glands).
                                Anus (with anal glands).

[Illustration:

  FIG. 300.—The three primary divisions of the alimentary canal of an
    embryonic orthopterous insect: _br_, brain; _sbg_, subœsophageal
    ganglion; _ng_, nervous cord; _st_, stomodæum; _pr_, proctodæum;
    _mv_, malphigian tubes; _mesen_, mid-intestine; _ht_, heart; _md_,
    mandibles; _mx_, _mx′_, 1st and 2d maxillæ.—After Ayers, with some
    changes.
]

The appendages of the alimentary canal are: (1) the salivary and poison
glands, which arise from the stomodæum in embryonic life; (2) while to
the chylific stomach a single pair of cœcal appendages (Orthoptera and
larval Diptera, _e.g._ Sciara), or many cœca may be appended; (3) the
urinary tubes, also the rectal glands and the paired anal glands. In a
Hemipter (_Pyrrhocoris apterus_) appendages arise from the intestine in
front of the origin of the urinary tubes. In certain insects a single
cœcal appendage (Nepa, Dyticus, Silpha, Necrophorus, and the
Lepidoptera) arises from the proctodæum.

[Illustration:

  FIG. 301—Larva of honey-bee: _g_, brain; _bm_, ventral nervous cord;
    _œ_, œsophagus; _sd_, spinning-gland; _cd_, mid-intestine or
    chyle-stomach; _ed_, hind-intestine, not yet connected with the
    mid-intestine; _vm_, urinary tube; _an_, anus; _st_, stigmata.—After
    Leuckart, from Lang.
]

In certain larval insects, as those of the Proctotrypidæ (first larval
stage), the higher Hymenoptera (ichneumons, ants, wasps, and bees, Fig.
301), in the Campodea-like larvæ of the Meloidæ and Stylopidæ, the larva
of the ant-lion (Myrmecoleo), and those of _Diptera pupipara_
(Melophagus), the embryonic condition of the separation of the
proctodæum and mid-gut (mesenteron) persists, the stomach ending in a
blind sac; in such cases the intestine, together with the urinary tubes,
is entirely secretory.

The anus is wanting in the larva of the ant-lion, as also in the wasps
(in which there is a rudimentary colon) and in freshly hatched bees,
though it becomes perfectly formed in the fully grown larvæ (Newport,
art. Insecta, p. 967, and H. Müller).

In the larvæ of lamellicorn Coleoptera (_Melolontha vulgaris_) the
digestive tube is nearly as simple as in bees, though there is a large
colon, which at its beginning forms an immense cœcum, and has also one
anal aperture (Newport).

The length and shape of the digestive canal is dependent on the nature
of the food and also on the mode of life, especially the ease or
difficulty with which the food is digested.

  Newport, while stating that the length of the alimentary canal in
  larvæ is not in general indicatory of the habits of the species,
  makes this qualification after describing the digestive canal of
  Calandra as compared with that of Calosoma: “The length and
  complication of the intestines, therefore, appear to have some
  reference to the quality of the food to be digested, since it is
  well known that the food of these latter insects (weevils) is of
  difficult assimilation, being as it is chiefly the hard ligneous
  fibres of vegetable matter; but they cannot be received as always
  indicatory of a carnivorous [or] vegetable feeder, since, as above
  remarked, the length of the canal is considerable in one entirely
  carnivorous larva, while it is much shorter in some herbivorous, and
  particularly in pollenivorous larvæ, as in the Melolontha and the
  apodal Hymenoptera.”

[Illustration:

  FIG. 302.—Digestive canal of a carabid beetle: _b_, œsophagus; _c_,
    crop; _d_, proventriculus; _f_, mid-intestine, or “chyle-stomach,”
    with its cœeca; _g_, posterior division of the stomach; _i_, the
    two pairs of urinary tubes; _h_, intestine; _k_, rectum; _l_, anal
    glands.—After Dufour, from Judeich and Nitsche.
]

  Newport also contends that the length of the alimentary canal is not
  more indicative in the perfect insect of the carnivorous or
  phytophagous habits of the species than in the larva. It is nearly
  as long (being from two to three times the length of the whole
  body), and is more complicated, in the rapacious Carabidæ (Fig. 302)
  than in the honey-sipping Lepidoptera, whose food is entirely
  liquid. Referring to the digestive canal of Cicindelidæ, which is
  scarcely longer than the body, he claims that “we cannot admit that
  the length of the digestive organs, and the existence of a gizzard
  and gastric vessels, are indicatory of predacity of habits in the
  insect, because a similar conformation of parts exists often in
  strictly vegetable feeders. The existence and length of these parts
  seem rather to refer to the comparative digestibility of the food
  than to its animal or vegetable nature.” Newport then refers to the
  digestive canal of Forficulidæ (in which the gizzard is present, the
  canal, however, passing in an almost direct line through the body,
  making but one slight convolution), “a farther proof that the
  _length_ of the canal must not be taken as a criterion whereby to
  judge of the habits of a species.” He adds this will apply equally
  well to the omnivorous Gryllidæ, in which there exists a short
  alimentary canal, but a gizzard of more complicated structure than
  that of the Dytiscidæ.

In larval insects and others (Synaptera, Orthoptera, etc.), in which the
digestive canal is simplest, it is scarcely longer than the body, and
passes through it as a straight tube.

In the caterpillar, which is a voracious and constant feeder, the
digestive canal is a large straight tube, not clearly differentiated
into fore-stomach, stomach, and intestine; but in the imago, which only
takes a little liquid food, it is slender, delicate, and highly
differentiated. In the larva the mid-gut forms the largest part of the
canal; in the imago, the intestine becomes very long and coiled into
numerous turns; at the same time the food-reservoir (the “sucking
stomach”) develops, and the excretory tubes are longer.


                        _a._ The digestive canal

[Illustration:

  FIG. 303.—Interior view of the bottom of the head of _Danais
    archippus_, the top having been cut away, showing, in the middle,
    the pharyngeal sac with its five muscles: the frontal (_f.m_),
    dorsal pair (_d.m_), and the lateral pair (_l.m_); _cl_, clypeus;
    _cor_, cornea; _œ_, œsophagus; _p.m_, one of the large muscles which
    move the labial palp.—After Burgess.
]

It will greatly simplify our conception of the anatomy of the digestive
canal if we take into account its mode of origin in the embryo, bearing
in mind the fact that during the gastrula condition the ectoderm is
invaginated at each pole to form the primitive mouth and fore-gut
(stomodæum) and hind-gut (proctodæum). The cells of the ectoderm secrete
a chitinous lining (intima), which forms the continuation of the outer
chitinous crust, and thus the lining of each end of the digestive canal
is cast whenever the insect molts; while the mid-intestine (mesenteron),
arising independently of the rest of the canal much later in embryonic
life from the mesoderm, is not the result of any invagination, being
directly derived from the mesoderm, and is not lined with chitin.

=The mouth, or oral cavity, and pharynx.=—This is the beginning of the
alimentary bounded above by the clypeus, and labrum, with the
epipharynx, and below by the hypopharynx, or tongue, as well as the
labium. Into it pour the secretion of the salivary glands, which passes
out through an opening at the base of the tongue or hypopharynx. On each
side of the mouth are the mandibles and first maxillæ.

=The sucking or pharyngeal pump.=—This organ has been observed by Graber
in flies and Hemiptera, but the fullest account is that by Burgess, who
was the first to discover it in Lepidoptera. In the milk-weed butterfly
(_Danais archippus_) the canal traversing the proboscis opens into a
pharynx enclosed in a muscular sac (Figs. 303, 304, and 310).

The pharyngeal sac, says Burgess, serves as a pumping organ to suck the
liquid food through the proboscis and to force it backwards into the
digestive canal.

[Illustration:

  FIG. 304.—Longitudinal section through the head of Danais, showing the
    interior of the left half: _mx_, left maxilla, whose canal leads
    into the pharynx; _hph_, floor of the latter, showing some of the
    taste-papillæ; _oe_, œsophagus; _ep_, epipharyngeal valve; _sd_,
    salivary duct; _d.m_, _f.m_, and _cl_, as in Fig. 302.—After
    Burgess.
]

Meinert (“Trophi Dipterorum”) has made elaborate dissections of the
mouth and its armature, including the pharynx of several types of
Diptera, with its musculature. He describes the pharynx as the
principal, and in most Diptera, as the only part of the pump (antlia),
and says: “By the muscles of the pump (_musculis antliæ_) the superior
lamina of the pharynx is varied that the space between the two laminæ
may be increased, and the liquid is thus led through the siphon formed
by the mouth-parts into the mouth” (Fig. 81).

=The œsophagus.=—This is a simple tube, largest in those insects feeding
on solid, usually vegetable, food, and smallest in those living on
liquid food. It usually curves upwards and backwards, passing directly
under the brain, and merges into the crop or proventriculus either at
the back part of the head or in the thorax, its length being very
variable. Its inner walls longitudinally are folded and lined with
chitin.

According to Newport, in the œsophagus of the Gryllidæ, of the two
layers of the mucous lining the second is distinctly glandular and
secretory, and in it there are many thousands of very minute granular
glandular bodies, which probably secrete the “molasses” or repellent
fluid often ejected by these and other insects when captured.

=The crop or ingluvies.=—This, when present, is an enlargement of the
end of the œsophagus, and lined internally with a muscular coat. It is
very large in locusts (Fig. 298), Anabrus (Fig. 299), and other
Orthoptera (the Phasmidæ excepted), in the Dermaptera, and most adult
Coleoptera. A crop-like dilatation in front of a spherical gizzard is
also present in the Synaptera (Poduridæ and Lepismidæ), as well as in
the Mallophaga (Nirmidæ).

[Illustration:

  FIG. 305.—Digestive canal of Calandra: _H_, pear-shaped œsophagus;
    _I_, crop; _K_, gastric cœca _L_, ilium; _MN_, colon; _P_, urinary
    tubes.—After Newport.
]

[Illustration:

  FIG. 306.—Section of the crop (_H_), gizzard (_I_), and stomach (_K_)
    of Athalia.-After Newport.
]

[Illustration:

  _Fig. 307._—Upper side of head and digestive canal of Myrmeleon larva:
    _a_, crop; _b_, “stomach”; _c_, free ends of two urinary tubes;
    _c′_, common origin of other six tubes; _d_, cœcum; _e_, spinneret;
    _ff_, muscles for protruding its sheath; _gg_, maxillary
    glands.—After Meinert, from Sharp.
]

In the larvæ of weevils (_Calandra sommeri_) there is a crop (Fig. 305),
but not in the larva of Calosoma; also, according to Beauregard, in the
pollen-eating beetles Zonitis, Sitaris, and Malabris it is wanting,
while in Meloe it is highly developed (Kolbe).

The crop forms a lateral dilatation of the end of the œsophagus in the
larvæ of weevils and of saw-flies (_Athalia centifoliæ_, Fig. 306).

=The “sucking stomach” or food-reservoir.=—This is a thin muscular pouch
connected by a slender neck with the end of the œsophagus or the crop,
when the latter is present. There is no such organ in Orthoptera, except
in Gryllotalpa. It is wanting in the Odonata and in the Plectoptera
(Ephemeridæ); in Platyptera (Perlidæ and Termitidæ), in Trichoptera, and
in Mecoptera (Panorpidæ). In most adult Neuroptera (Myrmeleonidæ,
Hemerobiidæ, and Sialidæ), but not in Rhaphidiidæ, the long œsophagus is
dilated posteriorly into a kind of pouch or crop, and besides there is
often a long “food-reservoir” arising on one of its sides, that of
Myrmeleon (Fig. 307) and Hemerobius being on the right side.

[Illustration:

  FIG. 308.—Digestive canal of _Sarcophaga carnaria_: _a_, salivary
    gland; _b_, œsophagus; _c_, food reservoir; _f-g_, stomach; _h_,
    intestine; _i_, urinary tubes; _k_, rectum.—From Judeich and
    Nitsche.
]

A true food-reservoir is present in most Diptera (Fig. 308) as well as
in the larvæ of the Muscidæ, but according to Dufour it is wanting in
some Asilidæ and in _Diptera pupipara_, and according to Brauer in the
Œstridæ. The food-reservoir in Diptera is always situated on the left
side of the digestive canal; there is usually a long neck or canal,
while the reservoir is either oval or more usually bilobed, and often
each lobe is itself curiously lobed.

In Lepidoptera (Figs. 309, 310) the so-called “sucking stomach” is, as
Graber has proved, simply a reservoir for the temporary reception of
food; though generally found to contain nothing but air, Newport has
observed that in flies it is filled with food after feeding. He has
found this to be the case in the flesh fly, and in Eristalis he has
found it “partially filled with yellow pollen from the flowers of the
ragwort upon which the insect was captured,” the pollen grains also
occurring in the canal leading to the bag, in the gullet, and in the
stomach itself. Graber has further proved by feeding flies with a
colored sweet fluid that this sac is only a food-receptacle. As he says:
“It can be seen filling itself fuller and fuller with the colored fluid,
the sac gradually distending until it occupies half the hind-body.”

The food-reservoir of the Hymenoptera is a lateral pouch at the end of
the long, slender œsophagus, and has been seen in the bee to be filled
with honey.

[Illustration:

  FIG. 309.—Digestive canal of _Sphinx ligustri_: _h_, œsophagus; _i_,
    rudiment of the gizzard; _k_, “stomach”; _q_, its pyloric end; _t_,
    food reservoir; _p_, urinary tubes; _l_, ilium; _m_, cœcum of colon;
    _n_, rectum; _v_, vent.—After Newport.
]

In the mole-cricket the hinder part of the crop is armed within with
hook-like bristles directed backwards so as not to prevent the energetic
pressure of the food backwards into the proventriculus, and to obviate
the possibility of a regurgitation. (Eberli.)

=The fore-stomach or proventriculus.=—This is especially well developed
in the Dermaptera, in the Orthopterous families Locustidæ, Gryllidæ, and
Mantidæ, while in the Thysanura (Lepisma) there is a spherical gizzard
provided with six teeth. It also occurs in many wood-boring insects, and
in most carnivorous insects, notably the Carabidæ, Dyticidæ, Scolytidæ,
in the Mecoptera (scorpion-flies), in the fleas, and in many kinds of
ants, as well as Cynips, Leucospis, and Xyphidria. It is very muscular,
lined within with chitin, which is usually provided with numerous teeth
arising from the folds. These folds begin in the œsophagus or crop, and
suddenly end where the mesenteron (“chylific stomach”) begins. It has
been compared with the gizzard of birds, and is usually called by German
authors the chewing or masticating stomach. (Kaumagen.)

  The proventriculus is best developed in the Gryllidæ (_Acrida
  viridissima_), where the six folds at the end of the crop close
  together to form a valve between the crop and proventriculus. “They
  are each armed with five very minute hooked teeth; and, continued
  into the gizzard, develop many more in their course through that
  organ. These first teeth are arranged around the entrance to the
  gizzard, and seem designed to retain the insufficiently comminuted
  food and to pass it on to that organ.

[Illustration:

  FIG. 310.—Anatomy of _Danais archippus_ after removal of right half
    of the body. _Lettering of the head_: _a_, antenna; _ph_, pharynx;
    _pl_, labial palpi; _r_, proboscis; _g_, brain; _usg_,
    subœsophageal ganglion. _Lettering of the thorax_: I. II. III.
    thoracic segments; _b_{1}_, _b_{2}_, _b_{3}_, the coxal joints of
    the three pairs of legs; _bm_, muscles of the wings; _ac_ cephalic
    aorta with its swelling; _œ_, œsophagus; _bg_, thoracic ganglia of
    the ventral cord; _sd_, salivary glands of one side, those of the
    other side cut off near their entrance into the common salivary
    duct. _Lettering of the abdomen_: 1–9. abdominal segments; _h_,
    heart; _sm_, so-called sucking-stomach (food-reservoir); _cm_,
    chyle-stomach; _ag_, abdominal ganglia: _ed_, hind intestine with
    colon (_c_) and rectum (_r_); _rm_, urinary vessels; _ov_, ovarial
    tubes, those of the right side cut off; _ove_, terminal filaments
    of the ovaries; _bc_, bursa copulatrix; _obc_, its outer aperture;
    _od_, oviduct; _vag_, vagina; _wo_, its outer aperture; _ad_,
    glandular appendages of the vagina partly cut away; _vk_,
    connective canal between the vagina and bursa copulatrix with
    swelling (receptaculum seminis); _an_, anus.—After Burgess, from
    Lang.
]

[Illustration:

  FIG. 311.—Transverse section of the proventriculus of _Gryllus
    cinereus_: _muc_, muscular walls; _r_, horny ridge between the
    large teeth (_sp_).—After Minot.
]

[Illustration:

  FIG. 312.—Transverse section of the proventriculus of the
    cockroach.—After Miall and Denny.
]

[Illustration:

  FIG. 313.—Digestive canal of the honey-bee: _A_, horizontal section
    of the body; _lp_, labial palpus; _mx_, maxilla; _e_, eye; _pro.
    t_, prothorax; _mesa. t_, mesothorax; _meta. t_, metathorax; _dv_,
    dorsal vessel; _v_, _v_, ventricles of the same; No. 1, No. 2, No.
    3, salivary gland systems; _œ_ œsophagus; _g_, _g_, ganglia of
    chief nerve-chain; _n_, nerves; _hs_, honey-sac; _p_, petaloid
    stopper or calyx of honey-sac or stomach-mouth; _c. s_, chyle
    stomach; _bt_, urinary tubes; _si_, small intestine (ilium); _l_,
    “lamellæ or gland-plates of colon,” rectal glands; _li_, rectum.
    _B_, cellular layer of stomach; _gc_, gastric cells, × 200. _C_,
    urinary tube; _bc_, cells; _t_, trachea. _D_, inner layer, with
    gastric teeth (_gt_).—After Cheshire.
]

  “Next to these in succession on each of the longitudinal ridges are
  four flat, broad, somewhat quadrate teeth, each of which is very
  finely denticulated along its free margin. These extend about
  half-way through the gizzard. They appear to be alternately elevated
  and depressed during the action of the gizzard, and to serve to
  carry on the food to the twelve cutting teeth, with which each ridge
  is also armed, and which occupy the posterior part of the organ.
  These teeth are triangular, sharp-pointed, and directed posteriorly,
  and gradually decrease in size in succession from before backward.
  Each tooth is very strong, sharp-pointed, and of the color and
  consistence of tortoise shell, and is armed on each side by a
  smaller pointed tooth. These form the six longitudinal ridges of the
  gizzard, between each two of which there are two other rows of very
  minute teeth of a triangular form, somewhat resembling the larger
  one in structure, occupying the channels between the ridges. The
  muscular portion of the gizzard is equally interesting. It is not
  merely formed of transverse and longitudinal fibres, but sends from
  its inner surface into the cavity of each of the large teeth other
  minute but powerful muscles, a pair of which are inserted into each
  tooth. The number of teeth in the gizzard amounts to 270, which is
  the same number in these Gryllidæ as found formerly by Dr. Kidd in
  the mole-cricket. Of the different kinds of teeth there are as
  follows: 72 large treble teeth, 24 flat quadrate teeth, 30 small
  single-hooked teeth, and 12 rows of small triangular teeth, each row
  being formed of 12 teeth. This is the complicated gizzard of the
  higher Orthoptera.” (Newport.)

  In the more generalized cockroach, there are six principal folds,
  the so-called teeth, which project so far inwards as to nearly meet
  (Fig. 312). The entire apparatus of muscles and teeth is, as Miall
  and Denny state, “an elaborate machine for squeezing and straining
  the food, and recalls the gastric mill and pyloric strainer of the
  crayfish. The powerful annular muscles approximate the teeth and
  folds, closing the passage, while small longitudinal muscles, which
  can be traced from the chitinous teeth to the cushions, appear to
  retract these last, and open a passage for the food.”

As in the fore-stomach or proventriculus of the lobster, the solid,
rounded teeth do not appear to triturate the solid fragments found in
the organ, but act rather as a pyloric strainer to keep such bodies out
of the chylific stomach. We accept the view of Plateau that this section
of the digestive canal in insects, which he compares to the psalterium
of a ruminant, is a strainer rather than a masticatory stomach, and both
Forel and Emery, as well as Cheshire, take this view.

The proventriculus of the honey-bee (Fig. 313, _hs_) is called by
apiarians the “honey-sac” or “honey-stomach.” Cheshire states that if it
be carefully removed from a freshly killed bee, its calyx-like
“stomach-mouth” may be seen to gape open and shut with a rapid snapping
movement. The entrance to the stomach is guarded by four valves, each of
which is strongly chitinous within, and fringed along its edge with
downward-pointing fine stiff bristles. By the contraction of the
longitudinal muscles (_lm_), the valves open to allow the passage of
food from the honey-sac to the “chyle-stomach.” It is closed at will by
circular muscles (_tm_). Then the bee can carry food for a week’s
necessities, either using it rapidly in the production of wax, or eking
it out if the weather is unfavorable for the gathering of a new store.

[Illustration:

  FIG. 314.—“Honey-sac stopper,” “stomach-mouth,” or calyx-bell of
    honey-bee, × 50. _A_, front view of one of the lobes of the
    calyx-bell; _l_, lip-like point, covered by down-turned bristles
    (_b_); _sm_, side membrane. _B_, longitudinal section of the
    stomach-mouth, with continuations into entrance of chyle-stomach;
    _l_, _l_, lip-like ends of leaflets; _s_, setæ; _lm_, longitudinal
    muscles; _tm_, transverse muscles in cross-section; _cl_, cell-layer
    of honey-sac; _LM_, _TM_, longitudinal and transverse muscles of
    same; _nc_, nucleated cells of tubular extension of stomach-mouth
    into chyle-stomach; _lm′_, _tm′_, longitudinal and transverse
    muscles of chyle stomach; _c_, _c_, cells covered within by an
    intima. _C_, cross-section of stomach-mouth; _m_, cross-section of
    muscles seen at _lm_ in _B_; _tm_, transverse muscles surrounding
    stomach-mouth. _D_, cross-section through small intestine; _a_ and
    _m_, longitudinal and surrounding muscles.—After Cheshire.
]

  Cheshire also shows that when bees suck up from composite and other
  flowers nectar together with much pollen, the outside wrinkled
  membrane (_sm_, _A_, Fig. 314) “is seen to continually run up in
  folds, and gather itself over the top of the stomach-mouth, bringing
  with it, by the aid of its setæ, the large pollen-grains the nectar
  contains.” The lips (_l_, _l_, _B_, Fig. 314), now opening, take in
  this pollen, which is driven forwards into the cavity made between
  the separating lips by an inflow of the fluid surrounding the
  granules. The lips in turn close, but the down-pointing bristles are
  thrown outwards from the face of the leaflet, in this way revealing
  their special function, as the pollen is prevented from receding
  while the nectar passes back into the honey-sac, strained through
  between the bristles aforesaid, the last parts escaping by the
  loop-like openings seen in the corners of _C_, Fig. 314. The whole
  process is immediately and very rapidly repeated, so that the pollen
  collects and the honey is cleared. “Three purposes, in addition to
  those previously enumerated, are thus subserved by this wondrous
  mechanism. First, the bee can either _eat or drink_ from the mixed
  diet she carries, gulping down the pollen in pellets, or swallowing
  the nectar as her necessities demand. Second, when the collected
  pollen is driven forwards into the chyle-stomach, the tube
  extension, whose necessity now becomes apparent, prevents the
  pellets forming into plug-like masses just below _p_, Fig. 313, for,
  by the action of the tube, these pellets are delivered into the
  midst of the fluids of the stomach, to be at once broken up and
  subjected to the digestive process. And third, while the little
  gatherer is flying from flower to flower, her stomach-mouth is busy
  in separating pollen from nectar, so that the latter may be less
  liable to fermentation and better suited to winter consumption. She,
  in fact, carries with her, and at once puts into operation, the most
  ancient, and yet the most perfect and beautiful, of all
  ‘honey-strainers.’”

  Forel’s experiments on the proventriculus of ants prove that through
  its valvular contrivance it closes the passage from the crop to the
  mid-intestine (“chylific stomach”), and allows the contents of the
  former to pass slowly and very gradually into the latter. Emery
  confirms this view, and concludes that the organ in the Camponotidæ
  and in the Dolichoderidæ provided with a calyx-bell, usually
  regarded as a triturating stomach (Kaumagen), but more correctly as
  a pumping stomach, consists of parts which perform two different
  functions. Under the operation of the muscles of the crop the
  entrance to the pumping stomach becomes closed, in order by such
  spasmodic contraction to prevent the flow of the contents of the
  crop into the proventriculus. By the pressure of the transverse
  muscles of the proventriculus its contents are emptied into the
  mid-intestine, while simultaneously a regurgitation into the crop is
  prevented. In the Dolichoderidæ and Plagiolepidinæ the closure in
  both cases is effected by the valves. In the true Camponotidæ there
  are two separate contrivances for closing; the calyx belonging to
  the crop-musculature, while the valves essentially belong to the
  proventricular pumping apparatus.

  Opinions vary as to the use of this portion of the digestive canal.
  Graber compares it to the gizzard of birds, and likens the action of
  the rosette of teeth to the finer radiating teeth of the sea-urchin,
  and styles it a chopping machine, which works automatically, and
  allows no solid bits of food to pass in to injure the delicate walls
  of the stomach (mid-gut).

  He also states that the food when taken from the proventriculus is
  very finely divided, while that found in the œsophagus contains
  large bits.

  Kolbe says that this view has recently been completely abandoned,
  and that the teeth are used to pass the food backwards into the
  chylific stomach. “But Goldfuss had denied the triturating action of
  the proventriculus of the Orthoptera (Symbolæ ad Orthopterorum
  quorundam Œconomiam, 1843), stating that the contents of the same
  are already fluid in the gullet, so that the fore-stomach (Kaumagen)
  does not need to comminute the food” (Kolbe). In the Gryllidæ and
  Locustidæ, just before the posterior opening of the proventriculus
  into the stomach the chitinous lining swells into a ring and
  projects straight back as the inner wall of the cylindrical chylific
  stomach. The muscular layer forms two sac-like outgrowths or folds,
  which separate on the circular fold from the chitinous membrane.
  This apparatus only allows very finely comminuted food to pass into
  the stomach.

  In the Acrydiidæ (_Eremobia muricata_) at the end of the
  proventriculus, where it passes into the stomach, is a small
  circular fold which hangs down like a curtain in the stomach.

=The œsophageal valve.=—Weismann[50] states that the origin of the
proventriculus in the embryo of flies (Muscidæ) shows that it should be
regarded as an intussusception of the œsophagus. While in the embryo the
invaginated portion of the œsophagus is short, after the hatching of the
larva it projects backwards into the mid-intestine. Kowalevsky also
observed in a young muscid larva, 2.2 mm. in length, that the œsophagus,
shaped like a tube, extends back into the expanded portion
(proventriculus) and opens into the stomach (Fig. 315, _A_). In a larva
10 mm. long the funnel is shorter, the end being situated in the
proventriculus (Fig. 315, _B_, _pr_). In the cavity between the outer
(_o_) and inner wall (_i_) no food enters, and the use of this whole
apparatus seems to be to prevent the larger bits of food from passing
into the chylific stomach (Kowalevsky).

[Illustration:

  FIG. 315.—Œsophageal valve of young muscid larva: _m_, its opening:
    _t_, thickening of the cells; _mes_, mesoderm.—After Kowalevsky.
]

  Beauregard has found a similar structure in the Meloidæ, and calls
  it the “cardiac valvule” (Fig. 318, _Kl_). It was observed by
  Mingazzini in the larvæ of phytophagic lamellicorn beetles, and
  Balbiani described it in a myriopod (Cryptops) under the name of the
  “œsophageal valvule.”

  Gehuchten describes a homologous but more complicated structure in a
  tipulid larva (_Ptychoptera contaminata_), but differing in
  containing blood-cavities, as a tubular prolongation of the
  posterior end of the œsophagus which passes through the
  proventriculus and opens at various positions in the anterior part
  of the chylific stomach (Fig. 316).

  The three layers composing this funnel are distant from each other
  and separated by blood-cavities, the whole forming “an immense
  blood-cavity extended between the epithelial proventricular lining
  and the muscular coat.”

  According to Schneider the longitudinal muscular fibres of the fore
  and hind gut in insects pass into the stomach (mid-gut). The
  anterior part of the fore-gut has generally only circular fibres.
  When, however, the longitudinal fibres arise behind the middle, then
  they separate from the digestive canal and are inserted a little
  behind the beginning of the chylific stomach. Hence there is formed
  an invagination of the proventriculus, which projects into the
  cavity of the stomach.

  Schneider describes this process, which he calls the “beak,” as an
  invagination of the fore-stomach which projects into the cavity of
  the stomach. The two layers of the invagination in growing together
  form a beak varying in shape, being either simple or lobed and armed
  with bristles or teeth. This beak is tolerably large in Lepisma,
  Dermaptera (Forficula), Orthoptera, and in the larvæ and adults of
  Diptera, but smaller in the Neuroptera and Coleoptera, while in
  other insects it is wanting.

  =Proventricular valvule.=—Gehuchten also describes in Ptychoptera
  what he calls “the proventricular valvule,” stating that it is “a
  circular fold of the intestinal wall” (Fig. 310, _vpr_). He claims
  that it has not before been found, the “proventricular beak” of
  Schneider being regarded by him as the œsophageal valvule.

[Illustration:

  FIG. 316.—Digestive canal of _Ptychoptera contaminata_: _gs_, salivary
    glands; _ra_, œsophagus; _pr_, proventriculus; _gt_, crown of eight
    small tubular glands; _im_, mid-intestine; _ga_, two accessory white
    glands; _vm_, urinary vessels; _ig_, small intestine; _gi_, large
    intestine; _r_, rectum; _A_, the proventriculus in which the hinder
    end of the œsophagus extends as far as the chyle-stomach. _B_,
    longitudinal section of the proventricular region; _sph_, muscular
    ring or œsophageal sphincter; _ppr_, wall of the proventriculus;
    _e_, circular constriction dividing the cavity of the proventriculus
    in two; _vpr_, circular fold of the wall of the mid-intestine
    forming the proventricular valve; _vœ_, œsophageal valve.—After
    Gehuchten.
]

=The peritrophic membrane.=—This membrane appears first to have been
noticed by Ramdohr in 1811 in _Hemerobius perla_. It has been found by
Schneider, who calls it the “funnel.” On the hinder end of the
fore-stomach, he says, the cuticula forms a fold enclosing the outlet of
the fore-stomach, and extending back like a tube to the anus. This
“funnel,” he adds, occurs in a great number of insects. It has been
found in Thysanura, but is wanting in Hemiptera. In the Coleoptera it is
absent in Carabidæ and Dyticidæ. It is generally present in Diptera and
in the larvæ of Lepidoptera, but not in the adults. In Hymenoptera it
has been found in ants and wasps, but is absent in Cynipidæ,
Ichneumonidæ, and Tenthredinidæ. All those insects (including their
larvæ) possessing this funnel eat solid, indigestible food, while those
which do not possess it take fluid nourishment. It is elastic, and
firmly encloses the contents of the digestive tract. Until Schneider’s
discovery of its general occurrence, it had only been known to exist in
the viviparous Cecidomyia larvæ (Miastor). Wagner, its discoverer,
noticed in the stomach of this insect a second tube which contained
food. Pagenstecher was inclined to regard the tube as a secretion of the
salivary glands. Metschnikoff, however, more correctly stated that the
tube consisted of chitin, but he regarded it as adapted for the removal
of the secretions. (Schneider.) Plateau, however, as well as Balbiani,
the latter calling it the “peritropic membrane,” considers this membrane
as a secretion of the chylific stomach, and that it is formed at the
surface of the epithelial cells. It surrounds the food along the entire
digestive tract, forming an envelope around the fæcal masses. On the
other hand, Gehuchten states that in the larva of Ptychoptera its mode
of origin differs from that described by Plateau and by Schneider, and
that it is a product of secretion of special cells in the
proventriculus.

=The mid-intestine.=—This section of the digestive canal, often, though
erroneously, called the “chylific stomach” or ventriculus, differs not
only in its embryonic history, but also in its structure and physiology
from the fore and hind intestine of arthropods, and also presents no
analogy to the stomach of the vertebrate animals. In insects it is a
simple tube, not usually lined with chitin, since it is not formed by
the invagination of the ectoderm, as are the fore and hind intestine,
the absence of the chitinous intima promoting the absorption of soluble
food. Into the anterior end either open two or more large cœcal tubes
(Fig. 299), or its whole outer surface is beset with very numerous fine
glandular filaments like villi (Fig. 317 and Fig. 329).

The mid-intestine varies much in size and shape; it is very long in the
lamellicorn beetles (Melolontha and Geotrupes), and while in Meloë it is
very large, occupying the greatest part of the body-cavity, in the
longicorn beetles and in Lepidoptera it is very small. The pyloric end
consists of an internal circular fold projecting into the cavity. In the
Psocidæ (Cæcilius) the pyloric end is prolonged into a slender tube
nearly as long as the larger anterior portion.

The limits between the mid and hind intestine are in some insects
difficult to define, the urinary tubes sometimes appearing to open into
the end of the mid-intestine (“stomach”). The latter also is sometimes
lined with an intima. The limits are also determined by a circular
projection, directly behind which is an enlargement of the intestine in
the shape of a trench (_rigole_), or circular _cul-de-sac_ (the “pyloric
valvule” of some authors, including Beauregard), while the walls of the
small intestine contract so as to produce a considerable constriction of
the cavity of the canal. This constriction exactly coincides with the
beginning of the double layer of circular muscles in the wall of the
small intestine. An internal layer, which is the continuation of the
circular muscles of the chylific stomach, and an external layer much
more developed probably belong to this part of the alimentary canal.
Since the homologue of the circular fold occurs in the locust as well as
in Diptera, it is probably common to insects in general.

[Illustration:

  FIG. 317.—Digestive canal of _Carabus monilis_: _h_, œsophagus; _i_,
    gizzard or proventriculus; _k_, “stomach,” with its cœca (_r_); _p_,
    urinary tubes; _q_, their point of insertion; _m_, _n_, colon, with
    cœcal glands; _s_, anal glands; _a_, _b_, _c_, a gastric cœcum; _a_,
    _b_, portion of lining of gizzard.—After Newport.
]

[Illustration:

  FIG. 318.—Digestive canal of Meloe: _sch_, œsophagus; _Kl_, œsophageal
    valve; _mD_, mid-intestine; _eD_, hind-intestine; _Ei_, eggs; _g_,
    sexual opening.—After Graber.
]

  Gehuchten adds that the limit set by the circular projection does
  not exactly coincide with the opening into the intestine of the
  urinary tubes and the two annexed glands. He shows by a section (his
  Fig. 133) that the tubular glands open into the alimentary canal in
  front of the circular fold. It is the same with the Malpighian
  tubes. They are not, therefore, he claims, dependences of the
  terminal intestine, but of the mid-intestine. Beauregard has
  observed the same thing in the vesicating insects (Meloidæ). The
  Malpighian tubes, he says, open into the “chylific stomach” before
  the valvular crown. This arrangement does not seem to be general,
  because, according to Balbiani, the Malpighian vessels open into the
  beginning of the intestine in Cryptops. Compare also Minot’s account
  of the valve in locusts separating the stomach from the intestine,
  and in front of which the urinary or Malpighian tubes open.

=Histology of the mid-intestine.=—The walls of the stomach are composed
of an internal epithelium, a layer of connective tissue, with two
muscular layers, the inner of which is formed of unstriated circular
muscular fibres, and the outer of striated longitudinal muscular fibres.

In the cockroach short processes are given off from the free ends of the
epithelial cells, as in the intestine of many mammals and other animals.
“Between the cells a reticulum is often to be seen, especially where the
cells have burst; it extends between and among all the elements of the
mucous lining, and probably serves, like the very similar structure met
with in mammalian intestines, to absorb and conduct some of the products
of digestion.” (Miall and Denny.)

Gehuchten shows that the epithelial lining of the mesenteron (chylific
stomach) of the dipterous larva Ptychoptera is composed of two kinds of
cells, _i.e._ secreting or glandular cells and absorbent cells, the
former situated at each end of the stomach, and the absorbent cells
occupying the middle region. The part played by these cells in digestion
will be treated of beyond in the section on digestion. (See p. 327.)

=The hind-intestine.=—In many insects this is divided into the ileum, or
short intestine, and the long intestine. The limit between the intestine
and stomach is externally determined by the origin of the urinary tubes,
which are outgrowths of the anterior end of the proctodæum. Like the
fore-intestine the hind-intestine is lined with a thick muscular layer,
and, as Gehuchten states, the passage from the epithelial lining of the
stomach (mid-intestine) to the muscular lining of the intestine is
abrupt.

=Large intestine.=—In Ptychoptera, as described by Gehuchten, there are
no precise limits between the small and large intestine; the epithelium
of the large intestine has a special character, and its constituents
present a close resemblance to the absorbed cells of the chylific
stomach, being like them large and polygonal. The muscular layer is not
continuous, and is formed of longitudinal and circular fibres, the
latter being the larger.

=The ileum=—Though in most insects slender, and therefore called the
small intestine, the ileum is in locusts (Fig. 298) and grasshoppers
(Anabrus, Fig. 299) as thick as the stomach. In many carnivorous beetles
(Dyticus, Fig. 320, _il_, and Necrophorus) it is very long, but rather
slender and short in the Carabidæ and Cicindelidæ, as well as those
insects whose food is liquid, such as Diptera. In the Lepidoptera it
varies in length, being in Sphinx quite long and bent into seven folds
(Fig. 309), while it is very short in the Psocidæ, Chrysomelidæ, and
Tenthredinidæ.

In the locust the ileum is traversed by six longitudinal folds with
intervening furrows; outside of each furrow is a longitudinal muscular
band. Seen from the inner surface the epithelium has an unusual
character, the cells in the middle of each of the flat folds being quite
large, polygonal in outline, while towards the furrows the cells become
very much smaller. The walls are double when seen in transverse section,
the inner layer consisting of epithelial cells resting on connective
tissue, the outer layer formed of circular muscles. The cuticula is
thin, but probably chitinous; it resembles that on the gastro-ileal
folds, except that there are no spinules, but unlike the cuticula of the
stomach it extends equally over the folds and the furrows. (Minot.) In
the cockroach the junction of the small intestine with the colon is
abrupt, a well-developed annular fold assuming the nature of a circular
valve. (Miall and Denny.)

=The gastro-ileal folds.=—In the locust the intestine is separated from
the chylific stomach by what Minot calls “the gastro-ileal folds,” which
form a peculiar valve. The urinary vessels open just underneath and in
front of this valve. In Melanoplus, and probably in the entire family of
Acrydiidæ, they are indicated as “dark spots, round in front and lying
at the anterior end of the ileum so as to form a ring around the
interior of the intestine.” They are 12 in number, and all alike. They
are pigmented and round in front where they are broadest and stand up
highest; they narrow down backwards, the pigment disappears, and they
gradually fade out into the ileal folds. Directly beneath them, and just
at the posterior end of the stomach, there is a strong band of circular
striated muscular fibres. The epithelium of these folds is covered with
minute conical spines, which are generally, but not always, wanting
between the folds. (Minot.)

=The colon.=—This section of the intestine (Fig. 319) is sometimes
regarded as a part of the rectum. In the locust the six longitudinal
folds of the ileum are continued into the colon, but their surface,
instead of being smooth as in the ileum, is thrown up into numerous
irregular curved and zigzag secondary folds. The cells of the epithelium
are of uniform size, and the layer is covered by a highly refringent
cuticula without spines; and, like that in the ileum, it rests on a
layer of connective tissue, beyond which follows (1) an internal coat of
longitudinal, and (2) an external coat of circular striated muscular
fibres. (Minot.)

[Illustration:

  FIG. 319.—Digestive canal of _Lucanus cervus_: _G_, anterior muscles
    of the pharynx; _H_, œsophagus; _I_, gizzard; _K_, chyle-stomach;
    _L_, ilium; _M_, colon (cœcal part of); _N_, colon; _O_, rectum;
    _a_, frontal ganglion of the vagus; _b_, vagus nerve; _c_, anterior
    lateral ganglion connected with the vagus.—After Newport.
]

In butterflies (_Pontia brassicæ_), in _Sphinx ligustri_, and probably
in most Lepidoptera the colon is distinct from the rectum, and is
anteriorly developed into a very large more or less pyriform or
bladder-like cæcum (Figs. 309, 310), which in certain Coleoptera
(Dyticus, Fig. 320, _d_; Silpha, Necrophorus, etc.) is of remarkable
length and shape; it also occurs in Nepidæ (Fig. 327). In the cockroach
a lateral cæcum “is occasionally, but not constantly, present towards
its rectal end,” and a constriction divides the colon from the rectum.
(Miall and Denny.)

=The rectum.=—The terminal section of the hind-gut varies in length and
size, but is usually larger than the colon, and with thick, muscular
walls. In Lepidoptera it is narrow and short.

The rectum is remarkable for containing structures called rectal glands
(Fig. 298). Chun describes those of _Locusta viridissima_ as six flat
folds, formed by a high columnar epithelium and a distinct cuticula;
there is a coat of circular bands corresponding to the furrows between
the glands. Minot states that this description is applicable to the
locusts (Acrydiidæ) he has investigated, the only difference being in
the structural details of the single layers. He claims that the rectal
folds “do not offer the least appearance of glandular structure,”
neither is their function an absorbent one, as Chun supposed. From their
structure and position Fernald regards the rectal glands of Passalus as
acting like a valve, serving to retain the food in the absorptive
portions of the digestive tract till all nutriment is extracted.

[Illustration:

  FIG. 320.—_Dyticus marginalis_, ♂ opened from the back: _a_, crop;
    _b_, proventriculus; _c_, mid-intestine beset with fine cœcal
    glands; _d_, long cœcal appendage of the colon; apodemes;
    _B_{1}_-_B_{3}_, apodemes; _vhm_, coxal extensor muscle, moving the
    hind leg; _ho_, testis; _dr_, accessory gland; _r_, penis; _e_,
    reservoir of the secretion of the anal gland.—After Graber.
]

The epithelial folds of the larvæ of dragon-flies serve as organs of
respiration, the water being admitted into this cavity, and when
forcibly expelled serving to propel the creature forward. Paired and
single anal glands (repugnatorial) enter the rectum of certain
Coleoptera (Figs. 302, _l_; 317, _s_; 320, _e_).

=The vent (anus).=—The external opening of the rectum is situated in the
end of the body, in the vestigial 10th or 11th abdominal segment, and is
more or less eversible. It is protected above in caterpillars, and other
insects with 10 free abdominal segments, by the suranal plate. It is
bounded on the sides by the paranal lobes, while beneath is the
infra-anal lobe.

The anus is wanting in certain insects, and where this is the case the
hind-gut, owing to a retention of the embryonic condition, is usually
separated from the mid-intestine. (See p. 300.)

[Illustration:

  FIG. 321.—Enteric canal of _Psyllopsis fraxinicola_: _œ_, œsophagus;
    _md_, mid-intestine; _ed_, hind-intestine; _vm_, urinary vessels;
    _s_, the coil formed by the hind-intestine and the most anterior
    part of the mid-intestine.—After Witlaczil, from Lang.
]

  Some remarkable features of the digestive canal in hemipterous
  insects are noteworthy. In the Coccidæ, according to Mark, the
  anterior end of the long mid-intestine forms, with the hinder end of
  the œsophagus, a small loop, whose posterior end is firmly grown to
  the wall of the rectum, and forms a cup-like invagination of the
  latter. Then the rest of the tube-like stomach turns sidewise and
  forms a large loop, which turns back on itself and occupies a large
  part of the body-cavity. This loop receives on the anterior end,
  near the œsophagus, the two urinary vessels, and forms just below
  the opening into the rectum a short cæcum.

  In other homopterous genera (Psyllidæ and some Cicadidæ) Witlaczil
  describes nearly the same peculiarity, the mid-gut and part of the
  intestine forming a loop growing together for a certain distance and
  winding round each other (Fig. 321).

=Histology of the digestive canal.=—In all the divisions of the
digestive canal of insects the succession of the cellular layers
composing it is the same: 1st, a cuticula; 2d, an epithelium; 3d,
connective tissue; 4th, muscular tissue. In the locust, the first
division of the canal (fore-gut), there are two muscular coats, an
internal longitudinal and an external circular coat; the fibres are all
striated. The lining epithelium is not much developed, but forms a
thick, hard, and refringent cuticula, which is thrown up into spiny
ridges. In the second division (mid-gut, “stomach”) the epithelium is
composed of very high columnar cells, which make up the greater part of
the thickness of the walls, while the cuticula is very delicate,
slightly refringent, with no ridges, and is probably not chitinous; the
fibres of the muscular coats are not striated, while this division is
also distinguished by the presence of glandular follicles and folds. The
stomach and the cæcal appendages have all these peculiarities in common,
while no other part of the canal is thus characterized.

The third division (intestine and rectum) is composed of an epithelium,
the cells of which are intermediate in size between those of the fore
and mid gut. The cells are often pigmented, and they are covered by a
much thicker cuticula than that of the stomach, but which is not so
thick and hard as that of the œsophagus and proventriculus. The very
refringent cuticula is not thrown up into ridges, though in some parts
it is covered with delicate conical spines, which are very short. “The
epithelium and underlying connective tissue (_tunica propria_) are
thrown up into six folds, which run longitudinally, being regular in the
ileum and rectum (as the rectal glands), but very irregular in the
colon. Outside the depression between each two neighboring folds there
is a longitudinal muscular band, these making six bands. This peculiar
disposition of the longitudinal muscles does not occur in any other part
of the canal; it is, therefore, especially characteristic of the third
division.” (Minot.)




                 LITERATURE ON THE ORGANS OF DIGESTION


  =Treviranus, G. R.= Resultate einiger Untersuchungen über den inneren
    Bau der Insekten. (Verdauungsorgane von _Cimex rufipes_.) (Annal. d.
    Wetterau. Gesells., 1809, i, pp. 169–177, 1 Taf.)

  =Ramdohr, C. A.= Abhandlungen über die Verdauungswerkzeuge der
    Insekten. 1811, vii, pp. 221, 30 Taf.

  =Dutrochet, R. J. H.= Mémoire sur les métamorphoses du canal
    alimentaire dans les insectes. (Journal de Physique, 1818, lxxxvi,
    pp. 130–135, 189–204; Meckel’s Archiv, 1818, iv, pp. 285–293.)

  =Suckow, F. W. L.= Verdauungsorgane der Insekten. (Heusinger’s
    Zeitschr. f. organ. Physik., 1828, iii, pp. 1–89.)

  =Doyère, L.= Note sur le tube digestif des Cigales. (Ann. Sc. nat.
    Zool., 1839, 2 Sér., xi, pp. 81–85.)

  =Grube, A. E.= Fehlt den Wespen- oder Hornissenlarven ein After oder
    nicht? 1 Taf. (Müller’s Archiv für Physiol., 1849, pp. 47–74.)

  =Sirodot.= Recherches sur les sécrétions chez les insectes. (Ann. Sc.
    nat. Zool., 1858, 4 Sér., x, pp. 141–189, 251–328, 12 Pls.)

  =Milne-Edwards, H.= Leçons sur la physiologie et l’anatomie comparée,
    v, 1859, pp. 498–536, 581–638.

  =Dufour, L.= Recherches anatomiques sur les Carabiques et sur
    plusieurs autres insectes Coléoptères. Appareil digestif. (Ann. Sc.
    nat., ii, 1824, pp. 462–482, 2 Pls.; iii, 1824, pp. 215–242, 5 Pls.,
    pp. 476–491, 3 Pls.; iv, 1824, pp. 103–125, 4 Pls.; iv, 1825, pp.
    265–283.)

  —— Recherches anatomiques sur l’Hippobosque des chevaux. (Ann. Sc.
    nat., 1825, vi, pp. 299–322, 1 Pl.)

  —— Description et figure de l’appareil digestif de l’_Anobium
    striatum_. (Ibid., xiv, 1828, pp. 219–222, 1 Pl.)

  —— Recherches anatomiques sur les Labidoures. Appareil de la
    digestion. (Ibid., xiii, 1828, pp. 348–354, 2 Pls.)

  —— Recherches anatomiques et considerations entomologiques sur
    quelques insectes Coléoptères, compris dans les familles des
    Dermestins, des Byrrhiens, des Acanthopodes et des Leptodactyles.
    Appareil digestif. (Ibid., Sér. 2, Zool., i, 1834, pp. 67–76, 2
    Pls.)

  —— Résumé des recherches anatomiques et physiologiques sur les
    Hémiptères. (Ibid., pp. 232–239.)

  —— Mémoire sur les métamorphoses et l’anatomie de la _Pyrochroa
    coccinea_. Appareil digestif. (Ibid., Sér. 2, Zool., xiii, 1840, pp.
    328–330, 334–337, 2 Pls.)

  —— Histoire comparative des metamorphoses et de l’anatomie des
    _Cetonia aurata_ et _Dorcus parallelepipedus_. Appareil digestif.
    (Ibid., Sér. 2, Zool., 1824, xviii, pp. 174–176, 2 Pls.)

  —— Anatomie générale des Diptères. Appareil digestif. (Ibid., Sér. 3,
    Zool., i, 1814, pp. 248, 249.)

  —— Histoire des métamorphoses et de l’anatomie du _Piophila
    petasionis_. Appareil digestif. (Ibid., Sér. 3, Zool., i, 1844, pp.
    372–377, 1 Pl.)

  —— Études anatomiques et physiologiques sur les insectes Diptères de
    la famille des Pupipares. Appareil digestif. (Ibid., Sér. 3, Zool.,
    iii, 1845, pp. 67–73, 1 Pl.)

  —— Recherches sur l’anatomie et l’histoire naturelle de l’_Osmylus
    maculatus_. Appareil digestif. (Ibid., Sér. 3, Zool., ix, 1848, pp.
    346–349, 1 Pl.)

  —— Études anatomiques et physiologiques, et observations sur les
    larves des Libellules. Appareil digestif. (Ibid., Sér. 3, Zool.,
    xvii, 1852, pp. 101–108, 1 Pl.)

  —— Recherches anatomiques sur les Hyménoptères de la famille des
    Urocerates. Appareil digestif. (Ibid., Sér. 4, Zool., i, 1854, pp.
    212–216, 1 Pl.)

  —— Fragments d’anatomie entomologique. Sur l’appareil digestif du
    _Nemoptera lusitanica_. (Ibid., Sér. 4, viii, 1857, pp. 6–9, 1 Pl.)

  —— Recherches anatomique et considerations entomologiques sur les
    Hémiptères du genre Leptopus. Appareil digestif. (Ibid., Sér. 4,
    Zool., 1858, x, pp. 352–356, 1 Pl.)

  —— Recherches anatomiques sur l’_Ascalaphus meridionalis_. Appareil
    digestif. (Ibid., Sér. 4, xiii, 1860, pp. 200–202, 1 Pl.)

  =Leydig, F.= Zur Anatomie von _Coccus hesperidum_. (Zeitschr. f.
    wissens. Zool., v, 1853, pp. 1–12, 1 Taf.)

  =Lubbock, J.= On the digestive and nervous System of _Coccus
    hesperidum_. (Proc. Roy. Soc., ix, 1886, pp. 480–486; also Ann. Mag.
    Nat. Hist., 1859, Ser. 3, iii, pp. 306–311.)

  =Scheiber, S. H.= Vergleichende Anatomie und Physiologie der
    Œstriden-Larven. V. Das chylo- und uropœtische System. (Sitzber. d.
    k. Akad. d. Wissens. Wien. Math.-naturwiss. Cl., 1862, xlv, pp.
    39–64, 1 Taf.)

  =Gerstaecker, A.= Bronn’s Klassen und Ordnungen des Tierreichs. V.
    Gliederfüssler. (Ernährungsorgane, pp. 87–105.)

  =Graber, V.= Zur naheren Kenntnis des Proventriculus und der
    Appendices ventriculares bei den Grillen und Laubheuschrecken.
    (Sitzber. d. k. Akad. d. Wissensch. Wien. Mathem.-naturwiss. Cl.,
    lix, 1869, pp. 29–46, 3 Taf.)

  —— Ueber die Ernährungsorgane der Insekten und nächstverwandten
    Gliederfüssler. (Mitteil. d. naturwiss. Vereins für Steiermark.
    Graz, 1871, ii, pp. 181, 182.)

  —— Verdauungssystem des Prachtkäfers. (Ibid., Graz, 1875.)

  —— Die Insekten., i, 1877. (Verdauungsapparat, pp. 308–328.)

  =Wilde, K. F.= Untersuchungen über den Kaumagen der Orthopteren.
    (Archiv f. Naturgesch., xliii Jahrg., 1877, pp. 135–172, 3 Taf.)

  =Simroth, H.= Ueber den Darmkanal der Larven von _Osmoderma eremita_
    mit seinen Anhängen. (Giebel’s Zeitschr. f. d. ges. Naturwiss.,
    1878, li, pp. 493–518, 3 Taf.)

  =Müller, H.= Ueber die angebliche Afterlösigkeit der Bienenlarven.
    (Zool. Anzeiger, 1881, pp. 530, 531.)

  =Schiemenz, Paulus.= Ueber das Herkommen des Futtersaftes und die
    Speicheldrüsen der Bienen, nebst einem Anhänge über das Riechorgan.
    (Zeitschr. f. wissens. Zool., xxxviii, 1883, pp. 71–135, 3 Taf.)

  =Rovelli, G.= Alcune ricerche sul tubo digerente degli Atteri,
    Ortotteri e Pseudo-Neurotteri. (Como, 1884, p. 15.)

  =Beauregard, H.= Structure de l’appareil digestif des Insectes de la
    tribu des Vésicants. (Compt. rend. Acad. Paris, 1884, xcix, pp.
    1083–1086.)

  —— Recherches sur les Insectes vésicants., 1 Part, Anatomie. (Journ.
    Anat. Phys. Paris, 1885, xxi Année, pp. 483–524, 4 Pls.; 1886, xxii
    Année, pp. 85–108, 242–284, 5 Pls.)

  —— Les Insectes vésicants, Paris, 1890, Chap. III, Appareil digestif,
    pp. 63–99; (Phénomènes digestifs, pp. 161–170; Pls. 6–9.)

  =Wertheimer, L.= Sur la structure du tube digestif de l’_Oryctes
    nasicornis_. (Compt. rend. Soc. Biol. Paris, 1887, Sér. 8, iv, pp.
    531, 532.)

  =Kowalevsky, A.= Beitrage zur Kenntniss der nachembryonal Entwicklung
    der Musciden. (Zeitschr. f. wissens. Zool., xlv, 1887, pp. 542–594,
    5 Taf.)

  =Schneider, A.= Ueber den Darm der Arthropoden, besonders der
    Insekten. (Zool. Anzeiger, 1887, x Jahrg., pp. 139, 140.)

  —— Ueber den Darmkanal der Arthropoden. (Zool. Beiträge von A.
    Schneider, ii, 1887, pp. 82–96, 3 Taf.)

  =Fritze, A.= Ueber den Darmkanal der Ephemeriden. (Berichte der
    Naturforsch.-Gesellsch. zu Freiburg i. Br., 1888, iv, pp. 59–82, 2
    Taf.)

  =Emery, C.= Ueber den sogenannten Kaumagen einiger Ameisen. (Zeitschr.
    f. wissens. Zool., 1888, xlvi, pp. 378–412, 3 Taf.)

  =Meinert, F.= Contribution à l’anatomie des Fourmilions. (Overs.
    Danske Vidensk. Selsk. Forhandl. Kjöbenhavn, 1889, pp. 43–66, 2
    Pls.)

  =Mingazzini, P.= Richerche sul canale digerente dei Lamellicorni
    fitofage (Larve e Insetti perfetti). (Mitteil. Zool. Station zu
    Neapel, ix, 1889–1891, pp. 1–112, 266–304, 7 Pls.)

  =Fernald, Henry T.= Rectal glands in Coleoptera. (Amer. Naturalist,
    xxiv, pp. 100, 101, Jan., 1890.)

  =Visart, O.= Digestive canal of Orthoptera. (Atti Soc. Toscana Scient.
    Natur., vii, 1891, pp. 277–285.)

  =Eberli, J.= Untersuchungen an Verdauungstrakten von _Gryllotalpa
    vulgaris_. (Vierteljahresschr. d. Naturforsch. Gesells. Zurich,
    1892, Sep., p. 46, Fig.)

  =Holmgren, Emil.= Histologiska studier öfver några lepidopterlarvers
    digestionskanal och en del af deras Körtelartade bildningar. (Ent.
    Tidskr. Årg. xiii, pp. 129–170, 1892, 6 Pls.)

  =Ris, F.= Untersuchung über die Gestalt des Kaumagens bei den Libellen
    und ihren Larven. (Zool. Jahrb. Abth. Syst., ix, 1896, pp. 596–624,
    13 Figs.)

  See also the works of Straus-Dürckheim, Newport, Mark, Witlaczil,
    Vayssière, Landois, Jordan, Oudemans, Berlese, List, Grassi, Verson,
    Miall and Denny, Leidy, Cheshire, Kowalevsky, Gehuchten, Locy, etc.


                       _b_. Digestion in insects

For the most complete and reliable investigation of the process of
digestion, we are indebted to Plateau, whose results we give, besides
the conclusions of later authors:

In mandibulate or biting insects, the food is conducted through the
œsophagus by means of the muscular coating of this part of the digestive
canal. Suctorial insects draw in their liquid food by the contractions
followed by the dilatations of the mid-intestine (chylific stomach).
Dragon-flies, Orthoptera, and Lepidoptera swallow some air with their
food.

Where the salivary glands are present, the neutral alkaline fluid
secreted by them has the same property as the salivary fluid of
vertebrates of rapidly transforming starchy foods into soluble and
assimilable glucose. In such forms as have no salivary glands, their
place is almost always supplied by an epithelial lining of the
œsophagus, or, as in the Hydrophilidæ, a fluid is secreted which has the
same function as the true salivary fluid.

Nagel states that the saliva of the larva of Dyticus is powerfully
digestive, and has a marked poisonous action, killing other insects, and
even tadpoles of twice the size of the attacking larva, very rapidly.
The larvæ not only suck the blood of their victims, but absorb the
proteid substances. Drops of salivary juice seem to paralyze the victim,
and to ferment the proteids. The secretion is neutral, the digestion
tryptic. Similar extra-oral digestion seems to occur in larvæ of
ant-lions, etc. (Biol. Centralbl., xvi, 1896, pp. 51–57, 103–112; Journ.
Roy. Micr. Soc., 1896, p. 184.)

In carnivorous insects and in Orthoptera, the œsophagus dilates into a
crop (ingluvies) ended by a narrow, valvular apparatus (or gizzard of
authors). The food, more or less divided by the jaws, accumulates in the
crop, which is very distensible; and, when the food is penetrated by the
neutral or alkaline liquid, there undergoes an evident digestive action
resulting, in carnivorous insects, in the transformation of albuminoid
substances into soluble and assimilable matter analogous to peptones,
and, in herbivorous insects, an abundant production of sugar from
starch. This digestion in the crop, a food-reservoir, is very slow, and,
until it is ended, the rest of the digestive canal remains empty.

“Any decided acidity found in the crop is due to the injection of acid
food; but a very faint acidity may occur, which results from the
presence in the crop of a fluid secreted by the cæcal diverticula of the
mesenteron.” (Miall and Denny.)

When digestion in the crop is accomplished, the matters are subjected to
an energetic pressure of the walls through peristaltic contractions, and
then, guided by the furrows and chitinous teeth, pass along or gradually
filter through the valvular apparatus or proventriculus, whose function
is that of a strainer.

At the beginning of the “chyle-stomach” (mesenteron) of Orthoptera are
glandular cæca which secrete a feebly acid fluid. This fluid emulsifies
fats, and converts albuminoids into peptones. It passes forwards into
the crop, and there acts upon the food.

In the mesenteron (mid-intestine) the food is acted upon by an alkaline
or neutral fluid, never acid, either secreted, as in Orthoptera, by
local special glands, or by a multitude of minute glandular cæca, as in
many Coleoptera, or by a simple epithelial layer. It has no analogy with
the gastric juices of vertebrates; its function differs in insects of
different groups; in carnivorous Coleoptera it actively emulsionizes
greasy matters; in the Hydrophilidæ it continues the process of
transformation of starch into glucose, begun in the œsophagus. In the
Scarabæidæ, it also produces glucose, but this action is local, not
occurring elsewhere; in caterpillars, it causes a production of glucose,
and transforms the albuminoids into soluble and assimilable bodies
analogous to peptones, and also emulsionizes greasy matters. Finally, in
the herbivorous Orthoptera there does not seem to be any formation of
sugar in the stomach itself, the production of glucose being confined to
the crop (jabot).

When digestion in the crop is finished, the proventriculus relaxes, and
the contents of the crop, now in a semi-fluid condition, guided by the
furrows and teeth, passes into the mesenteron, which is without a
chitinous lining, and is thus fitted for absorption.

The contents of the mid-intestine (chylific stomach) then slowly and
gradually pass into the intestine, the first anterior portion of which,
usually long and slender, is the seat of an active absorption. The
epithelial lining observed in certain insects seems, however, to
indicate that secondary digestion takes place in this section. The
reaction of the contents is neutral or alkaline.

The second and larger division of the intestine only acts as a stercoral
reservoir. (The voluminous cæcum occurring in Dyticidæ, Nepa, and
Ranatra, whether full or empty, never contains gas, and it is not, as
some have supposed, a swimming-bladder.) The liquid product secreted by
the Malpighian tubes accumulates in this division, and, under certain
circumstances, very large calculi are often formed. In his subsequent
paper on the digestion of the cockroach, Plateau states that in the
intestine are united the residue of the work of digestion and the
secretion of the urinary or Malpighian tubes, this secretion being
purely urinary.

These organs are exclusively depuratory and urinary, freeing the body
from waste products of the organic elements. The liquid they secrete
contains urea (?), uric acid and abundant urates, hippuric acid (?),
chloride of sodium, phosphates, carbonate of lime, oxalate of lime in
quantity, leucine, and coloring-matters.

The products of the rectal or anal glands vary much in different groups,
but they take no part in digestion, nor are they depuratory in their
nature.

Insects have nothing resembling chylific substances.[51] The products of
digestion, dissolved salts, peptones, sugar in solution, emulsionized
greasy matters, pass through the relatively delicate walls of the
digestive canal by osmose, and mingle outside of the canal with the
blood.

Whatever substances remain undigested are expelled with the excrements;
such are the chitin of the integuments of insects, vegetable cellulose,
and chlorophyll, which is detected by the microspectroscope all along
the digestive canal of phytophagous insects.

In his experiments in feeding the larvæ of Musca with lacmus, Kowalevsky
found that the œsophagus, food-reservoir, and proventriculus, with its
cæcal appendages, always remained blue, and had an alkaline reaction;
the mid-intestine, also, in its anterior portion, remained blue, but a
portion of its posterior half became deep red, and also exhibited a
strong reaction. The hind-intestine, however, always remained blue, and
also had an alkaline reaction. (Biol. Centralbl., ix, 1889, p. 46.)

=The mechanism of secretion.=—Gehuchten describes the process of
secretion in insects, the following extract being taken from his
researches on the digestive apparatus of the larva of Ptychoptera. The
products of secretion poured into the alimentary canal are more or less
fluid; for this reason, it is impossible to say when an epithelial cell
at rest contains these products. For the secreting nature of these cells
is only apparent at the moment when they are ready for excretion; then
the cellular membrane swells out, and a part of the protoplasmic body
projects into the intestinal cavity.

Before going farther, the terms _secretion_ and _excretion_ should, he
says, be defined. With Ranvier, he believes that the elaboration in the
protoplasm of a definite fluid substance is, _par excellence_, the
secretory act, while the removal of this substance is the act of
excretion.

[Illustration:

  FIG. 322.—Different phases of the mechanism of secretion and of
    excretion.—After Gehuchten.
]

A glandular cell of the chylific stomach, when at rest, is always
furnished with a striated “platform,” or flat surface, or face, on the
side facing the cavity of the stomach, and the free edge of the
platform, or plateau, is provided with filaments projecting into the
digestive cavity (Fig. 322, _f_). These glandular cells, when active,
differ much in appearance. In a great number, the platform (plateau) has
disappeared, and is replaced by a simple, regular membrane. During the
process of secretion, a finely granular mass, in direct continuity with
the protoplasm, swells, and raises the membrane over the entire breadth
of the cell, causing it to project into the intestinal cavity (Fig. 322,
_A_, _B_). These vesicles, or drops of the secretion, whether free or
still attached by a web to the cells, are clear and transparent in the
living insect, but granular in the portions of the digestive canal fixed
for cutting into sections. Gehuchten then asks: “How does a cell gorged
with the products of secretion empty itself?” Both Ranvier and also
Heidenhain believe that one and the same glandular cell may secrete and
excrete several times without undergoing destruction, but their
researches made on salivary glands have not answered the question.
Gehuchten explains the process thus: when the epithelial cell begins to
secrete, the clear fluid elaborated in the protoplasm of the cell
increases the intracellular tension, until, finally, the fluid breaks
through certain weak places in the swollen basal membrane of the
platform, and then easily passes through the closely crowded filaments,
and projects out into the intestinal cavity as a pear-shaped vesicle of
a liquid rich in albumens at first attached to the free face of the
cell, but finally becoming free, as at Fig. 322, _A_, _B_.

When the elaboration of the substance to be secreted is more active, the
mechanism of the secretion is modified. The basal membrane of the
platform may then be raised at several places at once; instead of a
single vesicle projecting into the intestinal cavity, each cell may
present a great number more or less voluminous. If all remain small and
rapidly detach themselves from the glandular cell, the filaments of the
platform are simply separated from each other at different points of the
free face, as in Fig. 322, _C_. On the other hand, when the different
vesicles of a single cell become larger, the filaments of the platform
are compressed and crowded against each other in the spaces between the
vesicles remaining free, and the undisturbed portions of the platform
appear homogeneous (Fig. 322, _D_). After the excretion of the secretory
products by this process of strangulation, the cell then assumes the
aspect of a glandular cell at rest, and may begin again to form a new
secretion.

To sum up: The process of excretion may occur in two ways:

1. Where the membrane ruptures and the substances secreted are sent
directly out into the digestive cavity. 2. Where the vesicles become
free by strangulation, floating in the glandular or intestinal cavity,
and ending by rupturing and coming into contact with the neighboring
vesicles or with the food.

=Absorbent cells.=—Besides the glandular or secreting cells in
Ptychoptera, there is between the two regions of the chyle-stomach lined
with these cells a region about a centimetre long composed of absorbent
cells. The absorbent cells are very large, polygonal, and contain a
large nucleus, in which is a striated convoluted chromatic cord.

The food on entering the chyle-stomach is brought into contact with the
products secreted in the proventriculus, in the first part of the
chyle-stomach, and in the tubular glands. These products of secretion
act on the food, extracting from them useful substances which they
render soluble. These substances, after having been absorbed by the
absorbent cells in the middle region of the stomach, undergo special
modifications, and are transformed into solid products, which are
situated at the bottom of these cells. Afterwards the alimentary
substances freed from a portion of their useful substances are again
placed in contact with the products of secretion in the distal part of
the chylific ventricle, and reach the terminal part of the intestine.

“The products of secretion,” adds Gehuchten, “diverted into the
intestinal canal do not come into immediate contact with the alimentary
substances; they are separated from it by a continuous, structureless,
quite thick membrane (the peritrophic membrane), which directly envelops
the cylinder of food matters, extending from the orifice of the
œsophageal valvule to the end of the intestine. Between this membrane
and the free face of the epithelial lining there exists a circular
space, into which are thrown and accumulate the excreted substances. The
latter then cannot directly mingle with the aliments; but when they are
liquid they undoubtedly pass through this membrane by osmose, and thus
come into contact with the nutritive substances. It is the same with the
products of absorption. The absorption of soluble products of the
intestinal cavity is not then so simple a phenomenon as it was at first
thought to be, since these products are nowhere brought into immediate
contact with the absorbent cells” (pp. 90, 91).

The most recent authority, Cuénot, states that absorption of the
products of digestion takes place entirely in the mid-intestine, and in
its cæca when these are present. The mid-intestine exercises a selective
action on the constituents of the food comparable to the action of the
vertebrate liver.




               LITERATURE ON THE PHYSIOLOGY OF DIGESTION


  =Davy, J.= Note on the excrements of certain insects, and on the
    urinary excrement of insects. (Edinburgh New Phil. Journ., 1846, xl,
    pp. 231–234, 335–340; 1848, xlv, pp. 17–29.)

  —— Some observations on the excrements of insects, in a letter
    addressed to W. Spence. (Trans. Ent. Soc. London, Ser. 2, iii, 1854,
    pp. 18–32.)

  =Bouchardat, A.= De la digestion chez le ver à soie. (Revue et Mag. de
    Zool., Sér. 2, 1851, iii, pp. 34–40.)

  =Lacaze-Duthiers, H., et A. Riche.= Mémoire sur l’alimentation de
    quelques insects gallicoles et sur la production de la graisse.
    (Ann. Scienc. natur., 1854, Sér. 4, ii, pp. 81–105.)

  =Basch, S.= Untersuchungen über das chylopoetische und uropoetische
    System von _Blatta orientalis_. (Sitzungsber. d. math.-naturwiss.
    Classe d. Akad. d. Wissensch. Wien., 1858, xxxiii, pp. 234–260, 5
    Taf.)

  =Lambrecht, A.= Der Verdauungsprozess der stickstoffreichen
    Nährmittel, welche unsere Bienen geniessen, in den dazu geschaffenen
    Organen derselben. (Bienenwirtschaftl. Centralbl., viii Jahrg.,
    1872, pp. 73–78, 83–89.)

  =Plateau, F.= Recherches sur les phénomènes de la digestion chez les
    insectes, (Mém. Acad. roy. de Belgique, Sér. 2, xli, 1 Part, 1873,
    pp. 124, 3 Pls.)

  —— Note additionelle au mémoire sur les phénomènes de la digestion
    chez les insectes. (Bull. Acad. roy. de Belgique, Sér. 2, xliv,
    1877, pp. 710–733.)

  =Tursini, G. Fr.= Un primo passo nella ricerca dell’ assorbimento
    intestinale degli artropodi. (Rend. d. R. Accadem. di Sc. fis. e
    matemat. di Napoli, 1877, xvi, pp. 95–99, 1 Pl.)

  =Jousset de Bellesme=, Physiologie comparée. Recherches expérimentales
    sur la digestion des insectes et en particulier de la blatte. Paris,
    1876, vii, and 96 pp., 3 Pls.

  —— Recherches sur les fonctions des glandes de l’appareil digestif des
    Insectes. (Compt. rend., lxxxii, Paris, 1876, pp. 97–99.)

  —— Travaux originaux de physiologie comparée. (i, Insectes, Digestion,
    Métamorphoses.) Paris, 1878, 5 Pls.

  =Simroth, H.= Einige Bemerkungen über die Verdauung der Kerfe.
    (Zeitschr. f. d. gesammten Naturwiss., xli, 1878, pp. 826–831.)

  =Krukenberg, C. Fr. W.= Versuche zur vergleichenden Physiologie der
    Verdauung und vergleichende physiologische Beiträge zur Kenntnis der
    Verdauungsvorgange. (Untersuch, a. d. physiolog. Institut d.
    Universität Heidelberg, 1880, i, 4, pp 327, Figs.; ii, 1, p. 1,
    Figs.)

  =Metschnikoff, E.= Untersuchungen über die intrazelluläre Verdauung
    bei wirbellosen Tieren. (Arb. d. zool. Instit. Wien., 1883, v, pp.
    141–168, 2 Taf.)

  =Locy, William A.= Anatomy and physiology of the family Nepidæ. (Amer.
    Naturalist, xviii, 1884, pp. 250–255, 353–367, 4 Pls.)

  =Vangel, E.= Beiträge zur Anatomie, Histiologie, und Physiologie des
    Verdauungsapparates des Wasserkäfers, _Hydrophilus piceus_.
    (Termész. Füzet., x, 1886, pp. 111–126 (in Hungarian); pp. 190–208
    (in German), 1 Pl.)

  =Schönfeld.= Die physiologische Bedeutung des Magenmundes der
    Hönigbiene. (Archiv f. Anat. u. Physiol., Physiol. Abt., 1886, pp.
    451–458.)

  =Faussek, V.= Beiträge zur Histiologie des Darmkanals der Insekten.
    (Zeitschr. f. wiss. Zool., 1887, xl, pp. 694–712, 1 Taf.; Abstract
    in Zool. Anz., Jahrg. x, pp. 322, 323, 1 Taf.)

  =Frenzel, J.= Ueber Bau und Thätigkeit des Verdauungskanals der Larve
    des _Tenebrio molitor_, mit Berücksichtigung anderer Arthropoden
    (Berlin. Ent. Zeitschr., 1882, pp. 267–316, 1 Taf.); Inaug.-Diss.
    Göttingen, 1882.

  —— Einiges über den Mitteldarm der Insekten, sowie über
    Epithel-regeneration. (Archiv f. Mikrosk. Anat., 1885, xxvi, pp.
    229–306, 3 Taf.)

  —— Zum feineren Bau des Wimperapparates. (Ibid., 1886, xxviii, pp.
    53–80, 1 Taf.)

  —— Die Verdauung lebenden Gewebes und die Darmparasiten. (Archiv f.
    Anat., 1891.)

  =Gehuchten, A. van.= Recherches histologiques sur l’appareil digestif
    de la _Ptychoptera contaminata_, I Part. Étude du revêtment
    épithélial et recherches sur la sécrétion. (La Cellule, 1890, vi,
    pp. 183–291, 6 Pls.)

  =Cuénot, L.= Études physiologiques sur les Orthoptères. (Arch. Biol.,
    xiv, 1895, pp. 293–341, 2 Pls.)

  =Needham, James G.= The digestive epithelium of dragon-fly nymphs.
    (Zool. Bull., i, 1897, Chicago, pp. 104–113, 10 Figs.)

  With the writings of Mingazzini (see p. 323), Kowalevsky, Ranvier,
    Haidenhain, Beauregard (p. 323), Sadones.




     THE GLANDULAR AND EXCRETORY APPENDAGES OF THE DIGESTIVE CANAL


Into each primary division of the digestive canal open important glands.
The salivary and silk-glands are offshoots of the œsophagus (stomodæum);
the cœcal appendages open into the stomach (mesenteron), while the
urinary tubes grow out in embryonic life from the primitive intestine
(proctodæum), and there are other small glands which are connected with
the end of the hind-intestine.


                        _a._ The salivary glands

We will begin our account of these glands with those of the Orthoptera,
where they are well developed. In the cockroach a large salivary gland
and accompanying reservoir lie on each side of the œsophagus and crop.
The gland is a thin, leaf-like, lobulated mass, divided into two
principal lobes. These open into a common trunk, which after receiving a
branch from a small accessory lobe, and from the salivary reservoir,
unites with its fellow to form the unpaired salivary duct which opens
into the under side of the lingua. Each salivary reservoir is a large
oval sac with transparent walls. (Miall and Denny, also Figs. 299, _sr_,
and 327.) The ducts and reservoirs have a chitinous lining, and the
ducts are, like the tracheæ, surrounded by a so-called spiral thread, or
by separate, incomplete, hooplike bands, which serve to keep the duct
permanently distended. In the locust (Fig. 298) the lobules are more
scattered, forming small separate groups of acinose glands. In the
embryo of Forficula Heymons has observed a pair of salivary glands
opening on the inner angle of the mandibles, a second pair opening in
the second maxillæ, while a third pair of glands, whose function is
doubtful, is situated in the hinder part of the head, opening to the
right and left on the chitinous plate (postgula) behind the submentum.
In Perla, there are two pairs segmentally arranged (Fig. 343).

[Illustration:

  FIG. 323.—Left side of the head of the silkworm: _a_, adductor muscle
    of the mandible, from which the muscular fibres have been removed;
    _b_, upper fibres of the same; _c_, lower fibres cut away to show
    the adductor muscle (_e_); _d_, fibres inserted on the accessory
    adductor lamella; _f_, œsophagus, much swollen; _g_, salivary gland;
    _h_, dorsal vessel; _i_, _l_, tracheæ of the mandibular muscles;
    _k_, trachea; _n_, optic nerve.—After Blanc.
]

[Illustration:

  FIG. 324.—Lower side of the head of the silkworm exposed, the spinning
    apparatus, the œsophageal ganglion, and the adductor of the left
    mandible removed: _M_, mandible; _P_, abductor of the mandible; _R_,
    adductor; _N_, salivary gland attached at _O_ to the edge of the
    adductor muscle; _o_, _o_, transverse portion of the “hyoid”; 3,
    masticator nerve and its recurrent branch (7); _L_, tongue cut
    horizontally.—After Blanc.
]

Here we might refer to a pair of glands regarded by Blanc as the true
salivary glands. They do not appear to be the homologues of the salivary
glands of other insects, though probably functioning as such. The
functional salivary glands of lepidopterous larvæ have been overlooked
by most entomotomists, and the spinning glands have been, it seems to
us, correctly supposed to be modified salivary glands. Lucas also
regards those of case-worms (Trichoptera) as morphologically salivary
glands. Those of the silkworm were figured by Réaumur (Tom. i, Pl. v,
Fig. 1), but not described; while those of Cossus, which are voluminous,
were regarded by Lyonet as “_vaisseaux dissolvans_.” Dr. Auzoux (1849),
in his celebrated model of the silkworm, represented them accurately,
while Cornalia briefly described them as opening into the mouth. The
first satisfactory description is that of Blanc (1891), who states that
in the silkworm “the two salivary glands” are small, flexuous, yellow
tubes, which occupy a variable position on the sides of the œsophagus
(Fig. 323). The glandular portion passes into the head, ending at the
level of the adductor plate of the mandibles (Fig. 324, _o_), and
entering the buccal cavity at the base of the mandible, as seen in Fig.
323. It is plain, when we recognize the direct homology of the
silk-glands of the caterpillars with the salivary glands of other
insects, and of the spinneret with the hypopharynx, that these so-called
“salivary glands” in lepidopterous larvæ are different structures. They
are probably modified coxal glands, belonging to the mandibular segment.

[Illustration:

  FIG. 325.—One of the two salivary glands of _Cæcilius burmeisteri_:
    _d_, excretory duct; _cn_, the lumen or canal; _cg_, gland-cells;
    _ct_, salivary fluid.—After Kolbe.
]

  The polygonal epithelial cells of these glands contain branched
  nuclei, recalling those of the spinning-glands. In those
  caterpillars which feed on leaves, the salivary glands are slightly
  developed, but in such as bore into and eat wood, as the Cossidæ,
  the glands are, as figured by Lyonet, very large, forming two
  sausage-shaped bodies passing back to the beginning of the
  mid-intestine, each ending in a long convoluted filament. The
  salivary glands of the imago are very long and convoluted (Fig. 310,
  _sd_).

  In the Panorpidæ these glands differ in the sexes, the males having
  three pairs of very long tortuous tubes, while, in the females, they
  are reduced to two indistinct vesicles. (Siebold.)

In the Diptera in general there are two pairs, one situated in the beak,
the other in the thorax. In the larvæ there is a single pair (Fig. 341).
Kraepelin describes a third pair in the Muscidæ at the point of
transition from the fulcrum to the œsophagus, but Knüppel has apparently
found only what may be fat cells at this point, so that the supposed
presence of a third pair in Diptera needs confirmation. In the Psocidæ
there are two salivary glands, of simple tubular shape (Fig. 325).

In the Nepidæ the salivary glands are four in number, and of
conglomerate structure, two being long and extending back into the
beginning of the abdomen, while the other two are about one-fourth as
long. (Figs. 327, 328.) In Cicada, besides a pair of simple tortuous
tubes, there is in the head another pair of glands, each composed of two
tufts of short lobes, situated one behind the other. (Dufour.) In many
Hemiptera (Pyrrhocoris, Capsus, etc.) there is but a single pair, each
gland consisting of four lobes; in the Coccidæ each gland is divided
into two lobes (Fig. 326); in the Aphidæ, according to Witlaczil, they
consist of two lobes grown together. In the Psyllidæ they are said to be
absent.

In _Phylloxera vastatrix_ the saliva is forced through a salivary
passage out of the duct and into the mouth by a pumping apparatus
furnished with special muscles. (Krassilstschik.)

In the Odonata acinose glands are present in the imago, but not in the
nymph until in its last stage, Poletaiew accounting for their absence in
the earlier stages by the fact that the larva swallows more or less
water while taking its food.

In the Coleoptera, as we have observed in Anopthalmus, there are three
pairs of salivary glands (Fig. 74). In the Blapsidæ these glands consist
of many ramifying tubes united on each side of the œsophagus into a
single duct; in others they are but slightly developed, while in still
others they are wanting.

The salivary glands are most highly differentiated in the Hymenoptera,
and especially in the bees (Bombus and Apis), where Schiemenz found not
less than five systems of glands (Fig. 329; also 87), of which four
systems are paired. One pair of these glands lies in the tongue, three
in the head, and one in the thorax.

[Illustration:

  FIG. 326.—Acinous salivary glands of _Orthezia cataphracta_. In some
    acini the nuclei and boundaries of the cells are shown.—After List,
    from Field’s Hertwig.
]

  System I is situated in the head, and consists of unicellular
  glands; the duct from each cell leads into a common, strongly
  chitinized duct, opening into the gullet.

  System II, composed of acinose glands, lies also in the head; its
  duct is united with that of System III, situated in the thorax.
  (Fig. 329, 2, 3.)

  System IV is situated at the base of the upper surface of the
  mandibles, and forms a delicate sac lined within with glandular
  cells; its duct opens at the insertion of the mandibles.

  System V lies in the beak, and is a single gland consisting of
  unicellular glands; it opens into the common opening of Systems II
  and III. This system is wanting in the honey-bee, but occurs in
  Bombus and other genera.

[Illustration:

  FIG. 327.—Appendages of digestive canal of Belostoma.—After Locy.
]

[Illustration:

  FIG. 328.—Salivary and other glands of Ranatra.—After Locy.
]

  In all the five systems there constantly occur three cellular
  layers: the intima, epithelial, and propria. As regards their origin
  Schiemenz states that Systems I and IV are new structures, that
  System III arises in part, and Systems II and V wholly, from the
  silk-glands of the larva. As the glands differ much in the sexes,
  and in different species and genera, Schiemenz believes that their
  function is very manifold.

  In addition to those previously discovered by Schiementz, Bordas has
  detected two additional pairs of salivary glands in the worker and
  male honey-bee, _i.e._ the internal mandibular and sublingual
  glands, so that in Apis there are in all six pairs, and apparently
  one unpaired.

The delicate chitinous external layer of the gland is perforated by many
very fine pores through which the salivary fluid secreted by the
epithelial cells passes into the salivary duct. The glands are
externally bathed by the blood.

In many insects, including lepidopterous larvæ, the single median
opening of the salivary duct is converted into a spraying apparatus.

In the adult Lepidoptera, according to Kirbach:—

[Illustration:

  FIG. 329.—Salivary glands of the honey-bee: systems No. 1–3, × 15:
    _sv_, salivary valve (of systems 2 and 3) at base of tongue; _lp_,
    labial palpus; _mx_, maxilla; _so_, salivary opening of system 1 in
    hypopharyngeal plate; _no_, openings in plate for termination of
    taste-nerve; _œ_, œsophagus; _sd_, salivary duct; _b_, junction of
    ducts of system No. 2; _c_, junction of ducts of system No. 3; _sc_,
    _sc_, salivary sacs; _fl_, front lobe; _bl_, back lobe; _a_,
    chitinous duct, with spiral thread. _B_, single acinus of system No.
    1, × 70: _n_, nucleus; _st_, salivary tract; _d_, large duct. _C_,
    single pouch, or acinus, from system No. 2: _a_, propria or outer
    membrane; _sc_, secreting cells. _D_, termination of system No.
    3:_{1},_{2},_{3},_{4}, lines marking end of section; _d_, duct in
    section; _sc_, secreting cells in section; _n_, nucleus.—After
    Cheshire.
]

  “Its lower half forms a thick chitinous gutter, with a concave cover
  above, in which the similarly shaped upper half lies encased, so
  that between the two only a small semicircular opening remains.
  Powerful muscles extend from the cover to the lower side and to the
  two ridges of the bottom plate; through their contraction the upper
  channel is elevated, and presses out of the hinder part of the ducts
  into the space thus formed a great quantity of the saliva, which by
  allowing the contraction of the cover-muscle through the
  crevice-like opening, which is situated in the lower edge of the
  mouth-opening, becomes squeezed out in order either to mix with the
  fluid where the 2d maxillæ fuse, passing up into the canal in the
  proboscis, or to penetrate into and thus dilute the semi-fluid or
  solid substances taken, into the proboscis.”

  The morphology and general relations of the salivary glands have
  been sketched out by Hatschek, Patten, and by Lucas, from
  observations on those of the case-worms or larval Trichoptera.

[Illustration:

  FIG. 330.—Eight pairs of glands of Andrena: I, thoracic; II,
    postcerebral; III, supracerebral; IV, lateropharyngeal; V,
    mandibular; VI, internomandibular; VII, sublingual; VIII, lingual;
    _Md_, mandible; _L_, tongue; _o_, eye; _œ_, œsophagus; _J_,
    honey-sac.—After Bordas.
]

  Patten states that the spinning-glands in Neophylax are formed by a
  pair of ectodermal invaginations on the ventral side of the embryo,
  between the base of the 2d maxillæ and the nervous cord. They
  increase rapidly in length, and “they also unite to form a common
  duct, which opens at the end of the upper lip.”

  The salivary glands in the same insect are “formed by invagination
  of the ectoderm on the inner sides of the mandibles, in the same
  manner as are the spinning glands.”

  Lucas has shown that in trichopterous larvæ (Anabolia) there are
  three pairs of salivary glands in the head, which are serially
  arranged. The first pair belong to the mandibular, the second pair
  to the 1st maxillary, and the third pair, or spinning glands, to the
  2d maxillary segment. The first or mandibular glands open into the
  mouth at the base of the mandibles directly behind the dorsal
  condyle. The second pair open between the 1st and 2d maxillæ; at the
  base of the latter, near the ventral condyle of the mandibles. The
  third pair open into the hypopharynx, which is modified to form the
  spinneret. Lucas agrees with Korschelt in regarding them as modified
  coxal glands, Schiemenz having previously regarded the headglands of
  the imago of the bee as belonging to the segments bearing the three
  pairs of buccal appendages, so that each segment originally
  contained a pair of glands. It is thus proven that the silk-glands
  are modified salivary glands adapted to the needs of spinning larvæ,
  and indeed in the imago the sericteries revert to their primitive
  shape and use as salivary glands.

  The serial arrangement of the salivary glands in the Hymenoptera,
  where the number varies from five to ten pairs, is clearly proved by
  Bordas. He has detected five more pairs than were previously known,
  and names the whole series as follows:—1, the thoracic salivary
  glands, which are larger than the others, and nine other pairs,
  which are all contained in the head as follows: 2, postcerebral; 3,
  supracerebral; 4, lateropharyngeal; 5, mandibular; 6,
  internomandibular, situated on the inner side of the base of
  mandible; 7, sublingual; 8, lingual (these and 1 to 7 common to all
  Hymenoptera); 9, paraglossal (in Vespidæ); 10, maxillary (very
  distinct in most wasps). These glands do not all occur in the same
  species, being more or less atrophied.

  Bordas further shows the segmental arrangement of the cephalic
  glands by stating that the supracerebral glands correspond to the
  antennal segment, the sublingual glands to the labial, the
  mandibular glands (external and internal) to the mandibular segment,
  the maxillary glands to the 1st maxillary segment, the lingual
  glands to the 2d maxillary segment, while the thoracic and
  postcerebral salivary glands, he thinks, correspond to the ocular
  segment, a view with which we are indisposed to agree, although
  conceding that each of the six segments of the head has in it at
  least one pair of salivary glands.

  =Functions of the different salivary glands in Hymenoptera.=—The
  secretion of the thoracic glands is feebly alkaline. The
  postcerebral salivary glands, considered by Ramdohr to be organs of
  smell, secrete, like the preceding, a distinctively alkaline fluid,
  which mingles with the products of the thoracic glands. The
  supracerebral glands, also equally well developed in all
  Hymenoptera, though much atrophied in the females and especially the
  males of _Apis mellifica_, also in the Vespinæ and Polistinæ,
  secrete an abundant, feebly acid liquid, which is actively concerned
  in digestion.

  As to the mandibular glands, which Wolf supposed to be olfactory
  organs, their acid secretion, though smelling strongly, acts
  energetically on the food as soon as introduced into the mouth.

  The sublingual glands, atrophied in most Apidæ, but relatively
  voluminous in Sphegidæ, Vespinæ, Polistinæ, Crabronidæ, etc., empty
  their secretion into a small prebuccal excavation, where accumulate
  vegetable and earthy matters collected by the tongue, and the saliva
  secreted by these glands, acts upon them before they pass into the
  pharynx. The lingual glands secrete a thick, sticky liquid, which
  causes foreign bodies to adhere to the tongue, and also agglutinates
  alimentary substances. The uses of the other glands, maxillary and
  paraglossal, are from their minuteness undetermined. (Bordas.)




                   LITERATURE ON THE SALIVARY GLANDS


  =Leydig, F.= Zur Anatomie der Insekten. (Archiv Anat. und Phys. 1859.)

  —— Untersuchungen zur Anatomie und Histiologie der Tiere. Bonn, 1883,
    pp. 174, 8 Taf.

  —— Intra- und interzellulare Gänge. (Biolog. Centralblatt, x, 1890,
    pp. 392–396.)

  =Dohrn, A.= Zur Anatomie der Hemipteren. (Stettin. Entom. Zeit., 1866,
    salivary glands, pp. 328–332.)

  =Kupffer, C.= Die Speicheldrüsen von _Periplaneta orientalis_ und ihr
    Nervenapparat. (Beiträge zur Anatomie und Physiol., 1875.)

  =Schiemenz, P.= Ueber das Herkommen des Futtersaftes und die
    Speicheldrüsen der Biene. (Zeitschr. f. wissens. Zool., xxxviii,
    1883, pp. 71–135, 3 Taf.)

  =Korschelt, E.= Ueber die eigentümlichen Bildungen in den Zellkernen
    der Speicheldrüsen von _Chironomus plumosus_. (Zool. Anzeiger, 1884,
    pp. 189–194, 221–225, 241–246.)

  =Hofer, B.= Untersuchungen über den Bau der Speicheldrüsen und des
    dazu gehörenden Nervenapparates von Blatta. (Nova Acta d. Kais.
    Leopold.-Carol. Deutsch. Akad. d. Naturforscher, li, 1887, pp.
    345–395, 3 Taf.)

  =Knüppel, A.= Ueber Speicheldrüsen von Insekten. (Archiv für Naturg.,
    1887, Jahrg. 52, pp. 269–303, 2 Taf.)

  =Blanc, Louis.= La tête du _Bombyx mori_ à l’état larvaire, anatomie
    et physiologie. (Extrait des Travaux du Laboratoire d’Études de la
    Soie, 1889–1890; Lyon, 1891, p. 180, many figs.)

  =Bordas, L.= Anatomie des glandes salivaires des Hyménoptères de la
    famille des Ichneumonidæ. (Zool. Anzeiger, 1894, pp. 131–133.)

  —— Glandes salivaires des Apides, _Apis mellifica_. ♂ and ♀. (Comptes
    rendus Acad. Sc., Paris, cxix, pp. 363, 483, 693–695, 1894; also two
    articles in Bull. Soc. Philomath. Paris, 1894, pp. 5, 12, 66.)

  —— Appareil glandulaire des Hyménoptères. (Ann. Sc. Nat. Zool., xix,
    Paris, 1894, pp. 1–362, 11 Pls.) (See also p. 366.)

  =Berlese, Antonio.= Le cocciniglie Italiane viventi sugli agrumi.
    Firenze, 1896, 12 Pls. and 200 Figs.

  With the writings of Mark, Minot, Locy, List, Krassilstschik, Nagel
    (1896).


      _b._ The silk or spinning glands, and the spinning apparatus

The larvæ of certain insects, chiefly those of the Lepidoptera, possess
a pair of silk or spinning glands (sericteries) which unite to form a
single duct opening in the upper lip at the end of the lingua, which is
modified to form the spinneret. (See pp. 71, 75.) All caterpillars
possess them, and they are best developed in the silkworms, which spin
the most complete cocoon. Silk-glands also occur in the larvæ of the
Tenthredinidæ, in the case-worms or larval Trichoptera, also in certain
chrysomelid beetles (Donacia, Hæmonia), and in a weevil (Hypera). In a
common caddis-worm (Limnophilus) the glands are of a beautiful pale
violet-blue tint, and two and a half times as long as the larva itself;
viz. the body is 20 mm. and the glands 55 mm. in length.

In caterpillars the glands are of tubular shape, shining white, and much
like the ordinary simple tubular salivary glands of the imago. When only
slightly longer than the body they are twice folded, the folds parallel
and situated partly beneath and partly on the side of the digestive
canal; not usually, when folded in their natural position, extending
much behind the end of the stomach; but in the silkworms they are so
long and folded as to envelop the hinder part of the canal. In geometrid
caterpillars the glands when stretched out only reach slightly beyond
the end of the body; in Datana they are half again as long as the body.
Helm thus gives their relative length in certain Eurasian caterpillars,
and we add that of _Telea polyphemus_:—

 _Vanessa io_        length of body  32 mm.; of the silk glands  26 mm.
 _Smerinthus tiliæ_  length of body  63 mm.; of the silk glands 205 mm.
 _Bombyx mori_       length of body  56 mm.; of the silk glands 262 mm.
 _Antheræa yamamaya_ length of body 100 mm.; of the silk glands 625 mm.
 _Telea polyphemus_  length of body  60 mm.; of the silk glands 450 mm.

Thus in Telea the silk-glands are about 18.50 inches in length, being
about seven times as long as the body.

For the most complete accounts of the spinning glands of Lepidoptera and
their mechanism we are indebted to Helm and to Blanc, and for that of
the Trichoptera to Gilson.

The unpaired portion, or spinning apparatus (_filière_ of Lyonet), is
divided into two portions; the hinder half being the “thread-press,” the
anterior division the “directing tubes.” The silk material, stored up in
the thickest portion of the glands, passes into the thread-press (Fig.
334, _A_), which is provided with muscles which force the two double
ribbon-like threads through the directing tube, as wire is made by
molten iron being driven through an iron plate perforated with fine
holes. The entire spinning apparatus, or _filator_, as we may call it,
is situated in the tubular spinneret. The opening of the spinneret is
directed anteriorly, and the anterior end of the directing tube passes
directly into this opening so that the directing tube may be regarded as
an invagination of the lingua.

The silk thread which issues from the mouth of the spinneret is, as
Leeuwenhoek discovered, a double ribbon-like band, as may be seen in
examining the silk of any cocoon.

=The process of spinning.=—Since the appearance of Helm’s account,
Gilson, and also Blanc, have added to our knowledge of the way in which
the silk is spun and of the mechanism of the process. Gilson has
arrived, in regard to the function of the press or filator, at the
following conclusions: 1, the press regulates the thread, it compresses
it, gives it its flattened shape; 2, it regulates the layer of gum[52]
(grès) which surrounds the thread; 3, it may render the thread immovable
by compressing it as if held by pincers.

The process of spinning in the silkworm, says Blanc, comprises all the
phenomena by which the mass of silk contained in the reservoir is
transformed into the silk fluid of which the cocoon is spun. The
excretory canals each contain a cylindrical thread of silk having a mean
diameter of 0.2 mm. and surrounded by a layer of gum (_grès_) which in
the fresh living organ exactly fills the annular space situated between
the fibroin cylinder and the wall. Arrived within the common duct, the
two threads receive the secretion of Filippi’s gland, where the silken
fluid is formed, but has not yet assumed its definite external
characters. The two threads press through the common canal and arrive at
the infundibulum (Fig. 334, _c_) of the press, at the bottom of which is
situated the orifice of the spinning canal, almost completely divided
into two by the sharp edge of the rachis (Figs. 334, _a_, 335, _l_). The
threads each pass into one of the two grooves, and the layer of gum
(_grès_) fills the rest of the canal of the press or filator.

[Illustration:

  FIG. 331.—Longitudinal section of the spinneret: _a_, horizontal
    portion of the tongue; _b_, vertical portion; _c_, _f_, circle of
    the tongue; _d_, tongue-pad; _e_, orifice of the spinneret; _g_,
    body of the lyre; _h_, prebasilar membrane forming a fold; _i_,
    internal canal of the spinneret; _k_, filator.—After Blanc.
]

[Illustration:

  FIG. 332.—The lower lip (labium) of _Bombyx mori_, isolated, seen from
    the left side: _A_, lyre; _B_, spinneret; _C_, labial palpus; _D_,
    vertical part of the labium; _E_, horizontal part of the same; _H_,
    _L_, silk-canal; _K_, right gland of Filippi; _L_, canal of the left
    gland; _N_, labial nerve; _a_, oblique fibre of the elevator of the
    labium; _b_, right fibre of the same; _c_, depressor of the labium;
    _d_, superior spinning muscles.
]

[Illustration:

  FIG. 333.—The labium in a horizontal position, seen from the side:
    _f_, the filator or press situated under the external part of the
    spinneret (_d_), between the branches (_b_), of the lyre (_a_); _e_,
    labial palp; _c_, tongue.
]

[Illustration:

  FIG. 334.—Longitudinal section of the spinneret and press (_filator_):
    _A_, filator or press; _B_, spinneret; _C_–_D_, body of the lyre;
    _F_, lower part of the labium; _E_, common canal; _eh_, its
    epithelium; _G_, superior muscle of the press; _a_, rachis; _b_, its
    posterior enlargement; _c_, infundibulum; _d_, cuticle; _o_, orifice
    of the spinning canal; _op_, central canal of the lyre and of the
    spinneret; _fi_, hypodermis of the lyre; _f_, _f_, hypodermic pad of
    the lyre.
]

The silken substance is then pressed by the more or less powerful
contractions of the muscles of the filator, so that the passage of the
threads is facilitated. If the muscles totally contract, the spinning
canal is opened wide, the threads pass easily upwards and assume the
form of a triangular prism (Fig. 336).

[Illustration:

  FIG. 335.—Spinning apparatus, seen from above: _A_, opening of the
    spinneret; _B_, central canal of the spinneret (_C_); _D_, common
    canal; _E_, canal of Filippi; _F_, excretory canal of a silk-gland;
    _i_, orifice of the canal of Filippi’s gland; _l_, rachis; _k_, ring
    of the infundibulum; _b_, _c_, _d_, _e_, _f_, cavity of the
    different canals; _h_, spur which separates the two excretory
    canals.—This and Figs. 331–334 after Blanc.
]

If this contraction diminishes, the chitinous wall of the spinneret
comes together, owing to its elasticity; the ceiling of the canal
approaches the floor; the cavity tends to take the form of a
semicircular slit, and the threads are compressed, flattened. As each
mass or thread of silk is much more voluminous than the canal, except
when the latter is extremely dilated, it follows that the two threads
are always compressed, or squeezed together, and that each of them is
compelled to mould itself in the groove it occupies and to take its
shape. Hence the variations in the appearance of the two masses or
divided portions of silk, which as stated present all grades between the
form of an isosceles-triangular prism and that of a nearly flat ribbon;
but this last case is quite rare. The use of the spinneret, then, is to
compress the thread and to change its form more or less considerably, at
the same time as it diminishes its diameter.

[Illustration:

  FIG. 336.—Diagram of the press and its muscles: _a_, lower; _b_,
    lateral; _c_, upper muscles of the press.—After Blanc.
]

Moreover, this constant compression of the thread as it passes through
the press keeps it in a certain state of tension so as to allow the
caterpillar while spinning to firmly hold its thread.

Finally, when the worm suspends the contraction of its spinning muscles,
the press flattens, vigorously compresses the thread, and arrests its
motion, in such a way that if there was a strain on the silken fluid
(_bave_), it would break rather than oblige the caterpillar to let go
any more of it.

The press does not act directly on the silken thread, but through the
gummy layer (_grès_) which transmits over the whole surface of the
silken fluid (_brin_) the pressure exerted on it. After having overcome
this difficult passage, the silk thread has acquired its definite form;
it rapidly passes out of the spinneret.

=How the thread is drawn out.=—Having seen, says Blanc, how the two
masses of silk (_brins_), in passing through the spinning apparatus (or
press), join each other, constituting the frothy silken fluid, thus
becoming modified in form, it remains to examine the way in which the
thread is drawn out of the spinneret. If we examine a caterpillar while
spinning, it will be seen that in moving its head it draws on the frothy
mass of silk fixed to the web of the cocoon. This traction certainly
aids very much the exit of the thread, but it is not the only cause.

The silk, Blanc affirms, is pushed out by a force _a tergo_, developed
by different agents, such as the pressure of the distended cuticle or
the silky mass contained in the reservoir, as seen in the section of a
worm which has spun its cocoon. But if we consider a caterpillar before
it has begun to spin, it is difficult to explain the mechanism of
spinning. As Blanc has often observed, in making sections of the heads
of silkworms, two cases arise. Sometimes the worm has already spun a
little, and a certain length of the frothy silk (_bave_) issues from the
orifice of the spinneret, where it forms a small twisted bundle. At
other times the worm has not spun since its last moult or the frothy
mass of silk has broken within the head, and we find the end in the
common tube. In the first of these two cases, the worm, dilating its
press, is able by a general contraction to discharge a little of the
gritty material (_grès_) which lines the ball of silk hanging at the end
of the spinneret. It can also reject a certain quantity of the secretion
of Filippi’s glands and thus soften the gritty substance. The little
plug of silk can then adhere to the body with which it comes in contact.

In the same case it is necessary that the two bits or portions of silk
traverse the press, and this normally has a calibre less than their
diameter. The worm should then distend the spinning tube as much as is
practicable, so as to make the openings as large as possible. It has
been stated that the press is, in this condition, at least as large as
the mass of frothy silk. This Blanc believes (although Gilson thinks
otherwise) is pushed by a force _a tergo_, and reaches the funnel of the
spinning canal; its two bits of silk (_brins_) unite there, penetrate
into the canal itself, and, owing to successive impulses produced by the
general contractions of the worm, press through and pass out of the
spinneret.

While the silkworm is engaged in spinning its cocoon, the spinneret and
press execute very varied movements, determined by the elevator,
depressor, retractor, and protractor muscles of the labium, as well as
those of the press. These movements, originally very numerous, may
combine among themselves, so that the spinneret is susceptible of
assuming during the process of spinning still more diverse positions.

[Illustration:

  FIG. 337.—Portion of the silk-gland of _Bombyx mori_: _p_, tunica
    propria; _i_, tunica intima; _s_, secretion-cell with branched
    nuclei; _a_, separate secretion-cell from the anterior part of the
    silk-gland of _Amphidasis betularia_; _b_, the same of _Vanessa
    urticæ_; _c_, the same in _Smerinthus tiliæ_.—After Helm.
]

[Illustration:

  FIG. 338.—_A_, section of gland of lepidopter: _B_, section of
    silk-gland of a saw-fly larva; _n_, nucleus; _i.d_, canals; _d.s_,
    cavity.—After Gilson.
]

Histologically the silk-glands are composed of three layers,—the outer,
or _tunica propria_ (Fig 337); the inner, the _tunica intima_; the
middle layer being composed of extraordinarily large epithelial cells
which can be seen with the naked eye, and are also remarkable for the
branched shape of the nuclei (_a_, _b_, _c_, 337), the branches being
more or less lobed, and the larger the cells the more numerous are the
branches of the nucleus. Gilson[53] finds that those of Trichoptera,
Lepidoptera, Diptera, and Hymenoptera ordinarily consist of a small
number of cells; and it is quite common, he says, to find only two cells
in a transverse section (Fig. 338, _A_). In the Tenthredinidæ, however,
“the organ still consists of a tube, the wall of which is composed of
flat cells, but in addition to that, two series of spheroidal cells are
attached to the sides. Each of these cells contains a system of tiny
canals running through their cytoplasm (_B_, _i. d_). These cells are
the secreting elements; they continually cast the silk substance into
the tube.” A peculiarity of the _tunica intima_ is its distinct
transverse striation.

[Illustration:

  FIG. 339.—Branching nucleus of spinning gland of Pieris larva.—After
    Korschelt, from Wilson.
]

[Illustration:

  FIG. 340.—Filippi’s glands (_G_) isolated and seen from above: _e_,
    _e_, its lobules; _d_, its excretory canal; _E_, silk-duct; _C_,
    common canal; _c_, upper spinning muscle; _b_, lower muscle; _a_,
    lateral muscle; _T_, spinneret.—After Blanc.
]

The lining of the glands and of their common duct is moulted when the
caterpillar casts its skin, and this, as well as the mode of
development, shows that the glands are invaginations of the ectoderm.
Gilson finds that the silk-glands and silk-apparatus of Trichoptera are
very similar to those of caterpillars, and that the silk is formed in
the same way.

=Appendages of the silk-gland (Filippi’s glands).=—In most larvæ there
is either a single or a pair of secondary glands which open into the
spinning glands near their anterior end. They are outgrowths of the
gland provided with peculiarly modified excretory cells or evaginations
of the entire glandular epithelium. Those of _Bombyx mori_ (Fig. 340)
are very well developed, and, according to Blanc, form two whitish,
lobulated masses in the labium on each side of the common duct of the
spinning gland. Externally they appear to be acinose; but their
structure, as described by Blanc and by Gilson, is very peculiar. Helm
thinks, with Cornalia, that the function of these glands is to secrete
the adhesive fluid which unites the silk threads, and also to make the
silk more adhesive in the process of spinning, but Blanc states that
this is done before the thread passes into the common excretory tubes,
and he is inclined to think that the secretion serves to lubricate the
spinneret, and thus to facilitate the passage of the thread. On the
other hand, in certain caterpillars these glands are situated quite far
from the spinning apparatus.

  The silk-glands in the pupa state undergo a process of degeneration,
  and finally completely disappear. They are specific larval organs
  evolved in adaptation to the necessity of the insect’s being
  protected during its pupal life by a cocoon. (Helm.)

  Morphologically the silk-glands are by Lang regarded as modified
  coxal glands, and homologues of the setiparous parapodial glands of
  chætopod worms, the coxal glands of Peripatus, and the spinning
  glands of spiders.

  In Scolopendrella, spinning glands are situated in the two last
  segments of the body, opening out at the end of the cercopods (Fig.
  15, _s.gl_), and the larvæ of the true Neuroptera (Chrysopa,
  Myrmeleon, etc.) which spin cocoons, have spinning glands opening
  into the rectum. The silk forming the cocoon of the ant-lion, as
  Siebold and the older observers have stated, is secreted by the
  walls of the rectal or anal sac. Siebold (Anatomy of the
  Invertebrates, p. 445) states that in the larva of Myrmeleon, the
  silk-apparatus is very remarkable, “for the rectum itself is changed
  into a large sac and secretes this substance which escapes through
  an articulated spinneret projecting from the opening of the
  anus”[54] (Fig. 307, _e_). The larvæ of the Mycetophilidæ have
  spinning glands at the hinder end of the body, as also the imago of
  the female of the tineid moth Euplocamus. (Kennel.) The larvæ of
  ichneumons, wasps, bees, of Cecidomyia, and other Diptera, spin
  silken cocoons, but their glands have not yet been examined.

  It should also be observed that during the process of pupation the
  larvæ of butterflies, of certain flies (Syrphus), and beetles
  (Coccinellidæ and some Chrysomelidæ) attach themselves by silk spun
  from the anus, so that the pupa is suspended by its tail; such
  glands are probably homogenetic with the coxal glands.

  The silk in its fluid or soft state is mucilaginous, and according
  to Mulder, in the silkworm consists of the following substances,
  varying somewhat in their relative proportions by weight:

                       Silk-fibre material 53.67
                       Glue (Leim)         20.66
                       Protoplasm          24.43
                       Wax                  1.39
                       Coloring matter      0.05
                       Fat and resin        0.10




                   LITERATURE ON THE SPINNING GLANDS


  =Helm, E.= Anatomische und histiologische Darstellung der
  Spinndrüsen der Schmetterlingsraupen. (Zeitschr. f. wissens. Zool.,
  xxvi, 1876, pp. 434–469, 2 Taf.)

  =Lidth de Jeude, Th. W. van.= Zur Anatomie und Physiologie der
  Spinndrüsen der Seidenraupe. (Zool. Anzeiger, 1878, pp. 100–102.)

  =Engelmann, W.= Zur Anatomie und Physiologie der Spinndrüsen der
  Seidenraupe. (Onderz. Phys. Lab. Utrecht, iii, 1880, pp. 115–119.)

  =Joseph, G.= Vorläufige Mitteilung über Innervation und Entwickelung
  der Spinnorgane bei Insekten. (Zool. Anzeiger, 1880, pp. 326–328.)

  =Poletajew, N.= Ueber die Spinndrüsen der Blattwespen. (Zool.
  Anzeiger, 1885, pp. 22–23.)

  =Meinert, Fr.= Contribution à l’anatomie des fourmilions. (Overs.
  Danske Vidensk. Selsk. Forh. Kjöbenhavn, 1889, pp. 43–66, 2 Pls.)

  =Blanc, Louis.= Étude sur la sécrétion de la soie et la structure du
  brin et de la bave dans le _Bombyx mori_. Lyon, 1889, pp. 48, 4 Pls.

  —— La tête du _Bombyx mori_ à l’état larvaire. Anatomie et
  physiologie. (Extrait du volume des Travaux du Laboratoire d’Études
  de la Soie. Années 1889–1890, Lyon, 1891, pp. 180, 95 figs.)

  =Gilson, G.= Recherches sur les cellules sécrétantes. La soie et les
  appareils séricigènes: I. Lépidoptères. (La Cellule, 1890, vi, pp.
  115–182, 3 Pls. I, Lépidoptères (suite); II, Trichoptères. Ibid., x,
  pp. 71–93, 1893, 1 Pl.)

  =Garman, H.= Silk-spinning dipterous larvæ (Science, xx, 1893, p.
  215).

  Also the writings of Meckel, Pictet, Duméril, Klapálek,
  Wistinghausen, Loew, Hagen, Fritz Müller, Kolbe, McLachlan, de
  Selys-Longchamps.


                       _c._ The cæcal appendages.

These diverticula of the mid-intestine (“stomach”) are appended to the
anterior end, and in the living, transparent larva of Sciara, which has
two large, long, slender cœca (Fig. 341), the partly digested food may
be seen oscillating back and forth from the anterior end of the stomach
into and out of the base of each cæcum. In the Locustidæ (Anabrus, Fig.
299) and Gryllidæ (Fig. 344, _e_) there are two large, short cæca, and
in the locusts (Caloptenus) there are six cæca, while cockroaches have
eight. In the Coleoptera (Carabidæ and Dyticidæ) these large cæca appear
to be replaced by very numerous slender, minute villi or tubules, which
arise from the anterior part of the stomach (Figs. 317, _r_, also 342).

These cæca differ in structure from the stomach, as shown by Graber, as
well as by Plateau and by Minot. The latter states that a single
transverse section of one of the diverticula of the locust demonstrates
at once that its structure is entirely different from that of the
stomach.

[Illustration:

  FIG. 341.—Larva of Sciara: _s.gl_, salivary gland; _ur.t_, urinary
    tubes; _i_ intestine; _st_, stomach; _cae_ cæcal appendages; _t_,
    testis.
]

  Its inner surface is thrown up into longitudinal folds, generally
  twelve in number. These folds shine through the outer walls, and are
  accordingly indicated in the drawings of Dufour, Graber, and others.
  The entire cæcum has an external muscular envelope, outside of which
  are a few isolated longitudinal muscular bands. The folds within are
  formed mainly by the high cylindrical epithelium which lines the
  whole interior of the cavity. Tracheæ ramify throughout all the
  layers outside the epithelium. There are appearances of glandular
  follicles in the bottom of the spaces between the folds. (Minot.)

Burmeister supposed that these cæca were analogous to the pancreas, and
this view has been confirmed by Hoppe Seyler, Krukenberg, Plateau, and
others, who claim that the digestive properties of the fluid secreted in
them agrees with the pancreatic fluid of vertebrates.

[Illustration:

  FIG. 342—Cross-section of mid-intestine of _Acilius sulcatus_, showing
    the arrangement of the cæca, two tracheæ passing into each
    cæcum.—After Plateau.
]


        _d._ The excretory system (urinary or Malpighian tubes)

The excretory matters or waste products of the blood tissue of worms are
carried out of the body by segmentally arranged tubes called
_nephridia_. As a rule they arise in the blood sinuses of the body and
open externally through minute openings in the skin. As there is a pair
to each segment (in certain oligochete worms two or three pairs to a
segment), they are often called segmental organs. In the annulate worms
each segment of the body, even the cephalic or oral segment, originally
contains a pair of these excretory organs. These vessels may have
survived in myriopods and perhaps do exist in insects as urinary tubes,
and also occur in many of the Arachnida, and thus are characteristic of
each important class of land arthropods, but are either wanting or are
very rudimentary or much modified in the marine classes, notably the
Crustacea and Merostomata (Limulus), where they are represented by the
shell-glands of Copepoda, green glands of the lobster, and the brick-red
glands of Limulus.

[Illustration:

  FIG. 343.—Digestive canal of _Perla maxima_: _l_, upper lip; _mh_,
    buccal cavity; _ap_, common end of salivary ducts (_ag_); _o_,
    œsophagus; _s_, _s_, salivary glands, arranged segmentally; _b_,
    cæca of chyle-stomach; _lg_, their ligaments of attachment; _mp_,
    urinary tubes; _r_, rectum; _af_, anal orifice.—After Imhof, from
    Sharp.
]

In the earliest tracheate arthropod, Peripatus, these tubes are well
developed and are highly characteristic, each segment behind the head
bearing a pair (Fig. 4, _so_{4}_-_so_{9}_). It has been suggested by
some, but not yet proved, that the urinary tubes of insects are
morphologically the same as the segmental organs of worms and of
Peripatus; but there are no facts directly supporting this view, and, as
Sograff states, it is a pure hypothesis and can only be confirmed or
disproved by very detailed researches on the development of the urinary
tubes of myriopods and of insects. Others regard them as probably
homologous with the tracheæ, since they have a similar origin. As,
however, they arise in the embryo as outgrowths of the proctodæum they
may have arisen in myriopods and insects independently, and not be
vermian heirlooms.

While in worms and in Peripatus a pair of these segmental organs occur
in each segment, in insects this serial arrangement is not apparent;
those with a purely excretory function are not segmentally arranged,
with outlets opening externally, but arise as outgrowths of the
hind-intestine or proctodæum of the embryo, not being segmentally
arranged. The place of their origin is usually the dividing line between
the mid and hind intestine (Fig. 343, _mp_); this applies to
Scolopendrella (Fig. 15, _urt_) as well as to insects.

The urinary tubes are usually long, slender, blind, tubular glands
varying in number from two to over a hundred, which generally arise at
the constriction between the mid and hind intestine, and which lie
loosely in the cavity of the body, often extending towards the head, and
then ending near the rectum (Figs. 301, 310, _vm_). They were first
discovered by the Italian anatomist Malpighi, after whom they were
called the Malpighian tubes. While at first generally regarded as
“biliary” tubes, they are now universally considered to be exclusively
excretory organs, corresponding to the kidneys of the higher animals.

[Illustration:

  FIG. 344.—Digestive canal and appendages of the mole-cricket; _a_,
    head: _b_, salivary glands and receptacle; _c_, lateral pouch; _d_,
    stomatogastric nerves; _e_, anterior lobes of stomach; _f_, peculiar
    organ; _g_, neck of stomach; _h_, plicate part of same; _i_, rectum;
    _k_, anal gland; _m_, urinary tubes.—After Dufour, from Sharp.
]

Usually arising from the anterior end of the hind-intestine where it
passes into the mid-intestine, in certain forms they shift their
position, in some Hemiptera (Lygæus, Cimex) opening into the rectum,
while in the Psyllidæ they arise from the slender hinder part of the
mid-intestine, being widely separated at their origin. (Fig. 321.)

The length varies in different groups; where they are few in number (two
to four, six to eight), they are very long, but where very numerous they
are often short, forming dense tufts, each tuft connecting with the
intestine by a common duct (ureter), or, as in the mole-cricket, the
numerous tubes empty into a single duct (Fig. 344); in the locusts
(Acrydiidæ), however, they are arranged in 10 groups, each group
consisting of about 15 tubes, making about 150 in all; and are much
convoluted and wound irregularly around the digestive canal, and when
stretched out being about as long as the entire body.

The urinary tubes occur in twos, or in multiples of two, though a
remarkable exception is presented in the dipterous genera Culex and
Psychodes, in which there are five tubes; the young and fully grown
larvæ, as well as the pupa and imago of Culex, having this number (Fig.
433, _mg_.)

  In many insects (Pentatoma, Cimex, Velia, Gerris, Haltica, Donacia,
  and often in caterpillars), the vessels open into a sort of urinary
  bladder connecting with the intestine on one side.

[Illustration:

  FIG. 345.—_A_, section of urinary tube of Periplaneta; _B_, part of
    tube of Perla; _p_, peritoneal membrane; _c_, cavity or lumen;
    _n_, nucleus of a secreting cell.—After Schindler.
]

  In the larvæ of some insects the blind ends of the tubes are often
  externally bound to the rectum, in the silkworms being attached by
  fine threads to the intestine, while in some flies (Tipula and
  Ctenophora), two vessels may unite to form a loop. In all larval
  Cecidomyiæ, the two tubes are united to form a loop which curves
  backward, opening near the vent, the proctodæum being very short.
  (Giard.)

[Illustration:

  FIG. 346.—Portion of a urinary tube of _Calliphora vomitoria_: _tr_,
    trachea; _l_, lumen; _k_, nucleus.—After Gegenbaur.
]

  While usually the urinary vessels form simple tubes, in many species
  of Lepidoptera and Diptera they are branched, thus resembling those
  of spiders and scorpions. Moreover, in many Lepidoptera and Diptera
  (Fig. 308), the tubes are not simple, but are lobulated, and in some
  Hemiptera (Pentatoma, Notonecta, and Tettigonia) are twisted or
  lace-like. In rare cases there are two kinds of urinary tubes; in
  _Melolontha vulgaris_, two of them are partly lobulated and yellow,
  while the other two are simple and white. Their color in beetles
  varies, some being whitish or yellowish; in Geotrupes, Dyticidæ,
  Hydrophilidæ, etc., reddish brown; in Gryllotalpa as well as
  _Locusta viridissima_, there are two different kinds of vessels,
  differing in contents and in color (white or yellow), as well as
  histologically. (Schindler.)

  The exterior of the tubes is richly provided with tracheæ, which
  often form a web around them, and the fine branches often seem to
  attach them to the intestine. In Acheta they are enveloped by a very
  delicate, loose network of muscular fibres. (Schindler.)

  The urinary tubes consist, according to Schindler, of at least three
  cellular layers (Fig. 345):—

  1. An external, connective, nucleated membrane, the peritoneal
  membrane.

  2. A very delicate homogeneous basal membrane, the _tunica propria_.

  3. A single layer of large polygonal excretory cells.

  4. Lining the internal canal a chitinous layer penetrated by
  pore-canals, the _intima_ often wanting.

  The secretory cells are usually of the same size, but in many cases
  are relatively small; sometimes four to six or more form the
  periphery of the canal, sometimes three or only two. In some insects
  the cells are so very large that a single cell forms the entire
  periphery. The nuclei in the Lepidoptera (Papilio, Pontia, Cossus)
  are large and irregularly branched.

  The excretions of the Malpighian vessels, derived from the blood and
  from the fat-body, are more or less fluid and granular, sometimes
  pulpy. From the cells they pass into the canal, thence into the
  intestine, and thence out of the body. How, says Kolbe, the
  secretion passes into the intestine, whether by the contraction of
  the fine fibrillæ of the peritoneal membrane, or by the external
  pressure of the other organs, or by the pressure of the secretory
  matter behind, is not yet known. Grandis observed in living
  Hydrophilus that the urinary tubes moved, without the muscles
  seeming to show what caused the motion. Moreover, the cells
  incessantly changed their form. At a lower temperature such motions
  ceased. The tracheæ, ending freely in the cells, did not anastomose.
  (Kolbe.)

  The different colors of the tubes (white, yellow, red, brown, or
  green) is due to the hue of the excretions, and is independent of
  the color of the blood and of the urinary substances held in the
  secreted matter.

  Schindler found that insects of different stages, collected in
  winter, differed very much in their urinary secretions, the tubes in
  the adults being entirely empty, while in the larvæ they were filled
  full, so that he concluded that in the former the process of
  excretion during the winter hibernation is very slow, but in the
  latter very rapid.

  As to the activity of the urinary vessels the following experiments
  will throw some light. Tursini fed a Pimelia with fuchsin; its
  urinary tubes were consequently colored red. Schindler fed insects
  with indigo-carmine, which was excreted by the urinary tubes;
  Kowalevsky arrived at the same results, which seems to prove that
  these vessels are analogous to the kidneys of vertebrates. Moreover,
  Schindler injected through the side of the first abdominal segment
  into the cavity of the body of a Gryllotalpa a concentrated solution
  of sodium salt of indigotin-disulphonic acid. After one or two hours
  the external portion of the epithelium of the urinary vessels was
  stained deep blue, while the inner portion remained of the normal
  transparency; the nuclei being for the most part deeply stained.
  Between one and two days after, the staining matter had not yet
  wholly passed through the central canal, the surface recently
  stained still appearing light blue.

The solid contents of the urinary tubes consist partly of crystals,
which occur singly in the epithelial cells, or form scattered masses
when situated in the central canal. Besides tabular rhombic crystals,
there occur concretions which contain uric acid, and probably consist of
urate of soda, also octahedral crystals of chloride of soda, and
quadro-pyramidal crystals of oxalate of lime. Also acicular prisms
occur; besides chloride of soda, phosphates, carbonate of lime, oxalate
of lime in quantity, leucine, coloring matters, etc.; while the fluid
secretion also contains urea (?), uric acid, and abundant urates; uric
acid crystals were precipitated by the addition of acetic acid, and by
adding hydrochloric acid crystals belonging to the dimetric system were
formed. The often numerous spheroidal small granules are biurate of soda
and biurate of ammonia. Pale, concentrically banded concretions are
leucine pellets.

  According to Kölliker the contents of the urinary vessels[55] in
  general are: (1) round granules of urate of soda and urate of
  ammonia; (2) oxalate of lime; and (3) pale transparent concretions
  of leucine. Crystals of taurin are also said to occur. (Claus’
  Zoölogy, p. 531.)

  Although uric acid is characteristic of the urinary tubes, yet
  sometimes it is wanting in them, while uric acid substances in
  quantity occur in the fat-body or in the mid-intestine.

  In the living insect the urinary tubes remove urates from the blood;
  “the salts are condensed and crystallized in the epithelial cells,
  by whose dehiscence they pass into the central canals of the tubules
  and thence into the intestine.” (Miall and Denny.)

  The process of excretion is carried on not only by the urinary
  tubes, but also, as Cuénot has recently shown (1896) in Orthoptera,
  by the pericardial cells and certain cells of the fat-bodies. In the
  last-named cells urates are stored throughout life; the pericardial
  cells apparently secrete but do not store waste products, which are
  finally eliminated by the urinary tubes, the latter constantly
  eliminating waste.

  =Primitive number of tubes.=—Wheeler considers the primitive number
  of urinary tubules to be six, other authors regarding two pairs as
  the primary or typical number; and while Wheeler agrees that the
  more ancestral tracheate arthropods had but a single pair,
  Cholodkowsky supposes the primitive number in insects themselves to
  be a single pair. This view is strengthened by the fact that
  Scolopendrella has but a single pair (Fig. 15).

  While Peripatus has no urinary tubes, in Myriopods a single pair
  arises, as in insects, from the hind-intestine.

[Illustration:

  FIG. 347.—Section of proctodæum of embryo locust, showing origin of
    urinary tubes (_ur.t_); _ep_, epithelial or glandular layer; _m_,
    cells of outer or muscular layer; _a_, section of a tube.
]

  When in insects the number of these tubes is few, they are, with
  rare exceptions, arranged in pairs, so that Gegenbaur and others
  have considered this paired arrangement as the primitive one. When
  the tubules are very numerous in the adult, as in Orthoptera, the
  embryos and larvæ have a much smaller number, Wheeler stating that
  “in no insect embryo have more than three pairs of these vessels
  been found.” We have observed 10 primary tubes in the embryo of
  Melanopus (Fig. 347), from each of which afterwards arise 15
  secondary tubules. In the Termites, only, do the young forms have
  more urinary tubes than the adults.

  In Campodea there are about 16 urinary tubes and in Machilis either
  12 (Grassi) or 20 (Oudemans); but in other Thysanura the number is
  much less, Lepisma having either four, six, or eight, according to
  different authors, and both Nicoletia and Lepismina having six,
  opening separately into the hind-intestine. On the other hand, these
  organs have not yet been detected in Japyx. Whether they exist at
  all in the Collembola, which are degenerate forms, is doubtful. The
  weight of opinion denies their existence, though they may yet be
  found existing in a vestigial condition. They are said by Tullberg
  and by Sommer to exist in Podura, but are of peculiar shape.

  Coming now to the winged insects, in what on the whole is perhaps
  the lowest or most generalized order, the Dermaptera, the number is
  over 30, and their insertions regularly encircle the intestine.
  (Schindler.) In the most ancient and generalized family of
  Orthoptera, the Blattidæ, Schindler detected from 60 to 70 tubes,
  but in a nymph of Periplaneta not quite 10 mm. in length he found
  from 16 to 18, and in nymphs 4 to 5 mm. long there were only eight
  vessels; while Wheeler has found in the embryo of _Phyllodromia
  germanica_ but four tubes. In the adult Acrydiidæ there are as many
  as 150, in the Locustidæ between 40 and 50, and in the Gryllidæ
  about 100.

  The Ephemeridæ with about 40, the Odonata with 50 to 60 tubules, the
  Perlidæ with from 50 to 60, are polynephrious; while the Termitidæ
  and Psocidæ are oligonephrious, the former having from six to eight
  and the Psocidæ only four tubes. So also all the other orders not
  mentioned, except the Hymenoptera, have few of these tubes. The
  Hemiptera, with none in Aphidæ, a single pair in the Coccidæ, and
  two in all the rest of the order, have the fewest number.

  In the Neuroptera there are from six to eight, while in a larva,
  possibly that of Chauliodes, Wheeler finds the exceptional number of
  seven.

  The closely allied order Mecoptera (Panorpidæ), and also the
  Trichoptera, agree with the Neuroptera (Sialis) in having six.
  According to Cholodkowsky all Lepidoptera have six of these vessels,
  except Galleria, which has but four. He finds that in _Tinea
  biselliella_ (also _T. pellionella_ and _Blabophanes rusticella_)
  the larva has six vessels, which, however, undergo histolysis during
  pupation, a single pair arising in their stead. On this account he
  regards the primitive number of urinary tubes as two, or a single
  pair, this return from six vessels in the larva to two in the imago
  being considered a case of atavism.

  In the Coleoptera, the number of urinary tubes is from four to six;
  in what few embryo beetles have been examined (Doryphora,
  Melolontha), there are six vessels, but in the embryo of _Dyticus
  fasciventris_, Wheeler has detected only four, this number being
  retained in the adult. He thinks that in beetles in general, a pair
  of vessels must be “suppressed during post-embryonic development,
  presumably in early larval life.”

  In Diptera and Siphonaptera, the number four is very constant, there
  being, however, a fifth one in Culex and Psychoda (Fig. 400.)

  The number of these vessels is very inconstant in the Hymenoptera,
  varying from six (Tomognathus, an ant, worker) to 12 (Myrmica), and
  in Apis reaching the number of 150.

  In the embryo of the honey-bee and wall-bee (Chalicodoma), there are
  only four; we still lack any knowledge of the number in embryo
  saw-flies.

The following is a tabular view of insects with few urinary tubes
(Oligonephria) and many (Polynephria). It will be seen that the number
has little relation to the classification or phylogeny, insects so
distantly related as the Orthoptera and Hymenoptera being
polynephrious:—


                             _Oligonephria_

 Collembola, 2 (Podura), Tullberg and also Sommer.
 Thysanura, 4 (Lepisma); in Campodea, 16; in Machilis, 12 or 20; wanting
    in Japyx.
 Psocidæ, 4.
 Termitidæ, 6 (many in the young, Rathke).
 Mallophaga, 4.
 Physapoda, 4.
 Hemiptera, 2 (Coccidæ, none in Aphidæ).
 Neuroptera, 6–8. (In Sialidæ and Rhaphididæ 6; in Myrmeleonidæ and
    Hemerobiidæ, 8).
 Trichoptera, 6.
 Mecoptera, 6.
 Lepidoptera, 2–4–6 (2 in Tinea, Tineola, and Blabophanes; in Pterophorus
    and Yponomeuta, 4).


                     _Coleoptera_, 4–6; never more.

                             4
                      Carabidæ,
                      Dyticidæ,
                      Staphylinidæ,
                      Gyrinidæ,
                      Palpicornes,
                      Lamellicornes,
                      Cantharidæ,
                      Buprestidæ
                      (in larva, 6; in beetle, 4).

                             6
                      Byrrhidæ,
                      Nitidulidæ,
                      Dermestidæ,
                      Cleridæ,
                      Meloidæ,
                      Pyrochroidæ,
                      Bruchidæ,
                      Bostricidæ,
                      Cerambycidæ
                      Chrysomelidæ,
                      Coccinellidæ.

  Diptera, branching into 4 (Gegenbaur); in Culicidæ, and Psychoda, 5.
  Siphonaptera, 4.


                             _Polynephria_

 Orthoptera, 100–150. (In embryo Blattids, 4; in embryo locust, 10; in
    nymph of Gryllotalpa, 4.)
 Dermaptera, “over 30” (Schindler).
 Perlidae, 50–60.
 Plectoptera (Ephemeridæ), 40.
 Odonata, 50–60.
 Hymenoptera, 20–150. (In embryo bees only 4; Cynipidæ, Ichnenumonidæ,
    and Formicidæ have the smallest number, 6–12.)

  Here should be mentioned the singular fact discovered by Koulaguine
  that in the larva of Microgaster, the urinary tubes have no
  connection with the intestine, but open dorsally on the outside of
  the body on each side of the anus. Ratzeburg had stated that the
  last segment of the body was in the form of a vesicle. Koulaguine
  now shows that this vesicle is in reality the end of the intestine
  opening upwards; as the result of this dorsal opening of the
  intestine the Malpighian vessels open on the sides of the oval vent,
  and have no connection with the intestinal canal. Whether this is of
  morphological import, or is only a secondary adaptation, Koulaguine
  does not state, his paper being a preliminary abstract.

Wheeler thus sums up our present knowledge regarding the number and
homologies of the Malpighian or urinary tubes:

  1. It is very probable that the so-called Malpighian vessels of
  Crustacea and Arachnida are not the homologues of the _vasa
  Malpighi_ of the Eutracheata (insects and myriopods).

  2. The Malpighian vessels of the Eutracheata arise as paired
  diverticula of the hind-gut and are, therefore, ectodermal.

  3. In no insect embryo are more than six vessels known to occur;
  although frequently only four are developed.

  4. The number six occurs either during embryonic or post-embryonic
  life in members of the following groups: Apterygota, Orthoptera,
  Corrodentia; Neuroptera, Panorpata, Trichoptera, Coleoptera,
  Lepidoptera, and Hymenoptera.

  5. The number four seems to be typical for the Corrodentia,
  Thysanoptera, Aphaniptera, Rhynchota, Diptera, and Hymenoptera.

  6. The embryonic number in Dermaptera, Ephemeridea, Plecoptera, and
  Odonata has not been ascertained, but will probably be found to be
  either four or six.

  7. There is evidence that in at least one case (Melolontha), the
  tetranephric is ontogenetically derived from the hexanephric
  condition by the suppression of one pair of tubules.

  8. It is probable that the insects which never develop more than
  four Malpighian vessels have lost a pair during their phylogeny.

  9. The post-embryonic increase in the number of Malpighian vessels
  in some orders (Orthoptera, Odonata, Hymenoptera) is secondary and
  has apparently arisen to supply a demand for greater excreting
  surface.[56]




              LITERATURE ON THE EXCRETORY (URINARY) ORGANS


  =Malpighi, M.= Dissertatio epistolica de Bombyce, Societati regiæ
    Londini ad scientiam naturalem promovendam institutæ dicata.
    (Londini, 1669, 12 Pls.)

  =Herold, M. J. D.= Entwicklungeschichte der Schmetterlinge. 1815.

  =Rengger, J. R.= Physiologische Untersuchungen über den tierischen
    Haushalt der Insekten. Tübingen, 1817, pp. 82.

  =Wurzer.= Chemische Untersuchungen des Stoffes in den Gallgefässen von
    _Bombyx mori_. (Meckel’s Archiv f. Physiol., iv, 1818, pp. 213–215.)

  =Gaede, H. M.= Physiologische Bemerkungen über die sogenannten
    Gallgefässe der Insekten. (Nova Acta Acad. Caes. Leopold.-Carolin.,
    1821, x, Pars II, pp. 186–196.)

  =Meckel, J. F.= Ueber die Gallen- und Harnorgane der Insekten.
    (Meckel’s Archiv, i, 1826, pp. 21–36.)

  =Audouin, J. V.= Calculs trouvés dans les canaux biliaires d’un cerf
    volant. (Ann. sc. nat., 2 Sér., 1836, v, pp. 129–137.)

  =Frey und Leuckart.= Anatomie und Physiologie der Wirbellosen. 1843.

  =Dufour, L.= Mémoire sur les vaisseaux biliaires ou le foie des
    Insectes. (Ann. sc. nat., 1848, Sér. 2, xix, pp. 145–182, 4 Pls.)

  =Karsten, H.= Harnorgane von _Brachinus complanatus_. (Müller’s Archiv
    f. Anat. und Physiol., 1848, pp. 367–374.)

  =Fabre, J. L.= Étude sur l’instinct et les metamorphoses des
    Sphégiens. (Ann. d. sc. nat., 4 Sér., 1856, vi, pp. 137–189.)

  —— Étude sur le rôle du tissu adipeux dans la sécrétion urinaire chez
    les Insectes. (Ibid., 4 Sér., xix, pp. 351–382.)

  =Schlossberger, J. E.= Untersuchungen über das chemische Verhalten der
    Krystalle in den Malpighischen Gefässen der Raupen. (Archiv f. Anat.
    und Physiol., 1857, pp. 61–62.)

  =Leydig, F.= Lehrbuch der Histiologie. 1857.

  =Sirodot, S.= Recherches sur les sécrétions chez les Insectes. (Ann.
    sc. nat., 4 Sér., Zool., 1858, x, pp. 141–189, 251–334, 12 Pls.)

  =Kölliker, A.= Zur feineren Anatomie der Insekten (Ueber die
    Harnorgane, u.s.w.) (Verhandl. d. Physikal.-medizin. Gesellsch. in
    Würzburg, viii, 1858, pp. 225–235.)

  =Schindler, E.= Beitrage zur Kenntnis der Malpighischen Gefässe der
    Insekten. 3 Taf. (Zeitschr. f. wiss. Zool., xxx, 1878, pp. 587–660.)

  =Chatin, G.= Note sur la structure du noyau dans les cellules
    marginales des tubes de Malpighi chez les Insectes et les
    Myriapodes. (Ann. d. sc. nat., 6 Sér., xiv., 1882, pp. 7, 1 Pl.)

  =Witlaczil, E.= Zur Anatomie der Aphiden. (Arbeiten a. d. Zool.
    Instit. d. Univers. Wien., iv, 1882, pp. 397–441, 3 Taf.)

  =Cholodkowsky, N.= Sur les vaisseaux de Malpighi chez les
    Lépidoptères. (Compt. rend. Acad. d. Sc., Paris, xcix, 1884, pp.
    631–633.)

  —— Sur la morphologie de l’appareil urinaire des Lépidoptères.
    (Archives de Biologie, 1887, vi, pp. 497–514, 1 Pl.)

  =Loman, J. C. C.= Ueber die morphologische Bedeutung der sogenannten
    Malpighischen Gefässe der echten Spinnen. (Tijdschr. Nederl. Dierk.
    Ver. (2) Deel 1, 1887, pp. 109–113, 4 Fig.)

  =Marchal, P.= Contribution à l’étude de la désassimilation de l’azote.
    L’acide urique et la fonction rénale chez les Invertébrés. (Mém.
    Soc. Zool. de France, 1889, iii, pp. 42–57.)

  =Kowalevsky, A. O.= Ein Beitrag zur Kenntnis der Exkretionsorgane.
    (Biol. Centralbl., ix, 1889–90, pp. 33–47, 65–76, 127–128.)

  —— Sur les organes excréteurs chez les arthropodes terrestres.
    (Congrès international de Zool., 2^{me} Session à Moscou, 1892, Pt.
    I, pp. 186–235, 4 Pls.)

  =Griffiths, A. B.= On the Malpighian tubules of _Libellula depressa_.
    (Proc. Roy. Soc., Edinburgh, 1889, xv, pp. 401–403, Figs.)

  =Grandis, V.= Sulle modificazioni degli epitelii ghiandolari durante
    la secrezione. (Atti Accad. Torino, 1890, xxv, pp. 765–789, 1 Pl.;
    Archiv Ital. Biol., 1890, xiv, pp. 160–182, 1 Pl.)

  =Koulaguine, N.= Notice pour servire à l’histoire du développement des
    hyménoptères parasites. (Congrès internat. de Zool., 2^{me} Session
    à Moscou, 1892, Pt. I, pp. 253–277.)

  =Sograff, Nicolas.= Note sur l’origine et les parentés des
    Arthropodes, principalement des Arthropodes trachéates. (Congrès
    internat. de Zool., 2^{me} Session à Moscou, 1892, Pt. I, pp.
    278–302.)

  =Giard, Alfred.= (Note on the urinary tubes of larval Cecidomyia.
    Annals Ent. Soc., France, lxii, 1893, pp. lxxx-lxxxiv, 1 Fig.)

  =Wheeler, William M.= The primitive number of Malpighian vessels in
    insects. (Psyche, vi, May-December, 1893, Parts 1–6, pp. 457–460,
    485–486, 497–498, 509–510, 539–541, 545–547, 561–564.)

  =Metalnikoff, C. K.= Organes excréteurs des insectes. (Bull. Acad.
    imp. Sci. St. Pétersbourg, 1896, iv, pp. 57–72, in Russian, 1 Pl.)

  See also the works of Straus-Dürckheim, Will (Müller’s Archiv. 1848,
    p. 502), Brugnatelli, Leidy, Dufour, Ramdohr, Basch, Davy, Grassi,
    Minot, Berlese, Adlerz, Marchal (Bull. Ent. Soc. France, 1896, p.
    257); Bordas (Appareil glandulaire des Hyménoptères, 1894), also C.
    R. Acad. Sc. Paris, 1897.


                           _e._ Poison-glands

Poison-glands are mainly confined to the stinging Hymenoptera, _i.e._
certain ants, and the wasps and bees, but also occur in the mosquito,
while many, if not most bugs, seem to instil a drop of poison into the
punctured wounds they make.

In the honey and other bees the poison apparatus consists of two
poison-glands whose secretion passes by a single more or less convoluted
efferential duct into the large poison-sac, and thence by the excretory
duct, which is enlarged at the base of the sting (Figs. 194, 195), out
through the sting by the same passage as the eggs. According to Carlet,
the poison apparatus of bees consists of two kinds of glandular organs,
of which one kind secretes a feebly alkaline fluid, the other an acid
product. The poison is only effective when both fluids are mixed. The
resultant venom is always acid. The action of this venom upon some
animals, as rabbits, frogs, and certain beetles, is slight; but the
domestic fly and the flesh-fly are immediately killed by it. The
inoculation of a fly with the secretion of one of the glands does not
produce death until after a considerable time, but death follows very
quickly if the same fly is subjected to a second inoculation, this time
with the secretion of the other gland. The alkaline glands are in bees
and all poisonous Hymenoptera strongly developed, but become vestigial
in those forms which sting their prey to serve as food for their larvæ.
The poison which the solitary sand and wood wasps and Pompilidæ inject
into their victims only paralyzes them.

[Illustration:

  FIG. 348.—The poison apparatus of Ichneumon: _T_, sting; _GA_ acid
    gland; _TG_, _R′_, its tubes opening into the common poison-sac or
    reservoir; _ce_, its efferent canal; _Ga_, the tubular alkaline
    gland; _R_, the glandular end; _a_, the reservoir; _ce_, its duct;
    _Gac_, the accessory gland—After Bordas.
]

[Illustration:

  FIG. 349.—Cephalic gland of Belostoma.
]

  Bordas has found both the alkaline gland (gland of Dufour) and the
  acid gland to occur in a hundred species of Hymenoptera, including
  not only Aculeata, but also Ichneumonidæ (Fig. 348), Tenthredinidæ,
  and they may be safely said to be of general occurrence. The acid
  gland consists of three parts, the glandular portion, the reservoir
  for the poison, and the secretory canal. The alkaline gland is an
  irregular tube, with a striated surface and without a reservoir. In
  most Hymenoptera there is still a third gland, which is unpaired,
  granular, rectangular or lanceolate, with a short filamentous duct
  which opens beside the orifice of the alkaline glands.

The poison in ants, wasps, and bees consists of two substances, _i.e._
formic acid and a whitish, fatty, bitter residue in the secretion of the
glands; the corroding active formic acid is the essential part of the
poison. (Will.)

In Melipona the sting and poison-glands are aborted; in certain ants
(Formica, Lasius, etc.) the sting is wanting, but the poison-sac is
extraordinarily large.

  Bordas finds in various species of Ichneumon three kinds of glands
  opening into the base of the sting. The first two correspond to the
  acid (Fig. 348, _G.A_) and alkaline (_G.A_) glands of bees and wasps
  (Vespidæ, etc.), and the third (_G.ac_) is situated between the two
  lateral muscular bundles which attach the base of the sting to the
  last abdominal segment. The poison-reservoir (Fig. 348, _V_) is
  recognized by its yellow color and diaphanous and striated
  appearance. It is situated on the left of the hind-intestine, a
  little in front of the rectum. The tubular gland (_Ga_) or alkaline
  gland of aculeate Hymenoptera is remarkably large; it is situated on
  the left side of the body. The accessory gland (_G.A_) is elongated,
  triangular, flat, its duct opening at the base of the alkaline
  gland; it is formed of small spherical cells. Bordas has met with
  well-developed poison-glands in forty species belonging to the
  Terebrantia, including that of Tenthredo, Emphytus, as well as
  various genera of Ichneumonidæ, but in all these species the
  accessory gland was wanting.

[Illustration:

  FIG. 350.—View from above of the cephalic gland of Belostoma, ×
    20.—This and Fig. 349 after Locy.
]

Under the name of cephalic glands (Fig. 349), Locy describes a pair of
glands in the head of Nepidæ. The epithelial or secreting cells are
8–sided (Fig. 350). “When these insects are irritated,” he says, “a
secretion is freely thrown out around the base of the beak, which
produces death very quickly when introduced on a needle point into the
body of an insect.” He infers that the cephalic glands may be the source
of this poisonous secretion. The poisonous salivary fluid of the larva
of Dyticus is referred to on p. 324.

That the mosquito injects poison into the wound it makes has been proved
by Macloskie, who discovering fine droplets of a yellow oily-looking
fluid escaping from the end of the hypopharynx, afterwards detected the
poison-glands. It appears that the two salivary glands are subdivided,
each into three lobes, the middle of which (Fig. 351, _pg_) differs from
the others in having evenly granulated contents and staining more deeply
than the others. Having examined the preparations, we agree with the
discoverer that these lobes secrete the poison. The poison is diluted by
the secretion of the salivary lobes, and the two efferent ducts, one
from each set of glands, “carry forward and commingle the
venomo-salivary products in the main duct; and the stream is then
carried by the main duct to the reservoir at the base of the
hypopharynx.”

[Illustration:

  FIG. 351.—_A_, median section of head, showing (_du_) the
    venomo-salivary duct, with its insertion in (_hy_) the hypopharynx;
    _cb_, brain; below is the pharyngeal pump, leading from (_œ_) the
    œsophagus; _lre_, base of labrum-epipharynx; _m_, muscle; _n_,
    commissure (other parts removed). _B_, the venomo-salivary duct,
    showing its bifurcation, and the three glands on one of its
    branches; _pg_, poison gland; _sg_, the upper of the two salivary
    glands. _C_, the bifurcation of the duct, with its nucleated
    hypodermis.—After Macloskie.
]


                     _f._ Adhesive or cement-glands

Dewitz has discovered in ants and bees, in close connection with the
poison-glands, and like them discharging their secretion through the
sting, cement-glands. They arise by budding at the base of the
poison-glands.

The two glands in these Hymenoptera correspond to the tubular glands of
the Orthoptera, which open at the base of the inner sheath of the
ovipositor (Fig. 299, _sb_), so that the secretion flows out through it
as the poison of bees, etc., out of the sting. The use of the secretion
of these glands is either to glue the eggs together, or to afford
material for the egg-case of cockroaches and Mantidæ and the gummy
egg-case of the locusts, etc. The contents of the cement-glands serves
for the fixture of the eggs after deposition. In the stinging
Hymenoptera one of the cement-glands is an accessory gland; the other
becomes the poison-sac. The cement-glands are in the Hemiptera only
short blind sacs, in the Lepidoptera and Diptera long convoluted tubes,
tubular and branched in the Coleoptera, or richly branched in the
Ichneumonidæ and Tenthredinidæ. In the cockroach there are two
cement-glands, but the right one is probably of no functional
importance. The left one is filled with a milky substance, containing
many crystals and a coagulable fluid, out of which the egg-capsule
(oötheca) is formed. (Miall and Denny.) In the locusts the sebific or
cement-gland (Fig. 298, _sb_) secretes a copious supply of a sticky
fluid, which is poured out as the eggs pass out of the oviduct and
agglutinates the eggs into a mass, forming a thin coating around each
egg, which from the mutual pressure of the eggs causes the tough coating
to be pitted hexagonally. In other insects also (Trichoptera,
Chrysopidæ, Lepidoptera, etc.) there are similar secretions for the
protection or fastening of the eggs when laid.[57] The Trichoptera lay
their eggs either in or on the surface of the water in bunches or in
strings or in annular gelatinous masses on stones or on plants. This
jelly-like substance is secreted by two highly developed paired anal
glands. (Weltner, in Kolbe, p. 621.) Also in certain dragon-flies
(Libellula, Diplax, and Epitheca) the eggs are laid in jelly-like
masses.

With a similar secretion, spun from the end of the abdomen, the Psocidæ
cover their little bunches of eggs laid on the under side of leaves; and
the silk thread forming the egg-sac of the great water-beetle
(Hydrophilus) is secreted from such anal glands.


                          _g._ The wax-glands

Besides the honey-bee, which secretes wax in little scales on the under
side of the abdomen, the bodies of many other insects, such as the plant
and bark lice, as well as the Psyllidæ, Cicadidæ (especially Flata and
Lystra), are covered with a waxy powder, or as in Chermes, Schizoneura,
Flata, etc., with wool-like filaments of wax.

[Illustration:

  FIG. 352.—Under side of worker honey-bee, carrying wax scales, ×
    3.—After Cheshire.
]

[Illustration:

  FIG. 353.—Nymph of Lachnus, showing position of wax-glands.—Gissler
    _del._
]

The wax is secreted by minute unicellular dermal glands, which in the
lower insects (Hemiptera) are distributed nearly all over the body, but
in the bees are restricted either to the under (Apis, Fig. 352) or upper
side (Trigona) of the end of the abdomen.

The wax-glands of Pemphigus, Chermes, etc., lie under the little warts,
seen in _Lachnus strobi_, the white-pine aphis, to be distributed in
transverse lines across the back and sides of the abdominal segments
(Fig. 353). These warts are surrounded by a chitinous ring, and divided
into delicately marked areas. Through the delicate numerous pits in the
chitinous membrane of these areas the little waxen threads project,
since under each area ends a duct leading from a large glandular cell,
which is a specially modified hypodermis cell (Claus). The wax threads
are hollow, and all those arising from a single glued cell form a
bundle, whose threads separate from each other and form a white woolly
down or bloom covering the body. Witlaczil also shows that gall-forming
Aphids secrete a wax-like substance, which, during the movements of the
insects in the gall, is rubbed off, becoming a watery layer mixed with
the fluid excrement, which forms a spherical impervious layer lining the
gall, and thus rendering possible the mode of life of the gall-lice.

In the Psyllidæ Witlaczil has discovered wax-glands which also secrete
slender waxen threads. They are situated in groups of two or three at
the end of the abdomen near the anus, and arise from hypodermis cells.
The wax threads surround the liquid excrement as it passes out of the
vent, covering it with a continuous layer of wax. The excrement
accordingly is discharged very slowly and gradually, in sausage-shaped
masses slightly strung together and rolled into close spirals. The body
becomes unavoidably smeared with the sticky excrement, since it is not
entirely covered by the waxy layer. Moreover, in the larvæ of many
Psyllidæ waxen threads are formed on the upper side of the abdomen; they
are for the most part tightly curled or frizzly, like wool, and form,
though partly torn, a waxen coat, chiefly on the side and back of the
thorax and abdomen. The insects appear therefore as if covered with
dust. The mature animals of many species are also covered with a waxen
down. The wax threads rapidly dissolve and disappear in alcohol. From a
wax-like substance more or less easily dissolved in alcohol arise
peculiar hair-like structures which, in the larvæ of Psyllidæ, are
situated on the side and end of the body and also on the rudiments of
the wings. They are readily distinguished from ordinary hairs, as they
arise from glandular cells, and are of very different lengths, more or
less like bristles, but hollow, and very brittle. They are leaf-like in
the first nymphal stages of _Trioza rhamni_, but in following stages
become narrow and form a row around the entire periphery of the body.

The waxen dorsal shield which protects the body of bark-lice (Coccidæ)
is a similar product.

[Illustration:

  FIG. 354.—Young nymph and developing scale of _Aspidiotus
    perniciosus_: _a_, ventral view of nymph, showing sucking beak with
    setæ separated, with enlarged tarsal claw at right; _b_, dorsal view
    of same, somewhat contracted, with the first waxy filaments
    appearing; _c_, dorsal and lateral views of same, still more
    contracted, illustrating further development of wax secretion; _d_,
    later stage of same, dorsal and lateral views, showing matting of
    wax secretions and first form of young scale; all greatly
    enlarged.—After Howard and Marlatt, Bull. 3, N. S., Div. Ent., U. S.
    Dept. of Agr.
]

  Witlaczil has described the way it is formed in Aspidiotus and
  Leucaspis. The freshly hatched nymph shows no signs of a waxy
  secretion. But eventually waxen threads arise first on the hinder
  and anterior end of the body, and then over the whole surface. These
  threads interlace into a sort of felting and thus form the shield,
  which is usually much larger than the body and lies closely upon it.
  The shield is formed after the first moult. It is noteworthy that
  these threads are matted together to form as thick a tissue as that
  of the shield itself. The shield is whitish or gray and rather thin.
  On the thinnest part of the edge the single threads may be drawn
  out. The growth of the shield advances with the increase in size of
  the nymph around the entire edge, but is greatest behind. The first
  two larval skins are retained on the back under the shield. Also a
  very thin waxen pellicle remains on the resting place of the insect
  when it is raised. The wax-glands open in the pitted fields, and
  appear as clear brownish cells which are distinguished from the
  ordinary hypodermis cells by their greater size. (Witlaczil. Compare
  also Fig. 354.)

[Illustration:

  FIG. 355.—Wax disks of social bees: _a_, _Apis mellifica_, worker;
    _b_, do., queen; _c_, Melipona, worker; _d_, Bombus, worker.—From
    Insect Life, U. S. Dept. Agr.
]

The wax-glands in the honey-bee are scale-shaped organs situated on the
under side of the four last abdominal segments (Fig. 355). These secrete
the wax, which appears as whitish scales, and secretion is only possible
when the bees have sufficient honey and pollen. The wax is secreted by
the hypodermal cells rather than by glands within the abdominal cavity;
the wax traverses the cuticular layer, and accumulates on its outer
surface (Carlet). According to Fritz Müller, in the stingless bees
(Trigona) which he observed, the wax-glands are situated on the back of
the abdomen, but Ihering states that in many species of Trigona and
Melipona there are also slightly developed wax-organs on the ventral
side.

  It has been found that certain caterpillars secrete wax. Thus the
  cells of the Tortrix of the fir (_Retinia resinella_) formed of
  resin are lined with wax, as on dissolving away the resin with
  alcohol, Dr. Knaggs found a slight film of wax; also a secretion of
  wax has been detected in the larva of a butterfly (_Parnassius
  apollo_). The bodies of certain saw-fly larvæ are covered with a
  white powdery secretion, while the remarkable larva of a Selandria
  is clothed with snow-white, long, flocculent, waxy masses, nearly
  concealing the body (Fig. 356).


                _h._ “Honey-dew” or wax-glands of Aphids

The so-called “honey-dew” of Aphids which oozes from two wart-like
tubercles or tubes situated near the end of the body, is secreted by
hypodermal unicellular glands which open into a modification of a
pore-canal, the tube itself being an outgrowth of the cuticula.

  Witlaczil states that both in the “honey” tubes and in the body
  beneath, the sugary matter exists in cells of the connective tissue
  in the form of granules. “These large ‘sugar-cells’ in contact with
  the air undergo destruction, while the sugar crystallizes into
  needles, and thus each cell is transformed into a radiated
  crystalline mass.”

  “A muscle extends from a horseshoe-shaped place (a valve?) in the
  middle of the flat terminal plate of the honey tube, through this
  and down through the abdomen to the ventral surface. By this muscle
  the honey tube is at times erected, and we then find, as also when
  we lightly press the body of the insect, lumps of crystallized sugar
  which have been expressed through the tips of the honey tubes.”
  (Zool. Anzeiger, 1882, p. 241.)

[Illustration:

  FIG. 356.—Wax-secreting larva of a saw-fly.
]

[Illustration:

  FIG. 357.—_Lachnus strobi_, and its two “honey” warts.—Gissler
    _del._
]

  Busgen, after careful research, denies that this is a sugar, but
  claims as the result of chemical analysis, that it is more like wax.
  He observed that on reaching the air the drops issuing from the
  “nectary” or “honey” tube stiffened almost instantly into a wax-like
  mass, which was easily crushed between the teeth, and had no taste
  at all. No sugar-like substance or urea could be detected. He
  therefore concludes that the secretion in question should be
  regarded as a wax-like mass, which agrees well with Witlaczil’s
  anatomical observations, and confirms the statements of previous
  observers. Thus, as early as 1815, Kyber stated that the Aphides
  expelled an excrementitious substance through the “sap tubes.”
  Burmeister states that the tubes give out a fluid which “dries
  gumlike, but, so far as I have observed, has no peculiar taste.”
  Réaumur, and also Kaltenbach, state that the “honey” does not issue
  from the tubes, but from the anus. Lastly, Forel emphatically states
  that “the two dorsal tubes of Aphides do not secrete a sweet fluid,
  but a gluey wax, which is not sought by the ants. Moreover the
  shield-lice and many leaf-lice have no such tubes, but yet are often
  sought by ants. The drops of sugar which the ants lick up are rather
  the excrement of the insects in question.” Hence the opinion first
  stated by Linné, that a sweet fluid is secreted by Aphides, must be
  abandoned.

  On the other hand, Busgen, after careful observations, finds that
  the use of the sticky, waxen secretion is in reality a protective
  one, as he observed that when a larval Chrysopa rudely attacks the
  Aphides, they smear its face with the sticky wax, causing at least a
  momentary interruption in its attacks. He also observed that Aphides
  when invaded by coccinellid larvæ set their tubes in motion and
  besmear their heads and front part of the body. He thus seems to
  establish the fact that these tubes secrete a protective, sticky
  fluid.


                     _i._ Dermal glands in general

We have seen that certain of the hypodermal cells may be modified or
specialized to form secretory unicellular glands. Such are those
(trichogens) which secrete chitinous setæ, hairs, and spines, certain
setæ in some insects being hollow and containing a poison (p. 187);
others secrete wax, certain ones in Aphids “honey-dew”; in some cases
dermal glands may excrete protective, sticky, or otherwise offensive
matters, or may be depuratory, or facilitate the process of moulting.

There are other minute, unicellular, or compound dermal glands whose
function is unknown.

Dermal glands may be segmentally or serially arranged. Thus Verson has
detected a series of one or two pairs of unicellular glands near the
stigmata in each thoracic, and the first eight abdominal segments of the
silkworm (_B. mori_). In the earliest stages of growth of the
caterpillar they give out oxalate of lime, and in later stages uric
acid. They thus appear to act interchangeably with the urinary tubes, as
excretory organs. They do not, however, carry their products directly
outwards, but leave them between the hypodermis and cuticula, in order
to facilitate the sloughing off of the latter in the process of
moulting.




                   LITERATURE ON THE SECRETORY GLANDS


                              _a._ General

  =Sirodot, S.= Recherches sur les sécrétions chez les Insectes. (Ann.
    sc. nat., 4 Sér., Zool., 1858, x, pp. 141–189, 251–328, 12 Pls.)

  =Gazagnaire, G.= Des glandes chez les Insectes. (Compt. rend. Acad.
    Sc., Paris, 1886, cii, pp. 1501–1503; Annal. Soc. Ent. France, 1886,
    Bull., pp. 104–106.)

  =Leydig, F.= Beitrage zur Anatomie und Histiologie der Insekten. 1887.

  =Hanow, Karl.= Ueber Kerfabsonderungen und ihre Benutzung im eigenen
    Haushalte. (Programm des Realprogymnasiums zu Delitzsch für das
    Schuljahr 1889, xc; Delitzsch, 1890, pp. 3–22.)

  =Verson, E.= Di una serie di nuovi organi escretori scoperti nel
    filugello. (Publ. R. Stazione Bacologica di Padova, v, 1890, pp. 30.
    4 Pls.)

  —— Altre cellule glandulari di origine postlarvale. (Ibid., vii, 1892,
    pp. 16, 1 Pl.)

  —— =ed E. Bisson=. Cellule glandulari ipostigmatiche nel _Bombyx
    mori_. (Ibid., vi, 1891.)

  =Borgert, H.= Die Hautdrüsen der Tracheaten. Jena, 1891, pp. 80.

  =Batelli, Andrea.= Di una particolarità nell’ integumento dell’
    _Aphrophora spumaria_. (Monitore Zool. Ital., 1891, Anno ii., pp.
    30–32, dermal gland in last segment.)

  =Koschewnikow, G. A.= On a new compound dermal gland found in the
    sting of the bee. (Journal of the Zoological Section of the Society
    of the Friends of Natural Science. Moscow, ii, Nos. 1, 2, 1892, p.
    36. Preliminary notice. In Russian.)

  =Willem, V., et H. Salbe.= Le tube ventral et les glandes céphaliques
    des Sminthurus. (Ann. Soc. Ent. Belg., 1897, xli, pp. 130–132.)

  =Henseval, Maurice.= Les glandes à essence der _Cossus ligniperda_.
    (La Cellule, 1897, xii, pp. 19–26, 27, 29.)

  —— Recherches sur l’essence der _Cossus ligniperda_. (La Cellule,
    1897, xii, pp. 169–181, 183.)


                           _b._ Poison-glands

  =Macloskie, George.= The poison-apparatus of the mosquito. (Amer.
    Nat., xxii, 1888, pp. 884–888. 1 Fig. Also in Science, 1887, p.
    106.)

  =Beyer, Otto W.= Der Giftapparat von _Formica rufa_, ein reduziertes
    Organ. (Jena. Zeitschr. f. Wissens, xxv, 1891, pp. 26–112, 2 Taf.)

  =Bordas, L.= Sur l’appareil venimeux des Hyménoptères. (Comtes rend.,
    cxviii, 1894, pp. 296–299 and 873–874; also Zool. Anzeiger, xvii
    Jahrg., 1894, pp. 385–387, Figs.)

  —— Appareil glandulaire des Hyménoptères. (Glandes salivaires; Tubes
    de Malpighi et glandes venimeuses.) Paris, 1894, pp. 362, 11 Pls.

  —— Description anatomique et étude histologique des glandes à venin
    des insectes Hyménoptères. Paris, 1897, pp. 53, 2 Pls.

  See Forel (p. 186); also the standard authors, Kolbe, etc.; also Locy
    (Amer. Nat., xviii, 1884, p. 355), Nagel (p. 324), Fenger.


                            _c._ Wax-glands

  =Brandt und Ratzeburg.= Medicinische Zoologie, ii, 1830, p. 179. Taf.
    xxv, Fig. 18.

  =Treviranus, George R.= Ueber die Bereitung des Wachses durch die
    Bienen. (Zeitschrift für Physiologie, etc., iii, 1832, pp. 62, 225.)

  =Dufour, L.= Note anatomique sur la question de la production de la
    cire des abeilles. (Comptes rend. Acad. Sc., Paris, 1843, xvii, pp.
    809–813, 1248–1253; Revue Zool. 1843, l’Institut, 1843, xi.)

  =Dujardin, F.= Mémoire sur l’étude microscopique de la cire, etc.
    (Ann. Sc. Nat., xii, 1849, pp. 250–259.)

  =Tarzione-Tozzetti, H.= Studii sulle cocciniglie. Milano, 1867, 7 Pls.

  —— Sur la cire qu’on peut obtenir de la cochenille du figuier (_Coccus
    caricæ_). (Comptes rend. Acad. Sc., Paris, lxv, 1867, pp. 246–247.)

  =Claus, C.= Ueber die wachsbereitenden Hautdrüsen der Insekten.
    (Sitzungsber. Gesells. z. Beförd. d. Gesammt. Naturw. zu Marburg,
    June, 1867, No. 8, pp. 65–72.)

  =Witlaczil, E.= Die Anatomie der Psylliden. (Zeitschr. wissens. Zool.,
    xlii, 1885, pp. 582–586.)

  —— Zur Morphologie und Anatomie der Cocciden. (Zeitschr. wissens.
    Zool., xliii, 1886, pp. 149–174, 1 Taf.)

  =Ihering, H. von.= Der Stachel der Meliponen. (Ent. Nachrichten, xii
    Jahrg., 1886, p. 185.)

  =Carlet, G.= Sur les organes sécréteurs et la sécrétion de la cire
    chez l’Abeille. (Comptes rendus, cx, pp. 361–363, 1890.)

  —— La cire et ses organes sécréteurs. (Le Naturaliste, 1890, pp.
    149–151, 2 Figs.)

  Also the works of Siebold, Cheshire, Kolbe, Howard and Marlatt (Bull.,
    3, N. S., Div. Ent. U. S. Dept. Agr., 1896, p. 40), Knaggs, Berlese.


                    _d._ Wax-like glands of Aphides

  =Huber et Forel, A.= Études myrmécologiques, 1875.

  =Witlaczil, E.= Zur Anatomie der Aphiden. (Zool. Anzeiger, v Jahrg.,
    1882, pp. 239–241; Arbeiten a. d. Zool. Institut der Univ. Wien.,
    iv, 1882, pp. 397–441, 3 Taf.)

  =Busgen, M. J.= Der Honigtau. Biol. Studien an Pflanzen u.
    Pflanzenlause. (Jena. Zeitschrift, xxv, 1891, pp. 339–428.)




                DEFENSIVE OR REPUGNATORIAL SCENT-GLANDS


While these eversible glands are not found in marine or aquatic
arthropods such as Crustacea or Merostomata (Limulus), they are often
present in the air-breathing forms, especially insects. In the winged
insects they are of frequent occurrence, existing under great variety of
form, varying greatly in position, and appearing usually to be in
immediate relation with their active volant habits. Their presence is in
direct adaptation to the needs and habits of their possessors, and being
repellent, warning, or defensive structures, the odors they secrete
being often exceedingly nauseous, they appear to have been called into
existence in direct response to their biological environment. The fact
that these singular organs do not exist in marine or aquatic Crustacea
suggests that the air-breathing, aërial, or volant insects by these
eversible glands, usually in the form of simple evaginable hypodermic
pouches, are enabled to protect themselves by emitting an infinitesimal
amount of an offensively odorous fluid or ether-like spray which charges
the air throughout an extent of territory which may be practically
illimitable to the senses of their enemies. The principle is the same as
in the mephitic sulphuretted oil ejected by the skunks, the slight
quantity these creatures give out readily mixing with and charging the
atmosphere within a radius of many miles of what we may call the centre
of distribution.

As is now well known, the very delicate, attenuated highly volatile
odors exhaled are perceived by insects with extreme ease and rapidity,
the degree of sensitiveness to such scents being enormously greater than
in vertebrates, their organs of sense being developed in a corresponding
degree. Professors Fischer and Penzoldt, of Erlangen, have recently
established the fact that the sense of smell is by far the most delicate
of the senses. They find that the olfactory nerve is able to detect the
presence of 1⁄2,760,000,000 of a grain of mercaptan.[58] The smallest
particle of matter that can be detected by the eye is sodium, when
observed by the spectroscope, and this particle is 250 times coarser
than the particle of mercaptan which can be detected by the human nose.

  In those Arachnida which are provided with poison-glands, these
  scent-glands are absent, but in certain Acarina and Linguatulidæ,
  which have no poison-glands, there are various oil-glands, stigmatic
  glands, as well as scent-glands, and in seizing a Thelyphonus with
  the forceps we have observed it to send out from each side of the
  body a jet of offensive spray.

  We not infrequently find in myriopods (Polydesmidæ, Julidæ, and
  Glomeris) repugnatorial or the so-called cyanogenic glands, which
  are either paired, opening on the sides of the body, or form a
  single row along the median line of the under side of the body.
  Leidy describes and figures the spherical glands of _Julus
  marginatus_, of which there are 50 pairs. These glands have been
  regarded as modified nephridia, but are more probably coxal glands,
  and the homologues of the parapodial glands of annelid worms.

[Illustration:

  FIG. 358.—Sternite of _Machilis maritima_, with the pair of coxal sacs
    (_cb_) on the right side everted; _hs_, coxal appendages; _m_,
    retractor muscles.—After Oudemans, from Lang.
]

=Eversible coxal glands.=—True coxal glands occur in _Scolopendrella
immaculata_ on the 2d to 11th segment, on the inner side of the base of
the legs (Fig. 15, _c.g._). Homologous glands also occur in the same
position in _Campodea staphylinus_ (also in _C. cookei_ and _C.
mexicana_) on the 1st to 8th abdominal segments, and Oudemans has
described a pair of eversible sacs on each side of segments one to seven
of Machilis. These eversible sacs in the synapterous insects are
evidently modified coxal glands, and are probably repugnatorial as well
as respiratory in function.

The apparatus consists of an eversible gland, composed of hypodermic
cells, usually retracted by a slender muscle and with an efferent
passage, but the glands vary greatly in shape and structure in different
insects. In some cases these fœtid glands appear not to be the
homologues of the coxal glands, but simply dermal glands.

These repugnatorial glands are of not infrequent occurrence in the lower
or more generalized winged insects, and in situation and appearance are
evidently the homologues of the coxal glands of the Symphyla and
Synaptera.

_Fœtid glands of Orthoptera._—In the ear-wigs (Forficula and Chelidura)
Meinert has detected a pair of what he calls fœtid glands at the
posterior margin of the dorsal plates of the 2d and 3d abdominal
segments.

Vosseler also describes the same glands as consisting of a retort-shaped
sac, in whose walls are numerous small hypodermal cells and large single
glandular cells provided with an efferent passage, the fluid being
forced out by the pressure of the dermal muscles, one acting specially
to retract the gland. The creature can squirt to a distance of 5 and
even 10 cm. (4 inches) a yellowish-brown liquid or emulsion with the
odor of a mixture of carbolic acid and creosote.

The large eversible dorsal glands of the Blattidæ, since they contain
numerous hairs, which, when everted, are fan-like or like tufts, serve,
as in the spraying or scent apparatus, to disseminate the odor, and
might be classified with the alluring unicellular scent-glands or
_duftapparat_ of other insects, as they are by some authors; but as the
glands are large and compound they may prove to be the homologues of the
coxal glands rather than of the dermal glands.

Evaginable organs in the Blattids were first observed by Gerstæcker in
both sexes of Corydia; they are yellowish white, covered with hairs, and
are thrust out from between the dorsal and ventral plates of the 1st and
2d abdominal segments.

[Illustration:

  _Fig. 359._—Under side of end of Aphlebia, showing the two eversible
    sacs; _V-X_, five last abdominal segments; _A_, portion showing the
    hairs; _B_, showing origin of a hair in its follicle.—After Krauss.
]

In the cockroach (_P. orientalis_) Minchin detected two pouch-like
invaginations of the cuticle, lying close on each side of the middle
line of the body between the 5th and 6th tergites of the abdomen. They
are lined by a continuation of the cuticle, which forms, within the
pouches, numerous stiff, branched, finely pointed bristles, beneath
which are a number of glandular epithelial cells. In the male nymph of
_P. decorata_ he also found beside these glandular pouches “an
additional gland, opening by a tubular duct under the intersegmental
membrane between the 5th and 6th terga above the glandular pouch of each
side, and extending forward into the body cavity. The gland and its duct
are proliferations of the hypodermis, and there is no invagination of
the cuticle.” These eversible glands are most complicated in
_Phyllodromia germanica_. While it is absent in the female, in the male
it is relatively of enormous size, extending over the 6th and 7th
somites, as well as projecting far into the body cavity (Minchin). Haase
states that these glands become everted by blood-pressure and give out
the well-known disagreeable smell of these insects. He states that in
the male of _P. germanica_ the dorsal glands in the 6th and 7th
abdominal segments are without hairs and produce an oily secretion; they
function as odoriferous organs in sexual union.

In the male of another Blattid (_Aphlebia bivittata_) of the Canary
Islands, Krauss has detected two yellowish dorsal sacs 1.5 mm. in
length, opening out on the 7th abdominal segment, and filled full of
long yellowish hairs, the ends directed towards the opening, where they
form a thick tuft. These eversible glands lined with hairs appear to be
closely similar to the long slender eversible hairy appendages or scent
organs of certain Arctian and Syntomid moths. (Fig. 359.)

[Illustration:

  FIG. 360.—External flaps (_gl_) of glands of Platyzosteria.
]

We have found the external median wart with lateral lids or flaps in
between the 5th and 6th tergites of _Platyzosteria ingens_ Scudder, a
large wingless Blattid living under the leaf scars of the cocoanut tree
in Southern Florida (Fig. 360), but were unable to detect them in
Polyzosteria or in the common Blabera of Cuba, or in another genus from
Cordova, Mexico.

In another group of Orthoptera, the Phasmidæ, occur a pair of dorsal
prothoracic glands, each opening by a pore and present in both sexes. In
the walking-stick, _Anisomorpha buprestoides_, ♂ and ♀, these openings
are situated on each side of the prothorax at its upper anterior
extremity, situated at the bottom of a large deep pit. When seized it
discharged a “milky white fluid from the pores of the thorax, diffusing
a strong odor, in a great measure like that of the common Gnaphalium or
life everlasting” (Peale in Say’s American Entomology, i, p. 84). Boll
states that the females when captured “spurt from the prothorax,
somewhat after the manner of bombardier beetles, a strong vapor, which
slightly burnt the skin; when the females were seized by the males a
thick fluid oozed from the same spot.” Scudder describes these glands in
another Phasmid (_Autolyca pallidicornis_) as two straight, flattened,
ribbon-like bodies, with thick walls, broadly rounded at the end, lying
side by side and extending to the hinder end of the mesothorax. In
_Anisomorpha buprestoides_ the glands are of the same size and shape
(Scudder). In _Diapheromera femorata_ the repugnatorial foramina are
very minute, and the apparatus within consists of a pair of small
obovate or subfusiform sacs, one on each side of the prothorax, about 1
mm. in length, with a short and very slender duct opening externally at
the bottom of the pit (Scudder).

In the Mantidæ these seem to be genuine coxal glands, as there is a pair
situated between the coxæ of the first pair of legs. An evaginable organ
like a wart, with a glandular appearance, occurs on the hind femora of
the Acrydiidæ in a furrow on the under side, into which the tibia fits,
about one-fourth from the base (Psyche, iii, p. 32).

In the male cricket, the anal odoriferous glands are small lobes opening
into a reservoir on each side of the rectum (Dufour). Homologous glands
also occur in the Coleoptera (Fig. 302, _l_ and 317, _s_).

[Illustration:

  FIG. 361.—Glands (_g_) of Lachnus; _h_, “honey” wart.—Gissler _del._
]

Most Hemiptera or bugs send out a fœtid or nauseous odor due to a fluid
secreted by a single or double yellow or red pear-shaped gland, situated
in the middle of the mesothoracic segment, and opening between the
hinder or third pair of coxæ. In Belostoma Leidy describes these glands
as consisting of two rather long cœcal tubes situated in the metathorax,
beneath the other viscera, extending backwards into the abdomen, and
opening between the coxæ of the third pair of legs. Locy states that the
smell arising from these glands is pleasant, resembling that of well
ripened pears or bananas. Other bugs, moreover, emit an agreeable odor,
that of Syromastes resembling that of a fine bergamot pear. (Siebold.)
The fluid given out by the European fire-bug (_Pyrrhocoris apterus_) has
a sweetish smell, like ether. In the nymph there are three pairs of
dorsal glands, on abdominal segments 2–5, which are atrophied in the
mature insect. In the bed-bug, the nymph has three odoriferous glands
each with paired openings in the three basal abdominal segments
respectively, and situated on the median dorsal line, being arranged
transversely at the edge of the tergites; but after the last moult these
are aborted, and replaced by the sternal metathoracic glands (Künckel).
Gissler has detected a pair of glands in _Lachnus strobi_ (Fig. 361).

=Anal glands of beetles.=—Certain beetles are endowed with eversible
repugnatorial glands. _Eleodes gigantea_ and _E. dentipes_ of both sexes
are said by Gissler to possess these glands. When teased “they stand on
their anterior and middle legs, holding the abdomen high up and spurting
the contents of the glands right and left.” The glands (Fig. 366, 1) are
two reddish brown, somewhat bilobed sacs, and extend from the base of
the last up to the middle of the 2d abdominal segment, with an average
length of 6.5 mm. The liquid stains the human skin, has an acid
reaction, with a peculiar, “intensely penetrant odor, causing the eye to
lachrymate. It is soluble in water, alcohol, and ether. Boiled with
concentrated sulphuric acid and alcohol an ethereal aromatic vapor is
produced, indicating the presence of one or more organic acids, though
neither formic or acetic acid could be detected.” Williston has observed
the same habits in seven other species of Eleodes, all ejecting a
pungent vile-smelling liquid, one species (_E. longicollis_) ejecting a
stream of fluid from the anal gland, backwards sometimes to the distance
of 10 cm. or more, and he regards these beetles as “the veritable skunks
of their order.” Leidy briefly describes the odoriferous glands of _Upis
pennsylvanica_.

The anal glands consist, according to Meckel and also Dufour, of two
long, simple, flexuous cœca with reservoirs having two short excretory
ducts situated near the anus (Siebold).

Glands like those of Eleodes found in _Blaps mortisaga_ are described in
detail by Gilson (Fig. 366, 2). They form two pouches or cuticular
invaginations situated in the end of the abdomen on the sides of the end
of the intestine and unite on the median line underneath the genital
organs, forming a very short tube with a chitinous wall, continuous with
the cuticle of the last abdominal segment. Into each pouch open a large
number of fine slender lobules varying in shape, giving a villous aspect
to the surface. These lobules are composed of as many as fifty
unicellular glands, each of which is composed of four parts: (1) A
radiated vesicle, (2) a central sac, giving rise (3) to a fine excretory
tube, and (4) a sheath near the origin of the excretory tube. These are
all modifications of the cytoplasm of the cell with its reticulum; the
nucleus with its chromosomes is also present, but situated on one side
of the central sac. The fine excretory tubules form a bundle passing
down into the mouth of each lobule.

  Similar glands, though usually smaller, which have not been
  carefully examined, occur in Carabus (Fig. 300, 3) and Cychrus,
  which eject from the vent a disagreeable fluid containing butyric
  acid (Pelouse). The bombardier beetle Brachinus, with its anal
  glands, ejects a jet of bluish vapor accompanied with a considerable
  explosion, which colors the human skin rust red; it is caustic,
  smells like nitrous acid, and turns blue paper red. Westwood states
  that individuals of a large South American Brachinus on being seized
  “immediately began to play off their artillery, burning and staining
  the flesh to such a degree that only a few specimens could be
  captured with the naked hand, leaving a mark which remained for a
  considerable time.” The fluid ejected by another species, in
  Tripoli, blackened the fingers of the collector. “It is neither
  alkaline nor acid, and it is soluble in water and in alcohol.”
  (Kirby and Spence, iv, p. 149.)

  Species of other genera (Agonum, Pheropsophus, Galerita, Helluo,
  Paussus, Ozæna) are also bombardiers, though less decidedly so than
  Brachinus. A Paussid beetle (Cerapterus) ejects explosively a fluid
  containing free iodine (Loman), while Staphylinus, Stenus, Ocypus
  olens, Lacon, etc., have similar anal fœtid glands, the liquid being
  more or less corrosive. The secretion of _Mormolyce phyllodes_ is so
  corrosive that it is said to paralyze the fingers for 24 hours
  after. (Cuénot.)

[Illustration:

  FIG. 362.—Median section through the femoro-tibial joint of leg of
    Coccinella, showing at _o_ the opening through which the blood oozes
    out; _f_, femur; _t_, tibia; _e_, extensor muscle of the tibia: _s_,
    sinew of the same; at _ch_, chitinized; _h_, articular membrane;
    _v_, tibial process.—After Lutz.
]

The two pairs of remarkably large, soft, eversible, forked,
orange-yellow glands of the European genus Malachius, are thrust out
from the side of the 1st and the 3d thoracic segments. They are everted
by blood-pressure, and retracted by muscles. The larva of _Hydrophilus
piceus_ ejects by the anus a black, fœtid fluid.

Claus has shown that the larva of _Lina populi_ and other Chrysomelidæ
possess numerous minute, eversible glands in each of the warts on the
upper surface of the body, each gland containing a whitish, repellent
fluid smelling like the oil of bitter almonds and containing salicylic
acid derived from its food-plant, which issues as pearl-like drops.
Candèze thinks the fluid may contain prussic acid. The fluid is secreted
by a variable number of glandular cells, each provided with an efferent
duct. The larvæ of saw-flies, notably _Cimbex americana_, also eject
droplets of a clear fluid from non-evaginable glands situated near each
stigma (Chlolodkovsky).

=The blood as a repellent fluid.=—In this connection it may be mentioned
that though there are no special glands present, many beetles emit drops
of blood from the femoro-tibial joints of their legs as a means of
defence. Such are the oil-beetles (Meloë), Cantharis, Lytta. The
cantharadine secreted by these beetles, according to Beauregard, is an
efficient means of defence, as birds, reptiles, and carnivorous insects
will not usually attack them. This substance is formed in the blood and
also in the genital organs, and is so extremely caustic that scavenger
insects which feed upon their dead bodies will leave untouched the parts
containing cantharadine, and if May-beetles or mole-crickets are washed
with the blood of Meloë or with cantharidate of potassa, it will for
several days render them safe from the attacks of the carabids which
usually prey upon them. The eggs even after deposition are strongly
vesicant, and are thus free from the attacks of egg-eating insects
(Cuénot). The Coccinellidæ are also protected by a yellow, mucilaginous,
disagreeable fluid oozing out of the ends of the femora; in our common,
two-spotted lady-bird (_C. bipunctata_) the yellow fluid is
disagreeable, smelling like opium. Lutz has found that the blood in
Coccinellidæ passes out through a minute opening situated at the end of
each femur (Fig. 362). The blood is very repellent to insectivorous
animals.

The Dyticidæ eject from the anus a colorless, disagreeable fluid, while
these beetles, and especially the Gyrinidæ, when captured send out a
milky fluid which appears to issue from the joints of the body. The
Silphidæ throw out both from the mouth and vent a fœtid liquid with an
ammoniacal odor. They possess but a single anal gland, the reservoir
opening on one side of the rectum (Dufour).

Other malodorous insects have not yet been investigated; such are the
very persistent odors of lace-winged flies (Chrysopa).

More agreeable secretions, but probably formed by similar glands, is the
odor of rose or hyacinth given out by Cicindelæ, or the rose fragrance
exhaled by the European _Aromia moschata_.

=Eversible glands of caddis-worms and caterpillars.=—Gilson, while
investigating the segmentally disposed thoracic glands of larval
Trichoptera, has found in the larva of _Limnophilus flavicornis_ that
the sternal prothoracic tubercle gives exit to an underlying tubular
gland. In _Phryganea grandis_ each thoracic sternum affords an exit to
an eversible gland. Many caterpillars, as our subjoined list will show,
are very well protected by eversible repugnatorial glands situated
either in the under or upper side of the body. Since the time of De Geer
(1750) the fork-tailed larva of Cerura has been known to throw out a
secretion, which was described by Bonnet in 1755 as a true acid, sharp,
sour, and biting. This spraying apparatus in _Cerura (Harpyia) vinula_
has been well described by Klemensiewicz (Fig. 366, 4), though Rengger
in 1817 noticed the general form of the secretory sac, and that it opens
out in two muscular eversible tubes, out of which the secretion is
ejected.

The fork-tailed larva of _Macrurocampa marthesia_, which is much like
that of Cerura, when teased sends out a jet of spray to the distance of
nearly an inch from each side of the neck. While examining the very
gayly-colored and heavily-spined caterpillars of _Schizura concinna_ we
observed that when a fully-grown one was roughly seized with the forceps
or fingers it sent out a shower of spray from each side of the
prothoracic segment, exactly like that of Cerura and Macrurocampa.

In the European _Cerura vinula_ the apparatus consists of a single sac,
which opens by a narrow transverse slit on the under side of the neck,
out of which is rapidly everted four lateral tubes, two on each side
(Fig. 366, 4, _t_), which are withdrawn within the opening by the
contraction of several fine muscles. The apparatus in the American _C.
multiscripta_ is as in the European _C. vinula_. In a living specimen
the large secretory sac was seen to be of the same size and shape as in
Macrurocampa, and of the color of raw silk. The sac when distended
extends back to a little behind the middle pair of legs, and is nearly
two-thirds as wide as the body. The caterpillar sent out the fluid when
handled, but we could not make it spray.

In the larva of _Macrurocampa marthesia_ the cervical or secretory gland
(Fig. 366, 5) is situated in the 1st and 2d thoracic segments, extending
to the hinder edge of the latter and lying between the nervous cord and
the œsophagus and proventriculus, and when empty the bulk of it lies a
little to one side of the median line of the body. It is partly held in
place by small tracheæ, one quite large branch being sent to it from
near the prothoracic spiracle. The short, large duct, leading from it to
the transverse opening in the membrane between the head and prothoracic
segment, is a little narrower than this opening, and is kept distended
by tænidia, or a series of short, spiral threads which are pale, not
honey-yellowish, in color. This duct lies on one side of the prothoracic
ganglion, resting just under the commissures passing up to the brain; it
is also situated between the two silk ducts.

The very distensible sac (Fig. 366, 5) is rendered elastic by a curious
arrangement of the cuticle, the tænidia of the duct itself being
represented by very thickly-scattered, irregular, separate, sinuous,
chitinous ridges, which stand up from the cuticular lining of the wall
of the sac (Fig. 366, 6). The secretory cells of the walls of this sac
in _Cerura vinula_ are said by Klemensiewicz to be large hexagonal
cells, resembling those of silk-glands, having like them large branched
nuclei.

The fluid thrown out is said by Poulton to be formic acid; it causes
violent effervescence when allowed to fall upon sodium-bicarbonate, and
colors blue litmus paper red. It also appears from the researches of
Latter that these creatures in the imago state secrete free potassium
hydroxid, a substance for the first time known to exist in the animal
kingdom.

In the caterpillar of _Astyanax archippus_ (_Limenitis disippus_) a
dark, bladder-like sac is everted, but the lateral tubes appear to be
wanting, and no spray is sent out; it occurs in the larvæ of many
Nymphalidæ and other butterflies and moths.

These glands are functionally active in Perophora, but obsolete (at
least the external openings) in Lacosoma.

=The osmeterium in Papilio larvæ.=—The caterpillars of the swallowtailed
butterflies (Papilio, Doritis, and Thais), as is well known, when
irritated thrust out from a transverse slit on the upper part of the
prothoracic segment a large orange-yellow V-shaped fleshy tubular
process (the osmeterium), from which is diffused a more or less
melonlike but disagreeable, in some cases insufferable, odor; the
secretion is acid and reddens litmus paper. The mechanism has been
described and figured by Klemensiewicz.

When at rest, or retracted, the osmeterium lies in the upper part of the
body in the three thoracic segments, and is crossed obliquely by several
muscular bundles attached to the walls of the body, and by the action of
these muscles the evagination of the osmeterium is strongly promoted.
After eversion the tubes are slowly retracted by two slender muscles
inserted at the end of each fork or tube, and arising from the sides of
the 3d segment behind the head, crossing each other in the median line
(Fig. 366, 7 _r.m._). The secretion is formed by an oval mass of
glandular cells at the base of the forks; in the glandular mass is a
furrow-like depression about which the secretory cells are grouped. The
secretion collects in very fine drops on the side of each furrow
opposite the glandular cells.

According to C. D. Ash the larva of an Australian Notodontid (_Danima
banksii_ Lewin) protrudes from the under side of the prothoracic segment
a Y-shaped red organ like that of Papilio; no fluid or odor is given
out.

=Dorsal and lateral eversible metameric sacs in other larvæ.=—The showy
caterpillars of Orgyia and its allies have a conspicuous coral-red
tubercle on the back of the 6th and also the 7th abdominal segment,
which on irritation are elongated, the end of the tubercles being
eversible. When at rest the summit is crateriform, but on eversion the
end becomes rounded and conical. These osmeteria are everted by blood
pressure, and retracted by a muscle. Fig. 366, 9, represents a section
of an osmeterium of _Orgyia leucostigma_ when retracted by the muscle
(_m_); at the bottom of the crater are the secreting or glandular cells
(_gc_), being modified hypodermal cells. These doubtless serve as
terrifying organs to ichneumons and other insect enemies, and though we
have been unable to detect any odor emanating from the tubercles, yet
possibly they give out a scent perceived by and disagreeable to their
insect assailants.

[Illustration:

  FIG. 363.—Freshly hatched larva of _Hyperchiria io_, with its two
    pairs of eversible glands (_g_).
]

[Illustration:

  FIG. 364.—Young larva of _Megalopyge crispata_, enlarged, showing the
    seven pairs of lateral processes (_lp_): _sp_, spiracle; _abl′_,
    _abl^6_, six pairs of abdominal legs besides the anal pair.
]

In the Hemileucidæ there is a pair of lateral osmeteria, on the 1st and
on the 7th abdominal segments, which however, are not highly colored
(Figs. 363, 366, 10). In Megalopyge (Lagoa, Fig. 364) there is a lateral
row of singular pale permanently everted processes which appear to be
the homologues of the osmeteria of larvæ of other lepidopterous
families. As these are repeated on seven segments, their metameric
arrangement is obvious. The relation of these curious glands to the
viscera is seen in Fig. 297, _lgp_, and their minute structure in Fig.
365.

  At _A_, the lumen (_l_) is a deep narrow cavity, with the secretion
  (_secr._), collected at the mouth of the cavity, composed of a thin,
  mucus-like, coagulated fluid, containing granules of varying degrees
  of fineness, which take the stain readily. Outside of these are
  collected fine nuclei (_bc_), stained dark, and enveloped in a
  slight, transparent, pale, protoplasmic envelope, which may be blood
  corpuscles. The glandular cells themselves are simply modified
  hypodermal cells, as seen at _C_. In some of the nuclei, indistinct
  nucleoli are seen, and deeply stained granules, especially around
  the periphery of the nuclei. At _B_ is represented a section on one
  side of the middle, but still showing the spacious lumen. In the
  section represented by _C_, the knife passed through the process
  still nearer the outer edge, and near the base; at _C^1_, three of
  the glandular cells, with their large, deeply stained nuclei, are
  drawn. A transverse section at _D_ shows the large lumen or cavity
  (_l_).

  As to the function and homologies of these structures, it is
  difficult to decide. We have never noticed that they give off any
  odor, though they may prove to be repugnatorial; they are not
  visible in the fully grown, living insect, being concealed by the
  long, dense hairs clothing the body; they are not spraying organs,
  as they are imperforate at the end, not ending as the lateral,
  eversible glands of _Hyperchiria io_, etc., in a crateriform
  orifice.

  They may be permanently everted glands, or osmeteria, which have, by
  disuse, lost their power of retraction and their crateriform
  opening, as well as the power of secreting a malodorous fluid.

[Illustration:

  FIG. 365.—Section of lateral processes of larva of Megalopyge.
]

In certain of the butterflies, the Heliconidæ (Colænis, Heliconius,
Euides, and Dione), there is thrust from the end of the abdomen a pair
of large, irregular, rounded, eversible glands, which give out a
disagreeable odor, and are consequently repellent, and which seem to be
the homologues of the odoriferous glands of other butterflies.

  The large, soft, rounded, eversible glands, looking like puff-balls
  or a rounded pudding (Fig. 366, 12), are everted, when the
  butterflies are roughly seized, from the dorsal side of the
  penultimate segment of the abdomen. The males possess two smaller
  tubercles on the inside of the anal claspers or lobes. Müller also
  detected, in the females of various species of the Heliconidæ
  enumerated above, a pair of club-shaped processes like the balancers
  of flies, which are thrust out on each side of and under the
  odoriferous puff-balls of the hinder edge of the penultimate segment
  (Fig. 366, 13). The club or head is armed with hairs or bristles,
  which, in Heliconius, are like the scales of a butterfly.

[Illustration:

  FIG. 366.—Scent-glands of insects: 1. Anal eversible glands of
    Eleodes.—After Gissler. 2. Anal eversible glands of Blaps.—After
    Gilson. 3. Anal glands (_agl_) of _Carabus hortensis_: _rs_,
    reservoir; _d_, excretory duct; _i_, intestine; _r_, rectum.—After
    Kolbe. 4. Prothoracic spraying apparatus of _Cerura vinula_: _gl_,
    the gland; _d_, its duct, with tænidia; _t_, the spraying tubes;
    _m_, muscles; _rm_, retractor muscles.—After Klemensiewicz. 5. The
    thoracic glandular sac of _Macrurocampa marthesia_: _gl_, the
    glandular sac; _d_, its duct; _e_, peritracheal epithelium; _t_, the
    spiral threads or tænidia. 6. Irregular separate masses of chitinous
    ridges on the cuticular lining of the wall of the sacs of
    _Macrurocampa marthesia_. 7. Osmeterium (_os_) of the larva of
    _Papilio machaon_ at rest: _rm_, the retractor muscles at the ends;
    _m_, the numerous oblique muscles; _dm_, dorsal longitudinal
    muscles; _t_, trachea; _oe_, œsophagus; _gang_, brain; 1, head; 2,
    3, 4. thoracic segments. 8. Osmeterium (_os_) of one side, enlarged:
    _g_, glandular portion at the base; _d_, depressions in the cuticula
    of the glandular portion; _t_, trachea.—This and Fig. 7 after
    Klemensiewicz. 9. Eversible dorsal glands (_ev. gl_) of larva of
    _Orgyia leucostigma_ in Stage II: _gc_, glandular cells at bottom of
    the crater-like depression; _m_, retractor muscle; _p_, poison
    gland-cells of the root of the seta (_s_); _c_, cuticula; _hyp_,
    hypodermis; _A_, portion of the cuticle and hypodermis enlarged. 10.
    Lateral eversible gland of _Hyperchiria io_, Stage II: _rm_,
    retractor muscle; _oen_, œnocytes. 11. The same as Fig. 10, but
    representing a section through one side of the eversible gland. 12.
    _A_, end of body of _Colænis julia_; _ev_, eversible anal gland;
    _oa_, odoriferous appendages; _B_, the same in _Heliconius
    apseudes_, side view; _C_, odoriferous appendages of _Colænis dido_
    in fresh condition; _D_, tested with alcohol and benzine. 13.
    Odoriferous appendages of _Heliconius eucrate_, head cleansed.—Figs.
    12, 13, after F. Müller. 14. Odoriferous glands (_ogl_) in the pupa
    of _Vanessa io_: _r_, rectum; _h_, the folds of hypodermis which
    forms the terminal papilla of the abdomen; _ov_, oviduct.—After
    Jackson.
]

In the caterpillars of certain blue butterflies (Lycænidæ) is an
internal osmeterium, being a very minute sac which is everted from a
transverse slit on the top of the 7th abdominal segment. Its function is
quite the opposite of those of the caterpillars of other families, since
the sac exudes a sweet fluid very attractive to ants, which may be
diffused more widely by the delicate spinulose bristles crowning the
summit. W. H. Edwards states that in several species of Lycæna, besides
that on the 7th abdominal segment, there is on the 8th segment a pair of
minute dorsal evaginable tubercles.

A pair of small ramose odoriferous glands are said by Siebold, who
regarded them as alluring glands, to occur in Argynnis, Melitæa, and
Zygæna, to be situated near the orifice of the oviduct, and Scudder has
detected them near the anus of the female pupa of _Danais archippus_.
The appearance of the odoriferous glands in the pupa of _Vanessa io_ is
well shown by Jackson (Fig. 366, 14). They develop as two tubular
ingrowths of the hypodermis, perfectly distinct one from the other, each
having its own separate aperture to the exterior. In Fig. 366, 14 the
condition of parts is nearly as in the imago, the glands being situated
below the rectum and opening of the oviduct. In both sexes of another
Brazilian butterfly (_Didonis biblis_) on the median line of the abdomen
between the 4th and 5th segments are two roundish vesicles covered with
short gray hairs, which emit a disagreeable smell.

  It is possible that the dark-green fluid in Parnassius, secreted by
  an evaginable gland, and which is moulded into shape by the
  scimetar-shaped peraplast (Scudder), is formed by the homologues of
  the anal glands of other butterflies.




 Distribution of repugnatorial or alluring scent-glands in insects[59]


                          _A._ LARVAL INSECTS

_a._ _Each thoracic segment; sternal. Phryganea grandis._

_b._ _Prothoracic, sternal, discharging a lateral jet of spray; with a
single large internal sack._




                              LEPIDOPTERA


                             Family TINEIDÆ

_Hyponomeuta evonymella._


                            Family NOCTUIDÆ

_Bryophila_, _Cucullia formosa_, _C. scrophulariæ_, _Habrostola_,
_Cleophana linariæ,_ Catocala (sp.), _Aporia cratægæ_, _Aplecta
nebulosa_, _Leucania staminea_, _L. hispanica_, _L. nonagrioides_,
_Plusia gamma_.


                           Family NOTODONTIDÆ

_Pheosia rimosa_, _Schizura concinna_, _Danima Banksii_ (Australia),
_Macrurocampa marthesia_, _Heterocampa pulverea_, _Cerura vinula_, _C.
furcula_, _C. borealis_, _C. multiscripta._


                           Family NYMPHALIDÆ

Probably all the species.

_c._ _Prothoracic, dorsal; sending out a_ V-_shaped odoriferous organ_
(osmeterium).


                           Family PAPILIONIDÆ

All the species as a rule.

_d._ _Thoracic sternal, evaginable glands._


                           Family PEROPHORIDÆ

_Lacosoma chirodota_, _Perophora melsheimerii_.


                             Family NOLIDÆ

In three, and probably in all the species of Nola.

_e._ _Lateral, abdominal, non-eversible glands, one near each spiracle,
emitting a clear fluid._


                          Family TENTHREDINIDÆ

_Cræsus septentrionalis_, _C. varus_, _Cimbex americana_, _C. betulæ,
Trichiosoma_.

_f._ _Lateral, abdominal, partly eversible glands emitting neither
moisture nor odor, but flesh-colored._


                            Family _Tineidæ_

Phyllocnistis? (eight pairs.)


                           Family HEMILEUCIDÆ

_Hyperchiria io_ (two pairs, viz. on 1st and 7th segments), _H._ sp.
(Mexico), _Hemileuca yavapai_, _pamina_, _H. maia_, _H. artemis_,
_Pseudohazis eglanterina_.

_g._ _Lateral, abdominal, permanently everted, metameric glands, not
known to secrete a fluid, nor to be odoriferous._


                          Family MEGALOPYGIDÆ

_Megalopyge crispata._

_h._ _Medio-dorsal, partly eversible glands, emitting a spray of liquid
but no odor(?), and colored coral-red or orange-yellow (P. auriflua),
but usually in the European species yellowish._


                            Family LIPARIDÆ

All the species except those of Demas.

_i._ _A single, median, abdominal, dorsal gland, emitting a fluid
attractive to ants, on 7th segment; with a pair of minute, index glands
on the 8th segment._


                            Family LYCÆNIDÆ

All the species.

_j._ _Protrusile organs near the anus._

_Myrmeleon_ larva (Hagen? Dimmock).


                 _B._ NYMPH OF HETEROMETABOLOUS INSECTS

_a._ _Paired, dorsal glands, on abdominal segments 1, 2, and 3._

_Cimex lectularius_ (Künckel).

_b._ _The same on abdominal segment 5._

_Lachnus strobi._


                     _C._ PUPA OF CERTAIN BOMBYCES

_At anterior end of certain pupæ, internal glands to moisten threads of
the cocoon for exit of moth._


                           _D._ ADULT INSECTS

_a._ _Occurring on the prothorax only; strongly repugnatorial, best
developed in_ ♂.

_Anisomorpha buprestoides_, _Autolyca pallidicornis_, _Phasma putidum_,
_Phyllium_ (sp.), _Heteropteryx_ (sp.), _Diapheromera femoratum_
(probably in all the species of the family), _Mantis carolina_.

_b._ _Occurring on the pro- and mesothorax, and on the middle of the
abdomen, orange-yellow, fleshy tubercles or evaginations._

_Malachius bipustulatus_, _Anthocomus equestris_, _Evæus thoracicus_.

_c._ _Segmental, eversible glands, homologues of the coxal glands of
other Arthropods, occurring on all, or nearly all, the abdominal
segments._

_Scolopendrella immaculata_ (coxal glands on 3d to 11th pair of legs),
_Campodea staphylinus_ (a pair of coxal glands on 1st to 8th abdominal
segments), _Machilis maritima_ (eversible, coxal glands on segments
1–7).

_d._ _Occurring in the abdomen._

_d^1._ _In the two first abdominal segments._

_Corydia carunculigera_ ♂ and ♀.

_d^2._ _Alluring (?) organs situated on the dorsal side of the abdomen,
in the 6th, or 6th and 7th, abdominal segment._

_Periplaneta americana_ ♂, _P. orientalis_ (nymph), _P. decorata_ ♂
(nymph), _Ectoblatta germanica_ ♂, _Ectobia lapponica_ ♂, _Phyllodromia_
♂, _Aphlebia bivittata_ ♂, _Platyzosteria ingens_ (on seventh segment).

_e._ _At the end of the body._

_Colænis julia_ ♀ (F. Müller), _Heliconius apseudes_ (F. Müller).




            LITERATURE ON DEFENSIVE OR REPUGNATORIAL GLANDS


  =Aldrovandus, U.= De animalibus insectis libri septem cum singulorum
    iconibus ad vivum expressis. (Denuo impress: Bonon. apud Clementem
    Ferronium, 1638, p. 273. The first edition was in 1602.)

  =Moufet, T.= Insectorum sive minimorum animalium theatrum, etc.
    London, 1634, pp. 185, 186.

  =Gœdart, J.= Metamorphosis naturalis sive insectorum historia, etc.
    Amstelodami, 1700, Pars ii, p. 136. (French ed. of 1700, ii, p. 162;
    Lister’s Latin ed., London, 1685, p. 60.)

  =Réaumur, R. A. F.= Mémoires pour servir à l’histoire des insectes,
    etc. Paris, 1736, ii, pp. 266–269, Pls. 21, 22. [ii, partie ii, pp.
    21–23, of the Amsterdam ed. of 1737–1748.]

  =De Geer, C.= Observation sur la propriété singulière qu’ont les
    grandes chenilles à quatorze jambes et à double queue, du saule, de
    seringuer de la liqueur. (Mém. sav. étrang, Paris, 1750, i, pp. 530,
    531, Pl.; Goetze und Bonnet, etc., Auserlesene Abhandlungen, 1774,
    p. 220.)

  =Schaeffer, J. C.= Neuendeckte Theile an Raupen und Zweyfaltern, etc.
    Regensburg, 1754.

  =Sulzer, J. H.= Die Kennzeichen der Insekten, etc. Zürich, 1761, pp.
    65–67, Taf. 5, Fig. 34.

  =Müller, O. F.= Pile-larven med dobbelt Hale, og dens Phalæne, etc.
    Kjöbenhavn, 1772, pp. 53–56, Pl. 2, Figs. 3–5.

  =Bonnet, C.= Mémoire sur une nouvelle partie commune à plusieurs
    espèces de chenilles. (Mém. math. d. savants étrang., Paris, 1755,
    ii, pp. 44–52; Collection complète des œuvres de C. Bonnet, 1779,
    ii, pp. 3–16.)

  —— Mémoire sur la grande chenille à queue fourchue du saule, dans
    lequel on prouve, que la liqueur que cette chenille fait jaillir,
    est un véritable acide, et un acide très-actif. (Mém. math, de
    savants étrang., Paris, 1755, ii, pp. 267–282; Collection complète
    des œuvres de C. Bonnet, 1779, ii, pp. 17–24.)

  =Amoreux, P. J.= Notice des insectes de la France, réputés venimeux,
    etc. Paris, 1789, pp. 282–285.

  =Schwarz, C.= Neuer Raupenkalender. Nürenberg, 1791, Abth. i, p. 59.

  =Petzhold, C. P.= Lepidopterologische Beyträge. (L. G. Scriba’s
    Beiträge zu der Insekten-geschichte, Frankfurt am Main, 1793, Heft
    3, pp. 230–251.)

  =Nouvelle Dictionnaire d’Hist. Nat.=, xv, p. 487. (Larva of
    Hydrophilus ejects with a slight noise a fœtid and blackish fluid.)

  =Rengger, Johann Rudolph.= Physiologische Untersuchungen über die
    thierische Haushaltung der Insecten. Tübingen, 1817. (In the chapter
    entitled Abgesonderte Säfte bei den Raupen, he speaks of the
    glandular apparatus of the larva of _B. vinula_, noticing the
    general form of the secretory sac, that it opens out in two muscular
    evertible points, out of which the secretion is ejected.)

  =Dufour, L.= Mémoire anatomique sur une nouvelle espèce d’insecte du
    genre Brachinus. (Ann. de mus. d’histoire nat., xviii, 1811, pp.
    70–81.)

  —— Recherches anatomiques sur les carabiques et sur plusieurs autres
    coléoptères. (Ann. d. Sci. Nat., 1826, viii, pp. 5–54.)

  —— Mémoire sur les métamorphosis et l’anatomie de la _Pyrochroa
    coccinea_. Glande odorifique. (Ibid., ii sér. Zoologie, xiii, 1840,
    pp. 340, 341.)

  —— Recherches anatomiques sur les Diptères. 1851, pp. 195, 313.
    (Alimentary canal of Sepsis contains the seat of a “glande
    odorifique.”)

  =Kirby and Spence.= Introduction to entomology, etc. (2d ed., i, 1815;
    London, 1818, ii, pp. 238, 239.)

  =Lyonet, P.= Recherches sur l’anatomie et les metamorphoses de
    différentes espèces d’insectes. Ouvrage posthume. Paris, 1832.

  =Morren, C.= Mémoire sur l’émigration du puceron du pêcher (_Aphis
    persicæ_), et sur les caractères et l’anatomie de cette espèce.
    (Ann. Sci. Natur. Zool., 1836, vi, pp. 65–93, Pls. 6, 7.)

  =Ratzeburg, J. T. C.= Die Forstinsekten, etc. (Theil i, Die Käfer,
    etc., 1837, p. 246.)

  =Aubé, C.= Note sur une sécrétion fétide d’_Eumolpus pretiosus_. (Ann.
    Soc. Ent. Fr., 1837, i, vi; Bull., p. 58.)

  =Lacordaire, J. S.= Introduction à l’entomologie. 1838, ii, p. 45.

  =Meckel, von Hemsbach, Johann Friedrich.= Mikrographie einiger
    Drüsenapparate der niederen Thiere. (Anat. Phys. u. wiss. Med.,
    1846, pp. 1–73, Taf. 1–3; p. 46, Müller’s Archiv.)

  =Stein, Friedrich.= Vergleichende Anatomie und Physiologie der
    Insekten. Berlin, 1847.

  =Leidy, Joseph.= History and anatomy of the hemipterous genus
    Belostoma. (Journ. Acad. Nat. Sci. Philadelphia, Ser. 2, 1847, i,
    Part i, pp. 57–67, Pl. 1.)

  —— Odoriferous glands of invertebrata. (Proc. Acad. Philadelphia,
    1849, iv, 234–236, 1 Pl.; Ann. and Mag. N. H., Ser. 2, 1850, v, pp.
    154–156.)

  =Chapuis et Candèze.= Catalogue des larves des coléoptères, etc. (Mém.
    Soc. Sci. de Liège, 1853, viii, pp. 351–653, Pls. 1–9, pp. 611,
    612.)

  =Siebold, Carl Theodor.= Lehrbuch der vergleichenden Anatomie der
    wirbellosen Thiere, 1848. (Burnett’s transl., Boston, 1854.)

  =Burnett, Waldo Irving.= Translation of Siebold’s Anatomy of the
    Invertebrates, 1854. (Note on the osmeteria of _Papilio asterias_,
    which he regards as an odoriferous and defensive, rather than
    tactile, organ, p. 415.)

  =Karsten, H.= Bemerkungen über einige shaarfe und brennende
    Absonderungen verschiedener Raupen. (Müller’s Archiv für Anat. Phys.
    u. wiss. Med., 1848, pp. 375–382, Taf. 11, 12.) Describes the
    poison-glands at the base of the spines of Saturnia larvæ.

  —— Harnorgane des _Brachinus complanatus_. (Müller’s Archiv, 1848, pp.
    367–376, Taf.)

  =Laboulbéne, Alexandre.= Note sur les caroncules thoraciques du
    Malachius. (Annales de la Soc. Ent. de France, 3^e Sér., vi, 1858,
    pp. 521–528.)

  =Saussure, Henri de.= Recherches zoologiques de l’Amerique centrale et
    du Mexique. (6^e Partie, Études sur les Myriopodes et les Insectes,
    Paris, 1860.)

  =Gerstaecker, C. E. A.= Ueber das vorkommen von ausstülpbaren
    Hautanhängen am Hinterleibe an Schaben. (Archiv f. Naturgesch.,
    1861, xxi, pp. 107–115.)

  =Liegel, Hermann.= Ueber den Ausstülpungsapparat von Malachius und
    verwandten Formen. Inaug. Diss., Göttingen, pp. 31, 1 Taf. (n. d.,
    since 1858 and before 1878.)

  =Leydig, F.= Zur Anatomie der Insecten. (Archiv f. Anat. Phys. u.
    wiss. Med., 1859, pp. 33–89, 149–183, Taf. 2–4, pp. 35 and 38.)

  —— Ueber bombardier Käfer. (Biolog. Centralbl., x, 1890, pp. 395,
    396.)

  =Claus, C.= Ueber die Seitendrüsen der Larve von _Chrysomela populi_.
    (Zeits. f. wissens. Zool., xi, 1861, pp. 309–314, Taf. xxv.)

  —— Ueber Schutzwassen der Raupen des Gabelschwanzes. (Würzburger
    Naturw. Zeitschrift, 1862, iii, xiv; Sitz. am., 28 Juni, 1862.)

  =Rogenhofer, Alois.= Drei Schmetterlingsmetamorphosen. (Verhandlungen
    der k. k. zoolog.-bot. Gesellschaft, Wien, xiii, 1862, pp.
    1224–1230.)

  =Fitch, Asa.= Eighth report on the noxious and other insects of ...
    New York. (Trans. N. Y. State Agric. Soc., 1862, xxii, pp. 657–684),
    p. 677. (Separate.)

  =Guenée, Achille.= D’une organe particulier que présente une chenille
    de Lycæna. (Annales Soc. Ent. de France, Sér. 4, 1867, pp. 665–668,
    Pl. 13.)

  =Landois, L.= Anatomie der Bettwanze, _Cimex lectularius_, mit
    berücksichtigung verwandter Hemipterengeschlechter. (Zeitsch. f.
    wissens. Zool., 1868, xvii, pp. 206–224, 218–223, Taf. 11, 12.)

  =Studer, Theodor.= Mittheilungen der naturforsch. Gesellschaft in
    Bern, 1872–1873, No. 792–811, p. 101.

  =Candèze, E.= Les moyens d’attaque et de défense chez les insectes.
    (Bull. Acad. royale de Belgique, 2 Sér., xxxviii, 1874, pp.
    787–816.)

  =Mayer, Paul.= Anatomie von _Pyrrhocoris apterus_. (Reichert und du
    Bois-Reymond’s Archiv f. Anat. Phys., etc., 1874, pp. 313–347, 3
    Taf.)

  =Scudder, Samuel Hubbard.= Odoriferous glands in Phasmidæ. (Psyche, i,
    pp. 137–140, Jan. 14, 1876; Amer. Nat., x, p. 256, April, 1876.)

  —— Prothoracic tubercles in butterfly caterpillars. (Psyche, i, pp.
    64, 168, 1876.)

  —— Organs found near the anus of the ♀ pupa of Danais, which recall
    the odoriferous organs mentioned by Burnett, transl. Siebold’s Comp.
    Anat. as occurring in Argynnis and other genera. (Psyche, iii, p.
    278, 1882, p. 453, note 22.)

  —— Glands and extensile organs of larvæ of blue butterflies. (Proc.
    Bos. Soc. Nat. Hist., xxxiii, pp. 357, 358, 1888.)

  —— Butterflies of Eastern United States. i-iii, 1889.

  —— New light on the formation of the abdominal pouch in Parnassius.
    (Trans. Ent. Soc. London for 1892, January, 1893, pp. 249–253.)

  =Müller, Fritz.= Die Stinkkölbchen der weiblichen Maracujáfalter.
    (Zeitschr. f. wissens. Zool., 1877, xxx, pp. 167–170, Taf. 9.)

  =Plateau, Félix.= Note sur une sécrétion propre aux coléoptères
    dytiscides. (Ann. Soc. Ent. Belg., 1876, v, xix, pp. 1–10.)

  =Edwards, William H.= Notes on _Lycæna pseudargiolus_ and its larval
    history. (Can. Ent., x, Jan., 1878, pp. 1–14. Fig.)

  —— On the larvæ of _Lycæna pseudargiolus_ and attendant ants. (Can.
    Ent., x, July, 1878, pp. 131–136.)

  —— Butterflies of North America, i-iv. Many Pls. Phil., 1868—.

  =Voges, Ernst.= Beiträge zur Kenntniss der Juliden. (Zeitsch. f.
    wissens. Zoologie, xxxi, p. 127, 1878. The scent-glands are
    retort-shaped bodies, the necks of which open into _foramina
    repugnatoria_.)

  =Rye, E. C.= Secretion of water-beetles. (Ent. Month. Mag., xiv,
    1877–1878, pp. 232, 233.)

  =Forel, A.= Der Giftapparat und die anal Drüsen der Ameisen. (Zeits.
    f. wissens. Zool., 1878, xxx, Suppl., pp. 28–68, Taf. 3, 4.)

  =Saunders, William.= Notes on the larva of _Lycæna scudderi_. (Can.
    Ent., x, Jan., 1878, p. 14.)

  =Weismann, A.= Ueber Duftschuppen. (Zool. Anzeiger, 26th Aug., 1878,
    Jahrg., i, pp. 98, 99.)

  =Gissler, Carl Friedrich.= On the repugnatorial glands in Eleodes.
    (Psyche, ii, Feb., 1879, pp. 209, 210.)

  —— Odoriferous glands on the 5th abdominal segment in nymph of
    _Lachnus strobi_. (Fig. 273, p. 804, of Packard’s Report on Forest
    and Shade Tree Insects, 1890.)

  =Brunner von Wattenwyl, K.= Ueber ein neues Organ bei den Acridiodeen.
    (Verhandl. k. k. Zool. Bot. Gesells. Wien., 1879, xxix;
    Sitzungsber., pp. 26, 27.)

  —— Verhandl. k. k. Zool. Bot. Gesells. Wien. (A peculiar organ on hind
    femora of Acridiidæ.)

  =Rougement, P.= Observations sur l’organe détonnant du _Brachinus
    crepitans_ Oliv. (Bull. Soc. Sci. Nat. Neuchâtel, 1879, xi, pp.
    471–478, Pl.)

  =Goossens, Th.= Sur une organe entre la tête et la première paire de
    pattes de quelques chenilles. (Ann. Soc. Ent. France, ix, p. 4,
    1809; Bull., pp. 60, 61.)

  —— Des chenilles vésicantes. (Ann. Ent. Soc. France, vi, pp. 461–464,
    1887.)

  =Coquillett, D. W.= On the early stages of some moths. (Can. Ent.,
    March, 1880, xii, pp. 43–46.)

  =Chambers, Victor Tousey.= Notes upon some Tineid larvæ. (Psyche, iii,
    July, 1880, p. 67. Certain retractile processes “from the sides of
    certain segments of the larva.”)

  —— Further notes on some Tineid larvæ. (Psyche, iii, p. 135, Feb. 12,
    1881. Larva of Phyllocnistis has eight pairs of lateral pseudopodia
    on first eight abdominal segments.)

  =French, G. H.= Larvæ of _Cerura occidentalis_ Lint, and _C. borealis_
    Bd. (Can. Ent., July, 1881, xiii, pp. 144, 145.)

  =Passerini, N.= Sopra i due tubercoli abdominali della larva della
    _Porthesia chrysorrhœa_. (Bull. Soc. Ent. Ital., 1881, xiii, pp.
    293–296.)

  =Klemensiewicz, Stanislaus.= Zur näheren Kenntniss der Hautdrüsen bei
    den Raupen und bei _Malachius_. (Verhandlungen d. Zool. Bot.
    Gesellsch. Wien., xxxii, pp. 459–474, 1882, 2 Taf.)

  =Weber, Max.= Ueber eine Cyanwasserstoffsäure bereitende Drüse.
    (Archiv für Mikr. Anat., xxi, pp. 468–475, xxiv, 1882.)

  =Bertkau, Philip.= Ueber den Stinkapparat von _Lacon murinus_ L.
    (Archiv f. Naturg., 1882, Jahrg., xlviii, pp. 371–373.)

  =Dimmock, George.= Organs, probably defensive in function, in the
    larva of _Hyperchiria varia_ Walk. (_Saturnia io_ Harris). (Psyche,
    iii, pp. 352, 353, Aug. 19, 1882. Account of lateral eversible
    glands on 1st and 7th abdominal segments; they emit neither moisture
    nor odor.)

  —— On some glands which open externally on insects. (Psyche, iii, pp.
    387–399, Jan. 15, 1883. Treats of poison-glands, glandular hairs,
    eversible glands of Cerura, etc.)

  =Coleman, N.= Notes on _Orgyia leucostigma_. (Papilio,
    November-December, 1882, Jan., 1883, ii, pp. 164–166.)

  =Müller, F.= Der Anhang am Hinterleibe der _Acræa_-weibchen. (Zool.
    Anzeiger, 6th Aug., 1883, Jahrg., vi, pp. 415, 416.)

  =Dewitz, H.= Ueber das durch die Foramina repugnatoria entleerte
    Secret bei Glomeris. (Biol. Centralblatt, iv, pp. 202, 203, 1884.)

  =Williston, S. A.= Protective secretion of Eleodes ejected from anal
    gland. (Psyche, iv, p. 168, May, 1884.)

  =Poulton, Edward Bagnall.= Notes in 1885 upon lepidopterous larvæ and
    pupæ, including an account of the loss of weight in the
    freshly-formed lepidopterous pupæ. (Trans. Ent. Soc., London, June,
    1886, pp. 156, 157, 159.)

  —— Notes in 1886 upon lepidopterous larvæ, etc. (Trans. Ent. Soc.,
    London, Sept., 1887, pp. 295–301.)

  —— Notes in 1887 upon lepidopterous larvæ, etc. (Trans. Ent. Soc.,
    London, 1888, p. 597.)

  =Künckel-d’Herculais, J.= La punaise de lit et ses appareils
    odoriférants. (Comptes rendus, ciii, 1886, pp. 81–83; Annals & Mag.
    Nat. Hist., 5th Ser., xviii, 1886, pp. 167, 168.)

  —— Étude comparée des appareils odorifiques dans les differents
    groupes d’Hemiptères hétéroptères. (Compt. rend. Acad. Sc., Paris,
    cxx, pp. 1002–1004.)

  =Packard, A. S.= The fluid ejected by notodontian caterpillars. (Amer.
    Nat., 1886, xx, pp. 811, 812.)

  —— An eversible “gland” in the larva of Orgyia. (Amer. Nat., 1886, xx,
    p. 814.)

  —— Fifth Rep. U. S. Ent. Comm. Insects injurious to forest and shade
    trees, p. 136, 1890.

  —— Hints on the evolution of the bristles, spines, and tubercles of
    certain caterpillars. (Proc. Boston Soc. Nat. Hist., xxiv, 1890, p.
    551.)

  —— Notes on some points in the external structure and phylogeny of
    lepidopterous larvæ. (Proc. Bost. Soc. Nat. Hist., xxv, 1890, pp.
    83–114.)

  —— A study of the transformations and anatomy of _Lagoa crispata_, a
    bombycine moth. (Proc. Amer. Phil. Soc., Philadelphia, xxxii, 1893,
    pp. 275–292, 7 Pls.)

  —— The eversible repugnatorial scent glands of insects. (Journ., N. Y.
    Ent. Soc. iii, 1895, pp. 110–127; iv, p. 896; pp. 26–32, 1 Pl.)

  =Loman, J. C. C.= Freies Jod als Drüsensecret. (Tijdschr. Neder.
    Dierk. Ver. Deel 1, 1887, pp. 106–108.)

  =Riley, Charles Valentine.= Proc. Ent. Soc., Washington, March 13,
    1888, i, pp. 87–89.

  —— Notes on the eversible glands of larvæ of _Orgyia_ and _Parorgyia
    leucopæa_ and _P. clintonii (achatina)_. (See 5th Rep. U. S. Ent.
    Comm., p. 137.)

  =Denham, Ch. S.= The acid secretion of _Notodonta concinna_. (Insect
    Life, i, p. 147, 1888; hydrochloric acid.)

  =Michin, Edward A.= Note on a new organ, and on the structure of the
    hypodermis, in _Periplaneta orientalis_. (Quart. Journ. Micros. Sc.,
    Dec., 1888, xxiv, 1 Pl.)

  —— Further observations on the dorsal gland in the abdomen of
    Periplaneta and its allies. (Zool. Anz., 27 Jan., 1890, pp. 41–44.)

  =Maynard, C. L.= The defensive glands of a species of Phasma,
    _Anisomorpha buprestoides_. (Contributions to Science, i, April,
    1889.)

  =Schaeffer, Cæsar.= Beiträge zur Histologie der Insekten. (Zool.
    Jahrb. Morph. Abth. iii, pp. 611–652, Taf. xxix, xxx, 1889; treats
    of the ventral glands in prothorax of caterpillars; scales and hairs
    are secretions from the very greatly enlarged hypodermic cells.)

  =Gilson, G.= Les glandes odorifères der _Blaps mortisaga_ et de
    quelques autres espèces. (La Cellule, v. pp. 1–21, 1 Pl., 1889.)

  —— The odoriferous apparatus of _Blaps mortisaga_. (Rep. 58th Meeting
    Brit. Assoc. Adv. Sc., 1889, pp. 727, 728.)

  =Haase, Erich.= Ueber die Stinkdrüsen der Orthoptera. (Sitzgsber. Ges.
    Naturf. Freunde, Berlin, pp. 57, 58, 1889.)

  —— Zur Anatomie der Blattiden. (Zool. Anz., xii Jahrg., pp. 169–172,
    1889.)

  =Herbst, Curt.= Anatomische Untersuchungen an _Scutigera coleoptrata_.
    Ein Beitrag zur vergleichenden Anatomie der Articulaten. Dissert.,
    Jena, pp. 36 (Hautdrüsen, Coxal-Organ.); p. 1, 1889.

  =Wheeler, William M.= Hydrocyanic acid secreted by _Polydesmus
    virginiensis_ Drury. (Psyche, v, p. 422.)

  —— New glands in the hemipterous embryo. (Amer. Nat., Feb. 1890, p.
    187; odorous(?) glands.)

  =Jackson, W. Hatchett.= Studies in the morphology of the Lepidoptera,
    Pt. i. (Trans. Linn. Soc., London, 2 Ser., Zoöl., v, May, 1890.)

  =Krauss, Hermann.= Die Duftdrüse der _Aphlebia bivittata_ Brullé
    (Blattidæ) von Teneriffa. (Zool. Anz., xiii Jahrg., 1890, pp.
    584–587, 3 Figs.)

  =Fernald, H. T.= Rectal glands in Coleoptera. (Amer. Nat., xxiv, pp.
    100, 101, Pls. 4, 5, 1890.)

  =Verson, E.= Hautdrüsen system bei Bombyciden (Seidenspinner). (Zool.
    Anzeiger, 1890, pp. 118–120.)

  =Vosseler, Julius.= Die Stinkdrüsen der Forficuliden. (Arch. Mikr.
    Anat., xxxvi, 1890, pp. 565–578, Taf. 29.)

  =Carrière, J.= Die Drüsen am ersten Hinterleibsringe der
    Insektenembryonen. (Biol. Centralblatt, xi, pp. 110–127, 1891.)

  =Borgert, Henry.= Die Hautdrüsen der Tracheaten. Inaugural Diss.,
    Jena, 1891, pp. 1–80.

  =Lang, Arnold.= Lehrbuch der vergleichende Anatomie, English Trans. by
    Henry M. and Matilda Bernard, 1891, pp. 458, 459.

  =Kennel, J. von.= Die Verwandtschaftverhältnisse der Arthropoden.
    (Schriften herausgegeben von der Naturforscher Gesellschaft bei der
    Universität Dorpat, vi, Dorpat, 1891.)

  =Patton, W. H.= Scent-glands in the larva of Limacodes. (Can. Ent.,
    xxiii, Feb. 1891, pp. 42, 43; eight pairs of glands with pores along
    the edges of the back.)

  =Batelli, Andrea.= Di una particolarita nell integumento dell’
    _Aphrophora spumaria_. (Monitore Zoöl. Ital. Anno 2, pp. 30–32,
    1891. Dermal glands in the hindermost segment.)

  =Ash, C. D.= Notes on the larva of _Danima banksii_ Lewin. (Ent.
    Month. Mag., Sept. 1892, p. 232, Fig.) notodontian larva protrudes
    from under side of prothoracic segment a Y-shaped, red organ like
    that of Papilio; no odor or fluid given out.

  =Bernard, Henry M.= An endeavor to show that the tracheæ of the
    Arthropoda arose from setiparous sacs. (Spengel’s Zool., Jahrbuch,
    1892, pp. 511–524, 3 Figs.)

  =Latter, Oswald.= The secretion of potassium hydroxide by _Dicranura
    vinula_, and the emergence of the imago from the cocoon. (Trans.
    Ent. Soc. London, 1892, 287, also xxxii; Prof. Meldola adds that the
    larva of _D. vinula_ secretes strong, formic acid, and is the only
    animal known to secrete a strong, caustic alkali.)

  —— Further notes on the secretion of potassium hydroxide by _Dicranura
    vinula_ (imago), and similar phenomena in other Lepidoptera. (Trans.
    Ent. Soc. London; Nature, 1895, p. 551, March 20, 1895.)

  =Zograff, Nicolas.= Note sur l’origine et les parentes des
    Arthropodes, principalement des Arthropodes trachèates. (Congrès
    Internationale de Zoologie, 2^e Session à Moscow, Aug. 1892; Part i,
    Moscow, 1892, pp. 278–302, 1892; cyanogenic glands in Myriopods, p.
    287.)

  =Swale, H.= Odor of _Olophrum piceum_. (Ent. Month. Mag., v, Jan.
    1896, pp. 1, 2.)

  =Cuénot, L.= Moyens de défense dans la série animale, Paris, n. d.
    (1892); the ejection of blood as a means of defence by some
    Coleoptera. (Comptes rendus, Acad. Sc. France, April 16; Nature,
    April 26, 1894.)

  —— Sur la saignée réflexe et les moyens de défense de quelques
    insectes. (Arch. Zool. expér. (3), 1897, iv, pp. 655, 679, 680.)

  =Holmgren, Emil.= Studier öfverhudens och de körtelartade hudorganens
    morfologi hos skandinaviska macrolepidopterlarver. (K. Svenska
    Vetenskaps-Akademiens Handlingar, xxvii, No. 4, Stockholm, 4º 1895,
    pp. 82, 9 Pls.)

  =Lutz, K. G.= Das Blut der Coccinelliden. (Zool. Anzeiger, 1895, pp.
    244–255, 1 Fig.)

  =Gilson, Gustav.= Studies in insect morphology. (Proc. Linn. Soc.
    London, March 5, 1896; Nature, p. 500.)

  —— On segmentally disposed thoracic glands in the larvæ of the
    Trichoptera. (Journ. Linn. Soc., London, xxv. 1897.)

  =Cholodkowsky, N.= Entomotomische Miscellen, v, Ueber die
    Spritzapparate der Cimbiciden Larven, pp. 135–143, 2 Taf. Ibid., vi.
    Ueber das Bluten der Cimbiciden Larven, pp. 352–357, 1 Fig. (Horæ
    Soc. Ent. Rossicæ, xxx, 1897.)

  Also the writings of Darwin, Wallace, Poulton, Weir, Beddard, Butler,
    Busgen, (pp. 365–367), Girard, Kolbe, Locy.




                      THE ALLURING OR SCENT-GLANDS


It is difficult to draw the line between repelling and alluring glands.
Attention was first definitely called to the alluring odors of
Lepidoptera by Fritz Müller, who showed that the males of certain
butterflies are rendered attractive to the other sex by secreting
odorous oils of the ether series. He pointed out that the seat of the
odor is the androconia (see p. 199), while either repellent or pleasant
odors are exhaled from abdominal glands.

  Those of _Pieris napi_ yield a scent like that of citrons, _Didonis
  biblis_ gives off three different odors from different parts of the
  body, besides having a distinctly odorous spot on the hind wings.
  Both sexes have a sac between the fourth and fifth abdominal
  segments which exhales a very unpleasant (protective) odor, while
  the males have on the succeeding segment a pair of glands from which
  proceeds an agreeable odor like that of the heliotrope. _Callidryas
  argante_ throws off a musky odor. In _Prepona laertes_ the odor is
  like that of a bat, in _Dircenna xantho_ it is vanilla-like, the
  androconia being situated on the front edge of the hind wings. In
  _Papilio grayi_ the odor is said to be as agreeable and intense as
  in flowers. Certain sphingids are known to exhale a distinct odor,
  which Müller has traced to a tuft of hair-like scales at the base of
  the abdomen, and which fits into a groove in the first segment, so
  as to be ordinarily invisible.

[Illustration:

  _Fig. 367._—Scent-tufts: 1, of _Leucarctia acræa_; 2, of _Pyrrarctia
    isabella_.—After Smith.
]

  In the noctuid genus, Patula, the costal half of the hind wing is
  modified to form a large scent-gland, and in consequence the
  venation has been modified. The still greater distortion of the
  veins in the allied genus, Argida, was attributed by the author to
  its once having possessed a similar scent-gland, now become
  rudimentary by disuse. (Hampson.)

  Peculiar white or orange-colored, hairy, thread-like processes have
  been found protruding from narrow openings near the tip of the
  abdomen of Arctian moths (Fig. 367), which throw off, according to
  J. B. Smith, “an intense odor, somewhat like the smell of laudanum.”
  We have perceived the same unpleasant odor emanating from the males
  of _Spilosoma virginica_ and _Arctia virgo_, as well as _Leucarctia
  acræa_.

  We are informed by C. Dury that similar but longer hairy appendages
  are thrust out by the male of _Haploa clymene_. Many glaucopid moths
  protrude similar glandular processes. Thus Müller tells us that on
  seizing a glaucopid female by the wings, nearly the whole body
  became enveloped in a large cloud of snow-white wool which came out
  of a sort of pouch on the ventral side of the abdomen.

  The male of a glaucopid was seen to dart out a pair of long hollow
  hairy retractile filaments which in some species exceed the whole
  body in length. The apparatus secretes a peculiar odor, probably
  serving to allure the female (Nature), and certain Zygænidæ have on
  the inner side of the paranal lobes (Afterklappen) glands filled
  with a sweetly scented fluid. Smith has detected a peculiar brush of
  hair-like scales in a groove between the dorsal and ventral parts of
  the basal two segments of the abdomen of _Schinia marginata_ (family
  Noctuidæ), and when removed it exhaled a laudanum-like smell.

  The pupa of _Citheronia regalis_ gives out from the end of the
  abdomen a scent reminding us of laudanum.

[Illustration:

  FIG. 368.—Scent-tufts on middle legs of _Catocala concumbens_.—After
    Bailey.
]

Another mode of disseminating pleasant, alluring odors is that of the
males of certain moths, which bear pencils and tufts on their fore or
hind legs, and in the case of an Indian butterfly on the greatly
elongated palpi. Those on the legs are ordinarily concealed in cavities
or furrows in the leg, and may be thrust out and expanded so as to
widely diffuse their odor. Such are those of the males of Catocala (Fig.
368), which resemble an artist’s fitch brush. In _Hepialus hecta_, where
the arrangements for protecting the tufts are quite abnormal, Bertkau
has detected the cells which secrete the odorous fluid. In the male of
another Hepialus (_H. humuli_) a peculiar scent proceeds from the
curiously aborted and altered hind tibiæ. (Barrett.) In one case, that
of a geometrid moth (_Bapata dichroa_ of New Guinea), these pencils
occur on all the legs. (Haase). In many species a distinct odor is
perceptible when the leg bearing the pencil or tuft is crushed.

These eversible scent-glands have been supposed to be mostly restricted
to the Lepidoptera, and to a single known case in the Trichoptera, but
similar alluring male glands also occur in the Orthoptera (Locustidæ).
H. Garman has described and figured in the cave cricket (_Hadenœcus
subterraneus_) “a pair of white fleshy appendages protruding from slits
between the terga of the 9th and 10th abdominal somites, the nature of
which is not clear,” adding, “the slits through which the organs appear
are situated one on each side anterior to and a little within the cerci.
When fully protruded, the glands are white, cylindrical, a little
tapering, and are about one-eighth of an inch long.” He believes that
they are protruded during the period of sexual excitement, and suggests
that “the sense of smell is certainly the one best calculated to bring
the sexes together in the darkness of caves.” We had previously noticed
these organs in alcoholic specimens, but supposed that they were fungous
growths. On dissecting and making microscopic sections of them, the
gland is, when extended (Fig. 369), seen to be a long, ensiform, sharp,
band-like process, with numerous retractor muscular fibres. When at rest
each gland is folded about five times, forming a bundle lying on each
side of the end of the intestine. The walls are formed of a single layer
of epithelium, as seen in Fig. 369, _B_.

[Illustration:

  FIG. 369.—Eversible scent-glands (_a_) of Hadenœcus, nat. size:
    Kingsley, _del._; _A_, a gland outstretched, with the retractor
    muscular fibres; _t_, part of the tergite. _B_, section of the
    gland, showing the single layer of epithelial cells, and the
    muscular fibres (_m_).—Author _del._
]

In the male of the common wingless cricket, _Ceuthophilus maculatus_, we
have discovered what appears to be a pair of scent-glands lying directly
over the last abdominal ganglion. They form two large white sacs
situated close together, with a short common duct which passes back and
opens externally upwards by a transverse slit on the under side of the
last segment of the body.




                     LITERATURE ON ALLURING GLANDS


  =Watson, J.= On the microscopical examination of plumules, etc. (Ent.
    Month. Mag., ii, 1865, p. 1.)

  —— On certain scales of some diurnal Lepidoptera. (Mem. Lit. and Phil.
    Soc. Manchester, Ser. 3, ii, 1868, p. 63.)

  —— On the plumules or battledore scales of Lycænidæ. (Mem. Lit. and
    Phil. Soc. Manchester, Ser. 3, iii, 1869, p. 128.) Further remarks,
    etc. (Ibid., p. 259.)

  =Anthony, J.= Structure of battledore scales. (Month. Microsc. Journ.,
    vii, 1872, p. 250; see also p. 200.)

  =Morrison, Herbert Knowles.= On an appendage of the male _Leucarctia
    acræa_. (Psyche, i, pp. 21–22, October, 1874.)

  =Müller, Fritz.= The habits of various insects. (Nature, June 11,
    1874, pp. 102–103.)

  —— Ueber Haarpinsel, Fitzflecke und ähnliche Gebilde auf den Flügeln
    männlicher Schmetterlinge. (Jena. Zeitschr. f. Naturw., 1877, xi,
    pp. 99–114.)

  —— Beobachtungen an brasilianischen Schmetterlingen, ii. I. Die
    Duftschuppen der männlichen Maracujáfalter. (Kosmos, 1877, i, pp.
    391–395, Figs. 5, 6.) II. Die Duftschuppen des männchens von _Dione
    vanillæ_. (Kosmos, ii, 1877, pp. 38–42, 7 Taf.)

  —— As maculas sexuaes dos individuos masculinos das especies _Danais
    erippus_ e _D. gilippus_. (Arch. Mus. Nac. Rio Janeiro, ii, 1877
    (1878), pp. 25–29, 1 Pl.)

  —— Die Duftschuppen der Schmetterlinge (nach dem “Kosmos” in Ent.
    Nachr., 1878, pp. 29–32, 109).

  —— Wo hat der Moschusduft der Schwärmer seinen Sitz? (Kosmos, ii
    Jahrg., 1878, pp. 84, 85.)

  —— Os orgaos odoriferos dos especias _Epicalia acontius_, Lin. e de
    _Myscelia orsis_, Dru. (Arch. Mus. Nac. Rio Janeiro, ii, 1879, pp.
    31–35.)

  —— Os orgaos odoriferos nas pernas de certos Lepidopteres. (Arch. Mus.
    Nac. Rio Janeiro, ii, 1879, pp. 37–46, 3 Pls.)

  —— Os orgaos odoriferos da _Antirrhœa archœa_. (Arch. Mus. Nac. Rio
    Janeiro, iii, 1878, pp. 1–7, 1 Pl.)

  —— A prega costal das Hesperideas. (Arch. Mus. Nac. Rio Janeiro, iii,
    1880, pp. 41–50, 2 Pls.)

  =Weismann, August.= Ueber Duftschuppen. (Zool. Anzeiger, i, 1878, pp.
    98, 99.)

  =Arnhart, L.= Sexundäre Geschlechtscharaktere von _Acherontia
    atropos_. (Verh. d. k. k. zool. bot. Ges. Wien, xxix, 1879, p. 54.)

  =Bertkau, Philipp.= Duftapparat an Schmetterlingsbeinen. (Ent.
    Nachrichten, 1879, Jahrg., pp. 223, 224.)

  —— Ueber den Duftapparat von _Hepialus hecta_. (Archiv f. naturg.,
    xlviii Jahrg., 1882, pp. 363–370, Figs.; also in Biol.
    Centralblatt., ii Jahrg., 1882, pp. 500–502.)

  —— Ergänzung (Duftvorrichtungen bei Lepidopteren). (Ent. Nachr., 1880,
    p. 206.)

  —— Entomologische Mizellen. 1. Ueber Duftvorrichtungen einiger
    Schmetterlinge. (Verh. d. naturhist. Ver. d. preuss. Rheinlande und
    Westf., 1884, pp. 343–350.)

  =Reichenau, W. von.= Der Duftapparat von _Sphinx ligustri_. (Ent.
    Nachr., 1880, p. 141; also Kosmos, iv Jahrg., 1880, pp. 387–390.)

  =Fügner, R.= Duftapparat bei _Sphinx ligustri_. (Ent. Nachr., 1880, p.
    166.)

  =Lelievre, Ernest.= (Note in Le Naturaliste, June 1, 1880. Both sexes
    of _Thais polyxena_ emit an odorous exhalation. Notes on exhalation
    from _Spilosoma fuliginosa._)

  =Hall, C. G.= Peculiar odor emitted by _Acherontia atropos_.
    (Entomologist, London, xvi, p. 14.)

  =Åurivillius, Christopher.= Ueber secundäre Geschlechtscharactere
    nordischer Tagfalter. (Stockholm, 1880, Bihang till K. Svensk. Vet.
    Akad. Handl., v, pp. 56, 3 Taf.)

  —— Des caractères sexuels secundaires chez les papillons diurnes.
    (Ent. Tidskrift, 1880, pp. 163–166.)

  —— Anteckningar om några skandinaviska fjärilarter. (Ent. Tidskr., iv,
    Årg., 1884, pp. 33–37; Résumé (French), ibid., pp. 55–57.)

  =Kirby, W. F.= Fans on the fore legs of _Catocala fraxini_. (Papilio,
    ii, p. 84, 1882.)

  =Bailey, James S.= Femoral tufts or pencils of hair in certain
    Catocalæ. (Papilio, ii, 1882, pp. 51, 52, 146; also in Stettin Ent.
    Zeitung, xliii, p. 392.)

  =Edwards, Henry.= Fans on the feet of Catocaline moths. (Papilio, ii,
    p. 146, 1882.)

  =Stretch, R. H.= Anal appendages of _Leucarctia acræa_. (Papilio, iii,
    pp. 41, 42, 1883, 1 Fig.)

  =Weed, Clarence M.= Appendages of Leucarctia. (Papilio, iii, 1883, p.
    84.)

  =Grote, Aug. R.= Appendages of _Leucarctia acræa_. (Papilio, iii,
    1883, p. 84.)

  =Haase, Erich.= Ueber sexuelle Charactere bei Schmetterlingen.
    (Zeitschr. f. Ent., Breslau, N. F., 1885, pp. 15–19, 36–44; also
    Ent. Nachr., xi Jahrg., pp. 332, 333.)

  —— Duftapparate indo-australischer Schmetterlinge. (Corresp. Blatt.
    Ent. Ver. Iris, Dresden, 1886, pp. 92–107, 1 Taf.; ibid., 1887, pp.
    159–178; ibid., 1888, pp. 281–336.)

  —— Ueber Duftapparate bei Schmetterlingen. (Sitzgsber. Nat. Ges. Iris,
    Dresden, 1886, pp. 9–10; Abstr. in Journ. R. Micr. Soc., vi, pp.
    969–970, 1886.)

  —— Der Duftapparate von Acherontia. (Zeitschr. f. Ent., Breslau, N.
    F., 1887, pp. 5–6.)

  —— Dufteinrichtung indischer Schmetterlinge. (Zool. Anzeiger, 1888,
    pp. 475–481.)

  =Dalla Torre, K. W. von.= Die Duftapparate der Schmetterlinge.
    (Kosmos, 1885, ii, pp. 354–364, 410–423; Abstr. by J. B. Smith in
    Proc. Ent. Soc., Washington, i, pp. 38, 1888.)

  =Smith, John B.= _Cosmosoma omphale._ (Entomologica Americana, i, pp.
    181–185, 1886. Describes and figures cavities in under side of
    2d–4th abdominal segments of male, filled with a silky substance.
    This may be for display to attract ♀, as the whole mass must be very
    conspicuous when protruded. No odor noticed.)

  —— Scent organs in some Bombycid moths. (Entomologica Americana, ii,
    No. 4, pp. 79–80, 1886. Describes and figures long, slender, forked
    hairy, orange or white, eversible glands, everted from between 7th
    and 8th segments of abdomen of ♂ of _Leucarctia acræa_, _Pyrrharctia
    isabella_, _Scepsis fulvicollis_, and _Cosmosoma omphale_.)

  —— [Notes on odors and odoriferous structures of various moths and a
    note by L. O. Howard on odor of Dynastes.] (Proc. Ent. Soc.,
    Washington, i, pp. 40, 55, 56.)

  =Müller, W.= Duftorgane der Phryganiden. (Archiv f. Naturgesch., 1887,
    Jahrg. liii, pp. 95–97.)

  =Pollack, W.= Duftapparate der _Hadena atriplicis_ und Litargyria. (xv
    Jahrb. Westphäl. Prov. Ver. Münster, 1887, p. 16.)

  =Patton, W. H.= Scent-glands in the larva of Limacodes. (Can. Ent.,
    1891, xxiii, pp. 42, 43.)

  =Garman, H.= On a singular gland possessed by the male _Hadenœcus
    subterraneus_. (Psyche, 1891, p. 105, 1 Fig.)

  =Barrett, C. G.= Scent of the male _Hepialus humuli_. (Ent. Month.
    Mag., Ser. 2, iii, 1892, p. 217. Arises from the curiously aborted
    and altered hind tibiæ.)

  Also the writings of Baillif, Duponchel, F. Müller, Scudder (Psyche,
    iii, p. 278, 1881), Burgess, Keferstein, Alpheraky, Plateau,
    Marshall and Nicéville, Wood-Mason, White, Hampson.




                       THE ORGANS OF CIRCULATION


Although Malpighi was the first to discover the heart in the young
silkworm, it was not until 1826 that Carus proved that there was a
circulation of blood in insects, which he saw flowing along each side of
the body, and coursing through the wings, antennæ, and legs of the
transparent larva of Ephemera, though three years earlier Herold
demonstrated that the dorsal vessel of an insect is a true heart,
pulsating and impelling a current of blood towards the head. This
discovery was extended by Straus-Dürckheim, who discovered the
contractile and valvular structures of the heart. It is noteworthy that
both Cuvier and Dufour denied that any circulation, except of air,
existed in insects; and so great an anatomist as Lyonet doubted whether
the dorsal vessel was a genuine heart, though he pointed out the fact
that there are no arteries and veins connected with this vessel. Another
French anatomist, Marcel de Serres, thought that the dorsal vessel was
merely the secreting organ of the fat-body.

  The so-called peritracheal circulation claimed by Blanchard and by
  Agassiz has been shown by McLeod to be an anatomical impossibility,
  the view having first been refuted by Joly in 1849.

  Except the aorta-like continuation in the thorax and head which
  divides into two short branches, there are, with slight exceptions
  (p. 405), no distinct arteries, such as are to be found in the
  lobster and other Crustacea, and no great collective veins, such as
  exist in Crustacea and in Limulus. This is probably the result of a
  reduction by disuse in the circulatory system, since in myriopods
  (Julidæ and Scolopendridæ) lateral arteries are said to diverge near
  the ostia.


                             _a._ The heart

The heart or “dorsal vessel” is a delicate, pulsating tube, situated
just under the integument of the back, in the median line of the body,
and above the digestive canal. It can be partially seen without
dissection in caterpillars. It is covered externally and lined within by
membranes which are probably elastic; and between these two membranes
extends a system of delicate muscular fibres, which generally have a
circular course, but sometimes cross each other. The heart is divided by
constrictions into chambers, separated by valvular folds. The internal
lining membrane referred to forms the valvular folds separating the
chambers. Each of these chambers has, at the anterior end, on each side,
a valvular orifice (Fig. 370, ostium, _i_) which can be inwardly closed.

  Miall and Denny thus describe the different layers of the wall of
  the heart of the cockroach:

  “There are: (1) a transparent, structureless intima, only visible
  when thrown into folds; (2) a partial endocardium, of scattered,
  nucleated cells, which passes into the interventricular valves; (3)
  a muscular layer, consisting of close-set, annular, and distant,
  longitudinal fibres. The annular muscles are slightly interrupted at
  regular and frequent intervals, and are imperfectly joined along the
  middle line above and below, so as to indicate (what has been
  independently proved) that the heart arises as two half-tubes, which
  afterwards join along the middle. Elongate nuclei are to be seen
  here and there among the muscles. The adventitia (4), or connective
  tissue layer, is but slightly developed in the adult cockroach.”

[Illustration:

  FIG. 370.—Part of the heart of _Lucanus cervus_: _a_, the posterior
    chambers (the anterior ones are covered by a part of the ligaments
    which hold the heart in place); _i_, auriculo-ventricular openings;
    _g_, _g_, the lateral muscles fixed by the prolongations _h_, _h_,
    to the upper side of the abdomen.—After Straus-Dürckheim.
]

Graber says that the heart of insects may be regarded not as an organ
_de novo_, but only as the somewhat modified contractile dorsal vessel
of the annelids, in which, however, the transverse arteries arising on
each side became, with the gradual development of the tracheæ,
superfluous and finally abortive. He describes it as a muscular tube
composed of very delicate annular fibres, which within and without is
covered by a relatively homogeneous, strong, elastic membrane.

  The division into separate chambers is effected by means of a
  folding inwards and forwards of the entire muscular wall. “A portion
  of each side of the heart is first extended inwards so as very
  nearly to meet a corresponding portion from the opposite side, and
  then, being reflected backwards, forms, according to Straus
  (Consid., etc., p. 356), the interventricular valve which separates
  each chamber from that which follows it. Posteriorly to this valve,
  at the anterior part of each chamber, is a transverse opening or
  slit (Fig. 371, _b_), the _auriculo-ventricular orifice_, through
  which the blood passes into each chamber, and immediately behind it
  is a second, but much smaller, _semilunar valve_ (_c_), which, like
  the first, is directed forwards into the chamber. It is between
  these two valves on each side that the blood passes into the heart,
  and is prevented from returning by the closing of the semilunar
  valve. When the blood is passing into the chamber, the
  interventricular valve is thrown back against the side of the
  cavity, but is closed when, by the contraction of the transverse
  fibres, the diameter of each chamber is narrowed, and the blood is
  forced along into the next chamber.” (Newport.)

[Illustration:

  FIG. 371.—A, heart of _Lucanus cervus_: _a_, valves or chambers;
    _bb_, alary muscles; _c_, supposed auricular space around the
    heart. _B_, division into arteries of the end of the aorta in
    larva of _Vanessa urticæ_. _C_, interior of the chamber, showing
    the transverse fibres; _b_, auriculo-ventricular opening and valve
    into the chambers; _c_, semilunar valve; _d_, interventricular
    valve.—After Straus-Dürckheim, from Newport.
]

[Illustration:

  FIG. 372.—Heart of Belostoma.—After Locy.
]

  According to Müller, there is but a single pair of ostia in Phasma,
  and, in the larva of Corethra, the heart is a simple, unjointed
  tube, not divided into chambers, and Viallanes states that, in the
  very young larva of Musca, there are no ostia (Kolbe). In the larva
  of Ptychoptera, Grobben found a short oval heart, with one pair of
  ostia situated in the 6th abdominal segment; a long aorta proceeds
  from it, the thoracic portion of which pulsates; from behind the
  heart arises a pulsating pouch, which connects with the hinder
  aorta, which does not pulsate, and ends at the base of two tracheal
  gills. Burmeister was able to find only four pairs of openings in
  the larva of Calosoma. Newport states that, while Straus figures
  nine chambers in Melolontha, and, consequently, eight pairs of
  openings, he has not been able to observe more than seven pairs of
  openings in _Lucanus cervus_. He has invariably found eight pairs of
  openings both in the larva and imago of _Sphinx ligustri_, as well
  as in other Lepidoptera. According to Béla-Dezso, the number of
  pairs of ostia corresponds to that of the pairs of stigmata.

  There also occur, on each side of each chamber, two so-called
  pear-shaped bodies which are separated from the tubular portion of
  the heart itself, but, by means of muscular fibres, are united with
  the chamber and with their valves. These pyriform bodies appear as
  vesicles or cells with granular contents, besides some nuclei with
  nucleoli. They are of very small size. According to the measurements
  of Dogiel, in the larva of _Corethra plumicornis_, they are 0.02 to
  0.1 mm. long, and 0.06 to 0.08 mm. broad. He regards these peculiar
  bodies as apolar nerve-cells of the heart. (Kolbe.)

[Illustration:

  FIG. 373.—A, part of the heart of _Dyticus marginalis_, showing the
    spiral arrangement of the muscular fibres; _c_, closed, _e_, open,
    valve; _a_, dorsal diaphragm with interwoven muscular fibres; _b_,
    arrangement of fibres, recalling the screw-like features of the
    fibres of the human heart; _d_, narrow end. _B_, diagrammatic
    figure of the valvular openings, with the terminal flap (_e_), and
    the cellular valve, of a May beetle; _a_, valvular opening of a
    dipterous larva, with the interventricular valve (_b_). _C_,
    abdomen of a mole-cricket, ventral view; _c_, the segmented heart;
    _a_, aorta; _b_, segmented diaphragm under it.—After Graber.
]

  Besides the venous openings of the heart which open into the
  pericardial region, Kowalevsky has discovered, in the heart of some
  Orthoptera (Caloptenus, Locusta, etc.), five pairs of openings by
  which the cardiac chambers receive the blood of the peri-intestinal
  region. Graber had divided the cœlom of insects into three regions
  (pericardial, peri-intestinal, and perineural regions), and hitherto
  only a union of the heart with the pericardial region by slit-like
  openings was known. These openings are symmetrically distributed on
  five abdominal segments; each section of the heart in this region
  has, therefore, four openings, which are all of a truly venous
  nature. These openings, called cardio-cœlomic apertures, are visible
  to the naked eye, being situated on conical papillæ of the walls of
  the heart. These papillæ pass through the outer diaphragm, and open
  into the peri-intestinal part of the cœlom, in the Acrydiidæ
  directly, in the Locustidæ through special canals. The cells of the
  papillæ are spongy, possessing large nuclei, and similar, as a
  whole, to glandular cells. (Comptes rendus, cxix, 1894.)

  The mechanism by which the ostia are closed consists, according to
  Graber, of an ∞-shaped muscle passing around the two openings, and
  which, being interlaced, is sufficient to close the openings. But
  this is not all. The fore and hinder edge of the ostia project,
  leaf-like, into the cavity of the heart, and thus form, with the
  outer walls, two valves which, during the systole, filled with the
  blood rushing in, not only hermetically close the lateral openings,
  but also, by the simultaneous closure of the entire chamber by the
  circular muscles in the middle of the same, the two valves,
  simultaneously approaching each other, so nearly touch that they
  form a transverse partition wall in the chamber. But, for the last
  purpose, _i.e._ for the separation of the chambers from one another,
  there is a very special contrivance. In the May beetle, we find,
  besides a valve (Fig. 373, _B_, _e_), opening into the middle of the
  chambers, a large, stalked cell (_d_), which, in the diastole,
  _i.e._ in the expansion of the heart, hangs down free on the walls
  of the heart; but, in the systole or contraction, like a cork,
  closes the middle of the valve, but does not wholly close the
  cavity. He has observed, in the larva of Corethra, formal,
  interventricular valves, which also are not in the middle, but are
  separated from one another in the interlaced ends. They consist of
  two longitudinally membranous flaps which move against each other
  like two valves (Fig. 373, _B_, _b_).

  “But what is the necessity for such a complicated mechanism? All the
  blood from behind passes into the heart, and, for its propulsion a
  simple muscular tube, whose circular fibres would draw together and
  contract it, would be thought to be sufficient. But the heart,
  except in some larvæ, ends posteriorly in a blind sac, and the blood
  can only pass into it by a series of pairs of lateral openings. Now,
  as regards the reception and the propulsion of the blood forwards,
  two modes are conceivable. The simplest way would be that the
  tubular heart should, along its whole length, contract or expand;
  that, moreover, the blood should be simultaneously sucked in through
  all the openings, and that then, also, the contraction, or systole,
  should take place in every part of the heart at the same moment. But
  this would, plainly, in so long and thin-walled a vessel, be highly
  impracticable, since, through such a manipulation, the mass of blood
  enclosed in the heart would be crowded together rather than really
  impelled forwards. Only the second case could be admissible, and
  that is this, that each chamber pulsates, one after another, from
  behind forwards. But, then, each segmental heart must be separated
  from the others by a valve. To make the matter wholly clear, we may
  observe an insect heart pulsating, and this is best seen in one of
  its middle chambers. This chamber expands (simply by the relaxation
  of its circular muscles), the ostia, also, consequently open, and a
  given quantity of blood is drawn in from the pericardial cavity.
  What now would happen after the succeeding contraction if there were
  no valves between? The blood would not flow forwards, but seek a way
  out backwards.

  “But, in fact, the valve of the hinder chamber, at this time, closes
  itself, while, by the simultaneous expansion of the anterior ones,
  their door opens, and this section of the heart, at the same time,
  causes a sucking in of the contents of the posterior chamber. This
  phenomenon is repeated, in the same way, from chamber to chamber,
  which also acts alternately as ventricle and auricle, or by a
  sucking and pumping action. One is involuntarily reminded of the
  ingenious manipulation by which, by the alternate opening and
  shutting of the flood-gates, a vessel is carried along a canal.

  “This wave-like motion of an insect’s heart also has the advantage
  that, just before a pulse-wave has reached the chambers farthest in
  front, the hinder ones are already prepared for the production of a
  second, for, as a matter of fact, often 60, and even 100, and, in
  very agile insects, 150, waves pass, in a single minute, through the
  series of chambers, which make it very difficult to follow the
  flowing of their waves.” (Graber.)

  =The propulsatory apparatus.=—But the heart itself is only a part of
  the entire propulsatorial apparatus to which belongs the following
  contrivance, the nature of which has been worked out by Graber.

  Under the dorsal vessel is stretched a sort of roof-like diaphragm,
  _i.e._ a membrane, arched like the dorsal wall of the hind-body
  which is attached, in a peculiar way, to the sides of the body. The
  best idea can be gained by a cross-section through the entire body
  (Fig. 374): _H_ is the true dorsal vessel; _S_, the diaphragm. A
  surface view is seen at 373, _C_, _b_, where it appears as a plate
  with the edge regularly curved outwards on each side. Its precise
  mode of working is thus: from each dorsal band of the sides of the
  abdomen arises a pair of muscles spreading out fan-like, and
  extending to the heart, so that the fibres of one side pass directly
  over to those of the other, often splitting apart, or, between the
  two, extends outwards a perforated, thin web, like an elastic,
  fibrous sheet (Fig. 373, _A_, _a_), with numerous perforations,
  forming a diaphragm.

[Illustration:

  FIG. 374.—Diagram of transverse section of pericardial sinus of
    _Ædipoda cœrulescens_: _H_, heart; _s_, septum; _m_, muscles,—the
    upper suspensory, the lower alary.—After Graber, from Sharp. (See
    also Fig. 377.)
]

  Graber has thus explained the action of the pericardial diaphragm
  and chamber, as freely translated by Miall and Denny: “When the
  alary muscles contract, they depress the diaphragm, which is arched
  upwards when at rest. A rush of blood towards the heart is thereby
  set up, and the blood streams through the perforated diaphragm into
  the pericardial chamber. Here it bathes a spongy or cavernous tissue
  (the fat-cells), which is largely supplied with air-tubes, and
  having been thus aerated, passes immediately forwards to the heart,
  entering it at the moment of diastole, which is simultaneous with
  the sinking of the diaphragm.”

  In the cockroach, however, Miall and Denny think that the facts of
  structure do not altogether justify this explanation: “The fenestræ
  of the diaphragm are mere openings without valves. The descent of a
  perforated non-valvular plate can bring no pressure to bear upon the
  blood, for it is not contended that the alary muscles are powerful
  enough to change the figure of the abdominal rings.... The diaphragm
  appears to give mechanical support to the heart, resisting pressure
  from a distended alimentary canal, while the sheets of fat-cells, in
  addition to their proper physiological office, may equalize small
  local pressures, and prevent displacement. The movement of the blood
  towards the heart must (we think) depend, not upon the alary
  muscles, but upon the far more powerful muscles of the abdominal
  wall, and upon the pumping action of the heart itself.”

  “The peculiar office,” says Graber, “performed by the heart has
  already been stated. It is nothing more than a regulator; than an
  organ for directing the blood in a determinate course in order that
  this may not wholly stagnate, or only be the plaything of a force
  acting in another way, as, for example, through that afforded by the
  body-cavity and the inner digestive canal. At regular intervals a
  portion of the blood is sucked through the same, and then by means
  of the anterior supply tube it is pushed onward into the head,
  whence it passes into the cavities of the tissues. The different
  conditions of tension under which the mass of blood stands in the
  different regions of the body then causes a farther circulation.
  Besides this, the blood passes through separate smaller pumping
  apparatuses, and through vessel-like modifications of cavities, also
  through hollow spaces between the muscles, as, for example, in the
  appendages where a regular backward and forward flow of the blood,
  especially in the limbs, wings, antennæ, and certain abdominal
  appendages takes place. Here and there may occasionally occur a
  narrow place where the flow of blood is obstructed by the
  accumulation of the blood corpuscles, causing a considerable
  stagnation.” (Graber.)

[Illustration:

  FIG. 375.—_Libellula depressa_, opened from the back, showing the
    nervous cord (_b_{1}_-_b_{3}_, thoracic, _h_{1}_-_h_{7}_, abdominal,
    ganglia), also the furrow-like ventral sinus closed by a muscular
    diaphragm.
]

[Illustration:

  FIG. 376.—_A_, part of the ventral furrow of _Libellula depressa_ more
    highly magnified: _a_, a sternal plate (urite); _c_, the septum
    stretched over it, at _s_ in a relaxed or collapsed state; _b_ and
    _d_, the wing-like, sternal processes from which the muscular
    bundles of the diaphragm arise. _B_, same in Acridium.
]

[Illustration:

  FIG. 377.—Diagrammatic section of the abdomen of _Acridium
    tartaricum_, showing the ventral septum (_i_, _p_, _l_) contracted,
    and (_i_, _k_, _l_) stretched out; _oh_, rib-like lateral processes
    of the urite; _f_, ganglia; _b_, heart, with its suspensorium (_a_);
    _c_, fat tissue in the pericardial tissue sinus; _d_, dorsal septum
    or diaphragm contracted. _q_, extended; _g_, fat-body; _e_, muscular
    part of diaphragm; _no_, expiration, _hm_, inspiration, muscle.—This
    and Figs. 375, 376, after Graber.
]

=The supraspinal vessel.=—In many insects there is a ventral heart
acting on the heart’s blood as an aspirator, or more correctly a ventral
sinus lying on the nervous cord, and closed by a pulsating diaphragm.
This was discovered by Réaumur in the larva of a fly, and by Graber in
the dragon-fly and locusts (Acrydiidæ). A glance at Figs. 375 and 376
will save a long description. The ventral wall forms a furrow, and
between its borders (Fig. 377, _e_) extends the diaphragm. During the
contraction of the muscles—and this, here, acts from before
backwards—the membrane rises up and makes a cavity for the blood, which
passes backwards over the nervous cord. The dorsal and ventral sinuses
together thus bring about a closed circulation.

It thus appears that the insects are well provided with the means of
distribution of their nutritive fluid, and that the blood is kept
continually fresh and rich in oxygen. (Graber.)

=The aorta.=—While the heart is mostly situated within the abdomen, it
is continued into the thorax and the head as a simple, non-pulsating
tube, called the aorta. In Sphinx the aorta, as described by Newport,
begins at the anterior part of the 1st abdominal segment, where it bends
downwards to pass under the metaphragma and enter the thorax; it then
ascends again between the great longitudinal dorsal muscles of the
wings, and passes onwards until it arrives at the posterior margin of
the pronotum; it then again descends and continues its course along the
upper surface of the œsophagus, with which it passes beneath the brain,
in front of which and immediately above the pharynx, it divides into two
branches, each of which subdivides. Newport, however, overlooked a
thoracic enlargement of the aorta called by Burgess the “aortal chamber”
(Fig. 310, _a_, _c_).

[Illustration:

  FIG. 378.—_A_, last three abdominal segments and bases of the three
    caudal processes of _Cloëon dipterum_: _r_, dorsal vessel; _kl_,
    ostia; _k_, special terminal chamber of the dorsal vessel with its
    entrance _a_; _b_, blood-vessel of the left caudal process. _B_,
    26th joint of the left caudal appendage from below: _b_, a portion
    of the blood-vessel; _o_, orifice in the latter.—After Zimmermann,
    from Sharp.
]

  “In Sphinx and _Vanessa urticæ_, immediately after the aorta has
  passed beneath the cerebrum, it gives off laterally two large
  trunks, which are each equal in capacity to about one-third of the
  main vessel. These pass one on each side of the head, and are
  divided into three branches which are directed backwards, but have
  not been traced farther in consequence of their extreme delicacy.
  Anterior to these trunks are two smaller ones which appear to be
  given to the parts of the mouth and antennæ, and nearer the median
  line are two others which are the continuations of the aorta. These
  pass upwards, and are lost in the integument. The whole of these
  parts are so exceedingly delicate that we have not, as yet, been
  able to follow them beyond their origin at the termination of the
  aorta, but believe them to be continuous, with very delicate,
  circulatory passages along the course of the tracheal vessels. It is
  in the head alone that the aorta is divided into branches, since,
  throughout its whole course from the abdomen, it is one continuous
  vessel, neither giving off branches, nor possessing lateral muscles,
  auricular orifices, or separate chambers.” (Newport, art. Insecta,
  p. 978.)

Dogiel observed in the transparent larva of _Corethra plumicornis_ that
the aorta extends only to the hinder border of the brain. Here it
divides into two lamellæ, each of which independently extends farther
on. One lamella is seen under the brain and under the eye, the other
reaches near the eye. The lamellæ are tied to the integument by threads.
At the point of division of the aorta is an opening. (Kolbe.)

  True blood-vessels appear to exist in the caudal appendages of the
  May-flies, as the heart appears to divide and pass directly into
  them (Fig. 378). The last chamber of the heart diminishes in size at
  the end of the body, and then divides into three delicate tubular
  vessels which pass into the three caudal appendages, and extend to
  the end of each one, along the upper side. While the valves of the
  heart, in all insects, are directed anteriorly because the blood
  flows from behind, in the larva of the Ephemeridæ the valves of the
  last chamber of the heart are directed backwards, because from this
  chamber the blood flows in the opposite direction, _i.e._ into the
  caudal appendages. During the contraction of the heart, the
  elongated section of the same in the last abdominal segment receives
  a part of the mass of blood contained in the last chamber, which is
  driven by independent contractions into the caudal appendages. These
  vessels have openings before the end through which the blood enters
  into the cavity of the appendages, and can also pass back, in order
  to be taken up by the body cavity. It is possible that these
  blood-vessels stand in direct relation to respiration. (Zimmermann,
  Creutzburg, in Kolbe, p. 544.)

  =The pericardial cells.=—Along the heart, on both sides, occur the
  so-called pericardial cells, which differ from the fat-cells, and
  also the peritracheal cells of Frenzel, and are mostly arranged in
  linear series, which have a close relation to the circulation of the
  blood. In the larva of Chironomus, they lie in groups; in that of
  Culex, they are arranged segmentally. In caterpillars, these
  pericardial cells are not situated in the region of the heart, but
  are arranged linearly on the side, and form a network of granulated
  cells situated between the fat-bodies. Other rows of these cells are
  situated near the stigmata and the main lateral tracheæ. (Kolbe.)

  According to Kowalevsky, the pericardial cells, and the
  garland-shaped, cellular cord consist of cells, whose function it is
  to purify the blood, and to remove the foreign or injurious matters
  mingled with the blood.

[Illustration:

  FIG. 379.—Diagram of the circulatory organs in the head of the
    cockroach, seen from above: _A_, ampulla; _V_, antennal vessel;
    _M_, chief muscular cord; _m_, muscular band; _Bs_, wall of the
    blood sinus; _am_, opening of the aorta (a); _rg_, anterior
    sympathetic or visceral ganglion; _hg_, hinder visceral ganglion;
    _F_, _F_, facetted eyes; _o_, vestigial ocellus; _G_, _G_, brain;
    _S_, œsophagus.—After Pawlowa.
]

  =Ampulla-like blood circulation in the head.=—In the head of the
  cockroach occurs, according to Pawlowa, a contractile vascular sac
  at the base of each antenna. The cavity has a valvular communication
  with the blood space below and in front of the brain, and
  muscle-fibres effect systole and diastole. Each sac is beyond doubt
  an independently active part of the circulatory system. These organs
  also occur in Locusta and other Acrydiidæ, and Selvatico has
  described similar structures in _Bombyx mori_ and certain other
  Lepidoptera.

  =Pulsatile organs of the legs.=—Accessory to the circulation is a
  special system of pulsatile organs in the three pairs of legs of
  Nepidæ, generally situated in the tibia just below its articulation
  with the femur, but in the fore legs of Ranatra, in the clasp-joint
  or tarsus, just below its articulation with the tibia. First
  observed by Behn (1835), Locy has studied the organ (Fig. 380) in
  Corixa, Notonecta, Gerris, besides the Nepidæ. It is a whip-like
  structure attached at both ends, with fibres extending upward and
  backward to the integument of the leg, separate from the muscular
  fibres and does not involve them in its motions, and is not affected
  by the muscles themselves. “As the blood-corpuscles flow near the
  pulsating body they move faster, and around the organ itself there
  is a whirlpool of motion.” The beating of these organs aids the
  circulation in both directions, and when the motion ceases, the
  blood-currents in the legs stop; the rate of the pulsating organ is
  always faster than that of the heart, and the action is automatic.

[Illustration:

  FIG. 380.—Pulsating organs in Hemiptera: _A_, Belostoma nymph, _B_,
    legs of Corixa. _C_, Ranatra, adult, to show the exceptional
    position of the pulsating organ in the fore legs. _D_, pulsatile
    organ in tibia of Ranatra.—After Locy.
]


                             _b._ The blood

The blood of insects, as in other invertebrates, differs from that of
the higher animals in having no red corpuscles. It is a thin fluid, a
mixture of blood (serum) and chyle, usually colorless, but sometimes
yellowish or reddish, which contains pale amœboid corpuscles
corresponding to the white corpuscles (leucocytes) of the vertebrates,
though they are relatively less numerous in the blood of insects. The
yellow fluid expelled from the joints of certain beetles (Coccinella,
Timarcha, and the Meloidæ) is, according to Leydig, only the serum of
the blood. In phytophagous insects the blood is colored greenish by the
chlorophyll set free during digestion. The blood of _Deilephila
euphorbia_ is colored an intense olive-green, and that of _Cossus
ligniperda_ is pale yellow. (Urech.) The blood of case-worms
(Trichoptera) is greenish. In some insects it is brownish or violet. The
serum is the principal bearer of the coloring material, yet Graber has
shown that in certain insects the corpuscles are more or less beset with
bright yellow or red fat-globules, so as to give the same hue to the
blood.

=The leucocytes.=—The corpuscles are usually elongated, oval, or
flattened oat-shaped, with a rounded nucleus, or are often amœbiform;
and they are occasionally seen undergoing self-division. When about to
die the corpuscles become amœbiform or star-shaped. (Cattaneo.) Their
number varies with the developmental stage of the insect, and in larvæ
increases as they grow, becoming most abundant shortly before pupation.
The blood diminishes in quantity in the pupal stage, and becomes still
less abundant in the imago. (Landois.) The quantity also varies with the
nutrition of the insect, and after a few days’ starvation nearly all the
blood is absorbed. Crystals may be obtained by evaporating a drop of the
blood without pressure; they form radiating clusters of pointed needles.
The freshly drawn blood is slightly alkaline. (Miall and Denny.)

  The size of the corpuscles has been ascertained by Graber, who found
  that the diameter of the circular blood-disks of the leaf-beetle,
  _Lina populi_, is 0.006 mm.; of _Cetonia aurata_ and _Zabrus
  gibbus_, 0.008 to 0.01 mm.; and those of certain Orthoptera
  _(Decticus verrucivorus_, _Ephippiger vitium_ and _Œdipoda
  cœrulescens_), 0.011 to 0.014 mm. The longest diameter of the
  elongated corpuscles of _Carabus cancellatus_ is 0.008 mm.; of
  _Gryllus campestris_, _Locusta viridissima_, _Cossus ligniperda_,
  _Sphinx ligustri_ (pupa), and others, 0.008 to 0.01 mm.; of
  _Caloptenus italicus_, _Saturnia pyri_, _Anax formosus_, and others,
  0.011 to 0.014 mm.; of _Ephippiger vitium_, _Œdipoda cœrulescens_,
  _Pezotettix mendax_, _Zabrus gibbus_, _Phryganea_, and others, 0.012
  to 0.022 mm.; in _Stenobothrus donatus_ and _variabilis_, 0.012 to
  0.035 mm. The largest known are those of _Melolontha vulgaris_,
  which measure from 0.027 to 0.03 mm.

[Illustration:

  FIG. 381.—Blood corpuscles, or leucocytes, of insects: _A_, _a-g_, of
    _Stenobothrus dorsatus_ (the same forms occur in most Orthoptera and
    in other insects). _B_, _a_, leucocyte of the same insect with the
    nucleus brought out by ether; _b_, another of serpentine shape. _C_,
    leucocytes of the same insect after a longer stay in ether. _D_,
    leucocytes of the same after being in glycerine 14 days.—After
    Graber.
]

As regards the nature of the corpuscles, Graber concludes that they are
more like the cells of the fat-bodies than genuine cells. That they are
not true cells is shown by the fact that after remaining in their normal
condition a long time they finally coalesce and form cords. After
shrivelling, or after the blood has been subjected to different kinds of
treatment, the nucleus is clearly brought out (Fig. 381).

Besides the blood corpuscles there have been detected in the blood round
bodies which are regarded as fat-cells. They are circular, and for the
most part larger than the blood corpuscles, have a sharp, even, dark
outline, and an invariably circular nucleus. (Kolbe.)

  The blood of Meloe, besides the amœboid corpuscles, according to
  Cuénot, contains abundant fibrinogen, which forms a clot; a pigment
  (uranidine), which is oxidized and precipitated when exposed to the
  air; a dissolved albuminoid (hæmoxanthine), which has both a
  respiratory and nutritive function; and, finally, dissolved
  cantharidine.

  The corpuscles arise from tissues which are very similar to the
  fat-bodies, and which, at given times, separate into cells. The
  position of these tissues is not always the same in different
  insects. In caterpillars, they occur in the thorax, near the germs
  of the wings; in the saw-flies (Lyda), in all parts of the thorax
  and abdomen; in larval flies (Musca), in the end of the abdomen,
  just in front of the large terminal stigmata. The place where the
  blood corpuscles are formed is usually near, or in relation with,
  the fat-bodies. But while the fat-bodies mostly serve as the
  material for the formation of the blood-building tissues, in
  caterpillars the tracheal matrix also, and, in dipterous larvæ, the
  hypodermis serve this purpose. (Cæsar Schaeffer in Kolbe. See also
  Wielowiejski, Ueber das Blutgewebe.)

  Other substances occur in the blood of insects. Landois (1864)
  demonstrated the existence of egg albumen, globulin, fibrin, and
  iron in the blood of caterpillars. Poulton found that the blood of
  caterpillars often contained chlorophyll and xanthophyll derived
  from their food plants. A. G. Mayer has recently found that the
  blood (hæmolymph) of the pupæ of Saturniidæ (_Callosamia promethea_)
  contains egg albumin, globulin, fibrin, xanthophyll, and
  orthophosphoric acid, and Oenslager has determined that iron,
  potassium, and sodium are also present. (Mayer.)


                   _c._ The circulation of the blood

Every part of the body and its appendages is bathed by the blood, which
circulates in the wings of insects freshly emerged from the nymph or
pupal state, and even courses through the scales of Lepidoptera, as
discovered by Jaeger (Isis, 1837).

In describing the mechanism of the heart we have already considered in a
general way the mode of circulation of the blood.

The heart pumping the blood into the aorta, the nutritive fluid passes
out and returns along each side of the body; distinct, smaller streams
passing into the antennæ, the legs, wings (of certain insects), and into
the abdominal appendages when they are present. All this may readily be
observed in transparent aquatic insects, such as larval Ephemeræ,
dragon-flies, etc., kept alive for the purpose under the microscope in
the animalcule box.

Carus, in 1827, first discovered the fact of a complete circulation of
the blood, in the larva of Ephemera. He saw the blood issuing in several
streams from the end of the aorta in the head and returning in currents
which entered the base of the antennæ and limbs in which it formed
loops, and then flowing into the abdomen, entered the posterior end of
the heart. Wagner (Isis, 1832) confirmed these observations, adding one
of his own, that the blood flows backward in two venous currents, one at
the sides of the body and intestine, and the other alongside of the
heart itself, and that the blood not only entered at the end of the
heart, but also at the sides of each segment, at the position of the
valves discovered by Straus-Dürckheim.

Newport maintains that the course of the blood is in any part of the
body, as well as in the wings, almost invariably in immediate connection
with the course of the tracheæ, for the reason that “the currents of
blood in the body of an insect are often in the vicinity of the great
tracheal vessels, both in their longitudinal and transverse direction
across the segments.”

The circulation of the blood in the wings directly after the exuviation
of the nymph or pupa skin, and before they become dry, has been proved
by several observers. As stated by Newport, the so-called “veins” or
“nervures” of the wings consist of tracheæ lying in a hollow cavity, the
peritracheal space being situated chiefly under and on each side of the
trachea.

[Illustration:

  FIG. 382.—Circulation of the blood in hind wing of _Periplaneta
    orientalis_: the arrows indicate the usual direction of the blood
    currents.—After Moseley.
]

  Newport gives the following summary of the observations of the early
  observers, to which we add the observations of Moseley. “A motion of
  the fluids has been seen by Carus in wings of recently developed
  Libellulidæ, _Ephemera lutea_ and _E. marginata_, and _Chrysopa
  perla_; among the Coleoptera, in the elytra and wings of _Lampyris
  italica_ and _L. splendidula_, _Melolontha solstitialis_ and
  Dytiscus.” Ehrenberg saw it in Mantis, and Wagner in the young of
  _Nepa cinerea_ and _Cimex lectularius_. Carus detected a circulation
  in the pupal wings of some Lepidoptera, and Bowerbank witnessed it
  in a Noctuid (_Phlogophora meticulosa_); Burmeister observed it in
  _Eristalis tenax_ and _E. nemorum_, and Mr. Tyrrel in _Musca
  domestica_, but it has not been observed in the wings of
  Hymenoptera.

  Bowerbank observed that in the lower wing of _Chrysopa perla_ the
  blood passes from the base of the wing along the costal,
  post-costal, and externo-medial veins, outwards to the apex of the
  wing, giving off smaller currents in its course, and that it returns
  along the anal vein to the thorax. He found that the larger veins,
  1⁄408 in. in diameter, contained tracheæ which only measured 1⁄2222
  of an inch in diameter; but in others the tracheæ measured 1⁄1340,
  while the cavity measured only 1⁄500 of an inch. He states, also,
  that the tracheæ very rarely give off branches while passing along
  the main veins, and that they lie along the canals in a tortuous
  course. (Newport, art. Insecta, p. 980.)

  Bowerbank, also, in his observations on the circulation in the wings
  of Chrysopa, “used every endeavor to discover, if possible, whether
  the blood has proper vessels, or only occupied the internal cavities
  of the canals; and that he is convinced that the latter is the case,
  as he could frequently perceive the particles not only surrounding
  all parts of the tracheæ, and occupying the whole of the internal
  diameter of the canals, but that it frequently happens that globules
  experienced a momentary stoppage in their progress, occasioned by
  their friction against the curved surface of the tracheæ, which
  sometimes gave them a rotatory motion.”

[Illustration:

  FIG. 383.—Parts of a vein of the cockroach, showing the nerve (_n_)
    by the side of the trachea (_tr_); _c_, blood corpuscles.—After
    Moseley.
]

  Moseley found, owing to the large size and number of the corpuscles,
  that the circulation of the blood in the wings of insects is most
  easily observed in the cockroach, especially the hind wings. As seen
  in Moseley’s figure, the blood flows outward from the body through
  the larger veins (I and II) of the front edge of the wings, which he
  calls the main arteries of the wings, and more generally returns to
  the body through the veins in the middle of the wing; the blood also
  flows out from the body through the inner longitudinal veins (those
  behind vein IV), and the blood is also seen to flow through some of
  the small cross-veins. Fig. 383 shows one of the main trunks during
  active circulation. The corpuscles change their form readily, “the
  spindle-shaped ones doubling up in order to pass crossways through a
  narrow aperture.... In the irregularly formed corpuscles, which seem
  to represent leucocytes amœboid movements were observed....
  Corpuscles pass freely above and under the tracheæ, showing that
  these latter lie free in the vessels.” The hypodermis lining the
  vessels is best seen in the small transverse veins.

The pulse or heart-beat of insects varies in rapidity in different
insects, rising at times of excitement, as Newport noticed in
_Anthophora retusa_, to 142 beats in a minute.

When an insect, as, for example, a tineid caterpillar, has been enclosed
in a tight box for a day or more, the pulsations of the heart are very
languid and slow, but soon, on giving it air, the pulsations will, as we
have observed, rise in frequency to about 60 a minute, Herold observed
30 to 40 in a minute in a fully-grown silkworm, and from 46 to 48 in a
much younger one. Suckow observed but 30 a minute in a full-grown
caterpillar of _Gastropacha pini_, and 18 only in its pupa.

  In a series of observations made by Newport on _Sphinx ligustri_
  from the fourth day after hatching from the egg until the perfect
  insect was developed, he found that before the larva cast its first
  skin the mean number of pulsations, in a state of moderate activity
  and quietude, was about 82 or 83 a minute; before the second moult
  89, while before the third casting it had sunk down to 63; and
  before its fourth to 45, while, before leaving its fourth stage, and
  before it had ceased to feed, preparatory to pupating, the pulse was
  not more than 39. “Thus the number gradually decreases during the
  growing larva state, but the force of the circulation is very much
  augmented. Now when the insect is in a state of perfect rest,
  previously to changing its skin, the number is pretty nearly equal
  at each period, being about 30. When the insect has passed into the
  pupa state it sinks down to 22, and subsequently to 10 or 12, and
  after that, during the period of hibernation, it almost entirely
  ceases. But when the same insect which we had watched from its
  earliest condition was developed into the perfect state in May of
  the following spring, the number of pulsations, after the insect had
  been for some time excited in flight around the room, amounted to
  from 110 to 139; and when the same insect was in a state of repose,
  to from 41 to 50. When, however, the great business of life, the
  continuation of the species, has been accomplished, or when the
  insect is exhausted, and perishing through want of food or other
  causes, the number of pulsations gradually diminishes, until the
  motions of the heart are almost imperceptible.” Insects, then, he
  remarks, do not deviate from other animals in regard to their vital
  phenomena, though it has been wrongly imagined that the nutrient and
  circulatory functions are less active in the perfect than in the
  larval condition.

  The heart of a larval _Gastrus equi_ taken the day previous from a
  horse’s stomach beat from 40 to 44 times a minute (Scheiber); while
  Schröder van der Kolk observed only 30 beats in the same kind of
  maggot.

  In the larva of Corethra, while at rest, the heart contracts from 12
  to 16 or 18 times a minute, but when active the number rises to 22.
  The systole and diastole last from 5 to 6 minutes. (Dogiel.)

  Temperature also affects the pulsations, as they increase in
  frequency with a rise and decrease with a fall in temperature.

  =Influence of electricity.=—The influence of electricity on the
  action of the insect’s heart, from Dogiel’s experiments, is such as
  to cause an acceleration in the frequency of the beats, while an
  increase in the strength of the electric currents either diminishes
  the frequency of the beats or entirely stops the heart’s action. A
  violent excitation with the induction current causes a systole when
  the heart’s action has stopped for a long time; and if the
  excitation lasts uninterruptedly, then the contractions after a
  while become noticeable, according to the strength of the current.
  In such a case there are, however, interruptions in the regularity,
  strength, and order of the contractions. (Kolbe.)

  =Effects of poisons on the pulsations.=—Dogiel has also experimented
  on the influence of poisons in the form of vapor or as liquid
  solutions on the pulsations of insects, which is much as in
  vertebrates. The application of carbonic oxide to the larva of
  Corethra, whose heart one minute previous to the poisoning beat 15
  times a minute, accelerated the heart-beats in about 55 minutes to
  25 pulsations in a minute. Afterwards there was a retardation in the
  pulse to the normal beat. Carbonic acid had a similar effect.

The following results obtained by Dogiel are somewhat as tabulated by
Kolbe:—


  I. Substances which cause the pulsations of the heart to accelerate.

       _a._  An induction current of electricity, acting feebly.
       _b._  Ammonia, acting feebly.
       _c._  Ethyl ether, acting feebly.
       _d._  Oxalic acid, acting feebly.
       _e._  Carbolic acid, acting feebly.
       _f._  Potassium nitrate, acting feebly.
       _g._  Aconite, acting feebly.


              II. Substances retarding the heart’s action.

    _a._  An induction current of electricity, acting energetically.
    _b._  Ammonia, acting energetically.
    _c._  Ethyl ether, acting energetically.
    _d._  Oxalic acid, acting energetically.
    _e._  Carbolic acid, acting energetically.
    _f._  Veratrine, acting energetically.
    _g._  Atropine, acting energetically.
    _h._  Aconitine, acting energetically.
    _i._  Potassium nitrate, acting energetically.
    _g._  Ethyl alcohol.
    _h._  Chloroform.
    _i._  Carbonic oxide.
    _j._  Carbonic acid.
    _k._  Sulphuretted hydrogen.


              III. Substances whose action is indifferent.

                     _1._  Muscarine.
                     _2._  Curare.
                     _3._  Atropine, acting slowly.
                     _4._  Strychnine.

  The above-named substances comprise those which in the vertebrates
  effect a change in the activity of the motor nerve-ganglia of the
  heart and the muscular fibres. Hence it follows that the heart of
  the larval Corethra consists of muscular fibres provided with
  ganglia, and that the contractions of the muscular fibres are
  provoked through the agency of the ganglia. But since muscarine,
  atropine, and curare, whose influence in stopping the heart’s action
  of vertebrates is known, in insects either have no action or only
  make the pulsations slower; it seems to follow that the heart of the
  larval Corethra possesses no similar apparatus for lessening the
  heart’s action, and this is also confirmed by anatomical studies. On
  the contrary, aconite acts, as we must from observations conclude,
  exclusively on the motor centres and the muscles, but not on the
  apparatus for lessening the heart’s action, which, as has been
  remarked, is not present in the larval Corethra. (Kolbe _ex_
  Dogiel.)

  Dewitz has discovered an onward movement of the blood corpuscles,
  somewhat independent of the general circulation. This independent
  motion of the blood corpuscles is not only a creeping one like the
  amœboid motion of the white corpuscles of vertebrates, but they have
  besides a peculiar swimming movement. Dewitz noticed this in the
  hind wings of a recently emerged meal-worm beetle (_Tenebrio
  molitor_), still white and soft, after they had been cut off. The
  tissues forming the matrix within the wings constitute a network
  filled with blood. The current of blood within the wing thus cut off
  may be stopped flowing by a tap on the firmly clamped object-bearer
  on which the wing is placed, or by drawing it by an apparatus
  described by the same author, to incite in one way or another the
  blood corpuscles to swim forwards. When a corpuscle is disposed to
  move, we see it first stirring restlessly, or wabbling about, in
  this way changing its form; then it moves forwards, and does not
  come to a standstill. If it remains still there, after a while, by
  tapping, it begins again its movements.

  “Should one yet doubt the fact of this spontaneous movement of the
  blood corpuscles, he will surely be convinced of its correctness by
  observing the so-to-speak reluctantly springing motion of a blood
  corpuscle in the wing of _Tenebrio molitor_ with the simultaneous
  change of appearance and shape of the corpuscle.”

  This spontaneous or independent motion of the blood corpuscles is
  also produced by the heating apparatus. As soon as the corpuscles
  lie still in the severed wing and they are warmed, the corpuscles
  begin to pass through the meshes of the tissue. When cooled, the
  motion ceases, but as soon as the temperature rises to a certain
  grade, the corpuscles again move onwards.

  To explain this independent motion Dewitz thinks that they take up
  and then expel the blood-fluid, and in this way cause their motion.
  This independent motion is necessitated, in order that the stream of
  blood may become so regulated, that the blood corpuscles shall not
  be arrested in their course, but even turn back again out of the
  farther end of the antennæ and limbs. The chief mechanical power for
  the blood circulation must go on independently of the propulsatorial
  apparatus and of the heart. (Kolbe.)




      LITERATURE ON THE HEART AND ON THE CIRCULATION OF THE BLOOD


                       _a._ Anatomy of the organs

  =Meckel, J. F.= Ueber das Rückengefäss der Insekten. (Meckel’s Archiv,
    i, 1815, pp. 469–476.)

  =Müller, J. G.= De vasi dorsali Insectorum. Berolini, 1816, pp. 22.

  =Serres, P. Marcel de.= Observations sur les usages du vaisseau dorsal
    ou sur l’influence que le cœur exerce dans l’organisation des
    animaux articulés, etc. (Ann. du Mus. d’hist. nat., 1818, iv, pp.
    149–192, 313–380, 2 Pls.; v, 1819, pp. 59–147, 1 Pl.)

  =Herold.= Physiologische Untersuchungen über das Rückengefäss der
    Insekten. (Schriften d. Gesellsch. z. Beförderung d. Naturk. in
    Marburg, 1823, i, pp. 41–107.)

  =Carus, C. G.= Entdeckung eines einfachen vom Herzen aus
    beschleunigten Blutkreislaufes in den Larven netzilügliger Insekten.
    Leipzig, 1827, pp. 40, 3 Taf.

  —— Fernere Untersuchungen über Blutlauf in Kerfen. (Acta Acad.
    Leopold. Carol., 1831, xv, pp. 1–18, 1 Taf.)

  =Stadelmayr, L.= Ansichten vom Blutlauf nebst Beobachtungen über das
    Rückengefäss der Insekten. Diss. München, 1829, pp. 24.

  =Berthold.= Beitrage zur Anatomie, Zoologie und Physiologie.
    Göttingen, 1831.

  =Treviranus, G. R.= Ueber das Herz der Insekten, dessen Verbindung mit
    den Eierstocken und ein Bauchgefäss der Lepidopteren. (Zeitschr. f.
    d. Physiologie, von F. Tiedemann. G. R. u. L. C. Treviranus, 1832,
    iv, pp. 181–184, 1 Taf.)

  —— Beobachtungen aus der Zootomie und Physiologie. Bremen, 1839.

  =Wagner, R.= Beobachtungen über Kreislauf des Blutes und den Bau des
    Rückengefässes bei den Insekten. (Isis, 1832, iii, p. 30; vii, pp.
    320–331, 778–783, Fig.)

  =Bowerbank, J. S.= Observations on the circulation of the blood in
    insects. (Ent. Mag., 1833, i, pp. 239–244, 1 Pl; also in Müller’s
    Archiv f. Physiolog., 1834, i, pp. 119–120.)

  —— Observations on the circulation of the blood and the distribution
    of the tracheæ in the wing of _Chrysopa perla_. (Ent. Mag., 1837,
    iv, pp. 179–185.)

  =Jaeger.= Ueber die Entdeckung von einer Bewegung in den Schuppen des
    Schmetterlingsflügel. (Isis, 1837, v, p. 512.)

  =Behn, W.= Découverte d’une circulation de fluide nutritif dans les
    pattes de plusieurs insectes hémipteres. (Ann. Sc. nat., 1835, Sér.
    2, iv, pp. 1–12.)

  =Newport, G.= Insecta, in Todd’s Cyclopædia of Anatomy and Physiology,
    1839. London, pp. 853–994. On the circulation of the blood, p. 976,
    Figs.

  =Duvernoy, G. L.= Résumé sur le fluide nourricier, ses réservoirs et
    son mouvement dans tout règne animal. (Ann. Sc. nat., 1839, Sér. 2,
    xii, pp. 300–346.)

  =Dufour, L.= Études anatomiques et physiologiques sur une mouche dans
    le but d’eclairer l’histoire des metamorphoses et de la prétendue
    circulation dans les insectes. (Ann. Sc. nat. Zool., Sér. 2, 1841,
    xvi, pp. 5–14.)

  —— Note sur la prétendus circulation dans les insectes. (Compt. rend.
    Acad., Paris, 1844, xix, pp. 188–189.)

  —— Études anatomiques et physiologiques sur une mouche, dans le but
    d’éclaircir l’histoire des metamorphoses et de prétendue circulation
    des insectes. (Mém. mathémat. des Savants étrangers, Paris, 1846,
    ix, pp. 545–628, 1 Pl.)

  —— Sur la circulation dans les insectes. Bordeaux, 1849, 8º, pp. 40.
    (Compt. rend. Acad. Sci., Paris, 1849, xxviii, pp. 28–33, 101–104,
    163–170.)

  —— De la circulation du sang et de la nutrition chez les insectes.
    Bordeaux, 1851. (Act. Soc. Linn., Bordeaux, 1851, xvii, p. 9.)

  —— Études anatomiques et physiologiques et observations sur les larves
    des Libellules, Appareil circulatoire. (Annal. Sci. nat., Sér. 3,
    Zool., xvii, 1852, pp. 98–101, 1 Pl.)

  =Schröder van der Kolk, J. S. C.= Mémoire sur l’anatomie et
    physiologie du _Gastrus equi_. (N. Verhandl. Kl. Nederl. Instit.,
    11, 1845, pp. 1–155, 13 Pl.)

  =Nicolet, H.= Note sur la circulation du sang chez les Coléoptères.
    (Ann. Sc. nat., 1847, Sér. 3, vii, pp. 60–64.)

  =Verloren, C.= Mémoire en réponse à la question suivante: éclaircir
    par des observations nouvelles le phénomène de la circulation dans
    les insectes, en recherchant si on peut la reconnaître dans les
    larves de différents ordres de ces animaux. (Mém. couronn. et Mém.
    d. savants étrang. de l’Acad. Roy. Belgique, xix, 1847.)

  =Blanchard, E.= De la circulation dans les insectes. (Ann. Sc. nat.,
    1848, Sér. 3, ix, pp. 359–398, 5 Pls.)

  —— Sur la circulation du sang chez les insectes et sur la nutrition.
    (Compt. rend. Acad. Sc., Paris, 1849, xxviii, pp. 76–78; 1851,
    xxxiii, pp. 367–370.)

  —— Nouvelles observations sur la circulation du sang et la nutrition
    chez les insectes. (Ibid., pp. 371–376.)

  =Joly, N.= Mémoire sur l’existence supposée d’une circulation
    péritrachéenne chez les insectes. (Ann. Sc. nat. Zool., Sér. 3,
    1849, xii, pp. 306–316.)

  =Bassi, C. A.= Rapporto alla sezione di zoologia, anatomia comparata e
    fisiologia del congresso di Venezia, sul passagio delle materie
    ingerite nel sistema tracheale degli insetti. (Gazette di Milano,
    1847, vi; also Ann. Sc. nat. Zool., Sér. 3, 1851, xv., 362–371.)

  =Agassiz, Louis.= On the circulation of the fluids in insects.
    (Proceed. Amer. Assoc. Adv. Sc., 1849, pp. 140–143; Ann. Sc. nat.
    Zool., Sér. 3, xv, 1851, pp. 358–362.)

  =Leydig, F.= Anatomisches und Histiologisches über die Larve von
    _Corethra plumicornis_. (Zeitschr. f. wissen. Zool., iii, 1851, pp.
    435–451.)

  =Wedl, C.= Ueber das Herz von _Menopon pallidum_. (Sitzungsber. der k.
    Akad. d. Wissensch. Wien., 1855, xvii, pp. 173–180.)

  =Scheiber, S. H.= Vergleichende Anatomie und Physiologie der Oestriden
    Larven. (Sitzungsber. d. k. Akad. d. Wiss. Wien. Math.-naturwiss.
    Cl., xli, 1860, pp. 409–496, 2 Taf.; The circulatory system, pp.
    463–490.)

  =Brauer, Fr.= Beitrag zur Kenntnis des Baues und der Funktion der
    Stigmenplatten der Gastrus-Larven. (Verhdl. d. k. k. zool.-bot.
    Gesellsch. Wien., xiii, 1863, pp. 133–136.)

  =Moseley, H. N.= On the circulation in the wings of _Blatta
    orientalis_ and other insects, and on a new method of injecting the
    vessels of insects. (Quart. Jour. of Micr. Science, xi, n. s., pp.
    389–395, 1871, 1 P1.)

  =Graber, V.= Ueber die Blatkörperchen der Insekten. (Sitzber. Akad.
    Wien. Math.-naturw. Classe, lxiv, 1871, pp. 9–44, 1 Taf.)

  —— Vorläufiger Bericht über den propulsatorischen Apparat der
    Insekten. (Sitzber. d. k. Ak. d. Wiss. Wien., lxv, 1872, pp. 16, 1
    Taf.)

  —— Ueber den propulsatorischen Apparat der Insekten. (Archiv f.
    mikroskop. Anatomie, ix, 1873, pp. 129–196, 3 Taf.)

  —— Ueber den pulsierenden Bauchsinus der Insekten. (Archiv f.
    mikroskop. Anat., xii, 1876, pp. 575–582, 1 Taf.)

  =Grobben, Carl.= Über bläschenförmige Sinnesorgane und eine
    eigenthümliche Herzbildung der Larve von _Ptychoptera contaminata_
    L. (Sitzb. k. Akad. Wissensch. Wien., 1875, lxxii, p. 22, 1 Taf.)

  =Liebe, Otto.= Ueber die Respiration der Tracheaten, besonders über
    den Mechanismus derselben und über die Menge der ausgeatmeten
    Kohlensäure. Inaug.-Diss. Chemnitz, 1872, pp. 28.

  =Dogiel, John.= Anatomie und Physiologie des Herzens der Larve von
    _Corethra plumicornis_. (Mém. Acad. imp. St. Petersbourg, 7 Sér.,
    xxiv, 1877, Nr. 10, pp. 37, 2 Pls.) Separate, Leipzig, Voss.

  =Bütschli, O.= Ein Beitrag zur Kenntnis des Stoffwechsels,
    insbesondere der Respiration bei den Insekten. (Reichert’s und Du
    Bois-Reymond’s Archiv f. Anatomie u. Physiologie, 1874, pp.
    348–361.)

  =Béla-Dezso.= Ueber den Zusammenhang des Kreislaufs und der
    respiratorischen Organe bei den Arthropoden. (Zool. Anzeiger, i
    Jahrg., 1878, p. 274.)

  =Plateau, F.= Communication préliminaire sur les mouvements et
    l’innervation de l’organe central de la circulation chez les animaux
    articulés. (Bull. Acad. roy. de Belgique, Sér. 2, xlvi, 1878, pp.
    203–212.)

  =Jaworovski, Ant.= Ueber die Entwicklung des Rückengefässes und
    speziell der Muskulatur bei Chironomus und einigen anderen Insekten.
    (Sitzgsber. d. k. Akad. d. Wissensch. Wien. Math.-naturwiss. Cl.,
    lxxx, 1879, pp. 238–258.)

  =Zimmermann, O.= Ueber eine eigentümliche Bildung des Rückengefässes
    bei einigen Ephemeridenlarven. (Zeitschr. f. wissens. Zool., 1880,
    xxxiv, pp. 404–406.)

  =Burgess, E.= Note on the aorta in lepidopterous insects. (Proc. Bost.
    Soc. Nat. Hist., xxi, 1881, pp. 153–156, Figs.)

  —— Contributions to the anatomy of the milk-weed butterfly, _Danais
    Archippus_ F. (Anniversary Memoirs Boston Soc. Nat. Hist., 1880, pp.
    16, 2 Pls.)

  =Vayssière, A.= Recherches sur l’organisation des larves des
    Éphémérines. (Ann. Sc. nat. Zool., Sér. 6, xiii, 1882, pp. 1–137, 11
    Pls.)

  =Viallanes, H.= Recherches sur l’histologie des insectes et sur les
    phénomènes histologiques qui accompagnent le développement
    post-embryonnaire de ces animaux. (Ann. Sc. nat., Sér. 6, xiv, 1882,
    pp. 1–348, 18 Pls.)

  =Schimkewitsch, W.= Ueber die Identität der Herzbildung bei den wirbel
    und wirbellosen Tieren. (Zool. Anzeiger, 1885, viii Jahrg., pp.
    37–40, Fig.)

  —— Noch etwas über die Identität der Herzbildung bei den Metazoen.
    (Zool. Anzeiger, 1885, pp. 384–386.)

  =Creutzburg, N.= Ueber den Kreislauf der Ephemerenlarven. (Zool.
    Anzeiger, 1885, pp. 246–248.)

  =Poletajewa, Olga.= Du cœur des insectes. (Zool. Anzeiger, 1886, ix
    Jahrg., pp. 13–15.)

  =Selvatico, S.= L’aorta nel corsaletto e nel capo della farfalla del
    bombice del gelso. (Padova, 1887, p. 19, 2 Pls.)

  —— Die Aorta im Brustkasten und im Kopfe des Schmetterlings von
    _Bombyx mori_. (Zool. Anzeiger, 1887, x Jahrg., pp. 562–563.)

  =Kowalevsky, A.= Ein Beitrag zur Kenntnis der Excretionsorgane. (Biol.
    Centralbl., 1889, ix, pp. 33–47, 65–76, 127–128.)

  =Tosi, Alessandro.= Osservazioni sulla valvola del cardias in varii
    generi della famiglia delle Apidi. (Ricerche Lab. Anat. R. Univ.
    Roma, v, 1895, pp. 5–26, 16 Figs., 3 Pls.)

  =Pawlowa, Mary.= Ueber ampullenartige Blutcirculationsorgane im Kopfe
    verschiedener Orthopteren. (Zool. Anzeiger, xviii Jahrg., 1895, pp.
    7–13, 1 Fig.)

  Also the writings of Kolbe.


     _b._ The blood, blood corpuscles, leucocytes, and blood tissue

  =Wagner, R.= Ueber Blutkörperchen bei Regenwürmern, Blutegeln und
    Dipterenlarven. (Müller’s Archiv f. Anatomie u. Physiologie, 1835,
    pp. 311–313.)

  —— Nachtrage zur vergleichenden Physiologie des Blutes. (Archiv f.
    Anat. u. Physiologie, 1838.)

  =Newport, G.= On the structure and development of the blood. First
    series. The development of the blood corpuscle in insects and other
    invertebrata, and its comparison with that of man and the
    vertebrata. (Abstr. of the paper in Roy. Soc., 1845, v, pp. 544–546:
    also in Ann. Mag. Nat. Hist., Ser. 3, 1845, iii, pp. 364–367.)

  =Landois, H.= Beobachtungen über das Blut der Insekten. (Zeitschr. f.
    wissens. Zool., xiv, 1864, pp. 55–70, 3 Pl.)

  ——, =and L. Landois.= Ueber die numerische Entwicklung der
    histiologischen Elemente d. Insektenkörpers. (Ibid., xv, 1865, pp.
    307–327.)

  =Rollett, A.= Zur Kenntnis der Verbreitung des Hämatins. (Sitzgsber.
    d. k. Akad. d. Wiss. Wien., lxiv, 1871.)

  =Wielowiejski, H. v.= Ueber das Blutgewebe der Insekten. Eine
    vorläufige Mitteilung. (Zeitschr. f. wissens. Zool., 1886, xliii,
    pp. 512–536.)

  =MacMunn, C. A.= Researches on myohæmatin and the histohæmatins.
    (Proc. Roy. Soc. London, 1886, xxxix, pp. 248–252.)

  =Peyron, J.= Sur l’atmosphere interne des insectes comparée à celle
    des feuilles. (Compt. rend. Acad. Sc., Paris, 1886, cii, pp.
    1339–1341.)

  =Cuénot, L.= Études sur le sang, son rôle et sa formation dans la
    série animale. Part 2, invertébrés; note préliminaire. (Arch. Zool.
    Expériment., 1888, Sér. 2, v, pp. xliii-xlvii. See also ibid., Sér.
    3, 1897, pp. 655, 679–680.)

  =Dewitz, H.= Die selbstandige Fortbewegung der Blutkörperchen der
    Gliedertiere. (Naturwiss. Rundschau. Braunschweig, 1889, iv Jahrg.,
    pp. 221–222.)

  —— Eigenthätige Schwimmbewegung der Blutkörperchen der Gliedertiere.
    (Zool. Anzeiger, 1889, xii Jahrg., pp. 457–464, Fig.)

  =Schäffer, C.= Beitrage zur Histiologie der Insekten. II, Ueber
    Blutbildungsherde bei Insektenlarven. (Spengel’s Zool. Jahrbücher
    Abt. f. Anat. u. Ontogenie, iii, 1889, pp. 626–636, 1 Taf.)

  =Cattaneo, G.= Sulla morfologia delle cellule ameboidi dei Molluschi e
    Artropodi. (Boll. Sc. Pavia. Anno 11, 1889, p. 59, 2 Pls.)

  =Wagner, W. A.= Ueber die Form der körperlichen Elemente des Blutes
    bei Arthropoden, Würmern und Echinodermen. (Biolog. Centralblatt,
    1890, x, p. 428.)

  =Preyer, W.= Zur Physiologie des Protoplasma. II, Die Funktionen des
    Stoffwechsels. Die Saftströmung. (Patanie’s Naturwiss. Wochenschr.,
    1891, vi, pp. 1–5.)

  =Cholodkowsky, N.= Ueber das Bluten der Cimbiciden-Larven.
    (Entomologische Miscellen, vi, Horæ Soc. Ent. St. Petersburg, 1897,
    pp. 352–357, 1 Fig. The fluid thrown out through pores or fissures
    in the skin is the blood.)

  With the writings of Korotaiev, Tichomeroff, Pékarsky, Balbiani,
    Korotneff, Cuénot, and others.


                          _c._ The fat-bodies

  =Dufour, L.= Recherches anatomiques sur les Carabiques et sur
    plusieurs autres insectes Coléoptères. Du tissu adipeux
    splanchnique. (Ann. Sc. nat., viii, 1826, pp. 29–35.)

  —— Histoire comparative des métamorphoses et de l’anatomie des
    _Cetonia aurata_ et _Dorcus parallelepipedus_. Tissu adipeux
    splanchnique. (Ann. Sc. nat., Zoologie, Sér. 2, 1842, xviii, pp.
    178–179.)

  =Meyer, H.= Ueber die Entwicklung des Fettkörpers, der Tracheen und
    der keimbereitenden Geschlechtsteile bei den Lepidopteren.
    (Zeitschr. f. wissens. Zool., 1849, i, pp. 175–179, 4 Taf.)

  =Fabre, J. H.= Étude sur le rôle du tissu adipeux dans la sécretion
    urinaire chez les insectes. (Ann. Sc. nat., Sér. 4, xix, 1862, pp.
    351–382.)

  =Leydig, Fr.= Einige Worte über Fettkörper der Arthropoden. (Reichert,
    u. du Bois-Reymond’s Archiv f. Anat., 1863, pp. 192–203.)

  =Lindemann, K.= Zoologische Skizzen. 1. Struktur des Fettkörpers.
    (Bull. Soc. Imp. d. Natural. Moscou, 1864, pp. 521–526, 1 Pl.)

  =Landois, Leonh.= Ueber die Funktion des Fettkörpers. (Zeitschr. f.
    wissens. Zoologie, xv, 1865, pp. 371–372.)

  =Wielowiejski, H. v.= Ueber den Fettkörper von _Corethra plumicornis_
    und seine Entwicklung. (Zool. Anzeiger, 1883, vi Jahrg., pp.
    318–322.)

  —— Ueber das Blutgewebe der Insekten. (Zeitschr. f. wissens. Zool.,
    1886, xliii, pp. 512–536.)

  =Kowalevsky, A. O.= Sur les organes excréteurs chez les arthropodes
    terrestres. (Congrès internat. Zool., 2^{me} Sess., pp. 196–205,
    Moscou, 1892.)




                            THE BLOOD TISSUE


Under this name Wielowiejski has included several important tissues or
cellular bodies intimately concerned with the nutrition of the insect.
These are:—

 1. The blood corpuscles. (See p. 407, leucocytes and phagocytes.)
 2. The fat-body proper (_Corpus adiposum_).
 3. The pericardial fat-body (pericardial cells).
 4. The œnocytes.
 5. The garland-shaped cord of muscid larvæ.
 6. The subœsophageal body, a peculiar organ found by Wheeler in the
    embryos and young larvæ of Blatta and Xiphidium.
 7. The phosphorescent organs.


                           _a._ The fat-body

In the body cavity of winged insects and of their larvæ occur yellowish
masses of large cells filled with small drops of fat, and forming the
“fat-body.” It is of various shapes, more or less lobulated or net-like,
and covers or envelops parts of the viscera, also forming a layer under
the integument (Fig. 143). The tracheal endings are usually enveloped by
the fat-body. It is larger in the larvæ than in the adults, especially
in Lepidoptera, in them forming a reserve of nutrition, used during
metamorphosis and during the formation and ripening of the eggs and male
cells.

  Wielowiejski has shown that there is a regular arrangement of the
  fat-body in the general cavity of the body. For example, in the
  larva of Chironomus occur the following forms of this tissue. Around
  the periphery, on each side of the body cavity, is a loose network
  of lobes with large meshes constituting the peripheral layer or
  external lobular fat-body; these lobular masses are segmentally
  arranged.

  Within these segmental lobes, on each side of and along the
  digestive tract, extending along through almost the entire body, is
  an unbroken strand of this tissue, forming the internal fat-body
  cords. From the first larval stage, and even before hatching, its
  cells are so unusually large, being filled with large, clear, mostly
  colorless fat-drops, that their limits cannot be defined, and their
  nuclei can only with great difficulty be detected. Only in some
  large larvæ of Chironomus has Wielowiejski found clearly defined
  cells; the protoplasm of these cells contain almost no fat-drops.

  The fat-body is of mesodermal origin, and as Wheeler insists, is not
  derived from the œnocytes, as supposed by Graber. Formed from the
  mesoderm, it is a differentiation of portions of the cœlomic walls,
  and therefore metameric in origin. That the fat-body gives origin to
  the blood corpuscles Wheeler is doubtful.

  The fat-cells are distinct, spherical, and as a rule possess only
  one nucleus, though in those of Apis and Melophagus there are two
  nuclei, and in Musca several. Sometimes the cells contain a
  substance like the white of an egg, and concretions of uric acid, or
  these take the place of the fat-drops. The presence of uric acid
  shows that a very active metabolism goes on in the fat-body. “In
  some cases it has been proved that the fat-body in the larva is rich
  in fat and poor in concretions of uric acid, while in the imago it
  is poor in fat and rich in concretions of uric acid” (Lang).

  Leydig, in 1857 (Lehrbuch der Histiologie), spoke of the presence of
  dark concretions in the fat-body, and afterwards (1864) showed that
  there was a wide distribution of uric acid salts and concretions.
  Witlaczil, also, has detected concretions in the fat-body of the
  Psyllidæ, in larval Cecidomyiidæ, in the larvæ and pupæ of ants, and
  in the pupa of Musca.

  The physiological processes which take place in the fat-bodies are
  obscure. Graber regarded the whole system of the fat-bodies as “a
  single, many-lobed lung,” while before him Landois, taking into
  account the intimate relation existing between the finer tracheal
  branches and the fat-body, considered that the latter was concerned
  in respiration. Marchal thinks that the fat-body is a urinary organ,
  as the urates are formed within the cells of this body.

  Moreover, Schäffer maintains that a special kind of fat-body cell
  has the important function of taking up and giving out nutritious
  matters during the internal processes of metamorphosis, while he
  also believes that there is a genetic connection between the
  fat-body and the blood corpuscles—a view combated by Wheeler.

  Kowalevsky finds that the fat-body remains absolutely insensible to
  the action of the substances which stained the Malpighian tubes (p.
  352). So long as the cells are healthy and living they are not
  stained and do not absorb the colors in question; and this
  insensibility persists, even when the cells are of a different
  nature, as those of the fly (adipose and “intercalary” cells).


           _b._ The pericardial fat-body or pericardial cells

We have already, on p. 405, called attention to these organs, but they
also have an intimate relation to the fat-body.

Kowalevsky (1892) remarks that the disposition of these cells varies
much in different insects and even in the same animal. Thus, in the
Diptera and the ordinary flies there are found around the lower part of
the dorsal vessel 13 pairs of large pericardial cells which lie next to
a crowded bed of small cells forming a compact mass around the anterior
part of the dorsal vessel. In caterpillars, notably silkworms, from the
compact layer of pericardial cells which surround the heart, pass off
trunks which are directed towards the lateral walls of the body, also
forming close networks around the tracheæ and then passing down into the
abdominal cavity of the body of the larva.

In the larvæ of certain Hymenoptera, the trunks which pass off from the
pericardial region form a loose cord, a sort of fatty tissue covering
the entire body cavity.

  This tissue, adds Kowalevsky, entirely differs from œnocytes, or
  from the so-called glandular body whose formation in Gryllotalpa has
  been described by Korotaiev, and in _Bombyx mori_ by Tichomiroff. In
  a recent work wherein has been collected everything known regarding
  these last-named cells, Pékarsky proves that they are unique in
  nature and cannot be regarded either as fat-cells, or as pericardial
  cells, or even as formative leucocytes.

As to the structure of the pericardial cells, Kowalevsky adds that they
are always attached to muscular fibres passing off from the heart, and
that they lie, so to speak, upon them. In the locusts the muscular
fibres supporting the pericardial cells appear distinctly like little
staves or sticks. The attachment of the pericardial cells to the
muscular fibres has been observed by Cuénot and reproduced by him in his
work, but his description somewhat differs from that observed by
Kowalevsky in the locust (_Acrydium migratorium_).

  As to the nature of the acid excretions which are formed in the
  pericardial cells, in spite of his attempts to solve the problem,
  Kowalevsky has been unsuccessful. The only observations in this
  direction are those of Letellier on the pericardial glands of
  lamellibranch molluscs, which he found to contain hypouric acid, and
  it is probable, says Kowalevsky, that the acidity of the pericardial
  cells in insects is due to the presence of the same acid.

=Leucocytes or phagocytes in connection with the pericardial cells.=—It
is thought by Schäffer that the leucocytes or phagocytes may be free or
wandering fat-body cells. They play an important part in metamorphosis,
while they absorb or feed upon the remains of the larval organs, and
thus prove of use in the building up of the organs of the adult insects.

While the faculty of _phagocytosis_ is wanting in the urinary tubes,
Balbiani and more recently Cuénot have expressed the opinion that the
pericardial cells of insects may have the power of absorbing hard
bodies, “acting as a phagocytic gland.” This, however, is called in
question by Kowalevsky, from studies made on different insects. On
introducing powdered carmine into the body of an insect it has not been
absorbed by the pericardial cells, as they have not been colored red. It
is the leucocytes which absorb the grains of carmine, and which, after
having dissolved them, transmit them to the pericardial cells. Hence,
then, the pericardial cells have not the phagocytic power of which
Cuénot speaks.

Returning to his own observations on hard bodies introduced into
insects, or large globules introduced under the form of a milk emulsion,
Kowalevsky has found that these bodies were absorbed in the first place
by the free-swimming leucocytes, and in the second place by whole groups
or nests of leucocytes situated in different parts of the body,
principally on the threads of the adipose body. In the Orthoptera the
absorption is immediately effected by means of the cells of the membrane
which separates the pericardium from the cavity of the body underneath
the heart. The regions where the hard bodies are absorbed in great
number coincide with the regions of formation of the blood corpuscles.
In his researches on the larvæ of Hyponomeuta and other Lepidoptera,
Schäffer describes these regions as forming a sort of island. The nests
where the blood globules are formed are the most active centres of
phagocytosis.

[Illustration:

  FIG. 384.-Section of the heart (_c_) and pericardial cells (_pc_,
    _pc_) from the posterior part of the heart of a fly: _l_, _l_, nests
    of leucocytes situated between the heart and pericardial cells.—From
    a microphotograph, after Kowalevsky.
]

[Illustration:

  FIG. 385.—Cross-section of the heart of _Truxalis nasata_ and of the
    structures around it: _c_, heart: _ep_, epithelium under the
    cuticula (hvpodermis); _or_, ovarian tubes; _pc_, pericardial cells,
    with one or two nuclei containing a deposit of carmine; _l_ and
    _l′_, group of leucocytes, which have absorbed granules of India
    ink.—After Kowalevsky.
]

  Balbiani, and also Cuénot, have supposed that the formation of the
  blood corpuscles takes place in the pericardial cells, but
  Kowalevsky insists that these cells cannot form the leucocytes,
  which “are probably formed in different parts of the body, notably
  in the special nests [_Herde_ of Jäger] situated near the heart, but
  outside of the pericardial cells.”

  In Fig. 384, where the nests of leucocytes (_l_) are shown, it is
  evident that they are formed where observed, and “could not have
  come from the pericardial cells, which have their own structure and
  their special function,” these cells being very large and
  characteristic.

  In Kowalevsky’s preparations of Truxalis, the pericardial cells with
  deposits of carmine and the groups of leucocytes (Fig. 385, _l_ and
  _l′_) stained with India ink, we have to deal with elements
  absolutely different. If the formation of leucocytes was caused by
  the pericardial cells, these last would be obliged to free
  themselves from their contents and to modify their essential nature.


                           _c._ The œnocytes

[Illustration:

  FIG. 386.—Cluster of œnocytes from a nearly mature Phryganeid larva:
    _o_, œnocytes; _t_, large tracheal branch; _tt_, smaller tracheal
    ramifications; _h_, tracheal hypodermis.
]

[Illustration:

  FIG. 387.—A nearly mature embryo of Xiphidium ensiferum: _o_, _o_,
    œnocyte clusters seen from the surface through the integument; _a_,
    pleuropodium of the right side (appendage of the first abdominal
    segment); _s_, styli; _c_, cercopods.—This and Fig. 386 after
    Wheeler.
]

These cells (Fig. 386), with the exception of the eggs, are the largest
in the body, and occur in most if not all winged insects. They were
called _œnocytes_ (_oinos_, wine; _kustis_, cyst), by Wielowiejski in
allusion to their wine-yellow color. These cells are arranged
segmentally (Fig. 387) in clusters, held in place by tracheæ, and are
situated mostly on each side of the abdomen, rarely being found in the
adjoining parts of the thorax. They are more or less intimately
associated with the blood and fat-body. Unlike the fat-body, however,
they arise in embryonic life from the ectoderm, either by delamination
or by immigration, just behind the tracheal involutions.

  The separate cells of each cluster are usually separate, but in rare
  cases may fuse in pairs or form smaller clusters. In shape they are
  round or oval, often sending out pseudopodia-like processes, by
  which they are attached to the tracheal twigs or to each other. “The
  cytoplasm, which is very abundant, is full of yellowish granules and
  is sometimes radially situated towards its periphery. The large
  spherical or oval nucleus contains a densely wound and delicate
  chromatic filament.” (Wheeler.)

  Graber first pointed out the identity of these clusters of cells
  with certain metameric cell-masses in insect embryos, observed by
  Tichomiroff in those of the silkworm, and by Korotneff in the embryo
  mole-cricket.

  Although they resemble the blood corpuscles in some insects, they
  are always much larger, and do not seem to be amœboid, while they
  are never seen to undergo self-division, or to exhibit any
  appearance of giving rise to the blood-cells (Wheeler). They have
  not yet been detected in Thysanura (Synaptera) or in Myriopoda.


                     _d._ The phosphorescent organs

Phosphorescence is not infrequent in the Protozoa, cœlenterates, worms,
and has been observed in the bivalve Pholas, in a few abyssal Crustacea,
in myriopods (Geophilus), in an ascidian, Pyrosoma, and in certain
deep-sea fishes.

[Illustration:

  FIG. 388.—_A_, sagittal section through the hinder end of a male
    Luciola, the organs above the phosphorescent plate only drawn in
    outline: _s_, integument of the last segment, somewhat removed by
    the section-knife from the phosphorescent tissues; _d_, dorsal layer
    of the phosphorescent plate penetrated by irregular tracheal
    branches, and rendered opaque by numerous urate concretions imbedded
    in it; _v_, ventral phosphorescent layer of the plate, with
    perpendicular tracheal stems whose branches, where they pass into
    capillaries, bear lumps which stain brown with osmic acid; _n_,
    structureless substance (coagulum?) filling the end of the last
    ventral segment. _B_, isolated portion of the ventral layer of the
    phosphorescent plate; _tr_, tracheal stem surrounded by a
    cylindrical lobe: _p_, parenchym cell attached to the cylinder; _c_,
    capillary, without the spiral threads; _m_, coagulum stained brown.
    _C_, a tracheal stem of the ventral layer: at the fork of the
    brown-stained capillaries are lumps stained brown with osmic acid.
    _D_, a part of _C_, more highly magnified, showing the remains of
    the tracheal end-cells (_tc_) enveloping the brown lumps
    (_m_).—After Emery.
]

In insects luminosity is mostly confined to a few Coleoptera, and
besides the well-known fireflies, an Indian Buprestid (_Buprestis
ocelata_) is said to be phosphorescent; also a telephorid larva. Other
luminous insects are the Poduran Anurophorus, Fulgora, certain Diptera
(_Culex_, _Chironomus_[60] and _Tyreophora_), and an ant (Orya).

The seat of the light is the intensely luminous areas situated either in
the head (Fulgora), in the abdomen (Lampyridæ), or in the thorax (in a
few Elateridæ of the genus Pyrophorus). The luminous or photogenic organ
is regarded by Wielowiejski and also by Emery as morphologically a
specialized portion of the fat-body, being a plate consisting of
polygonal cells, situated directly under the integument, and supplied
with nerves and fine tracheal branches.

In Luciola as well as in other fireflies, including Pyrophorus, the
phosphorescent organ or plate consists, as first stated by Kölliker, of
two layers lying one over the other, a dorsal one (Fig. 388, _d_) which
is opaque, chalky white, and non-photogenic, and a lower one (_v_), the
active photogenic layer, which is transparent. Through the upper or
opaque layer and on its dorsal surface extend large tracheæ and their
horizontal branches, from which arise numerous very fine branches which
pass down perpendicularly into the transparent or photogenic layer of
the organ. Each tracheal stem, together with its short branches, is
enveloped by a cylindrical mass of transparent tissue, so that only the
short terminal branches or very fine tracheal capillaries project on the
upper part of the cylinder. These finest tracheal capillaries are not in
Luciola filled with air, but with a colorless fluid, as was also found
by Wielowiejski and others in Lampyris.

These transparent cylinders, with the tracheæ within, forming
longitudinal axes, resemble lobules. These lobules are so distributed
that they appear on a surface section of this plate as numerous round
areas in which circular periphery the tracheal capillaries are arranged
with the axially disposed tracheal end-cells. These “tracheal end-cells”
are only membranous enlargements at the base of the tracheal capillaries
(Wielowiejski). The cylindrical lobules are separated from each other by
a substance consisting of abundant large granular cells (parenchym
cells) among which project the tracheal capillaries. The cylindrical
lobules extend to the hypodermis and come in contact only by their
lateral faces with the parenchym.

The structure of the upper opaque chalky white layer of the
phosphorescent organ is, compared with that of the photogenic lower
portion, very simple. In its loose, pappose, mass are no cellular
elements, but when treated with different reagents it is seen to be
filled with countless urate granules (guanine) swimming in the fluid it
contains, the cell plasma appearing to be dissolved, the cells having
lost their cohesion.

In comparing the phosphorescent plate or organ of Luciola with that of
Lampyris, the general structure, including the clear cell elements of
the cylindrical lobules, which envelop the perpendicular tracheal twigs
and their branches, and also the granular parenchymatous cells are alike
in both, though the arrangement and distribution of the elements in
Luciola is more regular, in Lampyris the tracheal stems being
irregularly scattered through the parenchym.

Wielowiejski found in the larval and female Lampyris a higher degree of
differentiation than in the male, and Luciola has a more differentiated
photogenic organ than Lampyris, as seen in the more regular structure of
the lobules.

As regards the light-apparatus of Pyrophorus, or the cucujo, Heinemann
shows that, as in the Lampyridæ, it consists of distinct cells, and may
be regarded as a glandular structure. It is rich in tracheæ and the
other parts already described. In still later researches on a Brazilian
Pyrophorus Wielowiejski shows that the phosphorescent plate consists of
two layers, the upper usually being filled with crystalline urate
concretions, and entirely like those of the Lampyridæ, consisting of
distinct polygonal cells, among which are numerous tracheal stems, with
tænidia, coursing in different directions, when freshly filled with air,
and sending capillaries into the underlying photogenic layer. The latter
shows in its structure a striking difference in the cellular arrangement
from that of Lampyrids. In the upper or non-photogenic layer are
tracheal capillaries which pass down into the underlying cellular plate
and which are in the closest possible relations with the single cells—a
point overlooked by Heinemann.

=Physiology of the phosphorescence.=—As is well known, the
phosphorescence of animals is a scintillating or glowing light emitted
by various forms, the greenish light or luminous appearance thus
produced being photogenic, _i.e._ without sensible heat.

Langley rates the light of the firefly at an efficiency of 100 per cent,
all its radiations lying within the limits of the visible spectrum.
“Langley has shown that while only 2.4 per cent of luminous waves are
contained in the radiation of a gas-flame, only 10 per cent in that of
the electric arc, and only 35 per cent in that of the sun, the radiation
of the firefly (_Pyrophorus noctilucus_) consists wholly of visible
wave-frequencies.” (Barker’s Physics, p. 385.)

The spectrum of the light of the cucujo was found by Pasteur to be
continuous. (C. R. French Acad. Sc. 1864, ii, p. 509.) A later
examination by Aubert and Dubois showed that the spectrum of the light
examined by the spectroscope is very beautiful, but destitute of dark
bands. When, however, the intensity diminishes, the red and orange
disappear, and the green and yellow only remain.

Heinemann studied the cucujo at Vera Cruz, Mexico. At night in a dark
room it radiates a pale green light which shows a blue tone to the
exclusion of any other light. The more gas or lamp light there is
present, the more apparent becomes the yellowish green hue, which in
clear daylight changes to an almost pure very light yellow with a very
slight mixture of green. “In the morning and evening twilight, more
constantly and clearly in the former, the cucujo light, at least to my
eyes, is an intensely brilliant yellow with a slight mixture of red. In
a dark room lighted with a sodium light the yellow tone entirely
disappears; on the other hand, the blue strikingly increases.” As
regards the spectrum he found that almost exactly half of the blue end
is wanting and that the red part is also a little narrower than in the
spectrum of the petroleum flame.

Professor C. A. Young states that the spectrum given by our common
firefly (_Photinus?_) is perfectly continuous, without trace of lines
either bright or dark. “It extends from a little above Fraunhofer’s line
C, in the scarlet, to about F in the blue, gradually fading out at the
extremities. It is noticeable that precisely this portion of the
spectrum is composed of rays which, while they more powerfully than any
others affect the organs of vision, produce hardly any thermal or
actinic effect. In other words, very little of the energy expended in
the flash of the fire is wasted. It is quite different with our
artificial methods of illumination. In the case of an ordinary gaslight
the best experiments show that not more than one or two per cent of the
radiant energy consists of _visible rays_; the rest is either invisible
heat or actinism; that is to say, over 98 per cent of the gas is wasted
in producing rays that do not help in making objects visible.”

Panceri also remarks that while in the spectroscope the light of some
Chætopteri, Beroë, and Pyrosoma exhibit one broad band like that given
by monochromatic light, that of Lampyris and Luciola is polychromatic.
(Amer. Nat., vii, 1873, p. 314.)

The filtered rays of Lampyris pass (like Röntgen and uranium rays)
through aluminium (Muraoka).

The physiology of insect phosphorescence is thus briefly stated by Lang:
“The cells of this luminous organ secrete, under the control of the
nervous system, a substance which is burnt during the appearance of the
light; this combustion takes place by means of the oxygen conveyed to
the cells of the luminous body by the tracheæ, which branch profusely in
it and break up into capillaries.”

Emery states that the males of Luciola display their light in two ways.
When at night time they are active or flying, the light is given out at
short and regular intervals, causing the well-known sparkling or
scintillating light. If we catch a flying Luciola or pull apart one
resting in the day time, or cut off its hind body, it gives out a
tolerably strong light, though not nearly reaching the intensity of the
light waves of the sparkling light. In this case the light is constant,
yet we notice, especially in the wounded insect, that the phosphorescent
plate in its whole extent is not luminous, but glows at different places
as if phosphorescent clouds passed over it.

It is self-evident that a microscopic observation of the light of the
glow-worm or firefly is not possible, but an animal while giving out its
light, or a separated abdomen, may readily be placed under the
microscope and observed under tolerably high powers. By making the
experiment in a rather dark room Emery saw clear shining rings on a dark
background. “All the rings are not equally lighted. Comparing this with
the results of anatomical investigation, it is seen that the rings of
light correspond with the previously described circular tracheal
capillaries, _i.e._ the limits between the tracheal-cell cylinder and
the parenchym-cells. The parenchym-cells are never stained of a deep
brown; this proves that its plasma may be the seat of the
light-producing oxidation. Hence this process of oxidation takes place
in the upper surface of the parenchym-cells, but outside of their own
substance. The parenchym-cells in reality secrete the luminous matter;
this is taken up by the tracheal end-cells and burnt or oxidized by
means of the oxygen present in the tracheal capillaries. Such a
combustion can only take place when the chitinous membrane of the
tracheæ is extraordinarily fine and easily penetrable, as is the case in
the capillaries of the photogenic plate; therefore the plasma of the
tracheal cells only oxidizes at the forking of the terminal tracheal
twigs and in the capillaries.” (Emery.)

The color of the light of Luciola is identical in the two sexes, and the
intensity is much the same, though that of the female is more
restricted. The rhythm of the flashes of light given out by the male is
more rapid, and the flashes briefer, while those of the female are
longer, more tremulous, and appear at longer intervals.

Emery then asks: What is the use of this luminosity? Is it only to
allure the females of Luciola, which are so much rarer than the males?
Contrary to the general view that it is an alluring act, he thinks that
phosphorescence is a means of defence, or a warning or danger-signal
against insectivorous nocturnal animals. If we dissect or crush a
Luciola, it gives out a disagreeable cabbage-like smell, and perhaps
this is sufficient to render it inedible to bats or other nocturnal
animals. An acrid taste they certainly do not possess.

It has long been known that the eggs of fireflies, both Lampyridæ and
Pyrophorus, are luminous. Both Newport and more recently Wielowiejski
attributes the luminosity not to the contents of the egg, but to the
portions of the fat-body cells or fluid covering on the outside of the
eggs, due to ruptures of the parts within the body of the female during
oviposition. The larvæ at different ages are also luminous.

The position of the luminous organs changes with age. In the larvæ of
Pyrophorus before moulting, according to Dubois, the luminous organs are
situated only on the ventral side of the head and prothoracic segment.
In larvæ of the second stage there are added three shining spots on each
of the first eight abdominal segments, and a single luminous spot on the
last segment. These spots are arranged in a linear series and thus form
three luminous cords. In the adult beetles there is a luminous spot in
the middle of the first abdominal sternite, but the greatest amount of
light is produced by the two vesicles on the hinder part of the
prothorax, the position of which varies according to the species.




                     LITERATURE ON PHOSPHORESCENCE


  =Peters, W.= Ueber das Leuchten der _Lampyris italica_. (Müller’s
    Archiv f. Anatomie, 1841, pp. 229–233.)

  =Kölliker, A.= Die Leuchtorgane von Lampyris, eine vorläufige
    Mittheilung. (Verhandl. d. phys. medizin. Gesellsch. Würzburg, 1857,
    viii, pp. 217–224.)

  =Schultze, Max.= Ueber den Bau der Leuchtorgane der Männchen von
    _Lampyris splendidula_. (Sitzber. d. niederrhein. Gesellsch. f.
    Natur. u. Heilkunde zu Bonn, 1864, Sep. p. 7.)

  —— Zur Kenntniss der Leuchtorgane von _Lampyris splendidula_. (Archiv
    f. mikroskop. Anat., 1865, i, pp. 124–137, 2 Taf.)

  =Wielowiejski, H. Ritter von.= Studien über die Lampyriden. (Zeits. f.
    wissens. Zool., 1882, xxxvii, pp. 354–428, 2 Taf.)

  =Emery, Carlo.= Untersuchungen über _Luciola italica_ L. (Zeits. f.
    wissens. Zool., 1884, xl, pp. 338–355, 1 Taf.)

  —— La luce della _Luciola italica_ osservata col microscopio. (Bull.
    Soc. Ent. Ital. Anno xvii, 1885, pp. 351–355, 1 Taf.)

  =Wheeler, William Morton.= Concerning the blood tissue of the Insecta,
    III; Psyche, vi, 1892, p. 255. (Structure of the light organ of
    _Photuris pennsylvanica_.)

  =Heinemann, C.= Zur Anatomie und Physiologie der Leuchtorgane
    Mexikanischer Cucujos, Pyrophorus. (Archiv f. mikroskop. Anat.,
    1886, xxvii, pp. 296–383.)

  =Dubois, R.= Contribution à l’étude de la production de la lumière par
    les êtres vivants. Les Elaterides lumineux. (Bull. Soc. Zool.
    France, 1886, Année ii, pp. 1–275, 9 Pls.)

  =Wielowiejski, H. Ritter von.= Beiträge zur Kenntniss der Leuchtorgane
    der Insekten. (Zool. Anzeiger, 1889, Jahrg. xii, pp. 594–600.)

  =Hudson, G. V.= The habits and life-history of the New Zealand
    glowworms. (Trans. N. Zealand Inst., xviii, 1890, pp. 43–49, 1 P1.)

  =Dubois, R.= Sur le mechanisme de la production de la lumière chez
    l’_Orya barbarica_ d’Algérie. (Comptes rend. Acad. Sc. France, 1892,
    cxvii, pp. 184–186. Also in Ann. and Mag. Nat. Hist., 1893 (6), xii,
    pp. 415–416.)

  =Schmidt, P.= Ueber das Leuchten der Zuckmücken (Chironomidæ). (Zool.
    Jahrb. Morph., Abth. viii, 1894, pp. 191–216, 2 Taf. Also Ann. and
    Mag. Nat. Hist., xv, pp. 133–141, 1895. The light is due to
    bacteria. Nature, 1897.)

  =Chun, C.= Leuchtorgane und Facettenaugen. (Biol. Centralbl., 1896,
    pp. 315–320.)

  =Muraoka, H.= (On the filtered rays of Lampyris, Wiedemann’s Annalen,
    Dec., 1896.)

  See also the writings of Audouin, Carus, D. Turner, Thompson.




                         THE RESPIRATORY SYSTEM


While land vertebrates breathe by inhaling the air through the mouth
into the lungs, insects respire by internal air-tubes (_tracheæ_), which
ramify throughout every part of the body and its appendages. The air
enters these tubes through a few openings, called spiracles or
_stigmata_, arranged segmentally in the sides of the body. These tracheæ
are everywhere bathed by the blood, and thus the latter is constantly
aërated or kept fresh; the blood not, as in vertebrates or as in
molluscs, seeking the lungs or gills, or any specialized respiratory
portion of the body where the oxygen combines with the hæmoglobin, but
the respiratory tubes, so to speak, themselves seek out the blood and
the blood-tissue in every part of the insect body, penetrating to the
tips of the antennæ and of the legs, entering the most delicate tissues,
even perhaps passing through the walls of epithelial cells. As Lang
remarks, the want of an arterial vascular system is compensated for as
well as conditioned by the extremely profuse branching of the tracheæ.

[Illustration:

  FIG. 389.—Rat-tailed larva of Eristalis.
]

The aquatic larvæ of certain dragon-flies (Agrionidæ), may-flies,
case-worms, etc., respire by means of tracheal gills or branchiæ, which
are either filamental or leaf-like appendages containing tracheæ.
Somewhat similar structures appended to the thorax of pupal aquatic
Diptera, as in the mosquito and its allies, enable them to breathe while
stationed a little beneath the surface of the water. Other larvæ, as the
rat-tail larva of Eristalis, etc., lying at the bottom of shallow pools
or in ditches, etc., can breathe by raising slightly above the surface a
long appendage with two spiracles at the end, through which the air
enters the tracheal system. (See p. 461.)

Although Aristotle, as well as the natural philosophers of the Middle
Ages, supposed that insects did not breathe, one can easily see that
they do by holding a grasshopper or dragon-fly in one’s hand and
observing the rhythmical rise and fall of the upper and lower walls of
the abdomen, during which the air enters and passes out of the
air-openings or spiracles on each side of the body.

It is plain that insects consume very little air, since caterpillars may
be confined in very small, almost air-tight tin boxes, and continue to
eat and undergo their transformations without suffering from the
confinement. According to H. Müller an insect placed in a small,
confined space absorbs all the oxygen. Insects can survive for many
hours when placed in an exhausted receiver, or in certain irrespirable
gases. “Cockroaches in carbonic acid speedily become insensible, but
after twelve hours’ exposure to the pure gas they survive and appear
none the worse.” (Miall and Denny, p. 165.) Insects of the swiftest
flight breathe most rapidly, their great muscular activity requiring the
absorption of an abundance of oxygen.

[Illustration:

  FIG. 390.—Section of Sphinx embryo, showing at _s_ the ectoderm
    invaginated, and forming the germ of a stigma and trachea
    (_t_).—After Kowalevsky.
]

Warmth, plenty of food, besides muscular activity, increases the demand
for oxygen and the quantity of carbonic acid exhaled.


                            _a._ The tracheæ

[Illustration:

  FIG. 391.—Portion of a trachea of a caterpillar, with its branches
    _B_, _C_, _D_: _a_, peritracheal membrane; _b_, nucleus.—After
    Leydig, from Gegenbaur.
]

[Illustration:

  FIG. 392.—Structure of a trachea, diagrammatic: portions of the
    peritracheal membrane (_hy_) and chitinous intima (_cc_) removed to
    show the structure; in the chitinous intima or endotrachea (cc) can
    be seen the spiral thickenings or tænidia.—After Lang.
]

It will much simplify our conception of the nature of the air-tubes when
we learn that they originate in the embryo as tubular ingrowths of the
integument (ectoderm), these branching and finally reaching every part
of the interior of the body. They are elastic tubes, and being filled
with air are silvery in color, though at their origin near the spiracles
they are reddish or violet bluish; or, in the larva of Æschna, reddish
brown, this tint being due to a finely granular pigment situated in the
peritoneal membrane.

[Illustration:

  FIG. 393.—Longitudinal section of the trachea of _Hydrophilus piceus_:
    _ep_, epithelium; _cu_, cuticula; _f_, spiral threads.—After Minot.
]

In their essential structure the tracheæ consist of the chitinous
intima, which is a continuation of the cuticle of the integument, and of
a cellular membrane or outer layer of cells (a continuation of the
hypodermis) called the peritoneal membrane, or ectotrachea (Figs. 392,
393).

Leydig discovered that the spiral filaments are not distinct and
separate, but intimately connected with the inner membrane (intima), and
he detected the outer or peritoneal membrane, which Chun afterwards
found to be epithelial in its nature, Minot stating that it is a true
pavement epithelium.

Figure 393 represents a longitudinal section of a large trachea of
Hydrophilus, showing the peritoneal membrane (_ectotrachea_, _ep_) and
the intima or _endotrachea_, divided into the cuticula (_cu_), with the
darker colored inner layer, in which are embedded the dark-colored
tænidia (_f_).

[Illustration:

  FIG. 394.—Testis of Anabrus, showing the ramifications of the
    tracheæ.—After Minot.
]

=Distribution of the tracheæ.=—The distribution of the air-tubes, as
Lubbock and also Minot state, depends first upon the shape of the
organs, and upon the size of those whose size is variable. Around the
large, hollow organs (digestive canal, sexual organs) the tracheæ ramify
in all directions, forking so that the branches diverge at a wide angle.
In the organs which have muscular walls, like the oviduct, the tracheæ
run straight when the walls are distended, but have a sinuous course
when the walls are contracted. (Minot.)

  “Around the organs of more elongated form the branches of the
  tracheæ run more longitudinally, as is shown by the air-tubes of the
  muscles, which also present some peculiarities worthy of especial
  notice.

  “A short, thick trunk arrives at the muscular bundle, and dividing
  very rapidly, breaks up into a large number of delicate tubes, which
  penetrate between the muscular fibres, then terminating in tubes of
  exceeding fineness, which at first sight seem to form a network that
  might well be called a _rete mirabile_. A closer examination,
  however, reveals that it is not a real network, but rather an
  interlacing confusing to the eye. The longitudinal direction of the
  tracheæ of the muscles presents a striking contrast to the system of
  divarication represented in Figs. 13 and 14. The course of the
  tracheæ of the Malpighian tubes is also very curious. There is one
  large trachea which winds around the tube in a long spiral, giving
  off numerous small branches which run to the surface of the tube,
  upon which they form delicate ramifications. Each tube has but a
  single main trachea, and I think the trachea continues the whole
  length of the tube, but of this last point I am not quite sure.”
  (Minot.)

While in the nymphs of Orthoptera the tracheæ very closely resemble
those of the adult, in larvæ of insects with a complete metamorphosis
the tracheæ differ very much in distribution from those of the adult.
The larval tracheæ are also more generalized and more like those of the
original type than the tracheæ of perfect insects. (Lubbock.)

In general there are two main tracheæ, one passing along each side of
the body, near the digestive canal, connected with its mate by a few
transverse anastomosing branches, and sending off a branch to each
spiracle, this arrangement being most simple and apparent in the maggots
of Diptera. From these two main branches smaller twigs branch off into
every part of the body with its appendages, passing among the different
organs, often serving as cables to hold them loosely in place; they also
penetrate into the component parts of the organ themselves, passing into
the fat-bodies, and among the fibres of muscles, where they become
finely attenuated and refined like the capillaries of the vascular
system of vertebrates. (Figs. 395, 396.)

[Illustration:

  FIG. 395.—_Melanoplus femur-rubrum_, showing distribution of air-tubes
    (tracheæ) and air-sacs; _V_, main ventral trachea (only one of the
    two shown); _S_, left stigmatal trachea, connecting by vertical
    branches with _D_, the left main dorsal trachea; _c_, left cephalic
    trachea; _oc_, ocular dilated trachea. From the first, second,
    third, and fourth spiracles arise the first four abdominal air-sacs,
    which are succeeded by the plexus of three pairs of dilated tracheæ,
    I, II, III, in Fig. 396. Numerous air-sacs and tracheæ are
    represented in the head and thorax. The two thoracic spiracles are
    represented, but not lettered.
]

[Illustration:

  FIG. 396.—_D_, left dorsal trachea; _S_, left stigmatal trachea; I,
    II, III, first, second, and third pairs of abdominal dilated
    tracheæ, forming a plexus behind the ovaries; 1, pair of enormous
    thoracic air-sacs; 2, pair of smaller air-sacs; 3–7, abdominal
    air-sacs; _oc_, ocular dilated trachea and air-sacs; _c_, cephalic
    trachea. The relations of the heart to the dorsal tracheæ are
    indicated.—Drawn by Emerton from dissections by the author.
]

  In the youngest larva of _Corethra plumicornis_ Weismann ascertained
  the thickness of the longitudinal stem to be 0.0017 mm. That of the
  finest tracheal endings in the silk-glands of the silkworm was found
  by Von Wistinghausen to be 0.0016 mm. (Zeits. f. Wiss. Zool. xlix,
  1890, p. 575.) Weismann states that in the larvæ of Corethra and
  Chironomus the tracheal system is only incompletely developed; the
  tracheæ are not united with each other, and in the youngest larvæ
  they do not contain air.

[Illustration:

  FIG. 397.—Tracheal system of the right side of _Machilis maritima_:
    _k_, head; I, II, III, thoracic segments; 1–10, abdominal
    segments; _s_, stigma.—After Oudemans, from Lang.
]

  Each of the two main tracheæ, as Kolbe states, sends off into each
  segment of the body three branches.

  1. An upper or dorsal branch, which supplies the muscles of the
  dorsal region.

  2. A middle (visceral) branch, whose twigs pass to the digestive
  canal and back to the organs of reproduction.

  3. A lower (ventral) branch, whose twigs are distributed to the
  ganglia and to the muscles of the ventral region.

  In certain Thysanura, as a species of Machilis (Fig. 397), we
  probably have the primitive condition of the tracheal system, the
  longitudinal and transverse anastomoses being absent, but in other
  Thysanura (Japyx, Nicoletia, Lepisma, and a few species of Machilis)
  they are present.

  As Kolbe remarks, whether the fine ends of the tracheæ are closed or
  open, whether after the analogy of the blood capillaries of
  vertebrates they anastomose with each other, whether the ends of the
  air-tubes pass between the cells or penetrate into them, these
  questions are not fully settled. According to Leydig’s[61] latest
  views the tracheæ penetrate into the cells and unite with the
  hyaloplasma. Hence the process of respiration in the last instance
  takes place in the hyaloplasma. This assumption accords with the
  fact that in the tracheate Arthropods the terminations of the
  tracheæ carry the atmospheric air into the space bounded by the
  cellular network, also to the hyaloplasma filling the spaces.
  Leydig[62] also thinks that the finest tracheal endings penetrate
  into the muscular tissue and unite with the primitive muscular
  fibres.

Kupffer is likewise of the opinion that the fine tracheæ penetrate into
the cells, and Lidth de Jeude asserts that they enter the epithelial
cells, “each cell containing several branches.” Kölliker, Emery, etc.,
maintain, however, that the tracheal endings lie between the cells.
Wielowiejski,[63] in describing the line tracheæ of the phosphorescent
organs, thinks that the tracheal endings (tracheal capillaries) rarely
end blindly, but anastomose with one another, forming an irregular
network. The latest observer, Gilson (1893), asserts that tracheal twigs
penetrate deeply into the epithelial cells of the silk glands of larval
Trichoptera as well as of caterpillars, passing through their
protoplasm.

[Illustration:

  FIG. 398.—Tracheal network of the male glands of _Lampyris
    splendidula_: _tec_, tracheal end-cells; _cap_, tracheal
    capillaries; at _a_, an expanded matrix.—After Wielowiejski.
]

[Illustration:

  FIG. 399.—Tracheal capillary end-network (_tr. c. n._) of silk glands
    of _Ocneria dispar_: _p_, peritoneal (peritracheal) membrane.—After
    Wistinghausen.
]

  A late investigator, C. von Wistinghausen, finds in the tracheæ of
  the spinning-glands of caterpillars a completely formed network
  between the terminal branches of two or several tracheal groups. The
  tracheal tubes of this series of terminal branches pass into this
  network, which he calls the tracheal capillary end-network (Figs.
  398, 400). This last varies in thickness and spreads out under the
  membrana propria of the glandular mass over the entire surface of
  the large gland-cells and on a level with the tracheal capillaries.
  The tracheal endings do not penetrate into the cells, but are
  separated from the plasma of the cells by a thin membrane. The
  tracheal capillary end-network appears as a system of fine tubes
  like the tracheal capillaries, consisting of a peritoneal layer and
  a chitinous intima (Fig. 400). The walls of these tubes are
  homogeneous, not porous, though readily permeable by the
  parenchymatous fluid. The interchange of gases consequently may go
  on easier and more vigorously in a system of richly anastomosing
  tubules of the net-like mass of tracheal capillaries, than in tubes
  ending blindly.

  While the diameter of the tracheal capillaries is 0.0016 mm. or 1 µ,
  that of the tubules composing the tracheal capillary end-network is
  scarcely measurable, but is less than 1 µ.

[Illustration:

  FIG. 400.—Tracheal end-cells of _Lampyris splendidula_: _tr_,
    trachea with tænidia; _tre_, tracheal capillaries.—After
    Wielowiejski.
]

  These tracheal capillaries also occur on the seminal and other
  sexual tubes, on the intestine, on the urinary tubes, on the
  fat-bodies, but are most easily detected on the silk-glands.

  The latest researches are those of E. Holmgren, who has studied the
  branching of the tracheæ in the spinning-glands of caterpillars. He
  prefers to call the end-cells “transition cells,” as they lead from
  the tracheal tubes proper to the capillary network. This latter is
  formed by slender nucleated cells, often with an intracellular
  lumen, and, according to the author, probably constituting a
  respiratory epithelium. He finds that both large and small tracheæ
  may penetrate the gland-cells. (Anat. Anzeiger, xi, 1895, pp. 340–6,
  3 figs.; Jour. Roy. Micr. Soc., 1896, p. 182.)


                     _b._ The spiracles or stigmata

The spiracles are segmentally arranged openings in the sides of the
thorax and abdomen, through which the air passes into the air-tubes. In
its essential structure a spiracle, or _stigma_, is a slit-like opening
surrounded by a chitinous ring, the lips or edges of the opening being
membranous and closed by a movable valve of the spiracle attached by its
lower edge, which is closed by an occlusor muscle (Fig. 401). The
aperture when open forms a narrow oval slit; and in most insects the
slit is within guarded by a row of projecting spines or setæ, which form
a lattice work or grate to keep out dust, dirt, fluids, etc.

[Illustration:

  FIG. 401.—Horizontal section of left third stigma and trachea of
    _Melolontha vulgaris_, showing the chamber or drum leading into the
    trachea: _a_, _a_, external frame or valve protecting the outer
    opening of the stigma; _b_, _c_, _c_, inner frame closing the
    entrance into the trachea (_l_, _k_); _m_, occlusor muscle closing
    the inner orifice.—After Straus-Dürckheim.
]

  Krancher[64] has described five leading types of stigmata, not,
  however, taking into account those of the Synaptera.

  I. _Stigmata without lips_ (Primitive or generalized stigmata).

  _a._ The simplest stigma is an aperture which is kept open by a
  chitinous ring (Acanthia). The opening may be round or elliptical.
  There are no lips nor any movement of the edges to be observed. Such
  air-holes occur in the abdomen of bugs (Hemiptera) and beetles
  (Coleoptera); within the opening of the stigmata in the same insects
  is a funnel-like contraction. Also in the Diptera the abdominal
  stigmata are of the same type.[65] The stigmata of the Pulicidae
  (Siphonaptera) are more complicated, as the edges of the openings
  are provided with setæ (Fig. 402).

[Illustration:

  FIG. 402.—First abdominal spiracle with a part of the trachea of the
    cat-flea: _sp_, spiracle; _t_, trachea.
]

[Illustration:

  FIG. 403.—Stigma of Melolontha larva, seen from without: _b_, bulla;
    _s_, sieve-like plate; _o_, curved slit-like opening.—After Boas.
]

  _b._ The stigma consists of a series of minute single stigmata,
  which are usually surmounted by a common chitinous ring, and whose
  tubular continuations unite within in a common trachea, so that the
  single tubes pass off from the stigma like the fingers on the hand.
  This form is found in the larvæ and puparia of Diptera.

  II. _Stigmata with lips_ (Secondary more specialized stigmata).

  _c._ The lips are represented by a single chitinous ring, with
  sparse spines. One side of the stigma is a little higher, and partly
  overlaps the other posteriorly; this form is peculiar to the
  Orthoptera and Libellulidae.

  _d._ The lips are roof-like, bent inwards and densely hairy, forming
  a peculiar kind of felting. The setæ of the lips are in most beetles
  and many Lepidoptera separate, and more or less branched. In
  caterpillars, the setæ are so finely branched as to form a loose
  felt, or sieve-like arrangement.

  _e._ The stigmata are round, with a very broad border and a
  concentric middle portion, the structure being complicated. The
  concentric middle portion is pouch-like and bears the occlusor
  muscle. This form occurs in the larvæ of lamellicorn beetles, and
  can be seen with the naked eye, or with a lens, in Oryctes, Cetonia,
  and Melolontha (Fig. 403).

  _f._ Over the outer opening of the spiracle is an incurved chitinous
  projection, on one side of which the trachea takes its origin. It is
  thus in the Hymenoptera.

  The remarkable grate-like stigma of the lamellicorn larvæ has the
  appearance as if the outer closing plate or valve were impenetrable.
  The earlier observers considered these stigmata to be open, but
  Meinert regards them as closed; Schiödte, however, has observed by
  pressing a preserved specimen of a Melolontha larva the alcohol
  within passing out in drops, through the grate-like plate, and hence
  he considers this a proof that the stigma is permeable (Kolbe).

  More recently (1893) Boas has examined the same structure in the
  same species of larva as examined by Schiödte, and he finds it to be
  open only during the process of moulting. He finds that on each side
  of the larva there are nine short and wide stigmatic branches, each
  of which is shut off from the exterior by a brown plate; this
  consists of a reniform sieve-plate, and of a curved bulla which fits
  into the cavity of the plate. The stigmatic branch, however, is
  provided with a large external opening, which is homologous with the
  stigma, but which is usually closed by the plate and bulla, and is
  only open during the moulting; at first it is circular, but later
  becomes a cleft. A transverse section shows that the bulla is a
  simple tegumentary fold, the outer chitinous layer of which has
  become especially firm. The plate forms a horizontal half-roof,
  which springs from one side of the tracheal orifice, and is
  supported by obliquely set bases, which spring from the adjoining
  part of the inner side of the tracheæ. The plate and bars are purely
  cuticular structures. (Zool. Anz., 1893; also Journ. Roy. Micr.
  Soc., p. 54.)

  The tracheal system of libellulid nymphs is not closed; on the other
  hand, in the fully-grown nymphs the anterior stigmata occurring on
  the dorsal side are large, and the tracheæ arising from them are
  thick. These stigmata are permeable by the air. In half-grown and
  still younger stages of Æschna the two anterior thoracic stigmata
  are undeveloped. In order to breathe, the fully-grown nymph either
  rises up on the upper side and elevates the end of the body to the
  surface in order to take the air into the rectum, or it rests with
  the back of the thorax at the surface in order to breathe through
  the large stigmata. The young nymphs take in air only through the
  rectum. The young nymphs of Libellula and its allies, on the other
  hand, possess large thoracic stigmata, but they prefer to breathe
  through the rectum. The fully-grown nymphs of Agrion breathe through
  the thoracic stigmata. (Dewitz, in Kolbe.)

=The position and number of pairs of stigmata.=—The spiracles are
usually situated in the soft membrane between the tergites and
pleurites, but their exact position varies in different groups. In the
Coleoptera they occupy on the thorax a more ventral position, and on the
abdomen are placed near the edge of the dorsal side, under the elytra.
In the dragon-flies, the first pair is situated much more dorsally than
the second and third pairs; the following seven pairs are almost wholly
ventral and lie concealed in the membranous fold near the external
plate. In the Hemiptera, also, the abdominal stigmata, though entirely
free and visible, are situated ventrally.

Primarily, in the embryo a pair of stigmata appear on each segment of
the thorax and abdomen, except the 10th and 11th, and even possibly in
the head, for a pair of stigmata are said to occur in the head of
Podurids (Smynthurus) (Lubbock), though this statement needs
confirmation. Scolopendrella, however, is known to possess a pair of
cephalic spiracles.

From the foregoing statement it will be seen that while in existing
winged insects no more than 10 (in Japyx 11) pairs of stigmata are to be
found in any one species, yet that 12 segments of the body, in different
groups taken collectively, bear them. The primitive number of pairs of
spiracles, therefore, in winged insects, was 12, _i.e._ a pair in each
thoracic segment, and a pair in each of the first nine abdominal
segments. Insects were originally all holopneustic, and gradually as the
type became differentiated into the different orders they became
peripneustic or amphipneustic, and, in certain aquatic forms, apneustic.
(See pp. 459, 461.)

In the still more primitive, probably wingless, ancestors of insects
there was a larger number of stigmata. Hatschek, in 1877, discovered a
pair of tracheal invaginations in each of the three posterior
head-segments of the embryo of a moth, with stigmatal openings in the
1st and 2d maxillary segments.

Thus early in embryonic life every segment of the body, except those
bearing the eyes and the last abdominal, bore a pair of stigmata, so
that the primitive insect had at least 15, and perhaps more, pairs of
stigmata.

The position of the stigmata is subject to much variation, the result of
adaptation to this or that mode of life. Examples are those insects
which live in dusty situations or usually more or less concealed in the
earth, as in most beetles, and in the Hymenoptera. In such beetles, the
stigmata are situated in the thin membrane between the segments; in the
Hymenoptera, on the upper edge of the segments. In the Siphonaptera,
Pediculina, bed-bug, and similar forms, which breathe an air freer from
dust, the spiracles lie free on the outside of the body.

  “When the stigmata are free and without any protection on the
  abdomen, there are other ways by which the entrance of foreign
  bodies into the tracheæ is prevented. In such cases the body is
  covered with dense hairs, as in most Diptera and Neuroptera, as well
  as many Lepidoptera; or there is situated in front of the stigma
  either a small fissure which is covered over by a number of hairs
  arising from the edge, as in many Orthoptera; or, as in most
  insects, a luxurious growth of hairs on the inside of the stigma
  forms a thick filter for the air. Thus we see that also in this
  respect each species of insect is completely adapted to its
  surroundings.” (Krancher.)

[Illustration:

  FIG. 404.—_A_, thoracic stigma of the house-fly: _Sb_, valve which
    closes the opening.
]

[Illustration:

  FIG. 405.—Diagrammatic figures of the internal apparatus which closes
    the trachea, in the stag-beetle: _A_, trachea open; in _B_, closed;
    _St_, the stigma, with its grated lips; _Ct_, cuticula of the
    body-walls; _Vk_, closing pouch; _Vbü_, closing bow; _Vba_, closing
    band; _M_, occlusor muscle.—From Judeich and Nitsche.
]

=The closing apparatus of the stigma.=—Whether the external opening of
the stigma is permanently open or closed, communication with the tracheæ
may be cut off at pleasure during respiration by an internal apparatus
of elastic chitinous bands and rods and the occlusor muscle.

The parts concerned in this operation are: 1. The closing bow; 2. The
closing lever or peg; 3. The closing band; 4. The occlusor muscle (Figs.
405, 406).

[Illustration:

  FIG. 406.—Stigma, with the closing apparatus, of _Smerinthus populi_
    (imago), seen from within: _b_, closing bow; _c_, closing band: _o_,
    stigmatic opening; _r_, external chitinous ring; _l_, closing lever;
    _m_, occlusor muscle; _s_, scales which lie like roofing tiles over
    the stigma.—After Krancher.
]

“The first three parts are chitinized; they form a ring around the
stigmatic opening, and are united to each other by joints. The bow is
usually crescentic and as a rule surrounds one-half of the trachea. On
the other side is the closing band which, by different contrivances,
representing the closing lever or peg, becomes closely pressed against
the closing bow. This lever is usually of the shape of a slender
chitinous rod, which causes the closure; but it can also bend
rectangularly, become converted into a typical lever as in the
Lepidoptera, or it may assume the form of two peg-like processes, which
press with their base against the closing bow.” (Krancher.)

“The closure of the spiracular opening is effected by the contraction of
the muscles, while the opening is due to the elasticity of the chitinous
parts. When at rest the spiracle is naturally open, so that the air in
the trachea can directly communicate with the external air. Usually one
end of the muscle is attached to the closing peg, and the other end to
the closing bow. Where, as in Melolontha, the closing apparatus is
provided with two levers, then naturally the muscle binds these two
together and brings about by powerful contractions a firm closure of the
trachea”; but, remarks Krancher, “this is not the only kind; there are
numerous modifications. Besides the form just described, the levers
assume the form of valves (Sirex), or of a brush (Pulex); or of a ring
(larvæ of Diptera) with a circular muscle attached to it; or of a ring
which simply becomes compressed (thoracic stigmata of Diptera).”


         _c._ Morphology and homologies of the tracheal system

As first shown by Bütschli, the tracheal system is a series of
segmentally arranged tubular invaginations of the ectoderm; a pair of
stigmata primitively occurring on every segment of the body except
perhaps the most anterior, and the last two or last one, a reduction in
their number having since taken place, until in the Podurans none have
survived. In the supposed ancestor of myriopods and insects, Peripatus,
there are tracheæ; but they are very fine, simple, not-branched
chitinous tubes which are united into tufts at the base of a
flask-shaped depression of the integument, the outer aperture of which
depression is regarded as a stigma. In one species (_P. edwardsii_)
these tufts and their openings are scattered irregularly over the body;
but in another kind (_P. capensis_) some of the stigmata at least show
traces of a serial arrangement, being disposed in longitudinal rows—two
on each side, one dorsally and one ventrally, those of each row,
however, being more numerous than the pairs of legs. (See p. 9 and Fig.
4, _D_.)

It should be observed that in Peripatus, which does not possess urinary
tubes, the segmental organs or nephridia are well developed, hence the
tracheal tubes coexisting with them cannot be their homologues. We are
therefore compelled to regard the tracheal system as of independent
origin, arising in the earliest terrestrial air-breathing arthropod, and
not indebted for its origin to any structure found in worms, unless
perhaps, as both Kennell and Lang suggest, to dermal glands, since,
according to Kennell, certain Hirudinea and many Turbellarian worms
possess long, mostly unicellular, glands which spread far through the
parenchyma of the body. (Kennell.)

  Thus Kennell supposes that the ancestors of the Tracheates had
  spiracles on every segment of the body where the internal
  organization allowed them to exist. “The reduction of the breathing
  holes to a smaller number, and their restriction of a pair only to a
  single segment, was brought about partly by adaptation to a peculiar
  mode of life,—as insect larvæ especially teach us,—partly also—I may
  say mechanically—as a result of the obstruction to their development
  made by the growth or excessive development of other organs.” Among
  these he reckons the thick, dense cuticula of the integument, the
  internal fusion of several segments to form body-regions, and the
  arrangement and great development of the muscles in the head and
  thorax, etc. (p. 29.)

[Illustration:

  FIG. 407.—Section through a tracheal pit and diverging bundles of
    tracheal tubes taken transversely to the long axis of the body:
    _tr_, tracheæ, showing rudimentary spiral fibre; _tr. c_, cells
    resembling those lining the tracheal pits, which occur at
    intervals along the course of the tracheæ; _tr. o_, tracheal
    stigma; _tr. p_, tracheal pit.—After Balfour, from Sedgwick.
]

  Kennell has suggested the origin of the tracheæ of Peripatus from
  the unicellular dermal glands of annelidan ancestors, since he has
  found glands in certain land-leaches of tropical America, which are
  provided with enormously long tubular passages united into bundles
  and opening externally, these tubes appearing to be slightly
  chitinized. Fig. 407 will show the appearance of a bundle of fine
  tracheal tubes of Peripatus ending at the bottom of a follicle
  formed by a deep invagination of the integument, which may be
  regarded as a primitive spiracle. (See Kennell, Ueber einige
  Landblutegel des tropical America, Zool. Jahrb. ii, 1886; also Die
  Verwandtschaftsverhältnisse der Arthropoden, 1891, p. 25.) We may
  add that Carrière supposes from his study of the embryology of the
  wall-bee (_Chalicodoma muraria_), published in 1890, that not only
  the salivary glands, but also the tentorium, are homologues of the
  tracheæ, while other structures than tracheæ may have evolved from
  unicellular dermal glands, which are widely distributed. It may in
  this connection be observed that some authors derive the book-lungs
  or book-leaf tracheæ of Arachnida from the gills of Limulus; hence
  if those of Arachnida arose from quite different and more
  specialized organs than dermal glands, it is not impossible that the
  tracheæ of Peripatus, Myriopods, and insects arose _de novo_, and
  then we need not look for any primitive structures in worms from
  which they arose.

  Although Bütschli in 1870 in his embryology of the honey-bee called
  attention to the “great similarity which the eleven pairs of
  invaginations in the eleven first trunk-segments in their first
  indication (_anlage_) have with the spinning-glands, and also with
  the segmental organs of Annelids,” he did not go further than this,
  and it is now known that in the 2d maxillary segment open not only
  spinning-glands, but in the embryo a pair of stigmata.

  Paul Mayer, however, regarded the tracheæ and urinary tubes as
  homodynamous structures, and this view was advocated by Grassi
  (1885) for the reason that while in the embryo honey-bee there are
  ten pairs of stigmata, the first thoracic and two last abdominal
  segments wanting them, the germs of the urinary tubes arise in a
  corresponding situation on the two last abdominal segments. To this
  view Emery (Biol. Centralb., 1886, p. 692) objects that in Peripatus
  the nephridia and tracheæ “have nothing to do with the segmental
  organs,” as Peripatus besides nephridia possesses both coxal glands
  and tracheæ.

  Both Kennell and Lang derive the coxal glands of Arthropoda from the
  setiparous or parapodial glands of annelid worms, and the recent
  endeavor of Bernard to show that the tracheæ arose from setiparous
  glands seems to be disproved by the fact that in insects as well as
  in other Arthropoda coxal glands with their outlets exist in the
  same segments as those bearing stigmata. Reasoning by exclusion, we
  are led to regard Kennell’s original view as the soundest.

  Patten, however, regards the tracheæ as modified ends of nephridia,
  remarking: “Since in Acilius some of the abdominal tracheæ at first
  communicate with the cavities of the mesoblastic somites, it is
  probable that all the tracheæ represent the ectodermic portions of
  the nephridia.” (Origin of Vertebrates from Arachnids, p. 355.)

It is probable, therefore, that the tracheæ first arose as modifications
of dermal glands, as in mites and Peripatus, and that at first they were
not provided with tænidia (as in Chilopoda), while in later forms
tænidia were developed. In the earliest tracheate forms the stigmata
were not segmentally arranged, probably appearing irregularly anywhere
in the body, but afterwards in the myriopods and insects became serially
arranged.


                   _d._ The spiral threads or tænidia

It is generally supposed that the so-called “spiral thread” forms a
continuous thread from one end of a tracheal branch to the other. This
was first shown not to be the case by Platner in 1844. Minot has proved
that “there is not a single spiral thread, but several, which run
parallel to one another and end after making a few turns around the
trachea.”

The tænidia we have found to be in some cases separate, independent,
solid rings, though when there is more than one turn the thread
necessarily becomes spiral. The tænidia of a main branch stop at the
origin of the smaller branches, and a new set begins at the origin of
each branch. The tænidia at the origin of the branch do not pass
entirely around the inside of the peritoneal membrane; in the axils they
are short, separate, spindle-shaped bands (Fig. 409).

  At one point in the main trachea of the larva of Datana the tænidia
  were seen to end singly on one side (at a considerable distance from
  any branch or axil) at intervals, with a tænidium situated between
  them, making four or five turns; then there is only one band
  situated between two ends; this band or thread is succeeded by a set
  with five turns between the two ends, this set being succeeded by
  one complete ring situated between two ends; in all cases the ends
  vary in length, some threads being short and others long, so that
  they apparently end anywhere along the circumference of the trachea,
  and this arrangement is seen to apparently extend along the whole
  length of the trachea. Hence it is seen that as a rule the tænidia
  vary much in length, and never, as generally supposed, pass
  continuously from one end to another of a tracheal branch, for there
  are many spirals in a branch, each making only from one to five
  turns, most usually four turns. Fig. 408, part of a trachea of
  _Dyticus marginatus_, shows that at a slight bend in a trachea the
  tænidia is interrupted, and short, incomplete, wedge-shaped tænidia
  (_e_) are interpolated; at _A_, _d_ is seen a split in one of the
  tænidia (compare also MacLeod, Pl. 1, Fig. 9). The threads are quite
  irregular in width. In the axils of the branches there is, as seen
  in Fig. 409, a basketwork of independent, short, often
  spindle-shaped tænidia; these are succeeded by longer ones, until we
  have threads passing entirely around near the base of each new
  branch; these being succeeded by others which make from two to five
  spiral turns.

[Illustration:

  FIG. 408.—Tænidia of Dyticus: _d_, a split tænidium; _e_, _e_, ends of
    tænidia.
]

The shape of the tænidia appears to vary to a great extent. In
lepidopterous insects we have observed them to be in their general shape
rather flat and slightly concavo-convex, the hollow looking towards the
centre of the trachea. Minot’s section (Fig. 393) shows that in
Hydrophilus they are cylindrical and solid, and Chun states that those
of Stratiomys are round, while in Eristalis they are round, with a ridge
projecting into the cavity of the trachea; in Æschna the thread is
quadrangular. MacLeod states that sometimes it is cylindrical, in other
cases flat, likewise prismatic; Macloskie believes that the spiral
threads of the centipede are “fine tubules, externally opening by a
fissure along their course.”

[Illustration:

  FIG. 409.—Tænidia of Dyticus in an axil of two branches: _e_, _e_,
    ends of tænidia.
]

Stokes confirms Macloskie’s statements, stating that in the hemipterous
_Zaitha fluminea_ “the tænidia are fissured tubules formed within and
from chitinized folds of the intima, the convexity of the folds looking
towards the lumen of the tracheæ.” In Fig. 414, 1, are represented
portions of several tænidia showing the fissure, which is sometimes
interrupted; at 2 are seen “the formation of what may be called
apertures in a chitinous bridge.” Stokes regards the tænidia as
“inwardly directed folds of the membrane.” Near the spiracles the
tracheal membrane is externally studded with minute papillæ, as shown at
3, where are represented three broad and incomplete tænidia, with the
tapering end, or the beginning, of another. Stokes adds, “Here they are
only broad grooves, with no appearance of the narrow fissure of the
completed tænidium. At 4 is figured a portion of the internal surface of
a large trachea near the external orifice, the tænidia being in an
incipient stage, evidently forming more or less of a network, as is
usually the case next to the stigma” (compare p. 451, and Fig. 414).

[Illustration:

  FIG. 410.—End of salivary duct in base of proboscis of _Stomoxys
    calcitrans_: _a_, incomplete and irregular tænidia; _b_, two tænidia
    making incomplete rings near the distal end of the duct.
]

  The tracheæ of chilopod myriopods appear to be like those of
  insects. A number of authors have failed to detect the spiral
  threads in the Julidæ. As to the Arachnida, several observers,
  including Menge and Bertkau, have denied the existence of the spiral
  thread in the spiders with the exception of the Attidæ; and MacLeod
  finds them “scarcely visible” in Argyroneta.

Besides the tracheæ, the salivary duct is kept permanently distended by
tænidia, which, however, are not spiral. They usually form incomplete
rings, as in Stomoxys, arranged as shown in Fig. 410.

The labella (proboscis) of flies are supported by incomplete chitinous
tubes or “pseudo-tracheæ,” the ends of which form the scraping teeth,
this being, according to Dimmock, their primary function. Dimmock
describes them as cylindrical channels opening on the surface in zigzag
slits. These channels are held open by incomplete rings, one end of
which is forked. “These rings are apparently arranged so that one has
its fork on one side of the opening of the channel, the next ring the
fork on the opposite side of the channel, and so on, in alternation.
Their true structure is revealed when flattened out.”

[Illustration:

  FIG. 411.—Abdominal spiracle (left side) of cockroach (_P.
    americana_), side view, showing the bow: _p_, lateral pouch of
    spiracle (in centre) seen from within. The tessellated structure of
    spiracle and trachea shown at _A_, and the margin of the external
    aperture at _B_.—After Miall and Denny.
]

The use of the elastic tænidia is to render the tracheæ elastic, and to
keep them permanently open, as is the case with the parallel rings of
the trachea of the higher vertebrates. The tracheæ are thus rendered
firm and solid, at the least expense of chitinous material. The spiral
thread, as MacLeod remarks, “is the realization in nature of what
engineers call a form of the greatest resistance.”

  The tænidia are wanting in the fine endings of the tracheæ (tracheal
  capillaries); also in the cockroach, according to Miall and Denny,
  they are not developed in the large tracheæ close to the spiracles,
  and the intima or wall of the tube has a tessellated instead of a
  spiral marking (Fig. 411). The same structure is seen in the Perlidæ
  (Nemoura, Gerstaecker, Zeit. f. wissen. Zool. xxiv, Taf. xxiii,
  Figs. 5 and 7); also in Æschna (Hagen, Zool. Anz. 1880, p. 159). In
  certain fine tracheæ of the eyes of the fly no spiral threads are
  developed. (Hickson.) The air-sacs or dilated tracheæ are also
  without tænidia.

While in the living insect the main and smaller tracheæ are filled, with
air, it is stated by Von Wistinghausen that the fine capillary ends
contain a fluid.


         _e._ Origin of the tracheæ and of the “spiral thread”

While we owe to Bütschli the discovery of the mode of origin and
morphology of the tracheæ, which as he has shown[66] arise by
invaginations of the ectoblast; there being originally a single layer of
epiblastic cells concerned in the formation of the tracheæ; we are
indebted to Weismann[67] for the discovery of the mode of origin of the
“intima,” from the epiblastic layer of cells forming the primitive
foundation of the tracheal structure.

  Weismann did not observe the earliest steps in the process of
  formation of the stigma and main trunk of the tracheæ, which
  Bütschli afterwards clearly described and figured.

  Weismann, however, thus describes the mode of development of the
  intima; after describing the cells destined to form the peritoneal
  membrane, he says: “The lumen is filled with a clear fluid and
  already shows a definite border in a slight thickening of the
  cell-wall next to it.

  “Very soon this thickening forms a thin, structureless intima, which
  passes as a delicate double line along the cells, and shows its
  dependence on the cells by a sort of adherence to the rounded sides
  of the cells (Taf. vii, 97 _A_, _a b c_). Throughout the mass, as
  the intima thickens, the cells lose their independence, their walls
  pressing together and coalescing, and soon the considerably enlarged
  hollow cylinder of the intima is surrounded by a homogeneous layer
  of a tissue, whose origin from cells is recognized only by the
  regular position of the rounded nuclei (Taf. vii, Fig. 97, _B_).

  “Then as soon as the wavy bands of the intima entirely disappear,
  and it forms a straight, cylindrical tube, a fine pale
  cross-striation becomes noticeable (vii, 97, _B_, _int_), which
  forms the well-known ‘spiral thread,’ a structure which, as Leydig
  has shown, possesses no independence, but arises merely from a
  partial thickening of the originally homogeneous intima.

  “Meyer’s idea that the spiral threads are fissures in the intima
  produced by the entrance of air is disproved by the fact that the
  spiral threads are present long before the air enters. Hence the
  correctness of Leydig’s view, based on the histological structure of
  the tracheæ, is confirmed by the embryological development, and the
  old idea of three membranes, which both Meyer and Milne-Edwards
  maintain, must be given up.”

  Weismann also contends that the elastic membrane bearing the “spiral
  thread” is in no sense a primary membrane, not corresponding
  histologically to a cellular membrane. On the contrary, the
  “peritoneal membrane comprises the primary element of the trachea;
  it is nowhere absent, but envelops the smallest branches, as well as
  the largest trunks, only varying in thickness, which in the embryo
  and the young larva of Musca stands in relation to the thickness of
  the lumen.”

The trachea, then, consists primarily of an epithelial layer, the
“peritoneal membrane,” or the invaginated epiblast; from this layer an
intima is secreted, just as the skin or cuticle is secreted by the
hypodermis. We may call the peritoneal membrane the _ectotrachea_, the
intima or inner layer derived from the ectotrachea the _endotrachea_.
The so-called “spiral threads” are a thickening of the endotracheal
membrane, sometimes arranged in a spiral manner. For these chitinous
bands we have proposed the name _tænidia_ (Greek, little bands).

As to the origin of the spiral thread our observations[68] have been
made on the caterpillar of a species of Datana, which was placed in
alcohol, just before pupation, when the larva was in a semipupal
condition, and the larval skin could be readily stripped off. At this
time the ectotrachea of the larva had undergone histolysis, nothing
remaining but the moulted endotrachea, represented by the tænidia, which
lay loosely within the cavity of the trachea. The ectotrachea or
peritoneal membrane of the pupa is meanwhile in process of formation;
the nuclear origin of the tænidia is now very apparent.

[Illustration:

  FIG. 412.—Longitudinal section of a trachea, showing the origin of the
    tænidia.
]

[Illustration:

  FIG. 413.—Origin of the tænidia from nuclei.
]

Fig. 412 represents a longitudinal section through a secondary tracheal
branch, showing the origin of the chitinous bands, or tænidia. At _t′_
are pieces of six tænidia which have been moulted; _ectr_ indicates the
nuclei forming the outer cellular layer, the ectotrachea or peritoneal
membrane. These nuclei send long slender prolongations around the inside
of the peritoneal membrane; these prolongations, as may be seen by the
figure, become the tænidia. The tænidia, being closely approximate, grow
together more or less, and a thin endotracheal membrane is thus
produced, of which the tænidia are the thickened band-like portions. The
endotracheal membrane is thus derived from the ectotrachea, or primitive
tracheal membrane, and the so-called “spiral thread” is formed by
thickenings of the nuclei composing the secondary layer of nuclei, and
which become filled with the chitin secreted by these elongated nuclei.
The middle portion of the tænidia, immediately after the moult, is clear
and transparent, with obscure minute granules, while the nuclear base of
the cell is filled as usual with abundant granules, and contains a
distinct nucleolus.

[Illustration:

  FIG. 414.—Tænidia and internal hairs of _Zaitha_.—After Stokes.
]

The origin of the tænidia is also well shown by Fig. 413, which is
likewise a longitudinal section of a trachea at the point of origin of a
branch. The peritracheal membrane or ectotrachea (_ectr_) is composed of
large granulated nuclei; and within are the more transparent
endotracheal cells; at _t′_ are fragments of the moulted tænidia. The
new tænidia are in process of development at _t_; at base they are seen
to be granulated nuclei, with often a distinct nucleolus, each sending a
long, slender, transparent, pointed process along the inside of the
trachea. These unite to form the chitinous bands or spiral threads.

=Internal hair-like bodies.=—In the large tracheæ of Lampyris very fine
chitinous bristles project free into the cavity of the tube
(Gerstaecher), while according to Leydig there are similar chitinous
points in the tracheæ of the Carabid beetle Procrustes. Dugardin had
previously (1849) called attention to such hairs, giving a list of the
insects in which he observed them. Emery figures a section of the
tracheæ of Luciola, “in wendig behaart.”[69] Stokes has described those
of _Zaitha fluminea_ (Fig. 414) as “internal chitinous, hair-like bodies
arising from the fold of the tænidia and projecting into the lumen of
the tubes.” They are hollow, their minute cavity distinctly
communicating with that of the tænidium, from which they arise by an
enlarged base. They end in an exceedingly fine point which is
occasionally bifid or trifid. In Fig. 414, 4, several are shown attached
to the wrinkles of the tracheæ near a spiracle, and at 5 is represented
a transverse section of a trachea with three hairs projecting into its
cavity.[70]

  Stokes has also described “certain minute, elliptical bodies in the
  tænidia, each with an internal, presumably glandular, appendage, to
  all appearance forming part of the tænidium from which it springs.”
  These are shown in Fig. 414, at 1, 3, and, more in detail, at 6;
  those at 7, whose thickness is about 1⁄8000 of an inch, appear as
  collections of exceedingly minute, rounded apertures in a
  cushion-like mass. Although not commonly occurring on the tracheal
  membrane between the tænidia, they may be found there, as at 4.


   _f._ The mechanism of respiration and the respiratory movements of
                                insects

By holding a locust in the hand one may observe the ordinary mode of
breathing in insects. During this act the portion of the side of the
body between the stigmata and the pleurum contracts and expands; the
contraction of this region causes the spiracles to open. The general
movement is caused by the sternal moving much more decidedly than the
tergal portion of the abdomen. When the pleural portion of the abdomen
is forced out, the soft pleural membranous region under the fore and
hind wings contracts, as does the tympanum, or ear, and the membranous
portions at the base of the hind legs. When the tergum or dorsal portion
of the abdomen falls, and the pleurum contracts, the spiracles open;
their opening is nearly but not always exactly coördinated with the
contractions of the pleurum, but as a rule they are. There were 65
contractions in a minute in a locust which had been held between the
fingers about ten minutes. It was noticed that when the abdomen
expanded, the air-sacs in the first abdominal ring contracted.

For expanding the abdomen no special muscles are required, since it
expands by the elasticity of the parts. For contracting its walls there
are two sets of muscles, viz., special vertical expiratory muscles
serving to compress or flatten the abdomen (Figs. 415–418), and other
muscles which draw together or telescope the segments.

It was formerly supposed that when the abdomen contracted the air was
expelled from the body and the tracheæ emptied; that, when the abdomen
again expanded by its own elasticity, the air-tubes were refilled, and
that no other mechanism was needed. But Landois insisted that this was
not enough; as Miall and Denny state: “Air must be forced into the
furthest recesses of the tracheal system, where the exchange of oxygen
and carbonic acid is effected more readily than in tubes lined by a
dense intima. But in these fine and intricate passages the resistance to
the passage of air is considerable, and the renewal of the air could, to
all appearance, hardly be effected at all if the inlets remained open.
Landois accordingly searched for some means of closing the outlets, and
found an elastic ring or spiral, which surrounds the tracheal tube
within the spiracle.” By means of the occlusor muscle this ring
compresses the tube, “like a spring clip upon a flexible gas-pipe.”
“When the muscle contracts, the passage is closed, and the abdominal
muscles can then, it is supposed, bring any needful pressure to bear
upon the tracheal tubes, much in the same way as with ourselves, when we
close the mouth and nostrils, and then, by forcible contraction of the
diaphragm and abdominal walls, distend the cheeks or pharynx.”

Thus an important point in the respiration of tracheate animals, whether
insects, myriopods, or arachnids, is, as Landois claimed, the closure of
the spiracles, in order that pressure may be brought upon the air in the
tubes, so that it may pass onward into the finest terminations.

The injection of air by muscular pressure into a system of very fine
tubes may, as Miall and Denny remark, appear extremely difficult or even
impossible. Graham (Researches, p. 44) applies the law of diffusion of
gases to explain the respiration of insects, but until physical
experiments have been made, we may, with Miall and Denny, “be satisfied
that an appreciable quantity of air may be made by muscular pressure to
flow along even the finer air-passages of an insect.”

As to the respiratory movements of insects, Plateau is the principal
authority, and the following account of the process is taken from his
elaborate memoir, and from the statements afterwards contributed by him
to Miall and Denny’s “The Cockroach.”

Although many observers have superficially described the respiratory
movements of various insects, Rathke was the first one to state precise
views as to the mechanism of respiration. His posthumous work, treating
of the respiratory movements of the movable chitinous plates of the
abdomen, and of the respiratory muscles characteristic of all the
principal groups, filled an important blank in our knowledge. But,
notwithstanding the skill displayed in this research, many questions
still remain unanswered which require more exact methods than mere
observations with the naked eye or the simple lens.

Plateau, who was followed a year later by Langendorff, conceived the
idea of studying, by such graphic methods as are now familiar, the
respiratory movements of perfect insects.

  “He has made use of two modes of investigation. The first, or
  graphic method, in the strict sense of the term, consisted in
  recording, upon a revolving cylinder of smoked paper, the
  respiratory movements, transmitted by means of very light levers of
  Bristol board attached to any part of the insect’s exoskeleton.
  Unfortunately, this plan is only applicable to insects of more than
  average size. A second method, that of projection, consisted in
  introducing the insect, carried upon a small support, into a large
  magic lantern fitted with a good petroleum lamp. When the
  amplification does not exceed 12 diameters, a sharp profile may be
  obtained, upon which the actual displacements may be measured, true
  to the fraction of a millimetre. Placing a sheet of white paper upon
  the lantern screen, the outlines of the profile are carefully traced
  in pencil so as to give two superposed figures, representing the
  phases of inspiration and expiration respectively. By altering the
  position of the insect so as to obtain profiles of transverse
  sections, or of the different parts of the body, and, further, by
  gluing very small paper slips to parts whose movements are hard to
  observe, the successive positions of the slips being then drawn,
  complete information is at last obtained of every detail of the
  respiratory movements; nothing is lost.”

  “This method, similar to that employed by the English physiologist,
  Hutchinson,[71] is valuable, because it enables us, with a little
  practice, to investigate readily the respiratory movements of very
  small arthropods, such as flies or lady-birds. It has this advantage
  over all others, that it leaves no room for errors of
  interpretation.”

  “Not satisfied with mere observation by such means as these, of the
  respiratory movements of insects, the writer has also studied the
  muscles concerned, and, in common with other physiologists (Faivre,
  Barlow, Luchsinger, Dönhoff, and Langendorff), has examined the
  action of the various nervous centres upon the respiratory organs.
  The result at which he has arrived may be summarized as follows:—

[Illustration:

  FIG. 415.—Muscles of right half of the abdomen of _Forficula
    auricularia_: _A_, _a_, longitudinal tergal and sternal muscles;
    _D_, _E_, oblique muscles; _a_ (in upper figure) vertical
    expirator muscles.
]

  “1. There is no close relation between the character of the
  respiratory movements of an insect and its systematic position.
  Respiratory movements are similar only when the arrangement of the
  abdominal segments, and especially when the disposition of the
  attached muscles, are almost identical. Thus, for example, the
  respiratory movements of the cockroach are different from those of
  other Orthoptera, resembling those of the heteropterous Hemiptera.
  Those of the Trichoptera are like those of the aculeate Hymenoptera,
  while the Locustidæ ally themselves in respect to these movements
  with the Neuroptera and Lepidoptera.

  “2. The respiratory movements of insects, when at rest, are
  localized in the abdomen. As graphically stated by Graber, in
  insects the chest is placed at the hinder end of the body. If
  thoracic respiratory movements exist, they do not depend on the
  action of special muscles.

  “3. In most cases the thoracic segments do not share in the
  respiratory movements of an insect at rest. The respiratory
  displacements of the posterior segments of the thorax are, however,
  less rare than Rathke believed. Plateau has observed them in certain
  Coleoptera (Staphylinus, Chlorophanus, Corymbites), and they are
  more feebly manifested in Hydrophilus, Carabus, and Tenebrio. Among
  the singular exceptions to this rule is the cockroach (_Periplaneta
  orientalis_), in which the terga of the meso- and metathoracic
  segments perform movements exactly opposite in direction to those of
  the abdomen (Fig. 419).

[Illustration:

  FIG. 416.—Muscles of the left half of abdomen of _Staphylinus
    olens_; _A_, _B_, longitudinal dorsal muscles; _D_, _E_, oblique
    fascia; _a_, longitudinal sternal muscles; _d_, respiratory
    muscles (vertical expirators).
]

  “4. Leaving out of account all details and all exceptions, the
  respiratory movements of insects may be said to consist of the
  alternate contraction and recovery of the figure of the abdomen in
  two dimensions, viz. vertical and transverse. During expiration both
  diameters are reduced, while during inspiration they revert to their
  previous amounts. The transverse expiratory contraction is often
  slight, and may be imperceptible. On the other hand, the vertical
  expiratory contraction is never absent, and usually marked. In the
  cockroach (_P. orientalis_) it amounts to one-eighth of the depth of
  the abdomen (between segments 2 and 3); in _Eristalis tenax_ to
  one-ninth (at the 2d segment).

  “5. Three principal types of respiratory mechanism occur in insects,
  and these admit of further subdivision:

  “_a._ Sterna usually short and very convex, yielding but little.
  Terga mobile, rising and sinking appreciably. To this class belong
  all Coleoptera, heteropterous Hemiptera, and Blattina (Fig. 420).

  “In the cockroach (Periplaneta), the sterna are slightly raised
  during expiration (Fig. 421).

  “_b._ Terga well developed, overlapping the sterna on the sides of
  the body, and usually concealing the pleural membrane, which forms a
  sunken fold. The terga and sterna approach and recede alternately,
  the sterna being almost always the more mobile. To this type belong
  Odonata, Diptera, aculeate Hymenoptera, and acrydian Orthoptera
  (Fig. 422).

[Illustration:

  FIG. 417.-Muscles of right half of abdomen of _Phryganea striata_,
    ♀: _A_, _B_, longitudinal dorsal muscles; _a_, _b_, longitudinal
    sternal muscles; _D_, _e_, oblique muscles; 1, 2, inspirator
    muscles.
]

  “_c._ The pleural membrane, connecting the terga with the sterna, is
  well developed and exposed on the sides of the body. The terga and
  sterna approach and recede alternately, while the pleural zone
  simultaneously becomes depressed, or returns to its original figure.
  To this type, Plateau assigns the Locustidæ, Lepidoptera, and the
  true Neuroptera (excluding Trichoptera) (Fig. 423).

[Illustration:

  FIG. 418.—Muscles of left half of abdomen of Melolontha, ♀: _A_,
    _B_, longitudinal muscles (prétracteurs of Straus); _a_, _a_, true
    respiratory muscles (expirators).—This and Figs. 415–417, after
    Plateau.
]

  “6. Contrary to the opinion once general, changes in length of the
  abdomen, involving protrusion of the segments and subsequent
  retraction, are rare in the normal respiration of insects. Such
  longitudinal movements extend throughout one entire group only, viz.
  the aculeate Hymenoptera. Isolated examples occur, however, in other
  zoölogical groups.

  “7. Among insects, such as large beetles, Locustidæ, dragon-flies,
  etc., sufficiently powerful to give good graphic tracings, it can be
  shown that the inspiratory movement is slower than the expiratory,
  and that the latter is often sudden.

[Illustration:

  FIG. 419.—Profile of trunk of cockroach (_P. orientalis_). The black
    surface represents the expiratory contour, while the inspiratory
    is indicated by a thin line. The arrows show the direction of the
    expiratory movement: _Ms. th_, mesothorax; _Mt. th_, metathorax.
    Reduced from a magic-lantern projection.—After Plateau.
]

  “8. In most insects, contrary to what obtains in mammals, only the
  expiratory movement is active; inspiration is passive, and effected
  by the elasticity of the body-wall.

  “9. Most insects possess expiratory muscles only. Certain Diptera
  (_Calliphora vomitoria_ and _Eristalis tenax_) afford the simplest
  arrangement of the expiratory muscles. In these types, they form a
  muscular sheet of vertical fibres, connecting the terga with the
  sterna, and underlying the soft, elastic membrane which unites the
  hard parts of the somites. One of the most frequent complications
  arises by the differentiations of this sheet of vertical fibres into
  distinct muscles, repeated in every segment, and becoming more and
  more separated as the sterna increase in length. Special inspiratory
  muscles occur in Hymenoptera, Acridiidæ, and Trichoptera.

  “10. The abdominal, respiratory movements of insects are wholly
  reflex. Like other physiologists who have examined this side of the
  question, Plateau finds that the respiratory movements persist in a
  decapitated insect, as also after destruction of the cerebral
  ganglia or œsophageal connectives; further, that in insects whose
  nervous system is not highly concentrated (_e.g._ Acridiidæ and
  dragon-flies), the respiratory movements persist in the completely
  detached abdomen; while all external influences which promote an
  increased respiratory activity in the uninjured animal, have
  precisely the same action upon insects in which the anterior,
  nervous centres have been removed, upon the detached abdomen, and
  even upon isolated sections of the abdomen.

  “The view formerly advocated by Faivre, that the metathoracic
  ganglia play the part of special, respiratory centres, must be
  entirely abandoned. All carefully performed experiments on the
  nervous system of Arthropoda have shown that each ganglion of the
  ventral chain is a motor centre, and, in insects, a respiratory
  centre, for the somite to which it belongs. This is what Barlow
  calls the ‘self-sufficiency’ of the ganglia.” (Miall and Denny.)

[Illustration:

  FIG. 420.—Transverse section of abdomen of a lamellicorn beetle. The
    position of the terga and sterna after an inspiration is indicated
    by the thick line; the dotted line shows their position after an
    expiration; and the arrow marks the direction of the expiratory
    movement.
]

[Illustration:

  FIG. 421.—Cross-section of abdomen of cockroach.
]

[Illustration:

  FIG. 422.—Cross-section of abdomen of bee (Bombus).
]

[Illustration:

  FIG. 423.—Cross-section of abdomen of Sphinx.—This and Figs. 420–422
    after Plateau.
]

  Plateau has made similar observations upon the respiration of
  spiders and scorpions; but, to his great surprise, he was unable,
  either by direct observation, or by the graphic method, or by
  projection, to discover the slightest respiratory movement of the
  exterior of the body. This can only be explained by supposing that
  inspiration and expiration in pulmonate Arachnida are
  “intrapulmonary,” and affect only the proper, respiratory organs.
  The fact is less surprising because of the wide zoölogical
  separation between Arachnida and insects.


                           _g._ The air-sacs

In flying insects the tracheæ are in certain parts of the body enlarged
into sacs of various sizes. These air-sacs were first observed by
Swammerdam in a beetle (Geotrupes) and afterwards by Sir John Hunter in
the bee, Sprengel subsequently discovering them in other insects. Those
of the cockroach were described and illustrated in a very elaborate and
detailed way by Straus-Dürckheim (Figs. 424 and 425). These vesicles are
without tænidia. In the locust (_M. femur-rubrum_) there is a pair of
very large vesicles in the prothorax (Fig. 396). The five pairs of large
abdominal air-sacs arise, independently of the main tracheæ, directly
from branches originating from the spiracles. All these large sacs are
superficial, lying directly beneath the hypodermis, while the smaller
ones are buried among the muscles. We have detected 53 of these vesicles
in the head.

In the honey-bee (Fig. 426) and humble bee (Fig. 427) as well as the
flies there are two enormous air-sacs at the base of the abdomen. In
larval and wingless insects these sacs are entirely absent.

[Illustration:

  FIG. 424.—Thorax and abdomen of the cockchafer (_Melolontha
    vulgaris_), showing the tracheæ and air-sacs.—This and Fig. 425
    after Straus-Dürckheim.
]

=The use of the air-sacs.=—It was supposed by Hunter as well as by
Newport, and the view has been generally held, that the use of these
sacs is to lighten the weight, _i.e._ lessen the specific gravity of the
body during flight. It has, however, been suggested to us by A. A.
Packard that this view from the standpoint of physics is incorrect. It
is evident that the wings have to support just as much weight when the
insect is flying, whether the tracheæ and vesicles are filled with air
or not, the body of the insect during flight not being lightened by the
air in the sacs. The use of these numerous sacs, some of them very
spacious, is to afford a greater supply of air or oxygen than that
contained in the air-tubes alone, and thus to afford a greater breathing
capacity. The sacs are largest in dragon-flies, moths, flies, and bees,
which are swift of flight. When we compare the active movements of these
insects on the wing with those of a caterpillar or maggot, it will be
seen that the far greater muscular exertions of the volant insect create
a demand for a sudden and abundant supply of air to correspond to the
increased rapidity of respiration; and the enlargements of the
air-tubes, rapidly filled with air at each inspiration, render it
possible to supply the demand.

[Illustration:

  FIG. 425.—Head of _Melolontha vulgaris_, showing the numerous
    air-sacs, represented only on the left side, front view.
]

[Illustration:

  FIG. 426.—Tracheal, nervous, and digestive systems of the honey-bee
    (the tracheal system on the right side only partially drawn): _tb_,
    the large vesicles in the abdomen; _st_, stigmata; _hm_, honey
    stomach; _cm_, chyle stomach; _vm_, urinary tubes; _rd_, rectal
    glands; _ed_, rectum; _a_, antenna; _an_, eye; _b_{1}_-_b_{3}_,
    legs.—After Leuckart, from Lang.
]

  The case is thus seen to be very different from that of those fishes
  which, having a swimming-bladder, can in the water change the
  specific gravity of their bodies. The case of insects is almost
  exactly paralleled by that of birds, where, as stated by
  Wiedersheim, the air-sacs appear to form integral parts of the
  respiratory apparatus: “a greater amount of air can by their means
  pass in and out during inspiration and expiration, especially
  through the larger bronchi, and consequently there is less necessity
  for the expansion of the lung parenchyma.” In other words, the
  supply of air in these sacs, as in insects, increases the breathing
  capacity of the bird during flight. Wiedersheim’s retention of the
  old idea that the specific gravity of the body is lessened (p. 262)
  seems, however, to be incorrect, as the weight of the bird’s body is
  not diminished by the air contained in the sacs.


            _h._ The closed or partly closed tracheal system

[Illustration:

  FIG. 427.—The lateral and lower series of sacs of _Bombus terrestris_,
    ♂: _a_, _c_, longitudinal tracheæ;, connected by _b_, and dilated at
    _f_, and again in the succeeding segments; _i_, _k_, funnel-shaped
    dilatations passing over the dorsal surface of the abdomen and
    anastomosing (_g_) with their fellows opposite; at _l_,
    communicating directly by a large branch.—After Newport.
]

There are two chief morphological tracheal systems: 1. The open or
normal and primitive (_holopneustic_) type, and 2. The closed, or
secondary and adaptive, _i.e._ _apneustic_, type. The open system is
characterized by the presence of the stigmata. Through them the air
directly enters into the tracheal tubes, whose delicate walls allow the
exchange of gases in the blood. This type occurs in all sexually mature
individuals, and also in the greater number of larvæ.

The closed or apneustic tracheal system is distinguished either by the
want of stigmata, or, if present, they are not open, and do not
function, so that the tracheæ cannot communicate with the air. In such
cases the direct oxygenation of the blood is effected through the
delicate integument, especially over the surface of the body in general,
or in certain specialized places where the gill-like expansions of the
skin are rich in tracheæ; such outgrowths, generally tubular or
leaf-like, are called by Palmén _tracheal gills_.

This closed form of the tracheal system only occurs in the larval stage
of aquatic or parasitic insects, as in the Plectoptera (Ephemeridæ),
Perlidæ, Odonata, and Trichoptera, besides single genera of other
orders, _i.e._ among Coleoptera, Gyrinus, Pelobius, Cnemidotus, and the
young larva of Elmis; in the aquatic caterpillar of Paraponyx; in
certain Diptera (Corethra, Chironomus, etc.), and some of the parasitic
Hymenoptera (Microgaster).

Palmén has discovered that in the nymphs of Ephemeridæ, Perlidæ,
Odonata, and the larvæ of most Trichoptera the tracheal branch
(stigmatal branch) sent from the longitudinal trachea to where the
thoracic stigmata would be situated if present, or where their vestiges
only exist, are aborted, becoming simple solid cords not filled with air
(Fig. 436, _vf_, and 447, _f_, funiculus or stigmatic cord). In the
imago, however, they resume their function, connecting with the open
functional stigmata. In Corethra, in its earliest stages, the entire
tracheal system is, like the stigmatic branch, a system of solid cords
and empty of air. (Palmén.)

  Embryology shows that these stigmatal branches are well developed,
  and are formed at the same time as the stigmata. It was also shown
  by Dewitz, in a posthumous paper (1890), that in the young larval
  stage of the Odonata and Ephemeridæ the tracheal system is at first
  an open one, and in some of the families (Libellulidæ, Agrionidæ,
  and Ephemeridæ) thoracic stigmata are seen at a very early stage.
  From numerous experiments Dewitz concludes that in the young stages
  of Odonata and Ephemeridæ there is an open tracheal system;
  certainly in very young nymphs the thoracic spiracles allow the air
  to pass out. Fully grown nymphs of Æschnidæ, Libellulidæ, and
  Agrionidæ are capable not only of forcing the air out, but also,
  like the perfect insect, of inhaling it. Moreover, he proved that
  the gills of Ephemeridæ and Agrionidæ are not indispensable for the
  maintenance of life, as the insects can live without them, breathing
  either through the skin or by the rectum, or in both ways. It would
  seem that while in freshly hatched or very young larvæ of aquatic
  insects of different orders the skin is so delicate as to allow of
  dermal respiration, in after life, when the skin becomes thicker and
  denser, these expansions (gills), provided with a very thin and
  delicate skin, of a necessity grow out from the walls of the body.

  It thus appears that the closure and total or partial abolition of
  the stigmata are in adaptation to aquatic life, and that such
  insects have descended from terrestrial air-breathing winged forms.
  This is an important argument against the view that the wings are
  modified tracheal gills.

  In this connection may be noticed the closure of the 2d and 3d
  thoracic stigmata in holopneustic insects. We have found on laying
  open the body of a Sphinx larva that a large number of tracheal
  branches are seen to arise from the prothoracic and from the first
  pair of abdominal stigmata. Now between these points there are no
  spiracles or any external signs of them, there being in Lepidoptera
  no mesothoracic or metathoracic spiracles. Yet the main lateral
  trachea between the prothoracic and first abdominal segments
  deviates from its course and bends down to send off a small
  shrivelled stigmatal branch or cord to a place where, did a spiracle
  exist, we should look for it. In the larva of _Platysamia cecropia_,
  a similar vestigial stigmata branch is present.

  In the larva of Corydalus, also, a trachea as large as the main
  longitudinal one takes its origin and passes directly under the main
  trachea. Now both tracheæ send a stigmatal branch opposite to where
  the mesothoracic stigma should be, if present, _i.e._ on the hind
  edge of the segment.

  Verson, moreover, has found in the freshly hatched silkworm vestiges
  of meso- and metathoracic stigmata, each consisting of a circle of
  high hypodermal cells radially arranged around a common centre. The
  stigmatal branch is long, but shrivelled; its peritoneum is widened
  out into several berry-like saccules filled with cell-elements. In
  profile these rudimentary stigmata appear as a series of high
  hypodermal cells, which form the basis of a short blind tube.

[Illustration:

  Lydia M. Hart _del._

  PLATE I.—Examples of metapneustic insects: 1, _Bittacomorpha
    clavipes_, larva; 1 _a_, false foot; 1 _b_, its pupa; 2,
    _Limnophila luteipennis_; 2 _a_, end of larva; 2 _b_, its pupa; 3,
    end of larva of _Tipula eluta_.—After C. A. Hart.
]

  After the second moult there begins a peculiar transformation of the
  rudimentary stigmata. The stigmatal branch connected with them sends
  off at various points thick tufts of capillary tracheæ which press
  against the base of the blind tube. Gradually lengthening, they form
  a fold which continues to increase in length. The numerous tufts of
  tracheal capillaries extend beyond the inner surface of the two
  layers of which the developing wing consists, the berry-like
  saccules are drawn into the wing and converted into more or less
  thick tubes, which finally form the “veins.” It is clear, therefore,
  says Verson, as Landois claimed, that the wings of Lepidoptera must
  be regarded as in the fullest sense organs of respiration. (Zool.
  Anz., 1890, p. 116.)

The number of pairs of stigmata varies, especially in maggots or larval
Diptera, in adaptation to their varied modes of life. The larvæ of most
flies (Muscidæ) have a pair of peculiarly shaped processes on the
prothoracic segment bearing spiracular openings, and two anal spiracles,
while in _Ctenophora atrata_ L. only the anal pair are present. In the
rat-tailed maggots (Eristalis) the long caudal process ends in two
stigmata forming a respiratory tube, which can be thrust out of the
water for the reception of air. In the larval mosquito (Fig. 433) and
its ally, Mochlonyx, a short thick dorsal tube arises from the
penultimate segment of the body, in which the two main tracheæ end,
opening outward by a single spiracular aperture. Other dipterous larvæ
(Simulium, Tanypus, and Ceratopogon) possess no spiracles, the tracheal
system being a closed one.

The larvæ of most water beetles (Dyticidæ, Hydrophilidæ) possess but two
spiracles, which, as in maggots, are situated at the end of the body.
The aquatic larva of Amphizoa, according to Hubbard, breathes much as in
the Dyticidæ, by means of two large valvular spiracles placed close
together at the end of the body; “closed or rudimentary stigmata also
occur on the mesothorax and on abdominal segments one to seven
inclusive.”

  Hubbard adds: “The larva of Pelobius is wholly aquatic and breathes
  by branchiæ, but the obsolete stigmata are indicated precisely as in
  Amphizoa, with the exception of the last pair, which in Amphizoa are
  open spiracles, but in Pelobius are suppressed; the terminal eight
  segments being prolonged in a swimming stylet.”

From a review of the distribution of spiracles, and their atrophy,
partial or total, it will be seen that there are intermediate stages
between the open (holopneustic) and closed (apneustic) systems. These,
following Schiner, Brauer, and Palmén, may be defined thus:

  1. _Metapneustic type._—The larvæ possess only a single pair of open
  stigmata situated at the end of the body. (The dipterous Eristalis,
  Tipula, Culex, Ptychoptera, Bittacomorpha (Plate I.) with certain
  Tachinidæ, and in Coleoptera, the larvæ of Dyticus, and allies of
  Hydrophilus and Cyphon.)

  2. _Propneustic type._—The pupæ of Corethra, Culex, etc., in which
  only the most anterior pair of spiracles are open.

[Illustration:

  FIG. 428.—Visceral tracheal system of the nymph of _Æschna
    maculatissima_: _o_, œsophagus; _E_, stomach; _M_, urinary tubes;
    _R_, rectum; _A_, anus; _tv_, visceral tracheal trunks; _td_,
    dorsal trunks.—After Oustalet.
]

  3. _Amphipneustic type._—Larvæ with a pair of open spiracles
  situated at each end of the body, the intermediate spiracles being
  closed. (Most dipterous larvæ, Musca, after the first moult,
  Œstridæ, Asilidæ, and Syrphus.)

[Illustration:

  FIG. 429.—Branchial tuft of nymph of Æschna.
]

  4. _Peripneustic type_; with prothoracic and abdominal spiracles,
  the mesothoracic pair atrophied or closed. (The larvæ of Neuroptera,
  Mecoptera, Trichoptera, Lepidoptera, of most Coleoptera,[72] of most
  Diptera, and of most of the Hymenoptera.[73])

[Illustration:

  FIG. 430.—Part of three rows of respiratory folds from cuticular
    living rectum of Æschna. The shaded parts are abundantly supplied
    with tracheal tubes. The leaflets appear to be connected with a
    central trachea, but this is not really the case.—After Miall.
]

  These differences in the number of functional spiracles are in
  direct relation with the surroundings of the insects, the physical
  conditions of existence evidently determining the position of the
  active functional open spiracles and the closure of those useless to
  the organism.


_i._ The rectal tracheal gills, and rectal respiration of larval Odonata
                           and other insects

The remarkable mode of respiration by tracheal gills situated within the
intestine of the nymphs of dragon-flies was first described by
Swammerdam and afterwards by Réaumur. The most complete and best
illustrated modern account is that of Oustalet. In these insects the
large rectum is lined with six double longitudinal ridges, in Æschna
bearing numerous delicate tubes or papillæ, each of which contains very
numerous (by estimate 24,000) tracheal branches (Fig. 431); while in
Libellula the gills are lamellate (Fig. 432). The tracheæ arise both
from the main dorsal and visceral longitudinal trunks, which give rise
to secondary branches passing into the walls of the rectum and sending
into the branchial papillæ fine twigs, which, extending to the distal
end of the papilla or lamella, recurve and anastomose with the efferent
twigs.

[Illustration:

  FIG. 431.—A small part of one leaflet, highly magnified, showing many
    fine tracheal branches. The portion shown is marked by a small
    circle in Fig. 430, lower left-hand corner.—After Miall.
]

[Illustration:

  FIG. 432.—Leaves, _mh_, from a lamellate tracheal gill of Libellula:
    _t_, trachea.—This and Fig. 429, after Oustalet.
]

The anal opening is externally protected by the suranal and lateral
triangular chitinous plates, three to five in all. When open, the water
passes into the rectum and bathes the rectal gills, where it may be
forcibly expelled as if shot out from a syringe, thus propelling the
insect forward. In Libellula the anus affords direct access to the
intestinal cavity, but in Æschna Oustalet describes “a sort of vestibule
separated from the rectum by a circular valvule.” He also states that
the inspiration and the repulsion of water is produced at irregular
intervals, and rather by the movements of the dorsal and sternal arches
of the abdomen than by the contractions of the rectum, since the walls
of this organ are less muscular than is supposed.

[Illustration:

  FIG. 433.—Larva of a mosquito (_Culex nemorosus_) of middle age, seen
    from above, the tracheal system omitted: _at_, antennæ; _ab_, their
    middle joint; _eg_, elastic articular membrane; _atm_, antennal
    muscle; _atn_, antennal nerve; _zau_, compound; _eau_, simple eye;
    _os_, brain; _oex_, extensor; _ofl_, flexor of labrum; _ha_, neck;
    _œ_, œsophagus; _spd_, salivary gland; _mau_, cœca; _ch_, chyle
    stomach; _di_, contents of intestine; _mg_, urinary tubes; _dd_,
    ileum; _ed_, rectum; _a_, anus; _s_, sipho; _z″_, its bristles;
    _kb_, tracheal gills: _k_{1}_, _k_{2}_, _k_{3}_, closing lobes of
    the sipho; _kn_, basal tubercle of tactile hair; _g_, its ganglion
    cell; _th_, tactile hair of the siphon valve.
]

  The nymph of Calopteryx (and probably of all the group
  Calopteryginæ) possesses rectal gills besides external caudal
  tracheal gills. There are three double rectal longitudinal folds or
  ridges, interpenetrated by tracheal twigs. (Dufour, denied by
  Poletaiew, but confirmed by Hagen.)

  Dewitz claims that the caudal gills of the Agrionidæ are not their
  sole means of respiration, since he cut off the caudal tracheal
  gills of an Agrionid nymph, which continued to live for a week.
  Hence he thinks that there may be a rectal respiration, since under
  the microscope he saw a stream of water pass in and out of the end
  of the intestine.

  Dewitz’ experiments prove that in young Ephemerids there may be
  besides branchial, both rectal and skin respiration. He saw under
  the microscope the anus for a while opened and then closed, causing
  the rectum to move; powdered carmine mixed with water was drawn into
  and then expelled from the rectum. There was, however, no
  enlargement and contraction of the abdomen as in the rectal
  respiration of Æschna. (Zool. Anz. 1890, p. 500.)

[Illustration:

  FIG. 434.—End of the body of the same larva as in Fig. 431, seen from
    the side, the branches of the main tracheæ (_htr_) omitted: _kbl_,
    excrementitial pellet in rectum; _kb_, tracheal gills; _b_, funnel
    of the closing apparatus; _hz_, hollow tooth of the closing
    apparatus; _k_{1}_, _k_{2}_, _k_{3}_, siphonal lobes; _th_, tactile
    hair; _as_, chitinous plate; _str_, rudder; _l_, its thickened edge;
    _sch_, its shank; _z′_, _z″_, bristles.—This and Fig. 433, after
    Raschke.
]

Eaton states that there is a rectal respiration in the nymphs of
may-flies, and Palmén observed in young larvæ of Bætis and Cloëon that
the rectum took in “by gulps” water colored by carmine and expelled the
whole of it at once, in order to fill it again in the same way. “This
rectal respiration therefore corresponds to that of Libellulid larvæ.”

[Illustration:

  FIG. 435.—Thorax and anterior abdominal segments of the nymph of a
    may-fly (_Cloëon dimidiatum_) with tracheal gills (_tk_{1}_,
    _tk_{2}_, _tk_{3}_) and the rudiments of the fore wings (_VF_) and
    hind wing (_HF_): _tl_, tracheal longitudinal trunks.—After Graber,
    from Lang.
]

[Illustration:

  FIG. 436.—Gills on the middle abdominal segments of larva of _Bætis
    binoculatus_: _trl_, longitudinal tracheal trunks; _vf_, stigmatic
    cord; _ktr_, gill-tracheæ; _trk_, tracheal gills.—After Palmén, from
    Lang.
]

Besides breathing by spiracles, by tracheal gills, as well as through
the integument, the larva of Culex has been observed by Raschke to have
a rectal respiration. At the anterior end of the rectum arises a
countless number of fine tracheæ, which pass through the walls and,
subdividing, end in numberless very fine twigs in the papilla-like folds
situated within the rectum. The supply of tracheal twigs is greatest
where the papillæ are largest. (Figs. 433, 434.)


              _j._ Tracheal gills of the larvæ of insects

In many aquatic insects respiration is carried on by tracheal gills.
These are delicate, hollow, leaf-like or tubular outgrowths of the
integument usually attached to the sides or end of the hind-body, and
containing a trachea which usually sends off numerous minute branches,
so that the exchange of gases readily takes place in them.

[Illustration:

  FIG. 437.—A, nymph of _Ephemerella ignita_, with gills of left side
    removed; _g_, gills. _B_, nymph of Tricorythrus (_sp_), with
    gill-cover of right side removed; _gc_, gill-cover; _g_, _g′_,
    gills.—After Vayssière.
]

[Illustration:

  FIG. 438.—Left maxilla of _Jolia weselii_, with the cephalic tracheal
    gill (_h_) inserted at the base on the under side.—After Vayssière.
]

Palmén has shown that these tracheal gills, as he calls them, are not
developed on the same segments as the stigmata, and that the two
structures have no genetic connection with each other. It is evident
that these gills are secondary, adaptive organs.

In some cases (see p. 475) the tracheæ are wanting, but as such gills
are filled with blood, the air contained in the water must pass in
through their delicate walls.

In the Plectoptera (Ephemeridæ) the tracheal gills are either foliaceous
or filamentous; when foliaceous they form simple or double leaves, with
or without branches, or with a fringe of tubules, or under the leaf-like
cover-bearing tufts of filaments. They are situated on the (usually)
basal seven abdominal segments, at their hinder edge (Figs. 435, 436).
In Oligoneuria and Jolia a pair occurs on the under side of the head,
attached to the maxillæ, while in Jolia there is a pair on the under
side of the first thoracic segment at the insertion of each of the legs.
In certain genera (Heptagenia, Oligoneuria, and Jolia), they are in the
form of a flat cover, under which lies a tuft of respiratory tubes, or
(Ephemerella) a small bifid cluster of very delicate leaves (Fig. 437,
_A_). In Cœnis and Tricorythus the tracheal gills of the second pair are
modified to form plates covering all the succeeding pairs, those of the
first pair being nearly atrophied and well-nigh functionless. (Fig. 437,
_B_.)

[Illustration:

  FIG. 439.—Inner side of a gill-cover of the first pair, of
    Ephemerella, with the tracheal gills.—After Vayssière.
]

[Illustration:

  FIG. 440.—Nymph of Bætisca: III, section of abdomen; _a_, gills; _b_,
    flap; 1–9, abdominal segments.—After Walsh.
]

[Illustration:

  FIG. 441.—Nymph of _Prosopistoma punctifrons_: _o_, upper orifice of
    the respiratory chamber.—After Vayssière.
]

[Illustration:

  FIG. 442.—Filamentous tracheal gill and part of a trachea of
    Pteronarcys.—After Newport from Sharp.
]

Finally, in the highly modified forms Bætisca and Prosopistoma the
tracheal gills are entirely concealed and protected by mesothoracic
projections so as to form a true respiratory chamber, to which the water
has access either by an opening behind, as in Bætisca, or by three
openings, two ventral and one dorsal (Fig. 441), as in Prosopistoma.

The slender cylindrical tracheal gills of Heptagenia in the third or
fourth nymphal stage are 2–jointed, and the first abdominal pair in
Cænis are said by Palmén to be finger-shaped and 2–jointed. In
_Polymitarcys virgo_ the gills do not appear until the eighth or tenth
day after hatching.

  Dewitz found that young nymphs of Ephemerids will well endure the
  amputation of their gills, while fully grown ones die. Amputation of
  the lateral gills hastens ecdysis. After the change of skin, the
  gills are smaller than before, and at first contain no tracheæ, but
  in a few weeks they develop as completely as in normal individuals.
  The caudal gills were also renewed.

[Illustration:

  FIG. 443.—_A_, larva of Sisyra, enlarged. _B_, one of the hinder
    gills, with its tracheæ.—After Westwood, from Sharp. _C_, a gill,
    showing the branched tracheæ.—After Grube.
]

In the nymphs of Perlidæ the tracheal gills are usually present, and are
either foliaceous (Nemoura) or more commonly filamentous in shape (Fig.
442). They are situated either on the prosternum (Nemoura and
Pteronarcys), or on each side of the thorax, or on the sides of the
abdomen, or are restricted to a tuft on each side of the anus at the
base of the caudal stylets (Pteronarcys and Perla). Unlike the
Ephemeridæ the gills persist in certain genera throughout life.

The larvæ of the aquatic Neuroptera, Sisyra, Sialis, and Corydalus
possess lateral pointed bristle-like tracheal gills, which in Sisyra are
2–jointed; those of Sialis are, in the living larva, curved upwards and
backwards (Fig. 444). Corydalus is also provided with a ventral tuft of
delicate filamentous gills, which, however, according to Riley, do not
appear until after the first moult.

While the nymphs of Agrionidæ (which have rectal gills) respire chiefly
by the large caudal foliaceous gills (Fig. 445), there are, according to
Hagen, two genera of the Calopteryginæ (Euphæa, Fig. 445, and
Anisopleura) whose nymphs possess seven pairs of external lateral
tracheal gills, in shape like those of Sialis, besides three caudal and
three rectal tracheal gills.[74]

[Illustration:

  FIG. 444.—Larva of _Sialis lutarius_.—After Miall.
]

[Illustration:

  FIG. 445.—Caudal tracheal gill of nymph of Agrion.
]

  Hagen has also detected in the under side of the 5th abdominal
  segment of Epitheca and Libellula a pair of sacs of the shape of a
  Phrygian bonnet, each of which contains a smaller sac lined with
  epithelium,—as in Æschna they occur in the 5th and 6th, and in
  Gomphus in the 4th, 5th, and 6th segments. This serial arrangement
  appears to confirm Hagen’s suggestion that they are survivals of
  abdominal gills, which in Euphæa are completely evaginated.

[Illustration:

  FIG. 446.—Nymph of Euphæa, showing the lateral gills: _a_, one
    enlarged.—Folsom _del._
]

In the Trichoptera, all of which, except Enoicyla, are apneustic, and
most of which have tracheal gills, the latter are filamentous, and arise
either from the dorsal and ventral sides of the abdominal segment, or
they grow out from the sides; while in certain genera (Neuronia,
Phryganea, etc.) the gills are represented by conical hooks on the sides
of the 1st abdominal segment, which are evidently respiratory, as they
contain numerous tracheæ. The tracheal gills are either single or more
rarely form tufts (Figs. 447, 448).

In Hydropsyche (Fig. 448) the tracheal gills persist throughout life,
while in other genera they only last through the pupal stage. When first
hatched, the larva of Phryganea lacks gills. The larvæ of most of the
Hydropsychidæ, Rhyacophilidæ, and Hydroptilidæ have no gills, though
they appear well developed in the pupal stage. (Klapálek.)

[Illustration:

  FIG. 447.—_A_, an abdominal segment of Hydropsyche, with the tracheal
    gills (_lbr_): _trl_, longitudinal tracheal trunk; _f_, stigmatal
    branch. _B_, 5th abdominal segment of pupa of the same; _l_, the
    three lateral flaps of the tergite; _br^1_, _br^2_, branchiæ.
]

[Illustration:

  FIG. 448.—Imago, abdominal segments IV to VI, with the gills at _a_
    concealed in their natural condition; at _b_, drawn out with the
    needle; at _c_, projecting abnormally and dried.—This and Fig. 447
    after Palmén.
]

[Illustration:

  FIG. 449.—Larva and pupa of _Paraponyx stratiolata_, enlarged; _s_,
    spiracle.—After De Geer (compare Hart’s figure of _P. obscularis_,
    living in the Illinois River).
]

The only lepidopterous larva known to be provided with tracheal gills is
that of the pyralid genus Paraponyx. Its thread-like gills, arranged in
tufts of three or four, arise from a common tubercle situated on the
sides of nearly all the segments. Wood-Mason describes the East Indian
_P. oryzalis_ as “covered with a perfect forest of soft and delicate
white filaments,” arranged in tufts disposed in four longitudinal rows.
“The stigmata of the 2d, 3d, and 4th abdominal somites only are clearly
discernible.” The caterpillar crawls “free and uncovered” over the
submerged leaves of the rice plant “in the very midst of the water.” In
a Brazilian species of Paraponyx described as _Cataclysta pyropalis_, by
W. Müller, the tufts are reduced to simple unbranched filaments, and the
case is more complex than in the European species (Fig. 449).

[Illustration:

  FIG. 450.—Anterior end of larva of _P. stratiolata_, showing the head
    and first two thoracic segments, with their gills: _A_, a tuft of
    gills, much enlarged.—After De Geer.
]

[Illustration:

  FIG. 451.—Larva (1) and pupa (2_a_) of _Paraponyx pyropalis_ enlarged:
    _st_, stigmata.—After W. Müller.
]

Of coleopterous larvæ breathing by tracheal gills there are but few. The
larva of Gyrinus (Fig. 454) respires by 10 pairs of slender, hairy
abdominal gills similar to those of Corydalus, and the stigmata are
entirely wanting. Somewhat similar are the tracheal gills of
_Hydrocharis caraboides_. Hydrobius has shorter setose gills, our
American species having seven pairs of short setose gills. It has two
spiracles at the end of the body, through which the air is taken by
thrusting the body out of the water. The larvæ of two other aquatic
coleopterous genera, Pelobius and Cnemidotus, also have gills; those of
the former situated at the base of the coxæ, and brush-like, but
containing no tracheæ, though filled with blood, while those of
Cnemidotus are very long, bristle-like, jointed, and arising from the
dorsal side of the thoracic and abdominal segments. The stigmata are
wanting. (Schiödte.)

The larva of the dipterous genus Tanypus respires by two caudal
papilliform processes, in each of which a trachea ramifies.

[Illustration:

  FIG. 452.—Freshly hatched larva of Hydrobius: _t_, enlarged tracheæ,
    the heart between them; _g^1_-_g^7_, the seven pairs of gills. _A_,
    end of body, enlarged, showing the two terminal stigmata.—Emerton
    _del._
]

Certain larvæ with both stigmata and tracheal gills are enabled either
to live in or out of water or on the surface, as in the case of certain
beetles (Cyphonidæ, Elmidæ, Hydrophilidæ, Fig. 452), or the larval
mosquito and Psychodes (Fig. 455); also the nymphs of dragon-flies.

The larvæ of the Cyphonidæ (Helodes, Cyphon, Hydrocyphon) possess but a
single pair of stigmata, situated in the penultimate abdominal segment,
while at the end of the abdomen are delicate tracheal gills. The two
main tracheal trunks are much swollen. When on the surface of the water
the larva breathes through the stigmata situated near the end of the
abdomen; when floating in the water, the larva, like that of Gyrinus,
carries along at the end of its body a bubble of air. The gills are only
of use, as Rolph thinks, when the insect is compelled to remain a long
time under water.

The larva of our native _Prionocyphon discoideus_ (Say) is described by
Walsh as “vibrating vigorously up and down a pencil of hairs proceeding
from a horizontal slit in the tail”; this pencil is composed “of three
pairs of filaments, each beautifully bipectinate. I presume it is used
to extract air from the water.” When the larva is at the surface the
pencil of hairs touches the surface of the water, and occasionally a
bubble of air is discharged from the tail. “The general habit is to
crawl on decayed wood beneath the surface, occasionally swimming to the
surface, probably for a fresh supply of air.” (Proc. Ent. Soc. Phil., i,
p. 117.)

[Illustration:

  FIG. 453.—Larva of Psephenus, enlarged.
]

[Illustration:

  FIG. 454.—Larva of Gyrinus.—After Westwood.
]

The larvæ of the small water beetles of the family Elmidæ (Elmis,
Potamophilus, Macronychus, and Psephenus) have similar habits. That of
Elmis has ten dorsally situated pairs of spiracles, and on the end of
the body bushy gills which are protruded at pleasure. The young larva is
without spiracles, its tracheal system being closed. Macronychus and
Potamophilus have similar habits. In the larva of the latter genus,
which has nine pairs of spiracles, there are at the end of the body on
each side three tufts of thread-like gills which are connected with the
two main horizontal tracheæ, while the branches of the abdominal tracheæ
are dilated into numerous (64) bladder-like sacs. The larva usually
breathes through the caudal gills. When the water is low or dried up,
the air is inhaled directly through the spiracles. (Kolbe.)

The larva of _Psephenus lecontei_, by its broad hemispherical body, is
adapted to adhere to the smooth surface of rounded stones, in which
situation we have found it. Although it is said by Rolph to have two
pairs of spiracles, one pair on the mesothoracic and the other on the
1st abdominal segment, it probably rarely rises to the surface to
breathe the air direct.

[Illustration:

  FIG. 455.—End of body of a Psychodes larva: _A_, end of body of a
    young, freshly moulted larva, side view: _a_, the three anal gills;
    _b_, the left air-cavity. _B_, older larva of the same species, with
    the open air-cavity seen from above. _C_, end of larva of another
    species as it goes down into the water with a bubble of air, _b_,
    between the crown of hairs of the air-cavity or tube: _a_, the two
    pairs of anal gills; _b_, the two main tracheæ.—After F. Müller.
]

It possesses five pairs of gills on the under side of the 2d to the 6th
abdominal segments. Each gill has finger-shaped processes on its hinder
edge, which are “from their constant motion evidently connected with
respiration.” Tracheæ may be seen, according to H. J. Clark, entering
the gills, and “the circulation of water among the branchiæ is kept up
by the flapping of the tail-pieces.” The larva of _Helichus fastigiatus_
is said by Leconte to be “very nearly allied, while the remotely allied
_Stenelmis crenatus_ has no external branchiæ.”[75]

  The larva of the mosquito also has two modes of respiration,
  breathing either at the surface of the water through the two
  spiracles situated on the projection (siphon) at the hinder end of
  the body which is thrust out into the air; or when at the bottom
  respiring by tracheal gills. The pupa also has a double mode of
  respiration, either taking in air at the surface by the two thoracic
  horns with stigmatic openings, or when submerged using its tracheal
  gills.

  Besides its long caudal tracheal air-tubes, the larval Eristalis is
  said by Chun to thrust out from the anus a number (20) of short
  tracheal filaments which float about in the water and serve to
  absorb the air.

An aquatic Brazilian larva of the family Psychodidæ has been found by
Fritz Müller to take down under the water a large bubble of air (Fig.
455, _C_), the main tracheal trunk ending each in an opening at the end
of the body (_A_, _B_); besides this, while at the bottom it breathes by
three digitiform tracheal gills; another species having two pairs (_C_,
_a_).

[Illustration:

  FIG. 456.—Under side of body of larva of Blepharocera. showing the
    position of the tracheal gills: _A_, section of the body through a
    sucker, showing position of the gills. _B_, section of a sucker:
    _br_, gill with numerous tracheæ; _gl_, outlet of excretory gland;
    _M_, _m_, muscles.—After F. Müller.
]

The remarkable larvæ of the Blepharoceridæ (represented in the United
States by _Blepharocera capitata_), which live permanently in swift
streams, attached by median suckers to stones, are apneustic, and
breathe solely by leaf-like tracheal gills (Fig. 456, _br_) attached to
the under side of the second to sixth abdominal segments. Those of the
European Liponeura are said by Wierzejski to be branched, tree-like.
Also immediately in front of the anus and behind the last sucker are
four membranous sacs provided with tracheæ, but which are not capable of
being withdrawn. These are said by Müller to be the same as what Dewitz
states to serve as gills, and by Wierzejski they are homologized with
the four anal gills of Chironomus.

  The double mode of respiration in the larva of the horse bot-fly has
  been described by Scheiber. On the hinder end of the body are the
  stigmatic plates, which contain two lateral gill-plates and the
  middle stigmatal leaf. Besides this there is a pair of slightly
  developed prothoracic spiracles. The embryo and also freshly hatched
  larva of _Gastrophilus equi_ do not possess these gill-plates, but
  on the end of the body are, according to Joli, two long thread-like
  gills. The freshly hatched larva of the allied _Cephenomyia
  rufibarbis_ bears two caudal projections. (Kolbe.) As in shrimps and
  other Crustacea the gills are kept in constant motion, the water
  being driven over them by the rapid movements of the telson, so in
  the larval may-flies, and in the case-worm (Macronema), the gills
  move more or less rapidly. In case-worms as well as larval Perlidæ,
  Sialidæ, Paraponyx, and Hydrophilidæ the abdominal region is
  constantly moved to promote respiration. (Kolbe.)

=Blood-gills.=—Fritz Müller describes in trichopterous larvæ certain
delicate anal tubular processes into which the blood flows, and which do
not as a rule contain tracheæ, though occasionally very fine tracheal
branches. Müller compares them with the gills of crabs and of shrimps.
They are eversible finger-like tubules. They are used when the tracheal
gills are temporarily not available. Their number varies even in the
same genus. There are six in certain Rhyacophilidæ; five in different
Hydropsychidæ; in Macronema there are four, and they are green when
filled with the green blood of that insect, the tracheal gills being
whitish. In the freshly hatched larva, while the tracheal gills are
present, no anal blood-gills are visible. Similar blood-gills also occur
in the pupæ of certain caddis-flies. (Pictet.)

Similar anal gills filled with blood occur in the larvæ of the fireflies
(Lampyris, etc.), and perhaps, Kolbe thinks, serve for respiration,
though other authors believe them to be adhesive organs.

The larva of Pelobius has true blood-gills. (Schiödte. See p. 461.)

  The eversible ventral segmental sacs of Scolopendrella, Campodea,
  and Machilis, as well as the ventral tube (collophore) of Podura,
  Smynthurus, etc., may, as Oudemans and Haase have suggested, serve a
  respiratory purpose, though they lack tracheæ, and differ from
  blood-gills in containing no gases; yet the blood is forced into
  them, causing their eversion. Oudemans observed that Machilis
  everted its sacs when the vessel in which it was put was filled with
  warm, damp air. The sacs are only thrust out when the creature is
  completely at rest.

Structures referable to blood-gills also occur temporarily in the embryo
of Orthoptera; Rathke observed them in the mole-cricket; Ayres observed
them in _Œcanthus niveus_, where they form two stalked broad oval
appendages on the first abdominal appendages, which he regarded as
gills. Patten observed them in _Phyllodromia germanica_, as pear-shaped
structures occurring in the same situation, but regarded them as
sense-organs, as did Cholodkowsky. Graber found these structures in the
embryo of the May-beetle, which looked like the other embryonic limbs,
but survived after the disappearance of the latter, being longer and
broader and unjointed. These disappeared shortly before birth. In
Hydrophilus they remain, Graber states, after birth. Nussbaum has seen
them in Meloë.

Finally, Wheeler has discussed at length these embryonic organs, which
he regards as glandular structures, and calls _pleuropodia_, their
primitive function having been that of limbs. He has detected them in
the embryo of _Periplaneta orientalis_, _Mantis carolina_, _Xiphidium
ensiferum_ (Fig. 387); also in the Hemiptera (_Cicada septemdecim_,
_Zaitha fluminea_), and in _Sialis infumata_. He discards the view that
they were once gills or sense-organs, and concludes that they were
glands. But, as we have suggested, their function once that of gills,
and still respiratory in Synaptera, has perhaps become in the winged
insects glandular and repugnatorial. Instead, then, of being modified
abdominal limbs afterwards serving as glands, as Wheeler claims, we are
inclined to believe that they functioned as blood-gills.


                  _k._ Tracheal gills of adult insects

Tracheal gills are known to be retained by a few insects in the imago
stage, the nymphs in all stages breathing by them. The most notable
example is the perlid genus Pteronarcys, in which, as Newport states,
there are eight sets, comprising 13 pairs of branchial tufts distributed
over the under surface of the thoracic and first two abdominal segments.

  The first set, consisting of three pairs of tufts, partly encircling
  the neck like a ruff, arises from the soft membrane connecting the
  head and prosternum. The thoracic tufts originate between and behind
  the coxæ, as well as on the front margin of the meso- and
  metathoracic segments. The number of filaments in each tuft varies
  from about 20 to 50 or more, the densest tufts being those of the
  two hinder thoracic segments. Each filament is usually simple,
  though in a few cases they are branched (Fig. 457, _A_).

  The adult Pteronarcys is nocturnal, flying only at dewfall or in the
  night, and Mr. Barnston observed it when on the wing, “constantly
  dipping on the surface of the water”; by day it hides “in crevices
  of rocks which are constantly wetted by the spray of falling water,
  under stones and in other damp places.” It may thus be compared with
  the Amphibians, Necturus and Proteus, whose gills are retained in
  adult life. A similar large Chilian Perlid (_Diamphipnoa lichenalis_
  Gerst.) differs in completely lacking the thoracic gills, though
  there are four pairs on the abdomen, _i.e._ a pair on each of the
  first four segments. In this form the number of individual filaments
  in the largest tufts may amount to about 200.

  Another Perlid (_Dictyopteryx signata_) is said by Hagen to have two
  pairs of gill-tufts on the under side of the head; the first pair
  situated on the base of the submentum, the second on the membrane
  connecting the head and prosternum.

  Kolbe states that in the imagines of _Perla marginata_ and _P.
  cephalotes_ on the hinder edge of the thoracic stigmata arise three
  very small chitinous plates, which, on their under side and on the
  edges are beset with numerous short white filaments. These
  completely correspond to the filaments of the tuft-like larval
  gills. Persistent anal gills also occur in the imagines of Perla.

[Illustration:

  FIG. 457.—Under side of _Pteronarcys regalis_, showing the situation
    of the gills _(g_, _b_, _f_) and the sternal orifices: _A_, a
    branchial filament showing the direction of the current of blood;
    _c_, _d_, tracheæ. _B_, end of the abdomen enlarged.—After
    Newport.
]

  In _Nemoura lateralis_ and _cinerea_ the tracheal gills are
  differently disposed. On each side of the anterior edge of the
  prosternum arise delicate tightly twisted filaments, like those of
  the larva. (Einführung, p. 536.)

  Hagen also states that in the dragon-fly, Euphæa, the gills of the
  nymphs are retained in the imago, and Palmén remarks that in Æschna
  the rectal gills of the nymph persist in the imago, though not used
  for respiration.

  Palmén gives an instance of a caddis-fly (Hydropsyche, Fig. 448)
  retaining its gills through the imago stage, but they are unfit for
  respiration, as they are minute and shrunken.

  A walking-stick (_Prisopus flabelliformis_) found in the mountains
  of Brazil has the remarkable habit, according to Murray, of spending
  “the whole of the day under water, in a stream or rivulet, fixed
  firmly to a stone in the rapid part of the stream,” with its head
  turned up stream; but leaving the water at dark. The under side of
  the body, including the head, is hollowed so that the creature may
  adhere, sucker-like, to smooth stones; the claws, claspers, and
  flaps on the legs aid in retaining its hold, while the outer margin
  of the legs is dentate and thickly fringed with hair to repel the
  water.

  Another form, closely related to Prisopus, from Borneo (_Cotylosoma
  dipneusticum_) is said by Wood-Mason to be even more profoundly
  modified for an aquatic life, since it has not only spiracles, but
  also, as he claims, tracheal gills. From each side of the body, in
  fact along the lower margins of the sides of the metathorax, there
  stand straight out five equal, small, but conspicuous ciliated oval
  plates, “which, when the insect is submerged and its stigmata are
  closed, doubtless serve for respiration.” The author did not note
  the actual presence of tracheæ in these plates.




         LITERATURE ON THE ORGANS AND PHYSIOLOGY OF RESPIRATION


                 _a._ On the tracheal system in general

  =Lyonet, P.= Traité anatomique de la chenille qui ronge le bois du
    saule. (La Haye, 1760; 2d edit., La Haye, 1762, pp. 616, 18 Pls.)

  =Treviranus, G. R.= Beiträge zur Anatomie und Physiologie der Tiere
    und Pflanzen, 1816.

  —— Das organische Leben. Bremen, 1831.

  =Rengger, J. R.= Physiologische Untersuchungen über die tierische
    Haushaltung der Insekten. Tübingen, 1817. (Germar’s Mag. f. Ent.,
    1818, iii, pp. 410–413.)

  =Dufour, L.= Recherches anatomiques sur les Carabiques et sur
    plusieurs autres insectes Coléoptères. Organes de la respiration.
    (Ann. Sc. nat., viii, 1826, pp. 19–27, 2 Pls.)

  —— Études anatomiques et physiologiques sur les insectes Diptères de
    la famille des Pupipares. Appareil respiratoire. (Ann. Sc. nat.
    Zool., Sér. 3, iii, 1845, pp. 56–64, 1 Pl.)

  —— Description et anatomie d’une larve à branchies externes
    d’Hydropsche. (Ann. Sc. nat, Zool., Sér 3, 1847, viii, pp. 341–354.)

  —— Études anatomiques et physiologiques, et observations sur les
    larves des Libellules. Appareil respiratoire. (Ann. Sc. nat. Zool.,
    Sér. 3, xvii, 1852, pp. 76–97, 3 Pls.)

  —— Recherches anatomiques sur les Hyménoptères de la famille des
    Urocerates. Appareil respiratoire. (Ann. Sc. nat. Zool., Sér. 4, i,
    1854, pp. 203–209, 1 Pl.; see also p. 344.)

  =Burmeister, H.= Handbuch der Entomologie, i, 1832, pp. 169–194,
    416–436.

  =Kirby, W., and W. Spence.= Introduction to entomology, 1833, iv, pp.
    35–81.

  =Bowerbank, J. S.= Observations on the circulation of blood and the
    distribution of the tracheæ in the wing of _Chrysopa perla_. (Ent.
    Mag., iv, 1837, pp. 179–185.)

  =Platner, E. A.= Mitteilungen über die Respirationsorgane in der Haut
    bei der Seidenraupe. (Müller’s Archiv f. Physiol., 1844, pp. 38–49.)

  =Filippi, F. de.= Alcuni osservazioni anatomico-fisiologische sugl’
    Insetti in generale, ed in particulare sul Bombice del Gelso. (Ann.
    R. Acad. d’ Agricoltura di Torino, 1850, ii, p. 25, 1 Pl.; Transl.
    by C. A. Dohrn, Stettin, Ent. Zeit., 1852, xiii, pp. 258–267; xiv,
    pp. 124–132, 1 Pl.)

  =Newport, G.= On the formation and the use of the air-sacs and dilated
    tracheæ in insects. (Trans. Linn. Soc. London, 1851, xx, pp.
    419–423.)

  =Lubbock, J.= Distribution of tracheæ in insects. (Trans. Linn. Soc.
    London, 1860, xxiii, pp. 23–50.)

  =Landois, L.= Anatomie des _Phthirius inguinalis_ Leach. (Zeitschr. f.
    wissens. Zool., xiv, 1864, pp. 1–26, 5 Taf.)

  —— Anatomie des _Pediculus vestimenti_ Nitzsch. (Ibid., xv, 1865, pp.
    32–55, 3 Taf.)

  —— Anatomie des Hundeflohs (_Pulex canis_). (Nova Acta Leop.-Carol.
    Akad. der Naturf., Dresden, 1866, xxxiii, 1867, pp. 67, 7 Taf.)

  —— Anatomie der Bettwanze (_Cimex lectularius_ L.). (Ibid., xviii,
    1868, pp. 206–224, xix, 1869, pp. 206–233, 4 Taf.)

  =Meinert, Fr.= Campodeæ: en familie af Thysanurernes orden.
    (Naturhistorisk Tidsskr., 3 Raek., iii, 1864–65, pp. 400–440, 1 Pl.)

  =Reinhardt, H.= Zur Entwicklungsgeschichte des Tracheensystems der
    Hymenopteren mit besonderer Bezeichung auf dessen morphologische
    Bedeutung. (Berlin. Ent. Zeitschr., 1865, ix, pp. 187–218, 2 Taf.)

  =Gerstaecker, A.= Bronn’s Klassen und Ordnungen des Tierreichs, v,
    1866–1879. Organs of respiration, pp. 119–131.

  =Pouchet, G.= Développement du système trachéen de l’Anophèle
    (_Corethra plumicornis_). (Archiv zool. expérimentale, i, 1872, pp.
    217–232, 1 Fig.)

  =Graber, V.= Ueber eine Art fibrilloiden Bindegewebes der Insektenhaut
    und seine lokale Bedeutung als Trachealsuspensorium. (Archiv f.
    Mikroskop. Anat. x, 1874, pp. 124–144, 1 Taf.)

  —— Die Insekten; München, 1877. Organs of respiration, pp. 346–369.

  =Packard, A. S.= On the distribution and primitive number of spiracles
    in insects. (Amer. Naturalist, viii, 1874, pp. 531–534.)

  —— On the nature and origin of the so-called “spiral thread” of
    tracheæ. (Amer. Naturalist, xx, 1886, pp. 438–442, 2 Figs., p. 558.)

  =Wolff, O. J. B.= Das Riechorgan der Biene nebst einer Beschreibung
    des Respirationswerkes der Hymenopteren, des Saugrüssels und
    Geschmacksorganes der Blumenwespen. (Nova Acta d. kais. Leop-Carol.
    Akad. der Naturf., xxxviii, 1876, pp. 1–251, 8 Taf.)

  =Palmén, J. A.= Zur Morphologie des Tracheensystems. Leipzig, 1877,
    pp. 140, 2 Taf.

  =Moseley, H. N.= Origin of tracheæ in Arthropoda. (Nature, xvii, 1878,
    p. 340.)

  =Poletajew, Olga.= Quelques mots sur les organes respiratoires des
    larves des Odonates. (Horæ Soc. Ent. Ross., xv, 1880, pp. 436–452, 2
    Pls.)

  =Viallanes, H.= Sur l’appareil respiratoire de quelques larves de
    Diptères. (Compt. rend. Acad. Sc., Paris, 1880, pp. 1180–1182.)

  =MacLeod, J.= La structure des trachées et la circulation
    peritrachéenne. Bruxelles, 1880, pp. 70, 4 Pls.

  =Hagen, H. A.= Beitrag zur Kenntnis des Tracheensystems der
    Libellenlarven. (Zool. Anzeiger, 1880, pp. 157–162.)

  —— Einwürfe gegen Palmens Ansicht von der Entstehung des geschlossenen
    Tracheensystems. (Ibid., 1881, pp. 404–406.)

  =Macloskie, G.= The structure of the tracheæ of insects. (Amer.
    Naturalist, 1884, xviii, pp. 567–573, Fig.)

  =Haase, E.= Das Respirationssystem der Chilopoden und Symphylen
    (Scolopendrellen) vergleichen mit dem der Hexapoden. (Zeitschr. f.
    Ent. N. F., ix, Breslau, 1884.)

  —— Das Respirationssystem der Symphylen und Chilopoden. (Zool.
    Beiträge, von A. Schneider, i, 1884, pp. 65–95, 3 Taf.; Zool.
    Anzeiger, 1883, pp. 15–17.)

  =Grassi, B.= I progenitori degli Insetti e dei Miriapodi. L’Japyx e la
    Campodea. (Atti d. Accad. Gioenia d. Sc. Nat. Catania, 1885, Sér. 3,
    xix, pp. 83, 5 Pls.)

  —— I progenitori dei Miriapodi e degli Insetti. Anatomia comparata dei
    Tisanuri. (Reale Accad. d. Lincei di Roma, Anno 284, 1887.)

  =Meinert, Fr.= De eucephale Myggelarver. Sur les larves eucephales des
    Diptères. (Vidensk. Selsk. Skrifter., 6 Raekke, naturvid. og mathem.
    Afd. Kjöbenhavn, 1886, iv, pp. 369–493, 4 Pls.)

  =Cajal, S. R.= Coloration par la méthode de Golgi des terminaisons des
    trachées et des nerfs dans les muscles des ailes des insectes.
    (Zeitschr. f. wiss. Microscopie, 1890, vii, pp. 332–342, 1 Pl.)

  =Wistinghausen, C. v.= Ueber Tracheenendigungen in den Sericterien der
    Raupen. (Zeitschr. wissensch. Zool., xlix, 1890, pp. 565–582, 1
    Taf.)

  =Stokes, Alfred C.= The structure of insect tracheæ, etc. (Science,
    1893, pp. 44–46, 7 Figs.)

  =Sadones, J.= L’appareil digestif et respiratoire larvaire des
    Odonates. (La Cellule, xi, 1895, pp. 271–325, 3 Pls.)

  =Holmgren, Emil.= Über das respiratorische Epithel der Tracheen bei
    Raupen. (Festschrift Lilljeborg. Upsala, 1896, pp. 76–79, 2 Taf.)
    See also p. 437.

  Also Gegenbaur’s Comparative Anatomy, Engl. Trans.


                          _b._ On the Stigmata

  =Loewe, C. L. W.= De partibus quibus insecta spiritus ducunt. Diss.
    inaug. Halæ, 1814, pp. 28.

  =Sprengel, C.= Commentarius de partibus quibus Insecta spiritus
    ducunt. Lipsiæ, 1815, pp. 38, 3 Taf.

  =Dufour, L.= Recherches anatomiques sur l’Hippobosque des chevaux.
    (Ann. Sc. nat., 1825, vi, pp. 299–322, 1 Pl.)

  —— Nouvelles observations sur la situation des stigmates thoraciques
    dans les larves des Bupresticides. (Ann. Soc. Ent. France, Sér. 2,
    1844, ii, p. 203.)

  =Landois, H.= Der Tracheenverschluss bei _Tenebrio molitor_. (Reichert
    u. Dubois-Reymond’s Archiv f. Anat., 1866, pp. 391–397, 1 Taf.)

  —— =und W. Thelen.= Der Tracheenverschluss bei den Insekten.
    (Zeitschr. wissensch. Zool., xvii, 1867, pp. 187–214, 1 Taf.)

  —— Der Stigmenverschluss bei den Lepidopteren. (Reichert u.
    Dubois-Reymond’s Archiv f. Anat., 1886, pp. 41–49, 1 Taf.)

  =Hagen, H. A.= Beitrag zur Kenntnis des Tracheensystems der
    Libellen-Larven. (Zool. Anzeiger, 1880, pp. 157–162.)

  —— Einwürfe gegen Palmén’s Ansicht von der Entstehung des
    geschlossenen Tracheensystems. (Ibid., 1881, pp. 404–406.)

  =Krancher, O.= Der Bau der Stigmen bei den Insekten. (Zeitschr.
    wissensch. Zool., xxxv, 1881, pp. 505–574, 2 Taf.; Zool. Anz., 1880,
    pp. 584–588.)

  =Meinert, Fr.= Spirakelpladen hos Scarabæ-Larverne. (Vid. Meddel. Nat.
    For. Kjöbenhavn (4), Aarg. iii, 1882, pp. 289–292.)

  —— Noget mere om Spiracula cribraria og Os clausum. En Replik. (Ibid.
    (4), Aarg. v, 1884, pp. 68–91, Fig.)

  =Schiödte, J. G.= Spiracula cribraria—os clausum: lidt om
    naturvidenskabelig Methode og Kritik. (Nat. Tidsskrift (3), xiii,
    1883, pp. 427–473; also Jahresber. Neapel, 1883, p. 105.)

  =Verson, E.= Il meccanismo di chiusura negli stimmati di Bombix mori.
    (Atti Istit. Veneto. Sc., 1887, p. 9, Pl.)

  —— Der Bau der Stigmen von Bombyx mori. (Zool. Anzeiger, 1887, x
    Jahrg., pp. 561, 562.) See also Zool. Anzeiger, 1890, p. 116.

  =Haase, E.= Die Stigmen der Scolopendriden. (Zool. Anzeiger, 1887, x
    Jahrg., pp. 140–142.)

  —— Holopneustie bei Käfern. (Biolog. Centralbl., 1887, vii, pp.
    50–53.)

  =Carlet, G.= Note sur un nouveau mode de fermeture des trachées,
    “fermeture operculaire” chez les insectes. (Comp. rend. Acad. Sci.
    Paris, 1888, cvii, pp. 755–757.)

  =De Meijere, J. C. H.= Über zusammengesetzte Stigmen bei
    Dipterenlarven [etc.]. (Tijd. Ent., xxxviii, 1895, pp. 65–100, 33
    Figs.)

  Also the other writings of Palmén, Dufour, Dewitz, Boas, Verson.


            _c._ On tracheal gills and tracheal respiration

  =Pictet, F. J.= Mémoires sur les larves des Némoures. (Annal. Sc.
    nat., 1832, xxvi, pp. 369–391, 2 Pls.)

  —— Recherches pour servir à l’histoire et à l’anatomie des
    Phryganides. Genève, pp. 235, 20 Pls.

  —— Histoire naturelle générale et particulière, des insectes
    Neuroptères. I, Monographie: Famille des Perlides, Genève, 1841,
    1842, pp. 423, 53 Pls.

  —— Histoire naturelle générale et particulière des insectes
    Neuroptères. II, Monographie: Famille des Ephémérines. Genève,
    1843–1845, pp. 300, 47 Pls.

  =Dufour, L.= Recherches anatomiques et considerations entomologiques
    sur les insectes Coléoptères des genres Macronychus et Elmis. (Ann.
    Sc. nat. Zool., Sér. 2, 1835, iii, pp. 151–174, 1 Pl.)

  —— Description et anatomie d’une larve à branchies externes
    d’Hydropsyche. (Ibid., Sér. 3, 1847, viii, pp. 341–354, Fig.)

  —— Recherches anatomiques sur la larve à branchies extérieures du
    _Sialis lutarius_. (Ibid., Sér. 3, 1848, ix, pp. 91–99, Fig.)

  —— De diverses modes de respiration aquatique chez les insectes.
    (Compt. rend. Acad. d. Sc. Paris, 1849, xxix, pp. 763–770; Ann. and
    Mag. Nat. Hist., Sér. 2, 1850, vi, pp. 112–118.)

  —— Études sur la larve du Potamophilus. (Ann. Sc. nat., Sér. 4, xvii,
    1862, pp. 162–173, 1 Pl., Bericht v. Gerstaecker f. 1862, pp. 16,
    17.)

  =Grube, A. E.= Beschreibung einer auffallenden an Süsswasser-schwammen
    lebenden Larve (Sisyra). (Wiegmanns Archiv f. Naturgesch., 1843, ix,
    pp. 331–337, Fig.)

  =Schröder van der Kolk, J. L. G.= Mémoire sur l’anatomie et
    physiologie de _Gastrus equi_. (Nieuwe Verhandl. d. K. Nederl.
    Instit. Amsterdam, 1845, ix, pp. 1–155, 13 Pls.; Erichson’s Bericht.
    f. 1845, p. 109.)

  =Scheiber, S. H.= Vergleichende Anatomie und Physiologie der
    Œstriden-Larven. Respirationssystem. (Sitzungsber. Akad. Wissensch.
    Wien. Math.-naturw. Cl., 1862, xlv, pp. 7–39.)

  =Lubbock, J.= On the development of _Chloëon dimidiatum_. (Trans.
    Linn. Soc. London, I, 1868, xxiv, pp. 61–78, 2 Pls.; II, 1866, xxv,
    pp. 477–492.)

  =Oustalet, E.= Note sur la respiration chez les nymphes des
    Libellules. (Ann. Sc. nat. Zool., Sér. 5, xi, 1869, pp. 370–386, 3
    Pls.)

  =Rolph, W. H.= Beitrag zur Kenntnis eininger Insektenlarven. 1 Taf.
    Inaug. Dissertat. Bonn, 1873.

  =Chun, C.= Ueber den Bau, die Entwicklung und physiologische Bedeutung
    der Rektaldrüsen bei den Insekten. Frankfurt a. M., 1875.

  =Haller, G.= Die Stechmückenlarve. Kleinere Bruchstücke zur
    vergleichenden Anatomie der Arthropoden. I. Ueber das Atmungsorgan
    der Stechmückenlarven. (Archiv f. Naturgesch., xliv, 1878, pp.
    91–96, 1 Taf.)

  =Vayssière, A.= Recherches sur l’organisation des larves des
    Ephémérines. (Ann. d. Sc. nat. Zool., Sér. 6, xiii, 1882, pp. 1–137,
    11 Pls.)

  —— Monographie zoologique et anatomique du genre Prosopistoma Latr.
    (Ibid., Sér. 7, ix, 1890, pp. 19–87, 4 Pls.)

  =Eaton, A. E.= Notes on some species of Cloëon. (Ann. Mag. Nat. Hist,
    Ser. 3, xviii, 1866, pp. 145–148.)

  —— A revisional monograph of recent Ephemeridæ or may-flies. (Trans.
    Linn. Soc. London, 1883–1887, Ser. 2, iii, 63 Pls.)

  =Müller, Wilh.= Ueber einige im Wasser lebende Schmetterlingsraupen
    Brasiliens. (Archiv f. Naturgesch., 1884, i Jahrg., pp. 194–212, 1
    Taf.)

  =Vogler.= Die Tracheenkiemen der Simulien-Puppen. (Mitteil. Schweiz
    Ent. Gesellsch., 1887, vii, pp. 277–282.)

  =Raschke, E. W.= Die Larve von _Culex nemorosus_. Ein Beitrag zur
    Kenntnis der Insekten-Anatomie und Histiologie. (Archiv für
    Naturgesch., 1887, liii Jahrg., pp. 133–163, 2 Taf.; Zool. Anz.,
    1887, x Jahrg., pp. 18, 19.)

  =Klapálek, Fr.= Untersuchungen über die Fauna der Gewässer Böhmens. I.
    Metamorphose der Trichopteren. (Archiv f. naturwissensch.
    Landesdurchforschung von Böhmen, Prag, 1888, vi, No. 5, pp. 63; No.
    6, 1893, pp. 145, Figs.)

  =Müller, Fritz.= Larven von Mücken und Haarflüglern mit zweierlei
    abwechselnd thätigen Atemwerkzeugen. (Ent. Nachr., 1888, xiv Jahrg.,
    pp. 273–277; also Zool. Anzeiger, iv, 1881, pp. 499–502.)

  =Kolbe, H. J.= Ueber den Kranzförmigen Laich einer Phryganea.
    (Sitzungsber. d. Gesellsch. naturforsch. Freunde in Berlin, 1888,
    pp. 22–26.)

  =Haase, Erich.= Die Abdominalanhänge der Insekten mit Berücksichtigung
    der Myriopoden. (Morpholog. Jahrbuch, 1889, xv, pp. 331–435, 2 Taf.)

  =Dewitz, H.= Einiger Beobachtungen, betreffend das geschlossene
    Tracheensystem bei Insectenlarven. (Zool. Anzeiger, xiii, 1890, pp.
    500–504, 525–531.)

  =Miall, L. C.= Some difficulties in the life of aquatic insects.
    (Nature, xliv, London, 1891, pp. 456–462.)

  —— Natural History of aquatic insects, 1895, 116 Figs., pp. 1–395.

  =Weltner, W.= (Note on Sisyra.) (Ent. Nachr., p. 145, 1894.)

  Also papers by Hagen, Dewitz, Williams, Tömösváry (1884).


                 _d._ Literature on rectal respiration

  =Suckow, F. W. L.= Respiration der Insekten, insbesondere über die
    Darmrespiration der _Æschna grandis_. (Zeitschrift f. d. organ.
    Physik, von Heusinger, 1828, ii, pp. 24–29, 4 Taf.)

  =Dufour, L.= Sur la respiration branchiale des larves des grandes
    Libellules comparée à celle des poissons. (Compt. rend. de l’Acad.
    Sc. Paris, 1848, xxvi, pp. 301–303.)

  —— Études anatomiques et physiologiques et observations sur les larves
    des Libellules. (Ann. Sc. nat. Zool., Sér. 3, 1852, xvii, pp. 76–97,
    3 Pls.)

  =Gilson, G. and J. Sadones.= Larval gills of Odonates. (Journ. Linn.
    Soc. London, 1897.)


                     _e._ Physiology of Respiration

  =Bonnet, Ch.= Recherches sur la respiration des chenilles. (Mém. Math.
    des Savants Étrangers, Paris, 1768, v, pp. 276–303.)

  =Treviranus, G. R.= Biologie, oder Philosophie der lebenden Natur, für
    Naturforscher und Aerzte. 6 Bände, Göttingen, 1802–1822. (Atmung, in
    Bd. iv.)

  —— Die Erscheinungen und Gesetze des organischen Lebens. 2 Bände,
    Bremen, 1831–1833. (Atmung der Insekten in Bd. i.)

  —— Versuche über das Atemholen der niederen Tiere. (Zeitschrift f. d.
    Physiologie, von F. Tiedemann, G. R. u. L. C. Treviranus, 1832, iv,
    pp. 1–39.)

  =Hausmann, J. F. L.= De animalium exsanguinum respiratione
    commentatio. Hannover, 1803, vi, p. 70.

  =Spallanzani, L.= Memoirs on respiration. London, 1804.

  =Sorg, F. L. A. W.= Disquisitiones physiologicæ circa respirationem
    insectorum et vermium. Rudolstadt, 1805, Part II, p. 146.

  =Nitzsch, C. L.= Commentatio de respiratione animalium. Vitebergæ,
    1808, 4º, pp. 56.

  —— Ueber das Atmen der Hydrophilen. (Reil’s Archiv f. Physiologie,
    1811, x, pp. 440–458.)

  =Reimarus, J. A. H.= Ueber das Atmen, besonders über das Atmen der
    Vögel und Insekten. (Reil u. Autenrieth, Archiv f. Physiologie,
    1812, xi, pp. 229–236.)

  =Dufour, L.= Anatomie de la Ranatre linéaire et de la Nèpe cendrée.
    (Annal. génér. Scienc. phys., Bruxelles, 1821, vii, pp. 194–213, 1
    Pl.)

  —— Mémoire pour servir à l’histoire du genre Ocyptera. (Annal. Scienc.
    natur., 1827, x, pp. 248–261, 1 Pl.)

  —— Recherches sur quelques entozoaires et larves parasites des
    insectes Orthoptères et Hyménoptères. (Ann. Sc. nat. Zool., Sér. 2,
    1836, vi, p. 55; Sér. 2, 1837, vii, pp. 5–20.)

  —— Note sur le parasitisme. (Compt. rend. Acad. Sc. Paris, 1851,
    xxxiii, pp. 135–139; Rev. et Mag. de Zool., 1851, pp. 408–412.)

  =Dutrochet, R. J. H.= Du mecanisme de la respiration des Insectes.
    (Ann. Sc. nat., 1833, xxviii, pp. 31–44; Mém. Acad. Sc. Paris, 1838,
    xiv, pp. 81–93.)

  =Newport, G.= On the respiration of insects. (Phil. Trans. Roy. Soc.,
    London, 1836, cxxvi, pp. 529–566.)

  =Coquerel, Ch.= Note pour servir à l’histoire de _l’Æpus robini_.
    (Ann. Soc. Ent. France, Sér. 2, 1850, viii, pp. 529–532.)

  =Davy, J.= On the effects of certain agents on insects. (Trans. Ent.
    Soc. London, 1851, pp. 195–212.)

  =Barlow, W. F.= Observations of the respiratory movements of insects.
    (Phil. Trans. Roy. Soc. London, cxlv, 1855, pp. 139–148.)

  =Rathke, H.= Anatomisch-physiologische Untersuchungen über den
    Atmungsprozess der Insekten. (Schriften d. k. phys.-ökon. Ges.
    Königsberg, i Jahrg., 1860, pp. 99–138, 1 Taf.)

  =Lubbock, J.= On two aquatic Hymenoptera, one of which uses its wings
    in swimming. (Trans. Linn. Soc. London, xxiv, 1863, pp. 135–142, 1
    Pl.)

  =Boyle, R.= New pneumatical experiments about respiration. (Phil.
    Trans., 1870, v, No. 63, pp. 2051–2056.)

  =Lambrecht. A.= Das Atmungsgeschäft der Bienen. (Bienenwirtschaftl.
    Centralblatt, vii Jahrg., 1871, pp. 20–25.)

  —— Luftverbrauch eines Biens und die damit zusammenhängenden
    Lebensprozesse der Glieder desselben. (Ibid., 1871, pp. 115–120.)

  =Monnier.= Sur la rôle des organes respiratoires chez les larves
    aquatiques. (Compt. rend. Acad. Sc. Paris, lxxiv, 1872, p. 235.)

  =Liebe, Otto.= Ueber die Respiration der Tracheaten, besonders über
    den Mechanismus derselben und über die Menge der ausgeatmenten
    Kohlensäure. (Inaug. Diss. Chemnitz, 1872, pp. 28.)

  =Plateau, F.= Recherches physico-chimiques sur les articulés
    aquatiques. (Bull. Acad. Roy. Belg., Sér. 2, xxxiv, 1872, pp.
    271–321.)

  —— Recherches expérimentales sur les mouvements respiratoires des
    Insectes. (Mem. Acad. Belg., 1884, xlv, pp. 219, 7 Pls., 56 Figs.)

  —— Recherches physico-chimiques sur les articulés aquatiques. Part I.
    Action de sels en dissolution dans l’eau. Influence de l’eau de mer
    sur les articulés aquatiques d’eau douce. Influence de l’eau douce
    sur les Crustacés marines. (Mém. cour. et Mém. des savants étrang.
    de Belgique, xxxvi, 1871, pp. 68.) Part II. Résistance à l’asphyxie
    par submersion, action du froid, action de la chaleur, temperature
    maximum. (Bull. Acad. Roy. de Belgique, Sér. 2, xxxiv, 1872, pp.
    271–321.)

  —— Les Myriopodes marins et la résistance des Arthropodes à
    respiration aërienne à la submersion. (Journ. de l’anatomie et de la
    physiologie, 1890, xxvi, pp. 236–269.)

  =Bütschli, O.= Ein Beitrag zur Kenntnis des Stoffwechsels,
    insbesondere die Respiration bei den Insekten. (Reichert und
    Dubois-Reymond’s Archiv f. Anatomie, 1874, pp. 348–361.)

  =Ritsema Cz., C.= _Acentropus niveus_ Oliv., in Zijne levenswijze en
    verschillende toestanden. (Tijdschr. voor Entom, 1876, xxi,
    separate, pp. 34, 2 Taf.)

  =Pott, Rob.= Chemical experiments on the respiration of insects.
    (Psyche, ii, 1878.)

  =Sharp, D.= Observations on the respiratory action of the carnivorous
    water-beetles. (Journ. Linn. Soc. London, xiii, Zoology, 1878, pp.
    161–183.)

  =Krancher, O.= Das Atmen der Biene. (Deutscher Bienenfreund., xvi
    Jahrg., 1880, pp. 49–51.)

  =Gissler, C. F.= Sub-elytral air-passages in Coleoptera. (Proc. Amer.
    Assoc. Advanc. Sc. 29 Meet. (1881), 1881, pp. 667–669.)

  =Langendorff, O.= Studien über die Innervation der Atembewegungen. 6.
    Das Atmungszentrum der Insekten. (Archiv f. Anatomie u. Physiol.,
    Physiol. Abteil., 1883, pp. 80–87.)

  =Macloskie, G.= Pneumatic functions of insects. (Psyche, iii, 1883,
    pp. 375.)

  =Chalande, J.= Recherches sur le mecanisme de la respiration chez les
    Myriopodes. (Compt. rend. Acad. Sc. Paris, civ, 1887, pp. 126, 127.)

  =Comstock, J. H.= Note on respiration of aquatic bugs. (Amer.
    Naturalist, 1887, xxi, pp. 577, 578.)

  =Fricken, W. v.= Ueber Entwicklung, Atmung und Lebensweise der Gattung
    Hydrophilus. (Tagebl., 60, Versamml. deutscher Naturf. u. Aerzte,
    1887, pp. 114, 115.)

  =Schmidt, E.= Ueber Atmung der Larven und Puppen von _Donacia
    crassipes_. (Berlin. Ent. Zeitschr., 1887, xxxi Jahrg., pp. 325–334,
    1 Taf.)

  =Dewitz, H.= Entnehmen die Larven der Donacien vermittelst Stigmen
    oder Atemrohren den Luftraumen der Pflanzen die sauerstoffhaltige
    Luft? (Ibid., 1888, xxxii Jahrg., pp. 5, 6, Fig.)

  =Müller, G. W.= Ueber _Agriotypus armatus_. (Spengel’s Zoolog.
    Jahrbücher. Abt. f. Systematik, etc., iv, 1889, pp. 1132–1134.)

  —— Noch einmal _Agriotypus armatus_. (Ibid., v, 1890, pp. 689–691.)

  =Devaux, H.= Vom Ersticken durch Ertrinken bei den Tieren und
    Pflanzen. (Naturwiss. Rundschau, vi Jahrg., 1891, p. 231; Compt.
    rend. Soc. de Biol., 1891, Ser. 9, iii, p. 43.)

  See also Dewitz, p. 482; Kolbe, p. 482.




                       THE ORGANS OF REPRODUCTION


Insects are without exception unisexual, the male and female organs
existing in different individuals, no insects being normally
hermaphroditic. The reproductive organs are situated in the hind-body or
abdomen, especially near the end, the genital glands opening externally
either in the space between the 7th and 8th, or 8th and 9th, or 9th and
10th abdominal segments, but as a rule between the 8th and 9th segments
(Fig. 299).

The primary or essential male organs are the testes, those of the female
being the ovaries. As we shall see, the primitive number of seminal
ducts and oviducts was two, this number being still retained in Lepisma
and the Ephemeridæ. The reproductive organs of both sexes are at first,
in their embryonic condition, of the same shape and structure, becoming
differentiated in form and function before sexual maturity. These glands
and ducts have a paired mesodermal genital rudiment, the ends of the
ducts being often connected with corresponding ectodermal invaginations
of the cuticle.

The secondary sexual organs mainly comprise the external genital
armature of the male, and the egg-laying organs, or ovipositor of the
female. Besides these structures there are other more superficial
secondary sexual characters, such as differences in the size and
ornamentation as well as coloring of the body, or of parts of it.

The primary sexual organs of insects have been conveniently tabulated by
Kolbe, thus:—

I. _Male reproductive organs._

             1. Two testes, with testicular follicles.
             2. Seminal ducts (_vasa deferentia_).
             3. Seminal vesicle.
             4. Accessory glands.
             5. The common seminal outlet, with the penis.
             6. The copulatory apparatus.

II. _Female reproductive organs._

               1. Two ovaries, with the egg-tubes.
               2. Two oviducts.
               3. Receptaculum seminis; bursa copulatrix.
               4. Accessory sac.
               5. The common oviduct, vagina, uterus.
               6. The ovipositor.

  The ducts of the sexual glands in Peripatus being transformed
  nephridia or segmental organs, it has been inferred that this is
  also the case with those of insects, though, as Lang states, there
  is a considerable difference in the two cases, as the greater part
  of the ducts in Peripatus arises out of the ectoderm, while in the
  Myriopoda and insects they come from the mesoderm; but he adds that
  in the Annelids the greater part of the nephridial duct is of
  mesodermal origin.

While in insects there is but a single pair of genital outlets, the
serial arrangement of the testicular (Fig. 458) and egg-tubes (Fig. 459)
in some Thysanura (Campodea, Japyx, and Lepisma), where the tubes (5 to
7 on each side) open singly one behind the other in segmental
succession, indicates that in their ancestors these egg-tubes opened out
on different segments situated one behind the other. Each egg-tube
independently opens into one of the two oviducts, which extend through
the abdomen as straight canals. The two oviducts open externally by a
short unpaired terminal portion, which in Machilis is said to be
wanting, only the outer aperture of the two oviducts being in this case
common to both. In Campodea and in the Collembola the ovaries and testes
on each side are simply tubes. It is to be observed that in the young
Lepisma Nassonow found that the external openings of the two ejaculatory
ducts are paired (Fig. 458 _B_, _ed._).

[Illustration:

  FIG. 458.—Male genital organs of Thysanura: _A_, Lepisma in which the
    testes are segmentally arranged.—After Grassi. _B_, _Lepisma
    saccharina_, young ♂: _vd_, vas deferens; _ed_, ejaculatory duct;
    _ga_, external appendages,—After Nassonow. _C_, Machilis, the testis
    lateral and separate, but not corresponding to the segments. _D_,
    Japyx, with an undivided testicular tube on each side; _tt_, testes;
    _cd_, vas deferens; _vs_, seminal vesicle; _ce_, ejaculatory
    duct.—After Grassi, from Perrier.
]

  In the Stylopidæ, also, though this may be the result of adaptation
  to the singular parasitic habits of the females whose bodies are
  mostly situated in the abdomen of their host, the ends of the
  oviducts are formed by the invagination of the integument of the 2d,
  3d, and 4th abdominal segments. In the 2d to 5th segments are
  situated tubes which open in the cavity of the body with funnel-like
  ends, so that the ducts have a close resemblance to the segmental
  organs of worms. (Nassonow.)

[Illustration:

  FIG. 459.—Ovaries of Thysanura: _A_, of Campodea. _B_, of Japyx.—After
    Grassi. _C_, of Machilis.—After Oudemans, from Sharp.
]

[Illustration:

  FIG. 460.—Female genital organs of _Lepisma saccharina_, adult: _ov_,
    ovaries; _a_, part of the oviduct, corresponding to the calyx of
    winged insects; _od_, oviduct; _vg_, vagina; _rs_, copulatory pouch;
    _gg_, accessory glands; _m_, muscles; _n_, nervous cord.—After
    Nassonow, from Perrier.
]

Among the winged insects the reproductive organs of the cricket (Fig.
466) are perhaps as simple as any. The testes are separate, and the vasa
deferentia very long. The seminal vesicles bear numerous large and short
utricles (_utriculi majores_ and _breviores_), the penis being simple
and dilated at the end; while in _Phyllodromia germanica_ the testes are
functional throughout life, and consist of four lobes each. In the
common cockroach (_P. orientalis_) (Fig. 461) the testes are functional
only in the young male; they afterwards shrivel and are functionally
replaced by the vesiculæ seminales and their appendages, when the later
transformations of the sperm-cells are effected. The accessory glands
are numerous and differ both in function and insertion. Two sets of
these glands (utriculi majores and breviores) are attached to the
vesiculæ seminales and the fore end of the ejaculatory duct, while
another appendage, called by Miall and Denny the _conglobate gland_,
opens separately on the exterior upon a double hook, which forms a part
of the external genital armature. The so-called penis is long, slender,
and dilated at the end, but is not perforated.

[Illustration:

  FIG. 461.—_A_, male organs of the cockroach, ventral view: _Ts_,
    testis; _VD_, vas deferens; _DE_, ductus ejaculatorius; _U_,
    utriculi majores; _u_, utriculi breviores; 2, dorsal view; 3, _CG_,
    conglobate gland and its duct. _B_, male organs, side view: _A_,
    titillator; _B_, penis; other letters as in _A_.—After Miall and
    Denny.
]

In the locusts (Acrydiidæ) the testes are, unlike those of most other
Orthoptera, closely united to each other so as to form a single mass of
tubular glands into which penetrate both simple and dilated tracheæ; the
entire mass is situated in the 3d, 4th, and 5th abdominal segments, and
above the intestine. The anterior end of the testicular mass is rounded
and held in place by a broad, thin band, one on each side; two similar
bands are situated a little behind the middle of the mass. From the
under side, and a little in advance of the middle of the mass, two
straight small ducts, as long as the testicular mass, pass obliquely to
the sides of the body, at the posterior end of the 7th segment of the
abdomen; these are the vasa deferentia. Each vas deferens, with its
mate, forms a convoluted mass of tubes, comprising twenty folded bundles
(_epididymis_ of Dufour), and two single, long, convoluted tubes, the
_vesiculæ seminales_, which are lobed in the 6th and 7th segments of the
abdomen. The two vesiculæ unite over the 5th abdominal ganglion, forming
a thick, very short canal (_ductus ejaculatorius_), which passes into a
large spherical muscular mass (præputium), behind which is the large
intromittent organ (_penis_), which forms a short chitinous cylinder,
quite complicated in structure, being armed with hooks and projections
and affording excellent specific characters. It can be seen in place
without dissection by drawing back the orbicular convex piece called the
_velum penis_.

In the Hymenoptera the reproductive system is quite simple, as seen in
Fig. 462.

[Illustration:

  FIG. 462.—Male organs of saw-fly (Athalia centifoliæ): _a_, _a_,
    testes; _b_, _b_, epididymis; _c_, _d_, vas deferentia; _e_,
    vesiculæ seminales; _f_, ductus ejaculatorius; _h_, penis (see also
    p. 180).—After Newport.
]

The general shape and relations of the female reproductive organs are
seen in Fig. 298, of the locust (Acrydiidæ). The ovaries consist of two
large bundles of tubes, each bundle tied to the other by slight bands,
with air-sacs and tracheæ ramifying among them. These tubes extend along
the intestine, passing into the prothorax. The ovarian tubes opening
into the oviducts unite to form the vagina, which lies on the floor of
the abdomen. (In the cockroach the vagina has a muscular wall and
chitinous lining.) Above the opening of the duct, and directly
communicating with it, is the copulatory pouch (_bursa copulatrix_), a
capacious pocket lined within with several narrow, longitudinal,
chitinous bands. Behind the bursa copulatrix lies, partly resting under
the fifth abdominal ganglion, the sebific, cement, or colleterial gland
(_colleterium_; compare Fig. 299, _sb_), which is flattened,
pear-shaped, a little over half as long as a ripe egg of the same
insect. From the under side, a little in advance of the middle, arises
the sebific duct, which, after making three tight coils next to the
ganglion, passes back and empties into the upper side of the bursa
copulatrix, dilating slightly before its junction with the latter.

The most primitive type of reproductive organs observed in insects is
that of the young Lepisma and the Ephemeridæ, in which the outlets of
the oviducts and of the vasa deferentia respectively are double or
paired, showing that insects have probably inherited these structures
from the segmental organs of their vermian ancestors.

Réaumur had already observed the process of oviposition and seen that
the female Ephemera had two openings near the end of the “6th” abdominal
segment, from which he saw two masses of eggs pass out at a time (Fig.
463). Eaton afterwards (1871) referred to the oviducts as terminating
between the 7th and 8th segments of the abdomen, and after him Joly; but
for a detailed monograph on the subject we are indebted to Palmén. He
found that the outlets of the sexual glands are paired, not only in the
larvæ of all stages, but also in the imagines, and in both sexes. In the
males the vasa deferentia pass on the ventral side of the 9th segment
through two external appendages, both reproductive organs, at whose tips
or sides the openings are situated. In the larvæ the female openings are
not formed until after the last moult. In the females the two oviducts
open on the ventral side of the hind-body between the 7th and 8th
segments.

[Illustration:

  FIG. 463.—Upturned end of body of Ephemera, with two egg-masses (_o_)
    issuing at the same time from the double oviducts; _q_, anus.—After
    Réaumur.
]

Palmén suggests that the Ephemerids represent, in respect to the
reproductive system among insects, a very primitive type of
organization, and he concludes that the inner sexual organs of insects
are built up of two different morphological elements; _i.e._ (_a_)
internal primitive paired structures (testes with vasa deferentia,
ovaria with oviducts), and (_b_) integumental structures, such as the
ductus ejaculatorius and vagina.

In the younger larvæ the vasa deferentia form slender cords along which
are situated the seminal glands; these cords are inserted in the
integument on the hinder edge of the 9th sternite, where afterwards,
during the last moult, the copulatory organs grow out. In the older
larvæ the sperm collects in the cavities of these cords. Their walls
become expanded, and this section then functions as vesiculæ seminales.
The ends of the cords remain contracted and act as ductus ejaculatorii.
Common unpaired glandular structures are not present. At the last moult
the copulatory organs reach their complete development, and the ducts
become open externally.

The oviducts in the larva are at first slender, string-like, and bear
the egg-follicles. As soon as the eggs pass out of the follicles and
collect in the oviducts, the walls of the latter become stretched, and
this portion forms two uterus-like structures. The terminal division of
the two passages forms their vaginal portions. But since there is no
common vagina, there are no unpaired glands and no receptaculum seminis.
The two ducts become open after the last shedding of the skin.

Palmén adds that this paired or double nature of the sexual glands and
their external ducts in this group of insects occurs in some Myriopoda
(Fig. 3, _E_, _F_) and a few Arachnida (Fig. 3, _C_, _D_, the outlets
being in this class unpaired), numerous Crustacea, and most worms; and
as already stated it is very marked in Limulus, where the paired outlets
are in both sexes very simple and wide apart (Fig. 3, _A_). In the worms
the paired genital ducts are modified segmental organs. As we have seen,
in the young male Lepisma there are two male genital openings. Hence
this double nature of the genital passages in the may-flies seems to be
very primitive.

In the Dermaptera, also, the genus Labidura was found by Meinert to have
two independent ductus ejaculatorii, opening externally in double
external slit-like processes (_penes_). The two ducts arise from a
single seminal vesicle, which is either paired (_L. advena_), or forms a
common passage (_L. gigantea_). In Forficula (Fig. 464, _B_) only one
ejaculatory duct persists, the other is obliterated, and one of the
penes is atrophied, the other assuming a position in the middle line of
the body. Thus the single ejaculatory duct and seminal vesicle arise
from the primitive vasa deferentia, and not from the integument of the
body, as is the case in the following examples.

  According to the researches of Dufour, Loew, etc., most species of
  Orthoptera (Œdipoda), Libellula, Perla, Panorpa, Rhaphidia,
  Myrmeleon, Sialis, and Trichoptera (Hydropsyche) have double vasa
  deferentia and seminal vesicles, and two ejaculatory ducts. The male
  genital passages of Rhaphidia have a double opening, Loew describing
  “the two seminal vesicles as lying near each other and at last
  uniting in a common passage, with an external opening, which,
  however, must be very short, since I could only once clearly observe
  it.” This opening is a deep invagination of the external integument,
  at the bottom of which the two ducts open independently of each
  other. In such insects, Palmén states that the single ejaculatory
  duct morphologically arises by an invagination of the integument.

  In another group, forming, as regards the genital apparatus, a step
  next above the Ephemeridæ, viz. the Perlidæ, the oviducts open near
  each other at the bottom of a median single “vagina,” situated
  between the 7th and 8th abdominal segment; it is covered beneath by
  a valve-like, enlarged sternite of the preceding segment, and Palmén
  homologizes it with the ovi-valvula of some Ephemeridæ. He regards
  this bell-shaped vagina as a cup-like, deep, intersegmental fold,
  which projects into the body-cavity and there receives the two
  ducts.

  This differentiation in the Perlidæ may be regarded as the type for
  several groups of insects. But in others occur a complication which
  in some degree modifies the type. Thus the invagination arises out
  from one segment alone, but several segments during metamorphosis
  may become so reduced that the ventral portions of all may be
  invaginated to form the vagina. Thus in the larva of Corethra,
  according to Leydig, and also Weismann, the two testes are attached
  by two cords to the integument; the hinder ones are inserted
  independently, and share in the development of the outlets.

  Graber has observed the same relations in the pupa of Chironomus,
  the efferent genital tubes in both sexes being separate, so that
  there are two vaginal passages and two penes present. Palmén
  comments on these relations in the dipterous insects, remarking that
  during metamorphosis certain parts of the terminal abdominal
  segments are reduced, while others are hypertrophied; hence the
  points of insertion of the cords referred to becoming the openings
  of the vasa are carried within the abdomen; and this part of the
  integument becomes an unpaired section. In these insects, also,
  there is an unpaired vesicula seminalis, but its morphological
  nature (whether formed from the integumental duct or the fused vasa
  deferentia) can only be settled after special investigation.

  In the Lepidoptera, also, it has been shown by Herold, Suckow,
  Bessels, and recently with full details by Jackson, that the paired
  larval oviducts are at first solid, but become tubular early in
  pupal life. A little later, their cavities open into that of the
  azygos or unpaired oviduct. The paired oviducts open in the female
  caterpillars on the hind edge of the 7th abdominal segment,
  afterwards uniting with the unpaired vagina of the 8th segment,
  which is developed from the hypodermis.

  Jackson adds that there are three stages traceable in the evolution
  of the genital ducts of Lepidoptera: “an ephemeridal stage, which
  ends towards the close of larval life; an orthopteran stage,
  indicated during the quiescent period preceding pupation; and a
  lepidopteran stage, which begins with the commencement of pupal
  life.”

As a summary of these results it appears that the genital organs of
insects consist of two morphologically different elements: 1. the
primitive internal paired structures (testes with the vasa deferentia;
ovaries with the ovarian tubes), and 2. integumental structures (Fig.
464). In the most primitive winged insects (Ephemeridæ) the latter
structures are only represented by the two external sexual openings, the
entire reproductive system being paired. The paired parts become in the
more highly differentiated forms united into single parts, while, _a_, a
common integumental division, grows in, forming the ductus
ejaculatorius, or the vagina; or, _b_, the inner passages anastomose
together, _i.e._ the openings fuse together; or, _c_, both of these
cases occur at once; or, finally, we have _d_, where the superfluous
paired parts by reduction become single.

[Illustration:

  FIG. 464.—Evolution of the unpaired from the paired sexual organs of
    insects: _A-E_, male organs. The parts arising by invagination of
    the integument indicated by thick black lines. _A_, an Ephemerid.
    _B_, _Forficula auricularia_. _C_, nymph of Orthoptera in general.
    _D_, Œdipoda. _E_, _Cetonia aurata_. _F_, female organs of
    Æschna.—After Palmén, from Lang.
]

The male ducts open behind the 9th, the female passages of Ephemerids
behind the 7th abdominal segment, those of other insects behind the 8th,
except in the Stylopidæ (Strepsiptera), in which they open much in
front.

Figure 464 graphically shows their relation. In the Odonata (_F_) the
chitinous lining or integumental invagination extends inwards where the
two oviducts begin, in the Coleoptera (_E_) the vagina, bursa
copulatrix, and receptaculum seminis being lined by a thick chitinous
layer. While in Perla the two seminal ducts pass directly into the
copulatory organ, in the Coleoptera they open into the unpaired ductus
ejaculatorius at a distance from the copulatory organ.

The morphological results obtained by Palmén, and for the Lepidoptera by
Jackson, were apparently confirmed from an embryological point of view
by Nusbaum, from observations on the development of the sexual passages
in two genera of Pediculidæ, and are as follows:—

  1. The prevalent impression that the larval ducts unite with each
  other and give origin to the whole system of sexual ducts is
  incorrect; they form only the vasa deferentia or the oviducts.

  2. All other parts of the efferent apparatus (uterus, vagina,
  receptaculum seminis, ductus ejaculatorius, penis, and appended
  glands) develop from the hypodermis.

  3. The connective tissue and the musculature of the efferent
  apparatus are derived from mesoblast cells present in the
  body-cavity.

  4. The efferent ducts originate as paired rudiments. All unpaired
  (azygos) parts (uterus, penis, receptaculum seminis, unpaired
  glands, etc.) are at first paired. The unpaired efferent apparatus
  of insects must therefore be regarded as morphologically a secondary
  and more complicated form.[76]

  5. The male and female efferent ducts are strictly homologous.

  6. The cavities of the oviducts, uterus, vagina in the female, of
  the vasa deferentia, appended organs, and ductus ejaculatorius of
  the male arise independently, and come into connection secondarily.

  The presence of two genital openings, viz. a bursa copulatrix or
  copulatory pouch, and of the primitive oviducal orifice behind the
  9th segment, is peculiar to Lepidoptera, and the inquiry naturally
  arises whether they represent the outlets of two pairs of segmental
  organs. The question has been fully set at rest, however, by
  Jackson, who shows that the copulatory pouch is a secondary
  invagination of the ectoderm, being derived from the hypodermis,
  while the second aperture is a special adaptation. It is, however,
  the partial homologue of the vaginal orifice in other orders of
  insects. It opens behind the sternite of the 8th abdominal segment,
  the typical position of the vaginal aperture as shown by
  Lacaze-Duthiers. The lateral position of the bursa and its
  separation from the azygos oviduct are probably late features in the
  phylogenetic history of the Lepidoptera, subsequent even to the
  closure of the furrow.

  “The existence of a second or posterior aperture is probably to be
  attributed to the advantage gained by a terminal position for the
  aperture through which the ova are laid. The remarkable way in which
  this aperture shifts backwards seems to point very distinctly to
  this explanation, especially as the Lepidoptera are entirely devoid
  of the outgrowths which form the ovipositor in some orders; _e.g._
  most Orthoptera.”

  The original condition of things appears to have been retained in a
  moth, _Nematois metallicus_, which, according to Cholodkowsky,
  possesses but a single external aperture, the bursa opening into the
  dorsal wall of the unpaired oviduct.


                  _a._ The male organs of reproduction

Bearing in mind that the testes with their efferent ducts are, like the
ovaries and egg-tubes, primitive structures, there are various secondary
or adaptive structures which are either due (1) to modifications of the
male efferent ducts, or of the ovarian tubes, or (2) to various
accessory organs, mostly glandular, resulting from the invagination of
the ectoderm.

[Illustration:

  FIG. 465.—_A_, diagram of male sexual organs of Carabus. _B_, of
    Blaps. _C_, of Hydrophilus. The heavy black lines represent the
    ectodermal organs; _t_, testis; _a. g._, accessory glands.—After
    Escherich.
]

The male organs are, then, the following:—

1. Two testes (Figs. 465–469, _t_, _H_, _ho_).

2. The two seminal ducts (_vasa deferentia_, _v_, _sl_, _SL_), whose
lower or outer (distal) division becomes enlarged and acts as a seminal
vesicle (_vesicula seminalis_; Figs. 467–469, _bl_, _SB_).

3. The common ejaculatory duct (_ductus ejaculatorius_), with the penis
(Figs. 467–469, _ag_, _uSG_).

4. Accessory glands at the base of the vasa deferentia (_glandulæ
mucosæ_, Figs. 465–469, _a. g._, _dr_, _D_), whose secretion mixes with
the semen or serves for the formation of the seminal packets
(sematophores).

  In his paper on the internal male organs of beetles, Escherich
  states that those of the Carabidæ illustrate the simplest, most
  primitive condition (Fig. 465). A simple blind tube on each side
  produces spermatozoa, stores the elements, and secretes mucus. Each
  of these tubes opens into a somewhat larger duct, and the two unite
  in a common ejaculatory canal. The terminal portion in these beetles
  is lined with chitin, and is therefore ectodermal, and not the
  result of the union of the mesodermic vasa deferentia. The region
  corresponding to the testes, vasa deferentia, and seminal vesicles
  are mesodermic. Blaps (Fig. 465, _B_) is intermediate between the
  Carabidæ and Hydrophilus (Fig. 465, _C_). The accessory glands (_a.
  g._) are developed, and the seminal vesicles are situated in the
  middle, and not at the lower end of the vasa deferentia, as in
  Hydrophilus.

=The testes.=—Each testis is composed of follicles or corresponding
parts, which according to the group of insects in which they occur are
united in different ways; or each testis consists of a single hank or
skein-like blind tube which is enveloped by a membrane, as in the
Carabidæ, Dyticidæ, or Lucanidæ.

[Illustration:

  FIG. 466.—_t_, testis; _v_, vas deferens; _g_, seminal vesicle of
    _Acheta campestris_.—After Carus, from Gegenbaur.
]

[Illustration:

  FIG. 467.—Male sexual apparatus of a bark-beetle: _sl_, vas deferens;
    _ho_, testis; _bl_, seminal vesicle; _dr_, accessory gland; _ag_,
    ductus ejaculatorius.—After Graber.
]

The number of testicular tubes is small in most Hemiptera, but very
great in the Cicadidæ, Orthoptera, Coleoptera, and many Hymenoptera.
Although the testes are usually separated from each other, they are
closely united in certain Orthoptera (Gryllotalpa, Ephippigera),
Coleoptera (Galerucella), in many Lepidoptera, and in a number of
Hymenoptera (Scolia, Pompilus, Crabro, and others).

The two testes of most Lepidoptera are so closely grown together or
coalesced into a single body that one might regard them as a single
testis. But in the different families there occur all grades, from the
unpaired testes of most Lepidoptera to Hepialus with separate testes.
Cholodkowsky therefore distinguishes four types:—

  1. The embryonal or primitive type, with two testes, whose seminal
  follicles are entirely separate. (Brandt.) These testes are
  contained, as in all other Lepidoptera, in a well-developed thick
  chitinous membrane or scrotum, analogous to that of the higher
  vertebrates, which envelops each separate seminal follicle
  (_Hepialus humuli_).

  2. The larval type, with two testes, whose four follicles are
  enclosed by a common scrotal membrane (_Bombyx mori_, _Gastropacha
  quercifolia_, _Ichthyura anachoreta_ and _anastomosis_, _Saturnia
  pyri_, _Aglia tau_).

  3. The pupal type (since it first occurs in the pupa state), with a
  single testis, which possesses an external median lace-like
  covering. (Adela, Lycæna.)

  4. The imaginal type, with a single testis enveloped by a lace-like
  scrotum, within which the follicles are wound around the
  longitudinal axis of the testis. (Most Lepidoptera.)

  In Nematois there are twenty seminal follicles, the number of
  ovarian tubes being the same. (Cholodkowsky.)

  In many insects the testes are not composed of tubes (follicles),
  but of button-like bodies, each of which has its own duct.

  The color of the testes is usually white, but they may be orange
  (Decticus), yellowish green (_Locusta viridissima_), or deep yellow
  (Chrysopa).

  The testes of Asilid flies are enveloped by a common dark-red
  membrane rich in tracheæ, like that in Lepidoptera which clothes the
  separate testicular follicles. The two testes of Calliphora are
  enveloped by an orange-yellow capsule, outside of which is a special
  membrane formed by the fat-body. (Cholodkowsky.)

  In the honey-bee the testis has two envelopes, the outer of which is
  formed by the fat-body, the inner coat of connective tissue. The
  entire testis corresponds to a portion only of that of _Bombyx
  mori_.

[Illustration:

  FIG. 468.—Male organs of a weevil, _Hylobius abietis_: _H_, testis;
    _SL_, vas deferens; _D_, slime gland; _SB_, seminal vesicle; _uSG_,
    ejaculatory duct.
]

[Illustration:

  FIG. 469.—Male organs of Tomicus. Lettering same as in Fig. 468.—This
    and Fig. 468 from Judeich and Nitsche.
]

=The seminal ducts.=—The vasa deferentia are fine tubes, which vary much
in length; being short in many beetles and locusts, very short in many
Diptera (Syrphidæ, etc.), very long in Cicada and many beetles;
according to Burmeister, being in Dyticus about five times, in
Necrophorus and Blaps eight to ten times, in Cicada 14 times, in
_Cetonia aurata_ 30 times, as long as the body. They either resemble a
skein of silk, or form a tangled mass.

The distal or lower end of the vasa is in many insects dilated into a
sac or seminal vesicle, which serves for the reception and storage of
the seminal fluid after it passes through the vasa deferentia. In the
honey-bee the vas deferens is given off from the reservoir, forms loops
in and outside of the testis, and passes to the seminal vesicle. The
canal into which the vesicle narrows does not open into the ductus
ejaculatorius, but into the glandulæ mucosæ; its epithelial cells are
much vacuolated, and have, therefore, a spongy appearance.
(Koschewnikoff.)

[Illustration:

  FIG. 470.—_A_, spermatozoön of a beetle (Copris), partly macerated to
    show structure of flagellum, which consists of a supporting fibre
    (_s.f_) and a fin-like envelope (_f_); _n_, nucleus; _a_, _a_,
    apical body divided into two parts. _B_, anterior part of that of
    Calathus, with barbed head and finmembrane.—After Ballowitz, from
    Wilson.
]

=The ejaculatory duct= during coition conducts the sperm into the
copulatory pouch of the female. In consequence of the stretching of the
integumental membrane the end of the duct can be erected and again
withdrawn. For this purpose the end of the duct is thickened and is said
to be provided with powerful muscles. The evaginable terminal portion is
covered by a strong chitinous membrane forming the penis or intromittent
organ (Fig: 462, _h_), which is externally enveloped by a pair of
chitinous lobes, which in many beetles are converted into a capsule. The
ductus ejaculatorius of the honey-bee is inserted by two chitinous
branches into the point of union of the two glandulæ mucosæ; it and the
entire copulatory apparatus are devoid of muscles, though it is,
however, well developed beneath the mucous glands. (Koschewnikoff.)

=The accessory glands= of the vasa deferentia are tubes whose secretions
either directly mix with the semen, or in many cases form seminal
packets (_spermatophores_). In Coleoptera, Lepidoptera, and Diptera
there is usually one pair. In many insects there are several pairs, as
in Hydrophilidæ and Elateridæ; they are branched in Hemiptera, and in
Orthoptera bushy. The single glandular tubules are very long, and form a
skein-like mass. In Orthoptera, in the larger number of accessory
glands, two forms may be distinguished, which differ from each other in
their contents (Siebold). In the cockroach (Fig. 461) these glands form
the “mushroom” shaped gland of Huxley, which was at first regarded as
the testis.

=The spermatozoa.=—These very minute bodies, the sexual homologues of
the eggs, abound in the seminal fluid, and are formed in the follicles
of the testes from a germinal layer or epithelium, as are the eggs. They
are hair- or thread-like, usually consisting of a head, a body or
middle-piece, and a long, thread-like tail (flagellum), which vibrates
rapidly, causing the spermatozoön to move actively forwards (Fig. 470).

  In beetles, according to Ballowitz, there are two main types of
  spermatozoa, connected, however, by intermediate forms. There is a
  double-tailed type, already described by Bütschli and v. la Valette
  St. George, and there are others which are single-tailed. Bütschli
  showed that in the double spermatozoön one tail-filament is straight
  and stiff, the other being undulating and contractile. Ballowitz
  describes this type in Calathus (Fig. 470, _B_), Chrysomela, and
  Hylobius, etc., and shows that the straight or supporting portion of
  the tail is elastic, but somewhat stiff, resistant to reagents, and
  without any fibrillar structure, while the contractile fringe
  consists of an extremely complicated system of fibrils (Fig. 470).
  The single-tailed type of spermatozoön, as seen, _e.g._, in
  Melolontha and Hydrophilus, has no supporting fibres. The tail is
  twisted in a spiral, corresponds to the contractile fringe of the
  double type, and exhibits a complicated fibrillar structure. The
  fringed type works its way ahead like the screw of a steamer.

[Illustration:

  FIG. 471.—_C_, anterior end of spermatid of a moth (Pygæra). _D_,
    young spermatozoön of the same; _af_, axial filament; _c_,
    centrosome; _m_, middle-piece or _mitosoma_; _n_, nucleus; _p_,
    paranucleus; _e_, envelope of the tail.—After Platner, from Wilson.
]

Each spermatozoön is a modified but complete cell, and the nucleus
contains the chromatin, a deeply staining substance of the nuclear
network and of the chromosomes and the supposed bearer of heredity.

=Formation of the spermatozoön.=—It arises from a primordial germcell
called _spermatogonium_. This cell contains a large, pale nucleus and a
dark body, the accessory nucleus of Bütschli. The _spermatogonia_
subdivide, but at a certain period pause in their subdivisions, and
undergo considerable growth. “Each spermatogonium is thus converted into
a _spermatocyte_, which, by two rapidly succeeding divisions gives rise
to four spermatozoa, as follows: The primary spermatocyte first divides
to form two daughter-cells, known as spermatocytes of the second order,
or sperm mother-cells. Each of these divides again—as a rule without
pausing, and without the reconstruction of the daughter-nuclei—to form
two _spermatids_ or sperm-cells. Each of the four spermatids is then
directly transformed into a single spermatozoön; its nucleus becoming
very small and compact, its cytoplasm giving rise to the tail and to
certain other structures.... As the spermatid develops into the
spermatozoön, it assumes an elongated form, the nucleus lying at one
end, while the cytoplasm is drawn out to form the flagellum at the
opposite end.” (Wilson’s The Cell, from La Valette St. George.)

Henking finds that the primordial sperm-cells correspond to the
primordial ova, both forms of cells in the insect he studied containing
the characteristic number of twenty-four chromosomes.

  The spermatogenesis of Laphria, according to Cholodkowsky, is very
  peculiar, and strongly resembles that described by Verson in _Bombyx
  mori_. In the blind end of the testicular tubes lies a colossal cell
  visible to the naked eye, the spermatogone, from which the entire
  contents of the testes originate. In Bombyx this spermatogone
  appears in the larva state. Such colossal spermatogones also occur
  in Lepidoptera of different families (Hyponomeuta, Vanessa, and in
  the pupa of _Chareas graminis_), in Trichoptera, and in Hemiptera
  (Syromastes); and Cholodkowsky inquires whether they may not be
  typical of insects. Toyama has observed these colossal cells not
  only in the testes but also in the ovaries of the silkworm. He
  regards them as supporting cells.

The spermatozoa are inclined to remain in bundles, and in this state are
expelled during copulation. These bundles are either root-like, bushy,
string-like, sinuous, or worm-like.

  Auerbach has observed the spermatozoa of _Dyticus marginalis_ in
  their passage through the convoluted seminal vesicles. All those
  arising from one testicular tube are united in a bundle. Each has a
  very complex structure, bilateral but unsymmetrical. The right side
  of the head is concave, the left convex; the whole head is
  longitudinally curved to right or left; and on the posterior half of
  the right side there is a projecting ridge bearing a hook-shaped
  cyanophilous “anchor,” at the free end of which an erythrophilous
  spherule appears. The most remarkable fact is that the spermatozoa
  unite in pairs in a perfectly definite way, opposed and crossed in a
  manner somewhat suggestive of a pair of scissors, with the right
  sides of the heads in contact. During this conjugation, or
  “dejugation” as Auerbach calls it, the anchors change their shape,
  and the little spherules are lost. Hundreds of these double
  spermatozoa are found together in little balls. The conjugation is a
  temporary one, but it may permit a molecular exchange of substance,
  perhaps with the result of mixing the hereditary qualities and
  limiting variability. (Journ. Roy. Micr. Soc., 1893, p. 622.)

In many insects which lack a true penis, the bundle of spermatozoa are
united in the ejaculatory duct, forming packets which are enveloped by
the secretion of the accessory glands which stiffens into a hard case.
These packets are called _spermatophores_. They are either introduced
into the vagina of the female or simply remain outside. Graber has
repeatedly observed that the male crickets, in the absence of the
female, let their spermatophores fall to the earth; whether it is
afterwards made available is not known, because hitherto no case is
reported that females seeking impregnation search, as in the case of the
Isopod crustacean, Porcellio, for the spermatophores.

  In the Gryllidæ and Locustidæ the spermatophore lies in a cup-like
  cavity under the penis. This is called the “spermatophore cup”
  (Chadima, 1871), into which the ejaculatory duct of the testis
  opens.

  According to the views of Schneider, the spermatophores, with their
  capsule, usually consist solely of seminal filaments, which stick
  closely to each other, and only exceptionally have a capsule formed
  by a glandular secretion. In Locusta, however, and perhaps also in
  Gryllus, the sperm is enveloped by the secretion of the accessory
  glands of the seminal ducts; the spermatophores pass, still fluid,
  out of the sexual opening of the male into that of the female, but
  become chilled on the outer surface, so that the sperm, without
  coming in contact with the air, passes into the receptaculum
  seminis.

  The mode of grouping of the spermatozoa of the Locustidæ as they
  occur in the spermatheca of the female is remarkable. Their heads
  lie so close to each other that they form a long shaft, while the
  numerous threads are arranged so as to look like the two vanes of a
  feather, the entire mass being like a very long heron’s feather.
  (Siebold.)

  In the honey-bee the spermatophore is likewise enveloped by the
  secretion of the accessory glands, and thereby becomes a sort of
  seminal cartridge. This is a peculiar oval body which is carried
  during the marriage-flight into the air within the upper part of the
  penis, the so-called penis-bulb. (Leuckart.)


                 _b._ The female organs of reproduction

The different parts of the female reproductive organs are the following:

1. The two ovaries.

2. The two oviducts.

[Illustration:

  FIG. 472.—Female organs of generation of a saw-fly (_Athalia
    centifoliæ_): _a_, _b_, _c_, the 18 ovarial tubes originating from
    each of the two oviducts (_d_), and containing the immature eggs;
    _e_, common oviduct; _f_, spermatheca; _g_, poison-sac; _h_,
    poison-glands; 10, last ganglion.—After Newport.
]

3. The common egg-passage in nearly all insects (its distal or
hindermost part forming the uterus or vagina).

4. The receptaculum seminis, or spermatheca.

5. The bursa copulatrix, or copulatory pouch.

6. The accessory glands (cement, sebific, or colleterial glands, or “oil
reservoirs,” glandulæ sebaceæ, coleterium).

=The ovaries and the ovarian tubes.=—As in the testes, so each ovary
consists of a variable number of ovarian tubes, by some called
_ovarioles_, united by a thread at the distal end, and at the lower or
hinder end opening into the oviduct. Each ovarian or egg tube is divided
into three sections: (1) the terminal thread; (2) the terminal chamber,
and (3) the actual ovarian tube, or chambered main division, this
forming the longest part of the egg-tube.

The slender terminal thread serves to attach or suspend each egg-tube
near the dorsal vessel (not directly to the heart, as formerly
supposed), becoming lost in the fat-body.

[Illustration:

  FIG. 473.—Ovarian tube of _P. orientalis_: _A_, section near the end;
    _tf_, base of terminal filament. _B_, section lower down; _ec_,
    egg-cells in egg-chamber.—After Brandt.
]

The terminal chamber contains undifferentiated cell elements, supposed
to be the remains of the ovarian rudiments. From these arise (either in
the embryo or larva) first, the follicle epithelium of the ovarian
tubes; and, second, the material for the formation of the new eggs, and
nutritive cells. “In the terminal chamber these cell-elements remain
undifferentiated, excepting when required for the removal of the
follicle epithelium, eggs, and nutritive cells in the adult insect.”
(Lang.) This portion of the ovariole is called the _germarium_. In
Blatta it is filled with protoplasm in which numerous small nuclei are
imbedded. (Wheeler.) The chambered main division of the egg-tube
contains the ripening eggs, one in each compartment, the tube appearing
like a string of beads.

The egg-tubes are of two types: (1) those without, and (2) those with
nutritive cells, the first kind being the simplest, and occurring in the
Synaptera (except Campodea) and in Orthoptera. As an example may be
cited that of the cockroach (Fig. 473), where in each tube there is a
simple continuous row of eggs from the terminal chamber to the oviduct.
The tube being constricted between these consecutive eggs, gives it a
beaded appearance.

  In the cockroach (_Periplaneta orientalis_) each egg-tube has a
  beaded appearance. Its wall consists of a transparent elastic
  membrane, lined by epithelium, with an external peritoneal layer of
  connective tissue. The terminal filament (_tf_) is filled with a
  clear protoplasm, with a few nuclei. In the terminal chamber (_tc_)
  are large nucleated cells, with separate nuclei, both entangled in a
  network of protoplasm. In the third, or egg-chamber (_ec_), are
  about twenty ripening eggs, arranged in a single row. “Between and
  around the eggs the nuclei gradually arrange themselves into
  one-layered follicles, which are attached, not to the wall of the
  tubes, but to the eggs, and travel downwards with them. As the eggs
  descend, the yolk which they contain increases rapidly, and the
  germinal vesicle and spot (nucleus and nucleolus), which were at
  first plain, disappear. A vitelline membrane is secreted by the
  inner surface and a chitinous chorion by the outer surface of the
  egg-follicle.

  “The lowest egg in an ovarian tube is nearly or altogether of the
  full size; it is of elongate-oval figure, and slightly curved, the
  convexity being turned towards the uterus. It is filled with a clear
  albuminous fluid, which mainly consists of yolk. The chorion now
  forms a transparent yellowish capsule, which, under the microscope,
  appears to be divided up into very many polygonal areas, defined by
  rows of fine dots. These areas probably correspond to as many
  follicular cells.” (Brandt, from Miall and Denny.)

  In the second type, _i.e._ those egg-tubes with nutritive cells,
  there are two kinds. In the first the egg-chambers and yolk- or
  nutritive chambers alternate, each of the latter containing one or
  more nutritive cells, which serve for the nourishment of the
  ripening egg contained in the neighboring chamber. “The egg- and
  yolk-chambers may be distinctly separated externally by
  constrictions (Hymenoptera and many Coleoptera), or one nutritive
  and one egg-chamber may lie in each section of the ovarian tube,
  which is externally visible as a swelling (Lepidoptera, Diptera).”

  In the second kind with nutritive cells, the actual tube consists
  (Fig. 474, _C_) of ovarian chambers only; the nutritive cells here
  remain massed together in the large terminal chamber. The single egg
  in the tube is united with the terminal chamber by connective
  strands (_d. s._), which convey the nutritive material to the eggs.
  (Lang.)

  Egg-cells, nutritive cells, and the cells of the follicle-epithelium
  (epithelium of the chambers of the ovarian tubes) are, says Lang,
  according to their origin, similar elements, like the egg and
  yolk-cells of the flat worms (Platodes); division of labor leads to
  their later differentiation. Only a few of the numerous egg-germs
  develop into eggs, the rest serving as envelopes and as food for
  these few.

  Korschelt considers that all the chief elements of the egg-tubes,
  viz. egg, nutritive, and epithelial cells, arise by a direct
  transformation of the elements of the terminal chamber, and that the
  last may be traced to the indifferent elements of the terminal
  thread, the elements in question originating from the nuclear
  elements by a breaking down of the syncytium (or masses of
  protoplasm with nuclei scattered through it) composing it (Fig.
  475).

  The latest work is that of Wielowiejski (Zoologische Anzeiger, ix,
  1886, p. 132), whose observations are based on a study of the
  ovarian tubes and the growing eggs of the Hemiptera (Pyrrhocoris),
  the Coleoptera (Telephorus, Saperda, Cetonia and Melolontha,
  Carabidæ, and Hydradephaga), etc.

  Wielowiejski divides the ovaries of insects into three groups:—

  1. Comprising such ovaries in the ends of whose egg-tubes (terminal
  filament) the embryonal cells in the early stages are accumulated,
  and are transformed into egg-, yolk-, and epithelial cells
  respectively. (Ovaries of Orthoptera, geodephagous and
  hydradephagous Coleoptera, Lepidoptera, Diptera, and Hymenoptera).

  2. Comprising ovaries whose ends above the egg-cells and egg-germs
  (_Eianlagen_) possess throughout life a more or less voluminous
  solid accumulation of cells (terminal chamber), but which stand in
  no close relation with the first. (Ovaries of Coleoptera, with the
  exception of the Geodephaga and Hydradephaga, and Aphidæ in part.)

[Illustration:

  FIG. 474.—Various types of ovarian tubes, diagrammatic: _A_, ovarian
    tube without nutritive cells. _B_, egg-tube with alternating
    nutritive and egg-compartments. _C_, ovarian tubes in which the
    terminal chamber (_ek_) is developed into a nutritive chamber,
    with which the developing eggs remain connected by means of
    threads (_ds_); _ef_, terminal filaments; _efa_, egg compartments
    or chambers; _fe_, follicle epithelium; _df_, yolk-chambers.—After
    Lang (_C_ from Claus).
]

[Illustration:

  FIG. 475.—Upper portion of the ovary in Forficula, showing eggs and
    nurse-cells; below, a portion of the nearly ripe egg (_e_) showing
    deutoplasm-spheres and germinal vesicle (_gv_). Above it lies the
    nurse-cell (n), with its enormous branching nucleus. Two
    successively younger stages of egg and nurse-cell are shown
    above.—After Korschelt, from Wilson.
]

[Illustration:

  FIG. 476.—_A_, ovarian egg of a butterfly (Vanessa), surrounded by
    its follicle; above are the nurse-cells (_n. c._), with branching
    nuclei; _g.v_, germinal vesicle. _B_, egg of Dyticus, living; the
    egg (_o.v._) lies between two groups of nutritive cells; the
    germinal vesicle sends amœboid processes into the dark mass of
    food-granules.—After Korschelt, from Wilson.
]

[Illustration:

  FIG. 477.—A, lower portion of one of the two ovaries of _Sphinx
    ligustri_, the four egg-tubes uniting to form the slightly
    developed calyx (_ov_). The egg-tubes above contain ripe eggs
    still surrounded by the follicle; _e. c_, the empty egg-chamber.
    Beyond the empty egg-chambers (_e. c_) are three egg-chambers with
    ripe eggs and the connecting cord. The whole tube is surrounded by
    the peritoneal membrane and musculature.—After Korschelt.
]

  3. Comprising ovaries whose ends above the egg-germs contain a
  well-developed mass of cells functioning as a yolk-forming organ,
  between whose special elements grow root-like offshoots of nearly
  ripe egg-cells. (Hemiptera.)

[Illustration:

  FIG. 478.—Ovary of a beetle, drawn somewhat diagrammatically: _o_,
    egg-tube; _s_, stalk of the same; _c_, egg-calyx; _ov_,
    oviduct.—After Korschelt.
]

When the egg is ripe the food-chamber disappears because its contents
have served for the formation of the egg below it. In Lepidoptera
especially, the egg-tubes resemble strings of pearls because most of the
numerous eggs ripen simultaneously and are likewise deposited at the
same period, which is naturally not the case in those insects whose eggs
gradually ripen (Fig. 477). In other cases the egg- or food-compartments
are transformed into each other, but only one egg- and one
food-compartment can be situated in the same dilatation of the ovarian
tube. Finally, there are insects in whose egg-tubes the egg-compartments
are arranged in a single row, while the capacious terminal chamber
contains a large mass of food-cells.

Egg-cells, nutritive cells, as well as the cells of the follicle
epithelium (epithelium of the chambers of the ovarian tubes), originate
as similar or homologous elements, division of labor leading to their
later differentiation. Only a few of the numerous egg-germs develop into
eggs, the rest serving as envelopes and also as food for these few.
(Lang.)

In many insects the egg-tubes open into an egg-calyx (Fig. 478, _c_), in
which the ripe eggs collect before passing into the oviduct (_ov_).

  As the result of his investigations on the origin of the cellular
  elements of the ovaries of insects Korschelt concludes:—

  1. The different cell-elements of the egg-tubes, eggs, nutritive
  cells, and epithelium arise from identical undifferentiated elements
  situated in the contents of the earliest germ of the egg-tubes.

  2. The first formation of the cellular elements present, and the
  differentiation of the individual compartments of the egg-tube,
  occur during embryonic and larval life.

  3. The undifferentiated elements of the terminal chamber correspond
  to the embryonic condition, while in post-embryonic time, and even
  during imaginal life, a new formation of the different kinds of
  cells takes place.

  4. The mode of origin of the different kinds of cells from the
  undifferentiated elements varies greatly in different insects.

  5. From their histological nature, and from the mode of origin of
  their elements, the most complex egg-tubes and those provided with
  nutritive compartments are phylogenetically derived from those
  without such nutritive compartments.

  6. The nutritive cells in certain cases originate in the same way
  and at the same time as the germ-cells, and are therefore to be
  regarded as germ-cells which have abandoned the function of
  egg-making, and exchanged it for the production of nutritive
  material.

  7. In the egg-tubes with numerous nutritive compartments the
  nutritive cells can originate at the same place as the egg-cells,
  and they afterwards still lie intermingled with these in the
  beginning or upper part of the egg-tubes.

  8. While the capability of egg-making of the germ-cells originally
  situated in the extremity of the terminal chamber gradually becomes
  transferred to those at the base of the terminal chamber, and the
  first transform into nutritive cells, egg-tubes with nutritive
  compartments at the base may be found.

  9. The nutritive cells of certain forms arise independently of the
  germ-cells and therefore could not have previously originated from
  them.

  10. The epithelium has in all forms nearly the same mode of
  formation; it everywhere shows a close similarity to the
  undifferentiated elements of the terminal chamber, out of which it
  directly develops. As to the fact of formation of epithelium through
  the germ-vesicles (_Keimblaschen_), nutritive-cell nuclei, or the
  so-called “oöblasts,” I could not feel certain.

  11. Neither the eggs of Hemiptera or of other insects arise through
  the agency of “oöblasts,” but like the epithelial and nutritive
  cells arise by a gradual differentiation from the indifferent
  elements of the ovarian tubes.

  12. The different elements of the egg-tubes, also the eggs, have the
  morphological value of cells.

  =Origin of incipient eggs in the germ of the testes.=—Heymons has
  detected in the germ of the testes of the male larvæ of
  _Phyllodromia germanica_ 7 mm. in length, young or incipient eggs,
  similar to those seen in the ovarian tubes of the female larva of
  the same size. In another male larva of the same size also occurred
  short cylindrical tubes each with a terminal thread, which had the
  appearance of rudimentary egg-tubes. Hence he thinks that every part
  of the genital germs (_Anlagen_) in the male, which are not
  concerned in the formation of testicular follicles, represents the
  germ of a female genital gland. As is well known, no insects are
  hermaphroditic, but this case of the practical origin of eggs and
  egg-tubes in the lowest division of the male efferent passage, which
  is homologous with the egg-producing division of the female ovarian
  tubes, points back to hermaphroditic ancestors. And Heymons suggests
  that the frequent occurrence of hermaphroditism in insects probably
  confirms this view.

[Illustration:

  FIG. 479.—Abdomen of queen bee, under side, × 8: _P_, petiole; _o_,
    _o_, ovaries; _hs_, position filled by honey-sac; _ds_, place
    through which the digestive canal passes; _od_, oviduct; _co.d_,
    common oviduct; _E_, egg passing oviduct; _s_, spermatheca; _i_,
    intestine: _pb_, poison-bag; _p.g_, poison-gland; _st_, sting; _p_,
    palpi. _B_, vestigial ovaries of ordinary worker; _sp_, vestigial
    spermatheca. _C_, partially developed ovaries of fertile worker;
    _sp_, vestigial spermatheca.—After Cheshire.
]

=The bursa copulatrix.=—The copulatory pouch in most insects is a
special cup-shaped appendage of the vagina adapted for the reception of
the male organ during sexual union. Its mode of formation in the
cockroach is thus described by Haase:—

“By the retreat of the female sexual aperture, situated in the 8th
ventral plate, a considerable space, the genital pouch, is produced;
this is formed chiefly by the extended connective membrane between the
elongated 7th and 8th ventral plates. This serves for the development of
the egg-cocoon, which is retained by the internal appendages of the
posterior gonapophyses.”

The fertilization of the female takes place once for all a long time
previous to oviposition; the semen in the receptaculum seminis passes
out as the eggs slip down the egg-passage, and a spermatozoön gains
entrance into the interior of the egg through the micropyle. In
Œcanthus, according to Ayers, fecundation probably takes place while the
egg is passing into the vagina, “since it is hardly possible that the
male element could gain access to the follicles before the chorion is
secreted.”

In the Lepidoptera, as has been stated, the copulatory pouch opens
separately from the opening of the oviduct (vagina), but a slender canal
connects the pouch with the vagina (Fig. 310, _bc_). The outlet
(“vagina” of Burgess) of the copulatory pouch opens between the 7th and
8th segments, that of the oviduct (vagina) on the 9th segment being
“situated immediately below the anus and hardly separated from it,
between the lappets of the 9th segment.” (Burgess.) The opening of the
copulatory pouch is, as we have seen, the genuine or primitive sexual
opening.

[Illustration:

  FIG. 480.—Spermatheca of the honey-bee, queen, × 40: _a_, space filled
    by a clear fluid; _b_, mass of spermatozoa; _c_, duct; _d_, _d_,
    active spermatozoa.— After Cheshire.
]

[Illustration:

  FIG. 481.—Female sexual organs of Scolytus: _ER_, egg-tubes; _pEL_,
    paired oviducts; _ST_, spermatheca; _BT_, copulatory pouch; _KD_,
    cement-glands; _Sch_, vagina.—After Lindeman, from Judeich and
    Nitsche.
]

=The spermatheca.=—This is a sac or pouch for the reception and storage
or preservation of the semen. While in most of the higher insects it
opens into the dorsal wall of the vagina (Fig. 472, _f_), in the
cockroach, locusts, and grasshoppers it opens into the bursa; but in
other European Orthoptera, as in most insects, it lies upon the dorsal
wall of the vagina. (Berlese.) In the cockroach, it is a short tube
dilated at the end and wound into a spiral of about one turn. “From the
tube a cœcal process is given off, which may correspond with the
accessory gland attached to the duct of the spermatheca in many insects
(_e.g._ Coleoptera, Hymenoptera, and some Lepidoptera). The spermatheca
is filled during copulation, and is always found to contain spermatozoa
in the fertile female. The spermatozoa are no doubt passed into the
genital pouch from time to time, and there fertilize the eggs descending
from the ovarian tubes.” In Meloë the spermatheca is exceedingly large.
(Miall and Denny, pp. 170, 171.)

=The colleterial glands.=—We have already briefly referred to these
glands. Those of the cockroaches form a number of long blind tubes
opening into the vagina. They furnish the material for the egg-capsule
or oötheca, viz. chitin and large crystals of oxalate of lime.

  In _Phyllodromia germanica_ “these glands are glistening white till
  the time of oviposition approaches, when they assume a yellow tint,
  and the octahedral crystals are seen imbedded in a viscid substance
  which fills their lumina. This viscid substance is soluble in
  potassium hydrate, and is consequently not chitin. When excreted to
  form the oötheca, it slowly hardens, deepens in color, and becomes
  insoluble in potassium hydrate. Light has nothing to do with this
  change, which is possibly produced by the oxygen in the air. It is
  the same change which is undergone by the cuticula of the insect
  itself immediately after ecdysis.” (Wheeler.)

=The vagina or uterus.=—This is simply the end of the common oviduct,
which, when dilated, is called the vagina, and, in the pupiparous forms,
the uterus.

In the cockroach the vagina opens by a median vertical slit situated in
the 8th sternite, into the genital pouch or bursa, upon the dorsal wall
of which the orifice of the spermatheca is situated. In the sheep-tick
the oviduct is enlarged to form the so-called uterus, which furnishes a
milk-like secretion for the nourishment of the larva during its
intra-uterine life.

In insects in general, the external opening of the vagina is simple, the
chitinous structures (valves) at the opening being adapted to receive
the male intromittent organ.

When the eggs are to be deposited deep below the surface of the earth,
or in wood, or in wood-boring larvæ, or in the body of caterpillars,
etc., they are inserted by the ovipositor (see p. 167).

  _Signs of copulation in insects._—Leydig has collected, partly from
  his own observations and partly from those of others, a number of
  cases in which female insects bear traces of having had sexual
  union, in the form of tags or plates attached to the body, and
  apparently formed from material secreted by the male. Such probably
  is the “pouch” on the abdomen of _Parnassius apollo_, and a somewhat
  similar structure in _Fulgora laternaria_, and such is the plate
  which is found on the hinder part of the abdomen of _Dyticus
  latissimus_ and _D. marginalis_. Leydig compares these structures
  with the white plate in _Astacus fluviatilis_, and with the little
  white lid on the spider Argenna, and finds analogues among
  vertebrates. (Arbeit. Zool. Zoot. Inst. Wurzburg, x, 1891, pp.
  37–55, 2 Figs.)




                LITERATURE ON THE ORGANS OF REPRODUCTION


                              _a._ General

  =Hunter, J.= Observations of bees. (Phil. Trans. Roy. Soc. London,
    1792, lxxxii, pp. 128–195.)

  =Hegetschweiler, J. J.= Dissertatio inauguralis zootomica de
    insectorum genitalibus. Turici, 1820, pp. 28, 1 Pl.

  =Audouin, V.= Recherches anatomiques sur la femelle du Drile jaunatre
    et sur le male de cette espèce. (Ann. Sc. nat., ii, 1824, pp.
    443–462, 1 Pl.)

  =Müller, Johannes.= Ueber die Entwickelung der Eier im Eierstock bei
    den Gespenstheuschrecken. (Nova Acta Acad. Leop.-Carol., xii, 1825,
    pp. 555–672, 6 Taf.)

  =Dufour, L.= Recherches anatomiques sur les Carabiques et sur
    plusieurs autres insectes coléoptères. Organes de la génération
    (Ann. Sc. nat., vi, 1825, pp. 150–206, 6 Pls.; pp. 427–468, 4 Pls.).

  —— Recherches anatomiques sur l’Hippobosque des cheveaux. (Ibid.,
    1825, vi, pp. 299–322, 1 Pl.)

  —— Recherches anatomiques sur les Labidoures. Appareil de la
    génération. (Ibid., 1828, xiii, pp. 354–359, 2 Pls.)

  —— Recherches anatomiques et considérations entomologiques sur
    quelques insectes coléoptères, compris dans les familles des
    Dermestins, des Byrrhiens, des Acanthopodes et des Leptodactyles.
    Appareil génital. (Ibid., Sér. 2, Zool. i, 1834, pp. 76–82, 2 Pls.)

  —— Résumé des recherches anatomiques et physiologiques sur les
    Hémiptères. (Ibid., Sér. 2, i, pp. 232–239.)

  —— Mémoire sur les métamorphoses et l’anatomie de la _Pyrochroa
    coccinea_. Appareil génital. (Ibid., Sér. 2, Zool., xiii, pp.
    337–339, 1 Pl.)

  —— Histoire des métamorphoses et de l’anatomie des Mordelles. (Ibid.,
    Sér. 2, xiv, pp. 235–238, 1 Pl.)

  —— Anatomie générale des Diptères. Appareil génital. (Ibid., Sér. 3,
    Zool., i, 1844, pp. 250–264.)

  —— Histoire des métamorphoses et de l’anatomie du _Piophila
    petasionis_. Appareil génital. (Ibid., Sér. 3, Zool., i, 1844, pp.
    378–386.)

  —— Études anatomiques et physiologiques sur les insectes diptères de
    la famille des Pupipares. Appareil génital. (Ann. Sc. nat., Sér. 3,
    Zool., iii, 1845, pp. 73–93, 2 Pls.)

  —— Recherches sur l’anatomie et l’histoire naturelle de _l’Osmylus
    maculatus_. Appareil génital. (Ibid., Sér. 3, Zool., ix, 1848, pp.
    349–356, 1 Pl.)

  —— Recherches anatomiques sur les Hyménoptères de la famille des
    Urocerates. Appareil génital. (Ibid., Sér. 4, Zool., i, 1854, pp.
    216–234, 1 Pl.)

  —— Fragments d’anatomie entomologique. Sur les ovaires du _Nemoptera
    lusitanica_. (Ibid., Sér. 4, viii, 1857, pp. 9–10, 1 Pl.)

  —— Fragments anatomiques sur quelques Élatérides. (Ibid., Sér. 4,
    viii, 1857, pp. 365–372, 1 Pl.)

  —— Recherches anatomiques et considérations entomologiques sur les
    Hémiptères du genre Leptopus. Appareil génital. (Ibid., Sér. 10,
    1858, pp. 356–362, 1 Pl.)

  —— Recherches anatomiques sur _l’Ascalaphus meridionalis_. Appareil
    génital. (Ibid., Sér. 4, xiii, 1860, pp. 203–206, 1 Pl.)

  —— Sur l’appareil génital male du _Coræbus bifasciatus_. (Thomson’s
    Archiv Ent., 1857, i, pp. 378–381.)

  =Suckow, F. W. L.= Geschlechtsorgane der Insekten. (Heusinger’s
    Zeitschr. f. organ. Physik., 1828, ii, pp. 231–264, 1 Taf.)

  =Rathke, M. H.= Miscellanea anatomico-physiologica. Fasc. 1. De
    Libellarum partibus genitalibus. Regiomonti, 1832, p. 38, 3 Pls.

  =Dutrochet, R. J. H.= Observations sur les organes de la génération
    chez les pucerons. (Ann. Sc. nat., 1833, xxx, pp. 204–209.)

  =Doyère, L.= Observations anatomiques sur les organes de la génération
    chez la Cigale femelle. (Ann. Sc. nat., 1837, vii, pp. 200–206,
    Fig.)

  =Siebold, C. Th. E. von.= Ueber die weiblichen Geschlechtsorgane der
    Tachinen. (Wiegmann’s Archiv f. Naturgesch., 1838, iv, pp. 191–201.)

  —— Ueber die inneren Geschlechtswerkzeuge der viviparen und oviparen
    Blattläuse. (Froriep’s Notizen, 1839, xii, pp. 305–308.)

  —— Ueber das Receptaculum seminis der Hymenopteren-Weibchen. (Germar’s
    Zeitschr. f. Ent., 1843, iv, pp. 362–388, 1 Taf.)

  =Loew, H.= Beitrag zur anatomischen Kenntniss der inneren
    Geschlechtsteile der zweiflügligen Insekten. (Germar’s Zeitschr. f.
    Ent., 1841, iii, pp. 386–406, 1 Taf.)

  —— Horæ anatomicæ, Abth. I. Entomotomien. Heft i-iii, Posen, 1841.

  —— Beiträge zur Kenntniss d. inneren Geschlechtstheile der zweifl.
    Insecten. (Germar’s Zeitschr. f. Entomologie, iii, 1841, pp.
    386–406, 1 Taf.)

  =Stein, F.= Vergleichende Anatomie und Physiologie der Insekten. I,
    Monographie. Ueber die Geschlechtsorgane und den Bau des
    Hinterleibes bei den weiblichen Käfern, Berlin, 1847, i, pp. 139, 9
    Taf.

  =Brauer, F.= Beitrag zur Kenntniss des inneren Baues und der
    Verwandlung der Neuropteren. (Verhandl. d. zool. botan., Vereins in
    Wien, 1855, pp. 1–26, 5 Taf.)

  =Haliday, A. H.= Note on a peculiar form of the ovaries observed in a
    hymenopterous insect, constituting a new genus and species of the
    family Diapriadæ. (Nat. Hist. Review, 1857, iv, pp. 166–174, 1 Pl.)

  =Laboulbène, A.= Recherches sur les appareil de la digestion et de la
    reproduction du _Buprestis manca_. (Thomson’s Archiv Ent., 1857, i,
    pp. 204–236, 2 Pls.)

  =Lubbock, John.= On the ova and pseudova of insects. (Phil. Trans.
    Roy. Soc., London, cxlix, 1860, pp. 341–369.)

  =Landois, H.= Ueber die Verbindung der Hoden mit dem Rückengefäss bei
    den Insekten. (Zeitschr. f. wissens. Zool., xiii, 1863, pp. 316–318,
    1 Taf.)

  =Leydig, F.= Der Eierstock und die Samentasche der Insekten. (Nova
    Acta Acad. Leop.-Carol., xxxiii, 1867, pp. 88, 5 Taf.)

  —— Beiträge zur Kenntniss des thierischen Eies im unbefruchteten
    Zustande. (Spengel’s Zool. Jahrbücher, 1889. Abth. f. Anat., iii,
    pp. 287–432, 7 Taf.)

  =Bessels, E.= Studien über die Entwicklung der sexual Drüsen bei den
    Lepidopteren. (Zeitschr. wissens. Zool., xvii, 1867, pp. 545–563.)

  =Rajewsky.= Ueber die Geschlechtsorgane von _Blatta orientalis_, etc.
    (Nachr. d. k. Gesellschaft d. Moskauer Universität, xvi, 1875.
    Testes of cockroach. In Russian; for abstract, see Hoffmann u.
    Schwalbe, Jahresbericht, 1875, p. 425.)

  =Brehm, Siegfr.= Comparative structure of the reproductive organs in
    _Blatta germanica_ and _Periplaneta orientalis_. (Horæ Ent. Soc.
    Rossicæ, St. Petersburg, viii, 1880.) (In Russian, male organs
    only.)

  =Cholodkowsky, N. A.= Ueber die Hoden der Schmetterlinge. (Zool.
    Anzeiger, iii Jahrg., 1880, pp. 115–117.)

  —— Ueber den Bau der Testikel bei Schmetterlingen. (Zool. Anzeiger,
    1880, iii, pp. 214–215.)

  —— Ueber die Hoden der Lepidopteren. (Zool. Anzeiger, 1884, pp.
    564–568.)

  —— Ueber den Geschlechtsapparat von _Nematois metallicus_. (Zeitschr.
    f. wissens. Zool., xliii, pp. 559–568, 1885.)

  =Tichomirow, A.= Ueber den Bau der Sexualdrüsen und die Entwickelung
    der Sexualprodukte bei _Bombyx mori_. (Zool. Anzeiger, iii, 1880,
    pp. 235–237.)

  =Nusbaum, F.= Zur Entwicklungsgeschichte der Ausführungsgange der
    Sexualdrüsen bei den Insekten. (Zool. Anzeiger, v, 1882, pp.
    637–643.)

  ——On the developmental history of the efferent passages of the sexual
    glands in insects. Lemberg, 1884 (in Czech).

  =Berlese, Ant.= Ricerde sugli organi genitali degli ortotteri. (Atti
    della R. Acad. dei Lincei. Ser 3, xi, 1882.) (Genital organs of
    European Orthoptera.)

  =Balbiani, G.= Le Phylloxera du chêne et le Phylloxera de la vigne.
    Paris, 1884, pp. 45, 11 Pls.

  —— Contribution à l’étude de la formation des organes sexuel chez les
    insectes. (Recueil Zool. Suisse, 1885.)

  =Schneider, Anton.= Die Entwicklung der Geschlechtsorgane bei den
    Insekten. Zool. Beiträge, Breslau, i, 1885.

  =Beauregard, H.= Recherches sur les insectes vésicants. Suite.
    (Journal Anat. Phys., Paris, 1887, xxii Année, pp. 528–548, 1 Pl.;
    xxiii Année, pp. 124–163, 6 Pls.)

  —— Les insectes vésicants. Paris, 1890. Chap. v, Appareil de la
    génération, pp. 103–159, Pls. 10–12.

  =Nassonow, N.= Études morphologiques sur les Lepisma, Campodea et
    Podura. (Mém. Soc. Imp. Anthropologie et d’Ethn. Moscou, iii, 1887,
    pp. 85, 2 Pls., 68 Figs.)

  —— _Xenos rossii_; seine Anatomie und Entwicklungsgeschichte. (Bull.
    de l’Université de Varsovie, 1892, pp. 74, 2 Taf.) (In Russian.)

  —— Position des Strepsiptères dans le systeme selon les données du
    développement post-embryonal et de l’anatomie, p. 11, Warsaw, 1892.

  =Grassi, B. J.= Progenitori dei Miriapodi e degli Insetti. Memoria
    vii, Anatomia comparata dei Tisanuri. (Atti d. R. Acad. de’ Lincei,
    Cl. scienc. e fis., Serie 4, iv, 1888, pp. 435–606, 5 Pls.)

  =Heymons, R.= Ueber die hermaphroditsche Anlage der Sexualdrüsen beim
    Mannchen von _Phyllodromia germanica_. (Zool. Anzeiger, 1890, pp.
    451–457.)

  =Koschewnikoff, G.= Zur Anatomie der männlichen Geschlechtsorgane der
    Honigbiene. (Zool. Anzeiger, xiv, 1891, pp. 393–396.)

  =Verhoeff, C.= (See p. 186.)

  =Ingenitzky, J.= Zur Kenntniss der Begattungsorgane der Libelluliden.
    (Zool. Anzeiger, 1893, xvi Jahrg., pp. 405–407, 2 Figs.)

  —— On the fauna and organization of dragon-flies of Russian Poland,
    1893, 1 Pl. (In Russian.)

  =Escherich, K.= Anatomische Studien über das männliche Genitalsystem
    der Coleopteren. (Zeitschr. f. wissens. Zool., lvii, pp. 620–641,
    1894.)

  =Kluge, Max H. E.= Das männliche Geschlechtsorgan von _Vespa
    germanica_. Inaug. Diss. Leipzig, 1895, pp. 1–45, 1 Taf.

  =Verson, E.= La borsa copulatrice nei Lepidotteri (Atti e Mem. Accad.
    Sc. Lett. ed Arti, Padova, 1896, xii, pp. 369–372, 4 Pls.).

  =Klapálek, Fr.= Über die Geschlechtstheile der Plecopteren, mit
    besonderer Rücksicht auf die Morphologie der Genitalanhänge.
    (Sitzungsb. k. Akad. Wissens. Wien. Math.-Naturw. Cl., cv, 1896, pp.
    56, 5 Taf.)

  =Fenard, A.= Recherches sur les organes complémentaires internes de
    l’appareil génital des Orthoptères. (Bull. Sc. France Belg., xxix,
    1897, pp. 390–527, 528–533, 5 Pls.)

  Consult also the Works of Ayres, Balfour, Burgess, Burmeister,
    Bütschli, Claus, Dzierzon, Gensch, Henking, Honert, Huxley, Kluge,
    Kramer, Landois, Leuckart (art. Zeugung), Leydig, Ludwig,
    Metschnikoff, H. Meyer, Minot, Müller, Pfitzner, Schneider,
    Seeliger, Scholz, Siebold, Suckow, Swammerdam, Tichomiroff, Wagner,
    Waldeyer, Weismann, Wheeler, v. Wielowiejski, Will, Witlaczil, and
    Ziegler.


                 _b._ Formation of the egg (oögenesis)

  =Claus, C.= Beobachtungen über die Bildung des Insekteneies.
    (Zeitschr. wissens. Zool., xiv, 1864, pp. 42–54.)

  =Brandt, A.= Ueber die Eiröhren der _Blatta orientalis_. (Mém. Acad.
    Imp. Scienc. de St. Petersbourg, Sér. 7, xxi, 1874, p. 30.)

  —— Vergleichende Untersuchungen über die Eiröhren und die Eier der
    Insekten. (Nachr. d. Gesellsch. Freunde d. naturwiss. Moskau, xxiii,
    1876; also xxiv, 1877, pp. 77–79.)

  —— Das Ei und seine Bildungsstatte. Ein vergleichenden-morphologischer
    Versuch mit Zugrundelegung der Insecteneies. Leipzig, 1878.

  =Kadyi, H.= Beiträge zur Vorgänge beim Eierlegen der _Blatta
    orientalis_. Vorläufige Mittheilung. (Zool. Anzeiger, 1879, pp.
    632–636.) (Formation of the egg-capsules of cockroach.)

  =Brass, Arn.= Das Ovarium und der Eibildung und der ersten
    Entwicklungsstadien bei viviparen Aphiden. Halle, 1883. (Zeits. f.
    Naturwiss. in Halle, Jahrg. 1882.)

  —— Zur Kenntniss der männlichen Geschlechtsorgane der Dipteren. (Zool.
    Anzeiger, 1892, pp. 178–180.)

  =Will, Ludvig.= Zur Bildung der Eies und des Blastoderms bei den
    viviparen Aphiden. (Arbeiten Zool. Inst. Univ. Würzburg, vi, 1882,
    pp. 217–258.)

  =Korschelt, E.= Zur Frage nach dem Ursprung der verschiedenen
    Zellenelemente der Insectenovarien. (Zool. Anzeiger, 1885, pp.
    581–586, 599–605.)

  =Wielowiejski, H. V.= Zur Morphologie des Insektenovariums. (Zool.
    Anzeiger, 1886, ix Jahrgang, pp. 132–139.)

  —— Zur Kenntniss der Eibildung bei der Feuerwanze (_Pyrrhocoris
    apterus_). (Zool. Anzeiger, 1885, pp. 369–375.)

  =Blochmann, F.= Ueber die Richtungskörper bei Insekteneiern. (Morph.
    Jahrb., 1887, xii, ix 544.)

  Also the writings of Leydig (p. 509).


                        _c._ On the spermatozoa

  =Treviranus, G. R.= Ueber die organischen Körper des tierischen Samens
    und deren Analogie mit dem Pollen der Pflanzen. (Zeitschr. f. d.
    Physiologie, von F. Tiedemann, G. R. und L. C. Treviranus, 1835, v,
    pp. 136–153, 2 Taf.)

  =Siebold, C. Th. E. von.= Ueber die Spermatozoen der Crustaceen,
    Insekten, Gasteropoden und einiger anderer wirbellosen Tiere.
    (Müller’s Archiv f. Anatomie, 1836, pp. 13–52, 2 Taf.)

  —— Fernere Beobachtungen über die Spermatozoen der wirbellosen Tiere.
    (Ibid., 1836, p. 232; 1837, pp. 381–432, 1 Taf.)

  —— Ueber die Spermatozoen der wirbellosen Tiere, iv. (Ibid. 1837, pp.
    392–433.)

  —— Lange Lebensdauer der Spermatozoen bei _Vespa rufa_. (Wiegmann’s
    Archiv f. Naturgesch., 1839, v, pp. 107, 108.)

  —— Ueber die Spermatozoiden der Locustinen. (Nova Acta Acad.
    Leop.-Carol., 1845, xxi, pp. 249–274, 1 Pl.)

  =Kölliker, A.= Beiträge zur Kenntnis der Geschlechtsverhältnisse und
    der Samenflüssigkeit wirbelloser Tiere, nebst einem Versuch über das
    Wesen und die Bedeutung der sogenannten Samentiere, Berlin, 1841,
    pp. 88, 3 Taf.

  —— Die Bildung der Samenfaden in Bläschen als allgemeines
    Bildungsgesetz. (Neue Denkschr. d. allg. Schweiz. Ges., viii, 1847,
    pp. 28, 3 Taf.)

  —— Physiologische Studien über die Samenflüssigkeit. (Zeitschr. f.
    wissens. Zool., vii, 1856, pp. 201–272, 1 Taf.)

  =Yersin, A.= Observations sur le _Gryllus campestris_. (Bull. Soc.
    Vaudoise sc. nat., 1853, iii, pp. 128.)

  =Lespès, Ch.= Mémoire sur les spermatophores des grillons. (Ann. Sc.
    nat., Sér. 4, iii, 1855, pp. 366–377, 1 Pl.; iv, pp. 244–249, 1 Pl.)

  =Landois, H.= Entwicklung der büschelförmigen Spermatozoiden bei den
    Lepidopteren. (Schultze’s Archiv f. Anat. u. Physiol., 1866, pp.
    50–58, 1 Taf.)

  =Bütschli, O.= Vorlaufige Mitteilungen über Bau und Entwicklung der
    Samenfaden bei Insekten und Crustaceen. (Zeitschr. f. wissens.
    Zool., xxi, 1871, pp. 402–415.)

  =Bütschli, O.= Nähere Mitteilungen über die Entwicklung und den Bau
    der Samenfaden der Insekten. (Ibid., xxi, 1871, pp. 526–534, 2 Taf.)

  =La Vallette St. George, A. V.= Ueber die Genese der Samenkörper, III.
    Mitteilung. (Archiv f. Mikroscop. Anat., 1874, x, pp. 495–504, 1
    Taf.)

  —— Spermatologische Beitrage: II. Mitteilung (Ibid., 1886, xxvii, pp.
    1–122 Taf.). IV. Mitteilung (Ibid., 1886, xxviii, pp. 1–13, 4 Taf.)
    V. Mitteilung (Ibid., 1887, xxx, pp. 426–434, 1 Taf.)

  =Schneider, A.= Das Ei und seine Befruchtung, pp. 88, 10 Taf.;
    Arthropoden, pp. 57–68 und 79. Taf. 8–10, 1883.

  =Wielowiejski, H. de.= Observations sur la spermatogénèse des
    Arthropodes. (Archiv Slav. de Biologie, 1886, ii, pp. 28–36.)

  =Ballowitz, E.= Zur Lehre von der Struktur der Spermatozoen. (Anat.
    Anzeiger, i Jahrg., 1886, pp. 363–376.)

  —— Untersuchungen über die Struktur der Spermatozoen, zugleiche in
    Beitrag zur Lehre von feineren Bau der kontraktilen Elemente. Die
    Spermatozoen der Insekten. I. Coleopteren. (Zeitschr. f. wissensch.
    Zool., 1, 1890, pp. 317–407, 4 Taf.)

  —— Zu der Mittheilung des Herrn Professor L. Auerbach in Breslau über
    merkwürdiger Vorgänger am Sperma von _Dytiscus marginalis_. (Anat.
    Anzeiger, 1893, viii Jahrg., pp. 505–506.)

  —— Die Doppelspermatozoen der Dyticiden. (Zeitschr. f. wissensch.
    Zool., lxvi, 1895, pp. 458–499, 5 Taf.)

  =Beauregard, H.= Note sur la spermatogénèse chez la cantharide.
    (Compt.-rend. Soc. Biol., Paris, 1888, iv, pp. 331–333.)

  =Gilson, G.= Étude comparée de la spermatogénèse chez les Arthropodes,
    in La Cellule, Recueil de Cytologie et d’Histologie gén., i, 1888, 8
    Pl.

  =Verson, E.= Zur Spermatogenesis. (Zool. Anzeiger, xii Jahrg., 1889,
    pp. 100–103, Fig.)

  —— La spermatogenesi nel _Bombyx mori_. (Padova, 1889, 25 pp. und 3
    Taf.)

  —— Zur spermatogenesis. (Zool. Anzeiger, xii, 1889, pp. 100–103.)

  —— Zur spermatogenesis bei der Seidenraupe. (Zeitschr. f. wissens.
    Zool., lviii, 1894, pp. 303–313, 1 Taf.)

  =Henking, H.= Ueber Reductionsteilung der Chromosomen in den
    Samenzellen von Insekten. (Internat. Monatsschr. f. Anat. und Phys.,
    1890, vii, pp. 243–248.)

  —— I. Untersuchungen über die erste Entwicklungsorgänge in der Eiern
    der Insekten. II. Ueber Spermatogenese und deren Beziehung zur
    Eientwickelung bei _Pyrrhocoris apterus_. (Zeits. wissens. Zool.,
    xlix, 1890, pp. 503–564; li, 1891, pp. 685–736.) III. Specielles und
    Allgemeines. (Ibid., 1892, liv, pp. 1–274, 12 Taf.)

  =Sabatier, A.= De la spermatogénèse chez les Locustides. (Comptes
    rend. Acad. Paris, 1890, cxi, p. 797.)

  =Cholodkowsky, N.= Zur Frage über die Anfangsstadien der
    Spermatogenese bei den Insecten. (Zool. Anzeiger, 1894, pp.
    302–304.) See also Zool. Anzeiger, 1892, p. 179.

  =Auerbach, Leopold.= Ueber merkwürdige Vorgänge am Sperma von
    _Dytiscus marginalis_. (Sitz. Ber. Akad., Berlin, 1893, pp. 185–203,
    2 Figs.)

  —— Zu dem Bemerkungen des Herrn Dr. Ballowitz betreffend das Sperma
    von _Dytiscus marginalis_. (Anat. Anzeiger, viii Jahrg., 1893, pp.
    627–630.)

  =Toyama, K.= On the spermatogenesis of the silkworm. (Bull, ii, No. 3,
    Coll. Agric. Imp. Univ. Tokyo, pp. 125–157, 1894, 2 Pls.)

  =Wilcox, E. V.= Spermatogenesis of _Caloptenus femur-rubrum_ and
    _Cicada tibicen_. (Bull. Mus. Comp. Zool., xxvii, 1895, pp. 32, 5
    Pls.)

  —— Further studies on the spermatogenesis of _Caloptenus
    femur-rubrum_. (Ibid., xxix, 1896, pp. 193–202, 3 Pls.)

  =Wilson, Edmund B.= The cell in development and inheritance. (New
    York, 1896.) Also the writings of Platner, Waldeyer.


              _d._ On the paired genital efferent passages

  =Loew, H.= Abbildungen und Bemerkungen zur Anatomie einiger
    Neuropterengattungen. (Linnæa Ent., 1848, pp. 345–385, 6 Taf.)

  =Meinert, F.= Anatomia Forficularum, i, Kjöbenhavn, 1863, 1 Pl.

  —— Om dobbelte Saedgange hos Insecter. (Naturhist. Tidsskrift, 3
    Raekke, v, 1868, pp. 278–294.)

  =Palmén, J. A.= Zur vergleichenden Anatomie der Ausführungsgänge der
    Sexualorgane bei den Insekten. (Morph. Jahrb., ix, 1883, pp.
    169–176.)

  —— Ueber paarige Ausführungsgänge der Geschlechtsorgane bei Insekten.
    Eine morphologische Untersuchung. Helsingfors, 1884, pp. 108 und 5
    Taf.

  =Nusbaum, J.= Zur Entwicklungsgeschichte der Ausführungsgänge der
    Sexualdrüsen bei Insekten. (Kosmos, Lemberg, 1884, ix Jahrg., pp.
    256–266, 393–408, 462–474, 2 Taf. In Polish with résumé in German.)

  =Spichardt, C.= Beitrag zur Entwickelung der männlichen Genitalien und
    ihrer Ausführgänge bei Lepidopteren. (Verhandl. d. naturwiss.
    Vereins zu Bonn, 1886, xliii Jahrg., pp. 1–34, 1 Taf.)

  =Jackson, W. H.= Studies in the morphology of the Lepidoptera. I.
    (Zool. Anzeiger, xii Jahrg., 1889, pp. 622–626.) (See p. 389.)

  =Lowne, B. Th.= On the structure and development of the ovaries and
    their appendages in the blow-fly (_Calliphora erythrocephala_).
    (Journ. Linn. Soc. London, 1889, xx, pp. 418–442, 1 Pl.)

  See also Meinert (1897), Heymons (1897).


                             END OF PART I




                     PART II. EMBRYOLOGY OF INSECTS


                              _a._ The egg

[Illustration:

  FIG. 482.—Female Dyticus, laying eggs: _A_, ovipositor extended. _B_,
    egg of Notonecta, attached to stem of rush. _C_, egg of Dyticus,
    laid in excavation in rush.—After Régimbart, from Miall.
]

Insects as a rule arise from eggs which are laid in a great variety of
situations, those species which are viviparous being exceedingly few in
number compared with the class as a whole. It is noteworthy that Leydig
has found in the same Aphis, and even in the same ovary, an egg-tube
producing eggs, while a neighboring tube was producing viviparous
individuals.[77] The viviparous species are confined to certain
May-flies, the Aphidæ, Diptera (Sarcophaga, Tachinidæ, Œstridæ, and
Pupipara), and to certain Coleoptera (Stylopidæ and some Staphylinidæ).

The number of eggs laid varies from a very few, as in the Collembola and
in the Psocidæ, or 15 or even less in certain fossorial wasps, and from
20 to 35 in some locusts to many thousands in the social insects, the
honey-bee laying by estimate over 1,000,000 eggs in the course of her
life. Dr. Sharp thinks that from 50 to 100 may perhaps be taken as an
average number for one female to produce. The eggs of insects with a
complete metamorphosis are said by Brauer to be smaller in proportion to
the parent than those laid by ametabolous or heterometabolous insects.
In this respect the insects are paralleled by the birds, the highest
forms laying smaller eggs than the water birds, ostrich, Apteryx, etc.

[Illustration:

  FIG. 483.—Eggs (_e_) of Hydrobius (?) and their capsules, from which
    the larva, Fig. 452, hatched.—Emerton _del._
]

The egg, or ovum, when laid is not always ripe or perfect, but, as in
those of ants, continues to grow after oviposition. Others are laid some
time after the embryo has begun to form; and in the flesh-flies the
larva hatches before the egg is deposited.

[Illustration:

  FIG. 484.—Egg-masses of Chironomus: _A_, string of eggs of _C.
    dorsalis_, divided into sections to show both sides. _B_, twisted
    fibres which traverse the string of eggs. _C_, egg-mass of
    Chironomus (_sp_). _D_, egg-mass of a third species. _E_, part of
    _D_, more highly magnified. _F_, developing eggs, two stages.—After
    Miall.
]

Insects as a rule instinctively lay their eggs near or upon objects
destined to be the food of the larva; those of caterpillars on leaves,
those of many flies on meat or carrion, those of Copris and other
dung-beetles in dung, those of aquatic insects in water, while many
oviposit in the earth or in plants (Fig. 482), or in the bodies of
animals destined to be the hosts of the parasitic larvæ. As the eggs are
preyed upon by mites and other animals, the contrivances and
modifications of the mode of egg-laying, and the situations in which
they are placed, are almost endless. Many insects lay their eggs in a
mass, covered with a gummy substance; or those laid in the water, as the
eggs of dragon-flies, caddis-flies, Chironomus (Fig. 484), etc., are
enveloped by a jelly-like mass.

[Illustration:

  FIG. 485.—Egg-capsule of _Periplaneta americana_: _a_, side; _b_, end
    view; _c_, natural size.—After Howard and Marlatt, Bull. 4, Div.
    Ent. U. S. Dept. Agr.
]

The _oötheca_ of the cockroach (Fig. 485) is a solid, dense case, which,
after being carried about by the mother, can be left without harm in the
crevices of the floors of houses. The oötheca of Mantis (Fig. 486) is
formed by a large mass of frothy matter, which hardens and is attached
to stems of plants.

[Illustration:

  FIG. 486.—Egg-capsules of _Mantis carolina_.—After Riley.
]

  On the other hand, the female “walking-stick” (_Diapheromera
  femoratum_) drops her eggs, says Riley, loosely upon the ground,
  from whatever height she may happen to be, and “one hears a constant
  pattering, not unlike drops of rain, that results from the abundant
  dropping of these eggs, which, in places, lay so thick among and
  under the dead leaves that they may be scraped up in great
  quantities.” (Report for 1879.)

The eggs of the lace-winged flies are supported on pedicels, above the
reach of ovivorous mites.

  The female Chrysopa usually lays between 40 and 50 eggs. In one
  case, we observed that 18 egg-stalks were deposited, but there were
  only nine well-formed eggs in the batch, and nine eggless stalks,
  some only half the usual height, others with the knob of cement at
  the end to which the egg is ordinarily fastened. The eggs are
  evidently stuck on to the end of the pedicel after the latter has
  been formed, as, in one instance, an egg was glued to the stalk very
  much out of centre, the insect’s abdomen not having been aimed
  straight, so to speak, at the mass of cement.

[Illustration:

  FIG. 487.—Eggs of Chrysopa, with larva and fly.
]

  The eggs of Rhodites are fixed to a long stalk thickened at the end;
  those of Inquilines and certain Chalcids (_Leucospis gigas_, Fig.
  489, _A_) are also stalked; and the use of this stalk in the eggs of
  Cynips (_E_) is thought by Adler to be respiratory, while, also, he
  states that the egg-cavity communicates with the egg-stalk, so that
  a part of the egg-contents can pass into the latter, and this
  happens at the laying of each egg. The egg of certain ichneumons
  (Paniscus, Fig. 488) ends in a short stalk, which is inserted in the
  skin of the caterpillar destined to serve as the host of the
  parasite, the eggs, as stated by De Geer, being retained more firmly
  in the integument by the stalk so swelling as to form two knobs
  (Fig. 498, _c_).

[Illustration:

  FIG. 488.—Young larva of Paniscus in position of feeding on the skin
    of a caterpillar: _a_, the egg-shell.—After Newport, from Sharp.
]

  Certain Homoptera also have stalked eggs, as those of _Psylla
  pyricola_ (Fig. 489, _B_), those of _Aleyrodes citri_ (_C_, _a_,
  _b_), and of an allied form, _Aleurodicus cocois_ (_D_), and those
  of Corixa (Fig. 493).

[Illustration:

  FIG. 489.—Stalked eggs: _A_, of a Chalcid (after Fabre); _B_, of
    Psylla (after Slingerland); _C_, of Aleyrodes; _D_, of Aleurodicus
    (after Riley and Howard); _E_, of _Dryophanta scutellaris_ (after
    Adler).
]

[Illustration:

  FIG. 490.—Eggs of ox bot-fly, enlarged.—After Riley.
]

  Reference should also be made to the eggs of lice, which are oval
  and attached to the hairs of their host. Those of the ox bot-fly
  (_Hypoderma lineata_) are usually placed four to six together, and
  fastened to a hair. The lower portion of the egg is admirably
  adapted for clasping a hair. “It consists of two lobes, forming a
  bulbous enlargement, which is attached to the egg by a broad, but
  rather thin, neck, so that, when the latter is viewed sidewise, it
  appears as a slender pedicel” (Fig. 490, _a-d_). (Riley in Insect
  Life, iv, p. 307.) The egg of another fly (_Drosophila ampelophila_,
  Fig. 491) bears a pair of long, slender appendages near the anterior
  end. “The egg is inserted into the soft pulp of the decaying fruit;
  these appendages leave the ovipositor last, and are spread out upon
  the surface of the mass. They, in this way, serve to keep the egg in
  place, and thus insure the emergence of the larva into the open air
  instead of into the more or less fluid mass in which the egg is
  situated. The larva issues from the egg just above the base of these
  appendages.” (Comstock.)

[Illustration:

  FIG. 491.—Egg of Drosophila.—After Comstock.
]

=Mode of deposition.=—The exact process of oviposition has been rarely
observed, or at least not observed in detail, and further observations
are much needed. In the cockroach (Phyllodromia), Wheeler has seen the
eggs pass out of the oviduct and become arranged in the oötheca, in a
way similar to that in the account published by Kadyi on Periplaneta.

[Illustration:

  FIG. 492.—Rocky Mountain locust (_aa_) depositing its eggs (_c_); _d_,
    the earth partially removed, showing (_e_) an egg-mass already in
    place, and (_d_) one being placed; _f_ shows where such a mass has
    been covered over. _A_, oviposition; _j_, position of oviduct; _g_,
    egg-guide; _e_, egg. _B_, egg-mass of the same; _a_, from side, _b_,
    from beneath, _c_, from above.—After Riley.
]

  “When about to form the capsule, the female Blatta closes the
  genital armature, and the two folds of the white membrane which
  lines the oöthecal cavity close vertically in the middle line. Then
  some of the contents of the colleterial glands are poured into the
  chamber, and bathe the inner surface of the posterior wall. The
  first egg glides down the vagina from the left ovary, describes an
  arc, still keeping its germarium-pole uppermost, after having
  pressed the micropylar area against the mouth of the spermatheca,
  passes to the right side of the back of the chamber, and is placed
  perpendicularly two-thirds to the right of the longitudinal axis of
  the insect’s body. The next egg comes from the right ovary,
  describes an arc to the opposite side of the body, decussating with
  the path of the first egg, and is placed completely on the left side
  of the median line. The third egg comes from the left ovary, and is
  made to lie completely on the right side of the median line; and so
  the process continues, the ovaries discharging the eggs alternately,
  and each egg describing an arc to the opposite side of the capsule.
  The oöthecal chamber soon becomes too small to contain all the
  constantly accumulating eggs, so the anal armature opens and allows
  the end of the capsule to project. A raised line, the impression of
  the edges of the white membrane, runs down the end of the capsule.
  The last egg deposited comes from the right ovary, and lies
  two-thirds on the left, and one-third to the right, of the median
  line. As soon as the egg is laid, a further discharge from the
  colleterial glands spreads over the vaginal or anterior wall of the
  cavity, and becomes evenly continuous with the secretion which has
  before been spread over the back and the sides of the capsule by the
  white membrane.

  “The crista, a cord-like ridge running the full length of the dorsal
  surface of the capsule, is a thick-walled tube, either half of which
  is formed by the edge of the side walls of the capsule split into
  two laminæ. The rhythmical clasping of the three pairs of palpi
  which guard the vaginal opening is registered in an exquisite
  pattern on the inner face of either half of the crista.”[78]

The mode of oviposition in the locust has been fully described by Riley,
who states that the eggs pass down and out of the oviduct, and “guided
by a little finger-like style” (Fig. 298), they pass in between the
horny valves of the ovipositor, and issue at their tips amid the mucous
fluid which forms the egg-capsule (Fig. 492).

=Vitality of eggs.=—It is well known that the eggs of phyllopod and
other fresh-water Crustacea have wonderful vitality, withstanding
extreme dryness for several years, at least from two to ten. Such cases
are unknown among insects. It has been observed, however, by T. W.
Brigham, and also by L. Trouvelot, that the eggs of the walking-stick
(_Diapheromera femorata_) for the most part hatch only after the
interval of two years.[79]

The eggs of Bittacus are said by Brauer to lie over unhatched for two
years; indeed, the first condition of their hatching is a complete
drying of the earth in which the eggs lie, the second is a succeeding
thorough wetting of the ground in spring.

_Appearance and structure of the ripe egg._—The eggs of insects are on
the whole rather large in proportion to the size of the parent,
especially so in many minute forms, as the fleas, lice, etc.

Their general shape is spherical or oval, often cylindrical; where the
eggs are long and cylindrical a dorsal and ventral side can be
distinguished (Fig. 502). They are in the Tortricidæ and Limacodid moths
flattened, thin, and scale-like. In the eggs of locusts and
grasshoppers, as well as certain Diptera, the ventral side of the embryo
corresponds to the convex side, and the concave side of the egg to the
dorsal region of the embryo (Figs. 502 and 493).

There is an anterior and posterior end or pole, the anterior end being
that which in the body of the parent lies towards her head, or towards
the upper or distal end of the ovarian tube. Towards this end lies in
the later stages of embryonic life the head-end of the embryo, while the
posterior end of the embryo is turned towards the hinder pole of the egg
(Figs. 493 and 520).

=The egg-shell and yolk-membrane.=—The ripe egg is protected by two
membranes: 1, an inner or _vitelline membrane_ or _oölemma_ (_dh_) (Fig.
500, _d_), produced in the egg by a hardening of the outer layer, and 2,
the outer or _chorion_ (_c_), which is secreted by the cells of the
ovarian follicle. The latter is divided into two layers: an inner, the
_endochorion_, and an outer, the _exochorion_.

[Illustration:

  FIG. 503.—Fertilization of the egg of a round-worm (_Ascaris
    megalocephala_): _A_, the ends (centrosomes) of the spindle formed.
    _B_, the spindle completed; _sp_, sperm-nucleus, with its
    chromosomes; _ei_, egg-nucleus; _p_, polar bodies.—After Boveri,
    from Field’s Hertwig.
]

[Illustration:

  FIG. 493.—Eggs of Corixa: _A_, early stage before formation of the
    embryo, from one side. _B_, the same viewed in the plane of
    symmetry. _C_, the embryo in its final position; _a_, anterior, _p_,
    posterior, end; _l_, left, _r_, right, _v_, ventral, _d_, dorsal,
    aspect. (The letters refer to the _final_ position of the embryo,
    which is nearly diametrically opposite to that in which it first
    develops); _m_, micropyle; _p_, pedicle.—After Metschnikoff, from
    Wilson.
]

[Illustration:

  FIG. 494.—Eggs of Phasmidæ: _A_, _Lonchodes duivenbodi_. _B_,
    _Platycrania edulis_. _C_, _Haplopus grayi_. _D_, _Phyllium
    siccifolium_.—After Kaup, from Sharp.
]

While the yolk-membrane is usually a completely homogeneous, thin,
structureless membrane, the chorion or shell of the egg is usually
covered with a network of ridges enclosing polygonal areas, varying in
shape according to the species or genus. These external markings are due
to the impress of the cellular structure of the epithelium of the
ovarian follicle.

In the chorion of the cockroach the surface appears to be finely
granular, the finest granules being arranged in large, more or less
regularly hexagonal areas, which are bounded by narrow, dark spaces,
containing somewhat larger though less dense granules. The surface of
the eggs of certain Phasmids are variously sculptured (Fig. 494).

  The true structure of the chorion can only be, as Wheeler observes,
  seen in cross-sections, as shown by Blochmann, and also by Wheeler.
  The chorion consists of two chitinous laminæ kept in close
  apposition by means of numerous minute trabeculæ or pillars. It is
  the ends of these pillars that look like granules. In the spaces
  between the hexagonal areas, the trabeculæ are more scattered and
  individually thicker than those of the hexagons.

[Illustration:

  FIG. 495.—Egg of cotton-worm moth, Aletia: _a_, top view, showing the
    micropyle.—After Comstock.
]

[Illustration:

  FIG. 496.—Egg of _Danais archippus_.—After Riley.
]

These markings are of singular beauty and complexity in the eggs of many
Lepidoptera, whose ova are variously ribbed, forming a beautiful
fretwork of raised lines (Figs. 495 and 496), while in the Diptera and
Hymenoptera the chorion is less solid, and usually smooth under low
powers. The exochorion of the egg of the house and meat fly (_C.
vomitoria_) is pitted with elongated hexagonal depressions, which cross
the egg transversely. That of the honey-bee is also divided into long
hexagonal areas (Fig. 497).

[Illustration:

  FIG. 497.—Egg with embryo of honey-bee, × 40: _ch_, chorion; _ga_,
    ganglia; _s. ga_, brain; _jm_, jaw-muscles forming; _c_, œsophageal
    collar; _fb_, fore intestine; _mb_, mid-intestine; _ab_,
    hind-intestine.—After Cheshire.
]

[Illustration:

  FIG. 498.—Micropyle (_Mk_) of eggs; _a_, of a fly, Antomyia; _b_,
    _Drosophila cellaris_; _c_, stalked egg of _Paniscus
    testaceus_.—After Leuckart, from Perrier.
]

When the eggs are deposited in exposed places, and remain in such
situations for several days, or weeks, or even through the winter, the
shell is either solid and strengthened by the ribs and ridges; or the
shell, if of winter eggs, is unornamented, and is dense and solid, to
resist extremes in temperature or the attacks of egg-eating birds,
mites, etc.

=The micropyle.=—This is an opening or canal, or, as in most insects, a
group of canals situated at the anterior end of the egg for the entrance
of the spermatozoa during the process of fertilization of the ovum (Fig.
498). In Acrydians, however, the micropyle is situated at the posterior
end of the egg. The micropyle (Fig. 499) is a complicated apparatus
within whose circumference the vitelline membrane appears to be firmly
attached to the chorion, so that the perforation passes through the
chorion as well as the yolk-membrane.

The micropyles of the cockroach are probably as simple and generalized
as in any insect. Wheeler states that they are in Phyllodromia scattered
over the end of the egg, “over a quadrant of the upper hemisphere, where
the beautiful hexagonal pattern of the chorion gives away to an even
trabeculation.” The micropyles are wide-mouthed, very oblique,
funnel-shaped canals, perforating the chorion, the apertures of the
funnels appearing under a low power as clear, oval spots, the long axis
of which is parallel to the long axis of the egg.

[Illustration:

  FIG. 499.—_a_, fragment of a micropylar papilla, showing its lumen;
    _b_, optical section of another papilla, in this one the lumen
    extends to the vitelline membrane, but does not pass beyond it; _c_,
    _d_, _e_, and _f_, papillæ of different forms. _A_, anterior end of
    an ovarian egg, showing mode of growth of the micropylar papillæ:
    _a_, _b_, two successive stages; _c_, surface view of modified
    papillæ from the lower edges of the cap; _d_, tunica propria of the
    ovariole; _e_, remnant of the cell-mass that secreted (?) the
    micropylar cap.—After Ayers.
]

[Illustration:

  FIG. 500.—Egg of _Perla maxima_: _c_, chorion; _d_, oölemma; _gs_,
    glass-like covering of micropyle; _l_, cavity under same; _g_,
    canals penetrating chorion.—After Imhof, from Sharp.
]

  “With a higher power the tube of each funnel is clearly visible as a
  thin canal which dilates rapidly into the large oval aperture on the
  outer face of the chorion. The narrow tube is sometimes fully as
  long as the large orifice. The micropylar perforations are all
  directed from the germarium to the vaginal pole of the egg. Hence a
  line, the hypothetical path of the spermatozoön, drawn through one
  of these oblique micropyles, and continued into the egg, would
  strike the equatorial plane. The female pronucleus, as we shall see
  further on, moves in this plane.” (Wheeler, p. 289.)

[Illustration:

  FIG. 501.—Micropyles: _a_, of _Nepa cinerea_; _b_, of _Locusta
    viridissima_; _c_, of a bug (_Pyrrhocoris apterus_).—From
    Gerstäcker.
]

The micropylar region is generally, at least in Orthoptera and Odonata,
covered by a gelatinous cap (Figs. 499 and 500, _gs_), which may form a
covering membrane which extends over a large part of the egg, or may
envelop the entire outer surface. In some cases micropyles are scattered
over the entire surface of the egg, but usually the perforation is
situated at the end, and is often guarded by raised processes, either
one or several, like bristles, or toadstools, etc., these being
especially characteristic of the eggs of certain Hemiptera (Nepa, Fig.
501, _a_, and Ranatra), or the region is variously sculptured, as in the
eggs of butterflies. In the micropylar apparatus of Œcanthus the papillæ
have a distinct lumen (Fig. 499), or a channel for the ingress of the
male filament.

[Illustration:

  FIG. 502.—Diagrammatic median section through egg of Musca in stage of
    fertilization (incorporating the figures of Henking and Blochmann):
    _ch_, chorion; _d_, dorsal; _v_, ventral side of the egg; _dh_,
    yolk-membrane; _do_, nutritive yolk; _g_, gelatinous cap over the
    micropyle (_m_); _K_, outer layer of plasma (Keimhautblastem); _p_,
    male and female pronucleus before copulation; _r_, directive body
    (Richtungskörper).—After Korschelt and Heider.
]

Another use of the micropylar apparatus noticed by Ayers in the egg of
the tree-cricket is that it “serves as a thick, roughened plate, against
which the insect may push when ovipositing, without injury to the egg,
and without danger that the ovipositor slips from its place.” In
Chrysopa eggs the micropyle forms a conspicuous button-like knob,
resembling the finely milled head of a certain kind of screw.

=Internal structure of the egg.=—The egg-contents are surrounded by an
outer layer of protoplasm or formative yolk, which is separate from the
inner parts of the egg (Fig. 502, _do_), the latter being mostly
composed of the nutritive yolk-element. The superficial protoplasmic
layer, called by Weismann _Keimhautblastem_ (_K_) is, in a few cases,
afterwards entirely lost, but in most instances forms a very thin layer
of clear protoplasm, slight in extent compared with the yolk-mass
within.

The eggs of insects are rich in yolk, only certain eggs, such as those
of the Aphides and the egg parasites (Proctotrypidæ) being poor in yolk.
The eggs of heterometabolous insects have been said by Brauer to contain
relatively more yolk than those of the Metabola, particularly the
Diptera; though, as Wheeler observes, this rule has some exceptions, the
eggs of the 17–year Cicada being very numerous and small.

  This he thinks is a greater advantage to the insect than the
  production of a few large eggs, “when we consider the extremely long
  period of larval life and the vicissitudes to which the larvæ may be
  subjected during all this time.” “Similarly, _Meloë angusticollis_
  produces a large number of very small eggs, while the eggs of the
  smaller beetles (Doryphora, _e.g._) are much larger. But Meloë is a
  parasitic form, and probably only a few of its many offspring ever
  succeed in gaining access to the egg of the bee.”

In the eggs of Chrysopa the yolk-granules are remarkably small, so that
the primitive band is in strong contrast to the yolk in color and
density. When crushed, the yolk does not flow out as a liquid, but in a
pasty mass, and we have questioned whether, as in the eggs of Limulus,
whose yolk is solid with fine granules, the denseness of the yolk is not
connected in the way of cause and effect with their exposed situation.

The central or yolk-mass (Fig. 502, _do_) consists chiefly of rounded
masses of yolk, with fat-globules, between which extends a fine network
of protoplasm.

The elements of the yolk are spherical and strongly refractive, by
pressure becoming polygonal structureless homogeneous bodies.

The germinal vesicle of the ripe insect-egg lies in the centre of the
yolk, where it appears as a large vesicle-like cell-nucleus containing a
few chromatin elements.


                 _b._ Maturation or ripening of the egg

Before the eggs of animals can be fertilized, they require in some
observed cases, and probably in animals in general, to undergo a series
of changes, which, as observed in the starfish, etc., consists in the
replacement of the germinal vesicle by a very much smaller egg-nucleus,
and also at the same time the construction at one pole of the egg of the
directive or polar bodies (Fig. 502, _r_). Towards the end of the
ripening process of the insect egg this vesicle, according to Blochmann,
passes to the dorsal surface of the egg, and is transformed into the
directive spindles (_Richtungspindel_).


                     _c._ Fertilization of the egg

The egg next requires the penetration and admission into the
yolk-interior of a spermatozoön.

This process is essentially in insects, as in other animals, the fusion
of the sperm-nucleus with the nucleus of the egg. Under normal
conditions but a single spermatozoön is required for fertilization. As
shown by Hertwig, in the sea-urchin, after the spermatozoön has
penetrated into the egg, the head, and the small rounded body, called a
_centrosome_, can still be recognized, but the tail becomes fused with
the yolk of the egg. In the protoplasm of the egg (called _cytoplasm_)
the achromatic end of the sperm-nucleus gives rise to conspicuous rays,
like those observed in ordinary cell-division. Preceded by these rays,
the sperm-nucleus or male pronucleus (Fig. 502, _p_) moves towards the
nucleus of the egg, and finally fuses with it, thus forming a new single
nucleus. This latter, which is called “the cleavage nucleus,” rapidly
forms a nuclear or “cleavage spindle” (Fig. 503). This act gives an
impulse to the cleavage of the egg, which is the first step in the
formation of the embryo. All these changes have yet to be worked out in
detail in insects by microscopic sections of the egg, whose generally
hard and opaque egg-shells present great obstacles to such work.


           _d._ Division and formation of the blastoderm[80]

In insects as in most other Arthropoda the segmentation of the yolk is
superficial and not total. The ovum is _centrolicithal_, _i.e._ the yolk
is concentrated at the centre of the egg, and surrounded by a peripheral
layer of transparent protoplasm (the _Keimhautblastem_).

[Illustration:

  FIG. 504.—Formation of the blastoderm of _Pieris cratægi_: _A_,
    longitudinal section through the egg, with two masses of protoplasm
    in the yolk. _B_, a blastoderm-cell at the upper end. _C_, a later
    stage, with more blastoderm-cells.—After Bobretsky.
]

The first step in segmentation is the movement of the first
division-nucleus (_i.e._ that in the fertilized egg arising from the
union of the sperm-nucleus with the female pronucleus) towards the
interior of the egg in order to multiply itself by the mode of indirect
nuclear division (Figs. 504, _A_, and 507).

[Illustration:

  FIG. 505.—Embryology of the mole-cricket: 1, egg in which the amœboid
    nuclei (_abc_) are moving toward the surface; 2, egg in which the
    nuclei (_abc_) have reached the surface, and show an active
    nucleus-formation; 3, the blastoderm-cells have no nucleus, and are
    placed at equal distances apart; 4, the blastoderm-cells now forming
    a continuous layer; 5, cross-section of the egg with blastodermic
    disk, also showing the disposition of the endodermal cells; 6,
    cross-section of the blastodermic disk, with the myoblast cells
    (_mb_) already formed; 7, cross-section through the thorax of the
    embryo, the body-cavity extended into the limbs.
]

                               LETTERING.

 _abc_,     amœboid blastodermic cells.

 _bc_,      blastoderm-cells.

 _bl_,      blastoderm.

 _en_,      endodermal cells.

 _M″_,      cavity of the myoblast.

 _mb_,      myoblast cells.

 _N_,       nerve-furrow.

 _P_,       primitive groove.

 _pd_,      primitive disk.

[Illustration:

  FIG. 505 _concluded_.—Later stages in the embryology of the
    mole-cricket: 8, longitudinal section of the embryo; the
    yolk-pyramids (_yp_) form a common inner yolk-mass (_y_). 9, section
    through the heart; _H_, cavity of the heart; the two halves of the
    heart-sinuses having united dorsally, ventrally they are still open
    and are bounded by the walls of the mesenteron. 10, cross-section of
    an embryo, showing the blood-lacunæ separated on the back by the
    dorsal organ (_do_); the intestinal fasciated layer
    (_Darmfaserblatt_) has not completely enclosed the yolk. 11, embryo
    completely segmented, with the rudiments of the appendages, labrum
    (_lab_), and nervous ganglia (_pc-ng_). 12, a more advanced embryo,
    showing the stomodæum (_st_) indicated as a frontal protuberance.
    13, section through the recently hatched larva, showing the cells of
    the mesenteron or chyle-stomach, and the cellular layer on the front
    surface, also the proventriculus or crop.
]

                               LETTERING.

 _ant_,     antenna.

 _ar_,      arterial sinus.

 _bl_,      blastoderm.

 _bla_,     abdominal vesicles.

 _cr_,      proventriculus, or crop.

 _dm_,      ventral diaphragm.

 _do_,      dorsal organ.

 _d pm_,    dorsal diaphragm.

 _ent_,     enteric layer.

 _f_,       fat-body.

 _g_,       ventral ganglion.

 _H_, _ht_, heart.

 _l_,       lacuna.

 _m_,       mouth.

 _md_,      mandible.

 _m.en_,    mesenteron.

 _mx′_,     1st maxilla.

 _mx″_,     labium, or 2d maxilla.

 _ml_,      leaf-like portion of mesenteron.

 _oe_,      œsophagus.

 _pc_,      procerebrum.

 _pm_,      proctodæum.

 _sg_,      subœsophageal ganglion.

 _sm_,      stomodæum.

 _tg_,      thoracic ganglion.

 _vm_,      ventral muscle.

 _y_,       yolk.

 _yp_,      yolk-pyramids.

 _I_,       1st pair of feet.

 _II_,      2d pair of feet.

 _III_,     3d pair of feet.

                           —After Korotneff.

The origin of numerous division-nuclei as the offspring of the first has
been observed to take place in the eggs of those insects (Aphides,
Cecidomyia, and Cynips) which have a slight amount of yolk. Yet in the
large, ordinary eggs of insects with an abundance of yolk there is no
doubt, say Korschelt and Heider, that these numerous division-nuclei,
which soon after the process of oviposition are scattered within the egg
between the yolk-spheres, and are enveloped by a star-shaped
protoplasmic layer, and which constitute the formative elements of the
blastoderm,—there is no doubt but that they have practically arisen
through indirect nuclear division from the first division-nucleus.

The process of formation of the blastoderm in ordinary eggs with
abundant yolk was first observed by Bobretsky in the eggs of a moth
(Porthesia) and Pieris, also by Graber, and more recently by Blochmann
in Musca, and by Heider in Hydrophilus.

In the earliest stage observed by Bobretsky there first appear after
fertilization a few (the smallest number four) cell-like, minute amœboid
masses of protoplasm, each with a distinct nucleus. A few (one at least)
of these bodies gradually pass out of the centre of the yolk to the
surface of the egg (Fig. 504, _A_, _n_), these becoming larger and
rounder, and from one or two of these nuclei (_B_, _bc_) the blastoderm
originates (_C_, _bl_). Those nuclei remaining in the yolk increase in
number and afterwards become the nuclei of rounded masses of
yolk-granules, forming the so-called yolk-spheres which Bobretsky
regards as true cells.

To the few blastoderm cells situated on the upper end of the egg are
added others which continue to pass from the yolk to the periphery, and
then the blastoderm spreads out farther and farther from the upper end
of the egg until finally it covers or envelops the whole yolk. This
layer of cells is called the _blastoderm_.

As to the origin of the primitive amœboid cells, Bobretsky is in doubt,
but is disposed to think that they are the result of the subdivision of
the germinative vesicle or nucleus of the ovarian egg-cell. In this
connection may be quoted the observations of Graber, who states that an
examination of the ovarian cell at an early period has revealed the
presence, in the centre of the yolk, of a number of amœboid cells, which
appear to have been formed by the division of the germinal vesicle.
These “primary embryonic cells” have a relatively large nucleus and a
number of nucleoli. Several may be seen to unite with one another by
means of their pseudopodia, and they may also be observed to undergo
division. With this account may be compared the results obtained by
Korotneff in his work on the embryology of the mole-cricket (Fig. 505).

[Illustration:

  FIG. 506.—Four successive stages in the formation of the blastoderm of
    _Calliphora vomitoria_ (the figures represent segments of
    cross-sections through the fly’s egg): _A_, the nuclei of the
    division-cells have arranged themselves parallel with the outer
    surface of the egg. _B_, the division-cells fused with the
    “keimhautblastem.” _C_, the outer surface becomes furrowed by
    indentations; all the nuclei of the blastoderm-cells in process of
    division. _D_, the blastoderm-cells form a high cylinder-epithelium:
    _b_, “keimhautblastem”; _bz_, blastoderm-cells; _d_, nutritive yolk;
    _dz_, yolk-cell; _fz_, so-called division-cell; _i_, inner
    “keimhautblastem.”—After Blochmann, from Korschelt and Heider.
]

The result of these and of later observations, especially those of
Blochmann on Musca, and those of Heider on Hydrophilus, show that the
division-nuclei lie near the centre of the egg, along the longitudinal
axis (Fig. 507, _A_). Each of these nuclei is enveloped by a star-shaped
mass of protoplasm, and on the whole resembles a wandering amœboid cell.
These isolated masses of protoplasm are all connected by a fine network
of rays, which unite to form within the yolk a syncytium. Afterwards, in
the later stages, these division-cells, as they may be, though somewhat
incorrectly, regarded, move nearer the periphery and arrange themselves
into a plane parallel with the surface (Figs. 506, _A_, 507, _B_).
Continuing to divide, they reach the surface and fuse with the
peripheral protoplasmic layer (Figs. 506, _B_, 507, _C_). Then follows
the division into single cell-territories (Figs. 506, _B_, 507, _C_),
corresponding to the division-nuclei, through the appearance of furrows
which pass in from the outer surfaces of the egg into the interior and
gradually penetrate the entire “keimhautblastem.” In this way the
surface of the egg is covered with an epithelium (blastoderm). In many
insects the so-called inner “keimhautblastem” (Fig. 506, _D_, _i_) is
formed by the separation of a layer of protoplasm which contains larger
granules and are accumulated between the blastoderm and the upper
surface of the central nutritive yolk-mass. By the addition of this
plasmic layer the cells of the blastoderm increase in height, and now
form a cubical or cylinder epithelium, which continuously envelops the
surface of the egg. (Korschelt and Heider.)


    _e._ Formation of the first rudiments of the embryo, and of the
                          embryonic membranes

[Illustration:

  FIG. 507.—Formation of the blastoderm in Hydrophilus: _b_, completed
    blastoderm; _d_, yolk; _f_, so-called division-cells; _k_,
    “keimhautblastem”; _z_, yolk-cells.—After Heider, from Korschelt and
    Heider.
]

The embryo first arises as a whitish streak or band-like thickening on
the ventral side of the egg, and is variously called the “primitive
streak,” “primitive band,” “germinal band,” or “embryonal streak.” In
most cases the primitive band is divided at regular intervals by
transverse furrows, indicating the limits of what are to be the body
segments.

Cross-sections (Fig. 509) show that the band is composed of several
layers, _i.e._ an outer layer (ectoderm) and an inner layer which
comprises the endoderm and mesoderm, and so long as these two layers are
not sharply differentiated from one another, this second layer may be
called, with Kowalevsky, “the inner lower layer, or ento-mesoderm”
(Figs. 508, 509, _B_, _C_, _u_).

It is characteristic of insects, only rarely occurring in other
arthropods (_e.g._ the scorpion), that the primitive streak is not
situated on the surface of the egg, but becomes overgrown by a folded
structure (Fig. 508, _af_) rising from its edges, the amnion-fold, so
that it appears somewhat depressed or sunken in under the upper surface
of the yolk. While the amnion-folds are extending from all sides over
the primitive band, there becomes formed under it, by the invagination
of the outer surface of the egg, a cavity, the amnion-cavity (_ah_),
which, when the amnion-fold has completely overgrown the primitive band
and united together (Fig. 509, _C_), appears completely closed from
without.

[Illustration:

  FIG. 508.—Two schematic median sections through an insect-embryo to
    represent the development of the embryonal membranes. In _A_ the
    primitive streak is not wholly overgrown by the amnion-fold. In _B_
    the amnion-folds have united with each other and completely
    overgrown the primitive streak: _a_, fore, _b_, hind, egg-pole; _v_,
    ventral side; _d_, dorsal side; _af_, amnion-folds; _ah_,
    amnion-cavity; _am_, amnion; _do_, yolk; _ec_, ectoderm; _k_,
    head-end, _k′_, hinder-end, of the primitive streak; _s_, the part
    of the serosa arising from the amnion-fold; _s′_, the part of the
    serosa arising from the unaltered blastoderm; _u_, lower
    layer.—After Korschelt and Heider.
]

=Formation of the embryonic membranes.=—The amnion-folds finally
completely overgrow the primitive band (Fig. 509, _B_ and _C_), and form
the embryonal membranes. The primitive band is seen after its completion
to be overgrown by a double cellular epithelial membrane. The outer of
these two membranes, that which arises from the outer leaf or layer of
the amnion-fold, is the _serosa_ (Figs. 508, _B_; 509, _C_, _s_; 510).
This passes continuously into the unchanged part of the blastoderm,
which has no part in the formation of the primitive band and
germ-layers, and which covers the outer surface of the yolk. Thus the
_serosa_, which is usually held to include this portion also of the
blastoderm, forms a closed sac which covers the whole surface of the
egg, with one part extending over the surface of the yolk, and the other
over the primitive band (Fig. 510).

[Illustration:

  FIG. 509.—Diagrammatic cross-section through three successive stages
    of the primitive streak, and growing embryonal membranes of
    insect-embryos. _A_, formation of the ventral plate and of the
    gastrula invagination (_g_). _B_, upward growth of the amnion-folds
    (_af_). _C_, complete overgrowth of the primitive band through the
    amnion-folds: _v_, ventral side; _d_, dorsal side; _af_,
    amnion-folds; _ah_, amnion-cavity; _am_, amnion; _bl_, blastoderm;
    _bp_, ventral plate; _do_, yolk; _ec_, ectoderm; _s_, serosa; _u_,
    under or inner layer.—After Korschelt and Heider.
]

The inner of the two layers, called the _amnion_ (Fig. 509, _am_), is
more closely connected with the embryo. The amnion and ectoderm of the
primitive band together form a completely closed sac, whose lumen forms
the amniotic cavity. Originally connected with the serous membrane, it
splits off from the primitive band about the time the appendages begin
to bud out, and continues to closely envelop the body and appendages, as
seen in Fig. 509. Both of these membranes are, before the time of
hatching, either absorbed, or, as in Lepidoptera, retained. The amnion
is retained until after hatching in the locust, etc. In certain
Coleoptera the serosa is retained, and the amnion is absorbed (Fig.
532), while in Chironomus and the Trichoptera the serosa is absorbed,
and the amnion retained, with the egg-shell or chorion. Hence we have
eight layers in the winged insects[81] during embryonic life:

[Illustration:

  FIG. 510.—Surface view of fresh serosa from an Œcanthus, treated with
    acetic carmine; the blastoderm completely formed, × 500: _p_, polar
    body; _rf_, radiating fibres; _nls_, nuclear substance; _nlm_,
    nuclear membrane.—After Ayers.
]

     1. Exochorion. (Remains of the epithelium of the ovarian follicle.)
       2. Chorion. (Egg-shell or cuticle secreted in the ovarian
       follicle.) 3. Vitelline membrane. (Primary egg-membrane.
       Yolk-skin or membrane.) 4. Serous or outer germ-membrane.
       (Serosa.) } 5. Amnion or inner germ-membrane. } Derived from the
       blastoderm. 6. Ectoderm. } } 7. Mesoderm. } Embryo. } 8.
       Endoderm. } }

  In the embryo of Xiphidium and Orchelimum Wheeler has found and
  described with much detail a membranous structure which he calls the
  _indusium_. “The organ,” he says, “appears to have been retained by
  the Locustidæ, and completely lost by the embryos of other winged
  insects.” It arises in Xiphidium, as a simple circular thickening of
  the blastoderm, between and a little in front of the procephalic
  lobes (Figs. 511, 512, _A-E_), and afterwards spreads over nearly
  the whole surface of the egg, leaving the poles uncovered, as in
  Fig. 513, where it is divided into two further membranes, the inner
  and outer indusium, the former lying in contact with the amnion.
  After this the serosa “is excluded from taking any part in the
  development of the embryo; both its position and function are now
  usurped by the inner indusium.”

  Hence in an egg of the Locustidæ Wheeler distinguishes, passing from
  within outward in a median transverse section of the egg, the
  following envelopes:

      1. The chorion. 2. The blastoderm-skin-like cuticle secreted by
        the serosa. 3. The serosa. 4. The outer indusium. 5. A layer
        of dark granular secretion (probably some urate). 6. The
        cuticle secreted by the inner indusium. 7. The inner indusium.
        8. The amnion. While envelopes 1–7 invest the whole egg; layer
        8, the amnion, covers only the embryo.

[Illustration:

  FIG. 511.—Diagrams illustrating the movements and envelopes of the
    embryo of Xiphidium: _A_, after the closure of the amnioserosal
    folds. _B_, during the embryo’s passage to the dorsal surface.
    _C_, just after the straightening of the embryo on the dorsal
    surface; _ind_, indusium afterwards forming _ind^1_, the inner,
    and _ind^2_, the outer indusium; _ch_, chorion; _sr_, serosa;
    _am_, amnion; _gb_, germ-band; _v_, yolk; _bl. c_, blastoderm
    membrane.
]

  Wheeler further suggests that the so-called micropyle of the
  Collembola (Anurida), which has been homologized with the “dorsal
  organ” of Crustacea, is a possible homologue of the indusium, as
  also the “primitive cumulus” of spiders, and the “facette” or
  “cervical cross” of Pentastomids described by Leuckart and also by
  Stiles.

=The gastrula stage.=—The primitive band invaginates so as to give the
opportunity for the formation of the inner layer. This invagination,
which at a certain stage is established along the whole length of the
primitive band, forms a median furrow and may be regarded as the
gastrula-invagination of insects. The lower (inner) layer thus arising
afterwards spreads out under the entire primitive band (Fig. 509, _B_
and _C_), the edges of which become bordered by the growing amnion-fold.
(Korschelt and Heider.)

  In certain forms the primitive band arises from several separate
  rudiments which afterwards unite. Thus in Musca and Hydrophilus the
  anterior and posterior ends develop first, and in Hydrophilus the
  procephalic lobes originate independently of the rest of the band.
  In the Aphides, also, according to Will, these lobes arise
  independently, afterwards uniting with the primitive band.

[Illustration:

  FIG. 512.—Diagrams illustrating the movements and envelopes of the
    embryo of Xiphidium: _D_, the stage of the shortened embryo on the
    the dorsal yolk. _E_, embryo returning to the ventral surface. _F_,
    embryo nearly ready to hatch; _ch_, chorion; _b. lc_, blastoderm
    membrane; _sr_, serosa; _ind^1_, outer indusium; _ind^2_, inner
    indusium; _ind^2 + am_, inner indusium and amnion fused; _am_,
    amnion; _ind^1 c_, cuticle of the inner indusium; _ind^2 s_,
    granular secretion of the inner indusium; _am. s_, amniotic
    secretion; _v_, yolk; _cl_, columella; _gb_, primitive band.
]

=Division of the embryo or primitive band into body-segments.=—Meanwhile
the primitive band grows at the expense of the yolk, spreading out more
and more over its surface, until in certain cases (Coleoptera, Diptera,
Siphonaptera, and Trichoptera) it lies like a broad ribbon over the
yolk, so that the two ends nearly meet on the dorsal side. By this time
it becomes divided by transversely impressed lines into segments, which
correspond to those of the larva and adult. The first of these segments
is divided into two broad and flaring flaps, which are called the
procephalic lobes. It becomes the antennal segment.

[Illustration:

  FIG. 513.—Two stages in the spreading of the indusium. _A_, lateral
    view of egg just after the arrival of the embryo on the dorsal yolk.
    _B_, lateral view of the egg with the indusium nearly reaching the
    poles. _C_, same egg seen from the dorsal surface.
]

The mouth (_stomodæum_) now develops, and is situated at the
anterior,[82] and the rectum (_proctodæum_,) at the posterior pole, or
end of the primitive band.

[Illustration:

  FIG. 514.—Median section of the egg of _Anurida maritima_: _do_,
    “micropyle”; _bld_, blastoderm.—This and Figs. 511–513, after
    Wheeler.
]

In Blatta, Hydrophilus, the Trichoptera, and the Lepidoptera the
hindermost part of the primitive band is turned in ventrally (Figs. 534,
_C_).

  The preceding account of the relations of the primitive band to the
  yolk does not apply to all insects, since there are variations which
  appear to depend on the form of the egg, and on the amount and
  distribution of the yolk-masses. In certain Coleoptera, the
  primitive band sinks down and thus becomes immersed into the yolk.
  In Donacia (Kölliker and Melnikow) and Hydrophilus (Heider), and in
  the Chrysomelidæ and Attelabus, a weevil, as we have observed, the
  primitive band rests on the outside of the yolk, but in _Telephorus
  fraxini_ it is immersed. In the Hemiptera it is immersed (Fig. 516),
  but there is much variation in this respect, the degree of immersion
  being most marked in the Coccidæ (Aspidiotus), and least so in
  Corixa. Besides the position of the primitive band, there are in
  Odonata and Hemiptera differences in the origin of the primitive
  band itself and of the embryonic membranes.

[Illustration:

  FIG. 515.—Ventral view of five developmental stages of Hydrophilus:
    _a_ and _b_, places at which the blastopore contracts; _af_, edge
    of the amnion-fold; _af′_, caudal fold; _af″_, paired head-fold of
    the amnion; _an_, antenna; _es_, last segment; _g_, pit-like
    invagination (first indication of the amniotic cavity); _k_,
    head-lobes; _r_, furrow-like invagination; _s_, portion of the
    primitive streak covered by the amnion.—After Heider, from Lang.
]

[Illustration:

  FIG. 516.—Embryo of the louse: _am_, serosa; _db_, amnion; _as_,
    antenna; _vk_, clypeus.—After Melnikow.
]

  Korschelt and Heider divide the early embryo of insects into two
  types:

  1. Into those with a superficial primitive band; viz., where there
  is no passage of yolk-elements into the space between the amnion and
  serosa. The primitive band has in such cases a relatively
  superficial position (Figs. 508, 509, 521, 535). Examples are
  certain Orthoptera (Blatta, Œcanthus, Mantis, Gryllotalpa), also
  certain Hemiptera (Corixa), certain Coleoptera, and the Trichoptera,
  Diptera, and Hymenoptera.

  2. Into those with an immersed primitive band, with the space
  between the serosa and amnion filled with yolk (Figs. 517, 518,
  534). Examples are the orthopterous Stenobothrus, Odonata, many
  Hemiptera (the Pediculina and Pyrrhocoris), the Coleoptera already
  mentioned, and Lepidoptera.

  It should be observed, however, that these differences are of little
  phylogenetic or taxonomic value, since genera of the same order,
  notably the Coleoptera, differ as to the position of the primitive
  band, so also two orders so nearly allied as the Trichoptera and
  Lepidoptera.

  =Differences between the invaginated and overgrown primitive
  band.=—In respect to the mode of origin of the primitive band and
  its relative position, there are two opposite types, though
  connected by transitional forms. In the one case the primitive band,
  _i.e._ its ventral portion, the “ventral plate” (Fig. 518, _b_, _p_)
  is pushed in or invaginated in the interior of the egg; in the other
  case it becomes overgrown by the folds of the amnion arising from
  its edges.

[Illustration:

  FIG. 517.—Primitive streak of a lepidopter in cross-section: _ah_,
    amniotic cavity; _am_, amnion; _c_, cœlomic cavity; _do_,
    nutritive yolk, divided into single nucleated masses; _ec_,
    ectoderm; _m_, mesoderm; _pr_, primitive thickenings of the
    ventral nervous cord; _s_, serosa.—Combined figure after those of
    Brobretsky and Hatschek, from Korschelt and Heider.
]

[Illustration:

  FIG. 518.—Five diagrammatic median sections representing the growth
    of a dragon-fly (Calopteryx). _A-C_, development of the primitive
    streak (_k_, _k′_) by invagination. _D_, the amnion-fold (_af_),
    growing over the head-end of the primitive streak. _E_, closing of
    the opening of the amnion-cavity (_ah_): _v_, ventral, _d_, dorsal
    side; _a_, fore, _b_, hind end of egg; _bl_, blastoderm; _bp_,
    ventral plate; _do_, yolk; _k_, head-end, _k′_, caudal end, of the
    primitive streak; _kh_, germinal thickening or initial point of
    invagination; _s_, serosa.—After Brandt, from Korschelt and
    Heider.
]

  In insects with an overgrown primitive band, the band at the
  beginning is generally short and always situated on the ventral side
  of the egg, with the head-end looking forward, and remains in this
  position throughout embryonic life. There is no revolution of the
  embryo. The embryonal membranes arise through the formation of
  folds.

[Illustration:

  FIG. 519.—Three embryonic stages of Calopteryx: _am_, amnion; _g_,
    edge of the ventral plate; _ps_, germ of primitive band; _se_,
    serosa.—After Brandt, from Balfour.
]

[Illustration:

  FIG. 520.—Three farther stages of growth of Calopteryx. _B_ and _C_
    show the inversion of the embryo: _a_, opening of the
    amniotic-cavity, out of which the embryo emerges; _ab_, abdomen;
    _am_, amnion; _at_, antenna; _md_, mandible; _mx^1_, _mx^2_, 1st
    and 2d maxillæ; _œ_, œsophagus; _p^1_, _p^2_, _p^3_, legs; _se_,
    serosa; _v_, anterior end of the primitive streak.—After Brandt,
    from Balfour.
]

  In insects with an invaginated primitive band, of which the Odonata
  afford examples, the first rudiment of the primitive band is in the
  form of a ventral plate of slight extent passing ventrally in the
  hinder half of the egg, in whose posterior section a process of
  invagination (Fig. 518, _A_, _kh_), soon occurs. The cavity of this
  invagination is the first indication of the amnion-cavity (Fig. 518,
  _B_, _ah_), while its wall in its thickened ventral part (_K′_) is
  concerned in the formation of the primitive band, and, in its dorsal
  thin part, in the formation of the amnion (_B_, _C_, _am_).

  =Revolution of the embryo where the primitive band is
  invaginated.=—At first the head-end of the embryo is directed
  towards the posterior end of the egg, as in dragon-flies (Fig. 518).
  Also that surface of the primitive band which afterwards faces the
  ventral, is at first turned towards the dorsal side of the egg. In
  order to bring the primitive band into the later relations, there
  must occur the process of revolution, or turning, of the embryo. The
  somewhat advanced embryo of the Odonata, after the appearance of the
  head and thoracic appendages, undergoes a rotating motion around its
  transverse axis, and at the same time turns out of the amniotic
  cavity (Fig. 520, _B_). This process is so managed that near the
  head-region, the amnion and serosa, there closely situated to each
  other, are fused together, and at this place tear or burst open.
  Through this rent (_a_), in the same place in which the original
  invagination-opening was situated, the amniotic cavity again opens,
  and through the opening thus formed first the head and then the
  succeeding segments of the primitive band (Fig. 520, _B_) pass out,
  and remain there while the head passes on to the anterior pole of
  the egg on the ventral side, the embryo thus assuming a position
  like that of other insects. (Kowalevsky.)

  In the parasitic Hemiptera (Pediculina), according to Melnikow, the
  opening in the membranes near the head remains permanent, and the
  embryo becomes everted through it, while the yolk, enclosed in the
  continuous membrane formed by the amnion and serous membrane, forms
  a yolk-sac on the dorsal surface. The same process occurs in
  Mallophaga, and also in Œcanthus, as described by Ayers (Fig. 521).
  Generally as soon as the embryo passes out of the amniotic cavity
  the latter soon becomes smaller and finally completely disappears.

[Illustration:

  FIG. 521.—Revolution of the embryo of Œcanthus (diagrammatic): _a_,
    fore, _b_, hind end of egg; _am_, amnion; _d_, dorsal, _v_,
    ventral side of egg; _k_, primitive streak; _r_, dorsal plate
    (originating by the contraction of the serosa (_s_)).—After Ayers,
    from Korschelt and Heider.
]

  As the embryo grows, and the sides grow up and the back closes over,
  the contents of the yolk-sac are soon taken up and absorbed in the
  intestinal cavity, which communicates with it.

  In Phyllodromia, according to Wheeler, the process of revolution is
  “hurried through by the embryo from the beginning of the 16th to the
  end of the 17th day.” Several successive stages are represented in
  Fig. 522. In the 15th day the embryo still occupies the middle of
  the ventral surface of the egg. Soon the envelopes (amnion and
  serosa, _as_) rupture, an irregular slit being formed, and soon the
  egg and embryo are as seen in Fig. 522, _B_, the embryo standing out
  free from its envelopes on the yolk, and the edges of its dorsal
  growing walls (_b_) are distinctly marked. The tail now lies at the
  caudal end of the egg (Fig. 522, _C_). By the 17th day the walls
  have closed in the median dorsal line, and the embryo has grown in
  length to such an extent as to bring its head to the cephalic pole
  (Fig. 522, _E_).

  Korschelt and Heider consider, since the primitive band of the
  chilopod myriopods (Geophilus) is curved in at the middle and sinks
  into the interior of the yolk, that in insects the invaginated
  primitive band is the ancestral or primitive one, the overgrown
  primitive band being derived from it. The overgrown primitive band
  by its position may also be better insured against certain
  mechanical attacks, perhaps also against the danger of drying up.

[Illustration:

  FIG. 522.—Embryo of Phyllodromia, 15 days old; revolution about to
    begin. The stages in revolution are represented, after the rupture
    of the amnion and serosa, in _A_ to _E_, which are from embryos 16,
    16½, 16¾, and 17 days old respectively: _as_, amnion and serosa;
    _s_, edge of serosa; _b_, dorsal growing body-wall; _d.o_, dorsal
    organ; _x_, clear zone covered with scattered amniotic nuclei.—After
    Wheeler.
]


            _f._ Formation of the external form of the body

=Origin of the body-segments.=—As we have seen, the first traces of
segments appear very early, the primitive band being divided by
superficial transverse furrows into segments. This segmentation into
arthromeres (somites or metameres) can be observed in Hydrophilus and
Chalicodoma at a time when gastrulation begins (Figs. 515, 536). The
segmentation extends not only across the median portion of the primitive
band, through whose invagination the inner layer (endomesoderm) results,
but also to the lateral portions which become a part of the ectoderm of
the primitive band. These transverse furrows correspond to thinner
places in the epithelium, which in this stage forms the embryonal
rudiment. It thus happens that, in the forms named, after the end of
gastrulation not only the ectoderm, but also the endomesoderm, is
already segmented.

  So early an appearance of segmentation as that observed in
  Hydrophilus and Chalicodoma we must regard as a falsification of the
  process of development due to heterochrony. We must consider the
  conditions observed in other forms as the primitive ones, in which
  (as, for example, in Lina and in Stenobothrus, according to Graber)
  the gastrulation and separation of the ectoderm occurs in the still
  unsegmented primitive band, the division into segments occurring in
  later stages (Fig. 524). In these forms, then, the segmentation
  affects the invaginated endomesoderm, as well as the ectoderm.
  (Korschelt and Heider, p. 789.)

[Illustration:

  FIG. 523.—Diagrammatic cross-section through three successive
    stages of Gryllotalpa, showing the formation of the heart.
    (Compare Fig. 505.) The germs of the glandular intestinal layer
    (_darmdrüsenblatt_) are omitted. _A_, earliest stage; the
    primitive streak extends from _*x_ to _y*_. The embryonal
    membranes are torn and pressed against the back: _am_, edge of
    the rent; _rp_, dorsal plate (serosa); _l_, lamella (amnion
    turned up) standing in connection with the ectoderm of the
    primitive streak. _B_, second stage; the primitive streak has
    completely grown around the yolk; the dorsal organ is absorbed.
    _C_, third stage, dorsal portion; the formation of the heart is
    finished: _am_, vestige of the amnion-fold; _bs_, blood-sinus;
    _dd_, rudiment of the dorsal diaphragm; _dv_, ventral diaphragm
    (compare Fig. 505); _do_, yolk; _dz_, yolk-cells; _ec_,
    ectoderm; _gr_, vascular groove (rudiment of the heart); _l_,
    lamella of the upturned amnion; _lh_, definite body-cavity; _m_,
    transverse muscle; _n_, nervous cord; _r_, heart; _rp_, dorsal
    plate; _sp_, splanchnic; _so_, somatic layer of the mesoderm;
    _us_, primitive segmental cavity; _*x_, _y*_, lateral
    terminations of the primitive streak.—After Korotneff, from
    Korschelt and Heider.
]

In the completely segmented primitive band may be distinguished two
regions of a peculiar appearance (Figs. 515, 527), one at the anterior,
and the other at the hinder end. The anterior, the primary head-section,
contains the mouth-opening, and is characterized by its lateral
expansions, or procephalic lobes. The other section, or posterior
section, the so-called anal segment or telson, contains the anus.
Between the two sections lies the segmented primary trunk-segment, which
in insects consists of 17 segments. Of these the three most anterior are
those destined to bear the mandibles and two pairs of maxillæ; the three
following are the thoracic, which are succeeded by 10 abdominal
segments, besides the 11th or telson (pygidium, or suranal plate).

  It is now generally believed that there are primarily eleven
  abdominal segments, while Heymons has detected twelve in the embryos
  of Blattids and Forficula (see p. 162). In the later stages of
  embryonic development the number of abdominal segments is
  diminished, the 10th and 11th abdominal segments becoming fused. In
  Hydrophilus and Lina this is the case, but according to Graber, in
  the Lepidoptera there is a fusion of the 9th and 10th abdominal
  segments, the llth remaining free.

  According to Wheeler, in Doryphora, and also in Chalicodoma
  (Carrière), between the primary head-region and the mandibular
  segment is interpolated a rudimentary and transitory body-segment,
  the premandibular segment. According to Carrière this segment
  corresponds to a rudimentary pair of limbs, and also to a ganglion,
  which participates in the formation of the œsophageal commissure
  (see p. 51).

[Illustration:

  FIG. 524.—Three embryonic stages of a leaf-beetle (Lina): _A_,
    unsegmented primitive streak; in _B_ and _C_ the segmentation
    becomes distinct on the lower layer (_u_). _B_, with the germs of
    the gnathal segments (_k′-k‴_), and in _C_ the three thoracic
    segments (_t’-t‴_), with the first two abdominal segments (_a_′,
    _a″_): _bl_, blastopore; _kl_, head-lobes; _th_, extension of the
    primitive streak into the thoracic region.—After Graber, from
    Korschelt and Heider.
]

=The procephalic lobes.=—The head-lobes, or procephalic lobes, appear at
a very early period (Fig. 524, _kl_), before any traces of the segments
of the trunk region. Ayers has shown that in Œcanthus the primitive
band, in its earliest condition and before the appearance of the
head-lobes, is a simple oval plate or almond-shaped thickening near the
posterior end of the egg (Fig. 525, 1, 2). This plate is “soon divided
into two tolerably well-marked regions by the enlargement of the
head-end,” the first indication of the head-lobes (3). A depression next
forms in what is to be the middle of the forehead. “It indicates the
position of the future labrum, and forms the inner boundaries of the two
cephalic ganglia, which are developed on either side of this depression
at a much later stage.” Almost simultaneously with the appearance of
this depression, two lateral folds are formed in the trunk portion of
the band, which are the first indications of the maxillary and thoracic
regions, the abdominal portion not yet showing traces of a division into
segments (Fig. 525, 5). The thickened outer edges of the head-fold next
gradually grow in towards the median line (Fig. 525, 5), and bend
forward towards the region of the future mouth. The rounded angle made
by the posterior end of the head-fold is the first indication of the
antennæ. The embryo is now composed of four well-marked regions:
cephalic, maxillary, thoracic, and abdominal. The primitive band then
grows much longer, the primitive mouth and anus appear, and the
appendages bud out, and eventually the embryo revolves and appears on
the ventral side of the egg (Fig. 525, 6).

[Illustration:

  FIG. 525.—Early stages in the embryology of _Œcanthus niveus_. Fig. 1,
    the youngest observed primitive band, the serosa not yet formed; 2,
    longitudinal optic section (diagrammatic) of Fig. 1; 3, the
    primitive band after the appearance of the head-fold, which is
    indicated at this time by the more rapid growth and consequent
    greater breadth of the lower end of embryo, x 25; 4, a young embryo
    after the appearance of the primitive segment-folds, x 50; 5, a more
    advanced embryo, with the antennal folds distinctly marked off; the
    free ends of the primitive folds have united across the embryo
    posterior to the antennal folds, x 50; 6, ventral view of the embryo
    with the appendages budding out, x 25 (the embryo in this stage lies
    dormant through the six colder months of the year): _am_, amnion;
    _m_, micropylar end; _ch_, chorion; _gb_, primitive band; _bf_,
    brain-fold; _yl_, yolk; _tf_, caudal fold; _kf_, head-fold
    (procephalic lobe); _p.fd.t_, primitive thoracic fold; _p.fd.m_,
    primitive maxillary fold; _p.fd.a_, primitive abdominal fold;
    _ab.c_, abdominal constriction; _t.c_, thoracic constriction;
    _at.l_, antennal lobe; _M_, mesoderm; _h.g_, head groove; _mo_,
    mouth; _sk_ invagination of ectoderm to form head-apodeme; _md_,
    rudiment of mandible; _m^1_, 1st, _m^2_, 2d maxilla; _T^1–T^5_,
    legs: _ab.p_, 1st abdominal appendage; _ap_, other appendages; _tb_,
    caudal expansion; _mf_ median furrow; _B_, primitive unpaired organ
    (metastomum).—After Ayers.
]

  These primitive regions of the primitive band, before the segments
  are formed, are called by Graber _macrosomites_, and the secondary
  segments into which they divide (which afterwards become the
  body-segments), _microsomites_. The macrosomites are peculiar to
  insects, and may be an inheritance from a hypothetical ancestral
  form. With Korschelt and Heider, we should hardly share this view.

[Illustration:

  FIG. 526.—Older embryo of Œcanthus with the appendages budded out,
    those of the abdomen distinct: _abp_, first pair; _a.s_, anal
    stylet; _pr_, proctodæum; _am_, amnion.—After Ayers.
]

Our observations on locusts show clearly (1) that the procephalic lobes
are the pleural portion of the first cephalic or antennal segment; (2)
that the antenna is an appendage or outgrowth of the procephalic lobes;
(3) that the eyes are a specialized group of epidermal cells of the
upper part of the procephalic lobes, and are not homologues of the
antennæ or of the appendages in general; and (4) it seems to follow from
a study of the relations and mode of development of the clypeus and
labrum, that they arise between the procephalic lobes, and probably
represent the tergal part of the antennal segment, forming the roof of
the mouth, _i.e._ closing in from above the pharynx.

In general the formation of the body-segments into the primitive band is
in succession from before to the hinder end. This successive appearance
has been observed by Graber in genera of different orders (Stenobothrus,
Lina, and Hylotoma). For example, in the beetle Lina, after the
appearance of the mandibular and two maxillary segments, appear the
three thoracic segments, together with the two anterior abdominal
segments, the other abdominal segments arising afterwards. In other
cases, the formation of segments seems to be simultaneous along the
entire length of the primitive band. An exception to the rule has been
noticed by Heider in Hydrophilus, as in this beetle the development of
the segments of the middle region appears somewhat delayed, while the
fore and hind parts of the primitive band are more rapid in development.
In Pieris, according to Graber, the thoracic segments are more rapidly
developed than the others; soon after, the gnathic segments (mandibles
and two pairs of maxillæ) appear, and finally the abdominal segments are
formed.

=Fore-intestine (stomodæum) and hind-intestine (proctodæum),
Labrum.=—The digestive canal of insects consists, as in other animals,
of three portions, the fore, mid, and hind gut or intestine. The next
change after the completion of the segments of the primitive band is the
development of the fore and hind intestine and the appendages. The
fore-intestine (stomodæum) arises as an invagination in the area of the
primary head-section, and the hind-intestine (proctodæum) in the
terminal section (Figs. 300 and 546).

  In insects generally the formation of the fore-intestine occurs
  earlier than that of the hind-intestine. An exception was discovered
  by Graber and also by Voeltzkow in Muscidæ, where the proctodæum
  appears earlier.

[Illustration:

  FIG. 527.—Rudiments of the appendages of the embryo of Hydrophilus:
    _an_, antenna; _md_, mandibles; _mx_{1}_, 1st, _mx_{2}_, 2d maxilla;
    _vk_, clypeal region; _m_, mouth; _p^1-p^3_, legs; _p^4-p^9_,
    rudiments of abdominal appendages, 1–9; _st_, stigma; _a_,
    anus.—After Heider, from Lang.
]

Usually at the time of origin of the stomodæum a projection arises at
the anterior edge of the primary head-region, the so-called forehead
(Fig. 527, _vk_), which is the common rudiment of the clypeus and
labrum. In many cases (certain Coleoptera and Lepidoptera) these
rudiments first assume the form of paired hooks (see Figs. 83, 102, 104,
105, of Graber’s Keimstreif der Insekten, also Figs. 529 and 546), which
afterwards, by fusion in the median line, become single, though notched
in the middle; but in the more generalized Blatta and Mantis, as well as
in bees, the rudiment is single at the outset.

  The view advanced by Patten, and also by Carrière, that the labrum
  is a first pair of antennæ, is scarcely tenable, and we quite agree
  with Korschelt and Heider in regarding the clypeo-labral region as
  homologous with the upper lip of Crustacea, and, we may add, of
  Merostomes and of Trilobites.

  It should be observed that in many insects, in their earlier
  embryonic state, directly behind the mouth arises, from paired
  rudiments, what seem provisional lower lip structures (not to be
  confounded with the 2d maxillæ of insects). This under lip structure
  was first discovered by Bütschli in the bee (his inner or 2d
  antennæ), and afterwards by Tichomiroff in Lepidoptera. Heider, in
  his work on Hydrophilus, describes it as the “lateral mouth-lips,”
  while, more recently, Nusbaum has observed it in Meloë. This under
  lip structure may be regarded as analogous to the paragnaths of
  Crustacea, although to attempt to homologize it with these seems
  useless. (Korschelt and Heider.)

=Completion of the head.=—Sufficient attention has not been paid to this
subject by embryologists. The head is at first, dorsally, mostly
composed of the head-lobes, or antennal segment only, and the dorsal or
tergal portion of the oral appendages develop at a later period. We have
observed in the embryo of dragon-flies (Æschna) that the tergites of the
mandibles and first maxillæ are simultaneously fused with the
head-lobes, while the much larger tergal region of the 2d maxillæ
remains for some time separate from the anterior part of the head, and
is continuous with the thoracic segments, and it is only just before
hatching that this segment becomes fused with the rest of the head (Fig.
36). In a sense, the 2d maxillary segment when it is free from the head
reminds us of the foot-jaw, or 5th segment of chilopod myriopods (see
also p. 53).


                          _g._ The appendages

As we have seen, nearly or quite simultaneously all the limbs as a rule
bud out from each side of the median line of the primitive band. They
arise as saccular evaginations or outgrowths of the ectoderm, directed a
little backwards. They are at first filled with mesoderm cells, and in
the Orthoptera diverticula of the cœlom-sac are taken up into the
rudimental limbs, as in Peripatus and Myriopoda. (Graber, Cholodkowsky.)
As the antennæ, mouth-parts, legs, and abdominal appendages are all
alike at first, their strict homology with one another is thus
demonstrated. In insects never more than a single pair of limbs is known
to arise from one segment.

=The cephalic appendages.=—The antennæ evidently arise from the hinder
edge of the procephalic lobes (Fig. 527, _an_). As in Limulus, the first
pair of appendages are at first postoral (Fig. 528, _at_), afterwards
moving forward owing to changes in the relative proportions of the parts
of the head, and they are in all respects, in their development and
position in relation to the segment from which they arise, homologous
with the appendages succeeding them.

  The occurrence of rudiments of a pair of preantennal appendages in
  Chalicodoma which is claimed by Carrière, needs confirmation, as
  other embryologists have not observed them.

The postoral appendages of the head are the mandibles and the 1st and 2d
maxillæ, besides the supposed premandibular segment already referred to
on pp. 50–54, which only temporarily exists.

The trophi or oral appendages are all alike at first, but soon differ in
shape, acquiring their characteristic form shortly before the embryo
leaves the egg. The mandibles of Œcanthus are said by Ayers at the time
of revolution of the embryo to be slightly bilobed, and in his Fig. 5,
Pl. 19, they are represented as deeply trilobed, but in general they are
undivided. The 1st maxillæ are at this time distinctly trilobed. The 2d
maxillæ are separate, and distinctly though unequally bilobed, becoming
united shortly before birth. In the embryos of dragon-flies they are at
an early date very large and long, and directed backwards, and are not
fused together until just before hatching, when the extraordinary
mask-shaped labium is fully developed.

[Illustration:

  FIG. 528.—Two embryonic stages of the primitive streak of Melolontha.
    _A_, younger stage, with rudiments of eight pairs of abdominal
    appendages (_a^1–a^8_). _B_, older stage, the primitive band now
    very broad: _a_, 1st abdominal appendage, in _B_ sac-like; _x_,
    place of adhesive disc; _g_, brain; _l_, clypeo-labrum; _s_, lateral
    cord of the ventral nervous cord; other lettering as in previous
    figures.—After Graber, from Korschelt and Heider.
]

The distal parts of the labium, such as the ligula, palpifer, and palpus
are elaborated before the mentum and submentum. Many details as to the
final changes in the mouth-parts before hatching remain to be worked
out.

=The thoracic appendages.=—The three pairs of legs arise at the same
period and in the same manner in all insects; it is not until the end of
embryonic life that they become jointed, and that the claws and onychia
are developed. Especial attention has not yet been given to the details
of the development of the parts of the last joint of the tarsus.

  In many forms the antennæ are the first to appear, the mandibles,
  maxillæ, and legs appearing at a latter date, though simultaneously.
  It is thus in Stenobothrus, Hydrophilus, and Melolontha. In Lina,
  according to Graber, the mandibles precede the antennæ in
  appearance. In the Libellulidæ, according to Brandt, the legs first
  appear, then the jaws, and lastly the antennæ. This did not seem to
  be the case in the embryos of Æschna observed by us, although our
  observations were more superficial.

  On the other hand, in those insects whose larvæ are footless, the
  rudiments of the legs are retarded and aborted just before hatching
  (fossorial Hymenoptera and Apidæ), or the rudiments of the legs are
  not developed at all.

=The abdominal appendages.=—These appear soon after the thoracic limbs,
corresponding in most cases to the latter in shape and position, and
their position in the embryo is a matter of the greatest interest. Von
Rathke was the first embryologist to detect those of the first abdominal
segment, in his examination of the development of Gryllotalpa. Long
afterwards Bütschli detected them in the embryo of the honey-bee,
observing a pair on each segment. Patten observed them in Trichoptera;
Kowalevsky first perceived them in Lepidoptera, Tichomiroff confirming
his observations. Graber, Ayers, and Wheeler have observed them in
Orthoptera and Coleoptera, and the latter has detected them also in
Hemiptera and Neuroptera; and while they do not arise in the embryos of
Diptera and of Siphonaptera, they are to be looked for in any or all the
lower or more generalized orders.

As the result of these discoveries of polypodous embryos occurring in
all but the most specialized order (Diptera), it appears to be a
rational deduction that the winged insects have descended from insects
in which there were functional legs on each abdominal segment. Such an
ancestor was the forerunner of the Thysanura, in which abdominal
locomotive appendages still survive, though in a modified, more or less
aborted condition. This polypodous ancestral form was apparently allied
to Scolopendrella, which has a pair of functional legs on each abdominal
segment.

  The subject, then, of polypodous embryo insects is one of special
  interest, and has attracted much attention from Graber, Wheeler,
  Haase, and others. That these are genuine, though transitory
  appendages, is shown by the fact that certain pairs persist
  throughout adult life. The embryology of the Thysanura when worked
  out will throw much light on this subject, but we know that the
  spring (elater) of Collembola (and possibly the collophore) and the
  cerci of the winged insects are survivals of these limbs. That the
  three pairs of appendages forming the ovipositor, or sting, are most
  probably derived from these appendages is claimed by Wheeler (p.
  167), and seems proved by the fact that Ganin and also Bugnion has
  detected three pairs of imaginal disks in the embryo of parasitic
  Hymenoptera. Hence the abdominal appendages may ultimately be found
  to arise in nearly all cases from imaginal disks like those giving
  origin to the cephalic and thoracic appendages.

  As regards the Diptera, Pratt has observed that each of the three
  thoracic and eight abdominal segments of the embryo brachycerous
  Diptera (seen especially well in Melophagus) has two pairs of
  imaginal disks, a dorsal and a ventral pair. He thinks there is no
  doubt but that the ventral abdominal disks are homologous with the
  rudimentary appendages which appear in the embryos of all other
  insects, though not in the brachycerous dipters.

=Appendages of the first abdominal segment (pleuropodia).=—As early as
1844 Rathke observed in the embryo of the mole-cricket a pair of
appendages on the 1st abdominal segment, which he described as
mushroom-shaped bodies, and supposed to be embryonic gills. They are
called _pleuropodia_ by Wheeler, who, with Patten, Graber, and Nusbaum,
ascribes a glandular function to them, while Wheeler suggests that they
were odoriferous repugnatorial organs. In Blatta (Phyllodromia) they are
of large size, in Melolontha enormous (Fig. 528, _B_) and filled with
blood. Wheeler distinguishes as varieties, beside the mushroom-shaped
appendages of Gryllotalpa and Hydrophilus, the reniform (Œcanthus), the
broadly pyriform (Blatta), and the elongate pyriform (_Mantis
carolina_). In the European Mantis they are most limb-like, with a
digitiform continuation divided by a constriction into two sections.
(Graber.) In Meloë they assume the shape of a stalked cup. (Nusbaum.) In
the bee and in Lepidoptera the pleuropodia are not present, though the
temporary appendages on the succeeding segments appear; Carrière,
however, found them on the two first abdominal segments of very young
larvæ of the wall-bee (Chalicodoma).

Their cellular structure is peculiar, and they are either formed by
evagination or invagination, those of the latter type being subspherical
and solid. Those of the former type have a cavity communicating by means
of a narrow duct through the peduncle with the body-cavity (Blatta). No
tracheæ, nerves, or muscles enter them, though blood-corpuscles have
been seen in the cavities. “In some species the pleuropodia produce a
secretion from the ends of their enlarged cells. This secretion may be a
glairy albuminoid substance (Cicada, Meloë), a granular mass
(Stenobothrus), a bundle of threads (Zaitha), or a thick, striated,
cuticula-like mass (Acilius).” They attain their greatest size during
the revolution of the embryo, and they are “mere rudiments of what were
probably in remote ages much larger and more complex organs.” (Wheeler.)

  Lameere has observed that in Phyllodromia the first pair of
  abdominal appendages, after becoming of considerable size, undergo
  an enlargement at their free end, become detached, and fall into the
  amnion.

  Wheeler also calls attention to the homology of these pleuropodia
  with the 1st abdominal appendages of Campodea, shown by Haase to be
  originally glandular, but with at present a respiratory function. In
  the embryos of later, higher orders of insects, these appendages are
  in size and shape similar to those of the succeeding segments. (See
  also p. 164.)

[Illustration:

  FIG. 529.—Primitive band of _Bombyx mori_, showing the temporary legs
    on abdominal segments 2–11: _A_, early stage, in which the abdominal
    legs _al^2–al^10_ appear. _B_, later stage, when they are very faint
    and all except _al^3–al^6_ and _al^10_ are about to disappear. _C_,
    the persistent abdominal legs _al^3–al^6_ and _al^10_; _st^2_,
    _st^9_, the 2d and 3d pair of stigmata; _sgl_, silk duct.—After
    Tichomiroff.
]

=Are the abdominal legs of larval Lepidoptera and phytophagous
Hymenoptera true limbs?=—The presence of these abdominal legs in the
embryos of Sphinx (Kowalevsky), of _Bombyx mori_ (Tichomiroff), and both
_Bombyx mori_ and _Gastropacha quercifolia_ (except those of the first
segment), as well as in Hylotoma, which has 11 pairs of such appendages,
has suggested that the prop or prolegs of caterpillars and saw-fly larvæ
are survivals of these outgrowths, and not secondary, adaptive
structures. Opinions on this point vary. Balfour, and also, more
recently, Cholodkowsky, hold that the prolegs are survivals of the
embryonic appendages. Graber cautiously, after a lengthy and interesting
discussion, says that the question cannot be, in the present state of
our knowledge, solved. He, however, seems inclined to believe that the
prolegs are not merely secondary structures, and that the rudiments of
limbs may remain for a long time in a latent state before their final
development. Korschelt and Heider are disposed to regard the abdominal
appendages of Lepidoptera and Hymenoptera as true limbs, referring to
Balfour’s statement that in the Crustacea there are different examples
of the loss and later appearance of limbs, such as the loss of the
mandibular palpi of the zoëa of decapods, and the loss in the zoëa of
appendages in the Erichthus form of the Squilla larva corresponding to
the third pair of maxillipedes and first two pairs of legs of Decapoda,
and which are afterwards reproduced; similar cases occurring in the
Acarina. In the wasps and bees also, as is well known, the imaginal
disks of the thoracic appendages appear, the legs themselves being
suppressed in the larva (the imaginal disk probably existing in an
indifferent state), to reappear in the pupa and imago. It does not,
however, necessarily follow that the numerous pairs of hooked ventral
tubercles of certain dipterous larvæ (Ephydra) are true appendages.

It seems to us that it is a strong argument for the view that these
prolegs are survivals of primitive limbs, that from similar embryonic
paired outgrowths on different segments arise the spring of Podurans,
the anal cerci, and three pairs of appendages forming the ovipositor,
and the anal legs of the Corydalus larva, as well as those of
caddis-worms; at least five abdominal segments throughout the class of
insects as a whole bearing appendages in the adult.

On the other hand the view of Haase, that the prolegs of caterpillars
are secondary, adaptive characters, is supported by the fact of the
rapidity with which two pairs on the 3d and 4th segments nearly
disappear in the larvæ of certain Noctuidæ (Catocala, etc.), a reduction
evidently due to disuse.

=The tracheæ.=—The tracheal system arises as ectodermal invaginations on
one side of the appendages, appearing soon after the latter. The
earliest condition of the tracheal invagination is seen in section at
Fig. 539, _E_, _tr_; as it deepens, it sends off diverticula or tracheal
branches, while the narrow mouth of the invagination forms the stigma.
The cup-like cavities situated serially one behind the other, and
arising from the single tracheal invaginations, become at the end or
bottom of the cup elongated along the length of the body and fused
together at their ends; then the two longitudinal stems of the system
arise, by a breaking through at the place where the original
invagination had become fused, thus forming a continuous tube, the
lumina opening into each other. (Bütschli.)

The cuticular tracheal intima is differentiated late in embryonic life.
The entrance of the air is accomplished in part before the embryo
hatches, the air being derived from the tissues and fluids of the body.

  The farther development of the tracheal branches is due to the
  progressive formation of diverticula. The branches thus arising are
  intercellular formations. On the other hand, the finest twigs are
  intercellular structures. However, as Schaeffer states, the
  differences between the two modes of formation are not important.

  Wheeler mentions the existence of “two pairs of very indistinct
  tracheal openings in the 10th and 11th somites” of the abdomen of
  Doryphora (Fig. 546, _t_{19}_, _t_{20}_), and Heider believes that
  they exist in Hydrophilus.

The tracheal invaginations as a rule begin to appear after the
appendages commence to bud out. An exception is met with in the bee
(Apis), where the tracheal ingrowths are seen before the rudiments of
the legs. Most of the tracheal invaginations appear simultaneously. Only
rarely do we see an indication of their successive development from
before backwards. Thus in Hydrophilus, Graber observed that the
mesothoracic stigmata appeared somewhat earlier than those of the other
segments.


                          _h._ Nervous system

The rudiments of the nervous and tracheal systems essentially contribute
to the building up of the relief of the primitive band of insects. The
nervous system is the earliest to appear, being indicated very early, in
fact before the appendages begin to grow out. The first traces of the
nervous system are two ridges extending along the primitive band, the
depression between them being called the primitive furrow. At an early
period the segmentation is observed in the primitive ridges, while
widened spaces (the rudiments of the ventral ganglia) alternate
segmentally with the narrow places which are the incipient longitudinal
commissures (Fig. 527, _A_, _g_).

The primitive ridges extend anteriorly into the head-lobes; this part
must be regarded as the rudiment of the œsophageal commissure. The
rudiments of the brain are from their first appearance directly
connected with the ventral chain of ganglia.[83]

=Completion of the definite form of the body.=—This is accomplished by
the growth of the primitive band around the yolk, the band widening, so
that its edges behind the head extend up, and finally meet on the back,
forming the back or tergum of the embryo, thus enclosing the yolk (Fig.
530, _F_). The tergal wall of the head is due to the dorsal growth of
the head-lobes, and of the clypeo-labral region. In the course of this
process the anterior end of the primitive band becomes turned up
dorsally, forming a dorsal curve or bend. By this bending up of the
primitive band the forehead nearest the mouth forms a transverse ridge,
the labrum, while the basal or earlier part of the forehead now is
differentiated into the clypeus. This clypeo-labral region likewise
forms the roof or palatal region of the mouth. The head-lobes cause by
this dorsal growth a rotating motion which carries the rudimental
antennæ back over the mouth.

[Illustration:

  FIG. 530.—Diagram of the formation of the dorsal organ in Hydrophilus.
    _A_, cross-section through an egg, whose primitive streak is still
    covered over by amnion (_a_) and serosa (_s_). _B_, amnion and
    serosa are grown together in the middle line, then separated and
    drawn back to form a fold on each side. _C_, by the contraction of
    the serosa (_s_),which becomes converted into the dorsal plate, the
    folds become drawn up dorsally. _D_, the contracted serosa becomes
    partly overgrown by the folds. _E_, the folds grow together to form
    the dorsal tube. _F_, the mid-gut has closed over dorsally and
    enclosed the dorsal tube (_s_): _a_, amnion; _d_, yolk; _ec_,
    ectoderm; _h_, heart; _l_, body-cavity; _m_, rudiment of the
    mid-gut; _n_, nervous system; _s_, serosa (in _C_ and _D_ = dorsal
    plate, in _E_ and _F_, dorsal tube); _tr_, the chief tracheal
    stem.—After Graber and Kowalevsky, from Lang, and Korschelt and
    Heider.
]

The gnathal or post-antennal segments at first bear but a small part in
completing the tergal region of the head, but shortly before hatching
the mandibles and their muscles enlarge, giving fulness to the upper and
back part of the head.


     _i._ Dorsal closure and involution of the embryonic membranes

[Illustration:

  FIG. 531.—Schematic figure of the formation of the dorsal tube by
    invagination of the dorsal plate (transformed serosa); following
    after stage Fig. 520, _C_, and Fig. 521, _D_; _am_, amnion (now
    forming the provisional dorsal closure); _r_, dorsal tube, whose
    cells are already breaking away.—After Korschelt and Heider.
]

In most other Arthropoda (Crustacea, Arachnida, Myriopoda, etc.)
development goes on by the formation of a so-called primitive band, but
without the appearance of peculiar embryonic membranes. The outer
surface of the entire egg becomes, then, in part covered by the
band-like embryonic germ, and partly by a portion of the blastoderm
which remains unchanged. The dorsal region is formed by the widening and
spreading of the primitive band over the greater part of the surface of
the egg, while the area of the unchanged section of the blastoderm
continually becomes more restricted. It is generally accepted that the
latter is concerned in the dorsal closure, because, together with a
histological transformation, it becomes involved in the formation of the
ectoderm of the primitive band.

  A similar form of retrograde structure possibly occurs in the
  embryos of Poduridæ, in which a dorsal organ has been observed to
  develop in an early embryonic stage, which bears some relation to
  the cuticula enveloping the embryo, but whose significance is in
  general rather obscure.

  In most insects the relations are more complicated, since in such
  cases, the amnion-folds rise on the edges of the primitive band and
  of the unchanged section of the blastoderm, whose retrograde
  development is intimately connected with the closure of the back.

  A very simple case of dorsal closure, but which certainly is not a
  primitive one, occurs in Muscidæ and certain other Diptera whose
  amnion-folds are developed in a rudimentary way. In this case
  (according to Kowalevsky and Graber), the amnion-folds become
  smoothed out again. Amnion and serosa become then a simple
  epithelium, which throughout corresponds to the unmodified type of
  blastoderm of Crustacea, Arachnida, and Myriopoda, and here seems to
  share in the formation of the back. More complicated and very
  manifold relations of dorsal closure and involution of the embryonal
  membranes occur in other insects, of which Korschelt and Heider
  distinguish four different types:

  1. Involution under the formation of a continuous dorsal
  amnion-serosa-sac (Odonata).

  2. Involution with exclusively dorsal absorption of the amnion
  (Doryphora).

  3. Involution with exclusively dorsal absorption of serosa and
  separation of the amnion (Chironomus and Trichoptera).

[Illustration:

  FIG. 532.—Diagram of the formation of the dorsal walls in Doryphora
    in cross-sections: _am_, amnion; in _B_, serving as a provisional
    dorsal closure, in _C_, about to break up; _k_, primitive band;
    _s_, serosa.—After Wheeler, from Korschelt and Heider.
]

  4. Involution with separation of both embryonic membranes
  (Lepidoptera and Hymenoptera, Hylotoma).

[Illustration:

  FIG. 533.—Involution of the embryonic membranes of Chironomus: _am_,
    amnion; _r_, dorsal umbilicus; _s_, serosa, which has withdrawn
    into the region of the dorsal umbilicus, and in _C_ has passed
    into the interior of the embryo.—After Graber, from Korschelt and
    Heider.
]

  The first type occurs in the most primitive order of winged insects.
  The second type (Coleoptera) appears to be an independently
  inherited form of dorsal closure. In the first type, the formation
  of the amnion-serosa-sac is initiated by a rupture of the two fused
  embryonic membranes. This rupture in the ventral middle line occurs
  in Odonata only in the region of the head-section. In the second
  type only the amnion, in the third only the serosa are concerned in
  this rupture, while in the fourth type both membranes remain intact
  until the slipping out of the larva. (Korschelt and Heider.)


                   _j._ Formation of the germ-layers

[Illustration:

  FIG. 534.—Diagram showing the formation of the embryonic membranes in
    Lepidoptera (_A_, after Kowalevsky, _B_ and _C_, after Tichomiroff):
    _k_, primitive band; _am_, amnion: _se_, serosa; _do_, yolk; _vd_,
    invagination of the fore-gut, _ed_, of the hind-gut; _m_, mouth;
    _an_, anus; _x_, dorsal umbilical passage.—From Korschelt and
    Heider.
]

The older views on the structure of the layers of the primitive band of
insects were thoroughly unsatisfactory. Bütschli first found that in the
bee, by a kind of folding process, an inner layer of the primitive band
arose. Soon afterwards Kowalevsky, by the employment of section-cutting
and thorough researches, laid the foundation of a more exact knowledge
of these layers. He found that in Hydrophilus a furrow extended along
the whole length of the primitive band (Fig. 515, _A_, _B_, _r_), which,
while invaginating or sinking in, gave rise to the inner layer of the
primitive band, i.e. the common rudiment of endoderm and mesoderm (Fig.
539, _A-C_).

Kowalevsky also found similar conditions in the honey-bee (Apis),
Lepidoptera, and other forms. The furrow above mentioned must be
regarded as a very long gastrula invagination, extending along the
entire ventral side of the embryo, and the edges of the furrow as a
long-drawnout blastopore. The tube arising in Hydrophilus through the
closing of the furrow we may regard as a primitive intestinal canal.

The first rudiment of the gastrula furrow appears in insects as two
folds extending along both sides of the median line in the thickened
ventral plate (Fig. 536, _f_), through whose formation a more median
section of the ventral plate, the so-called middle plate (_m_), becomes
separated from the side plates (_s_). As the middle plate curves in and
becomes overgrown by the folds forming the edges of the blastopore, the
gastrula-tube (Fig. 539, _A_, _r_) is formed, and furnishes the
rudiments of the lower (inner) layer. The ectoderm, then, according to
Heider, arises from the lateral plates of the primitive band. The growth
of the edges of the blastopore, by which the closure of the
gastrula-tube is effected, takes place latest in the region of the most
anterior part of the furrow (Fig. 515, _B_ and _C_), corresponding to
that place in the primitive band in which the stomodæum afterwards
develops.

[Illustration:

  FIG. 535.—Two embryonic stages of a saw-fly (_Hylotoma berberidis_) in
    schematic median section: _a^1–a^{10}_, 1st to 10th abdominal
    segments; _bg_, ventral nervous cord; _og_, brain; _ol_, germ of
    labrum; _sp_, salivary gland; _ed_, hind-gut; _x_, _x′_, inner folds
    of amnion: other letters as before.—After Graber, from Korschelt and
    Heider.
]

[Illustration:

  FIG. 536.—Gastrula stage of the wall-bee (Chalicodoma), so-called
    flask-shaped stage: _f_, folds which on each side border the middle
    plate (edge of the blastopore); _m_, the partly segmented middle
    plate (here = rudiment of the mesoderm); _s_, the segmented lateral
    plate (becoming afterwards the ectoderm of the primitive band);
    _ve_, fore, _he_, hinder entodermal rudiment.—After Carrière, from
    Korschelt and Heider.
]

[Illustration:

  Fig. 537.—Two successive stages in the gastrulation of Apis.
    Cross-section through the primitive band: _b_, lower (inner) layer;
    _ec_, ectoderm.—After Grassi, from Korschelt and Heider.
]

During the invagination of the middle plate and its transformation into
the gastrula-tube a change takes place in its histological character
(Fig. 539, _A_ and _B_). While it originally consists of a high cylinder
epithelium, which after farther changes becomes divided into several
layers, since the wedge-shaped single cells push themselves over each
other, the cells in later stages become more and more cubical or
irregularly polygonal (Fig. 539, _B_), and are irregularly arranged. At
the same time the gastrula-tube is compressed in a dorso-ventral
direction. While it in this way spreads out laterally under the side
plates (ectoderm), its originally circular primitive lumen passes into
the form of a horizontal fissure, which in Hydrophilus long remains as
the boundary between the two layers of the inner (or lower) membrane.
(Korschelt and Heider.)

  There are numerous variations of the process of gastrulation, which
  are by Korschelt and Heider divided into three types, as follows:—

  1. Through invagination and formation of a tube (Fig. 539, _A_,
  Hydrophilus, Musca, Pyrrhocoris, etc.).

  2. By a lateral overgrowth (Fig. 537, Lepidoptera and Hymenoptera).

  3. By an inward growth of cells from a median furrow (Aphides and
  Trichoptera).

  In Doryphora and Lina (Fig. 524) the hinder end of the gastrula
  furrow is forked.

[Illustration:

  FIG. 538.—Diagrammatic sketch of the formation of the germinal layers
    in Doryphora: _A_, view of upper surface. _B_, cross-section through
    the fore end of the primitive streak at the line _a-a_. _C_, section
    through the middle of the primitive streak corresponding to the line
    _b-b_. _D_, section through the hinder end of the primitive band
    corresponding to the line _c-c_: _bl_, blastopore; _ec_, ectoderm;
    _en′_, anterior U-shaped; _en″_, hinder U-shaped germ of the
    endoderm; _ms_, mesoderm.—After Wheeler, from Korschelt and Heider.
]

The cellular layer arising from the gastrula invagination (lower layer)
forms the common germ of the endoderm and mesoderm. It has only recently
become known how these two germ-layers of insects have become
differentiated. Kowalevsky first discovered in Musca that the greatest
part of the lower (inner) layer yielded mesoderm exclusively, and that a
cell-mass only corresponding to the most anterior and posterior end of
the primitive band was used in the formation of the endoderm. We must
therefore, in insects, speak of a fore and a hinder endodermal rudiment.
In proportion, now, as the ectodermal invaginations, which are destined
to form the stomodæum and the proctodæum sink beneath the surface of the
embryo, the cell-masses of which the two endodermal rudiments are
composed are pushed farther in, and a separation between them and the
mesoderm is thus effected. The two endodermal rudiments now form
accumulations of cells which lie closely adjacent to the blind ends of
the stomodeal and the proctodeal invaginations. They soon widen out into
two hour-glass-shaped rudiments, which are directed with their
concavities towards each other, but with their convex side towards the
nearest pole of the egg. They soon change their form; two lateral
stripes grow out from them, and each now assumes the form of a U (Fig.
538, _en′_). The limbs of the fore and hind U-shaped rudiment are
directed toward each other, and grow towards each other until they meet,
and are fused together. Thus the endodermal rudiments arising out of the
fusion of the two U-shaped rudiments form two stripes extending along
the primitive band and situated mostly under the primitive segments. At
the two ends the endodermal rudiment fuses with the stomodeal and
proctodeal invaginations. These lateral endodermal streaks now spread
out, and gradually begin to grow over the yolk, on whose outer surface
they lie. This overgrowth makes the greater advance on the ventral side,
so that the two endodermal streaks first unite in the ventral median
line and afterward in the dorsal. The yolk in this way passes completely
into the interior of the rudiment of the mid-intestine.

  Kowalevsky has already proved that it is the median parts only of
  the inner layer which at the two ends of the primitive band become
  separated as endodermal rudiments through the advance of the
  stomodeal and proctodeal invaginations: the lateral portions become
  mesoderm.

  Kowalevsky has compared the germ-layers of insects with those of
  Sagitta. This comparison is supported by the later researches of
  Heider and of Wheeler on Coleoptera. (See Korschelt and Heider, p.
  809.)

  Relations somewhat different from the common type of formation of
  germ-layers occur in Hymenoptera. Kowalevsky and also Grassi agree
  that here also the endoderm originally forms a part of the lower
  (inner) layer. But the separation of the endoderm from the mesoderm
  goes on in Apis in such a way that the two ends of the inner layer
  pass up to the dorsal side of the egg, where the fore and hind
  rudiments of the endoderm extending along the back of the embryo
  grow together. When the two horseshoe-shaped rudiments have met each
  other and become fused, the enclosing of the yolk begins, which
  accordingly here proceeds from the dorsal towards the ventral side,
  instead of _vice versa_. As a result the endodermal cell-layer in
  Apis (and also Chalicodoma) at first does not lie under the
  primitive band, but on the dorsal side of the egg under that flat
  epithelium, which, arising from the amnion-fold, completes the
  provisional closure of the back.

  The yolk-cells and secondary yolk-segmentation are discussed by
  Korschelt and Heider at this point. The yolk-cells are elements
  scattered throughout the yolk and which partly remain in the yolk
  during the formation of the blastoderm (Fig. 507, _C_ and _D_), but
  which in part through a later immigration pass out of the blastoderm
  into the yolk. Graber has proved the fact of the migration of cells
  from the lower layer into the yolk, and his observations have been
  confirmed by other authors. Indeed, in certain cases (Melolontha),
  these later immigrant cells are clearly distinguishable by their
  histological characters from those originally found in the yolk.

  The yolk-cells are regularly scattered throughout the yolk. Their
  use to the embryo lies in the fact that they absorb the particles of
  yolk, which they digest and thus reduce to a fluid condition. It
  usually happens that after the complete formation of the primitive
  band there results a delimitation of the areas enclosing each
  yolk-cell, and this occurrence is called _secondary yolk-division_.
  In special cases (Apis, Musca) this occurrence seems not to take
  place. The yolk-cells are still, after the complete formation of the
  mid-intestine, to be recognized in the yolk-remnants filling the
  interior of the same, and gradually become absorbed.


 _k._ Farther development of the mesoderm. Formation of the body-cavity

We have seen that by means of an invagination extending throughout the
entire length of the primitive band a layer of cells is produced which
soon spreads out on the inner side of the band and thus forms a second
lower (inner) layer (Fig. 539, _C_). From this inner layer is separated
at the anterior and posterior ends of the primitive band, the endoderm,
which lies in direct contact with the invaginations of the proctodæum
and stomodæum. The remainder, by far the most extensive part of the
inner layer, is the mesoderm.

The mesoderm now becomes divided into two lateral streaks (mesodermal
streaks), by the withdrawal of its cells from the median line (Fig. 539,
_D_). This withdrawal is not, however, always a complete one. In the
free median space thus formed, the yolk often forms the so-called
_median yolk-ridge_. Segmentally arranged cavities soon appear in the
lateral region of the mesoderm (the primitive segmental cavities), and
the bordering mesoderm-cells arrange themselves in the form of an
epithelium, and constitute the wall of the primitive segments or
cœlom-sac. (Korschelt and Heider).

  The primitive segmental cavities in general arise through a split in
  the mesoderm. In Phyllodromia, according to Heymons, the primitive
  segments are very extensive. The mesoderm, at the time of the
  formation of the rudiments of the appendages, is raised with the
  ectoderm from the surface of the yolk, and in this way there arise
  in each segment cavities, which, since they are surrounded by
  mesodermal elements, become the closed cœlom-sacs (Fig. 540, _c_,
  _c′_, _c″_).

  The cœlom-sacs differ in different groups. They are largest in
  Orthoptera (Phyllodromia), where they take up almost all the cell
  material of the mesoderm in their formation, and exhibit certain
  conditions recalling those of Peripatus. The very large primitive
  segmental cavities, which in Orthoptera also extend into the
  rudiments of the appendages (Fig. 540, _B_, _ex_), in their later
  stages are, through the formation of a constriction, divided into a
  dorsal and a ventral half (Fig. 540, _B_, _c′_, _c″_). The ventral
  portions of these cavities extending into the extremities soon
  disappear, while the cells of their walls lose their epithelial
  nature, and group themselves irregularly into a sort of mesenchym.
  In this tissue, then, arises, partly through a separation among its
  cells, partly through the elevation of the same from the upper
  surface of the yolk, _the definite body-cavity_. The dorsal portions
  of the primitive segmental cavities remain unchanged a longer time
  in order to play a rôle in the formation of the intestinal muscular
  layer, of the heart, pericardial septum, and sexual organs.

[Illustration:

  FIG. 539.—Cross-section through the primitive streak of Hydrophilus in
    six successive stages: _A_, gastrula-stage (compare Fig. 515, _A_,
    corresponding to the point _a_). _B_, cross-section through stage,
    Fig. 515, _D_, in the most anterior section of the primitive band,
    where the same is not completely overgrown by the amnion-folds. _C_,
    cross-section through the trunk-segment of stage, Fig. 515, _E_.
    _D_, _E_, _F_, cross-sections through later stages: _am_, amnion;
    _b_, lower (inner) layer; _d_, yolk; _dz_, yolk-cells; _ec_,
    ectoderm; _en_, entoderm; _l_, definite body-cavity; _pr_, primitive
    groove (= neural groove); _pw_, primitive roll, or strip, of the
    ventral nerve-cord; _r_, blastopore; _sp_, fissure in the mesoderm
    (remains of the cavity of the primitive intestine); _se_, serosa;
    _s_, lateral cord of the rudiment of the nervous cord; _spm_,
    splanchnic layer of the mesoderm; _tr_, rudiment of a trachea (in
    _E_ appearing as an invagination of the ectoderm) in _F_ in
    cross-section; _us_, primitive segment (= cœlomic sac).—After
    Heider, from Lang.
]

[Illustration:

  FIG. 540.—Cross-sections through the abdominal part of three
    successive stages of evolution of _Phyllodromia germanica_: _am_,
    amnion; _bg_, rudiment of the ventral nervous chord; _c_, cœlomic
    cavity; _c′_, dorsal, and _c″_, ventral, section of the cœlomic sac;
    _cz_, cells of the walls of the primitive segment, which are joined
    to the genital rudiments; _gz_, genital cells; _dw_, dorsal wall of
    the cœlomic sac; _d_, yolk; _ec_, ectoderm; _ep_, epithelium-cells;
    _ex_, rudiment of the abdominal appendages; _f_, germ of the
    fat-body; _lw_, lateral wall of the cœlomic sac; _m_, mesoderm
    cells, which take no part in the formation of the cœlomic sac; _mw_,
    median wall of the cœlomic sac; _so_, somatic mesoderm layer; _vm_,
    ventral longitudinal muscle.—After Heymons, from Korschelt and
    Heider.
]

  In the highest groups of insects (Coleoptera, Lepidoptera, and
  Hymenoptera) the primitive segments are not so extensively developed
  (Fig. 539, _D-F_, _us_). They here form only relatively small sacs
  situated in the lateral parts of the primitive band which correspond
  to the dorsal section of the cœlom-sacs of Orthoptera. The ventral
  part is here from the very outset replaced by a mesenchym. As a
  result in these forms also no cœlomic diverticula occur in the
  rudiments of the extremities.

  The definite body-cavity of insects arises entirely independent of
  the cœlom cavities, and in fact, as Bütschli showed, through the
  separation of the primitive band from the yolk (Fig. 539, _F_, _l_).
  It appears bounded on the one hand by the surface of the yolk, on
  the other side by the irregularly arranged mesenchym cells.
  Originally we can in cross-sections distinguish three separate
  cavities of the definite body-cavity (in Hydrophilus according to
  Heider), a median and two larger paired lateral ones which later
  fuse with each other and with wide lacunæ (_e.g._ in the appendages)
  arising by the separation of the mesenchym cells. We refer the
  compartments of the definite body-cavity, as in Peripatus, to the
  primary body-cavity or segmentation-cavity. They are only lacunæ in
  the area of the mesenchym, and throughout bear the character of a
  pseudocœl.

  In later stages of embryonic development the cœlom-sacs and the
  definite body-cavity enter into communication with one another (Fig.
  523, _A_, _us_, _lh_). (Korschelt and Heider.)

  Then the hinder cœlom-sacs unite through the degeneration of the
  transverse dissepiments which separate them. After this a fissure
  opens in the median wall of the cœlomic sac, through which its
  cavity unites with the definite body-cavity. In the subsequent
  changes which the wall of the cœlom-sacs undergoes, these can be
  recognised no longer as separate divisions of the whole body-cavity.


                        _l._ Formation of organs

=The nervous system.=—As we have already seen (p. 554), the rudiments of
the ventral nervous cord arise, after the gastrula invagination is
completed, as two ectodermal thickenings situated on each side of the
median line, the so-called primitive rolls or strips (Fig. 528, _s_),
which extend from the centre of the procephalic lobes of the head to the
last segment, enclosing between them the single median “primitive
groove” (Fig. 539, _C_, _pr_, and _pw_).

Soon after the appearance of the primitive strips, the first traces of
segmentation may be detected. The ventral cord is from the first in
direct connection and continuous with the brain. From the segmental
expansions of the primitive strip arise the ventral nervous ganglia, and
from the intersegmental constrictions are developed the paired
longitudinal commissures.

Transverse sections of the ectoderm in the region of the primitive
strips (Figs. 539, _C_, and 517) show several layers of cells. Of these
cellular layers the deeper ones afterwards, by a kind of delamination,
separate from the superficial ones and form the “lateral cords,” _i.e._
the germs of the longitudinal cords of the ventral ganglionic cord.
Meanwhile the primitive groove (_pr_) deepens and forms an invagination
extending between the lateral cords. The cells at the bottom of this
invagination form the so-called “median cord,” and give rise to the
transverse commissures connecting the ganglia.

[Illustration:

  FIG. 541.—Transverse section through the rudiment of the ventral
    nervous cord of Xiphidium: _f_, fibrous mass; _m_, neuroblast cells
    of the median cord; _n_{1}-n_{4}_, neuroblasts of the lateral cord;
    _z_, pillar of ganglion-cells arising from the neuroblasts.—After
    Wheeler.
]

  Wheeler has detected in the rudiment of the ventral cord of several
  Orthoptera, on the upper surface of the lateral cords, four large
  cells which he calls _neuroblasts_ (Figure 541, _n_{1}-n_{4}_), from
  which cells arise by budding and become arranged in vertically
  arranged layers or pillars (_z_). Graber has observed them in
  Stenobothrus and Viallanes in Mantis. These neuroblasts are only
  present in the inter-ganglionic region, and soon move back to the
  hinder side of the transverse commissures.

At first there is a pair of ganglia to each of the 16 trunk-segments of
the embryo, but afterwards these become more or less fused together;
thus those of the three gnathal segments unite to form the subœsophageal
ganglion of the adult, and the last abdominal ganglia are fused together
and move a little anteriorly (see also pp. 227, 228).

=Development of the brain.=—The supraœsophageal ganglion is due to the
spreading out of the procephalic lobes. The rudiment of the brain is due
to a thickening of the ectoderm on the sides of the mouth and of the
forehead, this expansion of germinal brain-cells being the direct
continuation of the primitive rolls or strips, and which finally becomes
differentiated into the protocerebrum, deutocerebrum, and tritocerebrum,
as stated on p. 228.

The ganglion opticum, now regarded as a part of the compound eye, arises
as an ectodermal thickening on each side of the rudimentary brain. The
optic ganglion belongs exclusively to the foremost division of the brain
(see also p. 227).

=Development of the eyes.=—Compound eyes do not appear until the
beginning of pupal life, the single eye (ocellus) being the primitive
organ of vision. The ocellus of Acilius, according to Patten, arises as
a pit or depression of the ectoderm (Fig. 542). The long hypodermal
cells which form the walls of this pit or hollow are arranged in a
single layer, and bear at their free ends a striated cuticular edge
(_c_), while from their inner or basal end arise the fibres destined to
form the common optic nerve.

At a later stage (Fig. 542, _B_), the eye-pit is closed over, the edges
growing over and covering the deeper part of the eye. In this way there
arises out of the pit-like rudiment a two-layered optic cup. The outer
or superficial layer (_l_) becomes in its central part the crystalline
lens, while the peripheral parts form the iris. From the cuticular
striated border of these cells arise the chitinous or corneal lens. On
its outer edge the superficial layer of the eye passes gradually into
the unmodified hypodermis (_h_).

[Illustration:

  FIG. 542.—Two stages of development of the 5th of the six ocelli of
    larva of Acilius: _c_, cuticular striated band; _cl_, germ destined
    to form the corneal lens; _h_, hypodermis; _l_, crystalline-lens
    layer; _n_, optic nerve; _r_, retinal germ; _sp_, vertical fissure
    of the retina; _x_, the retina-cells bordering this fissure.
]

[Illustration:

  FIG. 543.—Two later stages of development of the same eye as in Fig.
    542: _i_, iris; _m_, middle inverted layer of the eye; _r_, retina;
    _sp_, vertical fissure of the retina; _st_, rods; other letters as
    in Fig. 542.—This and Fig. 542 after Patten, from Korschelt and
    Heider.
]

The inner, deeper layer of the eye, which forms the contracted
cup-shaped portion, appears to be the rudimentary retina (_r_). From its
cuticular rod-like or fibrous edge arise the visual rods. There soon
arise certain peculiarities characteristic of the eye of Acilius, _i.e._
the fissure (_sp_) bordered by the horizontally situated rods of the
large retina-cells (_x_).

In the farther developed eye (Fig. 543) there is a flattening of the
cup-shaped inner edge, by which the bottom of the eye is levelled and
the little rods belonging to it stand up vertically (Fig. 543, _B_,
_st_). Then the cells belonging to the edge of the retinal cup (_m_) are
turned in, forming an inverted layer constituting the germs of a third
layer interpolated between the two chief layers of the eye. (Korschelt
and Heider, from Patten.) Patten concludes that the structure of the
retina in the larval ocelli of insects is much like that of myriopods,
and that the whole eye is constructed on the same plan as that of
Peripatus and most molluscs.

=Intestinal canal and glands.=—The intestinal or digestive canal is
primitively divided, as already stated on p. 299, into three sections,
of which the anterior and posterior are called respectively the
stomodæum and proctodæum, and are invaginations of the ectoderm, forming
sacs whose blind ends face the future site of the mid-intestine. The
fore-intestine (stomodæum) in most cases arises earlier than the
proctodæum. Its muscles are derived from the mesoderm. From the
stomodæum arises at an early date an unpaired dorsal invagination out of
which develops the ganglion frontale and the pharyngeal nerve.

  The absorption of the ends of the blind sacs of the fore and hind
  intestine, and opening up of the passage into the mid-intestine,
  occur rather early in embryonic life. In the wasps and bees, as well
  as the larva of the ant-lion, the mid-intestine remains closed at
  the end, not communicating with the proctodæum, which has an
  exclusively excretory function (Fig. 497).

The mid-intestine arises from two originally separate rudiments, _i.e._
the fore and hind endodermal rudiments, which at the outset stand in the
most intimate relation with the invagination of the fore and hind
intestine. Originating as a simple collection of cells, so closely
adjoining these invaginations that Voeltzkow, Patten, and Graber derived
them directly through outgrowths of them, they become extended by
advancing cell-multiplication until they assume a U-shaped form. The
legs of the U-shaped rudiment are in the anterior endodermal mass,
directed backwards; those in the posterior mass, on the other hand, are
directed anteriorly. These legs grow towards each other until they
become fused together, forming two paired endodermal streaks, which pass
under the primitive band along its whole length, and are fused with it
at the fore and hind ends. In these places they stand in intimate union
with the proctodeal and stomodeal invaginations.

The paired endodermal streaks belong to the lateral portions of the
primitive band. As a rule, they lie directly under the row of cœlom-sacs
(Fig. 539, _F_). The dorsal wall of the primitive segments stands
consequently in intimate contact with the endodermal streaks. On this
wall of the primitive segments an active cell-growth takes place, and
the cell-material produced in this way, which separates from the dorsal
wall of the primitive segments, forms the outer or splanchnic layer of
the rudiment of the mid-intestine (_spm_, Figs. 539, _F_, 544, _sp_).
What remains of the dorsal wall of the cœlom-sacs after this separation
joins the genital rudiments and gives rise to the so-called terminal
thread-plate (Fig. 544, _ef_). The endodermal streaks, with the
splanchnic layer lying next to them, may now be considered as the
rudiments of the mid-intestine (Fig. 530, _m_, etc.). These are
noticeable in the following stages by their considerable lateral growth;
they spread out over the upper surface of the yolk, around which they
finally entirely grow (Figs. 539, _C-F_, 544, 545). This growth around
the yolk goes on in most cases in such a way as to unite the two
mid-intestinal streaks in the region of the ventral median line with
each other. Then afterwards their union on the dorsal side takes place
(Figs. 539, _F_, 545). The yolk thus passes completely into the interior
of the mid-intestine, and with it the remains of the dorsal tube or
dorsal organ, when such an one is present.

[Illustration:

  FIG. 544.—Cross-section through the abdominal region of a somewhat
    older primitive band of _Phyllodromia germanica_: _bg_, rudiment of
    the nerve-cord; _c_, remains of the cœlomic cavity; _cz_, rudiment
    of the genital efferent passage; _ec_, ectoderm; _en_, endoderm;
    _ef_, terminal cord-plate; _fk_, fat-body tissue; _gz_, genital
    cells; _h_, rudiment of the heart; _p_, rudiment of the pericardial
    cavity; _ps_, rudiment of the pericardial septum; _so_, somatic
    mesoderm layer; _sp_, splanchnic mesoderm layer.
]

=The salivary glands.=—These segmentally arranged glands, which open by
pairs into the three gnathal segments of the head, arise as ectodermal
invaginations originally opening not into the stomodæum, but outwards on
the surface of the body; hence Korschelt and Heider suggest that they
were originally dermal glands, whose mouths became drawn into the buccal
cavity.

[Illustration:

  FIG. 545.—Cross-section through the abdominal region of an embryo of
    cockroach (_P. germanica_) after the yolk has been completely
    enclosed by the primitive band and the closure of the back; _s_,
    tracheal stigma; other letters as in Figs. 540, 544.—This and Fig.
    544 after Heymons, from Korschelt and Heider.
]

[Illustration:

  FIG. 546.—Embryo of Doryphora shortly after the appearance of the
    appendages, unrolled and isolated: _o_, stomodæum; _lb_, labrum;
    _b^1–b^3_, three brain segments; _og^1–og^3_, three segments of the
    optic ganglion; _op^1–op^3_, three segments of the optic plate;
    _f^1–f^5_, five pairs of invaginations which form the tentorium,
    etc.; _t^7–t^{20}_, tracheal invaginations; the two last pairs
    (_t^{19}-t^{20}_) either disappear or form the openings of the
    sexual ducts; _at_, antennæ; _md_, mandibles; _mx^1–mx^2_, maxillæ:
    _p^1–p^3_, legs; _c_, commissure connecting the two ganglionic
    thickenings (_g^4_) of the premandibular segment; _gl_, ganglia;
    _mst_, middlecord thickenings; _mpg^1–mpg^3_, rudiments of three
    pairs of urinary tubes; _a_, proctodæum.—After Wheeler.
]

  For their serial arrangement, see p. 337. Korschelt and Heider state
  that they would be inclined to homologize the salivary glands of
  insects with those glands of myriopods opening into the
  mouth-cavity, were it not that these glands in myriopods opening
  into the mouth are in reality transformed nephridia originating from
  the mesoderm, while the salivary glands of insects are clearly
  ectodermal structures. We must, therefore, they add, leave to later
  researches the question of the homology of these organs, also of
  their relations to the similar glands of Peripatus.

[Illustration:

  FIG. 547.—Section of proctodæum of embryo locust, showing origin of
    urinary tubes (_ur.t_): _ep_, epithelial or glandular layer; _m_,
    cells of outer or muscular layer; _a_, section of a tube.
]

=The urinary tubes.=—These excretory vessels arise as paired
evaginations of the hind intestine or proctodæum. They are ectodermal
structures arising as lateral diverticula of the intestinal cavity (Fig.
546). Figure 547 represents their mode of origin at the anterior end of
the proctodæum of a locust. It will be seen that there are 10 primary
tubes. There are 150 such tubes in locusts, or 10 groups of 15 each. The
15 secondary tubes probably arise from the primary ones in the manner
described by Hatschek for Lepidoptera (see his Taf. III, Fig. 7).

  While the Malpighian tubes usually first arise as diverticula of the
  proctodæum, in the Hymenoptera (Apis and Chalicodoma) they appear,
  even before the completion of the proctodæum, as invaginations of
  the ectoderm which at first open out on the outer surface of the
  primitive band. They seem, then, in some degree, to be similar to
  the tracheal rudiments, which perhaps is the reason why they have
  been homologized with them, a view which we do not share, and in
  which Carrière does not concur. They afterwards pass, with the
  growing proctodæum, into the interior of the embryo. (Korschelt and
  Heider.)

=The heart.=—The dorsal vessel is first indicated, according to
Korotneff, by a long string or row of cells (_cardioblasts_), which on
each side border the mesodermal layer of the primitive band (Figs. 544,
_h_, 548, _h_). In the advancing growth of the primitive band around the
yolk, this rudiment steadily passes up more towards the dorsal side. It
is in connection with the wall of the primitive segment (Figs. 544 and
548), and represents the point at which the dorsal wall of the cœlom-sac
passes into the lateral wall. According to Korotneff, the cardioblasts
arise directly through a migration out from the wall of the primitive
segment.

In Gryllotalpa the formation of the dorsal organ, which, as Korotneff
states, is in this insect nothing else than a stopper which fills up the
dorsal gap of the body-wall of the embryo, is effected by the rupture of
the embryonal membranes. The serosa is drawn together to form a thick
plate (Fig. 523, _A_, _rp_), and the much degenerated amnion-folds
(_am_) which are laterally attached to it have moved from the edges of
the primitive streak (_*x-*y_) far towards the dorsal side (see Fig.
539, _C_, which represents a similar stage). The distance between the
rudiment of the amnion-fold and the lateral edge of the primitive band
(_*x_, _*y_) is occupied by an epithelial lamella (_l_), in which we
recognize the earlier amnion. This lamella does not lie directly on the
yolk, but is separated from it by a spacious blood-lacuna (_A_, _bs_),
in which can be seen numerous blood-corpuscles which have migrated in
from the mesoderm of the primitive band. The cardioblasts which have
arisen from the wall of the primitive segment (_us_) are on each side
arranged into the form of a furrow (_gr_), which bounds the blood sinus
below.

[Illustration:

  FIG. 548.—Cross-section through the abdominal part of an older
    primitive band of _P. germanica_ when beginning to grow around the
    yolk: _vm_, ventral longitudinal muscle; other lettering as in Fig.
    545.—After Heymons, from Korschelt and Heider.
]

By the continuous growth of the primitive band around the yolk, after
the resulting invagination and degeneration of the dorsal plate, the two
blood-lacunæ unite together on the dorsal side into a single one (_B_,
_bs_). These constitute the first cavity of the heart. The vascular
furrows (_gr_) come in contact with each other and grow together, and
the wall of the heart is thus formed. Ayers states that in Œcanthus the
heart is formed in the head region only after the yolk-sac has passed
entirely within the body. The venous ostia arise by two paired
invaginations of the lateral walls, forming a split at their bottom.

The rudiment of the heart stands, as we have seen, in intimate union
with the primitive segments. Out of the lateral walls of these segments,
after giving off the elements of the somatic mesoderm, arises an
epithelial plate which becomes the rudiment of the pericardial septum or
dorsal diaphragm (Figs. 523, _A-C_, _dd_, 544–545, _ps_). As soon as the
two halves of the rudiments of the heart have united with each other in
the dorsal middle line, the two halves of the pericardial septum unite
with each other and form the wall to the pericardial cavity and shut it
off from the rest of the body-cavity. For a long time the pericardial
septum remains in union with the wall of the heart. Afterwards, however,
it separates from it (Fig. 523, _C_, _dd_). (Korschelt and Heider.)

  The statements of other authors (Ayers, Grassi, Patten, Tichomeroff,
  Carrière, Heider, Heymons, etc.) as to the mode of origin of the
  heart in insects of other orders are all similar to the type
  described in Gryllotalpa. The difference consists mostly in the fact
  that the two large blood-lacunæ are wanting or only exist to a
  slight extent. It results that the rudiment of the cavity of the
  heart in the earlier stages is of slight extent and often scarcely
  recognizable.

  In Œcanthus (Ayers) and in Gryllotalpa, the hinder section of the
  heart is the first to develop, the development advancing from behind
  forward.

=The blood-corpuscles.=—Blood-cells are said by Korotneff to be, in
Gryllotalpa, at an early period present almost everywhere between the
yolk and mesoderm; they are derived, as he states, from the cells of the
somatic mesoderm layer, which has lost its connection with the other
parts of the mesoderm, and fall into the body-cavity. Ayers states that
the blood-corpuscles arise from serosa nuclei which have passed into the
body-cavity, where they become more vesicular, and ultimately all of the
nuclear substance goes to form from one to three spherical bodies, which
are surrounded by the common membrane.

“These bodies are blood-corpuscles and are free nucleoli immediately on
the rupturing of the vesicle which surrounds them.” (Ayers, Pl. 22,
Figs. 1, 3, p. 250.) More recently, Schaeffer has observed in
caterpillars certain cell-complexes associated with the fat-body which
he has called blood-forming masses.

=Musculature, connective tissue, fat-body.=—The muscles of various parts
of the body, as well as the connective tissue, arise by histological
differentiation from the somatic layer of the mesoderm (Fig. 523, _so_).
The fat-body originates from the same source, as shown by the researches
of Kowalevsky, Grassi, and of Carrière. In Hydrophilus a dorsal band of
the fat-body passes over the digestive canal arising by direct
transformation of the wall of the cœlom-sacs. But also the other
portions of the fat-body, as the fat-body lobes accompanying the
tracheal system, are of undoubted mesodermal origin. Heymons’
observations on the cockroach (Phyllodromia) agree with the foregoing
view. In this insect at a very early period certain cells in the wall of
the cœlom-sacs undergo a change, and may be recognized as the rudiments
of what are afterwards fat-body tissues (Fig. 540, _B_ and _C_, _f_).

=The reproductive organs.=—Our knowledge of the mode of development of
the genital organs is in a less satisfactory state than that of the
other organs. It is now known that the rudiments of the sexual glands
belong to the mesoderm, and are developed from the wall of the
cœlom-sacs. In the cockroach (Phyllodromia), the most generalized of the
winged insects, as Heymons has shown, in the earlier stages of the
embryo separate genital cells are already distinguished by their
histologically different characters from the other mesodermal cells. The
genital cells are larger and show a feebly stained nucleus with a clear
nucleolus. These genital cells, which are transformed normal mesodermal
cells, lie originally within the mesoderm layer or on the surface of
this layer turned towards the yolk, on the edge of the segments. After
the complete formation of the cœlom-sacs we find them (Fig. 549, _gz_)
in the dissepiments which separate the successive cœlom-sacs from one
another. Here new genital cells are constantly formed through the
transformation of mesoderm cells. The development of the genital cells
takes place in the 2d to the 7th abdominal segments.

Afterwards the genital cells pass into the interior of the cœlom-sacs,
and soon pass to the dorsal wall of the same (Fig. 540, _A_, _gz_) and
enter between the cells of this wall. The cœlom-sacs (_c_) show in
cross-section in this stage a triangular outline, so that we can
distinguish a dorsal, lateral, and median wall. The dorsal wall lies
next to the surface of the yolk, and afterwards gives rise by separation
or splitting to the splanchnic mesoderm (Fig. 544, _sp_), while from its
remains the terminal thread-plate (_ef_) originates. The lateral wall,
which is turned towards the ectoderm of the primitive band, is
intimately concerned in the formation of the somatic layer (Fig. 540,
_C_, _so_) of the mesoderm. Out of what remains arises the pericardial
septum (Fig. 544, _ps_).

When the genital cells have entered into the dorsal wall of the
primitive segments, they are already so numerous that they form a
continuous series extending from before backward. The genital rudiment
consists, then, of a string of cells lying on each side in the dorsal
wall of the primitive segments, which extend from the 2d to the 7th
abdominal segments. In the formation of these strings or rows of cells
not only are the genital cells concerned, but also still
undifferentiated mesoderm cells (Fig. 540, _B_, _C_), which originate
from the dorsal wall of the cœlom-sacs and lie next to the genital
cells. Some of these last tend to envelop the genital cells. We
designate them the epithelial cells of the genital rudiments (_ep_),
while others form a cellular cord which takes a position medial and
ventral to the genital cells.

[Illustration:

  FIG. 549.—Sagittal (longitudinal) section through the abdominal part
    of a primitive band of cockroach (_Phyllodromia germanica_) after
    the end of the formation of the primitive segments: 1–7, 1st to 7th
    abdominal segments; from the 8th abdominal segment (8) to the last
    segment (_es_) extends the inturned ventral part of the primitive
    band; _am_, amnion; _c_, cœlom-sac; _d_, yolk; _gz_, genital cells,
    lying partly in the dissepiments, partly in the wall or in the
    cavity of the primitive segments.
]

From the genital cells in the female arise only the egg-cells (and the
nutritive cells in those forms which have such). The follicular
epithelium of the egg-tube, on the other hand, also the corresponding
cells of the terminal chamber, originate from the epithelial cells.
Phyllodromia and Orthoptera in general, to which this description
applies, show in this respect tolerably simple relations, since the
germinal or terminal compartment of the ovary in them is composed of
relatively few cells. In most other insects, and especially those which
have a great number of food-cells in the ovary, the germinal chamber
(Keimfach) is extraordinarily large.

The ventral cellular cord (_cz_) develops into the proximal part of the
oviduct, which widens out and receives the single egg-tubes.

The cœlom-sacs in the farther course of their development, through the
retrograde development of the parts extending into the appendages,
through the development of the fat-bodies and through the delamination
of the somatic and the splanchnic mesoderm layer, become greatly
diminished in size. Finally, there remains left of them only a rather
small cavity (_c_), which is bordered on the side by the rudiment of the
pericardial septum (_ps_) and within by the terminal thread-plate
(_ef_). The dorsally situated point where these two lamellæ pass into
each other seems to stand in intimate connection with the cells of the
rudiment of the heart (_h_). The cord-like genital rudiment hangs from
the terminal thread-plate as from a mesentery (Fig. 549, _gz_).

[Illustration:

  FIG. 550.—Longitudinal section through the female genital rudiments of
    _P. germanica_. _A_, with beginning, _B_, with farther advanced
    growth of the ovarian tubes: _cz_, rudiment of the genital efferent
    passage; _ef_, terminal threads; _ep_, nucleus of the epithelial
    cells; _gz_, genital cells.—After Heymons, from Korschelt and
    Heider.
]

Together with the growth of the primitive band around the yolk, and the
formation of the back, the paired rudiments of the heart gradually
extend to the neighborhood of the dorsal median line, followed by the
genital rudiments which are connected with them by the terminal
thread-plates. The genital rudiments advance thus to the dorsal side of
the developing mid-intestine (Fig. 545, _gz_).

The terminal thread-plate (_ef_) is at first a simple epithelial plate.
Soon, however, follows an arrangement of its cells whereby they appear
to be arranged in vertical rows, each one of which corresponds to a
developing ovarian tube. In this way the terminal thread-plate separates
into the separate terminal threads of the ovarian tubes (Fig. 550,
_ef_). In this process of division the uppermost dorsal edge of the
terminal thread-plate takes no part. From it afterwards grows a thread
which extends anteriorly, which becomes the common terminal thread of
all the ovarian tubes, the so-called Müller’s thread. This is originally
united with the pericardial septum, but seems in later stages to have no
longer an intimate connection with it.

The formation of the single ovarian tubes, which in Phyllodromia number
about 20, is accomplished by the extension of indentations from the
dorsal side towards the ventral side of the ovarian rudiment (Fig. 550).
At the same time the epithelial cells (_ep_), which were originally
situated in part between the genital cells, become arranged in the form
of an epithelium on the surface of the ovarian tubes, which soon forms
on its outer surface a structureless cuticular _tunica propria_. The
outer peritoneal membrane of the ovary becomes formed of the cells of
the surrounding tissue of the fat-body.

The genital rudiment originally extends, as already stated, from the 2d
to the 7th abdominal segment. In the last, however, the genital cells at
first occur only sparingly, and afterwards completely disappear, so that
here the genital cord appears composed of epithelial cells only. This
part is the rudiment of the oviduct proper, and forms a direct
continuation of the above-mentioned cell-cord which is situated
ventralward from the genital cells, from which, as we have seen, the
proximal cup-shaped section of the oviduct is formed. The hinder section
of the oviduct turns down ventrally in order to unite at the boundary
between the 7th and 8th abdominal segments with the hypodermis. The
rudiment of the oviduct originally forms a solid strand of cells.
Afterwards a cavity is formed by the separation of the cells.

  In later stages there is a considerable shortening of the genital
  rudiment, so that it occupies a smaller number of abdominal segments
  than at first. At the same time the single ovarian tubes pass out of
  their originally vertical position into one more horizontal.

The paired connections of the rudiments of the oviducts with the
hypodermis of the intersegmental furrow between the 7th and 8th
abdominal segments reminds us of the conditions in the Ephemeridæ. This
is the primitive condition in insects. In the female of Phyllodromia
there is developed during larval life, from an ectodermal invagination,
an unpaired terminal section of the genital passage, which becomes the
genital pouch in which the egg-case (oötheca) is held. This genital
pouch is formed, as Haase has already proved, by the withdrawal of the
chitinous ventral plate of the 8th and 9th abdominal segment by
invagination into the interior of the body.

The development of the efferent passages has been investigated by
Nusbaum in the cockroach (Periplaneta) and in the Pediculina. He found
that only the vasa deferentia and the oviducts arise from the hinder
cord of the germs of the sexual glands, that is, out of the mesodermal
rudiments, while the other parts of the sexual efferent apparatus
(uterus, vagina, receptaculum seminis, ejaculatory duct, penis, and all
the accessory glands) develop from the integumental epithelium and are
of ectodermal origin. In fact, the unpaired parts (uterus, penis,
receptaculum seminis, unpaired glands) have developed from paired
rudiments, being outgrowths of the hypodermis. The hinder portions of
the rudiments of the sexual glands approach these hypodermal growths and
fuse with them. Through a median fusion of the paired hypodermal growths
arise the germs of the unpaired organs. These observations are in
complete agreement with the results at which Palmén arrived by
anatomical investigation (see p. 492).

From the agreement of the position of the sexual openings in
Phyllodromia with the conditions observed in the Ephemeridæ, with which
the Perlidæ also agree, we conclude that in the entire group of insects
an opening between the 7th and 8th abdominal segments is the primitive
condition, and that only by a secondary shifting has a more posterior
position of the opening (in many forms) been brought about. In this
category we must certainly include the Thysanura, in which the sexual
opening is single and situated between the 8th and 9th abdominal
segments.

=Development of the male germinal glands.=—These rudiments arise in
exactly the same manner as those of the female. Sexual differentiation
takes place in the later embryonic stages. We then notice that in the
male four masses of genital cells become surrounded by epithelial cells.
These masses, which form the germs of the four testicular follicles of
Phyllodromia, stand in intimate union with the rudiment of the vas
deferens, and in the later stages move in connection with the latter,
away from and behind the original genital rudiment. There remains, then,
with the terminal thread-plate a remnant of the genital rudiment, which,
according to Heymons, forms the female part of the original
hermaphroditic genital rudiment, and in special cases may develop even
into rudimentary egg-tubes and eggs. The rudimentary organ arising out
of this genital rudiment may also be demonstrated in the adult male of
Phyllodromia.

In the female the oviduct arises directly out of the originally
established efferent passage. In the male, on the contrary, it is not,
along its whole length, transformed into the vas deferens, but its
distal terminal portion degenerates and is replaced by a newly formed
terminal portion of the vas deferens, which then unites with the
ectodermal ductus ejaculatorius. (Korschelt and Heider.)

  On reviewing the facts as to the origin of the sexual organs, as in
  Phyllodromia,[84] as just described, it will be seen that they
  afford proof that in the derivation of the genital cells from the
  epithelial cells of the cœlom-sacs, there is a direct agreement with
  the annelids. In the later development of the paired genital glands,
  and of an efferent passage standing in direct union with the glands
  themselves, there is a certain agreement with the conditions in
  Peripatus. In the first place, the dorsal position of the genital
  glands is the same in the two groups. On the other hand, the genital
  glands of Peripatus, according to Sedgwick, are formed by direct
  fusion of the successive cœlom-sacs (and a similar point of view has
  been taken by Heathcote for the myriopods), hence it results that in
  Peripatus the genital cavities arise out of the cœlom-cavities. In
  the insects, on the other hand, the genital rudiment lies, to be
  sure, in the wall of the cœlom-sac, but the genital cavity (lumen of
  the oviducts) in them arises separately from the cœlom-sacs, while
  the cœlom-cavities finally become a small part of the definite
  body-cavity. We must consider the conditions in Peripatus and the
  myriopods as the more primitive, directly pointing to the annelids;
  on the other hand, those of the insects as derived and secondary.

  If we attempt to homologize the sexual efferent passages of insects
  with those of Peripatus, we are compelled to refer them to a
  modified pair of nephridia, and the origin of the latter (Peripatus)
  from the mesoderm agrees with that of insects. In general, however,
  in the development of the sexual outlets of insects, there are no
  characters which can be regarded as favorable to such a view. We
  must here accept the fact that the mode of development is secondary.

  Mention should be specially made of the fact we owe to Heymons, that
  in the genital rudiment of Phyllodromia the genital cells and
  epithelial cells can be distinguished from each other from the very
  beginning. This fact does not favor the generally accepted view that
  the follicle-cells and egg-cells arise through a later
  differentiation from one and the same kind of cell. From their first
  origin, indeed, in Phyllodromia, both kinds of cells may be referred
  to the same source.

  The mode of origin of the genital rudiments in Diptera and Aphides
  deserve special mention. In these groups the sexual germs are
  present in very early stages of life. This certainly in part is the
  result of the parthenogenetic and pædogenetic mode of reproduction
  in the two groups, which leads to an early differentiation of the
  sexual germs.

  In the Diptera the first germs of the genital glands are represented
  by the polar cells (Fig. 551, _pz_). In the asexual developing eggs
  of the oviparous Cecidomyia larva, before the formation of the
  blastoderm, there separates from the hinder pole (_D_) a rather
  large cell rich in granules, which soon divides into two and
  afterwards four polar cells. After the completion of the blastoderm
  these polar cells then pass in among the blastoderm cells (_G_) and
  into the interior of the embryo, where they are in later stages
  symmetrically arranged in two groups, and, enveloped by the cells of
  surrounding tissues, transformed into the genital rudiments.
  (Metschnikoff.)

  In Chironomus (Fig. 552, _p_), according to Balbiani, two polar
  cells almost simultaneously separate from the hinder pole of the
  egg, which, by division, form a group of four and eight cells.
  Exactly as in the case in Cecidomyia, these cells are taken within
  the embryo, where they lie divided into two groups on each side of
  the proctodæum. In all the young, freshly hatched larvæ; these two
  spindle-shaped groups, whose cells soon increase in number, may be
  seen situated dorsally on the side of the heart, enveloped by a
  clear cellular membrane which ends before and behind in a
  ligament-like terminal thread. The anterior terminal thread is the
  rudiment of the so-called Müller’s thread. The thread at the
  posterior end is the rudiment of the paired efferent passage of the
  genital glands. Through a division of the cells lying in the
  interior of the rudiments of the ovaries, there results the
  formation of a rosette-shaped group of cells which corresponds to
  the contents of an ovarian tube. With this view of Balbiani the
  later observations of Ritter agree.

  As in the Diptera, so in the Aphides, the first germs of the genital
  organs are differentiated very early in life. In the early stage in
  which through an invagination from the hinder pole of the egg the
  first rudiment of the amnion-cavity is formed, a group of cells
  becomes separated from the wall of this invagination before the
  formation of the lower layer, which at this time lies as an unpaired
  roundish mass within the embryo. This group of cells, according to
  Balbiani and Witlaczil, has arisen by division of a single cell.
  Afterwards it becomes horseshoe-shaped and divides into a number of
  roundish masses of cells, which are arranged in similar numbers on
  each side of the median plane of the body, and form the rudiments of
  the terminal fan (Endfächer). They are covered by an epithelial
  envelope which passes anteriorly into the terminal threads,
  posteriorly into the efferent passage. The origin of this epithelial
  case is unknown. The efferent passages of the separate ovarian tubes
  are united into a common oviduct, and this fuses with an unpaired
  ectodermal invagination lying under the hind intestine from which
  the accessory sexual organs are formed. (Korschelt and Heider from
  Metschnikoff, Witlaczil, Will.)

[Illustration:

  FIG. 551.—First developmental stages of the parthenogenetic eggs of
    the larva of Cecidomyia: _b_, peripheral protoplasmic layer
    (Keimhautblastem); _bl_, blastoderm; _d_, central yolk; _f_,
    division-nuclei; _n_, nutritive cell (“corpus luteum”) about to
    break up; _pz_, polar cells.—After Metschnikoff, from Korschelt
    and Heider.
]

  In the Hymenoptera Ganin has observed in the embryo of Platygaster
  the rudiments of the sexual glands in the form of two rounded masses
  situated near the posterior intestine and apparently derived from
  the same blastems or buds as the latter.

  Uljanin studied these organs in the larva of the honey-bee. They are
  two reniform bodies in the middle of which will soon appear the
  ovarian tubes. They also give birth to the internal parts of the
  excretory ducts, while the external part of the genital tube, as
  also the accessory glands which are connected with it, are derived
  by an invagination of the hypodermis at the surface of the
  penultimate segment.

  Dohrn observed in the larva of ants the rudiments of the ovaries in
  the form of two pyriform masses, each with eight prolongations which
  he regarded as young ovarian tubes.

[Illustration:

  FIG. 552.—Three longitudinal sections through the embryo of
    Chironomus. In _A_, the blastoderm (_bl_) is beginning to form,
    the polar cells (_p_) outside of it; in _B_, the polar cells have
    pressed in between the blastoderm cells; in _C_, they lie in the
    interior of the embryo: _b_, protoplasmic layer (Keimhautblast);
    _d_, yolk; _k_, nucleus of the forming blastoderm.—After Ritter,
    from Korschelt and Heider.
]

  In Encyrtus Bugnion observed the rudiments of the sexual glands in
  the middle of the larval period; they were rounded and with no
  apparent connection with the neighboring organs. Afterwards these
  rudiments elongated, approached nearer to the ventral surface, and
  placed themselves in relation with some small cell-groups which
  appeared under the rectum, and seemed destined to form the efferent
  canal (vas deferens) and accessory glands of the genital organs. He
  thought the sex could be recognized in the second half of larval
  life, the male gland being distinguished by its rounded shape and
  smaller size; the ovary by its oval form and larger size. In larvæ
  ready to be transformed the testis formed a cellular mass enveloped
  by a cuticle, and at its hinder end prolonged into an epithelial
  cord, which is undoubtedly the vas deferens. The ovary had a similar
  envelope, and from its cellular mass arose epithelial cords which
  were destined to become the ovarian tubes.


                     _m._ Length of embryonic life

The duration of embryonic life varies greatly in different insects. The
embryo of the blow-fly is fully developed in less than 24 hours, that of
the house-fly in 24 hours. In the locusts and tree-cricket the embryos
begin to develop at the end of the summer, continuing to grow until the
cool weather of autumn, when growth is arrested, the later stages being
finished in the latter part of the spring. It is so, likewise, with the
embryos of many moths and other insects.


                      _n._ The process of hatching

This has been observed only in a few cases, and careful observations as
to the exact manner in which the embryo breaks the egg-shell and frees
itself from the amnion are much needed. Also the rapid changes of form
from that of the embryo within the egg-shell, and that which it
immediately assumes after breaking forth from the shell and membranes,
have yet to be observed; for these will undoubtedly be found to have
special phylogenetic significance. Indeed, the phylogenetic importance
of the latest embryonic changes in insects just entering on the nymph or
the larval stages is very great, though little attention has as yet been
bestowed upon the matter.

As regards the changes at the time of hatching, Wheeler tells us that
the cockroach (Phyllodromia), shortly after leaving its narrow place in
the egg-capsule, undergoes a peculiar change in shape. Before hatching,
and when confined in the egg-shell, the body is about one-third as wide
as thick; but soon after breaking out of the chorion its body is much
flattened, its dorso-ventral diameter being only about a third as great
as its greatest breadth. This shows that the flattened shape of the body
of cockroaches, which adapts them for their life under bark and stones,
is a very late inheritance, and that these insects have descended from
those with more cylindrical bodies. The end of the body, also, which in
the egg is bent underneath the abdomen, is, after hatching, bent
dorsally, as indicated by the anal stylets, which now point directly
upwards and outwards. The spines and claws are developed shortly before
hatching. In the Locustidæ (Xiphidium, etc.) Wheeler has observed that
the pleuropodia, or 1st pair of abdominal temporary embryonic
appendages, are shed during hatching. All the other embryonic appendages
have also disappeared, except those which persist and have rapidly
become modified to form the cercopods, or the ovipositor.

In locusts, as we have observed[85] in the case of _Melanoplus spretus_,
the egg-shell bursts open at the head end, when the nymph, immediately
after extricating itself from the egg, casts off a thin pellicle (the
amnion), as we have also noticed in the case of the larvæ of the flea,
currant saw-fly, and other insects. Before the amnion is cast off, the
young nymph is almost motionless, but by slight movements of the body
draws itself, in about five minutes, out of the amnion. The exact
process of extraction is as follows: While it lies motionless, it puffs
out the thin, loose skin connecting the back of the head with the front
edge of the prothorax. The distention of this part probably ruptures the
skin, which slips over the head, the body meanwhile curved over until
the skin is drawn back from the head; when the latter is thrown back, it
withdraws its antennæ and legs, and the skin is in a second of time
pushed back to near the end of the abdomen; finally, it draws its hind
tarsi out of the skin, and in a moment or two more the young locust
frees itself, kicks away the cast skin, which resembles a little white
crumpled pellet, and which has also been compared to a diminutive
mushroom, and walks actively off,—sometimes, however, with the cast skin
adhering to the end of the abdomen. Before the shedding of the amnion
the body and legs are soft and flabby; immediately after, it walks
firmly on its legs. All the eggs hatched—at least one or more
hundreds—at about the same time, _i.e._ before 11 A.M.

[Illustration:

  FIG. 553.—Locust just before the amnion is cast, enlarged.—Emerton
    _del._
]

The nymph of _Stagmomantis carolina_ also sheds an amnion-skin, like
that of the locust; but the embryo before casting it off is much
elongated, and probably, like the European _Mantis religiosa_, the
curious elongated embryos have the same singular habit of suspending
themselves by threads, as shown in Fig. 554.

  The account by Pagenstecher of the first ecdysis of the European
  Mantis was so extraordinary that we asked Professor Cockerell to
  collect the eggs of our Stagmomantis in New Mexico and send them to
  us. This he has kindly done, writing that he can “hardly recognize a
  true moult, since all that is cast off is the egg-membrane. In
  short, Pagenstecher’s account must be not a little fanciful, unless
  our insect differs very much in its development from _Mantis
  religiosa_. The main change is that after leaving the egg the thorax
  enormously elongates, producing a bulging out, and thrusting the
  head forward.” Our observations on the alcoholic specimens fully
  corroborate Cockerell’s conclusions. Pagenstecher’s figure of the
  embryo appears to be inaccurate. Sharp states that the hatching
  nymphs remain suspended for some days until the “first change of
  skin is effected.” This so-called “skin” is evidently the amnion.

The 17–year Cicada, after hatching, is enveloped by the amnion, from
which it soon extricates itself, and then drops deliberately to the
ground, “its specific gravity being so insignificant that it falls
through the air as gently and as softly as does a feather.” (Riley.)

Other insects, as caterpillars, have room enough to turn around within
their shell and to eat their way through the walls of the chorion.

The meat-fly, as we have observed, hatches in the following manner. The
embryo moves to and fro, the body twisting until the exochorion is
ruptured; the egg-shell splits longitudinally, and in one or two seconds
the larva pushes its way out through the anterior end, and in a second
or two more extricates itself from the shell. The latter scarcely
changes its form, and the larva slips out, leaving the amnion within.

[Illustration:

  FIG. 554.—Egg-case of Mantis with young escaping: _A_, the case with
    young in their position of suspension. _B_, cerci magnified, showing
    the suspensory threads.—After Brongniart, from Sharp.
]

In the case of a fossorial wasp, _Specius speciosus_, which carries
Cicadæ into its burrow, laying an elongated egg on the body under the
median thigh of its victim, the larva on hatching, Riley states, “does
not emerge from the skin of the egg, but merely protrudes its head and
begins at once to draw nourishment from between the sternal sutures of
the Cicada.”

=The hatching spines.=—Animals belonging to quite distinct classes are
provided late in embryonic life with hard knobs or spines, which are
temporary structures for the purpose of breaking or cutting open the
egg-shell, when it is too thick and solid to be ruptured by the
movements of the embryo. The embryos of certain lizards, turtles, the
blind worm and some snakes, of the crocodile, and even birds, as well as
the duckbill and Echidna, are provided with them, always occurring, so
far as we are aware, on the end of the upper jaw. In the Arthropoda
similar structures have thus far only been met with in myriopods and
insects, though an analogous structure on the cephalothorax of the
embryo of phalangids has been observed by Balbiani. Metschnikoff
describes and figures a low conical spine serving this purpose situated
on the embryonal cuticle over the head of the advanced embryo of
Strongylosoma, and one on the 3d pair of mouth-parts of Geophilus.

In the winged insects, the embryo of Forficula is said by Heymons to
bear a single spine between the eyes, which serves as an egg-tooth. The
embryo of the Hemerobiidæ, according to Hagen, “opens the egg with an
egg-burster like a saw.” (Proc. Bost. Soc. Nat. Hist., xv, p. 247.)
Riley states that the egg-burster, or _ruptor ovi_, as he calls it, of
_Corydalus cornutus_, has “the form of the common immature mushroom,”
and he adds that it is a part of the amnion, being “easily perceived on
the end of the vacated shell.” Wheeler has observed three pairs of
broad-based chitinous “hatching spines” used by Doryphora in rupturing
its embryonic envelopes, and which are secreted by pyramidal thickenings
of the hypodermis (Figs. 555, 556).

[Illustration:

  FIG. 555.—The three pairs of hatching spines (_hsp_) on the late
    embryo of Doryphora.—After Wheeler.
]

[Illustration:

  FIG. 556.—Rudiment of the hatching spine: _eb_, being a thickening of
    the ectoderm (_ec_) in embryo Doryphora after formation of the
    heart; _s_, serosa.—After Wheeler.
]

[Illustration:

  FIG. 557.—Head of freshly hatched larva of _Pulex canis_: _eb_,
    hatching spine; _ant_, antennæ; _md_, mandible; _mx_, maxilla;
    _mx′_, 2d maxilla; _lbr_, labrum.
]

The hatching spine of _Pulex canis_ (Fig. 557) is a thin vertical plate,
like the edge of a knife, situated in the median line of the head very
near the posterior end, and is somewhat cultriform, the upper edge
slightly hollow, and turned up a little at the anterior end. Though we
did not see it working, it is situated at just the point on the head
where it would come in contact with the egg-shell, and it was evident
that the larva, by moving its head back and forth, would produce a
slight split in the chorion and cause it to burst asunder. Later on in
larval life it disappears, probably at the first moult.




                        LITERATURE ON EMBRYOLOGY


  =Koelliker, Albert.= Observationes de prima insectorum genesi, etc.
    Turici, 1842, pp. 29, 3 Pls.

  =Rathke, H.= Zur Entwickelungsgeschichte der Maulwurfsgrille
    (_Gryllotalpa vulgaris_). (Müller’s Archiv, 1844, ii, p. 27, Figs.
    1–5.)

  =Zaddach, G.= Untersuchungen über die Entwicklung und den Bau der
    Gliederthiere. I. Die Entwicklung des Phryganideneies. Berlin, 1854,
    pp. 138, 5 Taf.

  —— Ueber die Entwicklung der Insekten. (Schrift, d. k. phys.-oekon.
    Gesell. Königsberg, viii Jahrg., 1867, Sitzb., p. 16.)

  =Leuckart, Rudolph.= Die Fortpflanzung und Entwicklung der Pupiparen.
    Nach Beobachtungen an _Melophagus ovinus_. (Abhandl. Naturf. Gesell.
    Halle, iv, pp. 1–82, 3 Taf.) Halle, 1858.

  =Huxley, T. H.= On the organic reproduction and morphology of Aphis.
    Pt. I, 1858; Pt. II, 1858. (Trans. Linn. Soc., xxii, pp. 193–219,
    221–236, 5 Pls.)

  =Weismann, A.= Die Entwicklung der Dipteren im Ei, nach Beobachtungen
    an Chironomus sp., _Musca vomitoria_ und _Pulex canis_. (Zeitschr.
    f. wiss. Zool., xiii, 1863, pp. i-xvi, 1–263, 14 Taf.)

  —— Zur Embryologie der Insecten. (Arch. f. Anat. u. Physiol., 1864.)

  —— Beiträge zur Kenntnis der ersten Entwicklungsvorgänge im
    Insectenei. In: Beiträge zur Anatomie und Embryologie, etc.
    (Festschrift für J. Henle. Bonn, 1882.)

  =Kupffer, C.= Ueber das Faltenblatt an den Embryonen der Gattung
    Chironomus. (Arch. Micr. Anat., ii, 1866.)

  =Metschnikoff, E.= Embryologische Studien an Insecten. (Zeitschr. f.
    wiss. Zool., xvi, 1866, pp. 389–500, 10 Taf.)

  =Kupffer, Carl.= De embryogenesi apud Chironomos Observationes, etc.
    Kiliæ, 1867, pp. 16, 1 Pl.

  =Packard, A. S.= On the development of a dragon-fly (Diplax)
    [Æschna?]. (Proc. Bost. Soc. Nat. Hist., xi, 1868, pp. 366–372, 8
    Figs.)

  —— Embryology of Isotoma, a genus of Poduridæ. (Proc. Bost. Soc. Nat.
    Hist., xiv, 1870, pp. 13–15, 4 Figs.)

  —— Embryological Studies on Diplax [Æschna?], Perithemis, and the
    thysanurous genus Isotoma. (Mem. Peabody Academy of Science, Salem,
    i, 1871, pp. 1–21, 3 Pls.)

  —— Embryological studies on hexapodous insects. (Ibid., 1872, pp.
    1–17, 3 Pls.)

  —— The embryological development of the locust. (Chap. X, Third Report
    U. S. Ent. Commission, Washington, 1883, pp. 263–282, 7 Pls.)

  =Brandt, A.= Beiträge zur Entwicklungsgeschichte der Libelluliden und
    Hemipteren. (Mém. Acad. St. Pétersbourg (7), xiii, 1869, pp. 1–33, 3
    Taf.)

  —— Ueber das Ei und seine Bildungsstätte. (Leipzig, 1878, pp. 196, 4
    Taf.)

  —— Commentare zur Keimbläschentheorie des Eies. I. Die
    Blastodermelemente und Dotterballen der Insecten. (Arch. f. Micr.
    Anat., 1880, xvii.)

  =Melnikow, N.= Beiträge zur Embryonalentwicklung der Insecten. (Arch.
    f. Naturg., xxxv, 1869, pp. 137–189, 4 Taf.)

  =Bütschli, O.= Zur Entwicklungsgeschichte der Biene. (Zeitschr. f.
    wiss. Zool., xx, 1870, pp. 519–564, 4 Taf.)

  —— Bemerkungen über die Entwicklungsgeschichte von Musca. (Morph.
    Jahrb., xiv, 1888, pp. 170–174, 3 Figs.)

  =Kowalevsky, A.= Embryologische Studien an Würmern und Arthropoden.
    (Mém. Acad. St. Pétersbourg (7), xvi, 1871, pp. 1–70, 12 Taf.)

  —— Zur embryonalen Entwicklung der Musciden. (Biol. Centralbl., vi,
    1886, pp. 49–54.)

  =Müller, Fritz.= Beiträge zur Kenntniss der Termiten. (Jena. Zeitschr.
    Wissens., ix, 1875, pp. 241–263, 4 Taf.)

  =Oulganine, W. N.= (also spelled Uljanin). Sur le développement des
    Podurelles. (Arch. Zool. Expér., iv, 1875, pp. xxxix-xl, und v,
    1876, pp. xvii-xix.)

  —— Beobachtungen über die Entwicklung der Poduren. (Russian.) (Nachr.
    k. Gesellsch. Freunde Naturw., Anthrop. und Ethnogr., xvi, 1875.)

  =Dohrn, A.= Notizen zur Kenntniss der Insectenentwicklung. (Zeitschr.
    f. wiss. Zool., xxvi, 1876, pp. 112–138.)

  =Hatschek, B.= Beiträge zur Entwicklungsgeschichte der Lepidopteren.
    (Jena. Zeitschr. f. Naturw., xi, 1877, pp. 38, 3 Taf., 2 Figs.)

  =Bobretzky, N.= Ueber die Bildung des Blastoderms und der Keimblätter
    bei Insecten. (Zeitschr. f. wiss. Zool., xxxi, 1878, pp. 195–215, 1
    Taf.)

  =Graber, Vitus.= Vorläufige Ergebnisse einer grösseren Arbeit über
    vergl. Embryologie der Insecten. (Arch. f. Micr. Anat., xv, 1878,
    pp. 630–640, 1 Fig.)

  —— Ueber die Polypodie bei Insectenembryonen. (Morph. Jahrb., xiii,
    1888, pp. 586–615, 2 Taf.)

  —— Ueber die primäre Segmentirung des Keimstreifs der Insecten.
    (Morph. Jahrb., xiv, 1888, pp. 345–368, 2 Taf., 4 Figs.)

  —— Vergleichende Studien über die Keimhüllen und die Rückenbildung der
    Insecten. (Denkschr. Acad. Wiss. Wien., lv, 1888.)

  —— Vergleichende Studien über die Embryologie der Insecten und insbes.
    der Musciden. (Denkschr. Acad. Wiss. Wien., lvi, 1889.)

  —— Ueber den Bau und die phylogenetische Bedeutung der embryonalen
    Bauchanhänge der Insekten. (Biol. Centralbl., ix, 1889, pp.
    355–363.)

  —— Vergleichende Studien am Keimstreif der Insecten. (Denkschr. Acad.
    Wiss. Wien., lvii, 1890.)

  —— Üeber die embryonale Anlage des Blut- und Fettgewebes der Insecten.
    (Biol. Centralbl., xi, 1891, pp. 212–224.)

  —— Zur Embryologie der Insekten. (Zool. Anzeiger, xiv, 1891, pp.
    286–291.)

  —— Über die morphologische Bedeutung der ventralen Abdominalanhänge
    der Insekten-Embryonen. (Morph. Jahrb., xvii, 1892, pp. 467–482.)

  =Barrois, J.= Développement des Podurelles. (Assoc. Franc. p.
    l’Avance. des Sc., 7^e Sess., 1879.)

  =Kadyi, H.= Beitrag zur Kenntnis der Vorgänge beim Eierlegen der
    _Blatta orientalis_. (Zool. Anzeiger, 1879, ii, pp. 632–636.)

  =Tichomiroff, A.= Ueber die Entwicklungsgeschichte des Seidenwurms.
    (Zool. Anzeiger, ii Jahrg., 1879, pp. 64–67.)

  —— Zur Entwicklungsgeschichte des Seidenspinners (_Bombyx mori_) im
    Ei. (Arb. Laborat. Zool. Mus. Moskau, i, 1882, pp. vii, v, 1–80, 3
    Taf., 48 Figs.) (Russian.)

  —— Ueber die Entwicklung der =Calandra granaria=. (Biol. Centralbl.,
    x, 1890, p. 424.)

  =Balfour, Francis M.= A treatise on comparative embryology. i, ii.
    London, 1880. 2d edit., London, 1885.

  =Hertwig, O. und R.= Die Coelomtheorie. Versuch einer Erklärung des
    mittleren Keimblattes. Jena, 1881. (Jena. Zeitschr., xv, pp. 150, 3
    Taf.)

  =Selvatico, D. S.= Sullo sviluppo embrionale dei Bombicini. (Boll.
    Bachicoltura, viii, 1881.)

  =Lemoine, V.= Recherches sur le développement des Podurelles. (Assoc.
    Franç. pour l’Avanc. d. Sc. Congrès de la Rochelle, 1882.)

  =Balbiani, E. G.= Sur la signification des cellules polaires des
    Insectes. (Compt. rend. Ac. Sc. Paris, xcv, 1882.)

  —— Contribution à l’étude de la formation des organes sexuels chez les
    Insectes. (Recueil Zool. Suisse, ii, 1885.)

  =Korotneff, A.= Entwicklung des Herzens bei Gryllotalpa. (Zool.
    Anzeiger, vi Jahrg., 1883, pp. 687–690, 2 Figs.)

  —— Die Embryologie der Gryllotalpa. (Zeitschr. f. wiss. Zool., xli,
    1885, pp. 570–604, 3 Taf.)

  =Nusbaum, J.= Vorl. Mittheilung über die Chorda der Arthropoden.
    (Zool. Anzeiger, vi Jahrg., 1883, pp. 291–295, 3 Figs.)

  —— Die Entwicklung der Keimblätter bei _Meloë proscarabæus_. (Biol.
    Centralbl., viii, 1888, pp. 449–452, 2 Figs.)

  —— Zur Frage der Segmentirung des Keimstreifs und der Bauchanhänge der
    Insectenembryonen. (Biol. Centralbl., ix, 1889, pp. 516–522, 1 Fig.)

  —— Zur Frage der Rückenbildung bei den Insectenembryonen. (Biol.
    Centralbl., x, 1890, pp. 110–114.)

  —— Ueber die Entwicklungsgeschichte der Ausführungsgänge der
    Sexualdrüsen bei den Insecten. (In Polish, with German _résumé_, pp.
    39–42.) (“Kosmos” Lemberg, 1884, ix Jahrg.)

  —— Zur Embryologie des _Meloë proscarabæus_, Marscham. (In Polish,
    with Latin explanation of plates.) (“Kosmos” Lemberg, 1891.)

  =Schneider, A.= Ueber die Entwicklung der Geschlechtsorgane der
    Insecten. (Zool. Beiträge, herausg. v. A. Schneider, i, 1883.)

  =Will, L.= Zur Bildung des Eies und des Blastoderms bei den viviparen
    Aphiden. (Arb. Zool. Zoot. Inst. Würzburg, vi, 1883, pp. 217–258, 1
    Taf.)

  —— Entwicklungsgeschichte der viviparen Aphiden. (Spengel’s Zool.
    Jahrbücher. Abth. f. Anat. und Ont., iii, 1888, pp. 201–286, 5 Taf.)

  =Patten, W.= The development of Phryganids, with a preliminary note on
    the development of _Blatta germanica_. (Quart. Journ. Micr. Sc.,
    xxiv, 1884, pp. 54, 3 Pls.)

  —— Studies on the eyes of Arthropods. I. Development of the eyes of
    Vespa, with observations on the ocelli of some insects. (Journ. of
    Morphol., Boston, i, pp. 193–226, 1 Pl.)

  —— Studies on the eyes of Arthropods. II. Eyes of Acilius. (Journ. of
    Morphol., Boston, ii, 1888, pp. 97–190, 7 Pl., 4 Figs.)

  —— Eyes of molluscs and Arthropods. (Mitth. Zool. Station Neapel., vi,
    1888, pp. 542–756, 5 Pls.)

  =Grassi, B.= Interno allo sviluppo delle api nell’ uovo. (Atti Acad.
    Gioenia. Scienc. Nat. Catania (3), xviii, 1884.)

  —— Breve nota intorno allo sviluppo degli Japyx. Catania, 1884, also
    in I progenitori degli Insetti e dei Miriopodi. 1. L’ Japyx e la
    Campodea. (Atti Acad. Gioenia Sc. Nat. Catania (3), xix, 1885.)

  =Ayers, H.= On the development of _Œcanthus niveus_ and its parasite
    Teleas. (Mem. Boston Soc. Nat. Hist., iii, 1884, pp. 225–281, 8
    Pls.)

  =Witlaczil, Em.= Entwicklungsgeschichte der Aphiden. (Zeitschr. f.
    wiss. Zool., xl, 1884, pp. 559–696, 9 Taf.)

  =Hallez, P.= Orientation de l’embryon et formation du cocon chez la
    _Periplaneta orientalis_. (Compt. rend. Ac. Sc. Paris, ci, 1885, pp.
    444–446.)

  —— Sur la loi de l’orientation de l’embryon chez les Insectes. (Compt.
    rend. Ac. Sc. Paris, ciii, 1886, pp. 606–608.)

  =Heider, K.= Ueber die Anlage der Keimblätter von _Hydrophilus
    piceus_. (Abh. k. Acad. Wiss. Berlin, 1885.)

  —— Die Embryonalentwicklung von _Hydrophilus piceus_ L. (I. Theil.
    Jena, 1889, pp. 1–98, 13 Taf., 9 Figs.)

  =Carrière, J.= Kurze Mittheilungen aus fortgesetzten Untersuchungen
    über die Sehorgane. 7, Die Entwicklung und die verschiedenen Arten
    der Ocellen. (Zool. Anzeiger, ix Jahrg, 1886, pp. 141–147, 479–481,
    496–500.)

  —— Die Entwicklung der Mauerbiene (_Chalicodoma muraria_ Fabr.) im Ei.
    (Arch. f. Micr. Anat., xxxv, 1890, pp. 141–165, 1 Taf.)

  —— Die Drüsen am ersten Hinterleibsringe der Insectenembryonen. (Biol.
    Centralbl., xi, 1891, pp. 110–127, 3 Figs.)

  =Miall, L. C., and Denny, A.= The structure and life-history of the
    cockroach (_Periplaneta orientalis_). London, 1886. (The section on
    embryology by J. Nusbaum.)

  =Ryder, J.= The development of _Anurida maritima_ Guerin. (Amer.
    Naturalist, xx, 1886, pp. 299–302, 1 Pl)

  =Stuhlmann, F.= Die Reifung des Arthropodeneies nach Beobachtungen an
    Insecten, Spinnen, Myriopoden und Peripatus. (Ber. Freib.
    Naturf.-Gesellsch., i, 1886.)

  =Blochmann, F.= Ueber die Richtungskörper bei Insecteneiern. (Morph.
    Jahrbuch, xii, 1887, pp. 544–574, 2 Taf.)

  =Bruce, Adam Todd.= Observations on the embryology of insects and
    arachnids. A memorial volume. Baltimore, 1887, 4º, pp. 31, 6 Pls.

  =Weismann, A., und Ischikawa, Ch.= Ueber die Bildung der
    Richtungskörper bei thierischen Eiern. (Ber. Naturf. Ges. Freiburg,
    iii, 1887.)

  =Cholodkowsky, N.= Ueber die Bildung des Entoderms bei _Blatta
    germanica_. (Zool. Anzeiger, xi Jahrg., 1888, pp. 163–166, 2 Figs.)

  —— Studien zur Entwicklungsgeschichte der Insecten. (Zeitschr. f.
    wiss. Zool., xlviii, 1889, pp. 89–100, 1 Taf.)

  —— Zur Embryologie von _Blatta germanica_. (Zool. Anzeiger, xiii
    Jahrg., 1890, pp. 137–138.)

  —— Zur Embryologie der Hausschabe (_Blatta germanica_). (Biol.
    Centralbl., x, 1890, p. 425.)

  —— Ueber die Entwicklung des centralen Nervensystems bei _Blatta
    germanica_. (Zool. Anzeiger, xiv Jahrg., 1891, pp. 115–116.)

  —— Zur Embryologie der Insecten. (Ibid., 1891, pp. 465–466.)

  —— Die Embryonalentwicklung von Phyllodromia (_Blatta germanica_).
    (Mém. Acad. St. Pétersbourg (7), xxxviii, 1891.)

  =Henking, H.= Die ersten Entwicklungsvorgänge im Fliegenei und freie
    Kernbildung. (Zeitschr. f. wiss. Zool., xlvi, 1888, pp. 289–336, 4
    Taf., 3 Figs.)

  —— Ueber die Bildung von Richtungskörpern in den Eiern der Insekten
    und deren Schicksal. (Nachr. Ges. Wiss. Göttingen, 1888.)

  =Platner, G.= Die erste Entwicklung befruchteter und
    parthenogenetischer Eier von _Liparis dispar_. (Biol. Centralbl.,
    viii, 1888, pp. 521–524.)

  =Schmidt, F.= Die Bildung des Blastoderms und des Keimstreifs der
    Musciden. (Sitz. Naturf. Ges., Dorpat, viii, 1889.)

  =Viallanes, H.= Sur quelques points de l’histoire du développement
    embryonnaire de la Mante religieuse. (Rec. Biol. du Nord de la
    France, ii, 1889–1890.)

  =Voeltzkow, A.= Entwicklung im Ei von _Musca vomitoria_. (Arb. Zool.
    Zoot. Inst. Würzburg, ix, 1889, pp. 1–48, 4 Taf.)

  —— _Melolontha vulgaris_, ein Beitrag zur Entwicklung im Ei bei
    Insecten. (Arb. Zool. Zoot. Inst. Würzburg, ix, 1889, pp. 49–64, 1
    Taf.)

  =Wheeler, W. M.= Ueber drüsenartige Gebilde im ersten Abdominalsegment
    der Hemipterenembryonen. (Zool. Anzieger, xii, 1889, pp. 500–504, 2
    Figs.)

  —— The embryology of _Blatta germanica_ and _Doryphora decemlineata_.
    (Journ. of Morph., Boston, iii, 1889, pp. 291–374, 7 Pls., 16 Figs.)

  —— On the appendages of the first abdominal segment of the embryo
    cockroach (_Blatta germanica_). (Proceed. Wis. Acad. Science, Arts,
    and Letters, viii, 1890, pp. 87–140, 3 Pls.)

  —— Ueber ein eigenthümliches Organ im Locustidenembryo (_Xiphidium
    ensiferum_). (Zool. Anzieger, xiii, 1890, pp. 475–480.)

  —— Neuroblasts in the Arthropod embryo. (Journ. of Morphol., iv, 1891,
    pp. 337–343, 1 Fig.)

  —— A contribution to insect embryology. In. Diss. (Journ. Morph.,
    Boston, viii, 1893, pp. 1–160, 6 Pls., 7 Figs.)

  =Heymons, R.= Ueber die hermaphroditische Anlage der Sexualdrüsen beim
    Männchen von _Phyllodromia_ (Blatta) _germanica_. (Zool. Anzeiger,
    xiii Jahrg., 1890, pp. 451–457, 3 Figs.)

  —— Die Entstehung der Geschlechtsdrüsen von _Phyllodromia_ (Blatta)
    _germanica_ L. In. Diss. Berlin, 1891.

  —— Die Embryonalentwickelung von Dermapteren und Orthopteren unter
    besonderer Berücksichtigung der Keimblätterbildung. Monographisch
    bearbeiteit, 12 Lith. Taf. und 33 Figs. Jena, 1895, 4º, pp. 136.

  —— Ueber die Fortpflanzung und Entwickelungsgeschichte der _Ephemera
    vulgata_ L. (Sitzungsb. Gesell. Naturf. Freunde, Berlin, 1896, pp.
    81–96.)

  —— Entwicklungsgeschichtliche Untersuchungen an _Lepisma saccharina_
    L. (Zeitschr. wiss. Zool., lxii, 1897, pp. 588–631, 2 Taf.)

  =Ritter, R.= Die Entwicklung der Geschlechtsorgane und des Darmes bei
    Chironomus. (Zeitschr. f. wiss. Zool., 1, 1890, pp. 408–427, 1 Taf.)

  =Tichomirowa, O. S.= Zur Embryologie von Chrysopa. (Biol. Centralbl.,
    x, 1890, p. 423.)

  =Pedaschenko, D.= Sur la formation de la bandelette germinative chez
    _Notonecta glauca_. (In Russian.) (Revue Sc. Natural. St.
    Pétersbourg, i, 1891.)

  =Korschelt, E., and Heider, K.= Lehrbuch der vergleichenden
    Entwicklungsgeschichte der wirbellosen Thiere. (Spec. Th. Heft ii,
    Jena, 1891, many Figs.)

  With the writings of Lang (Comp. Anatomy).


                             END OF PART II




                 PART III.—THE METAMORPHOSES OF INSECTS


We have seen that the embryo rapidly passes through extraordinary
changes of form, and now, after hatching, especially in the insects with
a complete metamorphosis, the animal continues to undergo striking
changes in form, in adaptation to different modes of life.

The life of a winged insect, such as a butterfly, fly, or bee, may be
divided into four stages: the embryo, or egg state, the larva, pupa, and
imago,—the term _metamorphosis_ being applied to the changes after
birth, or post-embryonic stages of life. The transformations of the more
specialized orders of insects involve wonderful changes of form, which
are only paralleled in other types of animals by the metamorphoses of
the echinoderms, of certain worms, and of the Crustacea, as well as by
those of the frog. An insect, such as a butterfly or bee, during its
post-embryonic life lives, so to speak, three different lives, having
distinct bodily structures and existing under quite dissimilar
surroundings and habits; so that a caterpillar is practically a
different animal from the pupa, and the latter from the imago, with
different organs, the appendages and other structures being so modified
as to be, so far as regards their functions, radically different. These
changes of functions or of habits have also been plainly enough the
exciting cause of the divergency in structure of what fundamentally is
one and the same organ, the change having been brought about by
adaptation of the same organs to quite different uses.

The changes are not only observable in the body and its appendages, but
also in the internal organs, and consequently are both structural and
physiological. The term _larva_, as applied to the first stage of
animals, is a very variable and indefinite one, that of insects in
general being a much more highly organized animal than the larva of a
worm, starfish, or crustacean.


         _a._ The nymph as distinguished from the larval stage

As there is no marked difference between the different stages of the
young in the insects with an incomplete metamorphosis (Heterometabola),
the chief difference being the possession of the rudiments of wings and
the absence of a resting stage, the terms _larva_ and _pupa_ are in
reality scarcely applicable to them, and we much prefer the term
_nymph_, first proposed by Lamarck for the active “pupa” of Orthoptera,
Hemiptera, the Odonata and Ephemeridæ, and adopted in part by many.
Indeed, in the more generalized and older orders, the larval and pupal
stages are not differentiated, though the term _larval_, in its general
sense, will probably always be used; just as we speak of the larval
stages of worms, echinoderms, or Crustacea.

  Eaton in his elaborate work on the Ephemeridæ employs the term
  _nymph_ to designate all the aquatic or early stages in the
  development of the young after hatching, and he urges that the
  old-fashioned usage of _larva_ and _pupa_ seem scarcely worth
  retention. “Nymphs are young which live an active life, quitting the
  egg at a tolerably advanced stage of morphological development and
  having the mouth-parts formed after the same main type of
  construction as those of the adult insect.” The word _nymph_ is used
  in the same sense by McLachlan, and by Cabot. Calvert also applies
  the term _nymph_ “to the stage of odonate existence between the egg
  and the transformation into the imago.” On the other hand, Brauer
  applies the term _nymph_ to the pupa of holometabolous insects. For
  larval Hyatt proposes the term _nepionic_.


                 _b._ Stages or stadia of metamorphosis

The intervals or periods between the moults or ecdyses of caterpillars
and other eruciform larvæ are called stages or _stadia_; thus, as most
caterpillars moult four times, we have five stages or stadia, or stage
(stadium) I to V. As observed by Sharp, there is, unfortunately, no term
in general use to express the form of the insect at the various stadia;
“entomologists say, ‘the form assumed at the first moult,’ and so on.”
Hence he adopts a term suggested by Fischer,[86] and calls the insect as
it appears after leaving the egg the first _instar_, and what it is
after the first moult the second instar, and so on; hence the pupa, or
chrysalis, which assumed that condition after moulting five times would
be the sixth instar, and the butterfly itself would be the seventh
instar.


                 _c._ Ametabolous and metabolous stages

In the Synaptera development is direct, the young differing neither in
form, structure, or habits from the adult. Hence they are said to be
_ametabolous_. Since there is an absence of even a tendency to a partial
metamorphosis, it is evident that the insects have not inherited a
tendency to undergo a transformation, but that it is an adaptation
induced in the hexapod type after the first winged insects appeared, and
which became more marked in the more specialized insects and at a period
comparatively late in geological history, _i.e._ perhaps at or soon
after the beginning of the Carboniferous period.[87]

The transformations of the pterygote insects vary greatly in degree, and
it is difficult to draw the line between the grades. Those in which the
adults differ from the freshly hatched young only or mainly in having
wings are generally said to have an incomplete or gradual metamorphosis.
There is no inactive, resting, or pupal stage, and the wings are
acquired only after successive moults. Insects with an incomplete
metamorphosis are the Orthoptera, Dermaptera, Platyptera (Mallophaga,
Plecoptera, Corrodentia, Embidæ), Ephemeridæ, Odonata, Thysanoptera, and
Hemiptera, with the exception of the male Coccidæ, in which there is a
resting or sub-nymph stage. As regards the number of moults in the
Synaptera, Grassi states that in Campodea there is a single fragmentary
ecdysis, while Sommers tells us that _Macrotoma plumbea_ sheds its skin
throughout life, even after attaining its full size.

As an example of the partial metamorphosis of the hemimetabolous insects
we may select that of the locust, in which there are five moults and six
stages (instars), as seen in Fig. 558, five of which are nymphal. In the
first two stages there are no rudiments of wings, these appearing after
the second moult. Besides the acquisition of wings there are slight
differences after each moult, both in structure and color, besides size,
so that we may always recognize the comparative age and the particular
stage of growth of any individual.[88]

  We have watched the development of _Melanoplus spretus_ from the egg
  to the imago, and examined thousands of specimens which show the six
  stages. On the other hand, European authors differ as to whether
  there are three, four, or five moults in the migratory locust.[89]
  It is not improbable that, as is the case with many other insects,
  the number of moults may vary according to the temperature and food,
  variation in these agencies causing either retardation or rapidity
  in development.

Those with a complete metamorphosis are said to be _metabolous_ or
_holometabolous_. (Lang.)

Leach[90] in 1815 gave the name of Ametabola to insects without, and
Metabola to insects with a metamorphosis.

[Illustration:

  FIG. 558.—Partial metamorphosis of _Melanoplus femur-rubrum_, showing
    the five nymph stages, and the gradual growth of the wings, which
    are first visible externally in 3, 3_b_, 3_c_.—Emerton _del._
]

  Latreille (1831) called insects with an incomplete metamorphosis
  _homotenous_ (which means similar to the end of life), and those
  with a complete metamorphosis, _polymorphous_. For the different
  degrees of metamorphosis of insects he employed two terms: for the
  incomplete degree, _metamorphosis dimidia_, and for the total or
  pupal, _metamorphosis perfecta_.

  Westwood in his Introduction to the Modern Classification of Insects
  (1839), taking into account the relation of the larva with the
  imago, divided insects into two divisions: the _Heteromorpha_, or
  those in which there is no resemblance between the parent and its
  offspring, and _Homomorpha_, in which the larva resembles the imago,
  except in the absence of wings.

[Illustration:

  FIG. 559.—Manometabolous metamorphosis of the cockroach (_Phyllodromia
    germanica_) with its four nymphal stadia _a-d_; _e_, _h_, adult;
    _f_, female with egg-case; _g_, egg-case.—From Riley.
]

From the point of view of the degree of metamorphosis, insects have been
divided into _Heterometabola_ and _Metabola_.

I. _Heterometabola._—This group may be divided as follows:

1. _Manometabola_,[91] embracing those forms with a slight or gradual
metamorphosis, but which are active in all the stages, without any
resting stage. The orders passing through this degree of metamorphosis
are the following: Orthoptera, Dermaptera, Platyptera, Thysanoptera, and
Hemiptera (Coccidæ excepted).

In all these groups, the only external differences of importance between
the freshly hatched nymph and the adult is the presence of wings. The
chief difference internally is the complete development of the sexual
glands.

It should be observed, however, that in the last nymph stage of the
Thysanoptera the articulations of the limbs are enveloped by a membrane
and the wings enclosed in short fixed sheaths; the antennæ are turned
back on the head, and the insect, though it moves about, is much more
sluggish than in the other state. (Haliday.) Hence here we have a close
approach to the following degree.

2. _Heremetabola_,[92] including those forms with a gradual though
slight or incomplete metamorphosis, but with a quiescent or resting
stage at the close of the nymph life. Lang has emphasized this stage,
calling attention to the fact that the fore legs of the nymph of the
17–year Cicada, which lives underground on the roots of trees, are thick
and adapted for digging. The transition from the nymph to the winged
adult is signalized by the decided change in form of the fore legs, as
well as by the acquisition of the wings. “The last larval stage is,
then, what is called _quiescent_, _i.e._ the organization of the imago
develops within the chrysalis at the expense of the accumulated reserve
material.” (Lang.) There seems to be a resting stage, when the insect
does not perhaps suck the sap from the roots, and awaits in its chamber
its approaching change to the imago; but we should scarcely apply the
term _pupa_ to this stage, though the antennæ of the freshly hatched
larva are larger and longer than in the fully grown nymph and are
distinctly 8–jointed.

3. _Hemimetabola._—In this division, so named by Brauer, the changes are
more marked, though there is no truly inactive pupa-like stage. The
orders are Perlaria (Plecoptera), Odonata, and Plectoptera (Ephemeridæ).
The freshly hatched nymphs of these three groups are much alike in
shape, that of Perlidæ, and indeed most of the Platyptera, being more
generalized, unless we except that of Chloëon; all closely recall
Campodea, and are therefore in the Campodea-stage. These nymphs are
indeed more generalized than the freshly hatched nymph of Blattidæ, or
any other of the orders mentioned except the Platyptera, to which
perlids belong. They all have feet, and the body is more or less
flattened. (Fig. 560.)

II. _Holometabola._—In this division we have for the first time a true
larva, and a pupa stage as distinguished from the imago. Moreover, the
insect at each stage is distinguished by radical differences in form,
surroundings, and in the nature of the food, while the pupa is inactive,
usually immovable, and incapable of taking any food, and is often
protected by a cocoon spun by the larva. The holometabolous orders are
the Neuroptera, Coleoptera, Mecoptera, Trichoptera, Lepidoptera,
Siphonaptera, Diptera, and Hymenoptera.

  As we have among worms, echinoderms, and Crustacea certain
  exceptional species in a metamorphic group whose metamorphosis is
  suppressed, their development being direct, so there is in pterygote
  insects, though in a very much less degree, cases of direct
  development. In the wingless cockroaches such as Pseudoglomeris,
  etc., of the tribe of Periphæriides, in some of which, however, the
  males are winged, and in the Hemiptera, occur wingless forms such as
  the lice and bed-bug. The Mallophaga are all wingless, while certain
  Dermaptera (Chelidura, Anisolabis) are also apterous. The absence of
  wings in such cases is due to disuse from parasitism, or to a life
  under stones or in cracks and fissures, where the insects are driven
  to avoid their enemies, and hence do not need wings. The growth of
  wings and consequently the development of a metamorphosis is
  suppressed, so that, as Lang says, “in contrast to the original
  ametabola of the Apterygota, we have here an _acquired ametabola_.”

  It is rare that, after the rudiments of wings have once appeared in
  the very young, they should disappear in the late nymph stage; this
  is, however, said by Walsh to be the case with the Ephemerid Bætisca
  (Fig. 440). This is a case of retardation in an acquired
  ametabolesis.




                               THE LARVA


The term _larva_ is peculiarly applicable to the young of the
holometabolous orders. The name (Latin, _larva_, a mask) was first given
to the caterpillar because it was thought by the ancients to mask the
form of the perfect insect. Swammerdam supposed that the larva contained
within itself “the germ of the future butterfly, enclosed in what will
be the case of the pupa, which is itself included in three or more
skins, one over the other, that will successively cover the larva.” What
led to his conception of the nature of these changes was probably his
observations on the semitransparent larva of the gnat, in which the body
and limbs of the pupa can be partially seen; for Weismann has shown that
the great Dutch observer’s belief that the pupal and imaginal skins were
in reality already concealed under that of the larva is partially
founded in fact. Swammerdam states: “I can point out in the larva all
the limbs of the future nymph, or Culex, concealed beneath the skin,”
and he also observed beneath the skin of the larvæ of bees, just before
pupating, the antennæ, mouth-parts, wings, and limbs of the adult. But,
as we shall see farther on, the discovery by Weismann in the larva of
the germs of the imago has completely changed our notions as to the
nature of metamorphosis, and revolutionized our knowledge of the
fundamental processes concerned in the change from larva to pupa, and
from pupa to imago.

Not only are the larvæ of each order of insects characteristic in form,
so that the grub or larva of beetles is readily distinguished from those
of other orders, or the maggot of flies from the apodous larva of wasps
and bees, but within the limits of the larger orders there is great
diversity of larval forms, showing that they are the result of
adaptation to their surroundings. This is especially the case with the
larvæ of the Coleoptera, Lepidoptera, Diptera, and Hymenoptera.

In general, the larvæ of insects may be divided into two types,—the
_Campodea-form_, or campodeoid, sometimes called thysanuriform, and the
_eruciform_.


                  _a._ The Campodea-form type of larva

This is the most primitive and generalized type of larva (Fig. 560). A
Campodeoid larva is one nearest in general shape to Campodea, the form
which we have seen to be the nearest allied to the probable ancestor of
the insects, and it also resembles the nymphs of the heterometabolous
insects, before the appearance of their rudimentary wings.

Brauer, in 1869,[93] first suggested that the larvæ of a great number of
insects may be traced back to Campodea and Iapyx. The Campodea-form
larva is active, with a more or less flattened body, well developed
mandibulate mouth-parts, and usually long legs. The nearest approach to
the form of Campodea is the freshly hatched nymph of cockroaches
(Blattidæ), Forficula, Perlidæ, Termitidæ, Psocidæ, Embidæ, Ephemeridæ,
Odonata, especially the more generalized Agrionidæ, the nymphs of
Hemiptera, the larvæ of certain Neuroptera, the active pedate larvæ of
the more generalized Coleoptera, such as those of Carabidæ, Cicindelidæ,
Dyticidæ, etc., and the first larva (instar) of Stylopidæ and Meloidæ
(Fig. 560, _d_).

While the Campodea-shape is retained throughout nymphal life, of the
orders above mentioned the Neuroptera and Coleoptera alone have a true
resting pupal stage.

It should also be observed that great changes in the form of the nymph
occur within the limits of the Orthoptera; the nymph of all the families
except that of the Blattidæ, evidently the most generalized and
primitive, being more or less specialized, while the nymphs of the other
orders all vary in degree of specialization and modification. The
process of adaptation once begun went on very rapidly, as it has in many
other orders of insects, as well as in animals of other phyla.

[Illustration:

  FIG. 560.—Examples of campodeoid nymphs and larvæ: _a_, Campodea; _b_,
    Podura (Degeeria); _c_, Lepisma; _d_, triungulin larva of Meloë;
    _e_, Perla; _f_, Forficula; _g_, Chloëon; _h_, May-fly (Palingenia);
    _i_, Æschna; _j_, Atropos; _k_, Myrmeleon; _l_, Sialis; _m_,
    Corydalus; _n_, Cicada.
]


                    _b._ The eruciform type of larva

Brauer also sagaciously pointed out that “a larger part of the most
highly developed insects assume another larva form, which appears not
only as a later acquisition, through adaptation to certain definite
conditions, but also arises as such before our eyes. The larvæ of
Lepidoptera, of saw-flies, and Panorpidæ show the form most distinctly,
and I call this the caterpillar form (_Raupenform_). That this is not
the primitive form, but one later acquired, we see illustrated in
certain beetles. The larvæ of Meloë and of Sitaris, in their fully grown
conditions, possess the caterpillar form, but the new-born larvæ of
these genera show the Campodea-form. The last form is lost as soon as
the larva begins its parasitic mode of life.... The larger part of the
beetles, the Neuroptera (in part), the bees and flies (the last with the
most degraded maggot form), possess larvæ of this second form.” In 1871
we adopted these views, giving the name _eruciform_ to this type of
larvæ, and afterwards Lubbock adopted Brauer’s views. Brauer considered
that the eruciform larva was the result of living a stationary
semi-parasitic life on plants, in carrion, or burrowing in the trunks
and branches or leaves and buds of trees, where they do not have to move
about in search of their food. The change from the Campodea-form to the
eruciform larva is a process of degeneration and often of atrophy of the
limbs, and, in the footless forms of dipterous and hymenopterous
insects, of the gnathites, accompanied by a tendency of the body to
become more or less cylindrical.

The first steps in the origination of the eruciform larva were
apparently taken in the order Neuroptera, as restricted by Brauer and by
myself, where, though the larvæ are campodeoid, there is a true resting
pupal stage. The most generalized larval form is perhaps that of the
Sialidæ (Fig. 560, _l_), in which the body tends to be slightly
cylindrical, though the legs are long, and the gnathites well developed
for seizing and biting their living prey. The terrestrial larvæ of the
Hemerobiidæ, though modifications of the sialid larval form, are
considerably specialized in adaptation to their active carnivorous
habits. But the life-history of Mantispa, where there are two larval
stages, gives us plainly enough the key to the mode in which the
complete metamorphosis was brought about. The larva, born a true
Campodea-like form, with large, long, 4–jointed legs, has a structure
which would enable it to move about freely after its prey, beginning at
once to live a sedentary life in the egg-sac of a spider; before the
first moult it loses the use of its legs, while the antennæ are partly
aborted. The result is that, owing to this change of habits and
surroundings from those of its active ancestors, it changes its form,
and the fully grown larva becomes cylindrical, with small slender legs,
and, owing to the partial disuse of its jaws, acquires a small, round
head.

[Illustration:

  EXAMPLES OF COLEOPTEROUS LARVÆ, SHOWING THE PASSAGE FROM THE
    CAMPODEOID TO THE ERUCIFORM TYPE OF LARVÆ.

  FIG. 561.—Coleopterous larvæ showing passage from campodeoid to
    eruciform larvæ: _a_, _b_,
  Harpalus; _c_, Dyticus; _d_, Staphylinus; _e_, Silpha; _f_,
    Melanactes; _g_, Ludius; _h_, Elater; _i_, Donacia;
  _j_, Chrysobothris; _k_, Orthosoma; _l_, Coccinella; _m_, Byrrhus;
    _n_, Trox; _o_, _p_, Lachnosterna; _q_,
  Labidomera; _r_, Ptinus; _s_, Anobium; _t_, Balaninus (entirely
    apodous).
]

Its antennæ, mouth-parts, and legs not only retarded in growth, but
retrograding and becoming vestigial, the body meanwhile becoming fat and
cylindrical, an apparent acceleration of growth goes on within, with
probably an enlargement of the intestine and fat-body, and thus the
pupal form is perfected while the larva is full-fed and quiescent. It is
not improbable that in the primitive neuropteron, as the result of a
mode of life like that of Mantispa, the quiescent life of the later
stages graduated into a quiescent, inactive pupal life, allowing the
changes going on in the internal organs to result in a complete
metamorphosis, which was transmitted to the later Neuroptera, thus
making the complete metamorphosis a fixed, normal condition. It thus
appears that a change of habits and of food, and more especially the
fact that the nymph became so surrounded with an abundance of food close
at hand that it did not have to run actively about and seize it in a
haphazard manner, were the factors bringing about a change from the
Campodea-form nymph to the eruciform larva, thus inducing a
hypermetamorphosis.

The larvæ of the Mecoptera (Panorpidæ, Fig. 562, _b_) are still more
caterpillar-like, and besides their cylindrical body, rounded head,
small short gnathites, small thoracic legs, they have what appear to be
2–jointed legs to each of the nine abdominal segments, and the close
resemblance to caterpillars is farther carried out by the presence of a
pair of prothoracic spiracles, none existing on the other two thoracic
segments.

[Illustration:

  FIG. 562.—Examples of eruciform larvæ: _a_, Phryganea; _b_, Panorpa;
    _c_, Sesia; _d_, _d_, caterpillars; _e_, Selandria; _f_, Tipula;
    _g_, Simulium; _h_, Chionea; _i_, Musca; _j_, Tachina; _k_, Braula;
    _l_, flea; _m_, Tremex; _n_, coarctate larva of Meloë; _o_, bee
    (Andrena).
]

In the Meloidæ (Fig. 560, _d_) and Stylopidæ the first larval stage is
Campodea-form; the changes will be described in the subsequent section
on Hypermetamorphosis, and while these cases of change from a campodeoid
to an inactive eruciform larva are very salient, if we compare the
graduated series of larval forms throughout the order of Coleoptera, as
represented by the illustrations in Fig. 561, we shall see that in
nearly, if not each, case the form of the boring or mining, or bark or
bud or seed-inhabiting grub is the result of a change of habit and
commissariat from active predaceous larvæ, like those of the Carabidæ
and other adephagous families, together with those of the Staphylinidæ,
with their flat body, big mandibles, and well-developed maxillæ, to the
cylindrical bodies of such larvæ as those of Dermestes and Anthrenus,
which live a more sedentary life, to the root-feeding wire-worm or
elaterid larvæ, and scarabæid grubs, onward to the phytophagous
Chrysomelidæ, with the mining and boring buprestids and cerambycids,—in
all these forms we see a gradual atrophy of the legs, which is fully
carried out in the vermiform or maggot-like larva of the weevils. These
changes throughout the members of the entire order are epitomized in the
life-history of the Meloidæ, in which there are three typical forms of
larva: the Campodea-form (triungulin stage), eruciform (second or
carabidoid stage), and vermiform (coarctate) larva.

[Illustration:

  FIG. 563.—_Prodoxus cinereus_: _a_, apodous larva; _b_, head and
    prothoracic segment; _c_, anal hooks; _d_, pupa; _e_, cast pupal
    shell protruding from stalk of Yucca; _f_, female; _g_, side view of
    ♂ clasper.—After Riley, from Insect Life.
]

[Illustration:

  FIG. 564.—Larva of _Limacodes scapha_, nat. size.
]

In the Lepidoptera the eruciform, pedate type is adhered to throughout
the order, with the rare exception of the nearly apodous mining larva of
Prodoxus (Fig. 563, _a_), Phyllocnistis, and Nepticula, which have no
thoracic legs, and the limacodid larvæ, whose abdominal legs are totally
aborted, while the thoracic ones are much reduced (Fig. 564).

In the Hymenoptera the phytophagous forms are eruciform, while by the
agency of the same factors as already mentioned, _i.e._ a sedentary or
parasitic life and abundance of food within constant reach, the larvæ
lose their legs and become vermiform.

In the Diptera, which are the most highly specialized of insects, the
maggot or vermiform shape, and absence of any legs, prevails throughout
the order, though the eucephalous larvæ show their origin from a
primitive eruciform type of larva. The highly specialized larvæ of the
Culicidæ and Simuliidae are undoubtedly related to the earliest and most
generalized types, while the maggots of the parasitic flies (Tachinidæ)
and other muscids are later degradational forms, and the result of
adaptation induced, as in the previous cases, by a sedentary or
parasitic mode of life, living as they do immersed in an abundance of
rich nitrogenous food, with the result that the mouth-parts have become
atrophied by disuse, while the limbs have become entirely aborted,
though the thoracic imaginal discs develop normally in the embryonic or
pre-larval stages.

It appears, therefore, highly probable that the metamorphoses of insects
are the result of the action of change of conditions, just as the
polymorphism of Termites is with little doubt the result of differences
of food and other conditions. These matters will be farther discussed
under the head of Causes of Metamorphosis.




                LITERATURE ON ANCESTRY OF INSECTS, ETC.


  =Müller, Fritz.= Für Darwin, 1869, pp. 144, 67 Figs.

  =Brauer, Friedrich.= Betrachtung ueber die Verwandlung der Insekten im
    Sinne der Descendenz-Theorie. (Verhandlung d. k.k. zool. bot.
    Gesell. Wien., 1869, 1 Taf., pp. 1–20.)

  =Packard, A. S.= Amer. Naturalist, iii, p. 45, March, 1869.

  —— Proc. Boston Soc. Nat. Hist., xiv, 1870, p. 61.

  —— Amer. Nat., iv, Feb. 1871, p. 756; v, 1871, pp. 52, 567.

  —— Embryological Studies. (Memoirs Peabody Acad. Sc. Salem, 1871–72.)

  —— Our common insects, 1873, Chapter on Ancestry of Insects, pp.
    175–178.

  —— Third Report U. S. Ent. Commission, 1883, pp. 295–304.

  =Lubbock, John.= On the origin of insects. (Journ. Linn. Soc., London,
    xl, 1873.)

  —— Origin and metamorphoses of insects. (Nature, 1873 [in book form,
    1874], pp. 108, 66 Figs.)

  =Mayer, Paul.= Ueber Ontogenie and Phylogenie der Insekten. (Jena.
    Zeitschr. Wissens., x, 1876, pp. 125–221, 4 Taf.)

  =Hyatt, A., and Arms, J. M.= Insecta. (Bost. Soc. Nat. Hist. Guides
    for science-teaching, viii.) Boston, 1890, pp. 300, 13 Pls., 223
    Figs.


             _c._ Growth and increase in size of the larva

The rapidity of growth and enormous increase in size in early life is
especially noticeable in caterpillars and other phytophagous larvæ. The
latest observations are those of Trouvelot on _Telea polyphemus_. When
this silkworm hatches, it weighs 1⁄20 of a grain.

When

  10 days old it weighs ½ a grain, or   10 times the original weight.
  20 days old it weighs      3 grains   60 times the original weight.
  30 days old it weighs     31 grains  620 times the original weight.
  40 days old it weighs     90 grains 1800 times the original weight.
  56 days old it weighs    207 grains 4140 times the original weight.

“When,” he says “a worm is 30 days old, it will have consumed about 90
grains of food; but when 56 days old, it is fully grown and has consumed
not less than 120 oak leaves, weighing ¾ of a pound; besides this it has
drank not less than ½ an ounce of water. So the food taken by a single
silkworm in 56 days equals in weight 86,000 times the primitive weight
of the worm. Of this about ¼ of a pound becomes excrementitious matter,
207 grains are assimilated, and over 5 ounces have evaporated.”[94]

  Dandolo stated that the Asiatic silkworm (_Bombyx mori_) weighs on
  hatching not over 1⁄100 of a grain, but when fully grown about 95
  grains. During this period, therefore, it has increased 9500 times
  its original weight, and has eaten 60,000 times its weight of food.
  Newport thought this estimate of the amount of food was a little too
  great. But comparing it with Trouvelot’s estimate for the American
  silkworm, which weighs when hatched five times as much, it would not
  appear to be so. Newport found that the larva of _Sphinx ligustri_
  at the moment of leaving the egg weighs about 1⁄80 of a grain, and
  when fully fed 125 grains, so that in the course of 32 days it
  increases about 9976 times its original weight. This proportion of
  increase is exceeded by the larva of _Cossus ligniperda_, which,
  boring in the trunks of trees, remains about three years in the
  larva state, and increases, according to Lyonet, to the amount of
  72,000 times its first weight.

  Newport adds that those larvæ in which the proportion of increase is
  the greatest, are usually those which remain longest in the pupa
  state, as in the silkworm. “Thus Redi observed in the maggots of the
  common flesh-flies a rate of increase amounting to about 200 times
  the original weight in 24 hours, but the proportion of increase in
  these larvæ does not at all approach that of the Sphinx and Cossus.”
  From his observations on the larva of one of the wild bees
  (_Anthophora retusa_) Newport believes that this is also the case
  with the Hymenoptera. The weight of the egg of this insect is about
  1⁄150 of a grain, and the average of a full-grown larva 68⁄10
  grains, so that its increase is about 1020 times its original
  weight; “which compared with that of the Sphinx of medium size, is
  but as 1 to 9¾, and to a Sphinx of maximum size only as 1 to a
  little more than 11.”

  The growth is most rapid after the last moult. “Thus a larva of
  _Sphinx ligustri_, which at its last change weighed only about 19 to
  20 grains, at the expiration of eight days, when it was fully grown,
  weighed nearly 120 grains.” (Newport.)


                 _d._ The process of moulting (ecdysis)

Insects periodically shed the exoskeleton, together with the chitinous
lining of their internal organs of ectodermal origin, which thus
sloughed off are called the _exuvia_. The process in the locust has been
described by Riley.[95] It occupies from half to three-quarters of an
hour (Fig. 565). This process has naturally, from the ease with which it
can be observed, been most frequently examined in the Lepidoptera,
though careful and detailed observations of the inner and outer changes
are still greatly needed, especially in other orders. In the caterpillar
of most moths, especially one of the more generalized bombycine moths,
on slipping out of its egg-shell the head is of enormous size as
compared with the body, but the latter soon fills out after the creature
has eaten a few hours; the head, of course, does not during this time
increase in size, and the larvæ of different instars may be exactly
distinguished, as Dyar has shown, by the measurements of the head.

[Illustration:

  FIG. 565.—Process of moulting from nymph to imago in the locust (_M.
    spretus_): _a_, nymph with skin just split on the back; _b_, the
    imago drawing itself out, at _c_, nearly free; _d_, the imago, with
    wings expanded; _e_, the same with all parts perfect.—After Riley.
]

Before the caterpillar moults, it stops feeding, and the head is now
small compared with the body; the head of the second instar is now
large, situated partly under the much-swollen prothoracic segment, and
pushes the head of the first instar forward.

Newport has well described the mode of shedding the skin in _Sphinx
ligustri_, and his detailed description will apply to most lepidopterous
larvæ.

  The whole body is wrinkled and contracted in length, and there are
  occasionally powerful contractions and twitchings of its entire
  body; the skin becomes dry and shrivelled, and is gradually
  separated from the new and very delicate one of the next instar
  beneath. After several powerful efforts of the larva the old skin
  cracks along the middle of the dorsal surface of the mesothoracic
  segment, and by repeated efforts the fissure is extended into the
  1st and 3d segment, while the covering of the head divides along the
  vertex and on each side of the clypeus. “The larva then gradually
  presses itself through the opening, withdrawing first its head and
  thoracic legs, and subsequently the remainder of its body, slipping
  off the skin from behind like the finger of a glove. This process,
  after the skin has once been ruptured, seldom lasts more than a few
  minutes. When first changed the larva is exceedingly delicate, and
  its head, which does not increase in size until it again changes its
  skin, is very large in proportion to the rest of the body.” (Art.
  Insecta, etc.)

  Trouvelot’s account is more detailed and an advance on that of
  Newport’s view. He explicitly states, and we know that he was a very
  close observer, that the old skin is detached by “a fluid which
  circulates between it and the worm.” His account is as follows: The
  polyphemus worm, like all other silkworms, changes its skin five
  times during its larval life. The moulting takes place at regular
  periods, which comes around about every 10 days for the first four
  moultings, while about 20 days elapse between the fourth and fifth
  moulting. The worm ceases to eat for a day before moulting, and
  spins some silk on the vein of the under surface of a leaf; it then
  secures the hooks of its hind legs in the texture it has thus spun,
  and there remains motionless; soon after, through the transparency
  of the skin of the neck, can be seen a second head larger than the
  first, belonging to the larva within. The moulting generally takes
  place after four o’clock in the afternoon; a little before this time
  the worm holds its body erect, grasping the leaf with the two pairs
  of hind legs only; the skin is wrinkled and detached from the body
  by a fluid which circulates between it and the worm; two
  longitudinal bands are seen on each side, produced by a portion of
  the lining of the spiracles, which at this moment have been partly
  detached; meanwhile the contractions of the worm are very energetic,
  and by them the skin is pulled off and pushed towards the posterior
  part; the skin thus becomes so extended that it soon tears just
  under the neck, and then from the head. When this is accomplished
  the most difficult operation is over, and now the process of
  moulting goes on very rapidly. By repeated contractions the skin is
  folded towards the tail, like a glove when taken off, and the lining
  of the spiracles comes out in long white filaments. When about
  one-half of the body appears, the shell still remains like a cap,
  enclosing the jaws; then the worm, as if reminded of this loose
  skull-cap, removes it by rubbing it on a leaf; this done, the worm
  finally crawls out of its skin, which is attached to the fastening
  made for the purpose. Once out of its old skin, the worm makes a
  careful review of the operation, with its head feeling the aperture
  of every spiracle, as well as the tail, probably for the purpose of
  removing any broken fragment of skin which might have remained in
  these delicate organs. Not only is the outer skin cast off, but also
  the lining of the air-tubes and intestines, together with all the
  chewing organs and other appendages of the head. After the moulting,
  the size of the larva is considerably increased, the head is large
  compared with the body, but 8 or 10 days later it will look small,
  as the body will have increased very much in size. This is a certain
  indication that the worm is about to moult. Every 10 days the same
  operation is repeated. From the fourth moulting to the time of
  beginning the cocoon the period is about 16 days. (Amer. Naturalist,
  i, pp. 37, 38.)

  Little has been recorded as to the exact mode of casting the larval
  skin in Coleoptera. Slingerland states that _Euphoria inda_ when
  pupating sheds the larval skin off the anal end in the same way as
  in caterpillars, while in _Pelidnota punctata_ the larval skin
  splits down the whole length of the back, retains the larval shape,
  and forms a covering for the pupa which remains inside. (Can.
  Entomologist, xxix, p. 52.) The old larval skin in the Coccinellidæ
  and certain Chrysomelidæ is retained crumpled up at the end of the
  body, while in Dermestes, Anthrenus, etc., it cloaks the pupa.

  Not only is the integument, with its hairs, setæ, and other
  armatures, as well as the cornea or facets of the eyes, shed, but
  also all the lining or intima of those internal organs which have
  been originally derived by an ingrowth or invagination of the
  ectoderm are likewise cast off, with the probable exception, of
  course, of the mid-intestine, which is endodermal in its origin.
  Even so early an observer as Swammerdam noticed that the internal
  lining of the alimentary canal comes away with the skin. He states
  that the larva of _Oryctes nasicornis_ sheds both the lining of the
  colon, and of the smaller as well as the larger branches of the
  tracheæ.

  Careful observations are still needed on the internal changes at
  ecdysis of most insects. Newport seems to have observed more closely
  than any one else, notwithstanding the great number who have reared
  caterpillars but have not carefully observed these points, the
  extent of the process internally. He informs us: “The lining of the
  mouth and pharynx, with that of the mandibles, is detached with the
  covering of the head, and that of the large intestines with the skin
  of the posterior part of the body, and besides these also the lining
  of the tracheal tubes. The lining of the stomach itself, or the
  portion of the alimentary canal which extends from the termination
  of the œsophagus to the insertion of the so-called biliary vessels,
  is also detached, and becomes completely disintegrated, and appears
  to constitute part of the _meconium_ voided by the insect on
  assuming its imago state.” (Art. Insecta, p. 876.) Newport states on
  another occasion that he had “noticed the remarkable circumstance
  [now explained by the fact that the mid-intestine is of endodermal
  origin] that the mucous lining of the true ventriculus was not cast
  off with the rest, but was discharged with the fecula.”[96]
  Burmeister also observed that the smaller tracheæ as well as the
  internal tunic of the colon of Libellulæ are shed.

  In the apodous larvæ of Hymenoptera which live in cells, as we have
  observed in those of Bombus, during the process of moulting, the
  delicate skin breaks away in shreds, probably owing to the tension
  due to the unequal growth of the different parts of the body. “Thus
  after the skin beneath has fully formed, shreds of the former skin
  remain about the mouth-parts, the spiracles, and anus. Upon pulling
  upon these, the lining of the alimentary tube and tracheæ can be
  drawn out, sometimes, in the former case, to the length of several
  lines.”[97] We then added, “As all these internal systems of vessels
  are destined to change their form in the pupa, it may be laid down
  as a rule in the moulting of insects and Crustacea, that the lining
  of the internal organs, which is simply a continuation of the outer
  tegument, or arthroderm, is, in the process of moulting, sloughed
  off with that outer integument.” We have satisfied ourself that in
  the larvæ of the Lepidoptera (_e.g._ Datana) the tracheæ at the time
  of ecdysis undergo a complete histolysis, and arise _de novo_ from
  hypodermal cells, the so-called spiral threads originating from
  elongated peritracheal nuclei. (See p. 449, Fig. 412.) This is
  undoubtedly also the case with the salivary ducts, which are
  strengthened and rendered elastic by tænidia like those of tracheæ.
  As the urinary tubes are diverticula of the proctodæum, itself an
  ectodermal invagination, they may also, though not lined with a
  chitinous intima, be renewed. With little doubt the intima of the
  ducts of poison, spinning, and most, if not all the other glands,
  though certainly the dermal glands, is exuviated. We have found that
  the lobster in moulting sheds, besides the skin with the most
  delicate setæ, the lining of the proventriculus, and the apodemes of
  the head and thorax, hence it is most probable that the tentorium of
  the head of insects as well as the apodemes and phragmas of the
  thorax are exuviated.

The formation of the inner skin, or that of any succeeding stage
(instar), is due to the secretion of the structureless chitinous layer
by the cells of the hypodermis, during the process of histogenesis.
These cells at this time are very active, and the formation of the new
layer of chitin arrests the supply of nourishment to the old skin, so
that it dries, hardens, and with the aid of the fluid thrown out at this
time separates from the new chitinous layer secreted by the hypodermis.

  Mention of this fluid, which Newport was the first to observe, and
  which he says causes the separation of the old from the underlying
  fresh integument of the caterpillar, recalls a passage in
  Hatchett-Jackson’s Studies in the morphology of the Lepidoptera,
  which we quote on a succeeding page, where he calls attention to the
  formation of such a liquid, which in the reptiles facilitates the
  process of moulting, adding, “Whether such is the case with the
  moult of the caterpillar, I do not know.” Is it not also possible
  that the growth of the setæ or tubercles on the cuticle of the
  caterpillar may likewise serve to loosen and detach the overlying
  skin about to be cast off? After writing the foregoing, we find that
  Miall and Denny have suggested that the setæ of the cockroach
  probably serve the same purpose as the casting-hairs of the crayfish
  and reptiles.

  It is well known that in the crayfish and in lizards the skin is
  first loosened by the growth of temporary hairs or setæ, which
  locally grow inward from the old cuticle and push the skin away when
  it is shuffled off by the movements of the body, jaws, and limbs, as
  well as the body in general.[98]

Such spines arise in the pupa of many insects, for Verhoeff finds that
the spines and teeth of pupal fossorial and other Hymenoptera, as well
as Coleoptera, function as moulting-processes for loosening and pushing
off the last larval skin, rather than for locomotion. He also claims
that the spines of the pupa of the dipterous Anthrax are both for
locomotion and for boring, especially the spines on the head and tail.
He therefore divides these pupal spines into helcodermatous (boring or
tearing) and locomotor spines.

Gonin has fully confirmed Newport’s discovery of the exuvial fluid. He
states that during pupation the outside of the pupa, especially the
parts of the head and thorax “is coated with a viscous liquid secreted
by special glands.” The parts only harden subsequent to pupation after
exposure to the air (p. 41). His observations were made under the
direction of Professor Bugnion, who kindly writes us:—

  “M. Gonin has proved the formation of a liquid which passes under
  the cuticle at the time of the last moult and facilitates
  exuviation. We think that this liquid is secreted by large cells
  (unicellular glands) which we see especially on the surface of
  segments 1–3. These cells form part of the hypodermis, and their
  pores open under the cuticle.”

  In a subsequent letter enclosing a sketch kindly made for me by M.
  Gonin (Fig. 566), Professor Bugnion writes me Aug. 24, 1897,
  regarding the functions of the large hypodermal cells (_l. hy_), as
  follows: “It seems to me, in fact, after having again examined the
  sections, that the function of these cells is not sufficiently
  elucidated. Indeed these cells occur only in the section passing
  through the 1st segment, between the head and 1st thoracic segment.
  It would seem, if these cells supply the liquid which lubricates the
  surface at the time of ecdysis, that they should be spread over the
  entire surface of the body. Moreover, these cells have no distinct
  orifice, and although there is seen at times to issue streams of a
  substance (coagulated by the reagents), they cannot be compared with
  true unicellular glands like those of the epidermis of fishes,
  amphibians, etc.

  “On the other hand, if it is the blood which oozes out on the
  surface (according to your hypothesis), it would seem that the loss
  of blood would cause the death of the larva. I believe then it is
  due to the secretion of the hypodermis which spreads over the whole
  surface when the cells are still soft (not yet hardened from contact
  with the air). At all events, there is a liquid spread over the
  surface; it is this liquid which glues the wings and the legs to the
  body at the moment the caterpillar issues from the rent in its skin.
  If at this instant we plunge the pupa in the water the liquid is
  dissolved, and the feet, wings, etc., are not glued to the body.”

  Dr. T. A. Chapman also writes us: “There is no question about the
  existence of a fluid between the two skins at moulting. In hairy
  larvae the hairs are always wet at first, or if the skin be renewed
  rather more quickly than the larva does it naturally, the wetness of
  both surfaces is obvious. I do not know the nature of the fluid, but
  it is related to that which hardens into the dense pupal case, and
  also hardens in a less degree the skin of the larva. I suppose it
  must contain some chitin in a soluble form. If a newly cast larva
  skin be taken, there is no difficulty in extending the shrivelled
  mass to its full length and dimensions, but if a short time elapses,
  this chitin hardens, and the skin cannot be extended after soaking
  in water, alcohol, ammonia, or any other solvent I have tried.”

It has been stated that there is a subimaginal pellicle in Lepidoptera,
but as Dr. Chapman writes me, “what has been observed has been some of
the inner pupal dissepiments, such as the pupal cases of the under
wings,” etc. They may be observed in the head of the tineid pupæ, and
other small moths. We have thought that the delicate, purplish, powdery
layer left in the cast shells of the pupæ of saturnians, Catocalæ, and
other moths, might possibly be such a pellicle, but this view has been
dispelled by the following statement of Professor Bugnion in a letter
answering an inquiry whether he had noticed such a pellicle.

  “A liquid which is secreted in a few minutes at the time of the last
  moult, forms in drying a yellowish layer spotted with black (in
  _Pieris brassicæ_). This layer extends around the entire pupa, and
  serves both to protect it and to glue together the wings, legs,
  etc., in their new position. The dried liquid on the surface of the
  pupa, and by means of which the appendages are glued to the surface,
  very likely corresponds to the pellicle of which you speak.” The
  newly exposed integument is at first pale and colorless, but soon
  assumes the hues peculiar to the species, and the insect, at first
  exhausted, after a short rest becomes active.

[Illustration:

  FIG. 566.—Transverse section through the prothoracic segment
    (ventral face) of larva of _Pieris brassicæ_, about 12 hours
    before pupation: _c_, cuticula; _l. hy_, large glandular (?)
    hypodermal cells; gradually passing into normal hypodermal cells
    (_hy_).—Gonin _del._
]

  E. Howgate has noticed under the microscope peculiar internal
  movements in a small immature transparent geometrid while moulting.
  “Each separate segment,” he says, “commencing at the head, elongated
  within the outer skin, whilst the next ones remained in their former
  state. Each segment in its turn behaved in this curious manner until
  the last was reached, when the motion was reversed and proceeded
  toward the head, when it was again reversed.... The whole proceeding
  appeared as if the larva was gliding within itself, segment after
  segment, the outer skin remaining as if held by the other segments,
  whilst the particular one in motion freed itself within. After
  remaining motionless for a short interval, the skin near the head
  swelled and burst, open at the back.... Presently out comes the head
  of the new caterpillar, pushing forward the old one.... After a
  short struggle the new true legs appear, pushing off and treading
  under foot the old ones. Then by violent wriggling movements the
  abdominal legs were extricated. Then all is clear, and the larva,
  which is quite exhausted, coils itself up and literally pants for
  breath.” (The Naturalist, November, 1885, No. 124, p. 366, quoted in
  Psyche, iv, p. 327, 1887.)

  Since the worms and most other ametabolous invertebrates are not
  known to moult their integument, the body steadily increasing in
  size without frequent changes of skin, it seems that growth may go
  on and still be accompanied by considerable changes in shape of the
  body without change of skin. Frequent ecdyses appear, then, to be
  the result of the great and sudden changes of the body, necessitated
  by the adaptation of the animal to new or unusual conditions of
  life. In young Daphnia, a cladocerous crustacean, as many as eight
  moults were observed in a period of 17 days, and spiders frequently
  moult even after reaching their full size. The swollen bodies of the
  gravid female of Gastrophysa, Meloë, or of Termites, and of the
  honey ant show that the skin can stretch to a great extent, but in
  the metamorphoses of Crustacea and of insects, whose young are more
  or less worm-like or generalized in form, with fewer segments and
  appendages, or with appendages adapted for quite different uses from
  those of mature life, the necessity for a change of skin is seen to
  be necessary for mechanical reasons. Hence Crustacea and insects
  moult most frequently early in life, when the changes of form are
  most thoroughgoing and radical, while simple growth and increase in
  size are most rapid at the end of larval life, as seen both in
  shrimps and crabs, and in insects.

  The hibernating caterpillars of certain butterflies are known to
  moult once oftener than those of the summer brood. Mr. W. H. Edwards
  has discussed the subject with much detail. “There seems,” he says,
  “to be a necessity with the hibernators of getting rid of the rigid
  skin in which the larva has passed the winter; that is, if the
  hibernation has taken place during the middle stages, as it does in
  Apatura and Limenitis. In these cases very little food is taken
  between the moult which precedes hibernation and the one which
  follows it, and the larva while in lethargy is actually smaller than
  before the next previous moult. The skin shrinks, and has to be cast
  off before the awakened larva can grow. Those species (observed)
  whose larva moults five times in the winter brood require but four
  moults during the summer.” He adds that while the larva is in
  lethargy, it is actually smaller than before the next previous
  moult. Dr. Dyar writes: “I think there is no doubt about the number
  of stages of arctian larvæ. They seem to have a great capacity of
  spinning out their life-history by interpolated stages (as regards
  width of head). I think it is because so many of them hibernate, and
  only a single brood extends throughout the season.” (Psyche iii, p.
  161.)

  On the other hand, it is difficult to understand why the
  caterpillars of arctians moult so frequently, nearly twice as often
  as in most other caterpillars, though the changes of form and
  armature are so slight.

  Dr. Chapman also writes me: “Arctians resemble bears (Arctos), polar
  and others, in having long hairs to protect them during winter, and
  are, in fact, typically hibernators. Many of them have to
  half-hibernate, having warmth enough to keep them awake, but not
  enough food for growth, but their tissues, at least the chitinous
  ones of the cutis, and also probably, and perhaps especially, of the
  alimentary canal, become old and effete, and require the
  rejuvenescence acquired by a moult. Other smooth-skinned hibernators
  have similar capabilities.”

  Chapman has shown in his paper on Acronycta that these caterpillars
  of this genus illustrate how larvæ may lose a moult, and they do so
  to acquire a sudden change of plumage.

=The number of moults in insects of different orders.=—It will be seen
from the data here presented that the number of moults is as a rule
greatest in holometabolic insects with the longest lives, and that an
excessive number of ecdyses may at times be due to some physical cause,
such as lack of food combined with low temperature.

In Campodea there is a single fragmentary moult (Grassi), while the
Collembola (_Macrotoma plumbea_) shed their skin throughout life.
(Sommer.)

In the winged insects, especially Lepidoptera, the number of moults is
dependent on climate. Insects of wide distribution growing faster in
warmer climates consequently shed their skins oftener; for example, the
same species may moult once oftener in the southern than in the northern
States, as in the case of _Callosamia promethea_, which in West Virginia
is double-brooded. Hibernating larvæ moult once oftener than those of
the summer brood. (W. H. Edwards.) Weniger by rearing the larvæ of
_Antheræa mylitta_ and _Eacles imperialis_, and which, when reared under
normal conditions, actually have six stages, found that when reared in a
warm moist atmosphere of about 25° C. they have but five stages, _i.e._
moult but four times. In the hot and moist climate of Ceylon, _A.
mylitta_ has but five stages. (Psyche, v, p. 28.)

Among Orthoptera Acrydians moult five times; _Diapheromera femorata_ but
twice (Riley); a katydid (_Microcentrum retinervis_) moults four times
(Comstock). _Mantis religiosa_, according to Pagenstecher, moults seven
times, having eight stages, including that before the amnion is cast,
but the first “moult” being an exuviation of the amnion, the number of
stages is seven. Cockroaches (_Periplaneta americana_) are said by
Marlatt to “pass through a variable number of moults, there being
sometimes as many as seven.”

In the Homoptera there are, in general, from two to four moults; thus in
Typhlocyba there are five stages, and in Aphis at least three, and in
Psylla four during the nymphal state. Psocus has four. Riley states that
the nymph of the female coccid, _Icerya purchasi_, sheds its skin three
times, and that of the male twice. Notwithstanding its slow growth,
Riley says, the 17–year Cicada moults oftener than once a year, and the
number of larval stages probably amounts to 25 or 30 in all. The bed-bug
sheds its skin five times; and with the last moult appear the minute
wing-pads characteristic of the adult. In _Conorhinus sanguisuga_ there
are “at least two larval stages and pupal stages.” (Marlatt.)

In the dragon-flies moulting occurs, Calvert thinks, many times, since
the rudiments of wings are said by Poletaiew to only appear in odonate
nymphs after the third or fourth moult.

In the May-fly, Chloëon, the number of ecdyses is 20. The neuropterous
_Ascalaphus_ (Helecomitus) _insimulans_ of Ceylon moults three times
before pupating. Among the Mecoptera Felt has shown that _Panorpa
rufescens_ moults seven times.

In Coleoptera the normal or usual number is not definitely known; Meloë
moults five times, but this is a hypermetamorphic insect; _Tribolium
confusum_ has been carried by Mr. Chittenden through seven moults.
_Phytonomus punctatus_, the clover-leaf weevil, moults three times,
according to Riley, who has observed that _Dermestes vulpinus_ passes
through seven larval stages.

  In the breeding jars, with plenty of food and a constant temperature
  of from 68° to 78° F., the larvæ cast their 1st skin in from four to
  nine days, the great majority moulting at seven days. Under the same
  conditions the 2d skin was cast at from four to seven days, the
  majority moulting at six days; the 3d skin at from three to six
  days, the majority moulting at five days; and the 4th skin at from
  three to six days, the majority moulting at five days; the 5th skin
  at from five to seven days, and the 6th skin at six days. There are
  thus seven larval stages. (Report for 1885, p. 260.)

  Riley has ascertained that by rearing isolated larvæ of _Tenebrio
  molitor_, one after being kept nearly a year had moulted 11 times,
  when it died. A second larva, hatched June 5, had moulted 12 times
  by June 10 of the following year, (1877), when it also died. Of _T.
  obscurus_ three larvæ were reared to the imago state. One moulted 11
  times by Aug. 30 of the same year, pupated Jan. 20, 1877, and
  finally became a beetle Feb. 7, 1877. The other two both moulted 12
  times, and reached the imago stage Feb. 18 and March 9,
  respectively. “All were, as nearly as possible, under like
  conditions of food and surroundings, and in all cases the moult that
  gave the pupa is not considered among the larval moults.”

  Two larvæ of the museum pest (_Trogoderma tarsale_) were kept by
  Riley in a tight tin box with an old silkworm cocoon. “They were
  half-grown when placed in the box. On Nov. 8, 1880, there were in
  the box 28 larva skins, all very much of a size, the larva having
  apparently grown but little. The skins were removed and the box
  closed again as tightly as possible. Recently, or after a lapse of
  two years, the box was again opened and we found one of the larvæ
  dead and shrivelled up; but the other was living and apparently not
  changed in appearance. There were 15 larva skins in the box. He
  could not tell when the one larva died, but it is certain that
  within a little more than three and a half years, two larvæ shed not
  less than 43 skins, and that one larva did not, during that time,
  appreciably increase in size. We know of no observations which
  indicate the normal or average length of life, or number of moults
  in either Tenebrio or Trogoderma, but it is safe to assume from what
  is known, in these respects, of allied species, that in both the
  instances here referred to, but particularly in the case of
  Trogoderma, development was retarded by insufficient nutrition, and
  that the frequent moulting and slow growth resulted therefrom, and
  were correlated.”[99] Further observations such as these are greatly
  needed.

Of the Siphonaptera the common cat and dog flea (_Pulex serraticeps_)
moults three times before pupating. (Howard.)

In Lepidoptera the usual or average number of moults is four, but the
number varies considerably, the greatest number yet known occurring in
_Phyrrarctia isabella_, which, Dr. Dyar informs me, moults 10 times.

From Dyar’s observations it appears that there are usually five larval
stages, but six and seven stages are not infrequent, while there are
seven in _Seirarctia echo_, eight in _Ecpantheria scribonia_, Scepis,
and Apatelodes, and nine and ten in arctians, while the European _Nola
centonalis_ moults nine times, other species of this genus shedding
their skins six times. (Buckler.) (Psyche, v, pp. 420–422.) _Callosamia
promethea_ appears, as a rule, to moult but three times. _Orgyia
antiqua_ was found by Hellins to moult from three to five times. Riley
found that in _O. leucostigma_ the males moult four times, the female
four, but sometimes five times, while Dyar states that in _O. gulosa_
the male larvæ moult three or four times, the female always four times;
in _O. antiqua_, however, there are six stages, and in the female seven.
Lithocolletis, Chambers thinks, as a rule, moults eight times, and
Comstock thinks that _L. hamadryadella_ casts its skin seven or eight
times.

In the blow-fly (Calliphora) Leuckart and Weismann have inferred at
least two moults, while Weismann suspected that there are as many as
four. In _Musca domestica_ we have observed that the larva moults three
times; in Œstridæ there are three larval stadia. (Brauer.) In Corethra
there are four larval moults, and Miall thinks there are probably as
many in Chironomus. Passing to the phytophagous Hymenoptera, there are
three moults or four larval stages in _Nematus erichsonii_, but Dyar
informs us that less than four stages in saw-fly larvæ is very rare,
that he has only one record of less than five, and that that is
doubtful; “five for nematid, six and seven for others, is certainly the
rule. The highest I have is the indication of 11 stages for _Harpiphorus
varianus_, but this again is an inference only, and attended with
doubt.” (Can. Ent., xxvii, p. 208.) In Bombus we have observed five
different sizes of larvæ, and hence suppose the least number of ecdyses
is five, while we are disposed to believe that this insect, as well as
wasps and bees, in general shed their skins as many as eight times
during their entire existence.

The honey-bee, Cheshire thinks, since he has found the old and ruptured
pellicles, probably moults six times before it spins its cocoon, or
passes into the semipupa condition. (Bees and Bee-keeping, p. 20.)

As to the cause of the great number of moults in the arctians and in the
beetles experimented with by Riley, it would seem that cold and the lack
of food during hibernation were the agents in arctians, and starvation
or the lack of food in the case of the beetles, such cause preventing
growth, though the hypodermis-cells retained their activity.

=Reproduction of lost limbs.=—Here might be discussed the subject of the
renovation or renewal of maimed or lost limbs, or the reparation of
other injuries. As is well known, the cœlenterates, echinoderms, and
worms under certain circumstances multiply by self-division, or if
artificially mutilated, the parts are gradually restored by
cell-proliferation or histogenesis. It is so with the antennæ and legs
of crustaceans as well as the digits and tail of salamanders. The
experiments first made by Le Pelletier[100] on spiders, and later by
Heineken,[101] and others after him, on different spiders, as well as on
Orthoptera and Hemiptera (Blatta, Reduvius, etc.), have proved that
antennæ and legs and other external parts which have been injured or
shortened, or entirely cut off in young individuals, are replaced at the
next, or after successive moults, though generally in diminished size.
This does not usually occur in adult life, and the process of reparation
of lost parts is apparently due to the active growth of the cells of the
parts affected during the process of moulting, when the histolysis of
the maimed or diseased parts is succeeded by the rapid development of
new cells, not only of the hypodermis, but also of the more specialized
tissues within. And this tends to prove that such histolysis and making
over of the muscles and other structures within occur especially in all
metamorphic insects, and also in ametabolous forms, though the process
has been most thoroughly examined in the Diptera, where these changes
are more marked.

Gonin has found that the thoracic legs of the caterpillar correspond
only to the tarsi of the imago (Fig. 608). It results, he says, from
this fact that in accordance with the observations of Réaumur (which
were wrongly interpreted by Newport and Künckel D’Herculais) that the
amputation of the legs of the larva does not involve the entire leg, but
only the extremity of the leg of the imago.

=Formation of the cocoon.=—While the larvæ of many insects, as those of
the butterflies, suspend themselves before transforming, and spin no
cocoon, or dig into the earth for protection and to secure an immunity
from too great changes of temperature, a large proportion of the larvæ
of metabolous insects which lead an inactive pupal life, line their
earthen cells with silk, or spin a more or less elaborate case of silk,
called the _cocoon_. We have seen that the inactive pupa of the male
scale-insects is covered by the scale itself, or even in one case the
insect forms a true cocoon of fibres of wax. The aquatic larvæ of the
Neuroptera and Coleoptera creep out of the water, and by the movements
of their bodies make a rude earthen cell in the bank, while that of
Donacia spins a dense, leathery cocoon (Fig. 567) in the earth. The
larvæ of the Embiidæ are protected by a cocoon, which they renew at each
moult. Coniopteryx spins an orbicular cocoon, the Hemerobiidæ a
spherical, dense, whitish one. The Trichoptera transform within their
larval cases, which thus serve as cocoons, as do certain case-bearing
Lepidoptera, notably the Psychidæ.

[Illustration:

  FIG. 567.—Cocoon (natural size) of _Donacia proxima_.
]

[Illustration:

  FIG. 568.—Cocoon and larva of _Lucanus dama_.
]

The pupa of certain leaf-eating beetles (Chrysomelidæ), as well as the
Coccinellidæ, Dermestidæ, Hister, etc., are usually protected by the
cast larval skin, which is retained, forming a rude shelter. While many
beetles spin an oval cocoon (Gyrinus, Silphidæ), the wood-boring species
make one of chips glued together, and that of Lucanus, which feeds on
decayed wood, is lined with silk (Fig. 568). Anobium constructs a silken
cocoon, interweaving the fine particles of its thin castings; the larvæ
of weevils also usually spin silken cocoons.

[Illustration:

  FIG. 569.—Larva (_a_), puparium (_b_), and imago (_c_) of Sarcophaga,
    enlarged.
]

[Illustration:

  FIG. 570.—_a_, _Erax bastardi_; _b_, pupa.—After Riley.
]

The larval skin of the coarctate Diptera is retained as a protection for
the soft-bodied pupa within, the old larval skin separating from the
integument of the semipupa. To this cocoon-like covering of the
coarctate pupa we have restricted the term _puparium_, originally used
by Kirby and Spence to designate the pupa. The puparium is usually
cylindrical or barrel-shaped, rounded at each end.

[Illustration:

  FIG. 571.—Puparium of _Hypoderma bovis_: _a_, side; _b_, ventral view,
    showing exit hole of adult; _c_, cap which splits off for exit of
    fly.—After Clark, from Osborn, Bull. 5, Div. Ent. U. S. Dept. Agr.
]

In the _Diptera cyclorhapha_, or common house and flesh flies, etc., the
puparium remains in vital connection, by means of four tracheæ, with the
enclosed pupa, which escapes from the case through a curved seam or lid
at the anterior end and not by a slit in the back, as do the
orthoraphous families, represented by the horse-fly (Tabanidæ, Asilidæ,
Fig. 570), etc., where in some cases the obtected pupa remains within
the loose envelope formed by the old larval skin, which Brauer calls a
false puparium. The dry, hard puparium is burst open at the cephalic end
when the fly emerges, by means of the frontal vesicle, which is
distended with fluid (Fig. 571).

The exact mode of spinning the cocoon by caterpillars has been carefully
observed by L. Trouvelot in the case of the polyphemus silkworm.

  “When fully grown, the worm, which has been devouring the leaves so
  voraciously, becomes restless and crawls about the branches in
  search of a suitable place to build up its cocoon; before this it is
  motionless for some time, holding on to the twig with its front
  legs, while the two hind pair are detached; in this position it
  remains for some time, evacuating the contents of the alimentary
  canal until finally a gelatinous, transparent, very caustic fluid,
  looking like albumen, or the white of an egg, is ejected; this is a
  preparation for the long catalepsy that the worm is about to fall
  into. It now feels with its head in all directions, to discover any
  leaves to which to attach the fibres that are to give form to the
  cocoon. If it finds the place suitable, it begins to wind a layer of
  silk around a twig, then a fibre is attached to a leaf near by, and
  by many times doubling this fibre and making it shorter every time,
  the leaf is made to approach the twig at the distance necessary to
  build the cocoon; two or three leaves are disposed like this one,
  and then fibres are spread between them in all directions, and soon
  the ovoid form of the cocoon distinctly appears. This seems to be
  the most difficult feat for the worm to accomplish, as after this
  the work is simply mechanical, the cocoon being made of regular
  layers of silk united by a gummy substance. The silk is distributed
  in zigzag lines of about one-eighth of an inch long. When the cocoon
  is made, the worm will have moved his head to and fro, in order to
  distribute the silk, about 254,000 times.

  “After about half a day’s work, the cocoon is so far completed that
  the worm can hardly be distinguished through the fine texture of the
  wall; then a gummy resinous substance, sometimes of a light-brown
  color, is spread all over the inside of the cocoon. The larva
  continues to work for four or five days, hardly taking a few minutes
  of rest, and finally another coating is spun in the interior, when
  the cocoon is all finished and completely air tight. The fibre
  diminishes in thickness as the completion of the cocoon advances, so
  that the last internal coating is not half so thick and so strong as
  the outside ones.” (Amer. Naturalist, i, p. 86.)

The mode of spinning the cocoon of an ichneumon (Microgaster) parasitic
on Philampelus has been well described by John P. Marshall, as follows:—

[Illustration:

  FIG. 572.—Microgaster larvæ; spinning their cocoons: _a_, enlarged
    view of 5.—After Marshall.
]

  The first appearance of the parasite is represented in Fig. 572, 1.
  A warty excrescence appears on the back of the caterpillar, which
  slowly emerges until it is seen to be a larva enclosed in a delicate
  transparent membrane, as represented in 2. This it soon succeeds in
  bursting, and, rising to its full length, balances itself a moment
  as in 3, then, bending double, it ejects from its mouth a glairy
  liquid, which instantly changes to silk, and fastens the posterior
  end to the skin of the caterpillar, as shown in 4, side view. It now
  begins to spin its cocoon by attaching a silken thread to the silky
  mass by which it had previously fastened itself to the caterpillar,
  and forming a series of loops of uniform size, first from right to
  left, and then back again from left to right, as represented in the
  front view, 5, and better in the enlarged view, _5^a_, the arrow
  heads showing the direction in which the head of the larva moved
  while forming the loops. The ends of the series, numbered 1, 2, 3,
  4, are fastened to the edges of the ventral side of the body, which
  thus serves as a measure of the width of the cocoon, and also acts
  as a support for the frail fabric in the first stages of spinning.
  After the larva has fastened the fabric as far up on its ventral
  surface as it can, conveniently, it then begins to spin free, as
  shown in the side view, 6, where it is represented as just
  completing the first half of its cocoon, which resembles in form a
  slipper. This accomplished, the larva ceases to spin for the time
  being, bends its head, as in 7, towards its ventral surface, and
  pushes the half cocoon free from its body. The form of the silken
  fabric enables it to stand unsupported, while the larva, sliding its
  head down to the base, holds on firmly until it swings its posterior
  end into the toe of the slipper.

  Figure 572, 8, shows it in the act of changing end for end, and in 9
  the larva is seen erect, beginning at the base to complete the other
  half of its cocoon; 10 shows the larva contracting its body as it
  spins upward for about half the length of the cocoon, when it again
  changes end for end, as shown in 11, where it is beginning at the
  upper part to unite the two sides, finally enclosing itself as
  represented in 12.

  It may now be seen, under the microscope, through the meshes of its
  cocoon actively engaged in lining the interior with layers of very
  fine silk ejected from its mouth in great abundance. One half of the
  cocoon is first lined by a forward and back movement of its head,
  and then reversing its position, it lines the other half in a
  similar manner.

  In one case the larva was disengaged from the skin of the
  caterpillar, after beginning its cocoon. It, however, began again,
  and spun a portion while lying on the table. This was removed, when
  it began a third time, and completed its cocoon.

  In about 10 days the insect made its appearance through a hole in
  the upper end, as represented in 13. The top was eaten off in a
  perfect circle and hung by a few threads, so as to resemble a lid as
  it was thrown back.

  One caterpillar observed had between 300 and 400 cocoons on its back
  and sides, and another was dissected after more than 30 larvæ had
  escaped, and 130 were discovered in the soft integuments of the
  back.

  The figures from 1 to 13 are magnified five diameters, but in order
  to observe the spinning of the cocoon a power of 50 is required.
  (Amer. Naturalist, xii, pp. 559, 560.)

  Certain differences observed by W. A. Buckhout in a Microgaster
  parasitic on the different species of Macrosila, are referred to in
  the same volume, p. 752.

[Illustration:

  FIG. 573.—Body of larva of Lithocolletis. swollen and filled with
    cocoons of Copidosoma, enlarged.
]

  While those chalcidid larvæ which feed internally on their host, as
  a rule, transform into naked, more or less coarctate pupæ, Howard
  states that the larvæ of Copidosoma, Bothriothorax, Homalotylus, and
  perhaps others, which are much crowded within their host, cause a
  marked inflation of the body of the latter (Figs. 573, 574). The
  nature of this cocoon-like cell, and how it is produced, is unknown.
  “Its structure shows it not to be silk, nor yet the last larval skin
  of the parasite, and whether it is an adventitious tissue of the
  host-larva or a secretion of the parasite, or is explicable upon
  other grounds, I cannot say.”

  The silken cocoon of an aphidiid ichneumon has been found by Miss
  Murtfeldt, and also by Dr. Riley, under a rose aphid in which it had
  lived, and referred by Howard to the genus Praon (Fig. 575).

=Sanitary conditions observed by the honey-bee larva, and admission of
air within the cocoon.=—Cheshire has observed that after the larva of
the honey-bee has spun its cocoon or silken lining of its cell, it
observes the following means of preserving cleanliness. The food given
to the larva, especially during the latter part of the growing period,
contains much pollen, the cases of the grains of which consist of
cellulose, which is indigestible.

[Illustration:

  FIG. 574.—Coccinellid larva infested by _Homalotylus obscurus_,
    enlarged.
]

[Illustration:

  FIG. 575.—Cocoon of Praon under the body of a dead Aphis,
    enlarged.—This and Figs. 573 and 574 after Howard, from Insect Life.
]

[Illustration:

  FIG. 576.—Pupation of Proctotrupes in the body of a larva of a beetle,
    representing a case mentioned by Dr. Sharp, where the parasites have
    pupated on the outside of the host, a pair of each attached to
    nearly each segment of the body of their host.—After Sharp.
]

  “These cases, with other refuse matters, collect in quantity within
  the bowel, which becomes distended, since it has no opening. The
  imprisoned larva, having little more than enough room for turning,
  must be freed of these objectionable residua.... In a word, the
  larva turns its head upon its stomach, and pushes the former towards
  the base of the cell until its position is reversed, the tail being
  outwards; and, thus placed, it laps up all residue of food,
  especially from its old clothes previously referred to, until they
  are dried, and practically occupy no space. It now throws up its
  stomach and bowel, with all their contents, and without detaching
  them from its outer skin, which is moulted as before, but in this
  instance to be pressed against the cell, so as to form for it an
  interior lining. The dejectamenta of the bowel in this way lie
  between the cast skin and cell-wall (as seen at _e_, Fig. 577), and
  so the larva remains absolutely unsoiled. It now turns its head and
  resumes its old position, joining its cocoon to the edges of its
  last cast skin, so that its habitation is relined, it is cleansed,
  and air can still pass to it through the imperceptible openings left
  by the bees in the sealing. This point is of radical importance,
  since breathing is carried on pretty rapidly during the latter part
  of its subsequent transformations, the absorbed oxygen permitting
  then of a production of heat, and causing also considerable
  diminution in weight.”

[Illustration:

  FIG. 577.—Larva and pupa of honey-bee in their cell: _SL_,
    spinning-larva; _N_, pupa; _FL_, young feeding larva; _co_, cocoon;
    _sp_, spiracles; _t_, tongue; _m_, mandible; _an_, antenna; _w_,
    wing; _ce_, compound eye; _e_, excrement; _ex_, exuvium.—After
    Cheshire.
]

As to the passage of air into the bee’s cocoon, Cheshire states that
before the cocoon can be built, a cover, technically called sealing, is
put over the larva by its nurses. These covers are made of pollen and
wax, and are pervious to the air. They are more convex and regular in
form than those sealing in the honey.[102]




                             THE PUPA STATE


The word _pupa_ is from the Latin meaning baby. Linnæus gave it this
name from its resemblance to a baby which has been swathed or bound up,
as is still the custom in Southern Europe. The term _pupa_ should be
restricted to the resting inactive stage of the holometabolous insects.

  Lamarck’s term _chrysalis_ was applied to the complete or obtected
  pupa of Lepidoptera and of certain Diptera, and _mumia_, a mummy, to
  the pupæ of Coleoptera, Trichoptera, and most Hymenoptera. Latreille
  (1830) also restricted the term pupa to the “oviform nymph,” or
  puparium, of Diptera. Brauer applies the term _nymph_ to the pupa of
  metabolous insects.

[Illustration:

  FIG. 578.—Pupa obtecta: _a_, of Sesia, with its cocoon-cutter on the
    head; _b_, of _Tortrix vacciniivorana_.
]

The typical pupa is that of a moth or butterfly, popularly called a
chrysalis. A lepidopterous pupa in which the appendages are more or less
folded close to the body and soldered to the integument, was called by
Linnæus a _pupa obtecta_; and when the limbs are free, as in Neuroptera,
Mecoptera, Trichoptera, and the lepidopterous genus Micropteryx it is
called a _pupa libera_ (Fig. 579). When the pupa is enclosed in the old
larval skin, which forms a pupal covering (puparium), the pupa was said
by Linnæus to be _coarctate_. The pupa of certain Diptera, as that of
the orthoraphous families, is nearly as much obtected as that of the
tineoid families of moths, especially as regards the appendages of the
head; the legs being more as in _pupæ liberæ_ (Fig. 580).

[Illustration:

  FIG. 579.—Pupa libera of neuropterous insects _a_, _Corydalus
    cornutus_; _b_, Sialis; _c_, Hemerobius.
]

The male Coccid anticipates the metabolous insects in passing through a
quiescent state, when, as Westwood states, it is “covered by the skin of
the larva, or by an additional pellicle.” The body appears to be broad
and flat, the antennæ and fore legs resting under the head, while the
two hinder pairs of legs are appressed to the under side of the body.
There is but a slight approach to the pupa libera of a metabolous
insect.

  Riley states that the male larva of _Icerya purchasi_ forms a cocoon
  waxy in character, but lighter, more flossy, and less adhesive than
  that of the female egg-cocoon. It melts and disappears when heated,
  proving its entirely waxy nature. When the mass has reached the
  proper length, the larva casts its skin, which remains in the hind
  end of the cocoon, and pushes itself forward into the middle of the
  cocoon. The pupa (Fig. 581) is of the same general form and size as
  the larva. All the limbs are free and slightly movable, so that they
  vary in position, though ordinarily the antennæ are pressed close to
  the side, as are the wing-pads; the front pair of legs are extended
  forward. “If disturbed, they twist and bend their bodies quite
  vigorously.” The pupa state lasts two or three weeks. A similar pupa
  is that of _Icerya rosæ_. (Riley and Howard.)

[Illustration:

  FIG. 580.—Pupa obtecta of Diptera: _a_, Ptychoptera; _b_, _Tabanus
    atratus_; _c_, _Proctacanthus philadelphicus_; _d_, _Midas
    clavatus_.
]

[Illustration:

  FIG. 581.—Pupa libera of _Icerya purchasi_, ventral view.—After
    Riley, Insect Life.
]

  The metamorphosis of _Aspidiotus perniciosus_ is of interest. The
  male nymph differs much after the first moult from the female,
  having large purple eyes, while the female nymph loses its eyes
  entirely. It passes into what Riley terms the _pro-pupa_ (Fig. 582,
  _b_), in which the wing-pads are present, while the limbs are short
  and thick. The next stage is the “true pupa” (Fig. 582, _c_, _d_),
  in which the antennæ and legs are much longer than before. There is
  no waxy cocoon, but only a case or scale composed of the shed larval
  skin, i.e. “with the first moult the shed larval skin is retained
  beneath the scale, as in the case of the female; with the later
  moultings the shed skins are pushed out from beneath the scale,” and
  when they transform into the imago they “back out from the rear end
  of their scale.”

[Illustration:

  FIG. 582.—_Aspidiotus perniciosus_, development of male insect: _a_,
    ventral view of larva after first moult; _b_, the same, after
    second moult (pro-pupa stage); _c_ and _d_, true pupa, ventral and
    dorsal views. All greatly enlarged.—After Riley.
]

The pupæ of Coleoptera and of Hymenoptera, though there is, apparently,
no near relationship between these two orders, are much alike in shape,
and, as Chapman pertinently suggests, those of both orders are helpless
from their quiescence, and hence have resorted for protection to some
cocoon or cell.

But it is quite otherwise with the pupæ of Lepidoptera and Diptera,
which vary so much in adaptation to their surroundings, and hence afford
important taxonomical and phylogenetic characters. This, as regards the
Lepidoptera, was almost wholly overlooked until Chapman called attention
to the subject, and showed that the pupæ had characters of their own, of
the greatest service in working out the classification, and hence the
phylogeny, of the different lepidopterous groups. We have, following the
lead of Chapman, found the most striking confirmation of his views, and
applied our present knowledge of pupal structures to dividing the
haustellate Lepidoptera into two groups,—Paleolepidoptera and
Neolepidoptera.

The pupæ of the Neuroptera, Coleoptera, and Hymenoptera differ
structurally from the imago, in the parts of the head and thorax being
less differentiated. Thus in the head the limits or sutures between the
epicranium and clypeus, and the occiput and gula, are obscurely marked,
while the tergal and pleural sclerites of the imago are not well
differentiated until the changes occurring just before the final
ecdysis.

It is easy, however, to homologize the appendages of the pupæ with those
of the imago of all the holometabolous orders except in the case of the
obtected pupa of the Lepidoptera (and probably of the obtected dipterous
pupæ), where the cephalic appendages are soldered together.

That the appendages of the lepidopterous pupa are, as generally
supposed, merely cases for those of the imago has been shown by Poulton
to be quite erroneous. He says: “If we examine a section of a pupal
antenna or leg (in Lepidoptera), we shall find that there is no trace of
the corresponding imaginal organ until shortly before the emergence of
the imago. In the numerous species with a long pupal period, the
formation of imaginal appendages within those of the pupa is deferred
until very late, and then takes place rapidly in the lapse of a few
weeks. This also strengthens the conclusion that such pupal appendages
are not mere cases for the parts of the imago, inasmuch as these latter
are only contained within them for a very small proportion of the whole
pupal period.” On the other hand, Miall and Hammond claim that there is
a strong superficial contrast as to the formation of the imaginal
organs, between Lepidoptera and tipularian Diptera, the appendages,
wings, and compound eyes being substantially those of the imago. “With
the exception of the prothoracic respiratory appendages and the
tail-fin, there is little in the pupa of Chironomus which does not
relate to the next stage.”

The exact homology of the “glazed eye” of the lepidopterous pupæ and of
the parts under the head, situated over the maxillæ, is difficult to
decide upon, and these points need farther examination. In the dipterous
pupa it is interesting to observe that the halteres are large and broad,
which plainly indicates that they are modified hind wings. The number
and arrangement of the spiracles is different in pupæ from those of the
larva and imago.

[Illustration:

  FIG. 583.—_Simulium piscicidium_: _a_, larva; _b_, _c_, _d_, pupa;
    _e_, thoracic leg; _f_, row of bristles at end of body. _A_, _S.
    pecuarum_, pupa; _a_, _b_, _c_, adminicula.—After Riley.
]

There are also secondary adaptive structures peculiar to the pupa, which
are present and only of use in this stage. These are the thoracic,
spiracular, or breathing appendages of the aquatic Diptera (Fig. 583),
the various spines situated on the head or thorax, or on the sides, or
more often at the end of the abdomen, besides also the little spines
arranged in more or less circular rows around the abdominal segments,
the cocoon-breaker, and the cremaster of many pupæ.

In the pupa of certain Diptera, there is a terminal cremaster-like
spine, as in that of _Tipula eluta_ (Fig. 584), _Tabanus lineola_ (Fig.
585), besides adminicula or locomotive spines like those of
lepidopterous pupæ (Fig. 580, _a_, _b_, _c_).

[Illustration:

  FIG. 584.—Pupa of _Tipula eluta_.
]

[Illustration:

  FIG. 585.—Pupa of _Tabanus lineola_.—This and Fig. 584 after Hart.
]

[Illustration:

  FIG. 586.—Pupa of _Galerita lecontei_, and of _Adelops hirtus_ (_a_,
    _b_, _c_).—After Hubbard.
]

The pupæ of Coleoptera are variously spined or hairy (Fig. 586). Those
of Hydrophilus and of Hydrobius are provided with stout spines on the
prothorax and abdomen which support the body in its cells, so that, as
Lyonet first showed, though surrounded on all sides by moist earth, it
is kept from contact with it by the pupal spines; other pupæ of beetles,
such as that of the plum weevil, which is also subterranean, possess
similar spines. The abdomen of many coleopterous pupæ, such as those of
Carabidæ, end in two spines, to aid them in escaping from their cells in
wood or in the earth; others have stiff bristles, and others spines
along each side of the abdomen (Fig. 586). All these structures are the
result of a certain amount of activity in what we call quiescent pupæ,
but most of these are for use at the end of pupal life, at the critical
moment when by their aid the insect escapes from its cocoon or
subterranean cell, or if parasitic, bores out of its host.

If we are to account for the causes of their origin, we are obliged to
infer that they are temporary deciduous structures due to the need of
support while the body is subjected to unusual strains and stresses in
working its way out of its prison in the earth, or its cell within the
stems and trunks of plants and similar situations. They are pupal
inheritances or heirlooms, and well illustrate the inheritance of
characters acquired during a certain definite, usually brief, period of
life, and transmitted by the action of synchronous heredity.

The pupæ of certain insects are quite active, thus that of Raphidia,
unlike that of Sialis, before its final ecdysis regains its activity and
is able to run about. (Sharp, p. 448.)


     _a._ The pupa considered in reference to its adaptation to its
               surroundings and its relation to phylogeny

The form of the pupa is a very variable one, as even in Lepidoptera it
is not entirely easy to draw the line between a pupa libera and a pupa
obtecta (Fig. 578); and though the period is one of inactivity, yet when
they are not in cocoons or in the earth in subterranean cells, their
form is more or less variable and adapted to changes in their
surroundings. Even in the obtected pupa of butterflies, there is, as
every one knows, considerable variability of shape and of armature,
which seems to be in direct adaptability to the nature of their
environment. Scudder has well shown that in certain chrysalids, such as
those of the Nymphalidæ, which are variously tuberculated, and hang
suspended by the tail, and often hibernate, these projections serve to
protect the body. All chrysalids with projections or ridges on different
parts of the body, being otherwise unprotected, move freely when struck
by gusts of wind, hence “the greater the danger to the chrysalis from
surrounding objects, the greater its protection by horny tubercles and
roughened callous ridges.” The greater the protection possessed in other
ways, as by firm swathing or a safe retreat, the smoother the surface of
the body and the more regular and rounded its contours. The tendency to
protection by tubercles is especially noticeable in certain South
American chrysalids of nymphalid butterflies. This response to the
stimuli of blows or shocks is also accompanied by a sensitiveness to the
stimulus of too strong light.

Previously Scudder[103] had made the important suggestion that the
smooth crescent-shaped belt of the “glazed eye” or “eyepiece” of
chrysalids is, as an external covering of the eye, midway between that
of the caterpillar and the perfect insect, and he asks: “May it not be a
relic of the past, the external organ of what once was? And are we to
look upon this as our hint that the archaic butterfly in its
transformations passed through an _active_ pupal stage, like the lowest
insect of to-day, when its limbs were unsheathed, its appetite
unabated?” etc. Scudder also shows that “the expanded base of the sheath
covering the tongue affords protection also to the palpi which lie
beneath and beside the tongue.”

All this tends to show the importance of studying the structure of the
pupa, in order to ascertain how the pupal structures have been brought
about, with the final object of discovering whether the pupæ of the
holometabolic insects are not descended from active nymphs, and if so,
the probable course of the line of descent.


            _b._ Mode of escape of the pupa from its cocoon

[Illustration:

  FIG. 587.—Pupa of _Micropteryx purpuriella_, front view: _md_,
    mandibles; _mx. p_, maxillary palpus, end drawn separately; _mx.′
    p_, labial palpi; _lb_, labrum.
]

“In all protected pupæ,” as Chapman says, “the problem has to be faced,
how is the imago to free itself from the cocoon or other envelope
protecting the pupa.” In the Coleoptera and Hymenoptera the imago
becomes perfected within the cocoon or cell, as the case may be, and as
Chapman states, “not only throws off the pupal skin within the cocoon,
but remains there till its appendages have become fully expanded and
completely hardened, and then the mandibles are used to force an outlet
of escape,” and he calls attention to the fact that “in many cases, even
in some entire families, they are of no use whatever to the imago except
in this one particular,” and he cites the Cynipidæ as perhaps the most
striking instance of this circumstance.

In those Neuroptera which spin a silken cocoon, _e.g._ the Hemerobiidæ,
the Trichoptera, and in Micropteryx (Fig. 588), the jaws used by the
pupa for cutting its way out of the cocoon are even larger in proportion
than in the pupa of caddis-flies (Fig. 588), being of extraordinary
size.

[Illustration:

  FIG. 588.—Mandibles (_md_) of _Micropteryx purpuriella_,
    enlarged.—Author _del._ _A_, pupal head of a hydropsychid
    caddis-fly, showing the large mandibles.—After Reaumur, from Miall.
]

In Myrmeleon the pupa pushes its way half out of the cocoon, and then
remains, while the imago ruptures the skin and escapes (Fig. 589, _a_).

Thus in the Neuroptera and Trichoptera we have already established the
more fundamental methods of escape from the cocoon, which we see carried
out in various ways in the more generalized or primitive Lepidoptera.

The most primitive method in the Lepidoptera of escaping from the cocoon
seems to be that of Micropteryx.

[Illustration:

  FIG. 589.—Larva of Myrmeleon with (_a_) its cocoon and cast pupa-skin.
]

  “In this genus,” says Chapman, “though it is nominally the pupa that
  escapes from the cocoon, it is in reality still the imago, the imago
  clothed in the effete pupal skin. To rupture the cocoon it uses not
  its own jaws, but those of the pupal skin, energizing them, however,
  in some totally different way from ordinary direct muscular action,
  their movements being the result of the vermicular movements of the
  pupa, acting probably by fluid pressure on the articular structure
  of the jaws, by some arrangement not altogether different perhaps
  from the frontal sac of the higher Diptera. In the Micropteryges the
  jaws of the pupa not only rupture the cocoon, but appear to be the
  most active agents in dragging the pupa through the opening in the
  cocoon and through any superincumbent earth, being merely assisted
  by the vermicular action of the abdominal segments, and we find in
  accordance with this circumstance that the pupal envelope is still
  very thin and delicate, and has little or no hardening or roughness
  by which to obtain a leverage against the walls of the channel of
  escape.” (Trans. Ent. Soc. London, 1896, pp. 570, 571.)

[Illustration:

  FIG. 590.—Pupa of Talæporia: _a_, cocoon-cutter; with vestiges of four
    pairs of abdominal legs, and the cremaster.
]

Some sort of a beak or hard process, more or less developed, according
to Chapman, adapted for breaking open the cocoon exists in nearly all
the Lepidoptera with incomplete pupæ (_pupæ incompletæ_), except the
limacodid and nepticulid section. “In all these instances the pupa
emerges from the cocoon precisely as in the Micropteryges, that is, the
moth it really is that emerges, but does so encased in the pupal skin.
To achieve this object, it seems to have been found most efficient to
have three, four, or five abdominal segments capable of movement, but to
have the terminal sections (segments) soldered together.”

This cocoon-breaker, as we may call it, is especially developed in
_Lithocolletis hamadryadella_. As described by Comstock, it forms a
toothed crest on the forehead which enables it to pierce or saw through
the cocoon.

  “Each pupa first sawed through the cocoon near its juncture with the
  leaf and worked its way through the gap, by means of the minute
  backward-directed spines upon its back, until it reached the upper
  cuticle of the leaf. Through this cuticle it sawed in the same way
  that it did through the cocoon. The hole was in each case just large
  enough to permit the chrysalis to work its way out, holding it
  firmly when partly emerged. When half-way out it stopped, and
  presently the skin split across the back of the neck and down in
  front along the antennal sheaths, and allowed the moth to
  emerge.”[104]

We have observed and figured the cocoon-breaker in Bucculatrix,
Talæporia (Fig. 590, _a_), Thyridopteryx, and Œceticus, and rough knobs
or slight projection answering the purpose in Hepialidæ, Megalopyge,
Zeuzera, and in Datana.[105] See also the spine on the head of _Sesia
tipuliformis_ (Fig. 578).

The imago of the attacine moths cuts or saws through its cocoon by means
of a pair of large, stout, black spines (_sectores coconis_), one on
each side of the thorax at the base of the fore wings (Fig. 591), and
provided with five or six teeth on the cutting edge (_C_, _D_).

[Illustration:

  FIG. 591.—Cocoon-cutter of the Luna moth: front view of the moth with
    the shoulders elevated and the rudimentary wings hanging down: _s_,
    cocoon-cutter; _p_, patagium. _B_, represents another specimen with
    fully developed wings: _ms_, scutum; _st_, scutellum of the
    mesothoracic segment; _s_, cocoon-cutter, which is evidently a
    modification of one of the pieces at the base of the fore wings; it
    is surrounded by membrane, allowing free movement. _C_ and _D_,
    different views of the spine, magnified, showing the five or six
    irregular teeth on the cutting edge.
]

[Illustration:

  FIG. 592.—Larva and pupa of a wood-wasp (Rhopalum), enlarged: _h_,
    temporary locomotive tubercles on head of pupa.—Trouvelot _del._
]

  Our attention[106] was drawn to this subject by a rustling, cutting,
  and tearing noise issuing from a cocoon of _Actias luna_. On
  examination a sharp black point was seen moving to and fro, and then
  another, until both points had cut a rough irregular slit, through
  which the shoulder of the moth could be seen vigorously moving from
  side to side. The hole or slit was made in one or two minutes, and
  the moth worked its way at once out of the slit. The cocoon was
  perfectly dry. The cocoon-cutter occurs in all the American genera,
  in _Samia cynthia_, and is large and well marked in the European
  _Saturnia pavonia-minor_ and _Endromis versicolora_. In _Bombyx
  mori_ the spines are not well marked, and they are quite different
  from those in the Attaci. There are three sharp points, being acute
  angles of the pieces at the base of the wing, and it must be these
  spines which at times perform the cutting through of the threads of
  the cocoon described by Réaumur, and which he thought was done by
  the facets of the eyes. It is well known that in order to guard
  against the moths cutting the threads, silkraisers expose the cocoon
  to heat sufficient to destroy the enclosed pupa. In Platysamia the
  cocoon-cutters, though well developed, do not appear to be used at
  all, and the pupa, like that of the silkworm and other moths
  protected by a cocoon, moistens the silk threads by a fluid issuing
  from the mouth, which also moistens the hairs of the head and
  thorax, together with the antennæ. It remains to be seen whether
  these structures are only occasionally used, and whether the
  emission of the fluid is not the usual and normal means of egress of
  the moth from its cocoon. Dr. Chapman remarks that throughout the
  obtected moths “there are many devices for breaking through the
  cocoon: specially constructed weak places in the cocoon, softening
  fluid, applied by the moth, assisted by special appliances of
  diverse sorts, such as in Hybocampa[107] and Attacus,” etc.

  As to the fluid mentioned above, Trouvelot states that it is
  secreted during the last few days of the pupa state, and is a
  dissolvent for the gum so firmly uniting the fibres of the cocoon.
  “This liquid is composed in great part of bombycic acid.” (Amer.
  Naturalist, i, p. 33.)

  The pupa of the dipterous genus Sciara (_S. ocellaris_ O. S.)
  resembles a tineid pupa, and before transforming emerges for about
  two-thirds of its length from the cocoon; the pupa-skin remaining
  firmly attached in this position.[108]

  Certain hymenopterous pupæ are provided with temporary deciduous
  conical processes. Thus we have observed in the pupa of _Rhopalum
  pedicellatum_ two very prominent acute tubercles between the eyes
  (_h_, Fig. 592). As the cocoon is very slight, these may be of use
  either in extracting itself from the silken threads or in pushing
  its way along before emerging from the tunnel in the stem of plants.
  (See also p. 611.)


                           _c._ The cremaster

Although this structure is in general confined to lepidopterous pupæ,
and is not always present even in them, since it is purely adaptive in
its nature, yet on account of its singular mode of development from the
larval organs, and the accompanying changes in the pupal abdomen, it
should be mentioned in this connection. The cremaster is the stout,
triangular, flattened, terminal spine of the abdomen, which aids the
pupa in working its way out of the earth when the pupa is subterranean,
or in the pupa of silk-spinning caterpillars its armature of secondary
hooks and curved setæ enables it to retain its hold on the threads of
the interior of its cocoon after the pupa has partially emerged from the
cocoon, restraining it, as Chapman well says, “at precisely that degree
of emergence from the cocoon that is most desirable.” He also informs us
that while in the “_pupæ incompletæ_ the cremaster is attached to an
extensible cable, which always allows some emergence of the pupa, in the
pupæ obtectæ there is no doubt but that in such cases as the Ichthyuræ,
Acronyctæ, and many others, it retains the pupal case in the same
position within the cocoon that the living pupa occupied; this is also
very usually the case in the Geometræ and in the higher tineids (my
pyraloids).”

  In many of the more generalized moths there is no cremaster
  (Micropteryx, Gracilaria, Prodoxus, Tantura, Talæporia, Psychidæ,
  Hepialidæ, Zeuzera, Nola, Harrisina), though in Tischeria and
  Talæporia (Fig. 590, but not in Solenobia) and Psychidæ, two stout
  terminal spines perform the office of a cremaster, or there are
  simply curved setæ on the rounded, unarmed end of the abdomen, as in
  Solenobia.

  In the obtected Lepidoptera, for example in such a group as the
  Notodontidæ, where the cremaster is present, though variable in
  shape, it may from disuse, owing to the dense cocoon, be without the
  spines and hooks in Cerura, or the cremaster itself is entirely
  wanting in Gluphisia, and only partially developed in Notodonta. In
  the butterflies whose pupæ are suspended (Suspensi), the cremaster
  is especially well developed. Reference might here be made to the
  temporary pupal structures in certain generalized moths, which take
  the place of a cremaster, such as the transverse terminal row of
  spines in Tinea, the two stout spines in Tischeria, and the dense
  rough integument and thickened callosities of the pupal head and end
  of abdomen of Phassus, which bores in trees with very hard wood;
  also the numerous stout spines at the end and sides of the abdomen
  in Ægerians. These various projections and spines, besides acting as
  anchors and grappling hooks, in some cases serve to resist strains
  and blows, and have undoubtedly, like the armature in the larvæ and
  imagines of other insects, arisen in response to intermittent or
  occasional pressure, stresses, and impacts.

=Mode of formation of the cremaster and suspension of the chrysalis in
butterflies.=—We are indebted to Riley[109] for an explanation of the
way the cremaster has originated, his observations having been made on
species of over a dozen genera of butterflies (Suspensi).

He shows that the cremaster is the homologue of the suranal plate of the
larva.[110] The preliminary acts of the larva have been observed by
various authors since the days of Vallisneri, _i.e._ the larva hanging
by the end of the abdomen, turning up the anterior part of the body in a
more or less complete curve, and the skin finally splitting from the
head to the front edge of the metathoracic segment, and being worked
back in a shrivelled mass toward the point of attachment. The critical
feat, adds Riley, which has most puzzled naturalists, is the independent
attachment of the chrysalis and the withdrawal from and riddance of the
larval skin which such attachment implies. Réaumur explained this in
1734 by the clutching of the larval skin between sutures of the terminal
segments of the chrysalis, and this is the case, though the sutures act
in a somewhat different way.

  Before pupation the larva spins a mass or heap of silk, the shape of
  which is like an inverted settee or a ship’s knee, and “one of the
  most interesting acts of the larva, preliminary to suspension, is
  the bending and working of the anal parts in order to fasten the
  back of the (suranal) plate to the inside of the back of the settee,
  while the crotchets of the legs are entangled in the more flattened
  position or seat.”

  In shedding the larval skin, the following parts are also shed, and
  have some part to play in the act of suspension: _i.e._ 1st, the
  tracheal ligaments (Fig. 593, _tl_), or the shed tracheæ from the
  last or 9th pair of spiracles; 2d, the rectal ligament (Fig. 593,
  _rl_), or shed intestinal canal; 3d, the Osborne or retaining
  membrane (_membrana retinens_, Fig. 593, _mr_), which is the
  stretched part of the membrane around the rectum and in the anal
  legs, and which is intimately associated with the rectal ligament.

[Illustration:

  FIG. 593.—Shrunken larval skin of _Vanessa antiopa_, cut open from
    the back and showing (_mr_) the retaining membrane, (_rl_) the
    rectal ligament, and (_tl_) the tracheal ligaments.
]

  The structures in the chrysalis are, first, the cremaster, with its
  dorsal (Fig. 594, _dcr_) and ventral (_vcr_) ridges, and the
  cremastral hook-pad (_chp_), said by Riley to be “thickly studded
  with minute but stout hooks, which are sometimes compound or
  furnished with barbs, very much as are some of our fishing-hooks,
  and which are most admirably adapted to the purpose for which they
  are intended.”

[Illustration:

  FIG. 594.—Ideal representation of the anal subjoint of _Vanessa
    antiopa_, from behind, with the spines removed, and all parts
    forced apart by pressure so as to show the homologies of the parts
    in the chrysalis which are concerned in pupation: homologies
    indicated by corresponding letters in Fig. 595, except that _r_
    (the rectum) corresponds with _pr_ in Fig. 595.
]

[Illustration:

  FIG. 595.—Anal parts of chrysalis of _Vanessa antiopa_, just prior
    to final extraction from shrunken larval skin: _c_, cremaster;
    _chp_, cremastral hook-pad; _h_, one of the hooks, more enlarged;
    _vcr_, ventral cremastral ridge; _dcr_, dorsal cremastral ridge;
    _lr_, larval rectum; _pr_, pupal rectum; _rp_, rectal plate; _sr_,
    sustentor ridges; _mr_, _membrana retinens_; _rl_, rectal
    ligament; _tl_, tracheal ligament; the 11th or last
    spiracle-bearing joint and the 12th joint being numbered.
]

  Secondly, there are the other structures, viz., the sustainers
  (_sustentors_), two projections which Riley states “homologize with
  the soles (_plantæ_) of the anal prolegs, which take on various
  forms (3), but are always directed forward so as easily to catch
  hold of the retaining membrane.” These sustentors are, however, as
  Jackson[111] has shown, and as we are satisfied, the vestiges of the
  anal legs.

[Illustration:

  FIG. 596.—_A_, chrysalis of Terias. _B_, posterior end of chrysalis
    of Paphia. _C_, posterior end of chrysalis of Danais. _E_, one of
    the sustainers of Terias, greatly enlarged to show its hooked
    nature. All the parts of subjoint lettered to correspond with Fig.
    595.
]

  Thirdly, the sustentor ridges, which, as Riley states, may be more
  or less obsolete in some forms, in Paphia (Fig. 596, _B_) and
  Limenitis form “quite a deep notch, which doubtless assists in
  catching hold of the larval skin in the efforts to attach the
  cremaster.”

[Illustration:

  FIG. 597.—Pupation of butterflies: _a_, attachment of larva of
    _Danais archippus_; _p_, attachment of larva of _Paphia
    glycerium_; _b_, ideal larva soon after suspension; _d_, ideal
    larva a few hours later, the needle (_n_) separating the forming
    membrane from the sustainers; _l_, ideal larva just before
    splitting of larval skin, with retaining membrane loosened from
    the sustainers and showing its connection both with the larval and
    pupal rectum. In all the figures the joints of the body are
    numbered; the forming chrysalis is shaded in transverse lines; the
    intervening space between it and larval skin is dotted: _h_, is
    the hillock of silk; _hl_, hooks of hind legs; _ap_, anal plate;
    _lr_, larval rectum; _pr_, pupal rectum; _mr_, retaining membrane;
    _c_, cremaster; _s_, sustainers.—This and Figs. 593–596 after
    Riley.
]

  “It is principally,” adds Riley, “by the leverage obtained by the
  hooking of the sustainers in the retaining membrane, which acts as a
  swimming fulcrum, that the chrysalis is prevented from falling after
  the cremaster is withdrawn from the larval skin. It is also
  principally by this same means that it is enabled to reach the silk
  with the cremastral hook-pads.”

  “Dissected immediately after suspension, the last abdominal segment
  of the larva is found to be bathed, especially between the legs and
  around the rectum, in an abundance of translucent, membranous
  material.”

  “An hour or more after suspension the end of the forming chrysalis
  begins to separate from the larval skin, except at the tip of the
  cremaster (Fig. 597, _b_). Gradually the skin of the legs and of the
  whole subjoint (10th segment) stretches, and with the stretching,
  the cremaster elongates, the rectal piece recedes more and more from
  the larval rectum, and the sustentor ridges diverge more and more
  from the cremaster, carrying with them, on the sustainers, a part of
  the soft membrane.” The rectal ligament will sustain at least 10 or
  12 times the weight of the chrysalis. That of Apatura seems to rely
  almost entirely on the rectal ligament, assisted by the partial
  holding of the delicate larval skin.




   FORMATION OF THE PUPA AND IMAGO IN THE HOLOMETABOLOUS INSECTS (THE
                           DIPTERA EXCEPTED)


We have seen that in the incomplete metamorphosis, although there may be
as many as five, and possibly seven moults, and in Chloëon as many as
20, and in _Cicada septemdecim_ perhaps 25 or 30, there is but a slight
change of form from one stage to another, and no period of inactivity.
And this gradual outer transformation is so far as yet known paralleled
by that of the internal organs, the slight successive changes of which
do not differ from those observed in the growth of ametabolous insects.
With the growth of the internal organs there probably goes on a series
of gradual regenerative processes, and Korschelt and Heider state that
we may venture to assume that each changed cell or group of cells which
have become exhausted by the exercise of the functions of life are
reabsorbed and become restored through the vital powers of the tissues,
so that as the result there goes on a constant, gradual regeneration of
the organs.

While the Hemiptera have only an incomplete metamorphosis, the males of
the Coccidæ are, as shown by O. Schmidt, remarkable for passing through
a complete or holometabolous development, with four stages, three of
which are pupal and inactive. Hence, as Schmidt observes, there is here
a hypermetamorphosis, like that of the Meloidæ, Stylopidæ, etc.

Shortly before the end of the larval stage of the male appear the
imaginal buds of the eyes, legs, and wings. In the 2d or 1st pupal stage
there is an atrophy of the antennæ and legs. On the other hand, at this
stage the female completes its metamorphosis.

The rudiments of the wings arise on the edge of the dorsal and ventral
side of the 2d thoracic segment, and this, we would remark, is
significant as showing a mode of origin of the wings intermediate
between that of the manometamorphic and holometamorphic insects. (See
pp. 137–142.) While Schmidt could not ascertain the exact structure of
the imaginal buds, he says “in general the process of formation of the
extremities is exactly as Weismann has described in Corethra.” The two
later pupal stages are “as in other metabolic insects.” (See p. 690,
Fig. 637.)

Thus far the internal changes in the metamorphosis of the Coleoptera
have not been thoroughly studied. They are less complete than in the
other holometabolous insects, the differences between the larva and
imago being much less marked than in the more specialized orders, and so
far as known all the larval organs pass, though not without some great
changes, directly into the imaginal ones, the only apparent exception
being the mid-intestine, which, as stated by Kowalevsky, undergoes a
complete transformation during metamorphosis. The following account,
then, refers almost wholly to the Lepidoptera, Hymenoptera, and Diptera.


                          _a._ The Lepidoptera

The first observations on the complete metamorphosis of insects which
were in any way exact were those of Malpighi, in 1667, and of
Swammerdam, in 1733. While the observations of Swammerdam, as far as
they extended, were correct, his conclusions were extraordinary. They
were, however, accepted by Réaumur and by Bonnet, and generally held
until the time of Herold in 1815, and lingered on for some years after.
The rather famous theory of incasement (“_emboîtement_”) propounded by
Swammerdam was that the form of the larva, pupa, and imago preëxisted in
the egg, and even in the ovary; and that the insects in these stages
were distinct animals, contained one inside the other, like a nest of
boxes, or a series of envelopes one within the other, or, to use his own
words: “_Animal in animali, seu papilio intra erucam reconditus._”

This theory Swammerdam extended to the whole animal kingdom. It was
based on the fact that by throwing the caterpillar, when about to
pupate, in boiling water, and then stripping off the skin, the immature
form of the butterfly with its appendages was disclosed. Malpighi had
previously observed the same fact in the silkworm, perceiving that
before pupation the antennæ are concealed in the head of the larva,
where they occupy the place previously taken by the mandibular muscles;
also that the legs of the moth grew in those of the larva, and that the
wings developed from the sides of the worm.

Even Réaumur (1734) remarked: “Les parties du papillon cachées sous le
fourreau de chenille sont d’autant plus faciles à trouver que la
transformation est plus proche. Elles y sont neanmoins de tout temps.”
He also believed in the simultaneous existence of two distinct beings in
the insect. “Il serait très curieux de connaître toutes les
communications intimes qui sont entre la chenille et le papillon.... La
chenille hache, broye, digere les aliments qu’elle distribué au
papillon; comme les mères préparent ceux qui sont portés aux fœtus.
Notre chenille en un mot est destineé à nourrir et à defendre le
papillon qu’elle renferme.” (T. i, 8^e Mémoire, p. 363.)

Lyonet (1760), even, did not expose the error of this view that the
larva enveloped the pupa and imago, and, as Gonin says, it was
undoubtedly because he did not use for his dissections of the
caterpillar of Cossus any specimens about to pupate. Yet he detected the
wing-germs and those of the legs, stating that he presumed the bodies he
saw to be the rudiments of the legs of the moth (p. 450).

Herold, in his work on the development of the butterfly (1815), was the
first to object to this erroneous theory, showing that the wings did not
become visible until the very end of larval life; that as the larval
organs disappear, they are transformed or are replaced by entirely new
organs, which is not reconcilable with a simple putting off of the outer
envelope. The whole secret of metamorphosis, in Herold’s opinion,
consisted in this fact, that the butterfly in the larva state increases
and accumulates a supply of fat until it has reached the volume of the
perfect state; then it begins the chrysalis period, during which the
organs are developed and take their definite form.[112] (Abstract mostly
from Gonin.) Still the old ideas prevailed, and even Lacordaire, in his
Introduction à l’Entomologie published in 1834, held on to Swammerdam’s
theory, declaring that “a caterpillar is not a simple animal, but
compound,” and he actually goes so far as to say that “a caterpillar, at
first scarcely as large as a bit of thread, contains its own teguments
threefold and even eightfold in number, besides the case of a chrysalis,
and a complete butterfly, all lying one inside the other.” This view,
however, we find is not original with Lacordaire, but was borrowed from
Kirby and Spence without acknowledgment. These authors, in their
Introduction to Entomology (1828), combated Herold’s views and stoutly
maintained the old opinions of Swammerdam. They based their opinions on
the fact, then known, that certain parts of the imago occur in the
caterpillar. On the other hand, Herold denied that the successive skins
of the pupa and imago existed as germs, holding that they are formed
successively from the “_rete mucosum_,” which we suppose to be the
hypodermis of later authors. In a slight degree the Swammerdam-Kirby and
Spence doctrine was correct, as the imago does arise from germs, _i.e._
the imaginal disks of Weismann, while this was not discovered by Herold,
though they do at the outset arise from the hypodermis, his _rete
mucosum_. Thus there was a grain of truth in the Swammerdam-Kirby and
Spence doctrine, and also a mixture of truth and error in the opinions
of Herold.

The real nature of the internal changes wrought during the process of
metamorphosis was first revealed by Weismann in 1864. His discovery of
the germs of the imago (imaginal buds) of the Diptera, and his theory of
_histolysis_, or of the complete destruction of the larval organs by a
gradual process, was the result of the application of modern methods of
embryology and histology, although his observations were first made on
the extremely modified type of the Muscidæ or flies, and, at first, he
did not extend his view to include all the holometabolous insects. Now,
thanks to his successors in this field, Ganin, Dewitz, Kowalevsky, Van
Rees, Bugnion, Gonin, and others, we see that metamorphosis is, after
all, only an extension of embryonic life, the moults and great changes
being similar to those undergone by the embryo, and that metamorphosis
and alternation of generations are but terms in a single series.
Moreover, the metamorphoses of insects are of the same general nature as
those of certain worms, of the echinoderms, and the frog, the different
stages of larva, pupa, and imago being adaptational and secondary.

While the changes in form from the larva to the pupa are apparently
sudden, the internal histogenetic steps which lead to them are gradual.
In the Lepidoptera a few days (usually from one to three) before
assuming the pupa stage, the caterpillar becomes restless and ceases to
take food. Its excrements are now hard, dry, and, according to Gonin,
are “stained carmine red by the secretions of the urinary tubes.” Under
the microscope we find that they are almost exclusively composed of
fragments of the intestinal epithelium. These red dejections were
noticed by Réaumur, and afterwards by Herold, and they are sure
indications of the approach of the transformations. It now wanders
about, and, if it is a spinner, spins its cocoon, and then lies quietly
at rest while the changes are going on within its body. Meanwhile, it
lives on the stores of fat in the fat-body, and this supply enables it
to survive the pupal period.

  The amount of fat is sometimes very great. Newport removed from the
  larva of _Cossus ligniperda_ 42 grains of fat, being more than
  one-fourth of the whole weight of the insect, he adds that the
  supply is soon nearly exhausted during the rapid development of the
  reproductive organs, “since, when these have become perfected, the
  quantity that remains is very inconsiderable.”

Although the larval skin of a lepidopterous insect is suddenly cast off,
the pupa quickly emerging front it, yet there are several intermediate
stages, all graduating into each other. If a caterpillar of a
Clisiocampa, which, as we have observed, is much shortened and thickened
a day or two before changing to a pupa, is hardened in alcohol and the
larval skin is stripped off, the semipupa (pro-nymph, pro-pupa of
different authors) is found to be in different stages of development,
and the changes of the mouth-parts are interesting, though not yet
sufficiently studied.

Newport attributes the great enlargement and changes in the shape of the
thoracic segments of the larva of _Vanessa urticæ_ at this time, to the
contraction or shortening of the muscles of the interior of those
segments, “which are repeatedly slowly extended and shortened, as if the
insect were in the act of laborious respiration.” This, he adds,
generally takes place at short intervals during the two hours
immediately preceding the change to the pupa, and increases in frequency
as that period approaches. He thus describes the mode of moulting the
larval skin: “When the period has arrived, the skin bursts along the
dorsal part of the 3d segment, or mesothorax, and is extended along the
2d and 4th, while the coverings of the head separate into three pieces.
The insect then exerts itself to the utmost to extend the fissure along
the segment of the abdomen, and, in the meantime, pressing its body
through the opening, gradually withdraws its antennæ and legs, while the
skin, by successive contortions of the abdomen, is slipped backwards,
and forced towards the extremity of the body, just as a person would
slip off his glove or his stocking. The efforts of the insect to get
entirely rid of it are then very great; it twirls itself in every
direction in order to burst the skin, and, when it has exerted itself in
this manner for some time, twirls itself swiftly, first in one
direction, then in the opposite, until at last the skin is broken
through and falls to the ground, or is forced to some distance from it.
The new pupa then hangs for a few seconds at rest, but its change is not
yet complete. The legs and antennæ, which when withdrawn from the old
skin were disposed along the under surface of the body, are yet
separate, and do not adhere together as they do a short time afterwards.
The wings are also separate and very small. In a few seconds the pupa
makes several slow, but powerful, respiratory efforts; during which the
abdominal segments become more contracted along their under surface, and
the wings are much enlarged and extended along the lateral inferior
surface of the body, while a very transparent fluid, which facilitated
the slipping off of the skin, is now diffused among the limbs, and when
the pupa becomes quiet dries, and unites the whole into one compact
covering.”

=The changes in the head and mouth-parts.=—The changes of form from the
active mandibulate caterpillar to the quiescent pupa, and then to the
adult butterfly, are, as we have seen, in direct adaptation to their
changed habits and surroundings, and they differ greatly in details in
insects of different orders. In many Lepidoptera and certain Diptera the
pupa and imago are without the mandibles of the larva, and, instead, the
1st maxillæ in the former order, and the 2d maxillæ in the latter, are
highly developed and specialized. The changes in the shape of the head,
with the antennæ, the latter rudimentary in the larvæ of the two orders
named, are noteworthy, and will be referred to under those orders. The
same may be said of the thorax with the legs and wings, and the abdomen
with the ovipositor. Every part of the body undergoes a profound change,
though in the Coleoptera, Trichoptera, and the more generalized and
primitive Diptera, each segment and appendage of the larva are directly
transformed into the corresponding parts of the pupa, and subsequently
of the imago. We shall see, however, beyond, that this general statement
does not apply to the Hymenoptera, in which there is a process of
cephalization or transfer of parts headward, peculiar to that order.

[Illustration:

  FIG. 598.—Internal organs of _Sphinx ligustri_: 1, head; 2–4,
    thoracic, 5–13, abdominal segments; _V_, fore-, _M_, mid-, _E_,
    hind-intestine; _gs_, brain; _gi_, infraœsophageal ganglion; _n_,
    ventral ganglion; _vm_, urinary tubes; _c_, heart; _G_, testis; _o_,
    œsophagus; _a_, anus; _m_, alary muscles of the heart.
]

[Illustration:

  FIG. 599.—Pupa of the same.
]

[Illustration:

  FIG. 600.—Imago of the same.—This and Figs. 598 and 599 after Newport,
    from Gegenbaur.
]

[Illustration:

  FIG. 601.—Nervous system of the larva of _Sphinx ligustri_.
]

[Illustration:

  FIG. 602.—Nervous system of the pupa of _Sphinx ligustri_, soon after
    pupation.—This and Fig. 601, after Newport.
]

=The change in the internal organs.=—These were especially, as regards
the nervous system, first carefully examined and illustrated by that
great English entomotomist, Newport, and those of the reproductive
organs by Herold as early as 1815. A glance at the figures (598–604),
reproduced from Newport’s article Insecta, will show the changes wrought
especially in the digestive and nervous systems of Sphinx and Vanessa,
his account of the alterations in the muscles having already been
quoted. As the pupal form is much nearer to that of the imago than of
the larva, so the digestive canal is seen to be nearly as much
differentiated in the pupa as in the imago, though the reservoir
(“sucking-stomach”) of the imago is not indicated in the pupa. These
changes are such as occur in an insect which is enormously voracious as
a larva, and which often, passing through a period of complete
inactivity, taking no food at all, finally becomes an insect which needs
to suck in only a minimum quantity of water or nectar, and which
practically abstains from all food. The head and genital glands also, as
well as the urinary vessels, are nearly the same. On the other hand, the
salivary glands have undergone, in the imago, a thoroughgoing reduction.

The changes undergone by the nervous system of _Sphinx ligustri_ and
_Vanessa urticæ_ have been described by Newport with fulness of detail.
An abstract of his observations on _Vanessa urticæ_, which undergoes its
changes in June in 14 days, and in August in eight days, we will now
give, in part verbatim, the subject being rendered much clearer by his
figures, which are reproduced.

  During the last larval stage, certain changes have already taken
  place in different parts of the cord, which shows that they had been
  a long time in progress. Besides the lateral approximation of the
  cords, the first change consists in a union of the 11th and 12th
  ganglia, the latter one being carried forwards; these two ganglia
  being entirely separate before the 3d moult.

  Two hours after the larva of _Vanessa urticæ_ has suspended itself
  in order to pupate, the brain is not yet enlarged, but the
  subœsophageal ganglion is nearly twice its original size and the
  ganglia behind are nearer together. “A little while before the old
  larval skin is thrown off there is great excitement throughout the
  body of the insect.” About half an hour (Fig. 603, 2) before this
  occurs the alary nerves and the cerebral, 2d, 3d, 4th, and 5th
  ganglia are slightly enlarged, and the 1st subœsophageal ganglion
  very considerably. Immediately after the insect has entered the pupa
  state (Fig. 603, 3), all the ganglia are brought closer together.
  One hour after (Fig. 603, 4) pupation the cerebral ganglia are found
  to be more closely united, the 4th and 5th ganglia are nearer, and
  the distance between the remaining ganglia is also reduced.

  Seven hours after pupation there is a greater enlargement of the
  cerebral ganglia, optic nerves, and ganglia and cords of the future
  thoracic segments.

  At 12 hours (Fig. 603, 5) the 5th pair of ganglia has almost
  completely coalesced with the cord and the 4th; at 18 hours (Fig.
  603, 6) the whole of the ganglia, cords, and nerves have become more
  enlarged, especially those of the wings, while the 4th and 5th
  ganglia of the cords have now so completely united as to appear like
  an irregular elongated mass. At 24 hours (Fig. 604, 7) the 4th and
  5th ganglia are completely united, the 5th being larger than the
  4th. At 36 hours (Fig. 604, 8) the optic nerves have attained a size
  almost equal to that of the brain. The 1st subœsophageal ganglion
  now forms, with the cerebral ones, a complete ring around the
  œsophagus, the crura having almost disappeared. The 6th ganglion has
  now disappeared, but the nerves arising from it remain. At 48 hours
  (Fig. 604, 9) the cord is straight instead of being sinuous, and the
  7th ganglion has disappeared, while the thoracic ganglia are greatly
  enlarged. At the end of 58 hours the 2d and 3d thoracic ganglia have
  united, and the double ganglion thus formed is only separated from
  the large thoracic mass composed of the 4th, 5th, and part of the
  6th ganglia, by the short but greatly enlarged cords which pass on
  each side of the central attachment of the muscles. “The optic and
  antennal nerves have nearly attained their full development, and
  those numerous and most intricate plexus of nerves in the three
  thoracic segments of the larva form only a few trunks, which can
  hardly be recognized as the same structures. The arrangement of the
  whole nervous system is now nearly as it exists in the perfect
  insect. The whole of these important changes are thus seen to take
  place within the first three days after the insect has undergone its
  metamorphosis; and they precede those of the alimentary canal,
  generative system, and other organs, which are still very far from
  being completed, and indeed, as compared with the nervous system,
  have made but little progress.” (Art. Insecta, pp. 962–965.)

[Illustration:

  FIG. 603.—Changes in the nervous system of _Vanessa urticæ_, during
    and after pupation.—After Newport.
]

[Illustration:

  FIG. 604.—Changes in the nervous system of _Vanessa urticæ_, from 24
    to 58 hours after pupation.—After Newport.
]

The initial steps and many of the subsequent internal changes escaped
the notice of Newport and others of his time, and it was not until the
epoch-making work of Weismann on the ultimate processes of
transformation of Corethra and of Musca, that we had an adequate
knowledge of the subject.

Weismann (1864) was the first to show for the Muscidæ and Corethra that
the appendages, wings, and other parts of the imago originate in
separate, minute, cellular masses called imaginal disks, buds, or folds
(histoblasts of Künckel). These imaginal buds, which arise from the
hypodermis, being masses of indifferent cells, are usually present in
the very young larva, and even in the later embryonic stages. It has
been shown that such imaginal buds exist for each part of the body, not
only for the appendages and wings (p. 126), but for the different
sections of the digestive canal. During the semipupal stage these buds
enlarge, grow, and at the same time there is a corresponding destruction
of the larval organs. The process of destruction is due to the activity
of the blood corpuscles or leucocytes (phagocytes), the larval organs
thus broken up forming a creamy mass, the buds from which the new organs
are to arise resisting the attacks of the virulent leucocytes, which
attach themselves to the weakened tissue and engulf the pigments (see p.
422). The two processes of destruction of the larval organs (histolysis)
and the building up of the imaginal organs (histogenesis) go hand in
hand, so that the connection of the organs in question in most cases
remains entirely continuous; while the last steps in the destruction of
the larval organs only take place after the organs of the imago have
assumed their definite shape and size. Other observers have corroborated
and confirmed his statements and observations, Gonin extending them to
the Lepidoptera and Bugnion to the Hymenoptera.

It is a pity that the observations, such as were set on foot by
Weismann, were not first made on the Trichoptera and Lepidoptera, which
are much more primitive and unmodified forms than the Diptera, but
mistakes of this nature have frequently happened in the history of
science.

[Illustration:

  FIG. 605.—Full grown larva of _Pieris brassicæ_ opened along the
    dorsal line: _d_, digestive canal; _s_, silk-gland; _g_, brain; _st
    I_, prothoracic stigma; _st IV_, 1st abdominal stigma; _a_, _a′_,
    germs (buds) of fore and hind wings; _p_, bud of thoracic
    segment;—those of the 3d pair are concealed under the silk-glands;
    _I_-_III_, thoracic rings.—After Gonin.
]

The latest and most detailed researches are those of J. Gonin on the
metamorphoses of _Pieris brassicæ_, made under the direction of
Professor E. Bugnion. They fill an important gap in our knowledge, and
show that the Lepidoptera transform in nearly the same manner as
described by Weismann in Corethra. We give the following condensed
account of Gonin’s observations.

On opening a caterpillar entering on the semipupa state (Fig. 605), the
relative position of the germs (imaginal buds or folds) of the wings and
of the legs are seen.

[Illustration:

  FIG. 606.—Section through thorax of a tineid larva on sycamore,
    passing through the 1st pair of wings (_w_): _ht_, heart; _i_,
    œsophagus; _s_, salivary gland; _ut_, urinary tube; _nc_, nervous
    cord; _m_, recti muscles; a part of the fat-body overlies the heart.
    _A_, right wing-germ enlarged.
]

[Illustration:

  FIG. 607.—Section of the same specimen as in Fig. 606, but cut through
    the 2d pair of wings (_w_): _i_, mid-intestine; h, heart; _fb_,
    fat-body; _l_, leg; _n_, nervous cord.
]

These imaginal buds in a more advanced stage are seen in our sections of
a tineid larva (Figs. 606, 607).

The number of 12 imaginal buds found by Weismann in the thorax of
Muscidæ does not occur in Lepidoptera, since, as in the Hymenoptera
(Bugnion), the dorsal buds of the prothoracic segment are wanting. Gonin
finds in Pieris that the ventral buds of the three thoracic segments are
each represented by several distinct folds attached to the femoro-tibial
bud and to the tarsal joints.

The imaginal buds serve in some cases for the formation of new organs
(wings, legs of insects with apodous larvæ); in others for the growth
and the transformation of organs already existing (legs, antennæ, 1st
and 2d maxillæ of Lepidoptera).

  As to the peripodal sac or hypodermic envelope which contains the
  imaginal bud, a portion persists and is regenerated, while the other
  part becomes useless and is detached under the form of débris, as
  shown by Weismann, Viallanes, and Van Rees in the Muscidæ. On this
  point Gonin disagrees with Dewitz, who stated that the walls of the
  wing-sacs are not destroyed, but are only gradually withdrawn at the
  time of pupation, in order to allow the orifice to distend and let
  the wing pass out to the exterior.

  The portion of the sac which persists (basal portion, peripheral pad
  of Bugnion, or annular zone of Künckel) serves at first to attach
  the appendage, while forming, to the hypodermis of the larva, then
  afterwards to more or less completely regenerate the adjoining
  portion of the integument. In this way the hypodermis of the thorax
  is partially, that of the head is almost entirely, replaced by the
  imaginal epithelium which proliferates at the base of the
  appendages,[113] while that of the abdominal segments persists, at
  least in a modified way, and only undergoes (at the end of the pupal
  period) transformations as regards the appearance of the scales and
  pigment.

=The wings.=—The imaginal buds of the wings do not participate in the
larval moults. Gonin has observed, contrary to Dewitz, that their
surface only forms a cuticle towards the end of the last larval stage.

The network of fine tracheæ of the wing-bud is drawn out at the time of
pupation with the internal cuticle of the large tracheæ. The permanent
tracheæ of the wing have already appeared at the time of the 3d moult
under the form of large rectilinear trunks, the position of which
corresponds afterwards to that of the veins, but they are not filled
with air until the time of pupation. There are from eight to ten of
these tracheæ in each wing (Fig. 159), and they give rise in the pupa to
a new system of fine tracheæ (tracheoles) which replaces that of the
larva. (For further details the reader is referred to pp. 126–137.)

=Development of the feet and the cephalic appendages.=[114]—In the
apodous larvæ of Diptera and Hymenoptera the rudiments of the legs are,
like those of the wings, developed within hypodermal sacs; at times they
remain there up to the end of larval life, but sometimes they appear
early at the surface. This origin of the legs, thanks to Weismann,
Künckel, and Van Rees, is well known in the Diptera; in the Hymenoptera
it has been proved to be the case with ants by Dewitz, and in Encyrtus
by Bugnion. As for the Lepidoptera our knowledge that the legs of the
imago arise from the six thoracic legs of the caterpillar, up to the
date of Gonin’s paper has not been in advance of that of Malpighi and
Swammerdam.

  Réaumur, moreover, was supposed to have furnished the proof, having
  from his experiments concluded that “if the legs of the pupa appear
  longer and larger than those of the caterpillar wherein they were
  contained, it is because they were folded and squeezed.” (8^e Mém.,
  p. 365.)

  This explanation of Réaumur’s has been generally accepted. Graber
  (Die Insekten, p. 506) accepted it, after examining microscopic
  sections of the legs, and Künckel averred that “Réaumur, having, in
  certain caterpillars, completely cut off one of the thoracic legs,
  had concluded that the butterfly which came from it lacked the
  corresponding member.” (Rech. sur l’org. et dév. des volucelles, p.
  160.)

  Newport, it is true, denied this disappearance of the legs, but did
  not wish to put himself in opposition to received ideas, and
  supposed that the member cut off was partly reformed in the imago.

  Künckel believes that he has found a better solution in his theory
  of histoblasts or imaginal buds; in his opinion, “Réaumur and
  Newport are both right,” but “when Réaumur cut off a caterpillar’s
  leg, he at the same time removed the histoblast, the rudiment of the
  leg of the butterfly. When Newport repeated this experiment, he
  simply mutilated the histoblast without completely destroying it: in
  the first case, the adult insect was born with one leg less; in the
  second case, it appeared with an atrophied foot.”

  “So ingenious an explanation,” says Gonin, “is not necessary.” To
  prove that the experiments of the two savants are not contradictory,
  it would have been sufficient to cite, as Künckel did not do, the
  exact words of Réaumur, for he having cut from a caterpillar “more
  than half of three of the thoracic legs on the same side,” says he
  found that the chrysalis had “the three limbs on one side _shorter_
  than the corresponding ones on the other side.” The same operation
  repeated on a somewhat younger caterpillar again showed in the
  chrysalis three maimed limbs, “so that they could not be said to be
  entirely absent. These results are like those of Newport; the
  interpretation only was faulty, as we shall attempt to prove.”

The real relations of the adult legs to the larval legs are thus shown
by Gonin.

“If we carefully strip off the skin of a caterpillar near the time of
pupation (Fig. 608), we see that the extremity only of the legs of the
imago is drawn out of the larval legs; the other parts are pressed
against each side of the thorax: near the ventral line a small pad
represents the coxa and the trochanter; the femur and the tibia are
distinctly recognizable, but soldered to each other and only separated
by a slight furrow; they form by their union a very acute knee or bend.
The femur is movable on the pad-like coxa, the tibia continues without
precise limits with the extremity concealed in the larval legs. The
three divisions of the latter do not appear to have any relation with
the live joints of the perfect state. Under the microscope the rudiment
appears very strongly plaited at the level of the tarsus, much less so
in the other regions. A large trachea penetrates into the femur with
some capillaries; reaching the knee it bends into the tibia at a sharp
curve, but does not become truly sinuous in approaching the extremity.
It is then the tarsus especially which is susceptible of elongation; it
may, on being withdrawn, give rise to the illusion that the whole organ
is disengaged from the larval leg.

“This disposition is, we believe, not known. It gives the key to the
experiments of Réaumur and of Newport.

“Even when we cut off the limb of the caterpillar at its base, we only
remove the tarsus of the imago; the femur and the tibia remain intact.
From an evident homology Réaumur has erroneously concluded that there is
an identity. His opinion, classical up to this day, that the limb of the
butterfly is entirely contained in the leg of the caterpillar, has been
found to be inexact and should be abandoned.”

=Embryonic cells and the phagocytes.=—Up to the last larval stage the
legs do not offer, says Gonin, any vestige of an imaginal germ, but they
contain a great number of embryonic cells (Fig. 145, _ec_). They are
almost always collected around a nerve or trachea; sometimes they are
independent, and sometimes retained in the peritoneal sheath, seeming to
arise by proliferation from this sheath. Some thus contribute to the
lengthening of the tracheal branches or nerves, and the others, becoming
detached, form leucocytes or phagocytes. They are very numerous in the
legs, at the beginning of the 4th stage, but are disseminated some days
later throughout the whole cavity of the body. At the time of histolysis
they attack the larval tissues and increase in volume at their expense;
in return they serve for the nutrition of the imaginal parts and
exercise no destructive action on them. Van Rees agrees with Kowalevsky
in comparing these attacks of the embryonic cells, sometimes victorious
and sometimes impotent, to the war which the leucocytes wage against
both the attenuated and the virulent bacteria.

=Formation of the femur and of the tibia, transformation of the
tarsus.=—Capillary tracheæ appear in the leg at the same time as in the
wing. They arise from the end of a tracheal trunk near the base of the
limb on the dorsal and convex side. After the 3d moult the hypodermis
thickens near this place; in a few days a pad is formed there and then a
large imaginal bud with a circular invagination. These buds were noticed
by Lyonet, who supposed them to be “les principes des jambes de la
phalène.” Nerves and a tracheal branch penetrate into the femoro-tibial
bud and form a small bay or constriction which marks the point of
junction of the femur with the tibia, and the body-cavity remains in
direct communication with the end of the limb.

[Illustration:

  FIG. 608.—Feet of the Pieris butterfly withdrawing from those of the
    larva.
]

[Illustration:

  FIG. 609.—Imaginal feet of Pieris uncovered with great care to
    preserve the position which they had in the larva: _ta_, tarsus;
    _t_, tibia; _g_, knee; _f_, femur; _h_, coxa.
]

The tarsus undergoes a series of changes; the surface is folded in a
very complicated way; at the level of each articulation, but only in the
internal and concave region of the leg, is developed a deep fold; on one
side there is a hypodermic thickening, on the other a simple leaf of the
envelope, which afterwards joins at its base with the parietal
hypodermis, and then two leaves are destroyed with the large cells of
the setæ. The internal part and end of the tarsus are then reconstituted
with the elimination of the débris, while the external and convex region
undergoes direct regeneration.

The coxa and trochanter are derived from the base of the larval leg, and
only the 1st pair are well separated from the base of the thorax. One or
two days before pupation the femoro-tibial bud, after having, until now,
preserved its antero-posterior direction, is placed transversely as
regards the larva, then becoming directed obliquely forward. This
rotatory movement of the coxa may be attributed to the great extension
of the fore wings, which push before them the two first pairs of legs.
The last pair in their turn are simply covered by the hind wings and are
but slightly displaced. This new position of the legs is that of the
imago: the knee of the 1st pair is situated in front of the tarsus; that
of the 2d a little outward; that of the 3d pair is directed backward.
(Gonin.)

=The antennæ.=—These appendages also have the same relation with those
of the caterpillar as in the case of the legs, the larval appendages
being only the point of departure of the imaginal growth. Weismann has
observed in Corethra how at the approach of each moult an invagination
like the finger of a glove allows the antenna to elongate from its base.
The process, says Gonin, is identical in the caterpillar of Pieris. At
the last moult the invagination is so pronounced that it is not effaced
with the renewal of the chitinous integument. Several days later it
again begins to grow larger. As the imaginal bud gradually sinks into
the cavity of the head, it presses back the hypodermic wall and thus
forms an envelope around it. Its base, widely opened, gives admission to
the nerves, besides capillaries and sometimes a large trachea.

[Illustration:

  FIG. 610.—Larva in same stage as Fig. 613; side view of head and
    thorax: _a_, _a′_, wings, with the folds on the surface, and the
    sinuous track of the tracheal bundles; _st I_, prothoracic stigma;
    _p_, _p′_, ends of the legs.
]

As soon as it reaches the posterior region of the head, the antenna in
lengthening becomes folded and describes the great curves which led
Réaumur to compare it to a ram’s horn (Fig. 613). The leaf of the
envelope thickens in the interior and all around the base of the organ.
Its ultimate rôle is closely like that of the two other hypodermic
formations. It is at the outset this layer of cells which in the larva
supports the ocelli. This layer, hidden on each side under the parietal
region, thickens and regenerates, forming a circular pad which becomes
more prominent and finally assumes the form of the compound eye of the
imago.

[Illustration:

  FIG. 611.—Head of the larva just before pupation: between the two
    mandibles (_m_) is seen the relief of the tongue or maxillæ (_m′_);
    _f_, spinneret; _l_, labrum; _a_, antenna.
]

[Illustration:

  FIG. 612.—Same stage as in Fig. 611, but after the removal of the
    larval skin, and including the lateral scale: _A_, side, _B_, front,
    view; _c_, “cimier” (the dotted line shows the position it takes in
    the pupa); _a_, antenna; _o_, eye; _t_, tongue.—This and Figs.
    608–611, after Gonin.
]

Finally, this layer gives rise to a conical prolongation (Fig. 612,
_c_), which after exuviation appears as a tuft of long hairs, and is
called by Gonin the crest (cimier, Fig. 612), which is characteristic of
the pupæ of Pieridæ. It is only differentiated towards the end of the
4th larval stage in a median depression of the vertex. It is an imaginal
bud in the most general sense of the word.

[Illustration:

  FIG. 613.—Larva of _Pieris brassicæ_ stripped of its skin some minutes
    before pupation: the antennæ (_a_) have been displaced, and the
    tongue cut off, to show the palpi (_p_); _c_, cimier: _o_, eye; _m_,
    vestige of a mandible; _t_, insertion of the tongue (see Fig. 612);
    _aa_, fore, _ap_, hind, wings; _g_, knee of a foot of the 3d pair.
]

On each side the base of the antenna comes in contact with the germ of
the crest. The envelopes approach each other, and their thickened part
constitutes with the ocular disks a new cephalic wall. The head of the
butterfly thus marked off is triangular; all the larval parts remaining
out of this area then disappear. The muscles and the nerves are resorbed
by histolysis, then the external part of the imaginal envelopes and the
old parietal hypodermis, reduced very thin and degenerated, is detached
in shreds. The antenna becomes external throughout its whole extent. The
transformation is in this case, then, almost as complete as in the
thorax of Diptera or Hymenoptera. It is necessitated by the change of
form and of volume of the head. The region of the ocelli persists
unchanged almost alone from the larva to the imago also. The limit is
not well marked between the portion which is the replacement or direct
renovation of the epithelium.

=Maxilla and labial palpi.=—The development of the tongue (1st maxillæ)
is so like that of the antennæ that it scarcely needs description.
Beginning at the last moult, the hypodermic contents of the maxillæ is
withdrawn in the cephalic cavity under the form of a hollow bud whose
base is turned inward. The invagination remains less distinct than in
the antennæ; it does not even reach to the anterior part of the
œsophagus. The two symmetrical halves of the tongue approach each other
and are thrice folded. When the caterpillar stops feeding, they each
curve in in the form of an S, remaining lodged under the floor of the
mouth (Fig. 613, _t_).

Underneath are to be seen two other buds, which by an identical process
become the labial palpi (Figs. 614, 615, _p_).

At the anterior part of the head, where the organs are very close
together, the envelopes form several folds without any further use (Fig.
615, _r_). The two leaves then fuse together and decay as at the surface
of the tarsus.

Finally, in the mandibles and the labrum, there is only a cellular
thickening without any invagination.

[Illustration:

  FIG. 614.—Section through the anterior region of the head of Pieris
    larva, four days after the 3d moult: _o_, œsophagus; _m_, _m_, 1st
    maxillæ containing the two imaginal buds of the tongue; _p_, _p_,
    labial palpi; _Tr_, trachea.
]

[Illustration:

  FIG. 615.—Section through the same place as in Fig. 614, 10 days after
    the 3d moult, the imaginal appendages having grown in size: _r_,
    _r_, caducous folds of the old hypodermis and of the envelopes.
    Other letters as in Fig. 614.—This and Figs. 613–614, after Gonin.
]

=Process of pupation.=—Notwithstanding the great number of persons who
have reared Lepidoptera, close and patient observations as to the exact
details are still needed. Gonin, who has made the closest observations
on Pieris, pertinently asks why the antennæ, which are appendages of the
head, are visible in the abdominal region, and why the tongue (maxillæ)
is extended between the legs as far as the 3d abdominal segment. To
answer these questions he made a series of experiments. Selecting some
caterpillars which were about to pupate, he produced an artificial
metamorphosis by removing the cuticula in small bits. Exposing the
appendages in this way, they preserved the position which they are seen
to take during growth. Each wing appeared within the limits of the
segment from which it grew out (Fig. 610), not extending beyond, as it
does in the normal pupa, so that Réaumur was wrong in saying that “the
wings are here gathered on each side into a kind of band, which is large
enough to lie in the cavity which is between the 1st and 2d segment.”
(8^e Mém., p. 359.)

All these parts are coated with a viscous fluid secreted by special
glands, which hardens after pupation upon exposure to the air. So long
as the parts are soft, they can easily be displaced. Gonin drew one of
the antennæ like a collar around the head, and one half of the tongue
upon the outer side of the wing.

“When pupation is normal, the integument splits open on the back of the
thorax, and the pupa draws itself from before backwards. Owing to the
feeble adherence which the chitinous secretion gives it, it draws along
with it the underlying organs. The legs, antennæ, the two halves of the
tongue (maxillæ) retained by their end, each in a small chitinous case,
can only disengage themselves from it when in elongating they have
acquired a sufficient tension. The curves straighten out and the folds
unbend. The chitinous mask of the head in withdrawing from the larval
skin follows the ventral line; the tongue and labial palpi free
themselves from its median part; the antennæ disengage themselves from
the two lateral scales. Between these different appendages a space is
left on the surface of the head for the eyes, and on the thorax for the
legs. These are not completely extended on account of the lack of
freedom of the femoro-tibial articulation; the femur preserves its
direction from behind forwards, and the knee in the two first pairs
remains at the same height. The wings overlie them and cover the under
side of the two basal abdominal segments; their surfaces in becoming
united increase much in size.”

As the chitinous frame of each spiracle gradually detaches itself, we
see a tuft of tracheæ passing out of the orifice. It is at this moment
that the provisional tracheal system is cast off, and it is easy to see
that the process is facilitated by the simultaneous elongation of all
the appendages. The permanent tracheæ can follow this elongation because
they are sinuous, and need only to straighten their curves. It is,
however, not the same with the tracheoles, as we have seen in the case
of the wings (p. 133), and their extension or stretching is thus
explained by a very simple mechanism.

  “The position which the organs assume in the chrysalis is not due to
  chance, everything is determined in advance, and the microscope
  shows us that the structure of the hypodermis is specially modified
  in all the parts which remain external. It is a fact well known to
  those who rear Lepidoptera that if this normal arrangement is
  disturbed there are few chances that the perfect insect will
  survive. A leg lifted up, or an antenna displaced, leaves a surface
  illy protected against external influences. Almost always this
  accident causes a drying of the chrysalis.

  “Several interesting experiments may be cited as bearing on this
  subject. If during transformation the chitinous mask of the head is
  separated from the integument beneath, it is arrested half-way in
  its development, and the antennæ and tongue are not fully extended.
  When the case or skin of the caterpillar is drawn, not from before
  backward, but in the opposite direction, all the appendages of the
  thorax are placed perpendicularly to the body. Dewitz and Künckel
  d’Herculais, in stating that the skin of the caterpillar splits open
  along its whole length, show that they were ignorant of the
  mechanism; for it is precisely because the chitinous larval skin
  splits open only in front that it preserves sufficient adherence to
  the organs beneath to draw them after it in the direction of the
  abdomen.

  “To only read modern authors, one would suppose that the mechanism
  of pupation had remained hitherto unknown. In reality, it did not
  escape the notice of Swammerdam or of Réaumur, both of whom have
  described it with care. The first attached too much importance to
  the flow of blood, the action of which would be rather to push the
  organs out than to extend them over the surface of the thorax; the
  second insists on the movements of the insect. This factor, very
  admissible in caterpillars, ‘whose under side is situated on a
  horizontal plane’ (iii, 9^e Mémoire, p. 395), cannot be invoked for
  those which suspend themselves by the tail, as in the genus
  Vanessa.” (Gonin.)


                          _b._ The Hymenoptera

In the Hymenoptera, Ratzeburg was the first to figure and describe the
numerous intermediate stages between the larva and pupa, his subjects
being the ants, Cynips, and Cryptus, which pass through five stadia
before assuming the final pupal shape.

In the bees, as we have observed in the larvæ of Bombus (Proc. Bost.
Soc. Nat. Hist., 1866), after hardening a series in alcohol of young in
different stages of development, it will be found difficult to draw the
line between the different stages since they shade insensibly into each
other, those represented in Fig. 616 being selected stages. The head of
the incipient semipupa distends the prothoracic segment of the larva
whose head is pushed forward and the thoracic segments are much
elongated, while the appendages and wings are well developed, and have
assumed the shape of those of the pupa. Development both in the head and
thorax begins in the most important central parts, and proceeds outwards
to the periphery. During this period the “median segment,” or 1st
abdominal, has begun to pass forward and to form a part of the thorax.

[Illustration:

  FIG. 616.—Transformation of the bumblebee, Bombus, showing the
    transfer of the 1st abdominal larval segment (_c_) to the thorax,
    forming the propodeum of the pupa (_D_) and imago: _n_, spiracle of
    the propodeum. _A_, larva; _a_, head; _b_, 1st thoracic, _c_, 1st
    abdominal, segment. _B_, semipupa; _g_, antenna; _h_, maxillæ; _i_,
    1st, _j_, 2d leg; _k_, mesoscutum. _l_, mesoscutellum; _m_,
    metathorax; _d_, urite (sternite of abdomen); _e_, pleurite; _f_,
    tergite; _o_, ovipositor; _r_, lingua; _q_, maxilla.
]

In what may be termed the 3d stage (Fig. 616, _C_), though the
distinction is a very arbitrary one, the change is accompanied by a
moulting of the skin, and a great advance has been made towards assuming
the pupal form. The abdomen is very distinctly separated from the
thorax, the propodeum being closely united with the thorax, and the head
and thorax taken together are nearly as large as the abdomen, the latter
now being shorter and perceptibly changed in form, more like that of the
completed pupa, while there are other most important changes in the
elaboration of the parts of the thorax, particularly the tergites, and
of the head and its appendages. Meanwhile the ovipositor has been
completed and nearly withdrawn within the end of the abdomen.

The next to study the transformations of the Hymenoptera was Ganin, who
discovered the early remarkable pre-eruciform larvæ, as we may call
them, of certain egg-parasites (Proctotrypidæ). He discovered the
imaginal buds of the wings in the third larva of Polynema (Fig. 185),
but his observations, and those of Ayers, need not detain us here, as
they have little to do with the subject of the normal metamorphosis of
the Hymenoptera, and will be discussed under the subject of
Hypermetamorphosis.

To Bugnion we owe the first detailed account of the internal changes in
the Hymenoptera, his observations being made on a chalcid parasite,
_Encyrtus fuscicollis_, a parasite of Hyponomeuta. The apodous larva
(Fig. 618) moults but once, the next ecdysis being at the time of
pupation. It passes through a semipupal stage.

[Illustration:

  FIG. 617.—Encyrtus larva: 1, 2, 3, ganglia in front of the brain; _m_,
    mouth; _s. gl_, silk-gland; _br_, brain; _n_, nervous cord; _w_, bud
    of fore, _w′_, bud of hind, wing.
]

Bugnion observed in the larva of Encyrtus three pairs of lower thoracic
or pedal imaginal buds, two pairs of upper or alary buds, a pair of
ocular or oculo-cephalic buds destined to build up all the posterior
part of the head, a pair of antennal buds, and three pairs of buds of
the genital armature (ovipositor). He also detected the rudiments of the
buccal appendages under the form of six small buds (Fig. 619), which do
not invaginate, and are not surrounded by a semicircular pad. Also in
the abdomen, behind each pair of stigmata, there is a group of
hypodermic cells (Fig. 617), which, without doubt, correspond to the
wing-buds, but are not differentiated into a central bud and its pad,
and does not merit the name of imaginal bud. In fact, except the
eye-buds, which are unlike the others, he only observed the imaginal
buds of the legs, wings, and ovipositor. The antennal buds are, like
those of the buccal appendages, without an annular zone.

The pedal buds were detected in the middle of larval life. They each
form a central bud surrounded by a circular thickening. They gradually
elongate and become tongue-like and somewhat bent; soon a linear opening
or slit appears, forming the mouth of a cavity which communicates with
that of the body, allowing the passage into them of the tracheæ,
muscles, and nerves, and afterwards of the blood. Finally, the buds grow
longer and slenderer, are bent several times, and show traces of the
articulations; and soon under the old larval skin, now beginning to rise
in anticipation of the moulting, we see the coxa, femur, tibia, and
tarsus of the perfect insect, the tarsal joints not yet being indicated.

[Illustration:

  FIG. 618.—Older Encyrtus larva, lateral view, showing the buds of the
    antennæ (_ant_), legs, and wings (_w_, _w′_): _oe_, œsophagus;
    _q^1_, _q^2_, _q^3_, buds of the genital armature; _o_, rudiment of
    the sexual gland (ovary or testis); _ur.t_, urinary tube; _st_,
    stomach; _i_, intestine (rectum); _n_, ventral nervous cord; _r_,
    rectum; _sp^1_-_sp^9_, spiracles.
]

[Illustration:

  FIG. 619.—A still older larva, ready to transform. The imaginal buds
    of the antennæ (_f_), eyes, wings (_a^1_, _a^2_), and legs have
    become elongated: _ch_, chitinous arch; _b_, mouth; _o_, eye-bud;
    _g_, brain; _e_, stomach; _x_, rudiment of the sexual glands (either
    the ovary or testis).—This and Figs. 617 and 618, after Bugnion.
]

The wing-buds (_a^1_, _a^2_) appear at the same time as those of the
legs, as racket-shaped masses of small cells situated directly behind
the 1st and 2d pair of stigmata, in contact with the tissue ensheathed
by the corresponding tracheal vesicle (Fig. 618). Afterwards they have
exactly the form of those of the Lepidoptera (Fig. 619).

The proliferation of the hypodermis is not limited to the thorax, but
takes place at corresponding points in the first seven abdominal
segments. These abdominal agglomerations of cells do not give rise to
true buds, but serve simply to reconstitute the hypodermis of the
abdominal segments at the time of metamorphosis.

=Ocular or oculo-cephalic buds.=—The eye of insects, as is well known,
is a modification of a portion of the integument, the visual cells being
directly derived from the hypodermis, the cornea being a cuticular
product of this last, like chitinous formations in general.

The ocular buds appear towards the end of larval life as a simple mass
of hypodermic cells, and form a compact layer on the dorsolateral face
of the prothoracic segment, and clothe the cephalic ganglion or brain
like a skull-cap. The central portion only is destined to form the eye,
while the peripheral pad, continuing to thicken, gives rise to a
voluminous and rounded mass, which meets on the median line that of the
opposite side, and forms the integument of all the posterior part of the
head.

Bugnion also observed on the median line a group of small hypodermic
cells which he regarded as the rudiment of the anterior ocellus, but he
did not detect those of the posterior ocelli.

=The antennal buds.=—These appear at an early date under the cuticle of
the head, as two distinct rounded cellular masses, with a central
cavity, but no annular zone (Fig. 619, _f_). Each one grows longer in a
transverse sense, and its summit, extended from the outer side, curves
downward. It now forms a hollow tube folded at the end, and terminated
by a disk whose centre is perforated (Fig. 619, _f_). Afterwards, when
the larva is ready to transform, it grows longer, becomes folded on
itself in its cavity, and, passing beyond on each side the limits of the
larval head, encroaches on the prothoracic segment.

=The buds of the buccal appendages.=—Towards the end of the larval
period, the buds of the mouth-parts appear as small digitiform
projections, situated on each side and below the mouth. Formed of small
epithelial cells pressed against each other, they are all directed
anteriorly, and possess no furrow or pad.

The 2d maxillæ (labium) is formed of two separate parts. The imaginal
buds of the lower lip appear on each side of the median line, with a
fissure indicating the differentiation of the palpus. On each side are
to be seen the 1st maxillary buds, bearing each a rudimentary palpus,
and, farther in front, the buds of the mandibles.

=The buds of the ovipositor.=—The six stylets of the ovipositor arise
from six small imaginal buds which become visible in the second half of
the larval period, on each side of the median line, on the lower face of
the three last segments (Fig. 620, _q^1_, _q^2_, _q^3_). The bud is
differentiated into a central discoidal bud, a furrow, and a marginal,
rather thick swelling or pad. Afterwards, these buds elongate and form
small papilliform projections directed backwards (Fig. 621); but only
during the pupal period do they, as already observed in Bombus, approach
each other and assume their definite shape as an ovipositor.

[Illustration:

  FIG. 620.—End of larva of Encyrtus of 2d stage, showing the three
    pairs of imaginal buds of the ovipositor _q^1_, _q^2_, _q^3_.
]

[Illustration:

  FIG. 621.—The same in an older larva ready to transform: _i_,
    intestine; _x_, genital gland; _a_, anus.—After Bugnion.
]

Finally, Bugnion states that while metamorphosis in the Hymenoptera is
less highly modified than in the Muscidæ, it is more marked than in the
Coleoptera and Lepidoptera. In these orders the pupa moves the abdomen,
but in Hymenoptera it is absolutely immovable throughout pupal life, as
long as the integument is soft.




                DEVELOPMENT OF THE IMAGO IN THE DIPTERA


The flies, particularly the Muscidæ and their allies (Brachycera), are
the most highly modified of insects, their larvæ having undergone the
greatest amount of reduction and loss of limbs, this atrophy involving
even most of the head. The following account has been prepared in part
from the works of Weismann, Ganin, Miall, and Pratt, but mostly from the
excellent general summarized account given by Korschelt and Heider.

In the holometabolic orders of insects, with their resting pupal stage,
during which no food is taken, the entire activity of life seems to be
turned to deep-seated and complicated internal developmental processes.
These inner changes involve an almost complete destruction of many
organs of the larva, and their renewal from certain germs (the imaginal
buds) already present in the larva, as will be seen in the highly
modified Muscidæ. Only a few larval organs become directly transferred
into the body of the pupa and imago. Such are the rudiments of the
genital system. The heart also, and the central portion of the nervous
system, suffer only slight and unimportant, almost trivial, internal
changes. On the other hand, most of the other organs of the larva become
completely destroyed: the hypodermis, most of the muscles, the entire
digestive canal with the salivary glands; while their cells, under the
influence of the blood corpuscles (leucocytes), which here act as
phagocytes, fall into pieces, which are taken up by them and become
digested. Simultaneously with this destructive, histolytic process, the
new formation of the organs by the imaginal buds, already indicated in
the embryo, is accomplished in such a way that the continuity of the
organs in most cases remains unimpaired. This process of transformation
can only be understood by considering that of the embryonal germs of the
organs, (1) only a part is destined for the use of the larva in growth,
and for the performance of certain functions which exhaust themselves
during larval life, so that it is no more capable of farther
transformation, and finally becomes destroyed; while (2) a second part
of the embryonal germs or rudiments persists first in an undeveloped
condition, as imaginal buds, in order to undertake during the pupa stage
the regeneration of the organs.

  Though Swammerdam knew that the rudiments of the wings were already
  present under the skin of the larvæ, we are indebted, for our
  present knowledge, to the thorough and profound investigations of
  Weismann on the metamorphosis of the Diptera, and also to the
  researches of Ganin and others who have worked on the pupæ of
  Muscidæ, in which the development is most complicated and modified.
  In the more generalized and primitive Diptera, such as Corethra, the
  processes of formation of the pupa and imago are much simpler than
  in the muscids and Pupipara. These processes are still simpler in
  the Lepidoptera and Hymenoptera, and for this reason we have given a
  summary of what has been done on these organs by Newport, Dewitz,
  and especially by Bugnion.

  Our knowledge of this subject is still very imperfect, only the more
  salient points having been worked out, and, as Korschelt and Heider
  state, there is still lacking certain proof as to how far the
  relations of the internal changes known to exist in the Muscidæ also
  apply to other orders of insects, though it must be considered that
  in the pupa of Lepidoptera, Hymenoptera, perhaps also the
  Coleoptera, and we would add in the Neuroptera as well as the male
  Coccidæ, very similar metamorphic processes take place.


                _a._ Development of the outer body-form

The form of the imago is completely marked out in the pupa, so that the
transition from the pupa to the imago is comparatively slight and only
depends on the modification and development of the parts already
present.

[Illustration:

  FIG. 622.—Anterior part of young larva of _Simulium sericea_, showing
    the thoracic imaginal buds: _p_, prothoracic bud (only one not
    embryonic); _w_, _w′_, fore and hind wing-buds; _l_, _l′_, _l″_,
    leg-buds; _n_, nervous system; _br_, brain; _e_, eye; _sd_, salivary
    duct; _p_, prothoracic foot.—After Weismann.
]

In most cases the modification in question consists of the changes
occurring during the passage from the larval form to the imago, the
reformation of parts already present being most marked, while the new
rudiments only participate in a limited way in the process. Thus, for
example, the head of the caterpillar together with the antennæ and
mouth-parts, also the thoracic limbs, pass directly and unchanged from
the larva into the pupa. The compound eyes and the wings are, however,
new formations, the latter arising from imaginal buds. The same is the
case with many other Heterometabola, where the passage of the larva into
the pupa in general is due to a transformation of parts already present.
The changes in the brain, the fusion of certain ganglia of the ventral
nervous cord, the changes in the abdomen, involving the reduction in the
number of segments and the remodelling of the end of the body, and the
formation of the ovipositor or sting, and in the higher Hymenoptera the
transfer of the 1st abdominal segment to the thorax, and the origin of
the genital armature,—all these should here be taken into account.

It should be observed that in every case where the larvæ are footless,
as in Diptera, all the Hymenoptera except the phytophagous ones and
certain coleopterous larvæ, the limbs of the imago stage are, in the
earliest stages, indicated as new structures in the form of imaginal
buds.

=Formation of the imago in Corethra.=—Corethra may serve as an example
of such a relatively simple metamorphosis. Its larva belongs to the
group of eucephalous dipterous larvæ. The head of the perfect insect is
already indicated in the larva, and its parts, with certain
modifications, pass directly into the pupa. The compound eyes, and this
is a rare exception among the Holometabola, are present in the larva. On
the other hand, the thoracic legs, the wings, and halteres are developed
out of new rudiments which are present in the last larval stage, before
pupation. Each thoracic segment has four of them, two ventral and two
dorsal (Fig. 622); the ventral buds becoming the legs. Of the dorsal
pairs, that of the mesothorax develops into wings, that of the
metathorax into halteres, while from the corresponding rudiments of the
prothorax in Corethra arise the stigma-bearing dorsal or respiratory
processes of the pupa, and in Simulium a tuft of tracheal gills (Fig.
623, _ra_; see also Fig. 582).

[Illustration:

  FIG. 623.—Late larva of Simulium, showing the rudiments of the pupal
    structures within the larval skin: _l^1_, _l^2_, _l^3_, fore,
    middle, and hind legs of the fly; _ra_, respiratory appendages of
    pupa; _w_, wing of fly; _h_, halter of fly.—After Miall.
]

[Illustration:

  FIG. 624.—Imaginal buds in larva of Corethra (diagrammatic
    cross-section of thorax): invaginations (_fe_ and _be_) of the
    larval hypodermis (_lhy_) in whose bases the rudiments of wings
    (_fa_) and legs (ba) arise; _lh_, chitinous integument of the
    larva.—After Lang.
]

These imaginal buds may be regarded as evaginations of the outer surface
of the body. The only difference is that the buds of the appendages as a
whole seem sunken below the level of the surface of the body, being
situated at the bottom of an evagination, as in the buds of the head and
trunk in the Pilidium larva of nemertean worms, and in the rudiments of
the lower surface of the body of Echinus present in the pluteus larva.

The lumen of the invagination in which the appendages of Corethra (and
other Holometabola) are situated is called by Van Rees the _peripodal
cavity_, and the external sheath bordering it, which is naturally
continuous with the hypodermis of the body, the _peripodal membrane_
(Fig. 636, _p_).

  We must adopt the view that the rudiments of the appendages
  (imaginal buds) are from the first divided into ectodermal and
  mesodermal portions, which are derived from the corresponding
  germ-layers of the larva. The ectoderm of the rudiments of the
  appendages is continuous with the peripodal membrane, and through it
  with the hypodermis. Weismann was inclined to derive the organs
  (tracheæ, muscles, etc.) developing within the germs of the
  appendages from a hypertrophy of the neurilemma of a nerve passing
  down from within into the imaginal bud, and held that nerves and
  tracheal branches soon after passed into the inner surface of the
  imaginal bud. (Korschelt and Heider.)

When the imaginal buds of the appendages enlarge, then the peripodal
membranes become correspondingly distended, and the limbs within assume
a more or less crumpled position, and in Corethra are spirally twisted,
while the rudiments of the wings are folded. The completion of the
rudimentary limbs is accomplished simply by their passing out of the
invagination in which they originated. The limbs thus gradually become
free, the peripodal membrane is seen to reach the level of the rest of
the hypodermis and become a part of it, and the base of the extremity is
no longer situated in a cavity.

The internal organs of Corethra undergo but to a slight degree the
destruction (histolysis) which is so thoroughgoing in the Muscidæ.
Kowalevsky states that in the mid-intestine of Corethra a histolysis of
the larval and reconstruction of the imaginal epithelium goes on in the
same way as has been described in Musca. Most of the larval organs pass
without histolytic changes directly over into those of the pupal and
imaginal stages; the muscles in general are also unchanged, but those of
the appendages and wings are made over anew. The last arise, according
to Weismann, in the last larval stage from strings of cells which are
already present in the embryo.

When we consider how insignificant the internal transformations are
during the metamorphosis of the Tipulidæ, of which Corethra serves as an
example, we can scarcely doubt that we here have before us conditions
which illustrate the passage between an incomplete and a complete
metamorphosis. Thus, among other things, should be mentioned the short
duration of the pupa stage and the activity of the pupa, as also the
early appearance of the germs of the compound eyes, a character which
Corethra has in common with the Hemimetabola. (Korschelt and Heider.)

=Formation of the imago in Culex.=—In respect to the formation of the
imaginal head, Culex is still more primitive than Corethra. Miall and
Hammond find from Hurst’s partly unpublished descriptions and
preparations that there are no deep invaginations for the compound eyes
or antennæ of the imago.

  “The compound eye forms beneath the larval eye-spots, and is at
  first relatively simple and composed of few facets. The number
  increases by the gradual formation of partial and marginal
  invaginations, each of which forms a new element. The imaginal
  antenna grows to a much greater length than that of the larval
  antenna, and its base is accordingly telescoped into the head, while
  the shaft becomes irregularly folded.[115] Culex, though more
  modified than Chironomus in many respects, _e.g._ in the
  mouth-parts, is relatively primitive with respect to the formation
  of the imaginal head, and shows a mode of development of the eye and
  antenna which we may suppose to have characterized a remote and
  comparatively unspecialized progenitor of Chironomus.”

=Formation of the imago in Chironomus.=—The development of the head of
the imago of _Chironomus dorsalis_ has been discussed by Miall and
Hammond. The invaginations which give rise to the head of the fly could
not be discovered even in a rudimentary state until after the last
larval moult.

  “Weismann has given reasons for supposing that invaginated imaginal
  rudiments could not come into existence before the last larval moult
  in an insect whose life-history resembles that of Corethra or
  Chironomus. If the epidermis were invaginated in any stage before
  the ante-pupal one, the new cuticle, moulded closely upon the
  epidermis, would become invaginated also, and would appear at the
  next moult with projecting appendages like those of a pupa or imago.
  This is actually the way in which the wings are developed in some
  larval insects with incomplete metamorphosis. In Muscidæ the
  invaginations for the head of the imago have been traced back to the
  embryo within the egg,[116] but the almost total subsequent
  separation of the disks from the epidermis renders their development
  independent of the growth of the larval cuticle and of the moults
  that probably take place therein.”

The pupal and imaginal cuticles do not follow at all closely the larval
skin, but, says Miall, become at particular places folded far into the
interior. “The folds which give rise to the head of the fly are two in
number and paired. They begin at the larval antenna on each side of the
head, and gradually extend further and further backwards. The object of
the folds is to provide an extended surface which can be moulded,
without pressure from surrounding objects, into the form of the future
head. On one part of each fold the facets of the large compound eyes are
developed; another part gives rise to the future antenna, a large and
elaborate organ, which springs from the bottom of the fold, and whose
tip just enters the very short antenna of the larva. The folds for the
head ultimately become so large that the larval head cannot contain
them, and they extend far into the prothorax. Here a difficulty occurs.
If the generating cuticle of the prothorax were also to be folded
inwards, the future prothorax would take a corresponding shape. But the
prothorax of the fly has a form dictated to it by the limbs which it
bears and by the muscles to which it gives attachment. These call for a
great reduction in its length, and a peculiar shape, which it is not
here necessary to describe. It will be enough to realize that the
epidermis of the future prothorax cannot be sacrificed to the folds
which are to give rise to the head of the fly. All interference between
the two developing structures is obviated by the provision of a
transverse fold, which pushes into the prothorax from the neck, and
forms a sort of internal pocket. The floor of the pocket forms two
longitudinal folds, which prolong the folds originating in the larval
head. The roof of the pocket shrinks up and forms the connection between
the head and thorax of the fly. Ultimately the head-part is drawn out,
leaving the prothoracic structures unaffected.”[117]

[Illustration:

  FIG. 625.—Process of formation of the parts of the head of the fly in
    the larva of Chironomus (male): _A_, the new epidermis thrown into
    complicated folds which have been cut away in places to show the
    parts within. _B_, the same parts in horizontal section; _lc_,
    larval cuticle; _tf_, transverse fold; _tf′_, upper wall of the
    same; _m_, cut edge of new epidermis; _ant_, larval antenna; _an_,
    nerve to the same; _ant′_, antenna of fly; _lf_, longitudinal fold;
    _o_, eye of fly; _on_, optic nerve; _an′_, root of antennary nerve;
    _br_, brain; _œs_, œsophagus; _b_, bulb of antenna of fly; _s_, _s_,
    _s′_, blood-spaces.—After Miall.
]

The development of the head of the fly of Chironomus appears, as Miall
and Hammond state, to be intermediate between the groups Adiscota and
Discota of Weismann; _i.e._ “between the types in which the parts of the
head of the fly are developed in close relation to those of the larva,
and the types in which deep invaginations lead apparently to the
formation of similar new parts far within the body, the seeming
independence of the new parts being intensified by thoroughgoing
histolysis,” and they suggest that possibly types may be discovered
intermediate between Chironomus and Muscidæ.

[Illustration:

  FIG. 626.—_A_, _B_, _C_, _D_, diagrams of transverse sections showing
    the development of the wings, legs, and the imaginal hypodermis of
    muscid flies from the imaginal buds of the larva during
    metamorphosis: _lh_, chitinous integument of larva from which the
    underlying hypodermis (_lhy_) has withdrawn; _iid_, imaginal buds of
    wings, _iiv_, of legs; _is_, the cords connecting them with the
    hypodermis; _fl_, wing-germs; _b_, leg-germs; _ihy_, imaginal
    hypodermis spreading out in _D_ from the imaginal buds. The imaginal
    rudiments of the hypodermis are indicated by thick, black outlines,
    the larval hypodermis by two thin, parallel lines.—After Lang.
]

We are now prepared to consider the extremely complicated changes, in
the Muscidæ, leaving out of consideration the origin of the wings from
imaginal buds, which has already been discussed on pp. 126–137.

=Formation of the imago in Muscidæ.=[118]—In the flesh, and undoubtedly
the house, and allied flies the germs or imaginal buds of the legs and
wings arise in the same way as in Corethra. But in the Muscidæ, the buds
are situated far within the interior of the body, the peripodal cavities
appear closed, and the peripodal membrane stands in connection with the
hypodermis merely by means of a delicate thread-like stalk. This
connecting cord, which was first detected by Dewitz, and whose
interpretation was entirely right, shows in its interior, as Van Rees
proved, a narrow cavity.

Though the earliest stages in the development of imaginal buds in the
embryo of the Muscidæ are still unknown, yet we shall not go far astray
if we refer them, like the imaginal buds of Corethra, to hypodermal
invaginations. We must, then, regard the stalk-like connection just
mentioned as the long drawn-out neck of this invagination.

In general, the development of the appendages (Figs. 626, 627) goes on
as described in Corethra. The rudiments of the legs enlarge and show at
an early date the first traces of the later joints. They are so packed
in the peripodal cavities that the single joints of the extremities
appear as if pushed in “like the joints of a travelling cup.” (Van
Rees.) The evagination of the completely formed buds of the limbs, which
occurs on the first day after the beginning of pupation, goes on in such
a way that the stalk of the imaginal bud (Figs. 626, _B_; 627, _B_)
shortens, while its cavity widens so that the limbs finally, as in
Corethra, pass out through the widely opened mouth of the peripodal
invagination, which at the same time gradually completely disappears.
The peripodal membrane is converted into a thickened part of the
hypodermis in the region adjoining the base of the leg, and from this
thickened hypodermal portion, the formation of the hypodermis of the
entire imaginal thorax goes on, as the larval hypodermis is gradually
destroyed.

[Illustration:

  FIG. 627.—Imaginal buds of muscid flies in process of development:
    _A_, brain (_c_) and ventral ganglion (_v_) of a larva, 7 mm. long,
    of _C. vomitoria_; _h_, head-rudiment; _rc_, portion of ventral
    cord; _pd_, prothoracic rudiment; _vc_{3}_, third nerve; _md_,
    mesothoracic rudiment. _B_, mesothoracic rudiment more advanced, in
    a pupa, just formed, of _Sarcophaga carnaria_, showing the base of
    the sternum and folds of the forming leg, the central part (_f_)
    representing the foot. _C_, the rudimentary leg of the same more
    advanced; _f_, femur; _t_, tibia; _f_{1}_, _f_{5}_, tarsal joints.
    _D_, two buds from a larva, 20 mm. long, of Sarcophaga, attached to
    tracheæ; _msw_, mesonotal and wing-germ; _mt_, metathoracic
    rudiment. _E_, _r_, mesothoracic germ of a 7 mm. long larva attached
    to a tracheal twig.—After Weismann and Graber, from Sharp.
]

  We must here settle the question as to the first origin of the
  mesodermal portions of the rudiments of the appendages. We can
  already distinguish in the imaginal buds of the fully grown muscid
  larva a clear separation between an ectodermal and an inner
  mesodermal part. Ganin derived the mesodermal part through a sort of
  differentiation and separation of the innermost layer of the
  ectodermal part, and Van Rees has, in general, confirmed this view.
  Kowalevsky, on the other hand, is inclined to the view that the
  mesodermal part of the imaginal bud is derived from the embryonal
  cells of the mesoderm. He finds scattered throughout the mesoderm,
  under the hypodermis of the larva, so-called wandering cells (Fig.
  632, _A_, _w_), which are different in appearance from the
  leucocytes and from the elements from which the formation of the
  mesodermal parts of the imaginal rudiments proceed. Kowalevsky is
  inclined to believe that there are in each segment rudiments of the
  imaginal mesoderm, but which are so delicate and indifferent that we
  cannot find them in the first stages of their origin. From these
  mesodermal imaginal rudiments the above-mentioned wandering cells of
  the mesoderm are derived, which afterwards come into connection with
  the ectodermal portion of the imaginal buds.

Still more complicated and difficult to understand is the development of
the head-section of muscids. We must remember that in muscid larvæ the
head-section exists in its most rudimentary form, being the result of
extreme modification and degeneration. The small size of the head is
also due to the fact that it is more or less retracted within the
thoracic region. Then, as shown by the researches of Weismann, in the
last embryonal stages, the forehead, mandibles, and the whole region of
the head around the mouth invaginate and form a sunken cavity (Fig. 628,
_p_), in which the chitinous supports of the hooks characteristic of
muscid larvæ are soon developed. This sunken part of the head, at whose
inner end is the œsophagus, is called by the not entirely appropriate
name of “pharynx,” and it must at present be remembered that the hollow
space thus named is not a part of the digestive canal. It is an
invaginated section of the head, _and the formation of the head of the
imago mainly depends on the evagination of this region_.

The first rudiments of the most important parts of the head (eyes,
antennæ, and forehead), occur in the youngest larvæ as paired masses of
cells which lie in the thorax next to the two halves of the brain (for
this reason called by Weismann “brain-appendages”), which are from their
first origin connected with the pharynx, and may be regarded as the
imaginal buds of the head. These appear very soon in later stages in the
shape of elongated sacs widening at the hinder end (Fig. 628, _A_ and
_B_, _h_), which from their origin are to be regarded as evaginations of
the pharynx. Very soon epithelial thickenings appear in the wall of this
sac-shaped brain-appendage, in which the rudiments of the parts of the
future head may be recognized.

Disk-shaped thickenings in the hinder widened part of the
brain-appendage form the rudiments of the compound eyes, which therefore
may be called the eye-buds. On the basal surface of the eye-buds is
situated a nervous expansion which is connected by a nerve with the
supraœsophageal ganglion. This nerve becomes the optic nerve of the
perfect animal, while the optic ganglion is clearly separated from the
brain.

In the anterior, more cylindrical or tube-like part of the
brain-appendage we find the “frontal buds” (_ss_), on which the antennal
rudiments (_at_) soon bud out, in exactly the same way as the rudiments
of the limbs arise from the imaginal buds.

[Illustration:

  FIG. 628.—Diagrammatic representation of the position of the imaginal
    buds in the larva (_A_) and pupa (_B_) of Musca (the wing rudiments
    omitted): _as_, eye-buds; _at_, antennal germs; _b^1_-_b^3_, germs
    of the legs; _bg_, central ganglionic cord: _g_, brain; _h_,
    so-called frontal appendage (_Hirnanhang_): _m_, peripodal membrane;
    _o_, opening of the frontal appendage into the pharynx; _oe_,
    œsophagus; _p_, so-called “pharynx”; _r_, rudiment of the proboscis;
    _ss_, frontal bud; _st_, stalk-like connection of the peripodal
    membrane with the hypodermis; _I_-_III_, 1st, 2d, and 3d thoracic
    segments.—Adapted from Van Rees, by Korschelt and Heider.
]

Originally (Fig. 628, _A_) the brain-appendages lie tolerably far behind
in the thorax of the larva, so that they connect the hindermost part of
the wall of the pharynx with the foremost section of the brain, which
they surround in the form of a mushroom. Afterwards, however, subsequent
to pupation, they move, together with the central nervous system,
farther forward (_B_), whereby they (if we have correctly understood the
descriptions of Weismann and Van Rees) laterally surround the pharynx
with their anterior end, which is somewhat ventrally bent. At the same
time, there becomes established a gradually widening communication (_B_,
_o_) between the brain-appendage and the pharynx, which soon extends in
the form of a lateral pharyngeal fissure along the entire length of the
brain-appendage. As a result, the cavity of the brain-appendage and the
pharynx so completely unite that the two soon form a single sac, the
head-sac or vesicle (Fig. 6–9, _k_). The walls of this head-vesicle are
the later head-wall, the most important parts of which can now be
recognized (the antennæ, eyes, rudiments of the beak). It is now
necessary that the head-vesicle (Fig. 629, +, +) be, by the eversion of
the pharynx, turned outward in order that the head of the pupa may be
completed. By this eversion of invaginated parts, the former
mouth-opening of the pharynx becomes a neck-section (Fig. 629, +, +) by
which head and thorax are now united. (Korschelt and Heider.)

[Illustration:

  FIG. 629.—Diagram of the changes to pupa of Musca before imago
    appears; the wing-germs not drawn: _as_, eye-buds; _at_, antennal
    germs; _b^1_-_b^3_, leg-germs; _bg_, ventral nerve-cord; _g_, brain;
    _k_, head-vesicle (originating from the union of the pharynx with
    the hypophysis, _Hirnanhängen_); _oe_, œsophagus; _r_, germ of the
    proboscis; _ss_, germ of the forehead; _I_, _II_, _III_, 1st, 2d,
    and 3d thoracic segments.—Based on Kowalevsky and Van Rees, with
    changes, after Korschelt and Heider.
]

  The cause of the eversion of the head-vesicle, which Weismann
  directly observed, appears to be due to an increase of the inner
  pressure through a contraction of the hinder parts of the body. The
  anterior end of the œsophagus now becomes turned down ventrally
  corresponding to the conformation of the head of the imago.

  It has been shown that the so-called pharynx is only an invaginated
  part of the outer surface of the larval head. The brain-appendage
  Korschelt and Heider consider to be the diverticulum of this
  invagination, in which the single parts of the body lie in an
  invaginated state. They may throughout be compared to the rudiments
  of the thoracic limbs. All these imaginal buds have been traced back
  to the invaginated parts of the outer surface of the body, _i.e._
  the ectoderm.

It should be borne in mind that the process of development of the head
of the highly-modified Muscidæ is much more complex than in the more
primitive Diptera.

In their essay on the development of the head of the imago of
Chironomus, Miall and Hammond arrange the dipterous types thus far
examined, in the order of complexity of the invaginations which give
rise to the head of the imago, in the following order:—

     1. Culex. Relatively simple. Invaginations of the imaginal buds,
       shallow. 2. Corethra, Simulium. } Intermediate. 3. Chironomus,
       Ceratopogon. } 4. Muscidæ. Relatively complex. Invaginations
       deep, and apparently, but not really, unconnected with the
       epidermis.


          _b._ Development of the internal organs of the imago

[Illustration:

  FIG. 630.—Median longitudinal section through larva of blow-fly during
    the process of histolysis: _an_, antenna; between _an_ and _w_,
    rudiments of eye; _w_ wings; _h_, halteres; _b_{1}_-_b_{3}_, legs;
    _f_, fat-body; _d_, middle of intestine; _n_, ganglia; _st_, stigma;
    6, 7, 6th and 7th body-segments.—After Graber, from Sharp.
]

It has already been observed that most of the organs of muscid larvæ
(and this applies to most Diptera, Lepidoptera, Coleoptera, and
Hymenoptera) are destroyed through the action of leucocytes, and that
their reformation is accomplished by definite groups of embryonal cells,
the imaginal buds or folds. Destruction and rebuilding occur during the
pupa stage in such a way that in many cases while this process is going
on the continuity of the organs does not seem to be disturbed. These
transformations especially concern the hypodermis, the digestive canal,
the muscles, the fat-body, and the salivary glands.

The transformation of the tracheal system is only partial, being in part
a simple process of regeneration through cell-division. Slighter changes
affect the heart, the central nervous system, and the reproductive
system (Fig. 630).

[Illustration:

  FIG. 631.—Diagram of the formation of the imaginal hypodermis on the
    abdomen of Muscidæ: _hi_, imaginal buds of the hypodermis; _lh_,
    larval hypodermis.—After Lang.
]

[Illustration:

  FIG. 632.—Section through the abdominal bud of the hypodermis of
    Musca: _A_, of the larva; _B_ and _C_, of the pupa; _h_, larval
    hypodermis; _h′_ separated portion of the same attacked by
    phagocytes; _i_, imaginal bud; _k_, phagocytes with what are called
    cell-wrecks or fragments (so-called granulated cells); _k′_,
    phagocytes enclosing hypodermal nuclei; _m_, mesoderm-germ of the
    imaginal bud; _w_, wandering cells.—After Kowalevsky, from Korschelt
    and Heider.
]

=The hypodermis.=—The hypodermis of the imago arises through an
extension of the ectodermal part of the imaginal buds. We have already
mentioned this for the thorax. As the appendages of the thorax in the
pupa gradually attain perfection, the hypodermis layer spreads from the
place of their insertion, the layer consisting of numerous small cells
whose origin we must refer to the peripodal membrane. This layer
continues to spread over the surface of the pupal thorax, while at the
same time the area of the larval hypodermis, consisting of large cells,
is seen to diminish. Hence the thin edge of the newly-formed hypodermis
(Fig. 631, _hi_) slowly grows into the space between the superficial
cuticula and the larval hypodermis (Fig. 632, _h_), so that at this
place the old hypodermis undergoing destruction eventually lies on the
inner side of the newly-formed epithelial layer (_B_). We therefore see
from this that, during the replacement of the old hypodermis by the new,
the continuity of the superficial epithelium is never interrupted. Since
the edges of the two kinds of hypodermis overlap, the surface of the
body is nowhere bare of epithelium. The dissolution of the larval
hypodermis is accomplished under the influence of the leucocytes (Fig.
632, _k_), which attack the larval hypodermis-cells and absorb their
contents piece by piece, and so fill themselves with bits of the
hypodermis-cells and their nuclei; since these fragments have the shape
of roundish granules, they were called by Weismann granule-balls. These
granule-balls, which fill the body-cavity of the later pupal stage, are
nothing else than the leucocytes (blood corpuscles) which have absorbed
the fragments of tissue of the larval body.

It should here be said that the destruction of the larval tissues is not
to be attributed to the previous death of the cells, but is the result
of the action of the leucocytes on tissues which, though weakened in
their vital power, are still living. While the completely healthy,
active tissues, _i.e._ those of the imaginal buds, withstand the attacks
of the leucocytes, the less healthy larval tissues are by the attacks of
the leucocytes divided into fragments and eaten and digested by them.
This process is most marked in the histolysis of the larval muscles. The
destruction of most of the larval organs depends, therefore, on the
capacity of the amœboid blood-corpuscles for taking food and on
intracellular digestion, as was first shown by Metschnikoff, who has
given to these leucocytes the name of “phagocytes.”

This process of histolysis goes on in the same way in the head and
abdomen as in the thorax. In the abdomen, as Ganin first proved, there
are in each of the eight segments of which it consists in the larva four
small cellular islets or imaginal buds (Figs. 631, _hi_, 632, _i_), from
which originate the new hypodermis.

Van Rees has lately found in the abdominal segments another pair of
smaller imaginal buds. The four imaginal buds occurring in the last
segment are situated close to each other, encircling the anal opening
(Fig. 633, _ims_), and take part in the formation of the hind-intestine,
the rectal pouches and rectal papillæ. To this segment also belong the
two pairs of imaginal genital buds (rudiments of the external sexual
organs) which were first found by Künckel d’Herculais in Volucella.

The newly formed hypodermis spreads rapidly over the outer surface of
the body, so that hypodermal areas corresponding to the separate
imaginal buds soon unite. Simultaneously with this completion of the
definite epithelial layer the larval hypodermis becomes completely
destroyed by the phagocytes.

=The muscles.=—A similar process of destruction by phagocytes affects
the greater number of the larval muscles, except the three pairs of
thoracic muscles employed in respiration, and which pass intact from the
larva to the imago. Indeed, the dissolution of the muscles is the first
process which occurs in the metamorphosis. The destruction of the larval
muscles is accomplished in such a way that, a great number of leucocytes
which have collected on the surface of the muscular fibres, press
through the sarcolemma and enter within the muscular tissue, filling the
spaces formed between them, By this means the muscles break up into a
number of rounded particles which are taken into the interior of the
leucocytes. Thus a collection of granule-balls arise from the muscles,
which finally separate from each other and become scattered throughout
the body-cavity of the pupa. In the same way as the muscular substance,
the muscle-nuclei are taken up and digested by the phagocytes.

The imaginal muscles develop from the definitive mesoderm which has
originated from the mesoderm of the imaginal buds (Fig. 632, _C_, _m_).

=The digestive canal.=—As in the hypodermis and muscles, the histolysis
of the larval digestive tract and its new formation from separate
imaginal buds go on simultaneously, so that the continuity of the
process is not interrupted.

[Illustration:

  FIG. 633.—Digestive tract of a Musca larva with the imaginal germs:
    _bd_, cœca; _s_, food-reservoir; _is_, imaginal ring of the salivary
    gland (_sp_); _f_, fat-cells at the end of the salivary gland; _pr_,
    proventriculus; _r_, its ring; _ie_, imaginal cells of the
    mid-intestinal epithelium; _ch_, chyle-stomach; _ma_, urinary tubes;
    _im_, imaginal cells of the mid-intestinal, muscular layer; _ims_,
    binder, abdominal, imaginal buds; _h_, hind intestinal, imaginal
    bud; _ht_, hind-intestine.—After Kowalevsky, from Korschelt and
    Heider.
]

The imaginal buds of the much-shortened pupal digestive canal occur in
the mid-intestine (stomach) in the form of numerous scattered groups of
cells (Fig. 633, _ie_), and in the fore- and hind-intestine in the form
of rings (_v_ and _h_) of imaginal tissue. The imaginal ring of the
fore-intestine (_v_) lies in the region of the proventriculus (_pr_,
compare Fig. 635, _im_), while that of the hind-intestine is situated
directly behind the base of the urinary tubes. The regeneration of these
two parts of the digestive canal is not entirely accomplished by these
two rings, but the imaginal rudiments of the neighboring parts of the
outer surface of the body also have a share in it. Thus it appears that
the foremost part of the œsophagus is built up from the imaginal buds in
the region of the mouth, while the imaginal buds surrounding the anus in
the 8th abdominal segment (Fig. 633. _ims_) produce by invagination the
rectal pouches, together with the rectal papillæ.

[Illustration:

  FIG. 634.—Cross-section through the mid-intestine of pupa of Musca:
    _e_, rejected and degenerate epithelium of the larval stomach; _f_,
    cellular layer newly formed around the same; _m_, muscular layer;
    _m′_, imaginal cell of _m_; _o_, imaginal bud of the mid-intestinal
    epithelium; _t_, tracheal stem.—After Kowalevsky, from Korschelt and
    Heider.
]

[Illustration:

  FIG. 635.—Longitudinal section through the proventriculus of a muscid
    larva: _im_, fore-intestinal, imaginal ring; _oe_, œsophagus; _pr_,
    proventriculus.—After Kowalevsky, from Korschelt and Heider.
]

The formation of the mid-intestine (stomach) takes place in such a way
that the island-like imaginal buds spread out by cell-multiplication
over the outer or basal surface of the larval mid-intestinal epithelium
(Fig. 634, _o_), until they finally unite, so as to form the wall of the
imaginal mid-intestine (stomach). At the same time the entire larval
epithelium (_e_) is cast in the interior and forms the so-called yellow
body, which becomes surrounded by a layer of small cells and a
jelly-like mass, and remains until its destruction in the pupal stomach.
The larval muscular layer (_m_) remains intact as long as the imaginal
mid-intestine is not fully developed, when it is attacked and destroyed
by phagocytes. The final muscular layer arises from single cells lying
on the outer surface of the imaginal buds (Figs. 633, _im_, 634, _m′_),
which should be regarded as special imaginal cells of the mid-intestinal
muscular layer.

The transformation of the fore-intestine is introduced by a degeneration
of the proventriculus and sucking stomach. The proventriculus (Fig. 635,
_pr_), which had been formed from a circular fold of the fore-intestine,
disappears by the smoothing out of this folded structure. The sucking
stomach also similarly degenerates by withdrawing gradually into the
œsophagus, so that instead of the original diverticulum there remains
only an enlargement of the œsophageal cavity. At the same time this part
of the canal is attacked and destroyed by phagocytes, while the
destroyed portions become replaced by the gradually extending imaginal
parts of the wall. The imaginal ring of the fore-intestine (Fig. 635,
_im_), which, according to Kowalevsky, is concerned in the formation of
a great part of the definitive œsophagus, becomes closed at its hinder
end so that the communication with the mid-intestine appears to be
interrupted.

The hind-intestine of the imago is rebuilt in exactly the same manner.
Here also the imaginal ring widens and forms a tube, which while it
grows around the openings into the urinary tubes, closes itself against
the mid-intestine, while behind it remains in connection with the larval
hind-intestine. In a similar way the larval hind-intestine is attacked
by the growth from behind of an imaginal ring, which proceeds from
imaginal buds near the anus, until finally, when the entire larval
hind-intestine is reduced to granule-balls, the two imaginal sections of
the tube are brought into contact with each other. (Kowalevsky in
Korschelt and Heider.)

  The larval salivary glands (Fig. 633, _sp_) are completely destroyed
  by phagocytes. Then succeeds the new formation of these glands from
  imaginal buds, which, according to Kowalevsky, form rings situated
  at their anterior ends.

  The nature of the transformation undergone by the urinary tubes is
  not yet well ascertained. According to Van Rees, there is in this
  case perhaps a regeneration of the larval cells by division, but on
  the other side there may be a histolysis of these elements.

The above-described method of transformation of the digestive canal
seems, according to Korschelt and Heider, to be very common among the
holometabolic insects. It has not only been observed in the Diptera, but
also in the Lepidoptera (Kowalevsky, Frenzel), Coleoptera (Ganin), and
Hymenoptera (Ganin). The stripping off of the epithelium of the
mid-intestine was found by Kowalevsky to occur also in Corethra, Culex,
and Chironomus.

=The tracheal system.=—As we have seen (p. 448), the tracheal system of
caterpillars just before pupation undergoes disintegration, accompanied
by a reformation of the peritoneal membrane and tænidia. The larval
ectotrachea undergoes histolysis, that of the imago being meanwhile
formed; the larval tænidia also break up, dissolve, and are replaced by
new tænidia which arise from the nuclei of the peritoneal membrane. That
the tracheal system in the Muscidæ during metamorphosis undergoes a
transformation is shown, as Korschelt and Heider claim, by the entirely
different shape of the system in the maggot, the pupa, and the fly. The
air is admitted to the tracheal system of the maggot, not by lateral
openings, but through the two stigmata at the end of the body. On the
other hand, the pupa breathes by prothoracic spiracles, while the fly
has six pairs of lateral stigmata of the normal structure. There may be
in the larva and pupa vestigial closed stigmata, as there are in the
thorax of caterpillars, with tracheal branches leading to where were
once functional stigmata. These stigmatal branches, as well as some
other portions of the tracheal system already observed by Weismann,
seem, according to Van Rees, to function as imaginal buds for the
regeneration of the tracheal matrix, while frequently also a
regeneration of this epithelium, by a simple repeated division of cells,
may be recognized. The disintegration of the tracheal system is
accomplished by means of phagocytes in the manner already described.

=The nervous system.=—The central nervous system passes directly from
the larval into the imaginal stage, since it must continue to exercise
most of its functions throughout metamorphosis, though it undergoes
important changes of form and position. At the same time, certain
histological transformations occur which may be regarded as a
histolysis. Such is the destruction and rebuilding in the interior of
the organs, which, however, preserve their continuity. Every case of
destruction of tissues in the pupa has come to be regarded as a
histolysis.

The problem of the transformation of the peripheral nervous system is
not yet well understood. Although during the destruction of the larval
muscles the motor nerves also in part degenerate, in the case of the
nerves distributed to the appendages the conditions are different, as
these may be recognized in the larva in the form of the nerve-threads
which place the imaginal buds in connection with the central nervous
system. These threads, according to Van Rees, pass from the larva into
the pupa and imago, so that with the farther development of the
rudiments of the extremities, only the distal part of the nerves
belonging to them are to be regarded as new formations. (Korschelt and
Heider.)

=The fat-body.=—The larval fat-body is also destroyed through the
activity of the leucocytes in the same way as the other tissues. The
reformation of the fat-body seems to begin in the mesoderm of the
imaginal buds. Possibly, also, the masses or collections of embryonic
cells which are regarded by Schaeffer as “blood-forming cells,” may
serve to regenerate the fat-body. At all events, they have been derived
from the mesodermal tissues. Though Wielowiejsky saw the fat-body of
Corethra arising from a cell-layer situated under the hypodermis, yet it
is not necessary to regard this observation as favorable to the view of
Schaeffer that in Musca the larval fat-body is derived in part from the
hypodermis, and in part from the tracheal matrix, thus from the
ectodermal tissues. (Korschelt and Heider.)

=Definitive fate of the leucocytes.=—We have seen that the formation of
the organs of the imago originates in the imaginal buds, in all cases
where these do not pass directly from the larva into the pupa. The
leucocytes, whose numbers in the pupa are greatly increased, take no
direct part in the formation of the tissues. Their importance seems to
lie in this, that they destroy those larval organs doomed to
destruction, the parts of which they take in and digest, and possibly,
by their powers of locomotion, convey particles of food to the
developing organs.

What, on the other hand, is the fate of the leucocytes after the
developmental processes in the pupa have ceased? There can be no doubt
that a part of the so-called granule-cells are again transformed into
normal blood-corpuscles. Another, and, as it seems, more considerable,
share suffer degeneration. Finally, the leucocytes themselves serve as
nourishment for the newly formed tissues. Of interest in this direction
is the observation of Van Rees, that numerous leucocytes finally pass
into the newly-formed hypodermis and then degenerate in crevices between
the hypodermis-cells. (Korschelt and Heider.)

  It has been suggested by Van Rees that the phagocytes attack all the
  larval organs indiscriminately, but that the active metabolism of
  the imaginal buds preserves them from these attacks. He also thinks
  that Kowalevsky is probably right in supposing that the buds render
  themselves immune by some poisonous secretion.

  Pratt, however, thinks that the supposition of a protecting or
  poisonous secretion is scarcely necessary to account for the
  phenomenon, and suggests that the larval tissues are a prey to the
  phagocytes, because at the end of larval life they become
  functionless and inactive, so as to become an easy prey to
  phagocytes or disintegrating influences of any sort. On the other
  hand, the imaginal buds “in which there is an exceedingly active
  metabolism, and _all the larval organs which remain functional
  during the metamorphosis_ are immune from the attacks of the
  phagocytes. The heart in the muscids continues to beat, as Künckel
  d’Herculais has observed, during the entire period of the
  metamorphosis, with the exception of a day or two in the latter half
  of it. The nervous system must continue functional during the entire
  time. The three pairs of thoracic muscles which pass intact from the
  larva to the imago are probably employed in respiration during the
  metamorphosis. The reproductive glands are, like the imaginal disks,
  rapidly growing organs.” He adds that among the other holometabolic
  insects many or most of the larval organs remain functional during
  metamorphosis, hence there is but little histolysis. “But the larval
  intestine would always necessarily become unfunctional, and, as we
  have seen, Kowalevsky is of the opinion that the larval mid-gut in
  all holometabolic insects contains imaginal disks, and undergoes
  degeneration during the metamorphosis.”

=The post-embryonic changes and imaginal buds in the Pupipara
(Melophagus).=—The sheep-tick (Melophagus) is still more modified than
the Muscidæ; the larva is apodous and acephalous, but, as Pratt
observes, much less highly specialized than those of muscids, and in
respect to the position of the thoracic buds it closely resembles
Corethra. They lie just beneath the hypodermis in two very regular rows,
and not in the centre of the body, as in Musca (Figs. 628, 636, _C_).
While, however, in Corethra all the thoracic buds are of larval origin,
arising after the last larval moult, in Melophagus, on the other hand,
each of these buds, except the dorsal prothoracic, arises in the embryo,
as is also the case in Musca.

In the cephalic buds the conditions are similar to those in Musca, but
still more complicated. Instead of a single pair of head-buds, there are
two pairs, one dorsal and one ventral. “The dorsal pair corresponds to
the muscidian head-disks in every respect; they are destined to form the
dorsal and lateral portions of the imaginal head, together with the
compound eyes. The ventral head-disks have no counterpart in Musca. The
fate of these disks or buds is to form the ventral portion of the head,
the paired projections forming the rudiments of the proboscis.

“The formation of the head-vesicle proceeds in a way similar to that in
Musca. The ventral disk fuses early at its lateral edges with the dorsal
pair. The communications between both ventral and dorsal disks and the
pharynx rapidly widen (in the old larva they have already become very
large), and soon the disks and pharynx form together a single vesicle,
which is the head-vesicle.” The imaginal buds of the abdomen Pratt finds
to be exactly as in the corresponding ones of Musca.

In the embryo of Melophagus the cephalic and thoracic imaginal buds
first appear as local thickenings, followed by the invagination of the
ectoderm; the cephalic buds first appear very early in the ontogeny of
the insect (Fig. 636, _C_), just as the germs of the digestive canal,
nervous system, and tracheæ are appearing. The single median thickening
(_v_) is destined to form the ventral cephalic bud, while the pair of
thickenings behind (_d_) become the dorsal buds, those homologous with
the cephalic buds of Musca.

[Illustration:

  FIG. 636.—Imaginal buds in Musca,—_A_, in Corethra,—_B_, in
    Melophagus,—_C_, in embryo of Melophagus: dorsal view of head; _b_,
    bud; _p_, peripodal membrane; _c_, cord; _hy_, hypodermis; _cl_,
    cuticula; _st_, stomodæum; _v_, ventral cephalic bud, behind are the
    two dorsal cephalic buds (_d_).—After Pratt.
]

The thoracic buds, which arise as hypodermic thickenings, do not appear
until late in embryonic life, until the time of the involution of the
head.

Pratt did not observe in the embryo the buds of the internal organs and
of the abdominal hypodermis, and thinks it probable that they appear
first in the larva.


                          _c._ General summary

We have seen that in Coleoptera, Lepidoptera, Diptera, and Hymenoptera,
and with little doubt in all the holometabolous insects, the parts of
the imago originate in single formative cellular masses (imaginal buds)
already present in the larva, and often even in the later embryonic
stages. There are such imaginal buds for each part of the body,—for the
appendages of the head, for the legs and wings, for the ovipositor, and
probably for the cercopods, for the hypodermis of the abdomen, and for
the different sections of the digestive canal. We have seen, as
Korschelt and Heider state, that the formation of the mesodermal organs
of the imago (muscles, connective tissue, fat-body) begins in the
mesodermal part of the imaginal buds, whose first origin is still
obscure. Simultaneously with the formation of the imaginal organs, there
goes on under the influence of the leucocytes the destruction of the
larval organs. Both processes (destruction and regeneration) therefore
go on hand in hand, so that the continuity of the organs in question in
most cases remains perfect, inasmuch as the complete destruction only
ensues after the formation of the final organs. The only exceptions are
most of the muscles of the larva, which are destroyed at a very early
period.

Moreover, it is evident that the sharp division into larval, pupal, and
imaginal stages only applies to the external surface of the body, since
they follow one another after successive moults. The processes of the
internal development, on the other hand, form an entirely continuous
series of transformations between which is no sharp line of demarcation.
Yet as a whole the form of the larva, pupa, and imago are kept distinct
in adaptation to their separate environments and habits.

Finally, as Pratt very truly remarks, the epigenetic period in insects,
when new organs are forming, does not end with the birth of the larva
from the egg, but extends through the larval, and even through the pupal
period. “The principal significance of the pupal period and the
metamorphosis is that it is the time when the larval characters which
were adapted for use during a period of free life in the midst of the
development, and which would be valueless to the imago, are corrected or
abandoned.”




                           HYPERMETAMORPHISM


When an insect passes through more than the three normal stages of
metamorphosis, _i.e._ the larval, pupal, and imaginal, it is said to
undergo a _hypermetamorphosis_. The best-known examples are the
supernumerary stages of Meloë, Stylops, etc.

[Illustration:

  FIG. 637.—Hypermetamorphosis of male of _Aspidiotus nerii_: 1, freshly
    hatched larva; 2, larva shortly before pupating; _b_, rudiments of
    the legs; _fl_, of the wings; 3, pupa before moulting; 4, the same
    after moulting; 6, larva farther advanced than in 2; _a_, antennal
    rudiments; _b_, rudiments of legs; _v_, stomach; _OG_, brain; _M.
    Fl_, rudiments of the elevator and depressor muscles of the wing;
    _M. Th_, rudiments of the dorsal muscles; _H_, rudiments of the
    testes; 7, pupa shortly before entering upon the imago state (5);
    _A_, eyes; _a_, antenna; _o_, mouth; _WD_, wax-glands; _BG_, ventral
    nervous cord; _Sb_, caudal setæ; _tr_, tracheæ; _p_, genital
    armature.—After Schmidt.
]

As has already been observed, Schmidt has shown that in the male of the
Coccidæ, there is a true hypermetamorphosis, as shown by Fig. 637. In
_Aspidiotus nerii_ there are five stages, there being two larval (1, 2)
and two pupal stages (3, 4, 7). Stage 3 (Fig. 637, 2) may be compared
with the pro-pupa stage of Riley (Fig. 581).

[Illustration:

  FIG. 638.—_Mantispa interrupta_, and side view of the same without
    wings: natural size.—Emerton _del._ _a_, freshly-hatched campodeoid
    larva of _Mantispa styriaca_, enlarged; _b_, the same, but older,
    before the first moult; enlarged.—Brauer.
]

We have already, on page 602, described the hypermetamorphosis of the
neuropterous insect Mantispa (Fig. 638).

[Illustration:

  FIG. 639.—Triungulin (_a_) of a Californian Meloë: _b_, the three
    triungulin claws; _c_, antenna; _d_, maxillary palpus; _e_, labial
    palpus; _f_, mandible; _g_, an abdominal joint; _h_, imago, ♀; _i_,
    antenna of ♂.—After Riley.
]

In Meloë the freshly hatched larva, or “triungulin” (Fig. 639, _a_), is
an active Campodea-like larva, which runs about and climbs up flowers,
from which it creeps upon the bodies of bees, such as Anthophora and
Andrena, who carry it to their cells, wherein their eggs are situated.
The triungulin feeds upon and destroys the eggs of its hostess.
Meanwhile its inactive life in the bee’s cell reacts upon the organism;
after moulting, the-second larval form (Fig. 640, _b_) is attained, and
now the body is thick, cylindrical, soft, and fleshy, and it resembles a
lamellicorn larva, with three pairs of rather long thoracic legs. This
is Riley’s carabidoid stage. This second larva feeds upon the honey
stored up for the young or larval bees. After another moult, there is
another entire change in the body; it is motionless, the head is
mask-like without movable appendages, and the feet are represented by
six tubercles. This is called the semipupa or pseudo-pupal stage. This
form moults, and changes to a third larval form (_c_), when apparently,
as the result of its rich, concentrated food, it is overgrown,
thick-bodied, without legs, and resembles a larval bee.

[Illustration:

  FIG. 640.—Oil-beetle: _a_, first larva; _b_, second larva; _c_, third
    larva; _d_, pupa.
]

[Illustration:

  FIG. 641.—History of Sitaris: _a_, triungulin or 1st larva; _g_, anal
    spinnerets and claspers of same; _b_, 2d larva; _e_, pseudo-pupa;
    _f_, 3d larva; _c_, true pupa; _d_, imago, ♀.—After V. Mayet, from
    Riley.
]

After thus passing through three larval stages, each remarkably
different in structure and in the manner of taking food, it transforms
into a pupa of the ordinary coleopterous shape (_d_).

The history of Sitaris, as worked out by Fabre and more recently by
Valery-Mayet, is a similar story of two strikingly different
adaptational larval forms succeeding the triungulin or primitive larval
stage. The first larva (Fig. 641, _a_) is in general like that of Meloë,
the second (_b_) is thick, oval, fleshy, soft-bodied, and with minute
legs, evidently of no use, the larva feeding on the honey stored by its
host. The pseudo-pupal stage is still more maggot-like than in the
corresponding stage of Meloë, and the third larva (_f_) is thick-bodied,
with short thoracic legs.

In the complicated life-history of another cantharid, _Epicauta
vittata_, as worked out by Riley (Fig. 642), we have the same
acquisition of new habits and forms after the first larval stage, which
evidently were at the outset the result of an adaptation to a change of
food and surroundings. The female Epicauta lays its eggs in the same
warm, sunny situation as that chosen by locusts (Caloptenus) for
depositing their eggs. On hatching, the active minute carnivorous
triungulin, ever on the search for eggs, on happening upon a locust egg
gnaws into it, and then sucks the contents. A second egg is attacked and
its contents exhausted, when, owing to its comparatively inactive habits
and rich nourishing food after a period of inactivity and rest, the skin
splits along its back, and at about the eighth day from beginning to
take food the second larva appears, with much smaller and shorter legs,
a much smaller head, and with reduced mouth-parts. This is the
carabidoid stage of Riley. After feeding for about a week in the egg a
second moult occurs, and the change of form is slight, though the
mouth-parts and legs are still more rudimentary, and the body assumes
“the clumsy aspect of the typical lamellicorn larva.” This Riley
denominates the scarabæidoid stage of the second larva.

[Illustration:

  FIG. 642.—_Epicauta cinerea_: _a_, end of 2d larval stage; _b_,
    portion of dorsal skin; _c_, _d_, coarctate larva; _e_, _f_,
    pupa.—After Riley.
]

After six or seven days there is another transformation, the skin being
cast, and the insect passes into another stage, “the ultimate stage of
the second larva.” The larva, immersed in its rich nutritious food,
grows rapidly, and after about a week leaves the now addled and decaying
locust eggs, and burrows into the clear sand, where it lies on its side
in a smooth cell or cavity, and where it undergoes an incomplete
ecdysis, the skin not being completely shed, and assumes the semipupa
stage, or coarctate larval stage of Riley.

In the spring the partly loose skin is rent on the top of the head and
thorax, and then crawls out of it the “third larva,” which only differs
from the ultimate stage of the second larva “in the somewhat reduced
size and greater whiteness.” The insect in this stage is said to be
rather active, and burrows about in the ground, but food is not
essential, and in a few days it transforms into the true pupa state.

These habits and the corresponding hypermetamorphosis are probably
common to all the Meloidæ, though the life-history of the other species
has yet to be traced.

In the genus Hornia described by Riley, the wings of the imago are more
reduced than in any other of the family, both sexes having the elytra as
rudimentary as in the European female glow-worm (_Lampyris noctiluca_).
These, with the simple tarsal claws and the enlarged heavy abdomen, as
Riley remarks, “show it to be a degradational form.”

Its host is Anthophora, and the beetle itself lives permanently in the
sealed cells of the bee, and Riley thinks it is subterranean, seldom if
ever leaving the bee gallery. The triungulin is unknown, but the
ultimate stage of the second larva, as well as the coarctate larva, is
like those of the family in general, the final transformations taking
place within the two unrent skins, in this respect the insect (Fig. 643)
approaching Sitaris.

It appears, then, that as the result of its semi-parasitic mode of life
the Campodea-form or triungulin larva of these insects, which has
free-biting mouth-parts like the larvæ of Carabidæ and other carnivorous
beetles, instead of continuing to lead an active life and feeding on
other insects; living or dead, and then like other beetles directly
transforming into the normal pupa, moults as many as five times, there
being six distinct stages before the true pupa stage is entered upon. So
that there are in all eight stages including the imaginal or last stage.

One cannot avoid drawing the very obvious conclusion that the five extra
stages constituting this hypermetamorphosis, as it is so well styled,
are structural episodes, so to speak, due to the peculiar parasitic mode
of life, and were evidently in adaptation to the remarkable changes of
environment, so unlike those to which the members of other families of
Coleoptera, the Stylopidæ excepted, have been subjected. The fat
overgrown body and the atrophied limbs and mouth-parts are with little
doubt due to the abundant supply of rich food, the protoplasm of the egg
of its host, in which the insect during the feeding time of its life is
immersed. Since it is well known that parthenogenesis is due to over, or
at least to abundant nutrition, or to a generous diet and favoring
temperature, there is little reason to doubt that the greatly altered
and abnormally fat or bloated body of the insect in these supernumerary
stages is the result of a continuous supply of rich pabulum, which the
insect can imbibe with little or no effort.

[Illustration:

  FIG. 643.—1, Egg-pod of _Caloptenus differentialis_ with the mouth
    torn open, exposing the newly hatched larva of _Epicauta vittata_ (1
    _a_) eating into an egg and the passage which it made through the
    mucous covering; natural size. 2, dorsal view of the 1st larva, or
    triungulin, of _E. vittata_; 2 _a_, one side of the head of same
    from beneath, greatly enlarged so as to show the mouth-parts; 2 _b_,
    terminal joint of maxillary palpus, showing imbrications and
    flattened inner surface armed with stout points; 2 _c_, leg, showing
    more plainly the tarsal spines; 2 _e_, labrum; 2 _d_, one of the
    abdominal joints from above, showing stout points, stigmata, and
    arrangement of spinous hairs. 3, eggs of _E. vittata_, the natural
    size indicated at side. 4, dorsal view of the carabidoid stage of
    the 2d larva of _E. vittata_: 4 _a_, its antenna; 4 _b_, its right
    maxilla; 4 _c_, its leg; 4 _d_, side view of same, showing its
    natural position within the locust-egg mass. 5, lateral view of the
    ultimate or full-grown stage of the 2d larva of _E. vittata_; 5 _a_,
    portion of the dorsal skin, showing short setaceous hairs. 6, third
    head, or that from the scarabæidoid stage of the 2d larva of _E.
    vittata_ from beneath, showing the reduction of mouth-parts as
    compared with the first head (2 _a_); 6 _a_, antenna of same; 6 _b_,
    maxilla of same; 6 _c_, mandible of same. 7, fourth head, or that of
    the full-grown larva of _E. vittata_, from above; 7 _a_, leg of
    same; 7 _b_, the breastplate or prosternal corneous piece. 8,
    lateral view of the pseudo-pupa or coarctate larva of _E. vittata_,
    with the partially shed skin adhering behind: 8 _a_, dorsal view of
    same; 8 _b_, its head, from the front; 8 _c_, same from side; 8 _d_,
    tuberculous leg; 8 _e_, raised spiracle; 8 _f_, anal part of same,
    9. lateral view of the true pupa of _Epicauta cinerea_ Forst: 9 _a_,
    ventral view of same. 10, _Epicauta vittata_ (lemniscata or
    trivittate var.). 11, _Epicauta cinerea_ Forst. (= _marginata_
    Fabr.). 12, antenna of the triungulin of _Epicauta pennsylvanica_:
    12 _a_, maxilla of same; 12 _b_, labial palpus of same. 13. ♂
    _Hornia minutipennis_, dorsal view; 13 _a_, lateral view of same; 13
    _b_, simple claw of same; 13 _c_, coarctate larva; 13 _d_, leg of
    ultimate stage of 2d larva.—After Riley.
]

[Illustration:

  FIG. 644.—Triungulin stage of _Stylops childreni_.
]

The life-history of the Stylopidæ is after the same general fashion,
though we do not as yet know many of the most important details. The
females are viviparous, the young hatching within the body of the
parent, as we once found as many as 300 of the very minute triungulin
larvæ issuing in every direction from the body of what we have regarded
as the female of _Stylops childreni_ in a stylopized Andrena caught in
the last of April. The larvæ differ notably from those of the Meloidæ in
the feet being bulbous and without claws, yet it is in general
Campodea-like and in essential features a triungulin (Fig. 644). The
intestine ends in a blind sac, as in the larvæ of bees, and this would
indicate that its food is honey. The complete life-history of no
Stylopid is completely known. It is probable that, hatched in June from
eggs fertilized in April, the larvæ crawl upon the bodies of bees and
wasps; finally, after a series of larval stages as yet unknown,[119]
penetrating within the abdomen of its host before the latter hibernates,
and living there through the winter. The females, owing to their
parasitic life, retain the larval form, while the free males are winged,
not leading in the adult stage a parasitic life, though passing their
larval and pupal stages in the body of their host, and are so unlike
ordinary beetles as to be referred by good authorities to a distinct
order (Strepsiptera).

[Illustration:

  FIG. 645.—_Stylops childreni_, ♂: _a_, abdomen of Andrena with ♀
    Stylops (_b_).
]

The triungulin stage of these insects corresponds in general to the form
of the larval Staphylinidæ and allied families, such as the
Tenebrionidæ, which are active in their habits, running about and
obtaining their food in a haphazard way, often necessarily suffering
long fasts. In the external-feeding, less active coleopterous larvæ,
like the phytophagous species, which have an uninterrupted supply of
nutritious food, we see that the body is thick and fleshy. So also in
the larvæ of the Scarabæidæ, Ptinidæ, and the wood-boring groups. In
internal feeders, like the larval weevils and Scolytidæ, which live
nearly motionless in seeds, fruits, and the sap-wood of plants and
trees, with a constant supply of nourishing, often rich food, the
eruciform body is soft, thick, and more or less oval-cylindrical. So it
is with the larvæ of Hymenoptera, especially in the parasitic forms, and
in the ants, wasps, and bees, which are nearly if not quite motionless,
at least not walking about after their food.

Now the change from the active triungulin stage to the series of
secondary, nearly legless, sedentary, inactive stages is plainly enough
due to the change of station and to the change of food. From being an
independent, active, roving triungulin, the young insect becomes a
lodger or boarder, fed at the expense of its host, and the lack of
bodily exertion, coupled with the presence of more liquid food than is
actually needed for its bare existence, at once induces rotundity of
body and a loss of power in the limbs, followed by their partial or
total atrophy.

That this process of degeneration may even occur in one and the same
stage of larval existence is very well illustrated by what we know of
the life-history of the wasp-parasite of Europe, _Rhipiphorus
paradoxus_. Thanks to the very careful and patient observations of Dr.
T. A. Chapman, we have a nearly complete life-history of this beetle,
the representative of a family in many respects connecting the Meloidæ
and Stylopidæ.[120] Where Rhipiphorus lays her eggs is unknown. Dr.
Chapman, however, found a solitary specimen of the young larva in the
triungulin stage. He describes it as “a little black hexapod, about 1⁄50
inch (.5 mm.) in length, and 1⁄120 inch in breadth, broadest about the
fourth segment, and tapering to a point at the tail; a triangular head
with a pair of three-jointed antennæ nearly as long as the width of the
head, with legs very like those of Meloë larvæ; the tibiæ ending in two
or three claws, which are supported and even obscured by a large
transparent pulvillus or sucker of about twice their length; this was
marked by faint striæ radiating from the extremity of the tibiæ, giving
it much the aspect of a lobe of a fly’s proboscis. Each abdominal
segment had a very short lateral spine pointing backwards; the last
segment terminated by a large double sucker similar to those of the
legs; and the little animal frequently stood up on this, and pawed the
air with its feet, as if in search of some fresh object to lay hold of.”

This almost microscopic larva finds a wasp grub and bores into its body,
probably entering at a point near the back of the first or second
segment behind the head. Dr. Chapman succeeded in finding the larva of
the beetle within that of the wasp, before the latter had spun up.
“Assuming that the wasp larva lives six days in its last skin before
spinning up, I should guess that the youngest of these had still two or
three days’ feeding to do. The Rhipiphorus larvæ were but a little way
beneath the skin of the back, about the fourth and fifth segments
[counting the head as the first], and indifferently on either side. The
smallest of these was 1⁄16 inch in length, and, except its smaller size,
was precisely like the larger ones I am about to refer to, having the
same head, legs, plates, etc. These were of the same size as those of
the larger larvæ, the difference in size of the latter being due to the
expansion of the intermediate colorless integument.”

After the wasp grub has spun the silken covering of its cell the larva
of Rhipiphorus may still be detected in some of them, being rendered
visible by its black legs and dark dorsal and ventral plates. “On
extracting this larva, it bears a general resemblance in size and
outline to the youngest larva of Rhipiphorus that I had found feeding
externally on the wasp grub, but with the very notable exception of the
already mentioned black marks. These are, in fact, a corneous head,
six-jointed legs, and a dorsal and ventral series of plates. I
immediately recognized the head and legs as identical with those of the
little black mite already described, but presenting a ludicrous
appearance in being widely separated from each other by the white skin
of the larva. I have no doubt that the dorsal and ventral series of
black marks are the corresponding plates of the mite-like larva floated
away from each other by the expansion of the intervening membrane. By
measurement also they agree exactly in size, although the larva
extracted from the wasp grub is ten times the length and six times the
width of the little Meloë-like larva. In length it is ⅙ inch (4.5 mm.),
and 1⁄28 inch in breadth.”

The remarkable changes thus described in the larva of this beetle after
it has begun its parasitic life within the body of its host are
especially noteworthy because the great increase in size and difference
in shape, as well as in habits, all take place before the insect has
moulted. The rapid development in size, and consequent distension of the
body and the separation of the sclerites of the segments behind the
head, are paralleled, as Chapman says, by the greatly swollen abdominal
region of the body in _Sarcopsylla penetrans_ and in the female of the
Termitidæ. In those insects this distension is due to the enlargement of
the ovaries and of the eggs contained within them, but in the
Rhipiphorus it is due to the comparative inactivity of the larva, and to
its being gorged with an unending supply of rich food, the blood and fat
of its host. It follows, then, that if a sedentary life and over, or at
least abundant, nutrition will have this effect within the short period
covered by the single first larval stage of the Rhipiphorus, it is
reasonable to infer that the hypermetamorphosis is also due to the same
factors.

Chapman then goes on to say that finally, within six hours of the time
of spinning up of the wasp grub, the Rhipiphorus larva at the end of
Stage 1., which is “usually in motion, and for its situation might be
called tolerably active, is seen to lay hold of the interior of the skin
with its anterior legs, and keeps biting and scratching with its strong
and sharp jaws until it is able to thrust through its head, when, in
less than a quarter of an hour, it completely emerges by a vermiform
movement; and at the same time it casts a skin, together with the black
head, legs, plates, etc.”

The larva, now in its second stage, passes forward and seizes hold of
the upper or lateral aspect of the prothoracic segment of the wasp grub.
On emerging it becomes shorter and thicker, “and very soon loses length
by that curving forward of its head which is so marked in the full-grown
larva, and which does not exist before its emergence.” The larva is now
found “lying like a collar immediately under the head of the wasp grub,
and is attached to it by the head, though not very firmly.” At this
stage the feeding of the young Rhipiphorus is rather sucking than
eating.

[Illustration:

  FIG. 646.—First larva (_a_) of _Bruchus fabæ_, greatly enlarged; _b_,
    thoracic processes; _c_, head, from front; _d_, from side; _e_,
    antenna; _f_, thoracic leg; _g_, rear view of tarsus; _h_, same,
    front view.—After Riley.
]

When about 6 mm. in length it moults a second time, and the full-grown
larva closely though superficially resembles a Crabro or Pemphredon
larva, the small head being bent over forwards. By the time it is ready
to pupate it has wholly eaten the wasp larva, and the temperature of the
cell being high, a larva 5 mm. long grows large enough in two days to
fill the top of the cell of its host, and the larva is ready to pupate
about a week after hatching, so that its development is very rapid. The
beetles themselves do not live in the cells. Chapman thinks they
hibernate, and that the eggs are laid in the spring or summer.

We thus have in this insect three larval stages, the triungulin, and two
later stages, the great differences between the first and the last two
being apparently due to their parasitic mode of life, the larva spending
its second stage within its host, involving an existence in a cell with
a high temperature, an uninterrupted supply of rich, stimulating food,
and a comparatively sedentary mode of life compared with that of the
triungulin at the beginning of its existence. It is quite obvious that
the hypermetamorphosis is primarily due to a great change in its
surroundings, _i.e._ the parasitic mode of life of the beetle, habits of
very rare occurrence in the Coleoptera, numerous in species as they are.

[Illustration:

  FIG. 647.—First larval stage of _Bruchus pisi_: _a_, egg in pea-pod;
    _b_, cross-section of opening of mine; _c_, young larva and opening
    on inside of pod by which it has entered, enlarged; _d_, _d_, _d_,
    eggs, natural size; _e_, 1st larval stage; _f_, a leg of same; _g_,
    prothoracic spinous processes.—After Riley.
]

In this connection attention may be drawn to a supernumerary larval
stage observed by Riley in the pea- and bean-weevils (Figs. 646 and
647). The larva on hatching has long slender legs, though differing from
those of an ordinary coleopterous larva in having but three joints (_j_,
_g_, _h_). This stage is very short, and the legs temporary, as, after
entering the bean or pea, it casts its skin, losing its legs, and
assuming the vermiform shape of the second larval stage. In this case
the change from a pedate to an apodous larva is plainly enough due to
the change from an external feeder, like a chrysomelid larva, to a larva
leading a boring, internal, almost quiescent life.

Certain ichneumons also appear to have two distinct larval stages, as
Ratzeburg inferred that in Anomalon there are four larval stages (Fig.
648).

[Illustration:

  FIG. 648.—History of _Anomalon circumflexum_: _A_, 1st instar or
    stage. _B_, 2d instar. _C_, larva in the 3d or encysted stage
    removed from its cyst. _D_, mature larva. _E_, pupa.—After
    Ratzeburg, from Sharp.
]

In another ichneumon, Klapálek detected what he calls the “sub-nymph.”
The insect pupates within the case of a caddis-fly, Silo (Fig. 649).

In the Proctotrypidæ there is also a hypermetamorphosis, though the
remarkable precocious stages they pass through are rather embryonic than
larval.

In a species of Platygaster which is parasitic in the larva of
Cecidomia, the first larva (Cyclops stage) is of a remarkable shape, not
like an insect, but rudely resembling a parasitic Copepod crustacean. In
this condition it clings to the inside of its host by means of its
hook-like jaws, moving about, as Ganin says, like a Cestodes embryo with
its well-known six hooks. In this stage it has no nervous, vascular, or
respiratory system, and the digestive canal is a blind one (Fig. 651).

[Illustration:

  FIG. 649.—Metamorphosis of Agriotypus: _A_, larva. _B_, “sub-nymph.”
    _C_, case of the Silo, with the string of attachment formed by
    Agriotypus. _D_, section of the case: _v^1_, operculum of case;
    _v^2_, cocoon; _ag_, pupa of Agriotypus; _e_, exuvia of same; _w^2_,
    wall of cocoon; _s_, remains of Silo; _w^1_, closure of case.—After
    Klapálek, from Sharp.
]

After moulting, the insect entirely changes its form; it is thick
oval-cylindrical, nearly motionless, with no appendages, but with a
digestive canal and a nervous and vascular system (Fig. 652).

After a second moult the third and last larval stage is attained, and
the insect is of the ordinary appearance of ichneumon larvæ.

Not less striking is the life-history of Polynema, which lays its eggs
in those of a small dragon-fly (_Agrion virgo_). The first larval stage
is most remarkable. It hatches as a microscopic immovable being,
entirely unlike any insect, with scarcely a trace of organization, being
merely a flask-shaped sac of cells. After remaining in this state five
or six days it moults.

[Illustration:

  FIG. 650.—Development of Platygaster: _A_, stalked egg: _a_ central
    cell giving origin to the embryo. _B_, _g_, germ; _b_, blastoderm
    cells. _C_, the same, farther advanced. _D_, cyclops-like embryo:
    _md_, rudiments of mandibles; _d_, rudimentary pad-like organs, seen
    more developed in _E_; _st_, bilobed tail.
]

The second stage, or Histriobdella-like form, as Ganin names it, is more
like that leech-like worm than an insect.

The third larval form is very bizarre, though more as in insects, having
rudimentary antennæ, mouth-parts, legs, and ovipositor. In this
condition it lives from six to seven days before pupating (Fig. 653).

The strange history of another egg-parasite (Ophioneurus) agrees in some
respects with that of the foregoing forms. It is when hatched of an oval
shape, with scarcely any organs, and differs from the genera already
mentioned in remaining within its egg-membrane, and not assuming their
strange shapes. From the cylindrical sac-like non-segmented larva
resembling the second larva of Platygaster it passes directly into the
pupa state.

A fourth form, Teleas (Fig. 654, _A-D_), is an egg-parasite of Gerris,
and in America one species oviposits in the eggs of Œcanthus.

[Illustration:

  FIG. 651.—First larva of Platygaster: _m_, mouth; _at_, rudimentary
    antenna; _md_, mandibles; _d_, tongue-like appendages.
]

[Illustration:

  FIG. 652.—Second larva of Platygaster: _œ_, œsophagus; _ng_, brain;
    _n_, nervous cord; _ga_ and _g_, genital organs; _ms_, muscular
    band.
]

The spindle-shaped larva in its first stage roughly resembles a
trochosphere of a worm rather than the larva of an insect so high in the
scale as a Hymenopter. It is active, but after moulting the second larva
is oval, still without segments. Dr. Ayers gives a profusion of details
and figures of the first and second stages of our Teleas, the second
strongly resembling the Cyclops stage of Ganin. He describes three
stages, and though he did not complete the life-history of the insect,
he thinks it changes to an ovoid flattened form which succeeds the
Cyclops stage in other Pteromalidæ, and that there are at least four
ecdyses.

It is difficult to account for these strange larval forms, unless we
suppose that the embryos, by their rich, abundant food, have undergone a
premature development, the growth of the body-walls being greatly
accelerated, the insects so to speak having been, under the stimulus of
over-nutrition and their unusual environment, and perhaps also the high
temperature of the egg, hurried into vermian existence on a plane
scarcely higher than that of an active ciliated gastrula.

Further observations, difficult though they will be, are needed to
enable us to account for the singular prematurity of the embryo of these
parasites. That these stages are reversional and a direct inheritance
from the vermian ancestors of these insects is not probable, but the
forms are evidently the result of adaptation in response to a series of
stimuli whose nature is in part appreciable but mostly unknown.

[Illustration:

  FIG. 653.—Third larva of Polynema: _at_, antenna; _fl_, imaginal bud
    of wing; _l_, rudimentary legs; _tg_, buds of one of the three pairs
    of styles of the ovipositor; _fk_, fat-body; _eg_, ear-like process.
]

[Illustration:

  FIG. 654.—_A-D_, development of Teleas; _A_, stalked egg; _B_, _C_,
    _D_, the 1st larval stage: _at_, antenna; _md_, hook-like mandibles;
    _mo_, mouth; _b_, bristles; _m_, intestine; _sw_, the tail; _ul_,
    under lip or labium. _E_, larva of another parasite,
    Ophioneurus.—This and Figs. 650–653 after Ganin.
]

It may be noted, however, that the appearance of a primitive band in the
second larval stage suggests the origin of these forms, as well as that
of insects in general, from a Peripatus-like, and again from an earlier
leech-like Annelid ancestor. Hence the first larval or Cyclops stage is
due to a precocious development caused by the unusual environment, and
is simply adaptational, and not of phylogenetic significance.




 SUMMARY OF THE FACTS AND SUGGESTIONS AS TO THE CAUSES OF METAMORPHISM


An explanation of the causes of metamorphosis is one of the most
difficult undertakings in biology, and the phenomenon has been
considered as one of the chief difficulties in the way of the acceptance
of the theory of descent.

A review, however, of the facts of hypermetamorphism, particularly the
life-history of Mantispa, throws much light on the subject, since it is
very probable that the supernumerary stages and marked changes of form
characterizing them are due to changes of environment, of habits, and of
food, causes which have exerted such a profound influence on organic
beings throughout all time. Besides these, as the result of changes in
the environment and nature of the food, we have the results brought
about by the use or disuse of structures brought into existence by the
action of stimuli from without, the class of insects abounding in
examples of temporary structures which perform a certain function, and
then disappear.

Again, if the origin of a hypermetamorphosis can thus be explained, it
follows that normal metamorphosis is most probably due to changes of
habitat, of seasons, of food, and to accelerated growth resulting from
the approach of sexual maturity.

The following facts and conclusions appear to be well established:—

1. The apterous insects (Synaptera) are ametabolous, only the winged
insects undergoing a metamorphosis.

2. The complete metamorphosis was not inherited from the primitive
ancestor of all insects, but acquired at a later period (F. Müller). The
eruciform type is a secondary, adaptive form, derived from the earlier,
campodeoid type of larva.

3. The earliest, most primitive pterygote insects passed through only a
slight metamorphosis. In other words, as soon as the wings were evolved
and insects became adapted to live or take refuge in a new medium, the
air, at the approach of the period of adult life, with the ripening or
perfection of the reproductive organs, a metamorphosis began to take
place, and the number of species greatly multiplied. On the other hand,
the Arachnida and Myriopoda, in which as a rule there is no
metamorphosis, being confined to a creeping life, with no change of
medium, remained poor in number of species.

4. At first the nymphs mainly differed from the adults in lacking wings,
though having the same habits; in holometabolous insects, the larva
became adapted to entirely different habits and environments, so that in
Hymenoptera, and especially Diptera, the larva became remarkably unlike
the imago.

5. Until the Mesozoic age, or late in the Carboniferous period, there
were, so far as we now know, only ametabolous and heterometabolous
insects, and these orders (Orthoptera, Dermaptera, Hemiptera,
Plectoptera, Odonata, and Neuroptera) were not numerically rich in
genera and species, while since early Mesozoic times geological
extinction has reduced their numbers.

6. During the Mesozoic age, and since then, the number of species,
genera, families, and orders has greatly increased, and insects have
become more and more holometabolous. The orders of Coleoptera,
Lepidoptera, Hymenoptera, and Diptera are many fold greater in number of
species and variety of form than the heterometabolous orders.

The rapid increase in the number and variety of types of insects
evidently is correlated with the profound geological changes which took
place at the end of the Paleozoic age, involving the appearance of
larger continental masses, or a greater land area, thus opening new
regions for settlement. Also the origin of flowering plants at about
this time undoubtedly had much to do with the genesis of new adaptive
structures, such as the changes in the mouth-parts and wings.

7. The process of metamorphosis, at least in the subtropical, temperate,
and polar regions, is largely dependent on the change from summer to
winter, and, in the tropics, from the rainy to the dry season.

As regards the organization of larval (nepionic) as compared with
imaginal forms, the nymphs and larvæ of insects are, with the exception
of many Diptera, nearly as perfectly developed as the adult. In this
respect the immature insect differs fundamentally from the larvæ of
certain worms (for example, the pilidium of Nemerteans) and from the
pluteus and brachiolaria stages of echinoderms, which possess only
digestive and water-vascular organs.

Insect nymphs and larvæ also differ from the nauplius and zoëa of
Crustacea in having at birth all the most important systems of organs
(digestive, circulatory, respiratory, nervous, muscular, with sometimes
a nearly perfected reproductive system) of the imago, also the same
number of cephalic, thoracic, and abdominal segments and appendages.
Metamorphism in insects involves (except in the Diptera) rather
modifications in the form and functions of organs and appendages already
present than the formation of new ones. In larval Crustacea, the
thoracic and abdominal appendages do not arise until some time after
hatching from the egg.

8. While cases of the suppression or abbreviation of larval characters
and direct development are not uncommon in echinoderms and crustaceans,
in insects this phenomenon occurs only so far as yet known in the
Diptera. In these insects the polypody in the embryo is outgrown, or
lost, the embryos and larvæ not having even the temporary rudiments of
abdominal appendages. The campodeoid characters also are entirely
suppressed, dropped, or lost in the more specialized holometabolous
orders, Lepidoptera, Hymenoptera, and Diptera, though retained in the
more primitive and generalized Coleoptera. (This proves that the
Coleoptera are lower or more primitive and generalized than the other
orders mentioned.) This abbreviation or loss of organs is, as Hyatt and
Arms claim, due to the prepotency of acquired characters in phylogeny,
and are also the result of homochronous heredity.

  “The Insecta of the more specialized orders, x.-xvi., afford, next
  to some parasites, the most notable examples of this mode of
  evolution. Their larval or nepionic, and pupal or neanic, stages are
  prolonged at the expense of the ephebic, winged stage, and the
  reasons for this prolongation are found in the great number of new
  features introduced into these stages of development in these orders
  as contrasted with those of the more primitive, and, in large part,
  more ancient orders, i.-ix. The law of tachygenesis has been at work
  here, as in the former cases alluded to above, and it is shown in
  the encroachments of the adaptive characteristics of the
  caterpillar, grub, and maggot upon the inherited characteristics of
  the Thysanuran stage, which loses its ancestral characteristics,
  until in most cases they are either obsolete or recognizable with
  difficulty.” (Hyatt and Arms, Natural Science, 1896, p. 400.)

9. In the holometabolous insects there is a resting, quiescent stage
during the pupal period, when the insect takes no food. In this respect
the more specialized insects differ from other metamorphic animals. The
larva has an abundant supply of fat lasting through pupal life, while in
the quiescent pupa, respiration and circulation is much lessened, the
animal being as a rule motionless. This resting stage is also necessary
for the histolysis and formation of the adult body from the imaginal
buds present in the larva.

10. The hypermetamorphosis of Mantispa, Meloë, Stylops, etc., indicate
very plainly that the eruciform type of larva is derived from the
campodeoid, since one and the same insect passes through these stages
before reaching sexual maturity.

11. As observed by Miall, the larva of insects differs from that of
other invertebrate animals in being larger than the adult.

12. The metamorphoses of insects are in some important respects
paralleled by those of the Amphibia. The case of pædogenesis of
Chironomus affords a parallel with that of the Siredon, or larva of
Amblystoma. Also the organs and appendages of the insects, such as
caterpillars, are present, just as the skeleton and other organs of the
tadpole are the homologues of those of the adult, although these parts
undergo a profound modification, and new structures are added. (See the
discussion of this point by Miall, and by Hyatt and Arms.)

=Theoretical conclusions; Causes of metamorphosis.=—It results from a
review of the known facts, together with reasonable inductions from such
facts, that so far from opposing the theory of descent, the facts of
metamorphosis, and particularly of hypermetamorphosis, seem to afford
solid foundation for the theory. While natural selection was not the
initiative cause, it plays a part as one of several factors; but the
fundamental causes are the same as those which have controlled the
origin of species and of the larger groups of animals in general. Owing
to the struggle for existence, due to overcrowding, the early insects
were forced to take to the air, acquiring wings to enable them to avoid
the attacks of creeping and running insects. In the end the insects
became, owing to this acquisition of wings, and afterwards to the
establishment of a complicated metamorphosis, numerically the most
successful type of life in existence, the number of species, extinct and
living, mounting into the millions.

All aquatic insects are evidently the descendants of terrestrial forms,
and the numberless contrivances and temporary larval organs,
particularly of dipterous larvæ, are evidently adaptations to the needs
of the insect during its aquatic life, and which are cast aside when the
creature passes to a different medium. The sudden or tachygenic
appearance of temporary structures, such as hatching spines, various
setæ, spines, respiratory organs, so characteristic of dipterous larvæ,
and of the protective colors and markings of caterpillars, and which are
discarded at pupation, or imagination, are evidently due to the action
of stimuli from without, to the primary neolamarckian factors, the
characters proper to each larval stadium, and to the pupal and imaginal
stadia,—characters probably acquired during the lifetime of the
individual,—becoming finally fixed by homochronous heredity.




       LITERATURE ON POSTEMBRYONIC DEVELOPMENT AND METAMORPHOSES


  =Herold, Moritz Johann David.= Entwicklungsgeschichte der
    Schmetterlinge anatomisch und physiologisch bearbeitet. (Cassel, u.
    Marburg, 1815. 33 Taf., 4º, pp. 1–118, i-xxxiv.)

  =Ratzeburg, F. T. C.= Ueber Entwickelung des fusslosen
    Hymenopteren-larven, mit besonderer Rücksigt auf die Gattung
    Formica. (Nova Acta Natur. Curios., xvi, 1832, pp. 145–176.)

  =Agassiz, Louis.= The classification of insects from embryological
    data. (Smithsonian Contr., ii, Washington, 1851, pp. 28, 1 Pl.)

  =Ganin, M.= Beiträge zur Erkenntniss der Entwicklungsgeschichte bei
    den Insecten. (Zeitschr. f. wiss. Zool., xix, 1869, pp. 381–451, 4
    Taf.)

  —— Ueber die Embryonalhülle der Hymenopteren- und
    Lepidopteren-embryonen. (Mém. Acad. St. Petersbourg (7), xiv, 1869,
    pp. 18, 1 Pl.)

  —— Materialien zur Kenntniss der post-embryonalen
    Entwicklungsgeschichte der Insecten. (Russian.) Warschau, 1876.
    (Abdruck aus den Arbeiten der V. Versammlung russischer Naturf. und
    Aerzte in Warschau, 1876. Abstract by Hoyer in Jahresber. der Anat.
    und Phys. von Hoffmann und Schwalbe, v, 1876, and in Zeitschr. f.
    wiss. Zool., xxviii, 1877, pp. 386–389.)

  =Weismann, A.= Die nachembryonale Entwicklung der Musciden nach
    Beobachtungen an _Musca vomitoria_ und _Sarcophaga carnaria_.
    (Zeitschr. f. wiss. Zool., xiv, 1864, pp. 101–263, Taf. 8–14.)

  —— Die Metamorphose von _Corethra plumicornis_. (Zeitschr. f. wiss.
    Zool., xvi, 1866, pp. 1–83, 5 Taf.)

  =Packard, A. S.= Observations on the development and position of the
    Hymenoptera, with notes on the morphology of insects. (Proceedings
    Boston Society of Natural History, 1866, pp. 279–295.)

  =Künckel d’Herculais=, J. Recherches sur l’organisation et le
    développement des Volucelles. Paris, 1875, Pt. I, pp. 208, 12 Pis.;
    II, 1881. Atlas of 15 Pls.

  =Dewitz, H.= Beiträge zur Kenntniss der post-embryonalen
    Gliedmaassenbildung bei den Insecten. (Zeitschr. f. wiss. Zool.,
    xxx, Suppl., 1878, pp. 78–105, 1 Taf.; Nachtrag, Ibid., pp. 25–28.)

  —— Ueber die Flügelbildung bei Phryganiden und Lepidopteren. (Berl.
    Ent. Zeitschr., xxv, 1881, pp. 53–66, 2 Taf.)

  =Lowne, B. Th.= Anatomy, physiology, morphology, and development of
    the blow-fly. London, Part I, 1880; Part II, 1891.

  =Viallanes, H.= Recherches sur l’histologie des Insectes et sur les
    phénomènes histologiques qui accompagnent le développement
    post-embryonnaire de ces animaux. (Ann. Sc. Nat. (6), xiv, 1882.)

  =Metschnikoff, E.= Untersuchungen über intracelluläre Verdauung bei
    wirbellosen Thieren. (Arb. a. d. zoolog. Inst. zu Wien., v, 1883.)

  —— Untersuchungen über die mesodermalen Phagocyten einiger
    Wirbelthiere. (Biol. Centralbl., iii, 1883.)

  =Wielowiejsky, H. v.= Ueber den Fettkörper von _Corethra plumicornis_
    und seine Entwicklung. (Zool. Anzeiger, vi Jahrg., 1883, pp.
    318–322.)

  =Pancritius, P.= Beiträge zur Kenntnis der Flügelentwicklung bei den
    Insecten. In.-Diss. Königsberg, 1884.

  =Rees, J. van.= Over intra-cellulaire spijsverteering en over de
    beteekenis der witte bloedlichampjes. (Maandblad voor
    Natuurwetenschappen, xi Jaarg., 1884, pp. 28.)

  —— Over de post-embryonale ontwikkeling von _Musca vomitoria_.
    (Maandblad voor Natuurwetenschappen, Juli, 1885.)

  —— Beiträge zur Kenntniss der inneren Metamorphose von _Musca
    vomitoria_. (Zool. Jahrb. Abth. f. Anat. u. Ontog., iii, 1888, pp.
    1–134, 2 Taf., 14 Figs.)

  =Frenzel, J.= Einiges über den Mitteldarm der Insecten, sowie über
    Epithel-regeneration. (Arch. Micr. Anat., xxvi, 1885.)

  =Kowalevsky, A.= Beiträge zur nachembryonalen Entwicklung der
    Musciden. (Zool. Anzeiger, viii, 1885, pp. 98, 123, 153.)

  —— Beiträge zur Kenntniss der nachembryonalen Entwicklung der
    Musciden. (I. Theil., Zeitschr. f. wiss. Zool., xlv, 1887, pp.
    542–594, 5 Taf.)

  =Schneider, Ant.= Ueber die Anlage der Geschlechtsorgane und die
    Metamorphose des Herzens bei den Insecten. (Zool. Beiträge, 1885, i,
    pp. 140–143, 1 Taf.)

  =Rehberg, A.= Ueber die Entwickelung des Insectenflügels (an _Blatta
    germanica_). (Marienwerder, 1886, pp. 12, 1 Taf.)

  =Schäeffer, C.= Beiträge zur Histologie der Insecten. (Spengel’s Zool.
    Jahrb., iii, Abth. f. Anat., 1889, pp. 611–652, 2 Taf.)

  =Hurst, H.= The post-embryonic development of a gnat (Culex).
    Manchester, 1890, pp. 26, 1 Pl.

  =Verson, E.= Der Schmetterlingsflügel und die sog. Imaginalscheiben
    desselben. (Zool. Anzeiger, xiii, 1890, pp. 116, 117.)

  =Bugnion, Edouard.= Recherches sur le développement post-embryonnaire,
    l’anatomie, et les mœurs de l’_Encyrtus fuscicollis_. (Recueil zool.
    Suisse, v, 1891, pp. 435–534, 6 Pls.)

  =Petersen, Wilhelm.= Die Entwicklung des Schmetterlings nach dem
    Verlassen der Puppenhülle. (Deutsch. Ent. Zeitschr., 1891, 2 lepid.
    Hft., pp. 199–214, 5 Figs.)

  =Kulagin, Nicolas.= Notice pour servir à l’histoire de développement
    des hyménoptères parasites. (Congrès international de Zoologie, 2^e
    Session, à Moscou, 1892, pp. 253–277. Also in Zool. Anzeiger, xv,
    1892, pp. 85–87.)

  —— On the development of Platygaster. (Journ. of Friends of Nat. Sc.
    Moscow. Zool., 1890.) (In Russian.)

  —— Beiträge zur Kenntniss der Entwicklungsgeschichte von Platygaster.
    (Zeitschr. f. wiss. Zool., lxiii, 1897, pp. 195–235, 2 Taf.)

  =Miall, L. C., and Hammond, A. R.= The development of the head of
    Chironomus. (Trans. Linn. Soc. London, 2d Ser., v, 1892, pp.
    265–279, 4 Pls.)

  =Pratt, Henry S.= Beiträge zur Kenntniss der Pupiparen. In.-Diss.
    Berlin, 1893, pp. 53 (Archiv f. Naturgesch., 1893), 1 Taf.

  —— Imaginal discs in insects. (Psyche, viii, 1897, pp. 15–30, 11
    Figs.)

  =Gonin, J.= Recherches sur la métamorphose des lépidoptères. De la
    formation des appendices imaginaux dans la chenille du _Pieris
    brassicæ_. (Bull. Soc. Vaud. sc. nat., xxx, 1894, pp. 1–52, 5 Pls.)

  =Heymons, R.= Ueber Flügelbildung bei der Larve von _Tenebrio
    molitor_. (Sitz. Ber. Gesell. Natf. Freunde. Berlin, Jahrg. 1896,
    pp. 142–144, 1 Fig.)

  Also the writings of Malpighi, Swammerdam, De Geer, Lyonet, Bonnet,
    Newport, Brauer, Chapman, Fabre, Valery-Mayet, Riley, Chobaut,
    Nassonow, Miall (Nature, 1895, pp. 152–158), Hyatt and Arms (Natural
    Science, 1896, pp. 395–403).


                            END OF PART III.




                                 INDEX

[Illustration]


 Abantiades, 57.

 Abbreviation of larval characters, 707.

 Abdomen, 162.

 Abdominal appendages, in the embryo, 164;
   embryonic appendages, 476;
   jointed appendages, 468.

 Acetabulum, 94.

 Acid, formic, 358;
   uric, 352.

 Acinose salivary glands, 334.

 Acoustic nerve, 290.

 Acronycta, 615;
   hastulifera, 194.

 Acrydium, 421.

 Actias luna, its cocoon-cutters, 634.

 Adelops, 630.

 Adhesive hairs, 111, 113;
   fluid, 113;
   glands, 360.

 Adiscota, 672.

 Adminicula, 629.

 Adranes cæcus, 57.

 Adult insects, tracheal gills of, 476.

 Æroscepsis, 265.

 Æschna, 53;
   rectal respiration in nymph of, 463.

 Agriotypus, hypermetamorphosis of, 701.

 Aileron, 124.

 Air-sacs, 456;
   use of, 457.

 Aletia xylina, tongue of, 66.

 Aleurodicus, 518.

 Aleyrodes, 518.

 Alitrunk, 90.

 Alluring glands, 391.

 Alula, 123, 125.

 Ametabola, acquired, 599.

 Ametabolia, 596.

 Amnion, 533;
   absence of, 534;
   cavity, 532;
   fold, 531;
   skin, shedding of, 584.

 Amphizoa, 461.

 Anabolia furcata, buccal organs of, 74.

 Anabrus, 49, 73;
   cuticula of, 187.

 Anal glands, 319, 326, 372;
   operculum, 181;
   silk glands, 346.

 Androconia, 197, 199.

 Anisomorpha, 371.

 Anisopleura, lateral gills of, 468.

 Anobium, 293, 620.

 Anomalon, hypermetamorphosis of, 701.

 Anophthalmus, brain of, 241;
   head of, 74;
   olfactory organs of, 276;
   salivary glands of, 334;
   tongue of, 74.

 Anoplus, 101.

 Ant, cement glands of, 360;
   organ of hearing in, 291;
   taste in, 282;
   phosphorescent, 424;
   poison sac of, 359;
   sounds produced by, 294;
   stingless, 359.

 Antefurca, 92.

 Antennæ, 57;
   imaginal buds of, 665;
   origin of imaginal from larval, 656;
   use of, 59, 270.

 Antennal auditory hairs, 292;
   lobes, 237;
   nerves, 650.

 Antheræa, 616.

 Anthrax, 612.

 Anurida, 51;
   maritima, 537.

 Anurophorus, 424.

 Anus, 319;
   absence of, 300, 320;
   of embryo, 537.

 Aphides, changes of color in, 205;
   honey dew of, 364;
   wax glands of, 364.

 Aphis, 616;
   reduction of tarsal joints of, 103.

 Aphrophora permutata, wings of, 141.

 Apis, premandibular segment in embryo of, 52;
   germ-layers of, 558.

 Apneustic type of tracheal system, 459.

 Apodemes, 92.

 Apodous larvæ, 103.

 Appendages, abdominal jointed appendages, 468;
   abdominal, origin of, 550, 551;
   abdominal, absence of, 550;
   cephalic, origin of, 548;
   of embryo, 548, 551;
   oral, 549;
   thoracic, origin of, 550.

 Aquatic insects, 459;
   descent of, from terrestrial, 708;
   life, adaptations to, 460.

 Arachnida, 6.

 Arctia, 391.

 Arctian larvæ, 615.

 Argida, 391.

 Armature, 187, 192.

 Arolium, 97, 100, 113.

 Arthromeres, 30.

 Arthropoda, classes of, 3.

 Articerus, 57.

 Ascalaphus, 616.

 Ash, on eversible glands, 377.

 Asilus, mouth-parts of, 79.

 Aspidiotus, 538, 627;
   nerii, hypermetamorphosis of male of, 690;
   nerii, metamorphosis of male of, 640, 690.

 Ateuchus sacer, 101.

 Attacus, mode of escape from its cocoon, 635.

 Attacine moths, 634.

 Attelabus, 538.

 Auditory organs, 287.

 Audouin, on the median segment, 163;
   on peritreme, 90.

 Autolyca, 371.

 Auzoux, on the salivary glands of silkworm, 332.

 Ayers, on embryonic abdominal appendages, 550;
   on fecundation of the egg, 505;
   on hypermetamorphosis of Teleas, 703;
   on origin of heart, 573.


 Bætisca, 467.

 Balancers, 124.

 Balbiani, on the polar cells of Chironomus, 580.

 Ballowitz, on spermatozoa, 497.

 Band, germinal, 531;
   invaginated, 538;
   overgrown, 538;
   primitive, 531, 536, 545.

 Bapata, 392.

 Basilar membrane of eye, 253.

 Bee, honey, air-sacs of, 458;
   breathing of, 456;
   cement glands of, 360;
   egg of, 521;
   flight of, 151;
   head, 80;
   moulting of, 611;
   mouth-parts of, 79;
   number of moults of, 618;
   premandibular segment in embryo of, 52;
   salivary glands of, 334;
   sanitary conditions observed by its larva, 623;
   seminal packet of, 500;
   spermatheca, 506;
   tongue of, 80, 81;
   tracheæ of, 458;
   wax glands of, 364.

 Bee’s foot, action of, in climbing, 114;
   sting, 172.

 Bees, twisted hairs of, 189.

 Beetles, anal glands of, 372;
   phosphorescent, 424;
   tongue of, 73;
   tracks of, 106;
   walking, 103.

 Benasus griseus, tongue of, 73.

 Bladder, urinary, 35.

 Blanc, on salivary glands of silkworm, 331, 332;
   on silk glands of silkworm, 340;
   on spinning glands of silkworm, 340.

 Blaps, 373;
   gait of, 109;
   tracks of, 109, 111.

 Blastoderm, 526, 529.

 Blatta, 43, 69;
   egg-tubes of, 501;
   embryology of, 537.

 Blattidæ, fœtid glands of, 370.

 Blepharocera, 474.

 Blochman, on embryology of Musca, 530.

 Blood, 407;
   corpuscles, 407, 419, 574;
   crystals from, 407;
   -forming cells, 574, 685;
   gills, 475;
   veins of wings, 121;
   lacunæ, 573;
   repellent nature of, 374, 407;
   serum, 407;
   tissue, 408, 419;
   vessels in the head, 405.

 Blow-fly, duration of embryonic life of, 582;
   egg of, 521.

 Boas, on spiracles of Melolontha larva, 439.

 Bobretsky, on embryology of Pieris, 529.

 Body, cavity, formation of, 563, 566;
   central, 232, 237;
   completion of embryonic, 555;
   form, development of outer, 668;
   mushroom, 233;
   pedunculated, 232, 233;
   stalked, 232, 233.

 Boll, on repellent glands, 371.

 Bombus, 219, 618;
   post-embryonic changes in, 661.

 Bombyx mori, 339, 366, 405, 496, 499, 608;
   embryonic abdominal legs of, 552.

 Bordas, on poison glands, 358;
   on salivary glands of Hymenoptera, 337.

 Bot-fly, of horse, 475;
   of ox, 518.

 Bothriothorax, 623.

 Brain, 222, 226;
   development of, 567;
   histology of, 238;
   modifications of, in different orders, 240.

 Brauer, on Campodea-form larvæ, 600–602;
   on metamorphosis, 598.

 Breathing, mechanism of, 451;
   rectal, 463.

 Brin, 342.

 Bristles, 188.

 Bruchus, hypermetamorphosis of, 700.

 Buccal appendages, 59.

 Bucculatrix, 634.

 Buckton, on change of color in aphides, 205.

 Buds, antennal, 665;
   buccal, 665;
   femerotibial, 656;
   frontal, 676;
   imaginal, 674;
   of Encyrtus, 663;
   Melophagus, 686;
   ocular, 665;
   of ovipositor, 665;
   of wings, 669.

 Bugnion, on composition of head of Hymenoptera, 55;
   on the germs of the sexual glands of Encyrtus, 582;
   on the imagined buds of ovipositor, 171;
   on the post-embryonic changes in Hymenoptera, 663.

 Burgess, on colors, 203;
   on hypopharynx, 76;
   on scales, 195.

 Burmeister, on organs of smell, 265.

 Bursa copulatrix, 505.

 Busgen, on honey dew, 365.

 Bütschli, on an under-lip structure in bee, 547;
   on origin and morphology of the tracheæ, 447;
   on premandibular segment, 52;
   on temporary abdominal appendages, 550.

 Butterfly, atrophy of tarsi of, 102;
   olfactory organs of, 274;
   larval, hibernating, 615.


 Caddis-worm, blood-gills of, 475;
   eversible glands of, 375;
   pupal mandibles of, 633.

 Cæca of mid-intestine, 300, 325, 347, 348;
   secretion of, 348.

 Calcar, 97.

 Calcaria, 97.

 Calculi in intestine, 325.

 Calliphora vomitoria, 618;
   eggs of, 521.

 Callosamia promethea, 192;
   number of moults of, 616.

 Callosune, 202.

 Caloptenus, 43.

 Calopteryx, 54, 464.

 Caltrops, 189.

 Calypta, 124.

 Calyx of brain, 233.

 Campodea, embryology of, 22, 52;
   ligula of, 721;
   moulting of, 616;
   premandibular segment of, 52.

 Campodea-form larva, 600.

 Campodeoid characters, loss of, in holometabolous insects, 707;
   larvæ, 600.

 Capillary tracheæ, 655.

 Carabidoid stage, 692.

 Carabus, walking, 107;
   tracks of, 109.

 Cardiac valvule, 312.

 Cardioblasts, 572.

 Cardo, 63.

 Carlet, on the poison apparatus of bees, 357;
   on walking in beetles, 109;
   on wax glands, 364.

 Carus, on the circulation, 397, 409.

 Case-worms, blood gills of, 475;
   functional salivary glands of, 331;
   spinning glands of, 337.

 Caterpillar, actions before pupation, 644;
   changes in mouth-parts during metamorphosis, 645;
   eversible sacs of, 375;
   excrement of, before pupation, 644;
   internal changes in, 645;
   moulting of, 609;
   number of moults in, 615.

 Catocala, 392.

 Cauliculus, 233.

 Cavity, peripodal, 669.

 Cecidomyia, 113;
   urinary tubes of, 351.

 Cells, absorbent, 328;
   amœboid egg, 529;
   egg, 502;
   embryonic, of buds of larval Lepidoptera, 655;
   genital, 575;
   setigenous, 191.

 Cement glands, 360.

 Centrosome, 525.

 Ceratopogon, 678.

 Cerci, 164, 178.

 Cercopoda, 164, 178.

 Cerura, 375.

 Ceuthophilus, 393.

 Chabrier, on use of elytra, 159.

 Chalicodoma, 542.

 Chambers, egg, 502;
   yolk, 502.

 Chapman, on cremaster, 636;
   the hypermetamorphosis of Rhipiphorus, 697;
   on mode of escape from cocoon, 632, 633;
   on the moulting fluid, moulting of arctians, 615;
   on value of pupal characters, 628.

 Chermes, 361.

 Cheshire, on admission of air into bee’s cocoon, 623;
   on bee’s foot, 114;
   on bee’s sting, 172;
   on bee’s tongue, 79, 82.

 Chiasma, 231.

 Chironomus, 36, 491;
   formation of the imago in, 671, 678;
   polar cells of, 580.

 Chitin, 29.

 Chlænius, brain of, 241.

 Cholodkowsky, on homologies of propleg or abdominal leg of
    caterpillars, 552;
   on patagia, 89;
   on testes of Lepidoptera, 496;
   on urinary tubes, 354.

 Chordotonal organs, 289.

 Chorion, 520, 534.

 Chromatin, 498.

 Chrysalis, 625;
   mode of suspension of, 637.

 Chrysopa, 517, 525.

 Chun, on the tænidia, 445.

 Cicada, shrilling organ of, 295.

 Cicada septemdecim, 616;
   hatching of, 584.

 Cimbex, 374.

 Circulation, of blood, 409;
   organs of, 397;
   peritracheal, 397.

 Citheronia, 392.

 Claspers, 176, 179.

 Claus, on eversible glands, 374.

 Clavola, 57.

 Climbing, mode of, 116.

 Closure, dorsal, of embryo, 556.

 Clypeus, 546, 547.

 Coarctate Diptera, 620.

 Coccidæ, male, 626;
   metamorphosis of, 641.

 Coccinella, moulting of, 611.

 Coccinellidæ, 375.

 Cockerell, on hatching of mantis, 584.

 Cockroach, 455, 456, 487;
   brain of, 229, 242;
   cement glands of, 360;
   chorion of egg of, 521;
   circulation of blood in wings of, 410;
   colleterial glands, 506;
   deposition of eggs of, 519;
   digestion of, 325;
   egg-tubes of, 501;
   fœtid glands of, 370;
   micropyle of eggs of, 523;
   mode of hatching, 583;
   oötheca of, 517;
   wingless, 598.

 Cocoon, admission of air in, 623;
   breaker, 634;
   cutter, 634;
   formation of, 619;
   mode of escape from, 635;
   spinning of, 621.

 Cœcal appendages of stomach, 300, 325, 347.

 Cœcum of colon, 318, 325.

 Cœlom-sac, 563, 566, 576.

 Coleoptera, embryology of, 537;
   gustatory organs of, 284;
   internal changes during metamorphosis of, 641;
   larval types of, 604, 606;
   number of moults of, 617;
   olfactory organs of, 275;
   phosphorescent, 421;
   pupa of, 630;
   salivary glands of, 334;
   seminal ducts of, 496;
   sounds produced by, 293;
   spermatozoa of, 497, 499;
   tongue of, 73.

 Colleterial glands, 506.

 Colon, 317;
   cæcum of, 318.

 Color sense, 260.

 Colors, 201;
   dermal, 203;
   interference, 201, 202;
   metallic, 204;
   natural, 203;
   optical, 201;
   order of development of, 208.

 Comb, tarsal, 97.

 Commissure of œsophageal ring, 237.

 Conditions of existence, 463.

 Cone, crystalline, 250, 251.

 Conglobate gland, 487.

 Coniopteryx, 620.

 Conjunctivus, 61.

 Conorhinus, 616.

 Cope, on causes of segmentation of body of arthropods, 33.

 Copidosoma, 623.

 Copris carolina, 61.

 Copulation, signs of, 507.

 Copulatory pouch, 505.

 Cord, stigmatic, 460;
   supraspinal, 240.

 Corethra, 433, 460, 618;
   auditory organs of, 291;
   formation of the imago in, 668, 678;
   plumicornis, wing-germs of, 129;
   tracheoles of, 133.

 Corixa, eggs of, 538.

 Cornea, 250.

 Corneal lens, 250.

 Corydalus, 46, 48, 59, 70, 460, 468.

 Corydalus cornutus, hatching spine of, 585.

 Coste, on pigments, 206.

 Cotylosoma, 478.

 Coxa, 95;
   origin of imaginal from larval, 656.

 Coxal glands, 369;
   sacs, 475;
   of myriopods, 14.

 Cremaster, 636;
   absence of, 636;
   mode of formation of, in butterflies, 637.

 Cremastral hook-spine, 638.

 Cricket, 487;
   anal glands of, 372.

 Crop, 303, 324.

 Crustacea, 4.

 Cucujo, 426.

 Cuénot, on blood, 374, 408;
   corrosive glands, 574;
   digestion, 329;
   leucocytes, 421;
   phagocytes, 421, 422.

 Cuilleron, 124.

 Culex, 461, 465, 474, 599;
   formation of the imago in, 668, 678;
   mouth-parts of, 78;
   phosphorescent, 424;
   sense of hearing in, 292;
   urinary tubes of, 350.

 Cup, spermatophore, 499.

 Cuticula, 187, 203;
   new layer of, formation of, 612.

 Cyclops stage of Proctotrypid parasites, 701.

 Cyphon, 472.

 Cytoplasm, 525.


 Dahl, on constancy of number of six legs, 100;
   on motion of insects on smooth, 100, 111, 113, 116.

 Danais, 197, 381;
   archippus, hypopharynx of, 75;
   plexippus, wings of, 137.

 Dandolo, on amount of food eaten by the silkworm, 608.

 Datana, 634.

 Datana ministra, moulting of, 611;
   section of larva, 131;
   setæ of, 188;
   tænidia of, 444, 448.

 Death-watch, 293.

 Deltochilum gibbosum, 101.

 De Moor, on tracks of insects, 106, 109.

 Dermal glands, 365.

 Dermaptera, wingless, 598.

 Deutocerebrum, 231, 237.

 Development, direct, 598.

 Dewitz’s discovery of imaginal bud-stalks, 673;
   locomotion of insects on smooth surfaces, 111;
   on movement of leucocytes independent of the circulation, 413;
   openings of glandular hairs, 192;
   ovipositor, 168, 170, 171;
   stigmata of odonate nymphs, 439;
   open tracheal system, 460, 464;
   wing-buds, 127, 142.

 Diapheromera, 371, 616;
   eggs of, 517, 520.

 Diaphragm, pericardial, 412.

 Didonis, 381, 391.

 Digestion, 324.

 Digestive canal of imago in the fly, appendages of, 297, 302, 331;
   formation of, 681;
   histology of, 320;
   length of, not a criterion of its habits, 301;
   primary regions, 299, 302.

 Dimmock, on hypopharynx, 71;
   on labrum epipharynx, 44;
   on pseudo-trachea, 446.

 Diplopoda, 12.

 Diptera, coarctate, 620;
   cyclorhapha, 621;
   development of imago of, 666;
   food reservoir of, 305;
   germs of genital glands, 580;
   hypopharynx of, 78;
   larval types of, 607;
   mouth-parts of, 78;
   olfactory organs of, 273;
   origin of legs of imago in, 654;
   orthoraphous, 621, 626;
   post-embryonic changes in, 666;
   salivary glands of, 333.

 Dipterous embryo, suppression of polypody in, 707.

 Direct development, 598.

 Discota, 672.

 Division nuclei, 530.

 Donacia, 620.

 Dorsal organ, 535.

 Doryphora, 50;
   embryology of, 544;
   hatching spine of, 586.

 Dragon-fly, 53;
   muscles of flight of, 157;
   number of moults of, 616.

 Draught power, 218.

 Drosophila, 518.

 Dryocampa, 187.

 Dubois on the cucujo, 424, 426.

 Ducts, ejaculatory, 496;
   seminal, 496.

 Dufour, gland of, 358.

 Dujardinia, 34.

 Dyar on the number of moults, 615.

 Dyticus, 461;
   foot of male, 93, 99, 114;
   larva, poisonous saliva of, 359;
   mode of swimming of, 116;
   tænidia of, 445;
   trail curves of, 108.


 Eacles, 616.

 Ears, 288.

 Eaton, on nymph stage, 594;
   of rectal respiration in nymphs of ephemerids, 465.

 Ecdysis, 609, 611.

 Ectoderm, 534;
   formation of, 558.

 Ectotrachea, 432, 448, 684.

 Egg, 515;
   burster, 585;
   capsule, 517, 520;
   cells, 504;
   chambers, 502;
   fertilization of, 525;
   germs, 502;
   guide, 183;
   internal structure of, 524;
   markings of, 521;
   maturation of, 525;
   mode of deposition of, 518;
   number laid, 515;
   ovarian, 501;
   ripe, 520;
   sacs, 361, 517, 520;
   shell, 520;
   smaller in holometabolous insects, 515;
   tubes, 501;
   vitality of, 520.

 Ejaculatory ducts, double openings of, 486;
   origin of, 578.

 Elater of Collembola, 551.

 Electricity, influence of, on action of heart, 412.

 Eleodes, 372.

 Elliott on color sense, 261.

 Elmis, 462, 473.

 Elytra, 124;
   glands in, 125.

 Embidæ, 620.

 Embryo, 531;
   revolution of, 540.

 Embryology of insects, 515.

 Embryonic life, length of, 582.

 Embryonic membranes, involution of, 556.

 Embryonic, post-, changes, 650.

 Emery, on homologies of the tracheæ, 443;
   on the firefly, 424, 427.

 Empodium, 97, 111, 116.

 Empretia stimulea, 192.

 Eucyrtus, 55, 171;
   post-embryonic changes in, 663.

 Endochorion, 520.

 Endoderm, 534, 561.

 Endomesoderm, 542.

 Endosternite, 94.

 Endotrachea, 432, 448.

 Entomoline, 29.

 Entothorax, 92.

 Environment, 463.

 Ephemera, circulation in, 409;
   double sexual openings of, 489;
   nymph of, lacinia of, 61;
   thorax of, 91.

 Ephemerella, 466.

 Ephemeridæ, 459, 460;
   double genital openings of, 492;
   gills of, 460;
   rectal respiration in, 464.

 Ephydra, 36, 553.

 Epicauta, life-history of, 692.

 Epilabrum, 13.

 Epimerum, 89.

 Epiopticon, 231, 253.

 Epipharynx, 43, 54.

 Epipleurum, 124.

 Episternum, 88.

 Erichson on sense of smell, 266.

 Eriocephala calthella, mandibles of, 62;
   maxillæ of, 68.

 Eristalis, 189, 461.

 Eruciform type of larva, 602, 605, 705.

 Escherich, on male genital organs of beetles, 495.

 Euphæa, lateral gills of, 468, 477.

 Euphoria inda, moulting of, 611.

 Eupolus, 113.

 Eupsalis minuta, 103.

 Eversible sacs, 475.

 Excretion, defined, 327;
   process of, 328.

 Excretory system, 348.

 Exner, on vision, 258.

 Exochorion, 520, 534.

 Exuvia, 609.

 Exuvium, 609.

 Eye, 249;
   acone, 251;
   buds, 665;
   compound, 250;
   embryonic development of, 567;
   eucone, 251, 252;
   facetted, origin of, 255;
   glazed, 629;
   pseudocone, 251, 252;
   simple, 249.


 Fabre, on life-history of Sitaris, 69.

 Facet, 250.

 Facets, number of, 249.

 Facetted eye, origin of, 255.

 Faivre, on function of brain, 244.

 Fat, amount of, in caterpillar, 644.

 Fat-body, 419;
   concretions in, 420;
   destruction and reformation of, in muscids during metamorphosis, 685;
   origin of, 574.

 Feet, post-embryonic development of, 653.

 Female reproductive organs, 485, 500;
   origin of, 575.

 Femur, 96;
   formation of, in imago of Lepidoptera, 655.

 Fernald, on rectal glands of Passalus, 318.

 Fertilization of the egg, 525.

 Filator, 340.

 Filippi’s glands, 345.

 Firefly, 426.

 Fischer, on color of butterflies, 200.

 Flagellum, 57.

 Flea, 438;
   hatching spine of, 586;
   hypopharynx of, 77;
   number of moults of, 617.

 Flies, syrphid, 189.

 Flight, 148, 219;
   theory of, 150.

 Fluid, exuvial, 612;
   moulting, 612;
   softening fluid of moths in escaping from the cocoon, 635.

 Fly, blow, hatching of, 585;
   development of imago in, 666;
   horse, mouth-parts of, 79;
   thorax of, 91;
   house, length of embryonic life of, 582;
   thorax of, 88;
   number of moults of, 618;
   meat, hatching of, 585;
   tenent hairs of, 111.

 Fold, amnion, 531.

 Folds, gastro-ileal, 317;
   giving rise to head of fly, 671.

 Folsom, on lateral gills of Euphæa, 468.

 Food-reservoir, 305.

 Foot, of beetle, 111;
   of fly, 111.

 Footprints of beetles, 106.

 Fore-stomach, 306.

 Forel, on gustatory organs, 281;
   on honey-dew, 365;
   on vision, 256.

 Forficula, 454, 491;
   fœtid glands of, 369;
   hatching spine of, 585.

 Formic acid, 358.

 Frenulum, 122.

 Fulgora, 424.

 Funiculus, 460.

 Funnel, 313.


 Galea, 63, 64.

 Galeruca, 113.

 Ganglia, function of, 244;
   fusion of, 225;
   optic, 231, 232;
   primitive number of, 567.

 Ganglion frontale, 569.

 Ganglion opticum, origin of, 567.

 Ganin, on abdominal imaginal buds, 170;
   on hypermetamorphosis of ichneumon parasites, 701.

 Gastro-ileal folds, 317.

 Gastropacha, 552;
   flattened hairs of, 194.

 Gastrophilus equi, 475.

 Gastrula, 558;
   stage, 535.

 Gegenbauer, on homology of wings with gills, 142.

 Gehuchten, on histology of muscles, 217;
   on the histology of mid-intestine, 316;
   on the pyloric valvule, 315;
   on secretion, mechanism of, 326.

 Gena, 46.

 Genital armature, male, 176;
   cells, 575;
   claspers, 176.

 Germarium, 501.

 Germ-layers, formation of, 558.

 Giard on urinary tubes, 351.

 Gills, blood, 475;
   in embryo insects, 476;
   tracheal, 459;
   adult insects, 476;
   rectal, 463.

 Gilson, on anal glands, 373;
   on spinning-glands, 340.

 Gissler on anal glands, 372.

 Gizzard, 311, 324.

 Glands, acid, 358;
   adhesive, 360;
   alkaline, 358;
   alluring, 391;
   of androconia, 199;
   anal, 319, 372;
   accessory of vasa deferentia, 497;
   cement, 360;
   colleterial, 506;
   conglobate, 487;
   corrosive, 374;
   coxal, 369, 383;
   dermal, 365;
   defensive, 368;
   eversible, 368, 382;
   Filippi’s, 346;
   fœtid, 369;
   mucous, 497;
   mushroom, 497;
   odoriferous, 381;
   repugnatorial, 368;
   salivary, 331, 570;
   setiparous, 444;
   sexual, origin of male, 579;
   unicellular, 366;
   wax, 361.

 Glandulæ mucosæ, 497.

 Gnathal segments of embryo, 556.

 Gonapophyses, 167, 168.

 Gonin, on moulting fluid, 613, 614;
   on the post-embryonic changes of Pieris, 651;
   on process of pupation, 659;
   on tracheæ of wings, 145;
   on the wing-germs, 131.

 Gorgeret, 170.

 Gottsche on vision, 257.

 Graber, on abdominal legs of caterpillars, 552;
   on blood, 408;
   on blood-gills of embryo, 476;
   on climbing, 116;
   on development of wings, 138;
   on flight, 153;
   on folding of wings, 155;
   on foot-tracks of beetles, 109;
   on heart, 398, 400, 402;
   on mechanics of segmented body and limbs of insects, 31, 38;
   on mechanics of walking, 103;
   on organs of hearing, 290;
   on organs of smell, 267;
   on premandibular segment, 52;
   on respiration, 454;
   on successive appearance of embryonic segments, 546;
   on swimming, 116.

 Grassi, on premandibular segment in Apis, 52;
   on Scolopendrella, 20.

 Grège, 340.

 Grès, 340.

 Griffiths, on pigments, 207.

 Grobben on heart, 399.

 Gromphas, 101.

 Gross, on color sense, 261.

 Gryllotalpa, 572;
   maxilla of, 64.

 Guide, egg, 183.

 Gula, 46, 68.

 Gulo-mental region, 46.

 Gummy layer of silk, 340.

 Gyrinus, 471, 620.


 Haase, on coxal sacs, 14;
   on eversible glands, 371;
   on the formation of the copulatory pouch, 505;
   on Scolopendrella, 21, 24;
   on the homology of the ovipositor, 171.

 Hadenœcus, 44, 392.

 Hagen, on colors, 201;
   on gills of Perlidæ, 477;
   on lateral gills of Euphæa nymph, 468, 477;
   on vestigial gills in other odonate nymphs, 469.

 Hair-fields, 197.

 Hair-forming cells, 188.

 Hair-scales, 197;
   tactile, 193.

 Hairs, 188;
   adhesive, 111, 113;
   development of, 193;
   gathering, 45;
   glandular, 190, 192;
   nettling, 191;
   plumose, 189;
   tenent, 99, 190;
   in tracheæ, 451;
   twisted, 189.

 Halteres, 124, 629.

 Hammond, see Miall.

 Hampson, on scent-glands, 391.

 Haplopus, 521.

 Harpes, 180.

 Harpiphorus, 618.

 Harpyia, 375.

 Hatching, process of, 583;
   spine, 585.

 Hauser, on organs of smell, 267, 269, 279.

 Head, 42;
   blood-vessels in, 405;
   completion of head of embryo, 548;
   formation of, in aculeate Hymenoptera, 57;
   lobes, 544;
   number of segments in, 50, 54;
   of Musca, post-embryonic development of, 675;
   post-embryonic development of appendages of, 653.

 Hearing, organs of, 287.

 Heart, 397;
   beat, 411;
   free from histolysis, 667;
   origin of, 572, 577;
   pericardial diaphragm, 402;
   propulsatory apparatus, 401;
   supraspinal vessel, 403.

 Heathcote, on double segments of Diplopods, 14.

 Heider, on embryology of Hydrophilus, 530.

 Heinemann, on the firefly, 426.

 Helcodermatous spines, 612.

 Helecomitus, 616.

 Helichus, 474.

 Heliconidæ, 379.

 Helm, on spinning-glands, 339.

 Hemelytra, 124.

 Hemerobiidæ, hatching spine of, 585.

 Hemimetabola, 598.

 Hemiptera, cardo of, 69;
   fœtid glands of, 372;
   lacinia of, 74;
   palps of, 68;
   salivary glands of, 333;
   stipes of, 69.

 Hepialus, 392, 495.

 Heptagenia, 467;
   lingua of, 73.

 Heredity, bearer of, 498;
   homochronous, 708.

 Heremetabola, 597.

 Herold, on the metamorphosis of the butterfly, 642;
   on wing-germs, 128.

 Heterochrony, 542.

 Heterometabola, 597.

 Heymons, on homologies of the labrum, 43;
   on nature of elytra, 126;
   on origin of fat-body, 575;
   on paired sexual openings, 493;
   premandibular segment, 52;
   on the primitive segments of Phyllodromia, 563;
   on reproductive glands, 575;
   tentorium, 50.

 Hicks, on auditory organs, 293;
   on olfactory organs, 266.

 Histogenesis, 650.

 Histolysis, 643, 650, 678, 680, 685, 688.

 Hoffbauer, on the structure of elytra, 125.

 Holmgren, on tracheal end-cells, 437.

 Holometabola, 598.

 Holometabolous insects, 595.

 Holopneustic type of tracheal system, 459.

 Holoptic head, 98.

 Homalotylus, 623.

 Homoptera, number of moults of, 616.

 Homotenous insects, 597.

 Honey-dew, 364;
   deterrent use of, 365;
   sac, 309;
   stomach, 309.

 Hopkins, on pigments, 207.

 Hornia, hypermetamorphosis of, 693.

 Horns, 187.

 Horn, on loss of tarsi, 101;
   on Platypsylla, 62.

 House-fly, thorax of, 88.

 Hum of bee, 295.

 Hunter, John, on the air-sacs, 456.

 Hurst, on the formation of the imago in Culex, 670.

 Hybocampa, 635.

 Hydrobius, 471.

 Hydrophilus, 374, 432;
   embryology of, 536, 537, 542, 546, 558, 575.

 Hydropsyche, blood-gills of, 475;
   gills of, at all stages, 469.

 Hydroüs, 49, 50.

 Hylotoma, 52.

 Hymenoptera, composition of the head in, 55;
   mouth-parts of, 79, 81;
   olfactory organs of, 277;
   open stigmata of, 462;
   poison glands of, 357;
   post-embryonic changes in, 661;
   salivary glands of, 334.

 Hyperchiria io, 187, 378;
   poisonous spines of, 192.

 Hypermetamorphosis, described, 688;
   causes of, 693.

 Hypodactyle, 73.

 Hypoderma, 518.

 Hypodermis, 188, 612;
   origin of an imago, 678.

 Hypopharynx, 13, 54, 68, 70.

 Hypoptère, 89.


 Icerya, 616, 626.

 Ichneumon, 622;
   hypermetamorphosis of, 701;
   poison glands of, 359.

 Ileum, 317.

 Imaginal buds, 653;
   disks, 653.

 Imago, formation of, in Chironomus, 671;
   Corethra, 668;
   Culex, 668;
   Hymenoptera, 661;
   Lepidoptera, 641;
   Melophagus, 686;
   Musca, 673;
   of fly, development of internal organs of, 678;
   Simulium, 668.

 Incasement theory, 641.

 Indusium, 534.

 Infra-anal lobe, 183.

 Infraœsophageal ganglion, 227.

 Ingluvies, 303.

 Insecta, diagnostic characters of, 26.

 Insects, ancestry of, 17;
   number of species of, 1;
   relation of, to other Arthropoda, 2;
   Symphyla, 18.

 Intestine, fore, embryonic development of, 547;
   formation, imaginal, 682;
   hind, 316, 547;
   histology of, 316;
   large, 316;
   mid, 314;
   origin of, 569.

 Invaginations of the imaginal buds, 678.

 Involution of the embryonic membranes, 556.

 Isotoma, 534.


 Jackson, on the structure of the cremaster and pupa, 639.

 Janet on muscular fibres, 216.

 Japyx, 486.

 Jolia, 466.

 Jugum, 123.

 Julus, larva of, 14.


 Katydid, 616.

 Kellogg, on Androconia, 199;
   on spinules, 197;
   on striæ of scales, 195, 198;
   on use of scales, 195.

 Kennel, on origin of tracheæ of Peripatus, 443.

 Kenyon, on double segments of Diplopods, 14;
   on mushroom bodies, 234.

 Kettelhoit, on specific characters of, 195.

 Kingsley, on classification of Myriopoda, 12.

 Kirbach, on salivary duct, 336.

 Kirby and Spence, views of, on metamorphosis, 642.

 Klapálek, on gills of case-worms, 467;
   on sub-nymph of Agriotypus, 701.

 Klemensiewiez, on eversible glands, 377.

 Kolbe, on atrophy of tarsi, 101;
   on blood, 408;
   on flight of Agrioninæ, 159;
   on tracheæ, 435;
   on tracheal gills of Perla, 477;
   on embryology of the mole-cricket, 529, 572.

 Korschelt, on the egg-tubes, 502;
   on egg-genesis, 504.

 Korschelt and Heider, on the embryology of insects, 531, 535, 538, 554,
    559, 570, 579;
   on formation of the imago of Corethra, 668;
   on position of genital glands in myriopods, 15;
   on stem form of myriopods, 17.

 Koulaguine, on dorsal opening of urinary tubes, 355.

 Kowalevsky, on embryonic abdominal appendages, 550;
   on embryonic membranes, 562;
   on origin of blood corpuscles, 574;
   experiments on feeding maggots with lacmus, 326;
   on fat-body, 420;
   on embryo origin of fat-body, 574;
   on the mesoderm of the rudiments of the appendages, 675;
   on openings of heart, 400;
   on pericardial cells, 420;
   on phagocytes, 421, 422, 655;
   phagocytosis, 686.

 Kraepelin, on homologies of the ovipositor, 168;
   on organs of smell, 267;
   on taste, 282.

 Krancher, on the stigmata, 438.

 Krauss, on eversible glands, 371.

 Krawkow, on chitin, 29.

 Krukenberg, on colors, 202, 205, 206.

 Künckel d’Herculais, on beating of heart throughout the post-embryonic
    changes, 686;
   on the origin of the imaginal from the larval legs, 654.

 Kupffer, on fine tracheæ, 435.


 Labella, 13, 446.

 Labial palpi, imaginal buds of, 658.

 Labidura, 491.

 Labium, 68, 549.

 Labrum, 42, 79;
   epipharynx, 43, 79;
   origin of, 546, 547;
   homologies of, 546, 547.

 Lacaze-Duthiers on the ovipositor, 167, 169.

 Lace-winged fly, 517.

 Lachnosterna, nervous system of, 225.

 Lachnus, 372.

 Lacinia, 63;
   of Eriocephala, 67;
   mandible of Copris, 61;
   Ephemera nymph, 61;
   mandible of Passalus, 61;
   mandible of Phanæus, 61;
   mandible of Staphylinus, 61.

 Lady-birds, 375;
   bug, 375.

 Lagoa crispata, 191, 378.

 Lamarckian factors, 708.

 Lamina supra-analis, 181.

 Lampyris, 425, 451.

 Landois, on pigment, 207;
   on sense of smell, 267;
   on origin of the tracheæ and veins of the wings, 145;
   on wings as respiratory organs, 461.

 Lang, on metamorphosis of seventeen-year Cicada, 698;
   on origin of coxal from setiparous glands, 444;
   on Peripatus, 9;
   on relation of myriopods to insects, 17;
   on segmented structure of arthropods, 32;
   on respiratory system, 430.

 Langley, on the light of the firefly, 426.

 Larva, defined, 599;
   Campodea-form, or campodeoid, 600;
   growth of, 608;
   voracity of, 608;
   eruciform, 705.

 Larvæ, apodous, 103, 653.

 Larval insects, tracheal gills of, 466;
   stage, 593.

 Latreille, on the median segment, 163;
   metamorphism, 597;
   on the term pupa, 625.

 Latzel, on coxal sacs, 14.

 Layer, superficial protoplasmic, of egg, 524, 526;
   germ, formation of, 558.

 Leach, on ametabola, etc., 596.

 Leaping power, 219.

 Legs, abdominal, of lepidopterous larvæ, and larval saw-flies, are they
    true legs?, 552;
   atrophy of, 102;
   mechanism of, 104;
   movements of, 105;
   muscles of, 215;
   post-embryonic development of, 653, 654;
   pulsatile organs in, 405.

 Lehrman, on organs of smell, 265.

 Leidy, on fœtid glands, 373.

 Lens, crystalline, 250, 251.

 Lendenfeld on flight, 149, 151, 159.

 Léon, on labial palpi of Hemiptera, 68;
   tongue of Hemiptera, 73.

 Lepidoptera, embryology of, 537;
   eversible sacs of, 375;
   maxillæ of, 65, 67;
   number of moults in, 616, 617;
   origin of legs of imago, 654;
   paired oviducts, 492;
   pupæ of, 628;
   testes of, 493, 495.

 Lepisma, 52;
   double sexual openings in, 486.

 Leptiform larvæ, 600.

 Leptis, thorax of, 91.

 Leucarctia, 391.

 Leucine, 352.

 Leucocytes, 407, 421, 650, 678, 680, 685;
   size of, 407.

 Leucopis, 113, 517.

 Leydig, on colors, 202, 204;
   on nerve-end apparatus in the wing, 153;
   on organs of smell, 266;
   on tracheæ, 432.

 Libellula, 463.

 Life, embryonic, length of, 582.

 Light of the firefly, 426;
   its use, 428.

 Ligula, 68.

 Limacodes scapha, 606.

 Limbs, homologies of, 39;
   mechanical origin of, 34, 35;
   lost, reproduction of, 619;
   result of disuse of, 101.

 Limnephilus pudicus, 46;
   maxilla of, 65.

 Limulus, 5.

 Lina, 374, 545, 546.

 Lingua, 68, 70.

 Lip, under, 68.

 Lipochrome, 206.

 Liponeura, 475.

 Lithocolletis, 618;
   its cocoon-cutter, 634.

 Litognatha nubilifasciata, 102.

 Lobe, axillary, 124;
   infra-anal, 183.

 Lobes, œsophageal, 237;
   antennal, 237;
   head, 544;
   procephalic, 544, 548;
   procerebral, 232.

 Lobulus, 124.

 Locomotion, 103;
   on smooth surfaces, 111.

 Locust, air-sacs of, 424, 456;
   brain of, 231;
   cæcal appendages of, 347;
   digestive canal of, 298;
   ear of, 288;
   head-segments of, 546;
   mode of breathing, 451;
   hatching, 583;
   moulting of, 609;
   nervous system of, 223;
   number of stages of, 595;
   number of moults of, 616;
   olfactory organs of, 272;
   oviposition of, 520;
   rectal glands of, 318;
   reproductive organs of, 488, 489.

 Locusta viridissima, rectal glands of, 318.

 Locy, on pulsatile organs in legs of Nepidæ, 405.

 Lonchodes, 521.

 Loop of wing, 122.

 Lophyrus, 59.

 Lora, 68.

 Lubbock, on color sense, 261;
   on vision, 256–258;
   on distribution of tracheæ, 433.

 Lucanus, 59, 620;
   thorax of, 94;
   dama, nervous system of, 225.

 Lucas, on segmental arrangement of salivary glands, 337.

 Luciola, 424, 427, 451.

 Luna moth, its mode of escape from its cocoon, 635.

 Lutz, on blood, 275.

 Lycæna, 381.

 Lymph, 206.

 Lyonet, on the imaginal buds, 656;
   on wing-germs, 128.


 Machilis, 164, 223, 369, 476;
   hypopharynx, 72.

 MacLeod, on the tænidia, 445.

 Macloskie, on the tænidia, 445.

 Macrotoma, 616.

 Macrurocampa, 375.

 Maggots, 607;
   rat-tailed, 461, 474.

 Malachius, 374.

 Male reproductive organs, 485, 494;
   origin of, 579.

 Malpighi, on germs of wings, 128;
   on the heart, 397;
   on the metamorphosis of silk moth, 641;
   on urinary vessels, 350.

 Malpighian tubes, 316, 348.

 Mandibles, 59;
   composite structure of, 60, 61;
   lacinia of, 60, 61;
   vestigial, 62.

 Manometabola, 597.

 Mantidæ, coxal glands of, 372.

 Mantis, oötheca of, 517.

 Mantis religiosa, embryo of, 584;
   hatching of, 584;
   number of moults of, 584, 616.

 Mantispa, 46, 48, 68, 95, 97;
   hypermetamorphosis of, 602, 690, 705;
   life-history of, 602.

 Marchal on the function of the fat-body, 420.

 Marey, on motion of insects, 111.

 Marey’s views on flight, 148, 151.

 Marshall, on the way Microgaster spins its cocoon, 622.

 Mason, Wood, on jointed structure of mandibles, 60;
   on Scolopendrella, 19, 22.

 Mastopoda pteridis, 103.

 Maxillæ, first, 62;
   second, 68;
   imaginal buds of, 658.

 Mayer, A. G., on the development of the wings of Pieris and Danais,
    136;
   on formation of scales, 196;
   on homologies of tracheæ, 443;
   pigments, 206–208.

 Mayer, Carl, on scales, 197.

 May-fly, lingua of nymph of, 73;
   number of moults of, 616;
   thorax of, 91.

 Mechanics of walking, 103.

 Mechanism of motion, 32;
   of limbs, 35;
   of secretion, 326.

 Meconium, 611.

 Mecoptera, maxillæ of, 65;
   number of moults of, 616.

 Median segment, 90, 163.

 Medifurca, 92.

 Megalopyge, 191, 378.

 Meinert, on buccal organs of myriopods, 13;
   on coxal sacs, 14;
   on elytra, 125;
   on hypopharynx, 78;
   on organs of taste, 281.

 Melanoplus, 43, 72, 86, 456;
   gastro-ileal folds of, 317;
   hatching of, 583;
   number of stages of, 595;
   rectal glands of, 318;
   tongue of, 72.

 Meldola, on yellow pigment, 206.

 Melipona, 359.

 Meloë, 110, 374;
   hypermetamorphosis of, 690;
   lacinia of mandibles of, 62;
   number of moults of, 617;
   small eggs of, 524.

 Melolontha, 213, 438, 455, 456, 458, 498, 549, 550, 551, 562.

 Melophagus, 507;
   post-embryonic changes in, 686.

 Membrane, peritrophic, 313;
   retaining, of pupæ, 638;
   serous, 532;
   vitelline, 520, 534.

 Membranes, embryonic, 531;
   formation of, 532;
   embryonic, involution of, 556.

 Menge on Scolopendrella, 18.

 Mentum, 54, 68.

 Merostomata, 5.

 Mesoderm, 534, 561, 563;
   cells, 576.

 Mesothorax, 86.

 Metabolia, 596.

 Metabolous insects, 595.

 Metameric structure, 33.

 Metamorphoses of insects, 593;
   stadia of, 594;
   stages, 594.

 Metamorphosis, causes of, 607, 705, 708;
   significance of, 688.

 Metapneustic type of tracheal system, 461.

 Metathorax, 87.

 Metschnikoff, on embryology of myriopods, 13, 16;
   on the germs of the genital glands, 580.

 Miall and Denny, on the blood, 407;
   on cement glands, 361;
   on chitin, 29;
   on digestive canal of cockroach, 316, 317;
   on heart, 398, 402;
   on labium, 53;
   on lingua, 72;
   on reproductive organs of the cockroach, 487;
   on respiration, 452;
   on salivary glands, 331;
   on tænidia, 447;
   on the tentorium, 49;
   on urinary tubes and products, 353.

 Miall and Hammond, on the differences between the pupa of Lepidoptera
    and Diptera, 629;
   on the formation of the imago of Chironomus, 671.

 Microcentrum, 616.

 Microgaster, mode of spinning its cocoon, 622.

 Micropteryx, 621, 626;
   escape from its cocoon, 633;
   hypopharynx of, 76;
   labium of, 76;
   pupal jaws of, 633.

 Micropyle, 522;
   use of, 524.

 Mid-intestine, 314;
   origin of, 569.

 Minchin, on eversible glands, 370.

 Minot, on cæca of stomach, 347;
   on colors, 203;
   on the cuticula, 187;
   on digestive canal, 318, 320, 321;
   on distribution of tracheæ, 433;
   on gastro-ileal folds, 317;
   on rectal glands, 318;
   on tænidia, 445.

 Molar, 61.

 Mole-cricket, 102, 527, 543, 572, 574;
   digestive canal of, 350;
   urinary tubes of, 350.

 Mosaic theory of vision, 257.

 Moseley, on circulation of blood, 410;
   on composition of chitin, 30.

 Mosquito, 461, 464, 465;
   poison gland of, 359;
   sense of hearing of, 292.

 Moulting, process of, 609;
   hairs and spines, 612.

 Moults, number of, 615.

 Mouth, 302;
   -appendages, buds of, 665;
   of embryo, 537.

 Müller, F., on blood-gills, 475;
   on larvæ of Psychodes and of Blepharocera, 474;
   on non-inheritance of the complete metamorphosis, 595.

 Müller, J., on alluring glands, 391;
   on heart, 399;
   on sense and organs of smell, 265;
   on the development of wings, 138, 143;
   on vision, 255.

 Müller, W., on gills of Paraponyx, 470.

 Müller’s, J., thread, 577.

 Mumia, 625.

 Musca, 88, 111;
   embryology of, 530, 536, 563;
   wing-germs of, 133.

 Muscidæ, appendages of imago, development of, 674;
   post-embryonic changes in, 666, 673, 678, 681.

 Muscles, 31;
   of caterpillars, 213;
   of cockchafer, 213;
   of Cossus, 211;
   destruction of, during metamorphosis, 680;
   of flight, 149;
   of Pygæra, 211;
   respiratory, 454;
   structure of, 215.

 Muscular fibres, 214;
   power, 217, 219;
   system, 211.

 Musculature, mode of origin of, 574.

 Mushroom bodies, 233.

 Mutilations, inheritance of, 102.

 Myriopoda, 11.

 Myrmeleon, maxilla of, 64;
   palpifer of, 69.

 Mystacides, scent scales of, 199.


 Nagel, on saliva of larval Dyticus, 324.

 Nassonow, on double sexual openings, 486.

 Necrophorus, tracks of, 109.

 Nematois, 496.

 Nematus, 53, 54, 618.

 Nemognatha, epipharynx of, 285;
   maxillæ of, 64, 65;
   organs of taste in, 285.

 Nemoptera, larva of, 42.

 Nemoura, 468.

 Neolamarckian factors, 708.

 Neolepidoptera, 628.

 Nepa, 523.

 Nephridia, 348.

 Nepionic stage or form, 706.

 Nepticula, 606.

 Nerve-centre, 222.

 Nerve-centres, functions of, 243;
   antennal, 650.

 Nerves, motor, 222;
   motor, degeneration of, during metamorphosis, 684;
   optic, 650;
   peripheral, transformation of, 684;
   sensory, 222;
   stomogastric, 238;
   sympathetic, 238;
   visceral, 238.

 Nervous system, 222;
   formation of, 566;
   free from histolysis during pupation, 667, 684;
   origin of, 554, 566;
   primitive rolls or strips, 566;
   slight changes in, during metamorphosis, 684.

 Neuroblasts, 567.

 Neuromeres, 227, 231.

 Neuroptera, 632;
   lingua of, 69.

 Newman, on the median segment, 163.

 Newport, on changes in nervous system of Sphinx during metamorphosis,
    648;
   on circulation, 409;
   on heart, 399;
   on larval Julus, 13;
   on the median segment, 163;
   on muscular power, 94;
   on muscles of Sphinx, 213;
   on number of segments of head, 50;
   on occiput, 48;
   on the process of moulting in Sphinx, 610, 611;
   Scolopendrella, 18;
   on sense of smell, 265;
   on tentorium, 49;
   on tracheal gills of Pteronarcys, 476.

 Nola, 618.

 Nucleus, division, 530;
   sperm, 525.

 Nusbaum, on origin of efferent sexual passages, 578.

 Nymphalid pupæ, 631.

 Nymph, 706;
   stage, 593, 600;
   sub-, 701.


 Occipital foramen, 46.

 Occiput, 48, 53.

 Ocellus, 249;
   development of, 567.

 Ockler, on feet of insects, 115.

 Odonata, 53;
   embryology of, 540;
   labium of, its mode of origin, 549;
   lateral gills of, 468;
   lingua of, 73;
   number of moults of, 616;
   nymphs, 460, 463.

 Odors, 368.

 Œcanthus, 476;
   embryology of, 541, 544, 549, 551, 573.

 Œceticus, 634.

 Œnocytes, 423.

 Œsophageal valve, 311;
   valvule, 312.

 Œsophagus, 303.

 Œstridæ, 618.

 Oken, on homology of maxillæ with legs, 39.

 Olfactory organs, 264.

 Oligonephria, 354.

 Oligoneuria, 466.

 Ommateum, 250.

 Onychium, 97.

 Oölemma, 520.

 Oötheca, 517.

 Operculum, 181.

 Optic nerves, 650.

 Optic tract, 253.

 Opticon, 253.

 Oral appendages, 549.

 Orchelimum, 534.

 Organ, dorsal, 535, 556.

 Organs, sensory, 249;
   of smell, 264.

 Orgyia, 377, 618;
   poisonous hairs of, 192.

 Orgyia antiqua, number of moults of, 618.

 Orthoptera, fœtid glands of, 369;
   number of moults of, 616;
   phagocytes in, 421;
   salivary glands of, 331;
   tongue of, 70.

 Orya, 424.

 Osmeterium, 377.

 Osten Sacken, on holoptic heads of Diptera, and on running flies, 98.

 Ostia, 397, 400.

 Otiocerus, 58.

 Oudemans, on relation of myriopods to insects, 17.

 Oustalet, on rectal respiration, 463.

 Ovarian tubes, 501;
   formation of, 578.

 Ovaries, 500, 502;
   formation of, 578;
   groups of, 502.

 Oviduct, 500, 503;
   double openings of, 486;
   origin of, 578, 579.

 Oviducts, segmental arrangement of, 486.

 Oviposition, 518, 520.

 Ovipositor, 167;
   germinal buds of, 665;
   imaginal buds of, 666;
   origin of, 551.

 Ovum, 515, 521, 524.


 Packard, A. A., on muscular power of insects, 219;
   use of air-sacs, 457.

 Pad, peripodal, 653.

 Pædogenesis, 708.

 Paleacrita vernata, maxilla of, 66.

 Paleolepidoptera, 628.

 Palmén, on double sexual openings, 490, 492;
   on tracheal gills, 459, 466;
   on the tentorium, 50.

 Palmula, 97.

 Palpifer, 63, 68.

 Palpus, first maxillary, 63, 64, 67;
   second, 68;
   in Hemiptera, 68.

 Palpi, labial, imaginal buds of, 658.

 Pancritius, on wing-germs, 130, 143.

 Paniscus, 517.

 Panorpa, abdomen of, 162;
   maxilla of, 65;
   number of moults of, 616.

 Panorpidæ, 602.

 Papilio, 377.

 Paraglossa, 54, 68.

 Parapodial glands, 444.

 Paraptera, 89.

 Parnassius, 381.

 Paronychium, 97.

 Paraponyx, 470.

 Passalus, 61;
   rectal glands of, 318.

 Pasteur, on the spectrum of the light of the firefly, 426.

 Patagia, 89.

 Patten, on embryonic abdominal appendages, 550;
   on the homologies of the tracheæ, 444;
   salivary glands, 337.

 Patula, 391.

 Pauropus, 18.

 Paussus, 57.

 Pawlowa, on blood-vessels in the head, 405.

 Pedicel, 57.

 Pediculina, embryology of, 541.

 Pelidnota, moulting of, 611.

 Pellicle, subimaginal, 613.

 Pelobius, 461, 475.

 Penis, 180;
   velum of, 181.

 Pericardial cells, 405;
   diaphragm, 402;
   septum, 574.

 Periopticon, 231, 253.

 Peripatus, 9, 580;
   nephridia of, 349;
   trachea of, 443.

 Peripodal cavity, 669;
   membrane, 669;
   sac, 653.

 Periplaneta, 72, 370;
   egg-tubes of, 501;
   tongue of, 72.

 Peritoneal membrane, 444.

 Peritracheal circulation, 397;
   membrane, 444.

 Peritreme, 90.

 Peritrophic membrane, 313.

 Perlidæ, 69, 468, 491, 493.

 Perris, on organs of smell, 266.

 Phagocytes, 421, 650, 655, 680, 683, 685.

 Phagocytosis, 421.

 Phanæus, 61;
   reduced tarsi of, 101.

 Phanæus pegasus, 188.

 Phaneroptera, 44.

 Pharynx, 302.

 Phasmidæ, eggs of, 521.

 Phosphorescence, 424;
   physiology of, 426.

 Photogenic organ of beetles, 424.

 Phragma, 93.

 Phryganea, 455.

 Phyllium, 521.

 Phyllocnistis, 606.

 Phyllodromia, 506;
   embryology of, 541, 563, 583;
   eversible glands of, 370;
   fat-body, origin of, 575;
   micropyle of eggs of, 523;
   pleuropodia of, 551;
   origin of sexual organs of, 576, 578–580;
   mode of hatching, 583;
   oötheca of, 519.

 Phytonomus, 617.

 Phytophagous larvæ, 606.

 Pictet, on blood gills, 475.

 Pieris, 39, 614;
   embryology of, 546;
   green pigment of, 206;
   post-embryonic changes of, 651;
   wing-germs of, 133.

 Pigment, 183, 203;
   chemical nature of, 206;
   of eye, 253;
   physical nature of, 206.

 Pits, olfactory, 271.

 Planta, 638.

 Plantula, 97.

 Plate, extensor, of foot, 116;
   pressure, 116.

 Plateau, on digestion, 324;
   on functions of ganglia, 244;
   on muscular power, 217;
   on respiration, 453;
   on vision, 256, 259.

 Platycrania, 521.

 Platygaster, hypermetamorphosis of, 701;
   sexual cells of, 581.

 Platypsyllus, 62.

 Platysamia, 460.

 Platyzosteria, 371.

 Plectoptera, 459, 466.

 Pleuropodia, 476, 551, 583.

 Pleurum, 87.

 Plumules, 198.

 Pocock, on classification of myriopoda, 12, 21.

 Poduridæ, 574.

 Poisonous spines, 191, 199.

 Poisons, effect of, on pulsations, 412;
   on hairs, 191, 199.

 Poison apparatus, 357;
   glands, 357;
   nature of, 357.

 Polar cells, 580.

 Polymitarcys, 467.

 Polymorphous insects, 597.

 Polynema, hypermetamorphosis of, 702.

 Polynephria, 354.

 Polyphemus silkworm, 621.

 Polypodous ancestor of insects, 22;
   embryos, 550.

 Polypody, 550;
   suppression of in dipterous embryos, 707.

 Pore-canals, 188.

 Porthesia, 529.

 Postfurca, 92.

 Post-gula, 54, 68.

 Post-retinal fibres, 231, 232.

 Postscutellum, 87.

 Pouch, copulatory, 505.

 Poujade, on flight, 159.

 Poulton, on the differences between the limbs of the pupa and imago,
    628.

 Præscutum, 87.

 Pratt, on absence of polypodous embryo in Muscidæ, 55;
   on the dorsal position of the stomodæum of Diptera, 537;
   on epigenetic period, 688;
   on fate of the leucocytes, 685;
   post-embryonic changes of Melophagus, 686;
   significance of metamorphosis, 688;
   on wing-buds, 127.

 Preantennal appendages, 548.

 Premandibular segment, 51, 52, 549.

 Press of spinning apparatus, 341.

 Primitive band, 531;
   streak, 531.

 Prionocyphon, 472.

 Prisopus, 69, 477.

 Proboscis, 446.

 Procephalic lobes, 544.

 Proctodæum, 537, 547, 569.

 Prodoxus, 606.

 Progoneate myriopods, 21.

 Pronotum, 87.

 Pronucleus, 526.

 Propodeum, 163.

 Propupa, 627.

 Prosopistoma, 467.

 Prostheca, 61.

 Prothorax, 86.

 Protocerebrum, 231, 232;
   its representative in annelids, 227.

 Proventricular valvule, 313.

 Proventriculus, 306;
   “beak” of, 312;
   formation of, in imago, 682;
   use of, 311, 324, 325.

 Prussic acid, 374.

 Psephenus, 462, 473.

 Pseudoglomeris, 598.

 Pseudonychium, 97.

 Pseudo-tracheæ, 446.

 Psocus, 616.

 Psychodes, 474.

 Psylla, 361, 518;
   glandular hairs of, 192;
   nymph of, 163.

 Pteronarcys, 468, 476.

 Pterygodes, 89.

 Pterygota, 27, 595.

 Pulex, number of moults of, 617.

 Pulex canis, hatching spine of, 586;
   hypopharynx of, 77.

 Pulling power, 218.

 Pulse, 411.

 Pulvillus, 97, 114, 116.

 Pump, pharyngeal, 302;
   adaptation of, to its surroundings, 631;
   armature of, 631;
   structure, 632.

 Pupa, coarctate, 621, 626;
   libera, 626;
   mode of escape of, from its cocoon, 632;
   mode of suspension, 637;
   nymphalid, 631;
   obtecta, 626;
   spines of, 629;
   state defined, 625.

 Pupa, semi, 691.

 Pupal, pseudo-, stage, 691;
   sustainers, 638.

 Puparium, 621.

 Pupation, mechanism of, 661;
   process of, 659.

 Pupipara, post-embryonic changes in, 686.

 Pushing power, 218.

 Pygidium, 163.

 Pyloric valvule, 315.

 Pyrophorus, 424.

 Pyrrarctia, 617.

 Pyrrhocoris, 372.


 Quiescent pupal life, 598.


 Ranatra, 523.

 Raphidia, 631.

 Raschke, on the rectal respiration of Culex, 465.

 Rath, Vom, on larval Julus, 13.

 Ratzeburg, on composition of head in Hymenoptera, 55.

 Réaumur, on the cremaster, 637;
   on the double sexual openings of Ephemera, 489;
   on germs of wings, 128;
   on the heart, 403;
   on the mechanism of pupation, 661;
   on metamorphosis, 642;
   on the origin of legs of imago from those of the larva, 654;
   on rectal respiration, 463;
   on sense of smell, 264;
   on vision, 256.

 Rectal glands, 318;
   tracheal gills, 463.

 Rectum, 318;
   of embryo, 577.

 Reinhard, on head of Hymenoptera, 55;
   on the median segment, 164.

 Reproduction, organs of, 485;
   origin of, 575.

 Repugnatorial glands, distribution of, 382.

 Respiration, 430, 451;
   rectal, 463.

 Respiratory system, 430;
   mechanism of, 451.

 Resting stage, 707.

 Retina, origin of, 568.

 Retinula, cells of, 250, 253.

 Rhabdites, 167, 517.

 Rhabdom, 250.

 Rhabdopoda, 176.

 Rhipiphorus paradoxus, 697.

 Rhopalum, 636;
   pupal spines of, 636.

 Rib, Semper’s, 121.

 Ribs of wings, 146.

 Ridges, primitive, of nervous system, 554.

 Riley, on the cremaster, 637;
   egg-burster, 585;
   hatching of seventeen-year Cicada, 584;
   life-history of Epicauta, 692.

 Rods of eye, 253.

 Rods, visual, of eye, origin of, 568.

 Rombouts, on locomotion of insects on smooth surfaces, 114.

 Ruptor ovi, 585.

 Ryder, on loss of tarsi, 101;
   on Scolopendrella, 19.


 Sacs, air, 456;
   use of, 457;
   coxal, 14;
   eversible, 369;
   hypodermal, 653.

 Saliva, 324;
   poisonous, 359.

 Salivary duct of Stomoxys, 446;
   glands, 331, 570;
   glands, formation of imaginal during metamorphosis, 683;
   homologues of coxal glands, 337;
   modified, 337;
   segmental arrangement of, 331.

 Savigny on epipharynx, 43;
   on homologies of appendages, 39, 71.

 Saw-fly, 374.

 Scale-hairs, 198.

 Scales, 187, 193, 202;
   battledore, 198;
   development of, 195;
   distribution of, 193;
   of fly’s wing, 124;
   scent, 198;
   flattened, 193;
   striæ of, 194, 202.

 Scape, 57.

 Scent-glands, 39.

 Scent-scales, 198.

 Scepsis, 618.

 Schæffer, on blood, etc., in the pupal wings, 146;
   the fat-body, 420;
   leucocytes, 421;
   on origin of scales, 196;
   on the rudimentary wings, 128.

 Schatz, on colors of butterflies, 202.

 Schiemenz, on salivary glands of bees, 334.

 Schindler, on urinary tubes, 351.

 Schiödte on blood-gills, 475.

 Schmidt, on the metamorphosis of male Coccidæ, 640;
   Scolopendrella, 21, 24.

 Schneider, on the funnel of the proventriculus, 313;
   on spermatophores, 500.

 Sciara, 348, 636.

 Sclerites, cervical, 46.

 Scolopendrella, 18;
   the ancestor of insects, 17;
   spinning glands, 346.

 Scudder on the glazed eye of pupal butterflies, 631.

 Scutellum, 87.

 Scutum, 87.

 Secretion, definition of, 327;
   mechanism of, 326;
   products of, 329.

 Sectores coconis, 634.

 Segment, antennal, 227;
   deutocerebral, 227;
   intercalary, 51;
   median, 90, 163;
   premandibular, 51, 52, 228;
   procerebral, 231.

 Segmental arrangement of genital glands, 486.

 Segments, number of, in head, 50, 68, 227, 229;
   optic, 231;
   origin of, 542.

 Seirarctia, 618.

 Selandria larva, mouth-parts of, 68.

 Seminal ducts, 496.

 Semipupa, 691.

 Semper, ground-membrane of, 136;
   on origin of hair-scales, 195.

 Sense organs, special, in flies, 293.

 Sensory organs, 249.

 Serosa, 532.

 Setæ, 188.

 Sexual differences, secondary, 59, 99, 101, 114;
   openings, double, 486, 490, 491.

 Sharp, on causes of segmentation of Crustacea, 33;
   on cervical sclerites, 46;
   on sternites, 92;
   homologies of elytra, 126.

 Sheep-tick, 507;
   post-embryonic changes of, 686.

 Sialidæ, 602.

 Sialis, 462, 468.

 Siebold, organ of, 290;
   on spermatophores, 500.

 Silk, 340;
   composition of, 346.

 Silk-fibre, composition of, 346.

 Silk-gland, 339;
   anal, 346;
   appendages of, 345;
   histology of, 334;
   modified coxal glands, 346;
   moulting of, 345.

 Silkworm, 331, 339, 366;
   amount of food eaten by, 608;
   functional salivary glands of, 331;
   mode of escape of, from its cocoon, 635;
   Polyphemus, 621;
   voracity of, 608.

 Simmermacher, on feet of insects, 113.

 Simulium, 668, 678;
   hypopharynx of, 78;
   wing-germs of, 129.

 Sinclair on double segments of Diplopods, 14.

 Sisyra, abdominal appendages of, 164.

 Sitaris, 691.

 Slug-worm, 188.

 Smell, experiments on, 269;
   organs of, 264, 271;
   physiology of, 268;
   sense of, 368.

 Smith, on lack of fore-tarsi in a moth, 102;
   jointed structure and lacinia of mandibles, 61;
   maxilla, 65;
   on scent-glands, 391.

 Sole, extensor, 116.

 Somites, 30.

 Sorby, on change of color in Aphides, 205.

 Sounds, 293.

 Specius, 585.

 Spengel, on color sense, 260.

 Spermatheca, 506.

 Spermatid, 498.

 Spermatocyte, 498.

 Spermatogonium, 498.

 Spermatophore cap, 499.

 Spermatophores, 497, 499.

 Spermatozoa, 497, 525;
   formation of, 498.

 Sperm-nucleus, 525.

 Sphinx, 456, 552;
   moulting of, 610.

 Sphinx ligustri, changes during metamorphosis, 646.

 Spilosoma, 391.

 Spindle, directive, 525.

 Spines, 187, 189;
   glandular, 190;
   helcodermatous, 612;
   locomotor, 612;
   moulting, 612;
   poisonous, 189.

 Spinneret, 342;
   of caterpillars, 75.

 Spinning apparatus, 340;
   at end of body, 346;
   glands, 339;
   process of, 340.

 Spinules, 189, 197.

 Spiracles, 437;
   types of, 438.

 Spiral thread, 444;
   absence of, 447;
   origin of, 448.

 Spraying apparatus, 370.

 Spring of Collembola, 551.

 Spuler, on pigments, 207, 208;
   on structure of scales, 195, 197.

 Squama, 123.

 Squamæ, 124.

 Squamula, 124.

 Squamule, 89.

 Stadia of metamorphosis, 594.

 Stage, carabidoid, 692;
   gastrula, 535;
   metabolous, 594;
   resting, 707;
   scarabæidoid, 692.

 Stages, ametabolous, 594.

 Stagmomantis carolina, embryo of, 584;
   hatching of, 584.

 Staphylinus, 61, 454.

 Stenobothrus sibiricus, swollen foretarsus in male, 113.

 Stenosternus, 101.

 Sternum, 87, 89.

 Stigmata, 437;
   closed, 460;
   closing apparatus of, 441;
   mesothoracic, 462;
   number of, in the embryo, 554;
   number of pairs of, 439, 461;
   position of, 440;
   vestigial, 460.

 Sting, bee’s, 172.

 Stipes, 63.

 Stokes, on the tænidia, 445;
   hairs in, 451.

 Stomach, chyle, 314, 325;
   absorbent cells of, 328;
   glandular cells of, 327;
   formation of, in imago fly, 681, 682;
   origin of, 569.

 Stomach-mouth, 309.

 Stomodæum, 537, 547, 569.

 Stomoxys, 446.

 Strauss-Dürckheim, on the heart, 397;
   on muscles of cockchafer, 213.

 Streak, embryonal, 531;
   primitive, 531.

 Streblopus, 101.

 Striæ of scales, 194, 202.

 Styles, abdominal, 176;
   of ovipositor, 167.

 Stylopidæ, 486;
   hypermetamorphosis of, 695.

 Stylops childreni, triungulin larva of, 695.

 Subgalea, 73.

 Submentum, 54, 63, 68, 69.

 Substance, fibrillar nerve, 238.

 Sucking stomach, 302, 305.

 Supra-anal plate, 181.

 Supra-œsophageal ganglion, 231.

 Supra-spinal vessel, 403.

 Suranal plate, 181.

 Surroundings, physical, 463.

 Sustainers of the pupa, 638.

 Swammerdam, on discovery of air-sacs, 456;
   on germs of wings, 128;
   on the mechanism of pupation, 661;
   on metamorphosis, 599;
   on rectal respiration, 463.

 Swimming, act of, 116.

 Symphyla, 18;
   characters of, 22.

 Synaptera, 27, 534, 594, 705.

 Syromastes, 372.


 Tabanus, 629;
   mouth-parts of, 79.

 Tænidia, 444;
   origin of, 447, 448.

 Talæporia, 634.

 Talocera, 57.

 Tanypus, 472.

 Tarsus, 96;
   changes of, from larva to imago of Lepidoptera, 655;
   reduction in or loss of, 101, 102.

 Taste, organs of, 281.

 Tegeticula yuccasella, 65, 66.

 Tegmina, 124.

 Tegula, 89, 124, 125.

 Telea polyphemus, amount of food eaten by, 608;
   cocoon of, 621;
   moulting of, 610;
   thorax of, 88.

 Teleas, hypermetamorphosis of, 703.

 Telephorus, 111, 538;
   tenent hairs of, 99.

 Tenebrio, 617.

 Tenent hairs, 99.

 Tentacle of maxilla, 65.

 Tentorium, 49.

 Tergum, 87.

 Termen, 122.

 Termes, abdomen, 162;
   origin of wings of, 140, 143.

 Termes flavipes, 64.

 Termopsis, 48, 64.

 Testes, 487, 495;
   incipient eggs in the germ of the testis, 504.

 Theory of incasement, 641.

 Thomas, on origin of scent-scales, 199.

 Thorax, 86, 95;
   gills on, 468.

 Thread-plate, 575.

 Thread, spiral, 444;
   origin of, 447, 448.

 Thyridium, 124.

 Thyridopteryx, 634.

 Thysanoptera, nymph of, 597.

 Thysanura, 72;
   cercopods of, 164;
   genital organs of, 486.

 Tibia, 96;
   formation of, in imago of Lepidoptera, 655.

 Tineid larva, wing-buds of, 652;
   wing-germs of, 129.

 Tipula, 629;
   flight of, 151, 152;
   thorax of, 91.

 Tissue, connective, 574.

 Tongue, 70.

 Torulus, 57.

 Tracheæ, 431, 442;
   capillary, 655;
   distribution of, 432;
   end-cells, 437;
   hairs in, 451;
   moulting of, 612;
   origin of, 553;
   origin of, in worms, 442;
   size of, 433;
   of wings, 144.

 Tracheal gills, of adult insects, 476;
   rectal, 463.

 Tracheal system, amphipneustic type, 462;
   apneustic type, 459;
   closed, 459;
   holopneustic type, 459;
   metapneustic type, 461;
   peripneustic type, 462;
   propneustic type, 462;
   reformation of, in metamorphosis, 683.

 Tracheoles, 126, 653.

 Tract, optic, 253.

 Trajectory made by wings, 150.

 Tribolium, 617.

 Trichodes, tracks of, 110.

 Trichogen, 188, 199, 366.

 Trichoptera, 469;
   appendages, 550;
   development of wings of, 142;
   embryology of, 537;
   eversible glands of, 375;
   haustellum of, 75;
   hypopharynx of, 74;
   pupal jaws of, 633;
   spinneret, 74.

 Trilobita, 5.

 Tritocerebrum, 231, 237.

 Triunguline larva, 100, 693, 695.

 Trochanter, 496;
   divided, 97.

 Trochantine, 95.

 Trogoderma, 617.

 Trophi, 54, 59, 549.

 Trouvelot, on the moulting fluid, 610;
   process of moulting of Telea, 610;
   spinning of cocoon by Telea, 621.

 Truxalis, 422.

 Tubercles, 187, 192.

 Tubes, egg, 501;
   ovarian, 501;
   urinary, 316, 317, 348, 353, 572.

 Tympanum, 289.

 Typhlocyba, 616.


 Uljanin, on the sexual cells of the honey bee, 582.

 Ungues, 96.

 Uranidin, 206.

 Urates, 352.

 Urea, 352.

 Urech, on pigments, 206, 208.

 Uric acid, 352.

 Urinary bladder, 351;
   tubes, 316, 317, 348;
   absent in Collembola, 353;
   excretions of, 351;
   origin of, 572;
   primitive number of, 352;
   solid contents of, 352.

 Urine, 206.

 Urite, 163.

 Uromeres, 163.

 Uro-patagia, 183.

 Urosome, 163.

 Urosternites, 163.

 Uterus, 507.

 Utriculi, 487.

 Uzel, on the embryology of Campodea, 51, 53;
   premandibular segment in Campodea, 51, 52.


 Vagina, 507.

 Valery-Mayet, on life-history of Sitaris, 691.

 Vallisneri, on the cremaster, 637.

 Valvule, cardiac, 312;
   proventricular, 313;
   pyloric, 315.

 Van Bemmelen, on colors, 208.

 Vanessa antiopa, 638.

 Vanessa io, 381.

 Vanessa urticæ, before pupation, 644;
   changes in nervous system during its metamorphosis, 646.

 Van Rees, on post-embryonic changes of Muscidæ, 673, 674, 676, 680,
    683, 684.

 Vas deferens, 496;
   origin of, 579.

 Vasa deferentia, 496;
   origin of, 579.

 Vayssière, on lingua of Ephemera, 73.

 Veins of wings, 121, 144.

 Velum penis, 181.

 Venomous glands, 358.

 Vent, 319.

 Ventriculus, 314.

 Vermipsylla, tongue of, 77.

 Verson, on serially arranged dermal glands, 366;
   vestigial stigmata, 460.

 Vesicles, air, 456;
   use of, 457;
   frontal, 621;
   of head of semipupal fly, 677.

 Vespa crabro, 217;
   olfactory organs of, 276.

 Vessel, dorsal, 397;
   urinary, 316, 348.

 Vestigial tracheæ, 460.

 Viallanes, on brain, 231;
   head segments, 51.

 Vision, mode of, by compound eyes, 256;
   by simple eyes, 255.

 Vitelline membrane, 520.

 Viviparous insects, 515.

 Volucella, thorax of, 91.

 Vosseler, on fœtid glands, 369.


 Wagner, on the circulation, 419.

 Walking, mechanics of, 103, 106.

 Walter, on epipharynx of moths, 44;
   on hypopharynx of moths, 75.

 Wasps, taste-organs of, 277, 286.

 Wax, of butterfly, 364;
   of caterpillar, 364;
   of saw-fly larva, 364.

 Wax-glands, 361, 364.

 Weevil, embryology of, 538;
   bean and pea, hypermetamorphosis of, 700.

 Weismann’s discovery of imaginal buds, 599, 643, 650;
   on formation of imago, 67;
   vesicle of semipupal fly, 677;
   on tracheæ, 434;
   on origin of tracheæ, 447;
   on origin of wings, 127, 129;
   theory of histolysis, 643.

 West, Tuffen, on feet of fly, etc., 100;
   on walking, 99.

 Westwood, on head of Hymenoptera, 55.

 Wheeler, on embryonic abdominal appendages, 550;
   on the homologies of the ovipositor, 167;
   homology and primitive number of urinary tubes, 354, 355;
   on œnocytes, 423;
   on pleuropodia, 476, 550, 551;
   on the premandibular segment, 51;
   on structure of chorion, 521.

 Wielowiejski, on blood tissue, 408;
   egg-tubes, 502;
   fat-body, 419;
   phosphorescence, 424;
   tracheæ and their endings, 436.

 Williston, on anal glands, 372.

 Will, on organs of taste, 282.

 Wing-buds, or discs, 127, 129;
   rods, 146;
   sheaths, 124.

 Wingless insects, 598.

 Wings, 120;
   buds of, 664;
   cells, 121;
   circulation of blood in, 410;
   development of tracheæ of, 144;
   development of veins of, 144;
   embryonic development of, 126;
   folding of, 154, 156;
   imaginal buds of, 653;
   mechanism of, 153, 156;
   origin of, 138;
   primitive origin of, 137;
   as respiratory organs, 461;
   rudimentary, ground-membrane of, 136;
   spreading of, 155;
   theory of, 144;
   tracheæ in, 122;
   veins of, 121.

 Wistinghausen, von, on tracheal endings, 436, 447.

 Witlaczil, on honey dew, 364.

 Wood-Mason, on gills of Paraponyx, 470;
   on Scolopendrella, 19, 22.


 Xiphidium, embryo of, 534;
   indusium of, 534, 535.


 Yersin, on results of section of commissures, 245.

 Yolk, amount of, 524, 529;
   membrane, 520;
   segmentation of, 526;
   cells, 562;
   ridge, median, 563.


 Zaitha, 431;
   pleuropodia of, 551.

 Zone, annular, 653.

-----

Footnote 1:

  Zool. Anzeiger, xvi, 1893, pp. 271–5.

Footnote 2:

  On the morphology of the Myriopoda, Proc. Amer. Phil. Soc. 1883, pp.
  197–209.

Footnote 3:

  Morphology and classification of the Pauropoda; also American
  Naturalist, 1897, p. 410.

Footnote 4:

  The term which we proposed for this hypothetical ancestor of insects,
  “Leptus-like” or “Leptiform,” was an unfortunate one, since the name
  Leptus was originally given to the six-legged larva of a mite
  (Trombidium), the origin of the mites and other Arachnida being
  entirely different from that of the myriopods and insects.

Footnote 5:

  Proc. Bost. Soc. Nat. Hist., xvi, 1873, p. 3.

Footnote 6:

  American Naturalist, May, 1880, pp. 375, 376.

Footnote 7:

  Zoologische Anzeiger, Bd. xx, 1897, pp. 125 and 129. He also states
  that Campodea resembles the myriopods, especially Geophilus, in the
  primitive band at first lying on the surface of the yolk, and in the
  absence of an amniotic cavity; also before hatching the abdomen is
  pressed against the thorax, as in myriopods.

Footnote 8:

  “Scolopendrella has very remarkable antennæ; they may be compared each
  to a series of glass cups strung upon a delicate hyaline and
  extensible rod of uniform thickness throughout; so that, like the body
  of the creature, they shrink enormously when the animal is irritated
  or thrown into alcohol, and they then possess scarcely two-thirds the
  length they have in the fully extended condition, their cup-like
  joints being drawn close together, one within the other. Peripatus,
  Japyx, many (if not all) Homoptera, and the S. Asiatic relatives of
  our common Glomeris have all more or less extensible antennæ.”
  (Wood-Mason, Trans. Ent. Soc., London, 1879, p. 155.)

Footnote 9:

  Lassaigne gave it the name of entomoline.

Footnote 10:

  Miall and Denny ex Krukenberg; Kolbe gives the formula as
  C_{9}H_{15}NO_{6} or C_{18}H_{15}NO_{12}. As the result of his recent
  researches, Krawkow (Zeits. Biol., xxix, 1892, p. 177) states that the
  chemical composition of chitin may prove to be somewhat variable.

Footnote 11:

  On allowing portions of a locust, a piece of the integument of
  Limulus, a scorpion, and a myriopod to soak for a month in white
  potash, neither were dissolved or affected by the reagent.

Footnote 12:

  We may add, while correcting the proofs of this book, that the
  important summary, by Uzel, of his work on the embryology of Campodea
  appears in the Zoologischer Anzeiger for July 5, 1897. He observes
  that the premandibular segment in the embryo is very distinct, and
  that the two projections arising from it persist in the adult.
  “Campodea is now the first example where these appendages are present
  in the sexually mature insect and function as constituents of the
  completed mouth parts. I propose for these hitherto overlooked
  structures the name of intercalary lobes.” They each form a slightly
  developed chitinous lobe covering a gap between the base of the labium
  and the fused external lobe and palpus of the first maxillæ (which are
  inclined near the labium) in place of the mandibles which have sunken
  inward. Uzel also homologizes these appendages with two similar
  projections (Höcker) observed in the embryo of Geophilus by Zograf to
  be situated in front of the mandibles. Heymons has also detected this
  segment in the embryo of Lepisma.

Footnote 13:

  While these pages are still in type, we may add, in confirmation of
  this view, that Uzel states, from his researches on the embryology of
  Campodea, that the maxillary tergites of the embryo only slightly
  share in building up the tergal region (occiput) of the head, but that
  they form the genæ of the maxillary segments. (Zool. Anzeiger, July 5,
  1897, p. 235.)

Footnote 14:

  Miall and Denny in their work on the cockroach, in describing the
  labium, remark: “The upper edge is applied to the occipital frame, but
  is neither continuous with that structure nor articulated thereto. By
  stripping off the labium upwards it may be seen that it is really
  continuous with the chitinous integument of the neck” (p. 95). This
  is, we think, a mistaken view, as proved by the embryology of the
  Odonata and of Nematus. Our statements on this subject were first
  published in part in 1871, and more fully in the third Report, U. S.
  Ent. Commission, 1883, pp. 284, 285. We also stated that all the gular
  region of the head probably represents the base of the primitive
  second maxillæ.

Footnote 15:

  After we had arrived at this conclusion, and written the above lines,
  we received the Zoologischer Anzeiger for March 29, 1897, in which Dr.
  N. Léon publishes the same view, stating that each side of the
  submentum is the homologue of the cardo, and each side of the mentum
  corresponds to the stipes of a single maxilla (p. 74).

Footnote 16:

  Miall and Denny were the first to homologize the paraglossæ with the
  galea and lacinia, showing the complete resemblance of the second
  maxillæ to the first pair, remarking that “the homology of the labium
  with the first pair of maxillæ is in no other insects so distinct as
  in the Orthoptera.” We have also independently arrived at a similar
  conclusion, but believe that the mentum corresponds to the first
  maxillary cardo, and the palpifer to the first maxillary stipes, the
  sclerite of each maxilla being fused to form the base of the labium,
  _i.e._ the unpaired mentum and submentum.

Footnote 17:

  Uzel states that what is regarded as the ligula of Campodea is formed
  from the sternite of the first maxillary segment; while the two parts
  regarded as paraglossæ grow out from the sternite of the mandibular
  segment, and these three structures together he regards as the
  hypopharynx. (Zool. Anzeiger, July 5, 1897, p. 234.)

Footnote 18:

  See, also, Breithaupt, Ueber die Anatomie und die Functionen der
  Bienenzunge, 1886. It confirms and extends Cheshire’s work.

Footnote 19:

  Cholodkowsky, Zool. Anz., ix, p. 615; x, p. 102.

Footnote 20:

  Zool. Anz., ix, p. 711.

Footnote 21:

  Ent. Amer., v, p. 110, Pl. II, Fig. 7.

Footnote 22:

  In his account of his studies on the locomotion of insects, De Moor
  states that he obtained the track of each of the feet in different
  colors by coating them with different pigments; the insect, as it
  moved, left its track on a strip of paper. (Archives de Biologie,
  Liège, 1890.)

Footnote 23:

  Carlet and also De Moor (1890) confirm Graber’s statement that in
  beetles the first and last appendages on the same side are in contact
  with the ground, while the middle one is raised. On the other side of
  the body the middle appendage is on the ground and the first and last
  one raised.

Footnote 24:

  Trans. Amer. Ent. Soc. xx, p. 168.

Footnote 25:

  Proc. Ent. Soc. London. Feb. 19, 1896. Heymons also shows that the
  germs of the elytra of the larva of _Tenebrio molitor_ in the prepupal
  stage are like those of other insects. (Sitzungs-Ber. Gesell. natur f.
  Freunde zu Berlin, 1896, pp. 142–144.)

Footnote 26:

  Zur Entwickelungsgeschichte und Reproductionsfähigkeit der
  Orthopteren. Von Vitus Graber. Sitzungsberichte d. math.-naturw.
  Classe der Akad. d. Wissensch., Wien. Bd. lv, Abth. i, 1867; also Die
  Insekten.

Footnote 27:

  On the transformations of the common house fly, by A. S. Packard, Jr.
  Proceedings Boston Society of Natural History, vol. xvi, 1874. See Pl.
  3, Figs. 12_a_, 12_b_.

Footnote 28:

  See our Guide to the Study of Insects, p. 66, Figs. 65, 66.

Footnote 29:

  Our Common Insects, 1873, p. 171.

Footnote 30:

  Compare the observations of Palmén, Gerstäcker, Vayssière, and others.

Footnote 31:

  Beiträge zur Kenntniss der Termiten. Jenaische Zeitschrift für
  Naturwissenchaft, Bd. ix, Heft 2, p. 253, 1875. Compare, however,
  Palmén’s Zur Morphologie des Tracheensystems, Helsingfors, 1877,
  wherein he opposes Müller’s view and adopts Gegenbaur’s. See p. 8,
  footnote.

Footnote 32:

  Pancritius, who also adopted Müller’s views, lays much stress on the
  fact that in larvæ of some orders the tracheæ do not enter the
  rudimentary wings until the end of larval life, and hence the wings
  have not originated from tracheal gills, but were originally “perhaps
  only protective covers for the body.”

Footnote 33:

  Reproduced from the author’s remarks in Third Report U. S. Ent.
  Commission, pp. 268–271, 1883.

Footnote 34:

  Von Lendenfeld, however, points out the fact that Straus-Durckheim
  proved that the wings of beetles are moved by a complicated system of
  numerous muscles. “In the Lepidoptera I have never found less than six
  muscles to each wing, as also in the Hymenoptera and Diptera.” “The
  motions of the wings of Libellulidæ are the combined working of
  numerous muscles and cords, and of a great number of chitinous pieces
  connected by joints.”

Footnote 35:

  Heymons, however, denies that the so-called cerci in Odonata are such,
  and claims that they are the homologues of the “caudal processes”
  (superior terminal appendages of Calvert), because they arise from the
  tenth abdominal segment.

Footnote 36:

  Amer. Nat., iv, December, 1870.

Footnote 37:

  Handbuch der Zoologie, p. 17, 1863, Fig. 162.

Footnote 38:

  In my account of the anatomy of _Melanoplus spretus_, 1st Report U. S.
  Entomological Commission, p. 259, I have called these the infra-anal
  flaps or _uro-patagia_.

Footnote 39:

  It has been suggested to us by A. A. Packard that the power possessed
  by insects of transporting loads much heavier than themselves is
  easily accounted for, when we consider that the muscles of the legs of
  an insect the size of a house-fly (¼ inch long), and supporting a load
  399 times its own weight, would be subjected to the same stress (per
  square inch of cross-section) as they would be in a fly 100 inches
  long of precisely similar shape, that carried only its own weight;
  from the mechanical law that, while the weight of similar bodies
  varies as the cube of the corresponding dimensions, the area of
  cross-section of any part (such as a section of the muscles of the
  leg) varies only as the square of the corresponding dimensions. In
  short, the muscles of a fly carrying this great proportional weight
  undergo no greater tension than would be exerted by a colossal insect
  in walking.

Footnote 40:

  This has been shown to be the case by Michels, who states that each
  commissure is formed of three parallel bundles of elementary
  nerve-fibres, which pass continuously from one end of the ventral or
  nervous cord to the other. “The commissures take their origin neither
  out of a central punctsubstanz (or marksubstanz), nor from the
  peripheral ganglion-cells of the several ganglia, but are mere
  continuations of the longitudinal fibres which decrease posteriorly in
  thickness, and extend anteriorly through the commissures, forming the
  œsophageal ring, to the brain.”

Footnote 41:

  The following extract from Newton’s paper shows, however, that the
  infra or subœsophageal ganglion, according to Faivre, has the power of
  coördinating the movements of the body; still, it seems to us that the
  brain is primarily concerned in the exercise of this power, as the
  nerves from the subœsophageal ganglion supply only the mouth-parts.
  “The physiological experiments of Faivre in 1857 (Ann. des Sci. Nat.
  tom. viii, p. 245), upon the brain of Dyticus in relation to
  locomotion, are of very considerable interest, showing, as they appear
  to do, that the power of coördinating the movements of the body is
  lodged in the infraœsophageal ganglion. And such being the case, both
  the upper and lower pairs of ganglia ought to be regarded as forming
  parts of the insect’s brain.”—Quart. Jour. Micr. Sc., 1879, p. 342.

Footnote 42:

  The arthropod protocerebrum probably represents the annelid brain
  (supraœsophageal ganglion). The antennal segment (deutocerebrum), with
  the premandibular (intercalary) segment (tritocerebrum) originally
  postoral, have, as Lankester suggests, in the Arthropoda moved forward
  to join the primitive brain. See Wheeler, Journ. Morphology, Boston,
  viii, p. 112.

Footnote 43:

  Viallanes’ assertion that the instincts of the horse-flies and
  dragon-flies are “lower” than those of the locusts, may, it seems to
  us, well be questioned.

Footnote 44:

  A. S. Packard, Experiments on the vitality of insects, Psyche, ii, 17,
  1877.

Footnote 45:

  Waterhouse, Trans. Ent. Soc., London, 1889, p. xxiv.

Footnote 46:

  J. Müller, Physiology of the Senses. Trans. by Baly, copied from
  Lubbock, p. 176.

Footnote 47:

  Hauser here uses the word _taster_, but this means palpus or feeler.
  It is probably a _lapsus pennæ_ for teeth (Kegeln).

Footnote 48:

  In 1870 I observed these sense-pits in the antennæ and also in the
  cercopoda of the cockroach (_Periplaneta americana_). I counted about
  90 pits on each cercus. They are much larger and much more numerous
  than similar pits in the antennæ of the same insect. I compared them
  to similar pits in the antennæ of the carrion-beetles, and argued that
  they were organs rather of the smelling than hearing. (Amer. Nat.,
  iv., Dec. 1870.) Organs of smell in the flies (Chrysopila) and in the
  palpi, both labial and maxillary, of Perla were described in the same
  journal (Fig. 270). Compare Vom Rath’s account of the organs in the
  cercopods of Acheta (Fig. 271); also the singular organ discovered by
  him on the end of the palpus of butterflies, in which a number of
  hair-like rods (_sh_) are seated on branches of a common nerve (_n_,
  Fig. 272).

Footnote 49:

  Forel, however (_Recueil Zoologique Suisse_, 1887), denies that these
  tympanic organs are necessarily ears, and thinks that all insects are
  deaf, with no special organs of hearing, but that sounds are heard by
  their tactile organs, just as deaf-mutes perceive at a distance the
  rumbling of a carriage. But he appears to overlook the fact that many
  Crustacea, and all shrimps and crabs, as well as many molluscs, have
  organs of hearing. The German anatomist Will believes that insects
  hear only the stridulation of their own species. Lubbock thinks that
  bees and ants are not deaf, but hear sounds so shrill as to be beyond
  our hearing.

Footnote 50:

  Weismann, Die nachembryonale Entwicklung der Musciden. Zeitschr. für
  wissen. Zoologie, xiv, p. 196, 1864.

Footnote 51:

  Plateau (1877) states that the digestive fluid of insects, as well as
  of Arachnids, Crustaceans, and Myriopods, has no analogy with the
  gastric juice of vertebrates; it rather resembles the pancreatic sugar
  of the higher animals. The acidity quite often observed is only very
  accessory in character, and not the sign of a physiological property.
  “Farther, I have found it in insects; Hoppe-Seyler has demonstrated in
  the Crustacea, and I have proved in the spiders, that the ferment
  causing the digestion of albuminoids is evidently quite different from
  the gastric pepsine of vertebrates; the addition of very feeble
  quantities of chlorhydric acid, far from promoting its action, retards
  or completely arrests it.” (Bull. Acad. roy. Belgique, 1877, p. 27.)

Footnote 52:

  The word _grès_ we translate as the layer of gum. Not sure of the
  English equivalent for _grès_, I applied to Dr. L. O. Howard, U. S.
  Entomologist, who kindly answers as follows: “I have consulted Mr.
  Philip Walker, a silk expert, who writes me the following paragraph:
  ‘_Grès_, as I understand it, is the gum of the silk fibre, hence the
  French name for raw silk, _grèye_, which is in distinction to the silk
  that has been boiled out in soap after twisting, or throwing, as it is
  called. As I understand it, the silk fibre is composed of the _grès_
  and fibroin. The former is soluble in alkali, like soap water, and the
  latter is not.’” While Blanc considers the _grès_ as the product of a
  special secretion of the wall of the reservoir, Gilson regards its
  production as simultaneous with that of the silk or of the fibroin
  (_l.c._ 1893, p. 74).

Footnote 53:

  On cytological differences in homologous organs. Report 63d meeting of
  British Assoc. Adv. Sc. for 1893. 1894. p. 913.

Footnote 54:

  See also Giard, Bull. Soc. Ent. France, p. viii, 1894.

Footnote 55:

  “The contents of the Malpighian tubules may be examined by crushing
  the part in a drop of dilute acetic acid, or in dilute sulphuric acid
  (10 per cent). In the first case a cover-slip is placed on the fluid,
  and the crystals, which consist of oblique rhombohedrons or derived
  forms, are usually at once apparent. If sulphuric acid is used, the
  fluid must be allowed to evaporate. In this case they are much more
  elongated, and usually clustered. The murexide reaction does not give
  satisfactory indications with the tubules of the cockroach.” (Miall
  and Denny, The cockroach, p. 129, footnote.)

Footnote 56:

  “There is a curious analogy between the excretory organs of these
  insects and the mesonephros of some vertebrates, where a second,
  third, etc., generation of tubules is added to the primitive metameric
  series. When the embryonic number of Malpighian vessels persists in
  insects, the demand for greater excreting surface is supplied by a
  lengthening of the individual vessels.”

Footnote 57:

  For the mode of adhesion of Cynips eggs, see Adler in Deutsche Ent.
  Zeits. 1877, p. 320.

Footnote 58:

  Mercaptan is a mercury, belonging to a class of compounds analogous to
  alcohol, having an offensive garlic odor. Methyl mercaptan is a highly
  offensive and volatile liquid.

Footnote 59:

  Embryonic or temporary glands, the “pleuropodia” of Wheeler, viz. the
  modified first pair of abdominal legs, occur in Œcanthus, Gryllotalpa,
  Xiphidium, Stenobothrus, Mantis (occasionally a pair on the second
  abdominal segment, Graber); Blatta, Periplaneta, Cicada, Zaitha,
  Hydrophilus, Acilius, Melolontha, Meloë, Sialis, Neophylax. (See
  Wheeler, Appendages of the First Abdominal Segment, etc., 1890.)

Footnote 60:

  These midges owe their phosphorescence to bacteria in their bodies
  during disease.

Footnote 61:

  Untersuchungen zur Anatomie und Histologie der Tiere, 1884, p. 72.

Footnote 62:

  Zelle und Gewebe, 1885, p. 43. (See also our p. 217.)

Footnote 63:

  Studien über die Lampyriden, Zeits. für wiss. Zool., xxxvii, 1882.
  Both Wielowiejski and M. Wistinghausen have completely disproved the
  view of Schultze, that the tracheæ end in star-like cells, where
  respiration takes place, as the “star-like cells” are simply net-like
  expansions of the peritoneal membrane of the tracheæ.

Footnote 64:

  The following summary compiled from Krancher, is translated, with some
  minor changes, from Kolbe’s work.

Footnote 65:

  Miall and Denny state that in the cockroach the abdominal spiracles
  are permanently open, owing to the absence of a valve, but
  communication with the tracheal trunk may be cut off at pleasure by an
  internal occluding apparatus.

Footnote 66:

  Zur Entwicklungsgeschichte der Biene, Zeitschr. wissens. Zoologie, xx,
  p. 519, 1870.

Footnote 67:

  Die Entwicklung der Dipteren im Ei, Zeitschr. wissens. Zoologie, xiii,
  1863.

Footnote 68:

  Amer. Naturalist, May, 1886, p. 438.

Footnote 69:

  Zeitschr. wissens. Zoologie, xl, 1884, Taf. xix, Fig. 8, _T_.

Footnote 70:

  _Science_, 1893, pp. 44–46.

Footnote 71:

  Art. Thorax, Todd’s Cycl. of Anat. and Phys.

Footnote 72:

  The mesothoracic stigmata are open in Carabus, Potamophilus, Elmis,
  Macronychus, Buprestis, Elater, Lampyris, Lycus, Triphyllus,
  Eucinetus, Dascillus, Psephenus, Ergates, Micralymna, and probably
  many others. The metathoracic stigmata are open in Lycus and Elmis.

Footnote 73:

  In the Hymenoptera the two pairs on the meso- and metathoracic
  segments are open in the Aculeata, also in the Siricidæ, among which
  sometimes that on the third segment is closed. In Pimpla and
  Microgaster (fully grown larvæ) only the mesothoracic stigmata are
  open.

  Palmén adds that most dipterous larvæ are amphipneustic; Cecidomyia,
  the Mycetophilidæ, Bibionidæ, and Stratiomys are typically
  peripneustic. (p. 92.)

  Moreover, a single insect, as Sialis, may be apneustic as a larva,
  peripneustic as a pupa, and holopneustic in the imago stage.

Footnote 74:

  Mr. J. W. Folsom, who has made the accompanying sketch of the nymph of
  _Euphæa splendens_ in the Cambridge Museum, finds only seven pairs of
  gills, there being no traces of them on segments 1, 9, and 10. A stout
  trachea, he writes us, enters the base of each gill, and subdivides
  into several long branches, which course along the periphery. Hagen in
  his original account said there were eight pairs on segments 1–8
  respectively.

Footnote 75:

  Harris, Correspondence, p. 226, Pl. III., Fig. 7.

Footnote 76:

  Nusbaum’s view has been questioned by Heymons, who, from his studies
  on the embryology of the cockroach (Periplaneta and Phyllodromia),
  Forficula, and Gryllus, concludes that the ectodermal ends of the
  sexual outlets owe their origin to an unpaired median hypodermal
  invagination, and that it is quite doubtful whether the ectodermal
  portions of the sexual passages of insects were ever paired (p. 104).
  On the other hand he appears, even throwing out the case of Ephemera,
  to have overlooked Nassonow’s discovery of paired outlets in the young
  of Lepisma.

Footnote 77:

  Acta Acad. German., xxxiii, 1867, No. 2, p. 81. Quoted by Dr. Sharp,
  Insecta, p. 142.

Footnote 78:

  Journ. Morph., iii, Boston, pp. 299, 300.

Footnote 79:

  Proc. Boston Soc. Nat. Hist., xi, pp. 88, 89.

Footnote 80:

  In the following general account of the embryology of insects, I have
  closely followed the admirable arrangement and description of
  Korschelt and Heider, in their Lehrbuch der vergleichenden
  Entwicklungsgeschichte der wirbellosen Thiere, pp. 764–846, often
  translating their text literally, though not omitting to state the
  results of other writers.

Footnote 81:

  Korschelt and Heider state that no cellular embryonal membranes are
  present in Synaptera, Uljanin finding none in the Podurids. In the
  embryo of _Isotoma walkerii_ we, however, observed a membrane which we
  compared to the larval skin of many Crustacea, and both Sommer and
  Lemoine have detected in eggs of the same group a cuticular larval
  skin which is provided with spines for rupturing the chorion. The
  amnion is also wanting in Proctotrupids (Ayers), and is rudimental in
  Muscidæ (Kowalevsky, Graber), in viviparous Cecidomyidæ, according to
  Metschnikoff, who also states that in certain ants of Madeira the
  envelopes are represented only by a small mass of cells in the dorsal
  region.

Footnote 82:

  In Diptera the stomodæum may be dorsal, Dr. Pratt tells us.

Footnote 83:

  Will (Aphis) and also Cholodkowsky’s statement (Blatta), as well as
  Balfour and Schimkewitch’s statements that the brain is at first
  disconnected from the ventral cord, are apparently erroneous.

Footnote 84:

  The description perhaps applies not only to the cockroaches, but, as
  seen from the similar but fragmentary notices of Heider and of Wheeler
  on the Coleoptera, may be common to insects in general.

Footnote 85:

  Report on the Rocky Mountain locust, etc. Ninth Annual Report U. S.
  Geol. and Geogr. Survey of the Territories for 1875, pp. 633, 634.

Footnote 86:

  Orthoptera Europæa, 1853, p. 37.

Footnote 87:

  In his Für Darwin (1863), Fritz Müller gives his reasons for the
  opinion that the so-called “complete metamorphosis” of insects was not
  inherited from the primitive ancestor of all insects, but acquired at
  a later period.

Footnote 88:

  For further details see the 1st Report of the U. S. Entomological
  Commission, 1878, pp. 279–281.

Footnote 89:

  See Köppen ueber die Heuschrecken in Südrussland, 1862, pp. 22, 23.

Footnote 90:

  In Samouelle’s The Entomologist’s Useful Compendium, 1819. See
  Westwood’s Class. Insects, i, p. 2; Leach’s Ametabolia comprised the
  Thysanura (Synaptera) and the lice.

Footnote 91:

  From the Greek μανός, scanty; μεταβολή, change.

Footnote 92:

  Greek, ἤρεμα, quiet; μεταβολή, change.

Footnote 93:

  At the same date (March, 1869) we independently suggested that the
  insects had originated from some form like the hexapodous young of
  Pauropus and Podura. In November, 1870, we suggested that the
  Thysanura and the hexapodous Leptus may have descended from some
  Peripatus-like worm. Afterwards (1871) we proposed for the ancestral
  form the term _leptiform_, which was later abandoned for Brauer’s term
  _Campodea-form_.

Footnote 94:

  Amer. Naturalist, i, p. 85, 1867.

Footnote 95:

  First Rep. U. S. Ent. Commission, p. 281–283.

Footnote 96:

  Trans. Ent. Soc. London, iii, p. xv. See also Ashton, R. J., Trans.
  Ent. Soc. London, iii, 1841–43, pp. 157–159.

Footnote 97:

  Proc. Bost. Soc. Nat. Hist., x, 1866, p. 283.

Footnote 98:

  See Max Braun’s article entitled Ueber die histologischen Vorgange bei
  der Hautung von _Astacus fluviatilis_, with a full bibliography, in
  Semper’s Arbeiten aus dem Zool. zoot. Institut in Würzburg, ii, pp.
  121–166. Also Semper’s Animal Life, p. 20. Trouvelot also discovered
  the moulting fluid. (Amer. Nat., i, p. 37.)

Footnote 99:

  American Naturalist, xvii, May, 1883, pp. 547, 548.

Footnote 100:

  Le Pelletier. A. M. L., Bulletin de la Société Philomathique, Paris,
  April, 1813.

Footnote 101:

  Heineken, Carl. Observations on the reproduction of the members in
  spiders and insects. (Zool. Journ., 1829, vi, pp. 422–432.)

Footnote 102:

  Bees and Bee-keeping, pp. 21, 22.

Footnote 103:

  Butterflies, their structure, changes, and life-histories. New York,
  1881, pp. 37–42. Butterflies of the Eastern United States and Canada,
  1888, 1889. Also, Frail children of the air, 1895, pp. 232, 233 _a_.
  Dr. Chapman, however, finds that this piece in micropupæ has no
  connection whatever with the head or eye, but belongs rather with the
  prothoracic segment. (Trans. Ent. Soc. London, 1893, p. 102.) We have
  been able to confirm his statements, but still this piece is peculiar
  to the pupal state.

Footnote 104:

  Rep. Ent. U. S. Dept. Agr., 1879, pp. 228, 229, Pl. IV, Fig. 4.

Footnote 105:

  Monograph of bombycine moths, Pt. I, 1897. Figs. 24, 28, 29, 33, 34,
  40, 77.

Footnote 106:

  Amer. Naturalist, xii, pp. 379–383.

Footnote 107:

  _Hybocampa milhauseni_, Dr. Chapman tells me, has a pupal spine
  (imperfectly present in Cerura) with which it cuts out a lid of the
  cocoon.

Footnote 108:

  Riley’s Report for 1892, p. 203.

Footnote 109:

  Philosophy of the pupation of butterflies, and particularly of
  Nymphalidæ, by Charles V. Riley. (Proc. Amer. Assoc. Adv. Science,
  xxviii, Saratoga Meeting, August, 1880, pp. 455–463.)

Footnote 110:

  The homology of the suranal plate of the larva with the cremaster of
  the pupa, established by Riley in 1880, is also affirmed by Jackson
  (1888) and by Poulton, and for some years we have been satisfied that
  this is the correct view; Professor Hatchett-Jackson discovered it, he
  states, in 1876.

Footnote 111:

  In his remarkable studies on the morphology of the Lepidoptera,
  Professor W. Hatchett-Jackson states his belief that Riley’s homology
  of the sustentors with the soles or plantæ of the anal prolegs, and
  the sustentor ridges with their limbs, is wrong, and that the
  eminences on either side the anal furrow, or the “anal prominences,”
  as they are termed by Riley, represent the prolegs, and that the
  sustentor ridges and sustentors are probably peculiar developments of
  the body of the 10th somite, found only in some Lepidoptera. From our
  examination of pupa of different families of moths, we are satisfied
  that Jackson’s view is the correct one. We have not found the
  sustentors and their ridges in the pupæ of the more generalized moths,
  but the vestiges of the anal legs are almost invariably present, their
  absence in the pupa of Nola and Harrisina being noteworthy.

Footnote 112:

  We copy from Kirby and Spence their abstract of Herold’s conclusions:
  “The successive skins of the caterpillar, the pupa-case, the future
  butterfly, and its parts or organs, except those of sex, which he
  discovered in the newly excluded larva, do not preëxist as germs, but
  are formed successively from the _rete mucosum_, which itself is
  formed anew upon every change of skin, from what he denominates the
  _blood_, or the chyle after it has passed through the pores of the
  intestinal canal into the general cavity of the body, where, being
  oxygenated by the air-vessels, it performs the nutritive functions of
  blood. He attributes these formations to a _vis formatrix_ (bildende
  Kraft).

  “The caul or epiploon (_fett-masse_), the _corps graisseux_ of
  Réaumur, etc., which he supposes to be formed from the superfluous
  blood, he allows, with most physiologists, to be stored up in the
  larva, that in the pupa state it may serve for the development of the
  imago. But he differs from them in asserting that in this state it is
  destined to two distinct purposes: first, for the production of the
  muscles of the butterfly, which he affirms are generated from it in
  the shape of slender bundles of fibres; and, secondly, for the
  development and nutrition of the organs formed in the larva, to effect
  which, he says, it is dissolved again into the mass of blood, and
  being oxygenated by the air-vessels, becomes fit for nutrition, whence
  the epiploon appears to be a kind of concrete chyle.”
  (Entwickelungsgeschichte der Schmetterlinge, pp. 12–27.) It seems that
  Herold was right in deriving the pupa and imago from the hypodermis
  (his _rete mucosum_), but wrong in denying that the germs did not
  preëxist in the young caterpillar, and wrong in supposing that the
  latter originated from the blood, also in supposing that the muscles
  owe their origin to the fat-body. Swammerdam, and also Kirby and
  Spence, were correct in supposing that the imago arose from “germs” in
  the larva, though wrong in adopting the “emboîtement” theory.

Footnote 113:

  In the regions where the imaginal buds are not present (dorsal aspect
  of the prothorax, and abdomen), the epithelium (hypodermis) may
  proliferate independently of these buds.

Footnote 114:

  We shall translate portions and, when the text allows, make an
  abstract of parts of Gonin’s clear and excellent account, often using
  his own words.

Footnote 115:

  C. Herbert Hurst, The Pupal Stages of Culex.

Footnote 116:

  Lowne on the Blow-fly, new edit., pp. 2, 41, Fig. 7.

Footnote 117:

  Miall, Natural History of Aquatic Insects, pp. 136–138. Also Trans.
  Linn. Soc. London, V, Sept., 1892.

Footnote 118:

  This account is translated from Korschelt and Heider, with some
  omissions and slight changes.

Footnote 119:

  Westwood in his excellent account of this group remarks: “Hence, as
  well as from the account given by Jurine, it is evident that the pupa
  of the Stylops is enclosed in a distinct skin, and is also in that
  state enveloped by the skin of the larva, contrary to the suggestion
  of Mr. Kelly.” (Class. Insects, II. 297.) This is all we know about
  the supernumerary larval stages.

Footnote 120:

  Some facts towards a life history of _Rhipiphorus paradoxus_. Annals
  and Magazine of Natural History for October, 1870.

------------------------------------------------------------------------




                          TRANSCRIBER’S NOTES


 ● P. 316, changed “abdominal cells” to “absorbent cells”.
 ● Silently corrected typographical errors and variations in spelling.
 ● Archaic, non-standard, and uncertain spellings retained as printed.
 ● Enclosed italics font in _underscores_.
 ● Enclosed bold font in =equals=.
 ● Superscripts are denoted by a caret before a single superscript
     character or a series of superscripted characters enclosed in curly
     braces, e.g. M^r. or M^{ister}.
 ● Subscripts are denoted by an underscore before a series of
     subscripted characters enclosed in curly braces, e.g. H_{2}O.