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                 The Cambridge Manuals of Science and
                              Literature



                      THE LIFE-STORY OF INSECTS



                      CAMBRIDGE UNIVERSITY PRESS
                      London: FETTER LANE, E.C.
                          C.F. CLAY, MANAGER

                            [Illustration]

                    Edinburgh: 100, PRINCES STREET
             London: H.K. LEWIS, 136, GOWER STREET, W.C.
            WILLIAM WESLEY & SON, 28, ESSEX STREET, STRAND
                       Berlin: A. ASHER AND CO.
                       Leipzig: F.A. BROCKHAUS
                     New York: G.P. PUTNAM'S SONS
             Bombay and Calcutta: MACMILLAN AND CO., LTD.




[Illustration: _Frontispiece._ Transformation of a Gnat (_Culex_).
    Magnified 5 times.
A.  Larva. (The head is directed downwards and the tail-siphon with
    spiracle points upwards to the surface of the water.)
B.  Pupal Cuticle from which the Imago is emerging. (The pair of
    'respiratory trumpets' on the thorax of the pupa are conspicuous. The
    wings of the Imago are crumpled, and the hind feet are not yet
    withdrawn.)
C.  Adult Gnat. Female.]



                            [Illustration]



                            THE LIFE-STORY

                              OF INSECTS



                                  BY

                          GEO. H. CARPENTER

                  Professor of Zoology in the Royal
                      College of Science, Dublin

                              Cambridge:
                       at the University Press
                              New York:
                          G.P. Putnam's Sons
                                 1913


                              Cambridge:
                      PRINTED BY JOHN CLAY, M.A.
                       AT THE UNIVERSITY PRESS


     With the exception of the coat of arms at the foot, the design on
     the title page is a reproduction of one used by the earliest known
     Cambridge printer John Siberch 1521




PREFACE

The object of this little book is to afford an outline sketch of the
facts and meaning of insect-transformations. Considerations of space
forbid anything like an exhaustive treatment of so vast a subject, and
some aspects of the question, the physiological for example, are almost
neglected. Other books already published in this series, such as Dr
Gordon Hewitt's _House-flies_ and Mr O H. Latter's _Bees and Wasps_, may
be consulted with advantage for details of special insect life-stories.
Recent researches have emphasised the practical importance to human
society of entomological study, and insects will always be a source of
delight to the lover of nature. This humble volume will best serve its
object if its reading should lead fresh observers to the brookside and
the woodland.

G.H.C.

DUBLIN,

_July_, 1913.




CONTENTS

CHAP.                                               PAGE

   I.  Introduction                                     1

  II.  Growth and Change                                8

 III.  The Life-stories of some Sucking Insects        16

  IV.  From Water to Air                               23

   V.  Transformations, Outward and Inward             35

  VI.  Larvae and their Adaptations                    49

 VII.  Pupae and their Modifications                   79

VIII.  The Life-story and the Seasons                  89

  IX.  Past and Present--the Meaning of the Story     105

       Outline Classification of Insects              122

       Table of Geological Systems                    123

       Bibliography                                   124

       Index                                          129




LIST OF ILLUSTRATIONS


Stages in the Transformations of a Gnat    _Frontispiece_

FIG                                                  PAGE
 1. Stages of the Diamond-back Moth (_Plutella          3
    cruciferarum_)

 2. Head of typical Moth                                5

 3. Head of Caterpillar                                 5

 4. Common Cockroach (_Blatta orientalis_)             12

 5. Nymph of Locust (_Schistocera americana_)          13

 6. _Aphis pomi_, winged and wingless females          19

 7. Mussel Scale-Insect (_Mytilaspis pomorum_)         21

 8. Emergence of Dragon-fly (_Aeschna cyanea_)      29-31

 9. Nymph of May-fly (_Chloeon dipterum_)              33

10. Imaginal buds of Butterfly                         39

11. Imaginal buds of Blow-fly                          43

12. Carrion Beetle (_Silpha_) and larva                51

13. Larva of Ground-beetle (_Aepus_)                   52

14. Willow-beetle (_Phyllodecta_) and larva            53

15. Cabbage-beetle (_Psylliodes_) and larva            54

16. Corn Weevil (_Calandra_) and larva                 55

17. Ruby Tiger Moth (_Phragmatobia fuliginosa_)        61

18. Larvae and Pupa of Hive-bee (_Apis mellifica_)     65

19. Larva of Gall-midge (_Contarinia nasturtii_)       68

20. Crane-fly (_Tipula oleracea_) and larva            69

21. Maggot of House-fly (_Musca domestica_)            71

22. Ox Warble-fly (_Hypoderma bovis_) with egg,
    larva, and puparium                                75

23. Pupa of White Butterfly (_Pieris_)                 85




CHAPTER I

INTRODUCTION


Among the manifold operations of living creatures few have more strongly
impressed the casual observer or more deeply interested the thoughtful
student than the transformations of insects. The schoolboy watches the
tiny green caterpillars hatched from eggs laid on a cabbage leaf by the
common white butterfly, or maybe rears successfully a batch of silkworms
through the changes and chances of their lives, while the naturalist
questions yet again the 'how' and 'why' of these common though wondrous
life-stories, as he seeks to trace their course more fully than his
predecessors knew.

[Illustration: Fig. 1. _a_, Diamond-back Moth (_Plutella
cruciferarum_); _b_, young caterpillar, dorsal view; _c_, full-grown
caterpillar, dorsal view; _d_, side view; _e_, pupa, ventral view.
Magnified 6 times. From _Journ. Dept. Agric. Ireland_, vol. I.]

Everyone is familiar with the main facts of such a life-story as that of
a moth or butterfly. The form of the adult insect (fig. 1 _a_) is
dominated by the wings--two pairs of scaly wings, carried respectively
on the middle and hindmost of the three segments that make up the
_thorax_ or central region of the insect's body. Each of these three
segments carries a pair of legs. In front of the thorax is the head on
which the pair of long jointed feelers and the pair of large,
sub-globular, compound eyes are the most prominent features. Below the
head, however, may be seen, now coiled up like a watch-spring, now
stretched out to draw the nectar from some scented blossom, the
butterfly's sucking trunk or proboscis, situated between a pair of short
hairy limbs or palps (fig. 2). These palps belong to the appendages of
the hindmost segment of the head, appendages which in insects are
modified to form a hind-lip or _labium_, bounding the mouth cavity below
or behind. The proboscis is made up of the pair of jaw-appendages in
front of the labium, the _maxillae_, as they are called. Behind the
thorax is situated the _abdomen,_ made up of nine or ten recognisable
segments, none of which carry limbs comparable to the walking legs, or
to the jaws which are the modified limbs of the head-segments. The whole
cuticle or outer covering of the body, formed (as is usual in the group
of animals to which insects belong) of a horny (chitinous) secretion of
the skin, is firm and hard, and densely covered with hairy or scaly
outgrowths. Along the sides of the insect are a series of paired
openings or spiracles, leading to a set of air-tubes which ramify
throughout the body and carry oxygen directly to the tissues.

[Illustration: Fig. 2. A. Head of a typical Moth, showing proboscis
formed by flexible maxillae (_g_) between the labial palps (_p_); _c_,
face; _e_, eye; the structure _m_ has been regarded as the vestige of a
mandible. B. Basal part (_b_) of maxilla removed from head, with
vestigial palp (_p_). Magnified.]

Such a butterfly as we have briefly sketched lays an egg on the leaf of
some suitable food-plant, and there is hatched from it the well-known
crawling larva[1] (fig. 1 _b, c, d_) called a caterpillar, offering in
many superficial features a marked contrast to its parent. Except on the
head, whose surface is hard and firm, the caterpillar's cuticle is as a
rule thin and flexible, though it may carry a protective armature of
closely set hairs, or strong sharp spines. The feelers (fig. 3 _At_) are
very short and the eyes are small and simple. In connection with the
mouth, there are present in front of the maxillae a pair of _mandibles_
(fig. 3 _Mn_), strong jaws, adapted for biting solid food, which are
absent from the adult butterfly, though well developed in cockroaches,
dragon-flies, beetles, and many other insects. The three pairs of legs
on the segments of the thorax are relatively short, and as many as five
segments of the abdomen may carry short cylindrical limbs or pro-legs,
which assist the clinging habits and worm-like locomotion of the
caterpillar. No trace of wings is visible externally. The caterpillar,
therefore, differs markedly from its parent in its outward structure, in
its mode of progression, and in its manner of feeding; for while the
butterfly sucks nectar or other liquid food, the caterpillar bites up
and devours solid vegetable substances, such as the leaves of herbs or
trees. It is well-known that between the close of its larval life and
its attainment of perfection as a butterfly, the insect spends a
period as a _pupa_ (fig. 1 _e_) unable to move from place to place, and
taking no food.

[1] The term _larva_ is applied to any young animal which differs
markedly from its parent.

[Illustration: Fig. 3. Head of Caterpillar of Goat-moth (_Cossus_) seen
from behind. _At_, feeler; _Mn_, mandible; _Mx_, maxilla; _Lm_, labium,
spinneret projecting beyond it. Magnified. After Lyonet from Miall and
Denny's _Cockroach_.]

Such, in brief, is the course of the most familiar of insect
life-stories. For the student of the animal world as a whole, this
familiar transformation raises some startling problems, which have been
suggestively treated by F. Brauer (1869), L.C. Miall (1895), J. Lubbock
(1874), R. Heymons (1907), P. Deegener (1909) and other writers[2]. To
appreciate these problems is the first step towards learning the true
meaning of the transformation.

[2] The dates in brackets after authors' names will facilitate reference
to the Bibliography (pp. 124-8).

The butterfly's egg is absolutely and relatively of large size, and
contains a considerable amount of yolk. As a rule we find that young
animals hatched from such eggs resemble their parents rather closely and
pass through no marked changes during their lives. A chicken, a
crocodile, a dogfish, a cuttlefish, and a spider afford well-known
examples of this rule. Land-animals, generally, produce young which are
miniature copies of themselves, for example horses, dogs, and other
mammals, snails and slugs, scorpions and earthworms. On the other hand,
metamorphosis among animals is associated with eggs of small size, with
aquatic habit, and with relatively low zoological rank. The young of a
starfish, for example, has hardly a character in common with its parent,
while a marine segmented worm and an oyster, unlike enough when adult,
develop from closely similar larval forms. If we take a class of
animals, the Crustacea, nearly allied to insects, we find that its more
lowly members, such as 'water-fleas' and barnacles, pass through far
more striking changes than its higher groups, such as lobsters and
woodlice. But among the Insects, a class of predominantly terrestrial
and aerial creatures producing large eggs, the highest groups undergo,
as we shall see, the most profound changes. The life-story of the
butterfly, then, well-known as it may be, furnishes a puzzling exception
to some wide-reaching generalisations concerning animal development. And
the student of science often finds that an exception to some rule is the
key to a problem of the highest interest.

During many centuries naturalists have bent their energies to explain
the difficulties presented by insect transformations. Aristotle, the
first serious student of organised beings whose writings have been
preserved for us, and William Harvey, the famous demonstrator of the
mammalian blood circulation two thousand years later, agreed in
regarding the pupa as a second egg. The egg laid by a butterfly had not,
according to Harvey, enough store of food to provide for the building-up
of a complex organism like the parent; only the imperfect larva could be
produced from it. The larva was regarded as feeding voraciously for the
purpose of acquiring a large store of nutritive material, after which it
was believed to revert to the state of a second but far larger egg, the
pupa, from which the winged insect could take origin. Others again,
following de Réaumur (1734), have speculated whether the development of
pupa within larva, and of winged insect within pupa might not be
explained as abnormal births. But a comparison of the transformation of
butterflies with simpler insect life-stories will convince the enquirer
that no such heroic theories as these are necessary. It will be realised
that even the most profound transformation among insects can be
explained as a special case of growth.




CHAPTER II

GROWTH AND CHANGE


The caterpillar differs markedly from the butterfly. As we pursue our
studies of insect growth and transformation we shall find that in some
cases the difference between young and adult is much greater--as for
example between the maggot and the house-fly, in others far less--as
between the young and full-grown grasshopper or plant-bug. It is
evidently wise to begin a general survey of the subject with some of
those simpler cases in which the differences between the young and
adult insect are comparatively slight. We shall then be in a position to
understand better the meaning of the more puzzling and complex cases in
which the differences between the stages are profound.

In the first place it is necessary to realise that the changes which any
insect passes through during its life-story are essentially
accompaniments of its growth. The limits of this little book allow only
slight reference to features of internal structure; we must be content,
in the main, to deal with the outward form. But there is an important
relation between this outward form and the underlying living tissues
which must be clearly understood. Throughout the great race of
animals--the Arthropoda--of which insects form a class, the body is
covered outwardly by a _cuticle_ or secretion of the underlying layer of
living cells which form the outer skin or _epidermis_[3] (see fig. 10
_ep_, _cu_, p. 39). This cuticle has regions which are hard and firm,
forming an _exoskeleton_, and, between these, areas which are relatively
soft and flexible. The firm regions are commonly segmental in their
arrangement, and the intervening flexible connections render possible
accurate motions of the exoskeletal parts in relation to each other,
the motions being due to the contraction of muscles which are attached
within the exoskeleton.

[3] The term 'hypodermis' frequently applied to this layer is
misleading. The layer is the true outer skin--ectoderm or epidermis.

Now this jointed exoskeleton--an admirably formed suit of armour though
it often is--has one drawback: it is not part of the insect's living
tissues. It is a cuticle formed by the solidifying of a fluid secreted
by the epidermal cells, therefore without life, without the power of
growth, and with only a limited capacity for stretching. It follows,
therefore, that at least during the period through which the insect
continues to grow, the cuticle must be periodically shed. Thus in the
life-story of an insect or other arthropod, such as a lobster, a spider,
or a centipede, there must be a succession of cuticle-castings--'moults'
or _ecdyses_ as they are often called.

When such a moult is about to take place the cuticle separates from the
underlying epidermis, and a fluid collects beneath. A delicate new
cuticle (see fig. 10 _cu'_) is then formed in contact with the
epidermis, and the old cuticle opens, usually with a slit lengthwise
along the back, to allow the insect in its new coat to emerge. At first
this new coat is thin and flabby, but after a period of exposure to the
air it hardens and darkens, becoming a worthy and larger successor to
that which has been cast. The cuticle moreover is by no means wholly
external. The greater part of the digestive canal and the whole
air-tube system are formed by inpushings of the outer skin (ectoderm)
and are consequently lined with an extension of the chitinous cuticle
which is shed and renewed at every moult.

In all insects these successive moults tend to be associated with change
of form, sometimes slight, sometimes very great. The new cuticle is
rarely an exact reproduction of the old one, it exhibits some new
features, which are often indications of the insect's approach towards
maturity. Even in some of those interesting and primitive insects the
Bristle-tails (Thysanura) and Spring-tails (Collembola), in which wings
are never developed, perceptible differences in the form and arrangement
of the abdominal limbs can be traced through the successive stages, as
R. Heymons (1906) and K.W. Verhoeff (1911) have shown for Machilis. But
the changes undergone by such insects are comparatively so slight, that
the creatures are often known as 'Ametabola' or insects without
transformation in the life-history. Now there are a considerable number
of winged insects--cockroaches and grasshoppers for example--in which
the observable changes are also comparatively slight. We will sketch
briefly the main features of the life-story of such an insect.

[Illustration: Fig. 4. Common Cockroach (_Blatta orientalis_). _a_,
female; _b_, male; _c_, side view of female; _d_, young. After Marlatt,
_Entom. Bull._ 4, _U.S. Dept. Agric._]

The young creature is hatched from the egg in a form closely resembling,
on the whole, that of its parent, so that the term 'miniature adult'
sometimes applied to it, is not inappropriate. The baby cockroach (fig.
4 _d_) is known by its flattened body, rounded prothorax, and stiff,
jointed tail-feelers or cercopods; the baby grasshopper by its strong,
elongate hind-legs, adapted, like those of the adult, for vigorous
leaping. During the growth of the insect to the adult state there may be
four or five moults, each preceded and succeeded by a characteristic
instar[4]. The first instar differs, however, from the adult in one
conspicuous and noteworthy feature, it possesses no trace of wings. But
after the first or the second moult, definite wing-rudiments are visible
in the form of outgrowths on the corners of the second and third
thoracic segments. In each succeeding instar these rudiments become more
prominent, and in the fourth or the fifth stage, they show a branching
arrangement of air-tubes, prefiguring the nervures of the adult's wing
(fig. 5). After the last moult the wings are exposed, articulated to the
segments that bear them, and capable of motion. Having been formed
beneath the cuticle of the wing-rudiments of the penultimate instar, the
wings are necessarily abbreviated and crumpled. But during the process
of hardening of the cuticle, they rapidly increase in size, blood and
air being forced through the nervures, so that the wings attaining their
full expanse and firmness, become suited for the function of flight.

[4] The convenient term 'instar' has been proposed by Fischer and
advocated by Sharp (1895) for the form assumed by an insect during a
stage of its life-story. Thus the creature as hatched from the egg is
the _first instar_, after the first moult it has become the _second
instar_, and so on, the number of moults being always one less than the
number of instars.

[Illustration: Fig. 5. Nymph of Locust (_Schistocera americana_) with
distinct wing-rudiments. After Howard, _Insect Life_, vol. VII.]

The changes through which these insects pass are therefore largely
connected with the development of the wings. It is noteworthy that in an
immature cockroach the entire dorsal cuticle is hard and firm. In the
adult, however, while the cuticle of the prothorax remains firm, that of
the two hinder thoracic and of all the abdominal segments is somewhat
thin and delicate on the dorsal aspect. It needs not now to be
resistant, because it is covered by the two firm forewings, which shield
and protect it, except when the insect is flying. There are, indeed,
slight changes in other structures not directly connected with the
wings. In a young grasshopper, for example, the feelers are relatively
stouter than in the adult, and the prothorax does not show the
specifically distinctive shape with its definite keels and furrows.
Changes in the secondary sexual characters may also be noticed. For
instance, in an immature cockroach both male and female carry a pair of
jointed tail-feelers or cercopods on the tenth abdominal segment, and a
pair of unjointed limbs or stylets on the ninth. In the adult stage,
both sexes possess cercopods, but the males only have stylets, those of
the female disappearing at the final moult.

Reviewing the main features of the life-story of a grasshopper or
cockroach, we notice that there is no marked or sudden change of form.
The newly-hatched insect resembles generally its parent, except that it
has no wings. Wing-rudiments appear, however, in an early instar as
visible outgrowths on the thoracic segments, and become larger after
each moult. All through its various stages the immature insect--_nymph_
as it is called--lives in the same kind of situations and on the same
kind of food as its parent, and it is all along active and lively,
undergoing no resting period like the pupal stage in the transformation
of the butterfly.

One interesting and suggestive fact remains to be mentioned. There are
grasshoppers and cockroaches in which the changes are even less than
those just sketched, because the wings remain, even in the adult, in a
rudimentary state (as for example in the female of the common kitchen
cockroach, _Blatta orientalis_, see fig. 4 _a_), or are never developed
at all. Such exceptional winglessness in members of a winged family can
only be explained by the recognition of a life-story, not merely in the
individual but in the race. We cannot doubt that the ancestors of these
wingless insects possessed wings, which in the course of time have been
lost by the whole species or by the members of the female sex. It is
generally assumed that this loss has been gradual, and so in many cases
it probably may have been. But there are species of insects in which
some generations are winged and others wingless; a winged mother gives
birth to wingless offspring, and a wingless parent to young with
well-developed wings. Such discontinuity in the life-story of a single
generation forces us to recognise the possibility of similar sudden
mutations in the course of that age-long process of evolution to which
the facts of insect growth, and indeed of all animal development, bear
striking testimony.




CHAPTER III

THE LIFE-STORIES OF SOME SUCKING INSECTS


We may now turn our attention to some examples of the remarkable
alternation of winged and wingless generations in the yearly life-cycle
of the same species, mentioned at the end of the last chapter.
Cockroaches and grasshoppers belong to an order of insects, the
Orthoptera[5], characterised by firm forewings and biting jaws; in all
of them the change of form during the life-history is comparatively
slight. A great contrast to those insects in the structure of the
mouth-parts is presented by the Hemiptera, an order including the bugs,
pond-skaters, cicads, plant-lice, and scale-insects. These all have an
elongated, grooved labium projecting from the head in form of a beak,
within which work, to and fro, the slender needle-like mandibles and
maxillae by means of which the insect pierces holes through the skin of
a leaf or an animal, and is thus enabled to suck a meal of sap or blood,
according to its mode of life. In many Hemiptera--the various families
of bugs both aquatic and terrestrial, for example--the life-history is
nearly as simple as that of a cockroach. It is the family of the
plant-lice (Aphidae) that affords typical illustrations of that
alternation of generations to which reference has been made.

[5] See outline classification of insects, p. 122.

The yearly cycle of the common Aphids of the apple tree has been lately
worked out in detail by J.B. Smith (1900) and E.D. Sanderson (1902). In
late autumn tiny wingless males and females are found in large numbers
on the withered leaves. The sexes pair together, and the females lay
their relatively large, smooth, hard-coated black eggs on the twigs;
these resistant eggs carry the species safely over the winter. At
springtide, when the leaves begin to sprout from the opening buds the
aphid eggs are hatched, and the young insects after a series of moults,
through which hardly any change of form is apparent, all grow into
wingless 'stem-mothers' much larger than the egg-laying females of the
autumn. The stem-mothers have the power, unusual among animals as a
whole, but not very infrequent in the insects and their allies, of
reproducing their kind without having paired[6] with a male. Eggs
capable of parthenogenetic development, produced in large numbers in the
ovaries of these females, give rise to young which, developing within
the body of the mother, are born in an active state. Successive broods
of these wingless virgin females (fig. 6 _a_) appear through the spring
and summer months, and as the rate of their development is rapid, often
the whole life-story is completed within a week. The aphid population
increases very fast. Later a generation appears in which the thoracic
segments of the nymphs are seen to bear wing-rudiments like those of the
young cockroach, and a host of winged females (fig. 6_b_) are produced;
these have the power of migrating to other plants. We understand that
wings are not necessary to the earlier broods whose members have plenty
of room and food on their native shoots, but that when the population
becomes crowded, a winged brood capable of emigration is advantageous to
the race.

[6] Such virgin reproduction is termed 'parthenogenesis.'

Many generations of virgin female aphids, some wingless, others winged
when adult, succeed each other through the summer months. At the close
of the year the latest brood of these bring forth young, which develop
into males and egg-laying females; thus the yearly cycle is completed.
Variations in points of detail may be noticed in different species of
aphids. The autumn males and egg-laying females are, for example,
frequently winged, and the same species may have constantly recurring
generations of different forms adapted for different food-plants, or for
different regions of the same food-plant. But taking a general view of
the life-story of aphids for comparison with the life-story of other
insects, three points are especially noteworthy. Virgin reproduction
recurs regularly, parthenogenetic broods being succeeded by a single
sexual brood. A winged parent brings forth young which remain always
wingless, and wingless adults produce young which acquire wings. The
wings are developed, as in the cockroach, from outward and visible
wing-rudiments.

[Illustration: Fig. 6. Apple Aphid (_Aphis pomi_), virgin females, _a_,
wingless; _b_, winged. Magnified 20 times.]

A family of Hemiptera, related to the Aphidae and equally obnoxious to
the gardener, is that of the Coccidae or scale-insects. These furnish an
excellent illustration of features noticeable in certain insect
life-histories. In the first place, the newly-hatched young differs
markedly from the parent in the details of its structure. A young coccid
(fig. 7 _c_) is flattened oval in shape, has well-developed feelers
(fig. 7 _d_) and legs, and runs actively about, usually on the leaves or
bark of trees and shrubs, through which it pierces with its long jaws,
so that it may suck sap from the soft tissues beneath. After a time it
fixes itself by means of these jaws and the characteristic scale or
protective covering, composed partly of a waxy secretion and partly of
dried excrement, begins to grow over its body. The female loses legs and
feelers, and never acquires wings, becoming little more than a sluggish
egg-bag (fig. 7 _e_). The male on the other hand passes into a second
larval stage in which there are no functional legs, but rudiments of
legs and of wings are present on the epidermis beneath the cuticle, as
shown by B.O. Schmidt for Aspidiotus (1885). The penultimate instar of
this sex in which the wing-rudiments are visible externally lies
passively beneath the scale, its behaviour resembling that of a
butterfly pupa. The adult winged male (fig. 7 _a_) leads a short, but
active life.

[Illustration: Fig. 7. Mussel Scale-insect (_Mytilaspis pomorum_). _a_,
male; _b_, foot of male; _c_, larva, ventral view; _d_, feeler of larva;
_e_, female, ventral view. After Howard, _Yearbook U.S. Dept. Agric._
1904. Magnified, _a, c, e_ x 20; _b, d_ x 120.]

Another family allied to the Aphidae is that of the Cicads, hardly
represented in our fauna but abundant in many of the warmer regions of
the earth. Here also the young insect differs widely from its parent in
form, living underground and being provided with strong fore-legs for
digging in the soil. After a long subterranean existence, usually
extending over several years, the insect attains the penultimate stage
of its life-story, during which it rests passively within an earthen
cell, awaiting the final moult, which will usher in its winged and
perfect state.

In the life-histories of cicads and coccids, then, there are some
features which recall those of the caterpillar's transformation into the
butterfly. The newly-hatched insect is externally so unlike its parent
that it may be styled a larva. The penultimate instar is quiescent and
does not feed. But while the caterpillar shows throughout its life no
outward trace of wings, external wing-rudiments are evident in the young
stages of the cicad. In the male coccid we find a late larval stage with
hidden wing-rudiments, the importance of which, for comparison with the
caterpillar, will be appreciated later.




CHAPTER IV

FROM WATER TO AIR


Insects as a whole are preeminently creatures of the land and the air.
This is shown not only by the possession of wings by a vast majority of
the class, but by the mode of breathing to which reference has already
been made (p. 2), a system of branching air-tubes carrying atmospheric
air with its combustion-supporting oxygen to all the insect's tissues.
The air gains access to these tubes through a number of paired air-holes
or spiracles, arranged segmentally in series.

It is of great interest to find that, nevertheless, a number of insects
spend much of their time under water. This is true of not a few in the
perfect winged state, as for example aquatic beetles and water-bugs
('boatmen' and 'scorpions') which have some way of protecting their
spiracles when submerged, and, possessing usually the power of flight,
can pass on occasion from pond or stream to upper air. But it is
advisable in connection with our present subject to dwell especially on
some insects that remain continually under water till they are ready to
undergo their final moult and attain the winged state, which they pass
entirely in the air. The preparatory instars of such insects are
aquatic; the adult instar is aerial. All may-flies, dragon-flies, and
caddis-flies, many beetles and two-winged flies, and a few moths thus
divide their life-story between the water and the air. For the present
we confine attention to the Stone-flies, the May-flies, and the
Dragon-flies, three well-known orders of insects respectively called by
systematists the Plecoptera, the Ephemeroptera and the Odonata.

In the case of many insects that have aquatic larvae, the latter are
provided with some arrangement for enabling them to reach atmospheric
air through the surface-film of the water. But the larva of a stone-fly,
a dragon-fly, or a may-fly is adapted more completely than these for
aquatic life; it can, by means of gills of some kind, breathe the air
dissolved in water.

The aquatic young of a stone-fly does not differ sufficiently in form
from its parent to warrant us in calling it a larva; the life-history is
like that of a cockroach, all the instars however except the final
one--the winged adult or _imago_--live in the water. The young of one of
our large species, a Perla for example, has well-chitinised cuticle,
broad head, powerful legs, long feelers and cerci like those of the
imago; its wings arise from external rudiments, which are conspicuous in
the later aquatic stages. But it lives completely submerged, usually
clinging or walking beneath the stones that lie in the bed of a clear
stream, and examination of the ventral aspect of the thorax reveals six
pairs of tufted gills, by means of which it is able to breathe the air
dissolved in the water wherein it lives. At the base of the tail-feelers
or cerci also, there are little tufts of thread-like gills as J.A.
Palmén (1877) has shown. An insect that is continually submerged and has
no contact with the upper air cannot breathe through a series of paired
spiracles, and during the aquatic life-period of the stone-fly these
remain closed. Nevertheless, breathing is carried on by means of the
ordinary system of branching air-tubes, the trunks of which are in
connection with the tufted hollow gill-filaments, through whose delicate
cuticle gaseous exchange can take place, though the method of this
exchange is as yet very imperfectly understood. When the stone-fly nymph
is fully grown, it comes out of the water and climbs to some convenient
eminence. The cuticle splits open along the back, and the imago, clothed
in its new cuticle, as yet soft and flexible, creeps out. The spiracles
are now open, and the stone-fly breathes atmospheric air like other
flying insects. But throughout its winged life, the stone-fly bears
memorials of its aquatic past in the little withered vestiges of gills
that can still be distinguished beneath the thorax.

The adult dragon-fly (fig. 8 _d_) is specialised in such a way that it
captures its prey--flies and other small insects--on the wing, swooping
through the air like a hawk and feeding voraciously. The head is
remarkable for its large globular compound eyes, its short bristle-like
feelers, and its very strong mandibles which bite up the bodies of the
victims. The thorax bears the two pairs of ample wings, firm and almost
glassy in texture, and its segments are projected forward ventrally, so
that all six legs, which are armed with rows of sharp, slender spines,
can be held in front of the mouth, where they form an effective
fly-trap. The abdomen is very long and usually narrow.

A female dragon-fly after a remarkable mode of pairing, the details of
which are beside our present subject, drops her eggs in the water, or
lays them on water-weeds, perhaps cutting an incision where they can be
the more safely lodged, or even goes down below the surface and deposits
them in the mud at the bottom of a pond. From the eggs are hatched the
aquatic larvae which differ in many respects from the imago. The
dragon-fly larva has the same predaceous mode of life as its parent, but
it is sluggish in habit, lurking for its prey at the bottom of the pond,
among the mud or vegetation, which it resembles in colour. The thoracic
segments have not the specialisation that they show in the imago; the
abdomen is relatively shorter and broader. The larval head has, like
that of the imago, short feelers, and the eyes are somewhat large,
though far from attaining the size of the great globular eyes of the
dragon-fly. But the third pair of jaws, forming the labium, are most
remarkably modified into a 'mask,' the distal central portion (mentum)
being hinged to the basal piece (sub-mentum) which is itself jointed
below the head. The mentum carries at its extremity a pair of lobes with
sharp fangs. Thus the mask can be folded under the head when the larva
lurks in its hiding place, or be suddenly darted out so as to secure any
unwary small insect that may pass close enough for capture. Dragon-fly
larvae walk, and also swim by movements of the abdomen or by expelling a
jet of water from the hind-gut. The walls of this terminal region of the
intestine have areas lined with delicate cuticle and traversed by
numerous air-tubes, so that gaseous exchange can take place between the
air in the tubes and that dissolved in the water. The larvae of the
larger and heavier dragon-flies (Libellulidae and Aeschnidae) breathe
mostly in this way. Those of the slender and delicate 'Demoiselles'
(Agrionidae) are provided with three leaf-like gill-plates at the tail,
between whose delicate surfaces numerous air-tubes ramify. These
gill-plates are at times used for propulsion. Thus air supply is ensured
during aquatic life. But occasionally, when the water in which the
larva lives is foul and poor in oxygen, the tail is thrust out of the
water so that air can be admitted directly into the intestinal chamber.
The aquatic life of these insects lasts for more than a year, and F.
Balfour-Browne (1909) has observed from ten to fourteen moults in
Agrion. Outward wing-rudiments are early visible on the thoracic
segments; when these have become conspicuous the insect, beginning in
some respects to approach the adult condition, is often called a nymph.
In an advanced dragon-fly nymph, H. Dewitz (1891) has shown that the
thoracic spiracles are open, and, as the time for its final moult draws
near, the insect may thrust the front part of its body out of the water,
and breathe atmospheric air through these. Thus before the great change
takes place the nymph has foretastes of the aerial mode of breathing
which it will practise when the perfect stage shall have been attained.
The emergence of the dragon-fly from its nymph-cuticle has been
described by many naturalists from de Réaumur (1740) to L.C. Miall
(1895) and O.H. Latter (1904). The nymph climbs out of the water by
ascending some aquatic plant, and awaits the change so graphically
sketched by Tennyson:

    A hidden impulse rent the veil,
    Of his old husk, from head to tail,
    Came out clear plates of sapphire mail.

'From head to tail,' for the nymph-cuticle splits lengthwise down the
back, and the head and thorax of the imago are freed from it (fig. 8
_a_), then the legs clasp the empty cuticle, and the abdomen is drawn
out (fig. 8 _b, c_). After a short rest, the newly-emerged fly climbs
yet higher up the water-weed, and remains for some hours with the
abdomen bent concave dorsalwards (fig. 8 _d_), to allow space for the
expansion and hardening of the wings. For some days after emergence the
cuticle of the dragon-fly has a dull pale hue, as compared with the dark
or brightly metallic aspect that characterises it when fully mature. The
life of the imago endures but a short time compared with the long
aquatic larval and nymphal stages. After some weeks, or at most a few
months, the dragon-flies, having paired and laid their eggs, die before
the approach of winter.

[Illustration: Fig. 8 _a, b_. Dragon-fly (_Aeschna cyanea_). Two stages
in emergence of fly from nymph-cuticle. From Latter's _Natural
History_.]

[Illustration: Fig. 8 _c_. Dragon-fly emerged, wings
expanding. From Latter's _Natural History_.]

[Illustration: Fig. 8 _d_. Dragon-fly (_Aeschna cyanea_) with
expanded wings.]

The life-story of a may-fly follows the same general course as that just
described for the dragon-flies, but there are some suggestive
differences. In the first place, we notice a wider divergence between
the imago and the larva. An adult may-fly is one of the most delicate
of insects; the head has elaborate compound eyes, but the feelers are
very short, and the jaws are reduced to such tiny vestiges that the
insect is unable to feed. Its aquatic larva is fairly robust, with a
large head which is provided with well-developed jaws, as the larval and
nymphal stages extend over one or two years, and the insects browse on
water-weeds or devour creatures smaller and weaker than themselves. They
breathe dissolved air by means of thread-like or plate-like gills
traversed by branching air-tubes, somewhat resembling those of the
demoiselle dragon-fly larva. But in the may-fly larva, there is a series
of these gills (fig. 9_b_) arranged laterally in pairs on the abdominal
segments, and C. Börner (1909) has recently given reasons, from the
position and muscular attachments of these organs, for believing that
they show a true correspondence to (in technical phraseology are
homologous with) the thoracic legs. One feature in which the larva often
agrees with the imago is the possession on the terminal abdominal
segment of a pair of long jointed cerci, and in many genera a median
jointed tail-process (see fig. 9) is also present, in some cases both in
the larva and the imago, in others in the larva during its later stages
only. The prolonged larval life in may-flies often involves a large
series of moults; Lubbock (1863) has enumerated twenty-one in the
life-history of Chloeon. In the second year of aquatic life
wing-rudiments (fig. 9 _a_) are visible, and the larva becomes a nymph.
When the time for the winged condition approaches the nymphs leave the
water in large swarms. The vivid accounts of these swarms given by
Swammerdam (1675), de Réaumur (1742) and other old-time observers are
available in summarised form for English readers in Miall's admirable
book (1895). May-flies are eagerly sought as food by trout, and the rise
of the fly on many lakes ushers in a welcome season to the angler.

The nymph-cuticle opens and the winged insect emerges. But this is not
the final instar; may-flies are exceptional among insects in undergoing
yet another moult after they have acquired wings which they can use for
flight. The instar that emerges from the nymph-cuticle is a sub-imago,
dull in hue, with a curious immature aspect about it. A few hours later
the final moult takes place, a very delicate cuticle being shed and
revealing the true imago. Then follow the dancing flight over the calm
waters, the mating and egg-laying, the rapid death. The whole winged
existence prepared for by the long aquatic life may be over in a single
evening; at most it lasts but for a few days.

[Illustration: Fig. 9. Nymph of May-fly (_Chloeon dipterum_) showing on
right side wing-rudiment (_a_), on left tracheal gills (_b_). Magnified
4 times. [Feelers and legs are cut short.] From Miall and Denny after
Vayssière.]

In the development of the may-flies, then, we notice not only a
considerable divergence between larva and imago, both in habitat and
structure; we see also what is to be observed often in more highly
organised insects--a feeding stage prolonged through the years of larval
and nymphal life, while the winged imago takes no food and devotes its
energies through its short existence to the task of reproduction. Such
division of the life-history into a long feeding, and a short breeding
period has, as will be seen later, an important bearing on the question
of insect transformation generally, and the dragon-flies and may-flies
afford examples of two stages in its specialisation. The sub-imaginal
instar of the may-fly furnishes also a noteworthy fact for comparison
with other insect histories. In two points, however, the life-story of
these flies with their aquatic larvae recalls that of the cockroach. All
the larval and nymphal instars are active, and the wing-rudiments are
outwardly visible long before the final moult.




CHAPTER V

TRANSFORMATIONS,--OUTWARD AND INWARD


We are now in a position to study in some detail the transformation of
those insects whose life-story corresponds more or less closely with
that of the butterfly, sketched in the opening pages of this little
book. In the case of some of the insects reviewed in the last three
chapters, the may-flies and cicads for example, a marked difference
between the larva and the imago has been noticed; in others, as the
coccids, we find a resting instar before the winged condition is
assumed, suggesting the pupal stage in the butterfly's life-story.

The various insect orders whose members exhibit no marked divergence
between larva and imago (the Orthoptera for example) are often said to
undergo no transformation, to be 'Ametabola.' Those with life-stories
such as the dragon-flies' are said to undergo partial transformation,
and are termed 'Hemimetabola.' Moths, caddis-flies, beetles, two-winged
flies, saw-flies, ants, wasps, bees, and the great majority of insects,
having the same type of life-story as the butterfly, are said to undergo
complete transformation and are classed as 'Metabola' or 'Holometabola.'
Wherein lies the fundamental difference between these Holometabola on
the one hand and the Hemimetabola and Ametabola on the other? It is not
that the larva differs from the imago or that there is a passive stage
in the life-history; these conditions are observable among insects with
a 'partial' transformation as we have seen, though the resting instar
that simulates the butterfly pupa is certainly exceptional. It has been
pointed out by Sharp (1899) that the most important indication of the
difference between the two modes of development is furnished by the
position of the wing-rudiments. In all Ametabola and Hemimetabola these
are visible externally long before the penultimate instar has been
reached; in the Holometabola they are not seen until the pupal stage.

Attention has already been drawn to the contrast in outward form between
a butterfly and its caterpillar. As in the case of dragon-fly or
may-fly, the larval period is essentially a time for feeding and growth,
and during this period the larval cuticle is cast four or five, in some
species even seven or eight times. After each moult some changes in
detail may be observable, for example in the proportions of the
body-segments or their outgrowths, in the colour or the closeness of the
hairy or spiny armature. But in all main features the caterpillar
retains throughout its life the characteristic form in which it left
the egg. From the tiny, newly-hatched larva to the full-fed caterpillar,
possibly several inches in length, there is all along the same crawling,
somewhat worm-like body, destitute of any outward trace of wings. When
however the last larval cuticle has split open lengthwise along the
back, and has been worked off by vigorous wriggling motions of the
insect, the pupa thus revealed shows the wing-rudiments conspicuous at
the sides of the body, and lying neatly alongside these are to be seen
the forms of feelers, legs, and maxillae of the imago prefigured in the
cuticle of the pupa (fig. 1 _e_). The pupa thus resembles the imago much
more closely than it resembles the larva; even in the proportions of the
body a relative shortening is to be noticed, and the imago of any insect
with complete transformation is reduced in length as compared with the
full-fed larva. Now these wings and other structures characteristic of
the imago, appear in the pupa which is revealed by the shedding of the
last larval cuticle. From these facts we infer that the wing-rudiments
must be present in the larva, hidden beneath the cuticle; and until the
last larval instar, not beneath the cuticle only, but growing in
such-wise that they are hidden by the epidermis. For if they were
growing outwardly the new cuticle would be formed over them, so that
they would be apparent after the next moult. But it is clear that only
in the pupa, forming beneath the cuticle of the last larval instar, can
they grow outwards.

Anatomical study of the caterpillar at various stages verifies the
conclusions just drawn from superficial observation. A hundred and fifty
years ago P. Lyonet in his monumental work (1762) on the caterpillar of
the Goat Moth (Cossus) detected, in the second and third thoracic
segments, four little white masses buried in the fat-body, and, while
doubtful as to their real meaning, he suggested that their number and
position might well give rise to the suspicion that they were rudiments
of the wings of the moth. But it was a century later that A. Weismann in
his classical studies (1864) on the development of common flies, showed
the presence in the maggot of definite rudiments of wings, and other
organs of the adult--rudiments to which he gave the name of _imaginal
discs_. We will recur later to these transformations of the Diptera. For
the present, we pursue our survey of changes in the life-history of the
Lepidoptera and can take to guide us the excellent researches of J.
Gonin (1894).

Careful study of the imaginal discs of the wings in a caterpillar (fig.
10) made by examining microscopically sections cut through them, shows
that the epidermis is pushed in to form a little pouch (_C, p_) and that
into this grows the actual wing-rudiment. Consequently the whitish disk
which seems to lie within the body-wall of the larva, is really a
double fold of the epidermis, the outer fold forming the pouch, the
inner the actual wing-bud. Into the cavity of the latter pass branches
from the air-tube system. In its earliest stage, the wing-bud is simply
an ingrowing mass of cells (fig. 10 _A_) which subsequently becomes an
inpushed pouch (_B_). Until the last stage of larval life the wing-bud
remains hidden in its pouch, and no cuticle is formed over it. When the
pupal stage draws near the bud grows out of its sheath, and projecting
from the general surface of the epidermis becomes covered with cuticle
to be revealed, as we have seen, after the last larval moult, as the
pupal wing. Thus all through the life of the humble, crawling
caterpillar, 'it doth not yet appear what it shall be,' but there are
being prepared, hidden and unseen, the wondrous organs of flight, which
in due time will equip the insect for the glorious aerial existence that
awaits it.

[Illustration: Fig. 10. A, B, C, Sections through epidermis and cuticle,
showing three stages in growth of the imaginal disc (_w_) of a wing in
the caterpillar of a White Butterfly (_Pieris_). _ep_, epidermis; _cu_,
cuticle; _t_, air-tube, whence branches pass into the developing wing.
In C, _cu'_ represents the new cuticle forming beneath the old one, and
(_p_) the pouch within which the wing-disc (_w_) lies. Highly magnified.
After Gonin, _Bull. Soc. Vaud._ XXX.]

As mentioned above, this hidden growth of the wing-rudiments, in
butterflies, beetles, flies, bees, and the great majority of the winged
insects, has been emphasised by Sharp (1899) as a character contrasting
markedly with the outward and visible growth of the wing-rudiments in
such insects as cockroaches, bugs, and dragon-flies. The divergence
between the two modes of development is certainly very striking, and a
conceivable method of transition from the one to the other is not easy
to explain. Sharp has expressed the divergence by the terms
_Endopterygota_, applied to all the orders of insects with hidden
wing-rudiments (the 'Metabola' or 'Holometabola' of most
classifications) and _Exopterygota_, including all those insects whose
wing-rudiments are visible throughout growth ('Hemimetabola' and
'Ametabola'). Those curious lowly insects, belonging to the two orders
of the Collembola and Thysanura, none of whose members ever develop
wings at all, form a third sub-class, the _Apterygota_ (see
Classificatory Table, p. 122).

Not the wings only, but other structures of the imago, varying in extent
in different orders, are formed from the imaginal discs. For example, de
Réaumur and G. Newport (1839) found that if the thoracic leg of a
late-stage caterpillar were cut off, the corresponding leg of the
resulting butterfly would still be developed, although in a truncated
condition. Gonin has shown that in the Cabbage White butterfly (_Pieris
brassicae_) the legs of the imago are represented, through the greater
part of larval life, only by small groups of cells situated within the
bases of the larval legs. After the third moult these imaginal discs
grow rapidly and the proximal portion of each, destined to develop into
the thigh and shin of the butterfly's leg, sinks into a depression at
the side of the thorax, while the tip of the shin and the
five-segmented foot project into the cavity of the larval leg. Hence we
understand that the amputation of the latter by the old naturalists
truncated only and did not destroy the imaginal limb. In the blow-fly
maggot, Weismann, B.T. Lowne (1890) and J. Van Rees (1888) have shown
that the imaginal discs of the legs (fig. 11--1, 2, 3) grow out from
deep dermal inpushings. Simple at first, these outgrowths by partial
splitting, become differentiated into thigh and shin.

[Illustration: Fig. 11. Front region of Maggot of Blow-fly
(_Calliphora_) showing diagrammatically the imaginal discs, which are
shaded. _e_, eye; _f_, feeler; _W_, fore-wing; _w_, hind-wing; 1, 2, 3,
legs. _H_ is the 'cephalic vesicle,' which becomes everted at the close
of the metamorphosis, so as to bring the feelers and eyes to the front,
the brain (_B_) moving forwards at the same time. After Van Rees, _Zool.
Jahrb._ 1894, and Lowne's _Blow-fly_.]

Similarly the feelers and jaws of the butterfly are developed from
imaginal discs, and this fact explains how it comes to pass that they
differ so widely from the corresponding structures in the caterpillar.
The larval feelers (fig. 3 _At_) are short and stumpy, those of the
butterfly long and many-jointed. The maxilla of the larva (fig. 3 _Mx_)
consists of a base carrying two short jointed processes; in the
butterfly a certain portion of the maxilla, the hood or galea, is
modified into a long, flexible grooved process, capable of forming with
its fellow the trunk through which the insect sucks its liquid food
(fig. 2). Nothing but some such provision as that of the imaginal discs
could render possible the wonderful replacement of the caterpillar's
jaws, biting solid food, into those of the butterfly sipping nectar from
flowers.

A curious segmental displacement of the imaginal discs with regard to
the larva is noticeable in some Diptera. In the larva of the
harlequin-midge (Chironomus) as described by Miall and Hammond (1900)
the brain is situated in the thorax, and the imaginal discs for the
head, eyes, and feelers of the adult lie in close association with it,
though they arise from inpushings of the larval head. These rudiments do
not appear until the last larval stage has been reached. In the gnats
Culex and Corethra, on the other hand, the imaginal discs for the
head-appendages retain their normal position within the larval head, and
appear in an early stage of larval life. Among the flies of the
bluebottle group (Muscidae) the brain (fig. 11 _B_) is situated, as in
Chironomus, in the thoracic region of the legless maggot, which is the
larva of an insect of this family, and the imaginal discs for eyes and
feelers (fig. 11 _e_, _f_) lie just in front of it. Here, the imaginal
buds of the legs (fig. 11--1, 2, 3) and wings (fig. 11 _W_, _w_) are
deeply inpushed, retaining their connection with the skin only by means
of a thread of cells. As the larva is legless and headless its outer
form is not affected by the discs and it is not surprising to learn that
they appear early. It has indeed been suggested that the pharyngeal
region of the larva, in connection with which the imaginal head-discs
are developed, should be regarded, though it lies in the thorax, as an
inpushed anterior section of the larval head. In any case this region is
pushed out during the formation of the pupa within the final larval
cuticle, so that the imaginal head with its contained brain, its
compound eyes, and its complex feelers, takes its rightful place at the
front end of the insect.

The mention of the brain suggests a few brief remarks on the changes in
the internal organs during insect transformation. There are no imaginal
discs for the nervous system; the brain, nerve-cords and ganglia of the
butterfly or bluebottle are the direct outcome of those of the
caterpillar or maggot. More than seventy years ago, Newport (1839)
traced the rapid but continuous changes, which, during the early pupal
period, convert the elongate nerve-cord of the caterpillar with its
relatively far-separated ganglia into the shortened, condensed
nerve-cord of the Tortoise-shell butterfly (_Vanessa urticae_) with
several of the ganglia coalesced. In many Diptera, on the other hand,
the nervous system of the larva is more concentrated than that of the
imago.

The tubular heart also of a winged insect is the directly modified
survival of the larval heart.

Similarly the reproductive organs undergo a gradual, continuous
development throughout an insect's life-story. Their rudiments appear in
the embryo, often at a very early stage; they are recognisable in the
larva, and the matured structures in the imago are the result of their
slow process of growth, the details of which must be reckoned beyond the
scope of this book. For a full summary of the subject the reader is
referred to L.F. Henneguy's work (1904) containing references to much
important modern literature, which cannot be mentioned here.

On the other hand, the digestive system of insects that undergo a
metamorphosis, passes through a profound crisis of dissolution and
rebuilding. This is not surprising when we remember that there is often
a great difference between larva and imago in the nature of the food.
The digestive canal of a caterpillar runs a fairly straight course
through the body and consists of a gullet, stomach (mid-gut),
intestine, and rectum; it is adapted for the digestion of solid food. In
the butterfly there is one outgrowth of the gullet in the head--a
pharyngeal sac adapted for sucking liquids; and another outgrowth at the
hinder end of the gullet (which is much longer than in the larva)--a
crop or food-reservoir lying in the abdomen. The intestine of the
butterfly also is longer than that of the larva, being coiled or
twisted. Towards the end of the last larval stage, the cells of the
inner coat (epithelium) lining the stomach begin to undergo
degeneration, small replacing cells appearing between their bases and
later giving rise to the more delicate epithelium that lines the mid-gut
of the imago. The larval cells are shed into the cavity of the stomach
and become completely broken down. J. Anglas (1902), describing these
microscopic changes in the transformations of wasps and bees, has shown
that the tiny replacing cells can be recognised in sections through the
digestive canal of a very young larva; they may be regarded as
representing imaginal buds of the adult gastric epithelium. In the
transformations of two-winged flies of the bluebottle group, A.
Kowalevsky (1887) has shown that these replacing cells are aggregated in
little masses scattered at different points along the stomach and thus
corresponding rather closely to the imaginal discs of the legs and
wings.

The gullet, crop, and gizzard of an insect, which lie in front of the
stomach, are lined by cells derived from the outer skin (ectoderm) which
is pushed in to form what is called the 'fore-gut.' Similarly the
intestine and rectum, behind the stomach, are lined with ectodermal
cells which arise from the inpushed 'hind-gut.' The larval fore- and
hind-guts are broken down at the end of larval life and their lining is
replaced by fresh tissue derived from two imaginal bands which surround
the cavity of the digestive tube, one at the hinder end of the fore-gut,
and the other at the front end of the hind-gut. The larval salivary
glands in connection with the gullet are also broken down, and fresh
glands are formed for the imago.

A large part of the substance of an insect larva consists of muscular
tissue, surrounding the digestive tube, and forming the great muscles
that move the various parts of the body, and of fat, surrounding the
organs and serving as a store of food-material. Very many of the
muscle-fibres and the fat-cells also become disintegrated during the
late larval and pupal stages, and the corresponding tissues of the adult
are new formations derived from special groups of imaginal cells, though
some muscles may persist from the larva to the adult. Similarly the
complex air-tube or tracheal system of the larva is broken down and a
fresh set of tubes is developed, adapted to the altered body-form of
pupa and imago.

The destruction of larval tissue and the development of replacing organs
from special groups of cells, derived of course from the embryo, and
carrying on the continuity of cell-lineage to the adult, are among the
most remarkable facts connected with the life-story of insects. The
process of tissue-destruction is known as 'histolysis'; the rebuilding
process is called 'histogenesis.' Considerable difference of opinion has
existed as to factors causing histolysis, and for a summary of the
conflicting or complementary theories, the reader is referred to the
work of L.F. Henneguy (1904, pp. 677-684). In the histolysis of the
two-winged flies, wandering amoeboid cells--like the white corpuscles or
leucocytes of vertebrate blood--have been observed destroying the larval
tissues that need to be broken down, as they destroy invading
micro-organisms in the body. But students of the internal changes that
accompany transformation in insects of other orders have often been
unable to observe such devouring activity of these 'phagocytes,' and
attribute the dissolution of the larval tissues to internal chemical
changes. The fact that in all insect transformation a part, and in many
a large part, of the larval organs pass over to the pupa and imago,
suggests that only those structures whose work is done are broken down
through the action of internally formed destructive substances, and one
function of the phagocytes is to act as scavengers by devouring what has
become effete and useless.




CHAPTER VI

LARVAE AND THEIR ADAPTATIONS


Among the insects that undergo a complete transformation, there is, as
we have seen in the preceding chapter, an amount of inward change, of
dissolution and rebuilding of tissues, that varies in its completeness
in members of different orders. It is now advisable to consider the
various outward forms assumed by the larvae of these insects, or rather
by a few examples chosen from a vast array of well-nigh 'infinite
variety.'

In comparing the transformations of endopterygote insects of different
orders, it is worthy of notice that in some cases all the members of an
order have larvae remarkably constant in their main structural features,
while in others there is great variety of larval form within the order.
For example, the caterpillars of all Lepidoptera are fundamentally much
alike, while the grubs of beetles of different families diverge widely
from one another. A review of a selected series of beetle-larvae will
therefore serve well to introduce this branch of the subject.

[Illustration: Fig. 12. _a_, Carrion-beetle (_Silpha_) with its larva,
_b_. Magnified, _a_ 3 times, and _b_ 4 times.]

[Illustration: Fig. 13. Larva of a Ground-beetle (_Aepus_). Magnified
6 times. After Westwood, _Modern Classification of Insects_.]

Beetles are as a rule remarkable among insects for the firm consistency
of their chitinous cuticle, the various pieces (_sclerites_) of which
are fitted together with admirable precision. In some families of
beetles the larva also is furnished with a complete chitinous armour,
the sclerites, both dorsal and ventral, of the successive body-segments
being hard and firm, while the relatively long legs possess well-defined
segments and are often spiny. Such a larva is evidently far less unlike
its parent beetle than a caterpillar is unlike a butterfly. Perhaps of
all beetle larvae, the woodlouse-like grub (fig. 12 _b_) of a
carrion-beetle (Silpha) or of a semi-aquatic dascillid such as Helodes
shows the least amount of difference from the typical adult, on account
of the conspicuous jointed feelers. The larval glow-worm, however, is of
the same woodlouse-like aspect, and in this case, where the female never
acquires wings, but becomes mature in a form which does not differ
markedly from that of the larva, the exceptional resemblance is closer
still. In all beetle-grubs the legs are simplified, there being only one
segment (a combined shin and foot) below the knee-joint, whereas in the
adult there is a shin followed by five, four, or at least three
distinct tarsal segments. The foot of an adult beetle bears two claws
at its tip, while the larval foot in the great majority of families has
only one claw. In one section of the order, however, the Adephaga
comprising the predaceous terrestrial and aquatic beetles, the larval
foot has, like that of the adult, two claws. Some adephagous larvae,
notably those of the large carnivorous water-beetles (Dyticus), often
destructive to tadpoles and young fish, have completely armoured bodies
as well as long jointed legs. More commonly, as with most of the
well-known Ground-beetles (Carabidae), the cuticle is less consistently
hard, firm sclerites segmentally arranged alternating with considerable
tracts of cuticle which remain feebly chitinised and flexible. Most of
the adephagous larvae (fig. 13) have a pair of stiff processes on the
ninth abdominal segment, and the insect, from its general likeness to a
bristle-tail of the genus Campodea, is often called a _campodeiform_
larva (Brauer, 1869). From such as these, a series of forms can be
traced among larvae of beetles, showing an increasing divergence from
the imago. The well-known wireworms--grubs of the Click-beetles
(Elateridae)--that eat the roots of farm crops, have well-armoured
bodies, but their shape is elongate, cylindrical, worm-like; and their
legs are relatively short, the build of the insect being adapted for
rapid motion through the soil. The grubs of the Chafers (Scarabaeidae)
are also root-eaters, but they are less active in their habits than the
wireworms, and the cuticle of their somewhat stout bodies is, for the
most part, pale and flexible; only the head and legs are hard and horny.
Usually an evident correspondence can be traced between the outward form
of any larva and its mode of life. For example, in the family of the
Leaf-beetles (Chrysomelidae) some larvae feed openly on the foliage of
trees or herbs, while others burrow into the plant tissues. The exposed
larvae of the Willow-beetles (Phyllodecta, fig. 14) have their somewhat
abbreviated body segments protected by numerous spine-bearing, firm
tubercles. But the grub of the 'Turnip Fly' (Phyllotreta) which feeds
between the upper and lower skins of a leaf, or of _Psylliodes
chrysocephala_ (fig. 15), which burrows in stalks, has a pale, soft
cuticle like that of a caterpillar.

[Illustration: Fig. 14. (_a_) Willow-beetle (_Phyllodecta vulgatissima_)
and its larva (_b_). Magnified 5 times. After Carpenter, _Econ. Proc. R.
Dublin Soc_. vol. I.]

[Illustration: Fig. 15. (_a_) Cabbage-beetle (_Psylliodes chrysocephala_)
magnified 5 times, and its larva (_b_) magnified 12 times.]

In the larvae of the little timber-beetles and their allies (Ptinidae),
including the 'death-watches' whose tapping in old furniture is often
heard, a marked shortening of the legs and reduction in the size of the
head accompany the whitening and softening of the cuticle. This
shortening of the legs is still more marked in the larvae of the
Longhorn Beetles (Cerambycidae) burrowing in the wood of trees or felled
trunks; here the legs are reduced to small vestiges.

[Illustration: Fig. 16. _a_, Grain Weevil (_Calandra granaria_); _b_,
larva; _c_, pupa. Magnified 7 times. After Chittenden, _Yearbook U.S.
Dept. Agric._ 1894.]

Finally in the large family of the Weevils (Curculionidae, fig. 16) and
the Bark-beetles (Scolytidae), the grubs, eating underground root or
stem structures, mining in leaves or seeds, or tunnelling beneath the
bark of trees, have no legs at all, the place of these limbs being
indicated only by tiny tubercles on the thoracic segments. Such larvae
as these latter are examples of the type called _eruciform_ by A.S.
Packard (1898) who as well as other writers has laid stress on the
series of transitional steps from the campodeiform to the eruciform type
afforded by the larvae of the Coleoptera.

A fact of much importance in the transformations of beetles as pointed
out by Brauer (1869) is that in a few families, the first larval instar
is campodeiform, while the subsequent instars are eruciform. We may take
as an example of such 'hypermetamorphosis' the life-story of the Oil or
Blister-beetles (Meloidae) as first described by J.H. Fabre (1857), and
later with more elaboration by H. Beaurégard (1890). From the egg of one
of these beetles is hatched a minute armoured larva, with long feelers,
legs, and cerci, whose task is, for example, to seize hold of a bee in
order that the latter may carry it, an uninvited guest, to her nest.
Safely within the nest, the little 'triungulin' beetle-grub moults; the
second instar has a soft cuticle and relatively shorter legs, which, as
the larva, now living as a cuckoo-parasite, proceeds to gorge itself
with honey, soon appear still further abbreviated. Later comes a stage
during which legs are entirely wanting, the larva then resting and
taking no food. The last larval instar again has short legs like the
grub of the second period. In connection with this life-history we
notice that the newly-hatched larva is not in the neighbourhood of its
appropriate food. Hence the preliminary armoured and active instar is
necessary in order to reach the feeding place; this journey
accomplished, the eruciform condition is at once assumed.

In all cases indeed we may say that the particular larval form is
adapted to the special conditions of life. A few examples from other
orders of endopterygote insects will illustrate this point. The
campodeiform type is relatively unusual, but most of the Neuroptera have
larvae of this kind, active, armoured creatures with long legs, though
devoid of the tail-processes often associated with similar larvae among
the Coleoptera. Such are the 'Ant-lions,' larvae of the exotic lacewing
flies, which hunt small insects, digging a sandy pit for their unwary
steps in the case of the best-known members of the group, some of which
are found as far north as Paris. In our own islands the 'Aphis-lions,'
larvae of Hemerobius and Chrysopa, prowl on plants infested with
'green-fly' which they impale on their sharp grooved mandibles, sucking
out the victims' juices, and then, in some cases, using the dried
cuticle to furnish a clothing for their own bodies. Among these insects,
while the mouth of the imago is of the normal mandibulate type adapted
for eating solid food, the larval mouth is constricted and the slender
mandibles are grooved for the transmission of liquid food.

Turning to eruciform types of larva, we find the _caterpillar_ (fig. 1
_b_, _c_, _d_) distinguished by its elongate, usually cylindrical body
with feeble cuticle, short thoracic legs and a variable number of pairs
of abdominal pro-legs, universal among the moths and butterflies forming
the great order Lepidoptera, and usual among the saw-flies, which belong
to the Hymenoptera. The vast majority of caterpillars feed on the leaves
of plants and their long worm-like bodies with the series of paired
pro-legs, are excellently adapted for their habit of clinging to twigs,
and crawling along shoots or the edges of leaves as they go in search of
food. Of great importance to a caterpillar is its power of spinning
silk, consisting of fine threads solidified from the secretion of
specially modified salivary glands whose ducts open in the insect's
mouth at the tip of the tubular tongue which forms a spinneret.

On the same bush caterpillars of moths and of saw-flies may often be
seen feeding together. The lepidopterous caterpillar, in our countries
at least, has never more than five pairs of pro-legs, situated on the
third, fourth, fifth, sixth, and tenth abdominal segments; each of these
pro-legs bears a number of minute hooklets, arranged in a circular or
crescentic pattern, which assist the caterpillar in clinging to its
food-plant. The saw-fly caterpillar, on the other hand, may have as many
as eight pairs of pro-legs, the series beginning on the second abdominal
segment; here, however, the pro-legs have no hooklets. Among the
Lepidoptera, we notice a reduction in the number of pro-legs in the
'looper' caterpillars of Geometrid moths. Here only two pairs are
present, those on the sixth and tenth abdominal segments. Consequently,
as the caterpillar can cling only by the thorax and by the hinder region
of the abdomen, the middle region of the body is first straightened out
and then bent into an arch-like form, as the insect makes its progress
by alternate movements of stretching and 'looping.'

[Illustration: Fig. 17. _c_, Ruby Tiger Moth (_Phragmatobia
fuliginosa_); _a_, caterpillar; _b_, cocoon. After Lugger, _Insect
Life_, vol. II.]

Caterpillars, with their relatively soft bodies, feeding openly on the
leaves of plants, are exposed to the attacks of many enemies, and the
various ways in which they obtain protection are well worth studying. A
clothing of hairs[7] or spines is often present, and it is interesting
to find that many species of our native Tiger and Eggar Moths (Arctiadae
and Lasiocampidae) which pass the winter in the larval stage, have
caterpillars with an especially dense hairy covering (fig. 17).
Experiments have shown that hairy and spiny insects are distasteful to
birds and other creatures that prey readily on smooth-skinned species, a
conclusion that might well have been expected. Certain smooth
caterpillars however appear to be protected by producing some nauseous
secretion, which renders them unpalatable. Many of these, as the
familiar cream yellow and black larva of the Magpie Moth (_Abraxas
grossulariata_), are very conspicuously adorned, and furnish examples of
what is known as 'warning coloration,' on the supposition that the gaudy
aspect of such insects serves as an advertisement that they are not fit
to eat, and that birds and other possible devourers thus learn to leave
them alone. On the other hand, smooth caterpillars which are readily
eaten by birds are usually 'protectively' coloured, so as to resemble
their surroundings and remain hidden except to careful seekers. Many
such caterpillars are green, the upper surface, which is naturally
exposed to the light, being darker than the lower which is in shadow.
When the caterpillar is large, the green area is often broken up by pale
lines, longitudinal as on the larvae of many Owl Moths (Noctuidae) or
oblique, as on the great caterpillars of most Hawk Moths (Sphingidae).
Such an arrangement tends to make the insect less easily seen than were
it to display a continuous area of the same colour. The 'looper'
caterpillars mentioned above afford remarkable examples of 'protective'
resemblance, for many of them show a marvellous likeness to the twigs of
their food-plant, tubercles on the insect's body resembling closely the
little outgrowths of the plant's cortex. It has been shown by E.B.
Poulton (1892) that many caterpillars are, in their early stages,
directly responsive to their surroundings as regards colour. Usually
green when hatched, they remain green if kept among leaves or young
shoots of plants, while they turn red, brown, or blackish if placed
among twigs of these respective hues. This effect appears to be due to a
direct response of the subcutaneous tissue to the rays of light
reflected from the surrounding objects. The sensitiveness dies away as
the caterpillar grows older, since little or no change of hue in
response to a change of environment could be induced after the
penultimate moult.

[7] The 'hairs' of an insect are not in the least comparable to the
hairs of mammals, being in truth, modified portions of the cuticle,
secreted by special cells.

Among those families of the Lepidoptera which are usually regarded as
low in the scale of organisation, caterpillars are very generally
protected by the habit of feeding in some concealed situation. For
example, the great larvae of the Goat Moth (Cossus) and the whitish
caterpillars of the Clearwing Moths (Sesiidae) burrow through the wood
of trees, eating the timber as they go. The little irritable
caterpillars of the Bell Moths (Tortricidae) roll leaves, fastening the
edges together with silk, and thus make for themselves a shelter; or
they bore their way into seeds or fruits, like the larva of the Codling
Moth that is the cause of 'worm-eaten' apples, too well-known to
orchard-keepers. Very many small caterpillars mine between the two skins
of a leaf, eating out the soft green tissue, and giving rise to a
characteristic blister in form of a spreading patch or a narrow sinuous
track through the leaf. The caterpillars of the Clothes-moths (Tineidae)
make for themselves garments out of their own excrement, the particles
fastened together by silk. In such curious cylindrical cases they wander
over the wool or fur, feeding and indirectly supplying themselves with
clothing at the same time.

The case-forming habit of the Clothes-moth caterpillars leads us
naturally to consider the similar habit adopted by their allies the
Caddis-larvae which live in the waters of ponds and streams, for the
Caddis-flies (Trichoptera) have much in common with the more primitive
Lepidoptera. The caddis-larva is as a rule of the eruciform type, but
with well-developed thoracic legs, and with hook-like tail-appendages;
by means of the latter it anchors itself to the extremity of its curious
'house.' It is of interest to note that in the earlier stages of some
caddises lately described and figured by A.J. Siltala (1907), the legs
are relatively very long, and the larva is quite campodeiform in aspect.
Some of these caddis-grubs retain the campodeiform condition and do not
shelter permanently in cases, as their relations do. Different genera of
caddises differ in their mode of building. Some fasten together
fragments of water-weeds and plant refuse, others take tiny particles of
stone, of which they make firmly compacted walls, others again lay hold
of water-snail shells, which may even contain live inhabitants, and bind
these into a limy rampart behind which their bodies are in safe hiding.

The silk with which the 'caddis-worms' fasten together the materials for
their houses is produced from spinning-glands which like those of the
Lepidoptera open into the mouth.

The survey of the various types of beetle-larvae enumerated above (pp.
50-56) concluded with a short description of the _legless grub_, which
is the young form of a weevil or a bark-beetle. This is a larva in which
the head alone has its cuticle firm and hard; the rest of the body is
covered with a pale, flexible cuticle, so that the grub is often
described as 'fleshy.' This type of larva is by no means confined to
certain families of the beetles, it is frequently met with, in more or
less modified form, in two other important orders of insects, the
Hymenoptera and the Diptera. Among the Hymenoptera this is indeed the
predominant larval type. We have just seen that a caterpillar is the
usual form of larva among the saw-flies, but in all other families of
the Hymenoptera we find the legless grub. A grub of this order may
usually be distinguished from the larva of a weevil or other beetle, by
its relatively smaller head and smoother, less wrinkled cuticle; it
strikes the observer as a feebler, more helpless creature than a
beetle-grub. And it is of interest to note that this somewhat degraded
type of larva is remarkably constant through a great series of
families--gall-flies, ichneumon-flies, wasps, bees (fig. 18), ants--that
vary widely in the details of their structure and in their habits and
mode of life. Almost without exception, however, they make in some way
abundant provision for their young. The feeble, helpless, larva is in
every case well sheltered and well fed; it has not to make its own way
in the world, as the active armoured larva of a ground-beetle or the
caterpillar of a butterfly is obliged to do.

[Illustration: Fig. 18. Young Larva (_FL_), Full-grown Larva (_SL_) and
Pupa (_N_) of Hive-bee (_Apis mellifica_). _co_, cocoon; _sp_,
spiracles; _ce_, eye; _an_, feeler; _m_, mandible; _l_, labium.
Magnified 4 times. After Cheshire, _Bees_.]

Among those saw-flies whose larvae feed throughout life in a concealed
situation, we find an interesting transition between the caterpillar
and the legless grub. For example, the giant saw-flies (so called
'Wood-wasps') have larvae that burrow in timber, and these larvae
possess relatively large heads, somewhat flattened bodies with pointed
tail-end, and very greatly reduced legs. The feeble legless grub,
characteristic of the remaining families of the Hymenoptera, is provided
for in a well-nigh endless variety of ways. The female imago among these
insects is furnished with an elaborate and beautifully formed
ovipositor, and the act of egg-laying is usually in itself a provision
for the offspring. Gall-flies pierce plant-tissues within which their
grubs find shelter and food, the plant responding to the irritation due
to the presence of the larva by forming a characteristic growth, the
_gall_, pathological but often regular and shapely, in whose hollow
chamber the grub lives and eats. Ichneumon-flies and their allies pierce
the skin of caterpillars and other insect-larvae, laying their eggs
within the victims' bodies, which their grubs proceed to devour
internally. Some very small members of these families are content to lay
their eggs within the eggs of larger insects, thus obtaining rich
food-supply and effective protection for their tiny larvae. In
Platygaster and other genera of the family Proctotrypidae, M. Ganin
(1869) showed the occurrence of hypermetamorphosis somewhat like that
already described as occurring among the Oil-beetles (Meloidae). The
larva of Platygaster is at first rather like a small Copepod crustacean,
with prominent spiny tail-processes; after a moult this form changes
into the legless grub characteristic of the Hymenoptera, among which
larvae even approaching the campodeiform type are very exceptional. The
species of Platygaster pass their larval stages within the larvae of
gall-midges.

Wasps, bees and ants, have the ovipositor of the female modified into a
sting, which is often used for the purpose of providing food for the
helpless grubs. Thus the digging wasps (Sphegidae and Pompilidae) hunt
for caterpillars, spiders, and other creatures which they can paralyse
with their stings, and bury them alongside their eggs to furnish a
food-supply for the newly-hatched young. The social wasps and many ants
sting and kill flies and other insects, which they break up so as to
feed their grubs within the nest. It is well known that the labour of
tending the larvae in these insect societies is performed for the most
part not by the mother ('Queen') but by the modified infertile females
or 'workers.' Other ants and the bees feed their grubs (fig. 18), also
sheltered in well-constructed nests, on honey elaborated from nectar
within their own digestive canals. In all cases we see that the
helplessness of the grub is associated with some kind of parental care.

[Illustration: Fig. 19. Larva of Gall-midge (_Contarinia nasturtii_),
ventral view showing anchor process (_a_), and spiracles projecting at
sides. Magnified 30 times. From Carpenter, _Journ. Econ. Biol_, vol.
VI.]

From the Hymenoptera we may pass on to the Diptera or Two-winged Flies,
an order of which the vast number of species and in many cases the
myriads of individuals force themselves on the observer's notice. F.
Brauer (1863) divided the Diptera into two sub-orders[8]; of the first
of these a Crane-fly or 'Daddy-long-legs' may be taken as typical, of
the second an ordinary House-fly or Bluebottle. All the larvae of the
Diptera are legless, those of the Crane-fly group have well-developed
hard heads, with biting mandibles, but in the House-fly section the
larva is of the degraded _vermiculiform_ type known as the _maggot_,
not only legless, but without a definite head, the front end of the
creature usually tapering to the mouth, where there are a pair of strong
hooks, used for tearing up the food. A few examples of each of these
types must suffice in the present brief survey. A few pages back (p. 66)
reference was made to the production of galls on various plants, through
the activity of larvae of the hymenopterous family Cynipidae. Many
plant-galls are due, however, to the presence of grubs of tiny dipterous
insects, the Cecidomyidae or Gall-midges. A cecid grub (fig. 19) has an
elongate body with flexible, wrinkled cuticle, tapering somewhat at the
two ends. The head, if rather narrow, is distinct, and beneath the
prothorax is a characteristic sclerite known as the 'anchor process' or
'breast bone.' Along either side of the body is a series of paired
spiracles, each usually situated at the tip of a little tubular
outgrowth of the cuticle; the hindmost spiracles are often larger than
the others. These little grubs live in family communities, their
presence leading to some deformation of the plant that serves to shelter
them. A shrivelled fruit or an arrested and swollen shoot, such as may
be due respectively to the Pear-midge (_Diplosis pyrivora_) or the
Osier-midge (_Rhabdophaga heterobia_), is a frequent result of the
irritation set up by these little grubs. In a larva of the crane-fly
family (Tipulidae, fig. 20) living underground and eating plant-roots,
like the well-known 'leather-jacket' grubs of the large
'Daddy-long-legs' (Tipula) or burrowing into a rotting turnip or swollen
fungus, like the more slender grub of a 'Winter Gnat' (Trichocera), the
student notices a somewhat tough cuticle, a relatively small but
distinct head, and frequently prominent finger-like processes on the
tail-segment. Further examination shows a striking modification in the
arrangement of the spiracles. Instead of a paired series on most of the
body-segments, as in caterpillars and the vast majority of insects
whether larval or adult, there are two large spiracles surrounded by the
prominent tail-processes, and a pair of very small ones on the
prothorax, the latter possibly closed up and useless. This restriction
of the breathing-holes to a front and hind pair (amphipneustic
condition) or to a hind pair only (metapneustic type) is highly
characteristic of the larvae of Two-winged flies.

[8] Known as the Orthorrhapha and the Cyclorrhapha; these terms are
derived from the manner in which the larval or pupal cuticle splits, as
will be explained in the next chapter (p. 88).

[Illustration: Fig. 20. Crane-fly (_Tipula oleracea_), _a_, female; _b_,
larva ('leather-jacket' grub). Magnified twice.]

[Illustration: Fig. 21. Maggot of House-fly (_Musca domestica_), _a_,
side-view, magnified 5 times; _b_, prothoracic spiracle; _c_, feeler;
_d_, hind-region with posterior spiracles; _e_, _f_, head-region with
mouth-hooks; _g_, head-region of young maggot; _h_, eggs. All magnified.
After Howard, _Entom. Bull._ 4, _U.S. Dept. Agric._]

Turning now to the _maggot_, characteristic of the House-fly section
(fig. 21) of the Diptera, we see the greatest contrast between the larva
and the imago that can be found throughout the whole class of the
insects. The Bluebottle's eggs, the well-known 'fly blow' laid in summer
time on exposed meat, not unnaturally arouse feelings of disgust, yet
they are the prelude to one of the most marvellous of all insect
life-stories. The fly--with its large globular head, bearing the
extensive compound eyes, the highly modified feelers with their
exquisitely feathered slender sensory tips, and the complex suctorial
jaws; with its compact thorax bearing the glassy fore-wings alone used
for flight, though the hind-wings modified into tiny drumstick-like
'halters' are the organs of a fine equilibrating sense--is perhaps the
most specialised, structurally the 'highest' of all insects. Yet in a
week or two this swift, alert, winged creature is developed from the
degraded maggot, white, legless, headless, that buries itself in putrid
flesh, 'feeding on corruption.'

The broad end of the maggot is the tail, while the narrow extremity
marks the position of the mouth. Above this are a pair of very short
feelers (fig. 21 _c_), while from the aperture project the tips of the
mouth-hooks (fig. 21 _e_, _f_), formidable, black, claw-like structures,
articulated to the strong pharyngeal sclerites and moved by powerful
muscles, tearing up the fibres of the flesh. On either side of the
prothorax is an anterior spiracle, a curious branching or fan-like
outgrowth (fig. 21 _b_), with a variable number of tiny openings which
are probably of little use for the admission of air to the tubes. In
many maggots the mouth-hooks and the front spiracles become more and
more complex in form in the successive instars. The cuticle, white and
smooth to the unaided eye, is seen on microscopic study to be set with
rows of tiny spines which assist the maggot's movements through its
food-mass. At the tail-end the large hind spiracles are conspicuous on a
flattened dorsal area of the ninth abdominal segment; each shows a hard
brown plate, traversed by three slits. And as we watch this curious
degraded larva thrusting its narrow head-end into the depths of its
ofttimes loathsome food-supply, we understand the advantage of access to
the air-tube system being mainly confined to the hinder end of the body.

Maggots, differing from that of the Bluebottle only in minor details,
are the larval forms of a vast multitude of allied species and display
great variation in the nature of their food. Most, however, hide their
soft defenceless bodies in some substance which affords shelter as well
as food. The Bluebottle maggot burrows into flesh, that of the House-fly
into horse-dung or vegetable refuse. The maggot of the Cabbage-fly eats
its way into the roots of cruciferous plants, that of the Mangel-fly
works out a broad blister between the two skins of a leaf, into which
the newly-hatched larva crawls directly from the egg. A large number of
species, forming an entire subfamily (the Tachininae) have larvae that
feed as parasites within the bodies of other insects.

The habit of parasitism by maggots in back-boned animals has led to some
remarkable modifications of the larva and to curious adventures in the
course of the life-story. The Bot-fly of the Horse (_Gastrophilus equi_)
and the Warble-fly of the Ox (_Hypoderma bovis_, fig. 22) lay eggs
attached to the hairs of grazing animals, which, at least in the case of
Gastrophilus, lick the newly-hatched larvae into their mouths. The
'bot,' or maggot of Gastrophilus, comes to rest in the horse's stomach;
often a whole family attach themselves by their mouth-hooks to a small
patch of the mucous coat of that organ. The maggot is relatively short
and stout, with rows of strong spicules surrounding the segments, and
with spiracles capable of withdrawal through a cup-like inpushing of the
tail-region of the body, so that the parasite is preserved from drowning
when the host drinks water. The young maggot of Hypoderma (fig. 22 _e_)
is elongate and slender, spends its first two stages burrowing in the
gullet wall and then wandering through the dorsal tissues of its host;
ultimately it arrives beneath the skin of the back and assumes for its
third and fourth instars a broad barrel-like form (fig. 22 _b_). The
supply of free oxygen within the ox's tissues being now insufficient,
the warble-maggot bores a circular hole through the skin and rests with
the tail spiracles directed upwards towards the outer air. When fully
grown the maggot works its way through the hole in the host's skin, and
falling to the ground pupates in some sheltered spot, the life cycle
occupying about a year. Similarly the Horse-bot escapes from the host's
intestine with the excrement, and pupates on the ground.

A curious modification of the maggot is noticeable in the larva of the
Hover-flies (Syrphus). These, unlike most of their allies, live exposed
on the foliage of plants, where they feed by preying on aphids.

[Illustration: Fig. 22. Ox Warble-fly (_Hypoderma bovis_), _a_, female;
_b_, full-grown maggot from back of ox, dorsal view; _c_, egg; _d_,
empty puparium, ventral view; _e_, young maggot from gullet, ventral
view. Magnified (lines show natural size). _a-d_, after Theobald, _2nd
Report Econ. Zool._ (_Brit. Mus._).]

In agreement with this manner of life, the cuticle is roughly
granulated, often greenish or reddish in hue, and the maggot, despite
its want of definite head and sense organs, moves actively and
purposefully about, often rearing up on its broad tail-end with an aphid
victim impaled on its mouth-hooks.

In a previous chapter reference was made to the exopterygote insects,
stone-flies, dragon-flies, and may-flies, whose preparatory stages live
in the water. Among the endopterygote orders many Neuroptera and
Coleoptera, all Trichoptera, a very few Lepidoptera and many Diptera,
have aquatic larvae. One or two examples of the adaptations of dipteran
larvae to life in the water may well bring the present chapter to a
close. Many members of the hover-fly family (Syrphidae) have maggots
with the tail-spiracles situated at the end of a prominent tubular
process. Among the best-known of syrphid flies are the drone-flies
(Eristalis), often seen hovering over flowers, and presenting a curious
likeness to hairy bees. The larva of Eristalis is one of the most
remarkable in the whole order, the 'Rat-tailed maggot' found in the
stagnant water of ditches and pools. It has a cylindrical body with the
hinder end drawn out into a long telescopic tube, a more slender
terminal section being capable of withdrawal into, or protrusion from, a
thicker basal portion. At the extremity of the slender tube is a crown
of sharp processes, forming a stellate guard to the spiracles. These
processes can pierce the surface-film of the water, and place the
tracheal system of the maggot in touch with the pure upper air; while
its mouth may be far down, feeding among the foul refuse of the ditch,
it can still reach out to the medium in which the end of its life-story
must be wrought out.

Reverting to the first great division of the Diptera, we find varied
adaptations to aquatic life among many grubs that possess a definite
head. The larva of a Gnat (Culex[9]) has projecting from the hind region
of the abdomen a long tubular outgrowth, at the end of which are the
spiracles, guarded by three pointed flaps forming a valve. When closed
these pierce the surface-film of the water in which the larva lives;
when opened a little cup-like depression is formed in the surface-film,
from which the larva hangs. Or having accumulated a supply of air, it
can disengage itself from the surface-film and dive through the water,
its tracheal system safely closed. Another mode of breathing is found in
the 'Blood-worms' and allied larvae of the Harlequin-midges
(Chironomidae) whose transformations are described in detail by Miall
and Hammond (1900). These larvae have two pairs of cylindrical,
spine-bearing pro-legs--one on the prothorax and the other on the
hindmost abdominal segment; the latter structures serve to fix the
larva in the muddy tube which it inhabits at the bottom of its native
pond. The penultimate abdominal segment has four long hollow outgrowths,
which contain blood, and have the function of gills, while the hindmost
segment has four shorter outgrowths of the same nature. Enabled thus to
breathe dissolved air, the Chironomus larva needs not, like the Culex or
the Eristalis, to find contact with the atmosphere beyond the
surface-film.

[9] See _Frontispiece_, A.

Most remarkable, in many respects, of all aquatic larvae are the grubs
of the Sand-midges (Simulium). These live entirely submerged and, having
no special gills, carry out an exchange of gases through the general
surface of the cuticle between the dissolved air in the water and the
cavities of the air-tube system. The body is shaped like a flask swollen
slightly at the hinder end and possesses a median pro-leg just behind
the head, also another at the tail, which serves to attach the larva to
a stone or to the leaf of an aquatic plant. The head has, in addition to
feelers and jaws, a pair of processes with wonderful fringes which by
their motion set up currents in the water, and bring food particles
within reach of the mouth. A number of the larvae usually live in a
community. Their power of spinning silken threads by which they can work
their way back when accidentally dislodged from their resting-place, has
been vividly described by Miall (1895).

Examples might be multiplied, but enough have been given to enforce the
conclusion that the forms of insect-larvae are wondrously varied, and
that frequently, within the limits of the same order or even family,
modifications of type may be found which are suited to various modes of
life adopted by different insects. A survey of the multitudes of insect
larvae--grubs, caterpillars, maggots--living on land, on plants,
underground, in the water; feeding on leaves, in stems, on roots, on
carrion, on refuse; by hunting or by lurking after prey; as parasites or
as scavengers, brings home to us most strongly the conclusion that each
larva is fitted to some little niche in the vast temple of life, each is
specially adapted to its part in the great drama of being.




CHAPTER VII

PUPAE AND THEIR MODIFICATIONS


The pupal stage is characteristic of the life-story of those insects
whose larvae have wing-rudiments in the form of inpushed imaginal discs,
and in all these insects there is, as we have seen, considerable
divergence in form between larva and imago. In the pupa the wings and
other characteristically adult structures are, for the first time,
visible outwardly; it is the instar which marks the great crisis in
transformation. The pupa rests, as a rule, in a quiescent condition, and
during the early period of this stage the needful internal changes, the
breaking down of many larval tissues, and their replacement by imaginal
organs, go on. Both outwardly and inwardly therefore, the insect
undergoes, at the pupal stage, a reconstruction necessitated by the
differences in form and often in habit, between the larva and the winged
adult; and the greater these differences, the more profound must be the
changes that mark the pupal stage.

From the prominence of imaginal structures in the pupa, it is at once
seen that the pupa of any insect must resemble the adult more nearly
than it resembles the larva. But in different groups of insects we find
different degrees of likeness between pupa and imago. In a beetle pupa
(see fig. 16 _c_), the appendages--feelers, jaws, legs, wings--stand out
from the body as do those of the perfect insect. This type is called a
_free_ pupa. The pupal cuticle has to be shed for the emergence of the
imago, but the pupa is already a somewhat reduced model of the final
instar, with abbreviated wings and doubled-up legs. A free pupa is
characteristic of the Coleoptera, Neuroptera, Trichoptera, Hymenoptera
and many Diptera. In some cases the pupa requires to be specially
adapted for a peculiar mode of life; for example, a special arrangement
of breathing organs may be necessary for life under water, and there
must needs be temporary pupal structures, not represented in the imago.

On the other hand, in the pupae of most Lepidoptera and of some Diptera,
there is more or less coalescence between the cuticle of the appendages
and the cuticle of the body generally, so that the appendages do not
stand out, but being, as it were, glued down to the body, are somewhat
masked (see fig. 1 _e_ and fig. 23). Consequently the _obtect_ pupa, as
this type is called, does not resemble its imago as fully as a free pupa
does. The outline of the wings for example in a butterfly's pupa can in
some cases be traced only with difficulty. T.A. Chapman has shown (1893)
that the completely obtect pupa characterises the more highly developed
families of Lepidoptera, while in the more primitive families the pupa
is incompletely obtect. If the pupa of a butterfly or moth be lifted and
held in the hand, a bending or wriggling motion of the abdomen can be
observed. In the incompletely obtect pupa, this motion is evident in a
greater number of segments than in the completely obtect, the number
concerned varying from five to two in different families. In the
nymphalid butterflies, the pupa is often called a 'chrysalis' on
account of the golden hue displayed by the cuticle, and the term
'chrysalis' is sometimes bestowed indiscriminately on any kind of pupa.
It has been shown by Poulton (1892) and others, that the colour of a
butterfly pupa is to some extent affected by the surroundings of the
caterpillar just before its last moult.

Reference has been made (p. 58) to the power of spinning silk possessed
by many larvae; often the principal use of this silk is to form some
protection for the pupa, the larva before its last moult constructing a
_cocoon_ within which the pupa may rest safely. Many larvae bury
themselves in the earth, and the pupa lies in an earthen chamber, the
lining particles of soil fastened together by fine silken threads.
Larvae that feed in wood, like the caterpillar of the Goat-moth (Cossus)
make a cocoon of splinters spun together, while hairy caterpillars, such
as those of the Tiger-moths, work some of their hairs in with the silk
to make a firm cocoon (fig. 17 _b_). On the other hand, those
caterpillars known as 'silkworms' make a dense cocoon of pure silk,
consisting of two layers, the outer of coarse and the inner of fine
threads. Silken cocoons very similar in appearance are spun by the
larvae of small Ichneumon-flies. Many pupae lie in a loose cocoon formed
of a few interlacing threads, as for example the conspicuous black and
yellow banded pupa of the Magpie-moth (_Abraxas grossulariata_) and the
pupae of various leaf-beetles. Others again spin together the edges of
leaves with connecting silken threads. The grubs of bees and wasps which
are reared in the comb-chambers of their nests seal up the opening of
the chamber with a lid, partly silk (fig. 18 _co_) and partly excretion,
when ready to pass into the pupal state. An additional external
'capping' may be also supplied by the workers.

The pupae of butterflies are especially interesting, as illustrating the
extreme reduction of the silken cocoon. The pupa of a 'swallowtail'
(Papilionid) or a 'white' (Pierid) butterfly (fig. 23) may be found
attached to a twig of its food-plant or to a wall, in an upright
position, its tail fastened to a pad of silk and a slender silken girdle
encircling its thorax. The pupa of a 'Tortoiseshell' or 'Admiral'
(Nymphalid) butterfly hangs head downwards from a twig, supported only
by the tail-pad of silk, which, useless as a shelter, serves only for
attachment. The pupa is fastened to this pad by a spiny hook or process,
the _cremaster_ (fig. 23 _cr_), on the last abdominal segment. The
cremaster is a characteristic structure in the pupa of a moth or
butterfly. C.V. Riley (1880) and W. Hatchett-Jackson (1890) have shown
that it corresponds with a spiny area, the suranal plate, which lies
above the opening of the caterpillar's intestine. The means by which the
suspended pupa of a nymphalid butterfly attaches its cremaster to the
silken pad which the larva has spun in preparation for pupation, is
worthy of brief attention. The caterpillar, hanging head downwards, is
attached to the silken pad by its hindmost pair of pro-legs or claspers
and by the suranal plate, and the cuticle is slowly worked off from
before backwards, so as to expose the pupa. Were the process of moulting
to be simply completed while the insect hangs by the claspers, the pupa
would of course fall to the ground. But there is enough adhesion between
the pupal and larval cuticles at the hinder end of the body, especially
by means of the everted lining of the hind-gut, for the pupa to be
supported while it jerks its cremaster out of the larval cuticle and
works it into the meshes of the silken pad. The moult is thus completed
and the pupa hangs securely all the time. In the numerous cases where
the pupa is enclosed in a cocoon, the cremaster serves to fix the pupa
to the surrounding silk. Chapman (1893) has drawn attention to the fact
that among the more highly organised moths the pupa remains in the
cocoon, the emergence being entirely left to the imago, while the pupae
of the more primitive moths work their way partly out of the cocoon
before the final moult begins. In the latter case, the cremaster is
anchored by a strand of silk which allows a certain degree of emergence,
and the pupa has rows of spines on its abdominal segments, of which a
greater number retain the power of mutual motion than in those pupae
which do not come out of their cocoons.

[Illustration: Fig. 23. Pupa of White Butterfly (_Pieris_), side view;
_f_, feeler; _w_, wing; _sp_, spiracle; _p_, anal pro-leg; _cr_,
cremaster. Magnified 8 times. In part after Hatchett-Jackson, _Trans.
Linn. Soc._ 1900, and Tutt's _British Butterflies_.]

While the pupa on the whole resembles the imago that is to emerge from
it, there are not a few cases in which a special structure necessary for
some contingency in pupal life is retained or adopted in this stage. A
butterfly pupa, like the imago, has no mandibles, but in the case of the
Caddis-flies (Trichoptera) and two families of small moths, the most
primitive of all Lepidoptera, the pupa, like the larva, has
well-developed mandibles. These enable the caddis pupa to bite its way
out of the shortened larval case in which it has pupated, and then to
swim upwards through the water ready for the caddis-fly's emergence into
the air. Pupae that are submerged require special breathing-organs. In
the previous chapter (p. 77) mention was made of the gnat's aquatic
larva with its tail-spiracles adapted for procuring atmospheric air
through the surface-film. The pupa of the gnat[10] also has 'respiratory
trumpets' serving the same purpose, but these are a pair of processes on
the prothorax, so that the pupa, which is fairly active, hangs from the
surface-film with its abdomen pointing downwards through the water. This
change of position is correlated with the necessity for the imago to
emerge into the air; were the pupa to hang head downwards as the larva
does, the gnat would perforce have to dive into the water. With the
beautifully adapted transfer of the functional spiracles, their position
is appropriately arranged for the gnat's emergence at the surface, and
the empty pupal cuticle floats serving the insect as a raft. On this it
rests securely and the crumpled wings have opportunity to expand and
harden before the insect takes to flight.

[10] See _Frontispiece_, B.

The aquatic pupae of other Diptera, many species of the midges
Chironomus and Simulium for example, breathe dissolved air by means of
tufts of thread-like gills, which arise on either side of the prothorax.
The pupae of Simulium rest in their curious little cup-like dwellings,
attached to submerged stones or plants. The Chironomus pupa is usually
found in an elongate gelatinous case adhering to a stone. From this case
the pupa rises to the surface of the water, that the midge may emerge
into the air. Miall and Hammond (1900) describe the arrangement by
which, when the pupal stage ends, and these gills are no longer
required, their connection with the air-tube system is severed 'without
undue violence.' The walls of the fine air-tubes that pass into the
gills are specially strengthened, but just below the pupal cuticle these
walls are exceedingly thin and delicate. Thus when the pupal cuticle is
cast, they are readily broken there, and the cuticle of the midge
forming beneath has a spiracular opening into the main air-trunk, ready
for use during the insect's aerial life.

Among those Diptera whose larva is the headless maggot a most
remarkable arrangement for protecting the pupa is to be found. The last
larval cuticle, instead of being as usual worked off and cast, after
separation from the underlying structures, becomes hard and firm,
forming a protective case (_puparium_) within which by the processes of
histolysis and histogenesis already described the organs of the pupa and
imago are built up. This puparium (fig. 22 _d_) is usually dark in
colour, often brown and barrel-shaped, and a subcircular lid splits off
from it at the head-end to allow the emergence of the fly[11]. While the
maggot breathes by its tail-spiracles, the functional spiracles of the
puparium (connected with the tracheal system of the enclosed pupa) are
far forward, and these may be situated at the tips of long sometimes
branching processes, which recall the thoracic gills of the aquatic
pupae mentioned a few pages above. Adaptations, various and beautiful,
to special modes of life, are thus seen to characterise pupae as well as
larvae.

[11] The presence of this sub-circular lid characterises Brauer's
suborder Cyclorrhapha. Those Diptera in which the pupal cuticle splits
in the normal, longitudinal manner are included in the Orthorrhapha (see
p. 67).




CHAPTER VIII

THE LIFE-STORY AND THE SEASONS


A number of interesting questions are associated with the seasonal cycle
of an insect's life-history. In a previous chapter (IV. pp. 30, 34)
reference has been made to the contrast between the long aquatic life of
the larval dragon-fly or may-fly, extending over several years, and the
short aerial existence of the winged adult restricted in the case of the
may-flies to a few hours. Here we see that the feeding activities of the
insect are carried on during the larval stage only; the may-fly in its
winged condition takes no food, pairing and egg-laying form the whole of
its appointed task. A similar though less extreme shortening of the
imaginal life may be noticed in many endopterygote insects. For example,
the bot- and warble-flies have the jaws so far reduced that they are
unable to feed, and the parasitic life of the maggot (see p. 74)
extending over eight or nine months in the body of the horse or ox,
prepares for a winged existence of probably but a few days. Again in
many moths the jaws are reduced or vestigial so that no food can be
taken in the winged state, as for example in the 'Eggars'
(Lasiocampidae) and the 'Tussocks' (Lymantriidae). It is noteworthy
that in these short-lived insects the male is often provided with
elaborate sense-organs which, we may believe, assist him to find a mate
with as little delay as possible; the male may-fly has especially
complex eyes, while the feelers of the male silk-moth or eggar are
comb-like or feathery, the branches bearing thousands of sensory hairs.
A box with a captive living female of one of these moths, if taken into
a wood haunted by the species becomes rapidly surrounded by a swarm of
would-be suitors, attracted by the odour emitted from the prisoner's
scent-glands.

Very exceptionally the imaginal stage may be omitted from the life-story
altogether. Nearly fifty years ago N. Wagner (1865) made the remarkable
discovery that in the larvae of certain gall-midges (Cecidomyidae) the
ovaries might become precociously mature and unfertilised eggs might be
developed into small larvae observable within the body of the
mother-larva; ultimately these abnormally reared young break their way
out. In this case therefore there may be a series of larval generations,
neither pupa nor imago being formed. Extended observations on the
precocious reproductive processes of these midges have lately been
published by W. Kahle (1908). A less extreme instance of an abbreviated
life-story was made known by O. Grimm (1870) who saw pupae of
Harlequin-midges (Chironomus) lay unfertilised eggs, which developed
into larvae. Here the imaginal stage only is omitted from the
life-history. Not always however is it the imaginal stage of the
life-history which is shortened. Reference (p. 18) has already been made
to the case of the virgin female aphids, whose eggs develop within the
mother's body, so that active, formed young are brought forth. Among the
Diptera it is not unusual to find similar cases, the female fly giving
birth to young maggots instead of laying eggs. Such is the habit of the
great flesh-fly (Sarcophaga), of some allied genera (Tachina, etc.)
whose larvae live as parasites on other insects, and occasionally of the
Sheep Bot-fly (Oestrus). In such cases we recognise the beginning of a
shortened larval period, and Brace's investigations in 1895, summarised
by E.E. Austen (1911), have shown that females of the dreaded African
Tsetse flies (Glossinia) bring forth nearly mature larvae, which pupate
soon after birth. In another group of Diptera, the blood-sucking
parasites of the Hippoboscidae and allied families, the whole larval
development is passed through within the mother's body, and a full-grown
larva is born the cuticle of which hardens and darkens immediately to
form a puparium; hence these flies are often called, though incorrectly,
Pupipara. Still more astonishing is the mode of reproduction in the
allied family of the Termitoxeniidae, curious, degraded, wingless
'guests' of the termites, or 'white ants,' lately made known through the
researches of E. Wasmann (1901). Here the individual is hermaphrodite--a
most exceptional condition among insects--and lays a large egg, whence
is usually hatched a fully-developed adult! Here then we find that all
the early stages, usual in the higher insects, are omitted from the
life-story.

Interesting comparison may be made between the total duration of various
insect life-stories. To some extent at least, the length of an insect's
life is correlated with its size, its food, the season of the year when
it breeds. Small insects have, as a rule, shorter lives than large ones;
those whose larvae devour highly nutritive food generally develop more
quickly than those which have to live on dry, poor, substances;
life-cycles follow one another most rapidly in summer weather when
temperature is high and food plentiful.

In early chapters we have already noticed the long aquatic life of the
larva and nymph of a dragon-fly, relatively a large insect, and the
rapid multiplication of the repeated summer broods of virgin aphids (p.
18). Within the one order of the Coleoptera it is instructive to compare
the small jumping leaf-beetles, the 'turnip-flies' of the farmer, whose
larvae mine in the green tissues, and complete their transformations so
rapidly that several successive broods appear in the spring and early
summer, with the larger click-beetles whose larvae, the equally
notorious 'wireworms,' feed on roots for three or four years before they
become fully grown. Among the Diptera, the 'leather-jacket' grub of the
crane-fly, feeding like the wireworm on roots, has a larval life
extending through the greater part of a year, while the maggot of the
bluebottle, feeding on a rich meat diet, becomes mature in a few days.
As examples of excessively long life-cycles the 'thirteen-year' and
'seventeen-year' cicads of North America, described by C.L. Marlatt
(1895), are noteworthy. Certain specially populous 'broods' of these
insects are known and localised, so that the appearance of the imagos in
future years can be accurately predicted. Here again we have to do with
bulky insects whose subterranean larvae and nymphs feed on comparatively
innutritious roots.

In our own climate, it is of interest to notice the variation among
insects as to the stage which carries the race over the winter. The
click-beetles, mentioned just above, emerge from their buried pupae in
summer, hibernate under stones or clods, and lay eggs among the herbage
next spring. At the same time of course, owing to the extended term of
the larval life, many more individuals of the species are wintering
underground as 'wireworms' of various ages, and these, except in very
severe frosts, can continue their occupation of feeding on roots. But in
the case of the 'turnip-flies' the food-supply is cut off in winter, and
all those beetles of the latest summer brood that survive hibernate in
some sheltered spot, waiting for the return of spring, that they may lay
their eggs, and start the life-cycle once again. Among the Diptera, most
species pass the winter as pupae, the sheltering puparium being a good
protection against most adverse conditions, or as flies. But where there
is a prolonged parasitic larval life, as with the bot- and warble-flies,
the maggot, warm and well-fed within the body of its mammalian host,
affords an appropriate wintering stage.

Among the Hymenoptera an especially interesting seasonal life-cycle is
afforded by the alternation of summer and winter generations in many
Gall-flies (Cynipidae) as H. Adler (1881, 1896) demonstrated for most of
our common species. The well-known 'oak-apples' are tenanted in summer
by grubs, which after pupation develop into winged males and wingless
females. The latter, after pairing, burrow underground and lay their
eggs in the roots, the larvae causing the presence there of globular
swellings or root-galls within which they live, pass through their
transformations and develop into wingless virgin females. These shelter
until February or March in their underground chambers, then climb up
the tree and lay on the shoots eggs, from which will be hatched the
grubs destined to grow within the oak-apples into the summer sexual
brood of flies.

The Lepidoptera afford examples of hibernation in all stages of the
life-history. In this order a few large moths with wood-boring
caterpillars, the 'Goat' (Cossus) for example, undergo a development
extending over several years, while at the other extreme a few small
species may have three or more complete cycles within the twelve months.
But in the vast majority of Lepidoptera we find either one or two
generations, definitely seasonal, within the year; the insect is either
'single-brooded' or 'double-brooded.'

Almost every winter one or more letters may be read in some newspaper
recording the writer's surprise at seeing on a sunny day during the cold
season, one of our common gaily-coloured butterflies of the Vanessa
group, a 'Tortoiseshell' or 'Red Admiral,' flitting about. Surprise
might be greater did the observers realise that the imaginal is the
normal hibernating stage for these species. Emerging from the pupa in
late summer or autumn, they shelter during winter in hollow trees, under
thatched eaves, in outbuildings or in similar situations, coming out in
spring to lay their eggs on the leaves of their caterpillars'
food-plants. The larvae feed and grow through the early summer months,
in the case of the Small Tortoiseshell (_Vanessa urticae_) pupating
before midsummer and developing into a July brood of butterflies whose
offspring after a late summer life-cycle, hibernate; while for the
larger species of the group there is, in our islands, only one complete
life-cycle in the year, though the same insects in warmer countries may
be double-brooded. C.G. Barrett records (1893, vol. I. pp. 153-4) how in
the August of 1879 hundreds and thousands of 'Painted Ladies' (_Pyrameis
cardui_) migrated into the south of England from the European continent
where in many places great swarms had been observed early in the summer.
'These August butterflies, the progeny of the June swarms, coming from a
warmer climate, had no intention of hibernating, but paired and laid
eggs. Some of the larvae were collected and reared indoors [butterflies]
emerging in November and December, but out of doors all must have been
destroyed by damp or frost, in either the larva or pupa state, for no
freshly emerged specimens were noticed in the spring, and no trace of
the great migration remained.'

In September and October the pedestrian, even in a suburban square, may
see moths with pretty brown, white-spotted wings flying around trees.
These are males of the common 'Vapourer' (_Orgyia antiqua_), in search
of the females which, wingless and helpless, rest on the cocoons
surrounding the pupae whence they have just emerged, the cocoons being
attached to the branches of the trees where the caterpillars have fed.
After pairing, the female lays her eggs among the silk of the cocoon,
partly covering them with hairs shed from her body, and then dies. The
eggs thus protected remain through the winter, the larvae not being
hatched till springtide, when the young leaves begin to sprout forth.
The caterpillars, adorned and probably protected by their 'tussocks' of
black or coloured bristles, feed vigorously. Their activity and habit of
occasional migration from one tree to another, compensates, to some
extent, as Miall (1908) has pointed out, for the females' enforced
passivity; only in the larval state can moths with such wingless females
extend their range. The caterpillars spin their cocoons towards the end
of summer, and then pupate, the moths emerging in the autumn and the
eggs, as we have seen, furnishing the winter stage.

After midsummer, the conspicuous cream, black and yellow-spotted
'Magpie' moth (_Abraxas grossulariata_) is common in gardens. The female
lays her eggs on a variety of shrubby plants; gooseberry and currant
bushes are often chosen. From the eggs caterpillars are hatched in
autumn, but these, instead of beginning to feed, seek almost at once for
rolled-up leaves, cracks in walls, crannies of bark, or similar places,
which may afford winter shelters. Here they remain until the spring,
when they come out to feed on the young foliage and grow rapidly into
the conspicuous cream, yellow and black 'looper' caterpillars mentioned
in a previous chapter (p. 60). These, when fully-grown, spin among the
twigs of the food-plant a light cocoon, in which the black and
yellow-banded wasp-like pupa spends its short summer term before the
emergence of the moth.

An equally familiar garden insect, the common 'Tiger' moth (_Arctia
caia_) with its 'woolly bear' caterpillar, affords a life-cycle slightly
differing from that of the 'Magpie.' The gaudy winged insects are seen
in July and August, and lay their eggs on a great variety of plants. The
larvae hatched from these eggs begin to feed at once, and having moulted
once or twice and attained about half their full size, they rest through
the winter, the dense hairy covering wherewith they are provided forming
an effective protection against the cold. At the approach of spring they
begin to feed again, and the fully-grown 'woolly bear' is a common
object on garden paths in May and June. Before midsummer it has usually
spun its yellow cocoon under some shelter on the ground and changed into
a pupa.

Another modification with respect to seasonal change is shown by the
Turnip moth (_Agrotis segetum_) and other allied Noctuidae (Owl-moths).
These are insects with brown-coloured wings, flying after dark in June.
The dull greyish larvae feed on many kinds of low-growing plants,
usually hiding in the earth by day and wandering along the surface of
the ground by night, biting off the farmer's ripening corn, or burrowing
into his turnips or potatoes. On account of the burrowing habits of this
insect it can feed throughout the winter, except when a hard frost puts
a temporary stop to its activity. By April it has become fully grown and
pupates in an earthen chamber a few inches below the surface. The Turnip
moth in our countries is partially double-brooded, a minority of the
autumn caterpillars growing more rapidly than their comrades so that
they pupate, and a second brood of moths appear in September. These pair
and lay eggs, the resulting caterpillars going as Barrett suggests
(1896, vol. III. p. 291) 'to reinforce the great army of wintering
larvae.'

Such underground caterpillars, to a great extent protected from cold,
can continue to feed through the winter. With other species we find that
the larva becomes fully grown in autumn, yet lives through the winter
without further change. This is the case with the Codling moth
(_Carpocapsa pomonella_), a well-known orchard pest, which in our
countries is usually single-brooded. The moth is flying in May and lays
her eggs on the shoots or leaves of apple-trees, more rarely on the
fruitlets, into which however the caterpillar always bores by the upper
(calyx) end. Here it feeds, growing with the growth of the fruit,
feeding on the tissue around the cores, ultimately eating its way out
through a lateral hole, and crawling upwards if its apple-habitation has
fallen, downwards if it still remains on the bough, to shelter under a
loose piece of bark where it spins its cocoon about midsummer and
hibernates still in the larval condition. Not until spring is the pupal
form assumed, and then it quickly passes into the imaginal state. In the
south of England, as F.V. Theobald (1909) has lately shown, and also in
southwestern Ireland, this species may be double-brooded, the usual
condition on the European continent and in the United States of America.
There the midsummer larvae pupate at once and the moths of an August
brood lay eggs on the hanging or stored fruit; in this case, again,
however, the full-grown larva, quickly fed-up within the developed
apples, is the wintering stage.

Several of the insects mentioned in this survey, like the last-named
codling moth, are occasionally double-brooded. As an example of the many
Lepidoptera, which in our islands have normally two complete life-cycles
in the year, we may take the very familiar White butterflies (Pieris) of
which three species are common everywhere. The appearance of the first
brood of these butterflies on the wing in late April or May is hailed as
a sign of advanced spring-time. They pair and lay their eggs on
cabbages and other plants, and the green hairy caterpillars feed in June
and July, after which the spotted pupae may be found on fences and
walls, attached by the silken tail-pad and supported by the
waist-girdle. In August and September butterflies of the second brood
have emerged from these and are on the wing; their offspring are the
autumn caterpillars which feed in some seasons as late as November,
doing often serious damage to the late cruciferous crops before they
pupate. The pupae may be seen during the winter months, waiting for the
spring sunshine to call out the butterflies whose structures are being
formed beneath the hard cuticle.

Reviewing the small selection of life-stories of various Lepidoptera
just sketched, we notice an interesting and suggestive variety in the
wintering stage. The vanessid butterflies hibernate as imagos; the
'vapourer' winters in the egg, the magpie as a young ungrown larva, the
'tiger' as a half-size larva; the Agrotis caterpillar feeds through the
winter, growing all the time; the codling caterpillar completes its
growth in the autumn, and winters as a full-size resting larva; lastly,
the 'whites' hibernate in the pupal state. And in every case it is
noteworthy that the form or habit of the wintering stage is well adapted
for enduring cold.

Our native 'whites' afford illustration of another interesting feature
often to be noticed in the life-story of double-brooded Lepidoptera. The
butterflies of the spring brood differ slightly but constantly from
their summer offspring, affording examples of what is called _seasonal
dimorphism_. All three species have whitish wings marked with black
spots, larger and more numerous in the female than in the male. In the
spring butterflies these spots tend towards reduction or replacement by
grey, while in the summer insects they are more strongly defined, and
the ground colour of the wings varies towards yellowish. In the
'Green-veined' white (_Pieris napi_) the characteristic greenish-grey
lines of scaling beneath the wings along the nervures, are much broader
and more strongly marked in the spring than in the summer generation,
whose members are distinguished by systematic entomologists under the
varietal name _napaeae_. The two forms of this insect were discussed by
A. Weismann in his classical work on the Seasonal Dimorphism of
butterflies (1876). He tried the effect of artificially induced cold
conditions on the summer pupae of _Pieris napi_, and by keeping a batch
for three months at the temperature of freezing water, he succeeded in
completely changing every individual of the summer generation into the
winter form. The reverse of this experiment also was attempted by
Weismann. He took a female of _bryoniae_, an alpine and arctic variety
of _Pieris napi_, showing in an intensive degree the characters of the
spring brood. This female laid eggs the caterpillars from which fed and
pupated. The pupae although kept through the summer in a hothouse all
produced typical _bryoniae_, and none of these with one exception
appeared until the next year, for in the alpine and arctic regions this
species is only single-brooded. Weismann experimented also with a small
vanessid butterfly, _Araschnia levana_, common on the European
continent, though unknown in our islands, which is double (or at times
treble) brooded, its spring form (_levana_) alternating with a larger
and more brightly coloured summer form (_prorsa_). Here again by
refrigerating the summer pupae, butterflies were reared most of which
approached the winter pattern, but it was impossible by heating the
winter pupae to change _levana_ into _prorsa_. Experiments with North
American dimorphic species have given similar results. Weismann argued
from these experiments that the winter form of these seasonally
dimorphic species is in all cases the older, and that the butterflies
developing within the summer pupae can be made to revert to the
ancestral condition by repeating the low-temperature stimulus which
always prevailed during the geologically recent Ice Age. On the other
hand, a high temperature stimulus applied to one generation of the
winter pupae cannot induce the change into the summer pattern, which has
been evolved still more recently by slow stages, as the continental
climate has become more genial. In tropical countries where instead of
an alternation of winter and summer, alternate dry and rainy seasons
prevail, somewhat similar seasonal dimorphism has been observed among
many butterflies. Not a few forms of Precis, an African and Indian genus
allied to our Vanessa, that had long been considered distinct species
are now known, thanks to the researches of G.A.K. Marshall (1898), to be
alternating seasonal forms of the same insect. The offspring when adult
does not closely resemble the parent; its appearance is modified by the
climatic environment of the pupa. The experiments of Weismann just
sketched in outline show at least that the same principle holds for our
northern butterflies.

We are thus led to see from the life-story of such insects, that the
course of the story is not rigidly fixed; the creature in its various
stages is plastic, open to influence from its surroundings, capable of
marked change in the course of generations. And so the seasonal changes
in the history of the individual from egg to imago point us to changes
in the age-long history of the race.




CHAPTER IX

PAST AND PRESENT; THE MEANING OF THE STORY


In the previous chapter we recognised how the seasonal changes in
various species of butterflies as observable in two or three
generations, indicate changes in the history of the race as it might be
traced through innumerable generations. The endless variety in the form
and habits of insect-larvae and their adaptations to various modes of
life, which have been briefly sketched in this little book, suggest
vaster changes in the class of insects, as a whole, through the long
periods of geological time. Every student of life, influenced by the
teaching of Charles Darwin (1859) and his successors, now regards all
groups of animals from the evolutionary standpoint, and believes that
comparisons of facts of structure and life-history of orders and classes
evidently akin to each other, furnish at least some indications of the
course of development in the greater systematic divisions, even as the
facts of seasonal dimorphism, mentioned in the last chapter, give hints
as to the course of development in those restricted groups that we call
species or varieties. A brief discussion of the main outlines of the
life-story of insects in the wide, evolutionary sense may thus fitly
conclude this book.

In the first place we turn to the 'records' of those rocks, in whose
stratified layers[12] are entombed remains, often fragmentary and
obscure, of the insects of past ages of the earth's history. Compared
with the thousands of extinct types of hard-shelled marine animals, such
as the Mollusca, fossil insects are few, as could only be expected,
seeing that insects are terrestrial and aerial creatures with slight
chance of preservation in sediments formed under water. Yet a number of
insect remains are now known to naturalists, who are, in this
connection, more particularly indebted to the researches of S.H. Scudder
(1885), C. Brongniart (1894), and A. Handlirsch (1906).

[12] See Table of Geological Systems, p. 123.

We are now considering insects from the standpoint of their
life-histories, and the individual life-story of an insect of which we
possess but a few fragments of wings or body, entombed in a rock formed
possibly before the period of the Coal Measures, can only be a matter of
inference. Still it may safely be inferred that when the structure of
these remains clearly indicates affinity to some existing order or
family, the life-history of the extinct creature must have resembled, on
the whole, that of its nearest living allies. And all the fossil
insects known can be either referred to existing orders, or shown to
indicate definite relationship to some existing group.

Passing over some doubtful remains of Silurian age, we find in rocks
usually regarded as Devonian[13] the most ancient fossils that can be
certainly referred to the insects, while from beds of the succeeding
Carboniferous period, a number of insect remains have been disinterred.
These Palaeozoic insects were frequently of large size, and they show
distinct affinities with our recent may-flies, dragon-flies,
stone-flies, and cockroaches. In the Permian period, the latest of the
divisions of the Palaeozoic, lived Eugereon, an insect with hemipteroid
jaws and orthopteroid wings. All these insects must have been
exopterygote in their life-history, if we may trust the indications of
affinity furnished by their structure. In the Mesozoic period, however,
insects with complete transformations must have been fairly abundant.
Rocks of Triassic age have yielded beetles and lacewing-flies, while
from among Jurassic fossils specimens have been described as
representing most of our existing orders, including Lepidoptera,
Hymenoptera and Diptera. In Cainozoic rocks fossil insects of nearly six
thousand species have been found, which are easily referable to
existing families and often to existing genera. We may conclude then,
imperfect though our knowledge of extinct insects is, that some of the
most complex of insect life-stories were being worked out before the
dawn of the Cainozoic era. Some instructive hints as to differences in
the rate of change among different insect groups may be drawn from the
study of parasites. For example, V.L. Kellogg (1913) points out that an
identical species of the Mallophaga (Bird-lice) infests an Australian
Cassowary and two of the South American Rheas; while two species of the
same genus (Lipeurus) are common to the African Ostrich and a third kind
of South American Rhea. These parasites must have been inherited
unchanged by the various members of these three families of flightless
birds from their common ancestors, that is from early Cainozoic times at
latest. On the other hand, the various kinds of such highly specialised
parasites as the warble-flies of the oxen and deer, must have become
differentiated during those later stages of the Cainozoic period which
witnessed the evolution of their respective mammalian hosts.

[13] The 'Little River' beds of St John, New Brunswick, Canada, by some
modern geologists however considered as Carboniferous.

The foregoing brief outline of our knowledge of the geological
succession of insects shows that the exopterygote preceded, in time, the
endopterygote type of life-history. We have already seen that those
insects undergoing little change in the life-cycle, and with visible,
external wing-rudiments, are on the whole less specialised in structure
than those which pass through a complete transformation. These two
considerations, taken together, suggest strongly that in the evolution
of the insect class, the simpler life-history preceded the more complex.
Such a conclusion seems reasonable and what might have been expected,
but we are confronted with the difficulty that if the most highly
organised insects pass through the most profound transformations, then
insects present a remarkable and puzzling exception to the general rules
of development among animals, as has already been pointed out in the
first chapter of this volume (p. 7). A few students of insect
transformation have indeed supposed that the crawling caterpillar or
maggot must be regarded as a larval stage which recalls the worm-like
nature of the supposed far-off ancestors of insects generally. Even in
Poulton's classical memoir (1891, p. 190), this view finds some support,
and it may be hard to give up the seductive idea that the worm-like
insect-larva has some phylogenetic meaning. But the weight of evidence,
when we take a comprehensive survey of the life-story of insects, must
be pronounced to be strongly in favour of the view put forward by Brauer
(1869), and since supported by the great majority of naturalists who
have discussed the subject, that the caterpillar or the maggot is itself
a specialised product of the evolutionary process, adapted to its own
particular mode of larval life.

The explanation of insect transformation is, in brief, to be found in an
increasing amount of divergence between larva and imago. The most
profound metamorphosis is but a special type of growth, accompanied by
successive castings and renewings of the chitinous cuticle, which
envelopes all arthropods. In the simplest type of insect life-story,
there is no marked difference in form between the newly-hatched young
and the adult, and in such cases we find that the young insect lives in
the same way as the adult, has the same surroundings, eats the same
food. This is the rule (see Chapters II and III) with the Apterygota,
the Orthoptera, and most of the Hemiptera. In the last-named order,
however, we find in certain families marked divergence between larva and
imago, for example in the cicads, whose larvae live underground, while
in the coccids, whose males are highly specialised and females degraded,
there succeeds to the larva--very like the young stage in allied
families--a resting instar, which in the case of the male, suggests
comparison with the pupa of a moth or beetle.

Turning to the stone-flies, dragon-flies and may-flies, whose
life-stories have been sketched in Chapter IV, we find that the early
stages are passed in water, whence before the final moult, the insects
emerge to the upper air. Except for the possession of tufted gills,
adapting them to an aquatic life, the stone-fly nymphs differ but
slightly from the adults; the grubs of the dragon-flies and may-flies,
however, are markedly different from their parents. In connection with
these comparisons, it is to be noted that the dragon-flies and may-flies
are more highly specialised insects than stone-flies, divergent
specialisation of the adult and larva is therefore well illustrated in
these groups, which nevertheless have, like the Hemiptera and
Orthoptera, visible external wing-rudiments.

From the vast array of insects that show internal wing-growth and a true
pupal stage, a few larval types were chosen for description in Chapter
VI, and a review of these suggests again the thought of increasing
divergence between larva and imago. Reference has been made previously
to the many instances in which the former has become pre-eminently the
feeding, and the latter the breeding stage in the life-cycle. It seems
impossible to avoid the conclusion that the active, armoured
campodeiform grub differing less from its parent than an eruciform larva
differs from its parent, is as a larval type more primitive than the
caterpillar or maggot. A. Lameere has indeed, while admitting the
adaptive character of insect larvae generally, argued (1899) with much
ingenuity that the eruciform or vermiform type must have been primitive
among the Endopterygota, believing that the original environment of the
larvae of the ancestral stock of all these insects must have been the
interior of plant tissues. He is thus forced to the necessity of
suggesting that the campodeiform larvae of ground-beetles or lacewings
must be regarded as due to secondarily acquired adaptations; 'they
resemble Thysanura and the larvae of Heterometabola only as whales
resemble fishes.' There are two considerations which render these
theories untenable. The Neuroptera and Coleoptera among which
campodeiform larvae are common, are less specialised than Lepidoptera,
Hymenoptera, and Diptera, in which they are unknown. And among the
Coleoptera which as we have seen (pp. 50 _f._) display a most
interesting variety of larval structure, the legless, eruciform larva
characterises families in which the imago shows the greatest
specialisation, while in the same life-story, as in the case of the
oil-beetles (pp. 56-7), the newly-hatched grub may be campodeiform,
changing to the eruciform type as soon as it finds itself within reach
of its host's rich store of food.

A certain amount of difficulty may be felt with regard to the theory of
divergent evolution between imago and larva, in the case of those
insects with complete transformation whose grubs and adults live in much
the same conditions. By turning over stones the naturalist may find
ground-beetles in company with the larvae of their own species. On the
leaves of a willow tree he may observe leaf-beetles (Phyllodecta and
Galerucella) together with their grubs, all greedily eating the foliage;
or lady-bird beetles (Coccinella) and their larvae hunting and devouring
the 'greenfly.' All of these insects are, however, Coleoptera, and the
adult insects of this order are much more disposed to walk and crawl and
less disposed to fly than other endopterygote insects. Their heavily
armoured bodies and their firm shield-like forewings render them less
aerial than other insects; in many genera the power of flight has been
altogether lost. It is not surprising, therefore, that many beetles,
even when adult, should live as their larvae do; since the acquirement
of complete metamorphosis they have become modified towards the larval
condition, and an extreme case of such modification is afforded by the
wingless grub-like female Glow-worm (Lampyris).

With most insects, however, the larva must be regarded as the more
specially modified, even if degraded, stage. Miall (1895) has pointed
out that the insect grub is not a precociously hatched embryo, like the
larvae of multitudes of marine animals, but that it exhibits in a
modified form the essential characters of the adult. Comparison for
example can be readily made between the parts of the caterpillar and the
butterfly, whose story was sketched in the first chapter of this book,
widely different though caterpillar and butterfly may appear at a
superficial glance. And the survey of variety in form, food, and habit
of insect larvae given in Chapter VI enforces surely the conclusion that
the larva is eminently plastic, adaptable, capable of changing so as to
suit the most diverse surroundings. In a most suggestive recent
discussion on the transformation of insects P. Deegener (1909) has
claimed that the larva must be regarded as the more modified stage,
because while all the adult's structures are represented in the larva,
even if only as imaginal buds, there are commonly present in the larva
special adaptive organs not found in the imago, for example the pro-legs
of caterpillars or the skin-gills of midge-grubs. The correspondence of
parts in butterfly and caterpillar just referred to, may still be
traced, though less easily, in bluebottle and maggot. The latter is an
extreme example of degenerative evolution, and its contrast with the
elaborately organised two-winged fly marks the greatest divergence
observable between the larva and imago. With this divergence the resting
pupal stage, during which more or less dissolution and reconstruction of
organs goes on, becomes a necessity, and it has already been pointed out
how the amount of this reconstruction is greatest where the divergence
between the larval and perfect stages is most marked. Whatever
differences of opinion may prevail on points of detail, the general
explanation of insect metamorphosis as the result of divergent evolution
in the two active stages of the life-story must assuredly be accepted.
No other explanation accords with the increasing degree of divergence to
be observed as we pass from the lower to the higher insect orders.

The successive incidents of the life-story of most insects are largely
connected with the acquisition of wings. Wings, and the power of flight
wherewith they endow their possessors, are evidently beneficial to the
race in giving power of extending the range during the breeding period
and thus ensuring a wide distribution of the eggs. In no case are wings
fully developed until the closing stage of the insect's life, they are
always acquired after hatching or birth. We have already noticed (p. 40)
how Sharp (1899) has laid stress on the essential difference between the
exopterygote and endopterygote insects, the wing-rudiments of the former
growing outwards throughout life while those of the latter remain hidden
until the pupal instar. Sharp considers that there is some difficulty in
bridging, in thought, the gap between these two methods of wing-growth,
and has put forward an ingenious suggestion to meet it (1902). Reference
has already been made to insects of various orders in which one sex is
wingless, the Vapourer Moth (p. 96) for example, or all the individuals
of both sexes are wingless, as the aberrant cockroaches mentioned in
Chapter II (p. 15), or certain generations of virgin females are
wingless, for example aphids (pp. 18-19) and gall-flies (pp. 94-5).
Insects may thus become secondarily wingless, that is to say be
manifestly the offspring of winged parents, and such wingless forms may
on the other hand give rise to offspring or descendants with
well-developed wings. Frequently, as in the case of the aphids, many
wingless generations intervene between two winged generations. A
striking illustration of this fact is afforded by an aquatic bug, _Velia
currens_, commonly to be seen skating over the surface of running water.
The adults of Velia are nearly always wingless, but now and then the
naturalist meets with a specimen provided with functional wings, the
possession of which enables the insect to make its way to a fresh
stream. Moreover there are whole orders of parasitic insects, such as
the lice and fleas, which, showing clear affinity to orders of winged
insects, are believed to be secondarily wingless. These orders are
designated by Sharp 'Anapterygota.' And from the analogy of the periodic
loss and recovery of wings in various generations of the same species,
he has concluded that the gap between the exopterygote and the
endopterygote method of development may have been bridged by an
anapterygote condition; that the ancestors of those insects with
complete transformations were the wingless descendants of primitive
insects which grew their wings from visible external rudiments, and
that in later times re-acquiring wings, they developed these organs in a
new way, from inwardly directed rudiments or imaginal buds.

This theory of Sharp's is original, daring, and ingenious, but the loss
and re-acquisition of wings which it presupposes is difficult to imagine
in large groups during a prolonged evolutionary history, while the
sudden appearance of a totally new mode of wing-growth in the offspring
of wingless insects would be an extreme example of discontinuity in
development.

On the whole the most probable suggestion which can be made as to the
origin of 'complete' transformation in insects is that the instar in
which wings were first visible externally became later and later in the
course of the evolution of the more highly organised groups. In this way
a gradual transition from the exopterygote to the endopterygote type of
life-story is at least conceivable. It will be remembered that a may-fly
(p. 33) undergoes a moult after acquiring functional wings, emerging
into the air as a 'sub-imago.' In not a few endopterygote insects, the
pupa shows more or less activity, swimming through water intermittently
(gnats) or just before the imago has to emerge (caddis-flies); working
its way out of the ground (crane-flies) or coming half-way out of its
cocoon (many moths). The pupa of the higher insects almost certainly
corresponds with the may-fly's sub-imago, and the facts just recalled as
to remnants of pupal activity suggest that in the ancestors of
endopterygote insects what is now the pupal instar was represented by an
active nymphal or sub-imaginal stage, possibly indeed by more than one
stage, as Packard and other writers have stated that pupae of bees and
wasps undergo two or three moults before the final exposure of the
imago. Such an early pupal instar has been defined as a 'pro-nymph' or a
'semi-pupa.' Examples have been given of the exceptional passive
condition of the penultimate instar in Exopterygota. The instars
preceding this presumably had originally outward wing-rudiments in all
insect life-histories, and the endopterygote condition was attained by
the postponement of the outward appearance of these to successively
later stages. The leg and wing rudiments of the male coccid (pp. 20-1)
beneath the cuticle of the second instar are strictly comparable to
imaginal buds, and these are present in one instar of what is generally
regarded as an exopterygote life-history. The first instar in all
insects has no visible wing-rudiments, but when they grow outwardly from
the body, they necessarily become covered with cuticle, so that they
must be visible after the first moult. There is no supreme difficulty in
supposing that the important change was for these early rudiments to
become sunk into the body, so that the cuticle of the second, and,
later, of the third and succeeding instars, showed no outward sign of
their presence. This suggestion is confirmed by Heymons' (1896, 1907)
observation of the occasional appearance of outward wing-rudiments on
the thoracic segments of a mealworm, the larva of the beetle _Tenebrio
molitor_, and by F. Silvestri's discovery (1905) of a 'pro-nymph' stage
with short external wing-rudiments between the second larval and the
pupal instars of the small ground-beetle _Lebia scapularis_. Whatever
may be the exact explanation of these abnormalities, they show that in
the life-story of the higher insects outward wing-rudiments may even yet
appear before the pupal stage, confirming our belief that such
appearance is an ancestral character. The inward growth of these
wing-rudiments may well have been correlated with a difference in form
between the newly-hatched insect and its parent. As this difference
persisted until a constantly later stage, and the pre-imaginal instar
became necessarily a stage for reconstruction, the present condition of
complete metamorphosis in the more highly organised orders was finally
attained.

To explain satisfactorily these complex life-stories is however
admittedly a difficult task. The acquisition of wings is, as we have
seen, a dominating feature in them all, but if we try to go yet a step
farther back and speculate on the origin of wings in the most primitive
exopterygote insects, the task becomes still more difficult. Many years
ago Gegenbaur (1878) was struck by the correspondence of insect wings to
the tracheal gills of may-fly larvae, which are carried on the abdominal
segments somewhat as wings are on the thoracic segments. But Börner has
recently (1909) brought forward evidence that these abdominal gills
really correspond serially with legs. Moreover Gegenbaur's theory
suggests that the ancestral insects were aquatic, whereas the presence
of tubes for breathing atmospheric air in well-nigh all members of the
class, and the fact that aquatic adaptations, respiratory and otherwise,
in insect-larvae are secondary force the student to regard the ancestral
insects as terrestrial. It is indeed highly probable that insects had a
common origin with aquatic Crustacea, but all the evidence points to the
ancestors of insects having become breathers of atmospheric air before
they acquired wings. How the wings arose, what function their precursors
performed before they became capable of supporting flight, we can hardly
even guess.

Our study of the life-story of insects, therefore, while it has taught
us something of what is going on around us to-day, and has given us
hints of the course of a few threads of that long life-story which runs
through the ages, brings us face to face with the most instructive, if
humbling fact that 'there are many more things of which we are
ignorant.' The passage from creeping to flight, as the caterpillar
becomes transformed into the butterfly, was a mystery to those who first
observed it, and many of its aspects remain mysterious still. Perhaps
the most striking result of the study of insect transformation is the
appreciation of the divergent specialisation of larva and imago, and it
is a suggestive thought that of the two the larva has in many cases
diverged the more from the typical condition. The caterpillar crawling
over the leaf, or the fly-grub swimming through the water, may thus be
regarded as a creature preparing for a change to the true conditions of
its life. It is a strange irony that the preparation is often far longer
than the brief hours of achievement. But the light which research has
thrown on the nature of these wonderful life-stories, the demonstration
of the unseen presence and growth within the insect, during its time of
preparation among strange surroundings, of the organs required for
service in the coming life amid its native air, confirm surely the
intuition of the old-time students, who saw in these changes, so
familiar and yet so wonderful, a parable and a prophecy of the higher
nature of man.




OUTLINE CLASSIFICATION OF INSECTS


Class INSECTA or HEXAPODA.

Sub-class A, APTERYGOTA.

Order 1. _Thysanura_ (Bristle-tails).
      2. _Collembola_ (Spring-tails).

Sub-class B, EXOPTERYGOTA.

Order 1. _Dermaptera_ (Earwigs).
      2. _Orthoptera_ (Cockroaches, Grasshoppers, Crickets).
      3. _Plecoptera_ (Stone-flies).
      4. _Isoptera_ (Termites or 'White Ants').
      5. _Corrodentia_
         (_a_) _Copeognatha_ (Book-lice).
         (_b_) _Mallophaga_ (Biting-lice).
      6. _Ephemeroptera_ (May-flies).
      7. _Odonata_ (Dragon-flies).
      8. _Thysanoptera_ (Thrips).
      9. _Hemiptera_
         (_a_) _Heteroptera_ (Bugs, Pond-skaters)
         (_b_) _Homoptera_ (Cicads, 'Greenfly,' Scales).
     10. _Anoplura_ (Lice).

Sub-class C, ENDOPTERYGOTA.

Order 1. _Neuroptera_ (Alder-flies, Ant-lions, Lacewings).
      2. _Coleoptera_ (Beetles).
      3. _Mecaptera_ (Scorpion-flies).
      4. _Trichoptera_ (Caddis-flies).
      5. _Lepidoptera_ (Moths and Butterflies).
      6. _Diptera_ (Two-winged flies)
         (_a_) _Orthorrhapha_ (Crane-flies, Midges, Gnats)
         (_b_) _Cyclorrhapha_ (Hover-flies, House-flies, Bot-flies, &c).
      7. _Siphonaptera_ (Fleas).
      8. _Hymenoptera_
         (_a_) _Symphyta_ (Saw-flies)
         (_b_) _Apocrita_ (Gall-flies, Ichneumon-flies, Wasps, Bees, Ants).




TABLE OF GEOLOGICAL SYSTEMS


These names, given by geologists to the various divisions of rocks, as
indicated by the fossils entombed in them, are arranged in 'descending'
order, the more recent formations above, the more ancient below, as
newer deposits necessarily lie over older beds.

CALNOZOIC OR TERTIARY GROUP.

Pleistocene.
Pliocene.
Miocene.
Eocene.


MESOZOIC OR SECONDARY GROUP.

Cretaceous.
Jurassic.
Triassic.


PALAEOZOIC OR PRIMARY GROUP.

Permian.
Carboniferous.
Devonian.
Silurian.
Cambrian.




BIBLIOGRAPHY


The following list of some books and papers, referred to in this little
volume or of especial service to the author in its preparation, is
needless to say very far from exhaustive. To save space, titles are
often abbreviated. Most of the works in the general list (A) contain
extensive lists of literature on insects and their transformations,
these should be consulted by the serious student.


A. GENERAL WORKS.

1909.     C. Börner. Die Verwandlungen der Insekten. _Sitzb. d. Gesellsch.
          naturforsch. Freunde, Berlin._

1869.     F. Brauer. Betrachtung über die Verwandlung der Insekten.
          _Verhandl. der K.K. zool.-bot. Gesellschaft in Wien._ XIX.

1899.     G.H. Carpenter. Insects, their Structure and Life. London.

1859.     C. Darwin. The Origin of Species. London.

1909.     P. Deegener. Die Metamorphose der Insekten. Leipzig.

1906.     J.W. Folsom. Entomology. London.

1878.     C. Gegenbaur. Grundriss der Vergleichende Anatomie. Leipzig.

1906.     A. Handlirsch. Die fossilen Insekten. Leipzig.

1904.     L.F. Henneguy. Les Insectes. Paris.

1907.     R. Heymons. Die verschiedenen Formen der Insectenmetamorphose.
          _Ergebnisse der Zoologie._ I.

1899.     A. Lameere. La raison d'être des Metamorphoses chez les Insectes.
          _Ann. Soc. Entom. Bruxelles._ XLIII.

1874.     J. Lubbock. The Origin and Metamorphoses of Insects. London.

1895.     L.C. Miall. (_a_) The Transformations of Insects. _Nature._ LIII.

1895.     ---- (_b_) The Natural History of Aquatic Insects. London.

1908.     ---- Injurious and Useful Insects. 2nd edition. London.

1839.     G. Newport. Insects. _Todd Cyclopaedia._ II. London.

1898.     A.S. Packard. Text book of Entomology. New York.

1734-42.  R.A.F. de Réaumur. Mémoires pour servir à l'Histoire naturelle
          et à l'anatomie des Insectes. Paris.

1895-8.   D. Sharp. The Cambridge Natural History, V, VI. London.

1899.     ---- Some points in the Classification of Insects. IV. _Internat.
          Zoolog. Congress._

1902.     ---- Insects in _Encycl. Brit._ 10th Edition, XXIX. London.

1910.     ---- and G.H. Carpenter. Hexapoda in _Encycl. Brit._ 11th
          Edition. Cambridge.

1737.     J. Swammerdam. Biblia Naturae. Leyden (incorporates works on
          Insects published during the author's lifetime 1669-75).

1909.     F.V. Theobald. Insect Pests of Fruit. Wye.


B. SPECIAL WORKS.

1881.     H. Adler. Ueber den Generationswechsel den Eichen-Gallwespen.
          _Zeitsch. f. wissensch. Zoologie._ XXXV.

1896.     ---- and C.R. Straton. Alternating Generations. Oxford.

1902.     J. Anglas. Nouvelles Observations sur les Métamorphoses Internes.
          _Arch. d'Anat. Microscop._ IV.

1911.     E.E. Austen. Handbook of the Tsetse-Flies. London (Brit. Museum).

1909.     F. Balfour-Browne. Life-History of Agrionid Dragonfly. _Proc.
          Zool. Soc. Lond._

1893, &c. C.G. Barrett. Lepidoptera of the British Islands. London.

1890.     H. Beaurégard. Les Insectes Vésicants. Paris.

1909.     C. Börner. Die Tracheenkiemen der Ephemeriden. _Zoolog. Anz._
          xxxiii.

1863.     F. Brauer. Monographie der Oestriden. Wien.

1894.     C. Brongniart. Récherches pour servir à l'histoire des Insectes
          fossiles des Temps Primaires. St Etienne.

1893.     T.A. Chapman. Structure of Pupae of Heterocerous Lepidoptera.
          _Trans. Entom. Soc. Lond._

1891.     H. Dewitz. Das geschlossene Tracheensystem bei Insektenlarven.
          _Zoolog. Anz._ xiii.

1857-8.   J.H. Fabre. L'Hypermétamorphose et les Moeurs des Meloides.
          _Ann. Sci. Nat._ (_Zool._), (4). VII. IX.

1869.     M. Ganin. Die Entwicklungsgeschichte bei den Insekten. _Zeitsch.
          f. wissensch. Zoolog._ xix.

1894.     J. Gonin. La Métamorphose des Lepidoptères. _Bull. Soc. Vaud.
          Sci. Nat._ xxx.

1870.     O. Grimm. Die ungeschechtliche Fortpflanzung einer Chironomus.
          _Mem. Acad. Impér. St Pétersbourg_ (7). xv.

1890.     W. Hatchett-Jackson. Morphology of the Lepidoptera. _Trans. Linn.
          Soc. (Zool.) Lond._ (2). v.

1896.     R. Heymons. Flügelbildung bei der Larve von Tenebrio molitor.
          _Sitzb. d, Gesellsch. Naturforsch. Freunde, Berlin._

1906.     ---- Ueber die ersten Jugendformen von Machilis alternata. _Ib._

1908.     W. Kahle. Die Paedogenesis der Cecidomyiden. _Zoologica._ IV.

1913.     V.L. Kellogg. Distribution and Species-forming of Ectoparasites.
          _Amer. Naturalist._ XLVII.

1887.     A. Kowalevsky. Die nachembryonale Entwicklung der Musciden.
          _Zeitsch. f. wissensch. Zool._ XLV.

1904.     O.H. Latter. Natural History of Common Animals (chaps. III, IV,
          V). Cambridge.

1890-95.  B.T. Lowne. The Blowfly, 2 vols. London.

1863.     J. Lubbock. Development of Chloeon. _Trans. Linn. Soc. Lond._
          XXIII.

1762.     P. Lyonet. Traité anatomique de la Chenille. Haag.

1669.     M. Malpighi. De Bombyce. London.

1898.     C.L. Marlatt. The periodical Cicada. _Entom. Bull._ 14, _U.S.
          Dept. Agric._

1898.     G.A.K. Marshall. Seasonal Dimorphism in Butterflies. _Ann. Mag.
          Nat. Hist._ (7). II.

1900.     L.C. Miall and A.B. Hammond. The Harlequin Fly. Oxford.

1901-3.   R. Newstead. Coccidae of the British Isles. London.

1877.     J.A. Palmén. Zur Morphologie des Tracheensystems. Leipzig.

1891.     E.B. Poulton. External Morphology of the Lepidopterous Pupa.
          _Trans. Linn. Soc. Zool._ (2). V.

1892.     ---- Colour-relation between Lepidopterous Larvae &c. and their
          surroundings. _Trans. Entom. Soc. Lond._

1880.     C.V. Riley. Pupation of Butterflies. _Proc. Amer. Assoc._ XXVIII.

1902.     E.D. Sanderson. Report of Entomologist. Delaware. U.S.A.

1885.     E.O. Schmidt. Metamorphose und Anatomie des männlichen
          Aspidiotus. _Archiv f. Naturgeschichte._ LI.

1885.     S.H. Scudder. Insekten in Zittel's Paleontologie. II.

1907.     A.J. Siltala. Die postembryonale Entwicklung der
          Trichopteren-Larven. _Zoolog. Jahrb. Suppl._ IX.

1905.     F. Silvestri. Metamorfosi e Costumi della Lebia scapularis.
          _Redia._ II.

1900.     J.B. Smith. The Apple Plant-louse. _New Jersey Agric. Exp.
          Station Bull._ 143.

1888.     J. Van Rees. Die innere Metamorphose von Musca. _Zoolog. Jahrb.
          Anat._ III.

1911.     K.W. Verhoeff. Ueber Felsenspringer, Machiloidea. _Zoolog. Anz._
          XXXVIII.

1865.     N. Wagner. Die viviparen Gallmückenlarven. _Zeitsch. f.
          wissensch. Zoolog._ XV.

1901.     E. Wasmann. Termitoxenia. _Zeitsch. f. wissensch. Zoolog._ LXX.

1864.     A. Weismann. Die nachembryonale Entwicklung der Musciden.
          _Zeitsch. f. wissensch. Zoolog._ XIV.

1865.     ---- Die Metamorphose von Corethra. _Ib._ XVI.

1876.     ---- Studien zur Descendenz-Theorie. Leipzig. (English
          Translation by R. Meldola, London, 1882.)




INDEX


_Abraxas grossulariata_, 60, 83, 97-8

Adaptation of larvae, 57, 79, 114

Adephaga, 51

Adler, H., 94

Aeschnidae, 27, 29, 31

Agrionidae, 27, 28

_Agrotis segetum_, 98

Air-tubes, 2, 11, 23, 47, 70, 77, 87, 120

Alternation of generations, 17, 94

Ametabola, 11, 35

Anapterygota, 116

Anglas, J., 46

Ant-lions, 57

Ants, 64, 66

Aphidae, 17-20, 116

_Aphis pomi_, 18-19

Aphis-lion, 57

Apterygota, 41, 110

Aquatic insects, 23-34, 76-9, 120

_Araschnia levana_ and var. _prorsa_, 103

_Arctia caia_, 98

Arctiadae, 59

Arthropoda, 9

Austen, E.E., 91

Avebury, Lord, _see_ Lubbock, J.


Balfour-Browne, F., 28

Bark-beetles, 55

Barrett, C.G., 96, 99

Beaurégard, H., 56

Bees, 40, 46, 64, 83

Beetles, 40, 50-7, 80, 107, 112-3, 119

Bell Moths, 62

Bird-lice, 108

Birth, 18, 91

_Blatta orientalis_, 15

Blister-beetles, 56

Blowfly or Bluebottle, 43, 44, 46, 67, 71-3, 93, 114

Börner, C., 32, 120

Bot-flies, 73-4, 89, 91

Brain, 44

Brauer, F., 6, 52, 56, 67, 109

Bristle-tails, 11

Brongniart, C., 106

Butterflies, 1, 83, 95-6, 114


Cabbage-butterflies, 39, 41, 85, 100-1

Cabbage-fly, 73

Caddis-flies, 62-3, 86, 117

Cainozoic insects, 107

Calliphora, 43.
  _See also_ Blowfly

Campodeiform larvae, 52, 56, 111

Carabidae, 52

Carboniferous insects, 107

_Carpocapsa pomonella_, 99-100

Carrion-beetles, 50

Caterpillar, 4, 36, 49, 58-62, 95-101, 109, 114

Cecidomyidae, 68-70, 90

Cerambycidae, 55

Cercopods, 12, 15

Chafers, 52

Chapman, T.A., 81, 84

Chironomus, 43, 77, 87, 91

Chloeon, 33

Chrysalis, 82.
  _See also_ Pupa

Chrysomelidae, 53.
  _See also_ Leaf-beetles

Chrysopa, 57

Cicads, 22, 93, 110

Classification, 122

Clearwing Moths, 62

Click-beetles, 52, 93

Clothes-moths, 62

Coccidae, 20, 110, 118

Coccinella, 113

Cockroaches, 11, 14, 15, 107, 115

Cocoons, 82

Codling Moth, 62, 99

Coleoptera, 50-6, 80, 112, 119

Collembola, 11

Complete transformation, 35, 107, 119.
  _See also_ Endopterygota

Corethra, 43

Cossus, 38, 62, 82, 95

Crane-flies, 67, 70, 93, 117

Cremaster, 83

Crustacea, 7, 120

Culex, 43, 77, 86

Curculionidae, 55

Cuticle, 2, 9, 29, 37, 40, 50, 81, 87, 110

Cynipidae, 94.
  _See also_ Gall-flies


Daddy-long-legs, 69-70

Darwin, C., 105

Deegener, P., 6, 114

Devonian insects, 107

Dewitz, H., 28

Digestive system, 10, 45-7

_Diplosis pyrivora_, 70

Diptera, 42, 64, 67-79, 81, 86-8, 91, 94, 107

Divergence between larva and imago, 110, 114, 121

Double-brooded Lepidoptera, 95, 100-4

Dragon-flies, 26-31, 107, 110

Drone-flies, 76

Duration of life, 34, 89, 92-3, 95

Dyticus, 51


Ecdysis, 10.
  _See also_ Moult

Ectoderm, 9, 11, 47

Eggar Moths, 59, 89

Eggs, 6, 17-18, 26, 34, 65-7, 71, 90, 94-5, 97

Elateridae, 52

Endopterygota, 41, 49, 108, 112, 115-6

Ephemeroptera, 24.
  _See also_ May-flies

Epidermis, 9, 40

Eristalis, 76

Eruciform larvae, 56, 58-70, 111

Evolution, 16, 103, 105-21

Exopterygota, 41, 108, 115-6, 118

Exoskeleton, 9


Fabre, J.H., 56

Fat-body, 47

Feeding-period, 27, 32, 36, 89, 111

Feelers, 1, 4, 42, 71

Fleas, 116

Fore-gut, 47

Free pupa, 80


Gall-flies, 64-6, 94, 115

Gall-midges, 68-70, 90

Ganin, M., 66

_Gastrophilus equi_, 73-4

Gegenbaur, C., 120

Geological history, 106-8, 123

Geometridae, 59

Gills, 24, 27, 32, 78, 87, 114, 120

Glossinia, 91

Glow-worm, 50, 113

Gnats, 43, 77, 86

Goat Moth, 38, 62, 82, 95

Gonin, J., 38, 41

Grasshoppers, 11, 14, 15

Grimm, O., 90

Ground-beetles, 52, 112

Growth, 9

Grub, 63-70.
  _See also_ Caterpillar, Larva


Hairs, 59, 82, 98

Hammond, A.R., 43, 77, 87

Handlirsch, A., 106

Harvey, William, 7

Hatchett-Jackson, W., 83

Hawk Moths, 60

Heart, 45

Helodes, 50

Hemerobius, 57

Hemimetabola, 35

Hemiptera, 17, 110

Henneguy, L.F., 45, 48

Heymons, R., 6, 11, 119

Hibernation. _See_ Wintering stages

Hind-gut, 47

Hippoboscidae, 91

Histogenesis and Histolysis, 48

Holometabola, 35

House-fly, 67, 71, 73

Hover-flies, 74-6

Hymenoptera, 58, 64, 94, 107

Hypermetamorphosis, 56

_Hypoderma bovis_, 73-5

Hypodermis, 9


Ichneumon-flies, 64, 66, 82

Imaginal buds or discs, 34-48, 114, 117-8

Imago, 24, 34, 114

Instar, 13, 33, 56, 117-9


Jaws of imago and larva, 2, 4, 5, 32, 42, 89

Jurassic insects, 107


Kahle, W., 90

Kellogg, V.L., 108

Kowalevsky, A., 46


Labium, 2, 27

Lacewing-flies, 57, 107

Ladybirds, 113

Lameere, A., 111

Lampyris, 113

Larva, 4, 22, 26-7, 32, 49-79, 110-15

Larval reproduction, 90

Lasiocampidae, 59, 89

Latter, O.H., 28

Leaf-beetles, 53, 83, 92-3, 113

_Lebia scapularis_, 119

Lepidoptera, 1, 36, 38, 49, 58, 81, 95-104, 107

Libellulidae, 27

Lice, 116

Lipeurus, 108

Longhorn Beetles, 55

Looper caterpillars, 59, 61

Lowne, B.T., 42

Lubbock, J., 6, 32

Lymantriidae, 90

Lyonet, P., 38


Machilis, 11

Maggot, 44, 67, 71-6, 109, 114

Magpie Moth, 60, 82, 97-8

Mallophaga, 108

Mandibles, 4, 17, 26, 58, 67, 86

Mangel-fly, 73

Marlatt, C.L., 93

Marshall, G.A.K., 104

Maxillae, 2, 17, 37, 42

May-flies, 31-4, 107, 110, 117, 120

Meloidae, 56

Mesozoic insects, 107

Metabola, 35

Metamorphosis (in general), 6, 109;
  (degrees of in insects) 8, 35, 109, 117-19

Miall, L.C., 6, 28, 33, 43, 77, 78, 87, 97, 113

Mosquito. _See_ Culex, Gnats

Moths, 1, 58-62, 84, 95-100, 117

Moult, 10, 32, 36, 41

_Musca domestica_, 71

Muscidae, 44

Muscles, 47


Nervous system, 44-5

Neuroptera, 57, 80, 112

Newport, G., 41, 44

Noctuidae, 60, 98

Nymph, 15, 28, 33


Oak-apples, 94

Obtect pupa, 81

Odonata, 24.
  _See also_ Dragon-flies

_Oestrus ovis_, 91

Oil-beetles, 56, 112

_Orgyia antiqua_, 96-7

Orthoptera, 17, 35, 110

Owl Moths, 60, 98


Packard, A.S., 56, 118

Paedogenesis. _See_ Larval reproduction

Painted Lady Butterfly, 96

Palaeozoic insects, 107

Palmén, J.A., 25

Parasitic insects, 73-4, 108, 116

Parental care, 64-6

Parthenogenesis, 18

Partial transformation, 35, 37

Perla, 24

Permian insects, 107

Phagocytes, 48

Phyllodecta, 53, 113

Phyllotreta, 53

_Pieris brassicae_, 39, 41, 85, 100

_Pieris napi_ and var. _bryoniae_, 102-3

Platygaster, 66

Plecoptera, 24.
  _See also_ Stone-flies

Pompilidae, 66-7

Poulton, E.B., 61, 82, 109

Precis, 104

Proctotrypidae, 66

Pro-legs, 4, 58-9, 84, 114

Pro-nymph, 118, 119

Protective coloration, 60-1

_Psylliodes chrysocephala_, 54

Ptinidae, 54

Pupa, 4, 37, 40, 79-88, 114, 117

Puparium, 88

Pupipara, 91

_Pyrameis cardui_, 96


Rat-tailed maggot, 76

Réaumur, R.A.F. de, 8, 28, 33, 41

Reproductive larvae, 90;
  pupae, 91

Reproductive organs, 45

_Rhabdophaga heterobia_, 70

Riley, C.V., 83


Sanderson, E.D., 17

Sand-midges, 78

Sarcophaga, 91

Saw-flies, 58-9

Scale-insects, 20.
  _See also_ Coccidae

Scarabaeidae, 52

Schmidt, E.O., 21

Scolytidae, 55

Scudder, S.H., 106

Seasonal changes, 89-104

Seasonal dimorphism, 102

Semi-pupa, 118

Sesiidae, 62

Sexual differences, 15, 20-1, 90

Sharp, D., 13, 36, 40, 115

Silk-spinning, 58, 62-3, 82

Silkworms, 82

Silpha, 50

Siltala, A.J., 63

Silvestri, F., 119

Simulium, 78, 87

Smith, J.B., 17

Sphegidae, 66-7

Sphingidae, 60

Spinneret, 58

Spiracles, 2, 23, 70, 72, 77, 86, 87

Spring-tails, 11

Stone-flies, 24, 107, 110

Sub-imago, 33, 117

Sucking insects, 17

Swammerdam, J., 33

Syrphus, 74-6


Tachininae, 73, 91

_Tenebrio molitor_, 119

Termitoxeniidae, 92

Theobald, F.V., 100

Thysanura, 11

Tiger Moths, 59, 82, 98

Timber-beetles, 54

Tineidae, 62

Tipulidae, 70

Tortoiseshell Butterfly, 45, 95

Tortricidae, 62

Tracheal system. _See_ Air-tubes, Spiracles

Transformation. _See_ Metamorphosis

Triassic insects, 107

Trichocera, 70

Trichoptera, 62-3, 76, 80, 86

Tsetse Flies, 91

Turnip-fly, 53, 92, 94

Turnip Moth, 98-9

Tussock Moths, 90, 97


_Vanessa urticae_, 45, 95

Van Rees, J., 42

Vapourer Moth, 96-7, 115

_Velia currens_, 116

Verhoeff, K.W., 11

Vermiculiform larvae, 67, 71-6, 111

Virgin stem-mothers, 18

Viviparous reproduction. _See_ Birth


Wagner, N., 90

Warble-fly, 73-4, 89, 108

Warning coloration, 60

Wasmann, E., 92

Wasps, 46, 64, 66-7, 83

Water-insects. _See_ Aquatic insects

Weevils, 55

Weismann, A., 38, 42, 102

White Butterflies, 41, 83, 85, 100-3

Willow-beetles, 53

Wingless insects, 15, 18, 20, 96, 115

Wing-rudiments, 13, 18, 20, 22, 24, 28, 33, 36-8, 40, 111, 115, 117-19

Wings, 1, 14, 115, 119-20

Winter broods, 102-3

Wintering stages, 93-101

Wireworms, 52, 93

Wood-wasps, 65




CAMBRIDGE: PRINTED BY JOHN CLAY, M.A. AT THE UNIVERSITY PRESS




                                 THE
                          CAMBRIDGE MANUALS
                      OF SCIENCE AND LITERATURE

             Published by the Cambridge University Press

                           GENERAL EDITORS
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     The Early Religious Poetry of the Hebrews. By the Rev. E.G. King,
     D.D.

     The Early Religious Poetry of Persia. By the Rev. Prof. J. Hope
     Moulton, D.D., D.Theol. (Berlin).

     The History of the English Bible. By John Brown, D.D.

     English Dialects from the Eighth Century to the Present Day. By
     W.W. Skeat, Litt.D., D.C.L., F.B.A.

     King Arthur in History and Legend. By Prof. W. Lewis Jones, M.A.

     The Icelandic Sagas. By W.A. Craigie, LL.D.

     Greek Tragedy. By J.T. Sheppard, M.A.

     The Ballad in Literature. By T.F. Henderson.

     Goethe and the Twentieth Century. By Prof. J.G. Robertson, M.A.,
     Ph.D.

     The Troubadours. By the Rev. H.J. Chaytor, M.A.

     Mysticism in English Literature. By Miss C.F.E. Spurgeon.


PHILOSOPHY AND RELIGION

     The Idea of God in Early Religions. By Dr F.B. Jevons.

     Comparative Religion. By Dr F.B. Jevons.

     Plato: Moral and Political Ideals. By Mrs A.M. Adam.

     The Moral Life and Moral Worth. By Prof. Sorley, Litt.D.

     The English Puritans. By John Brown, D.D.

     An Historical Account of the Rise and Development of
     Presbyterianism in Scotland. By the Rt Hon. the Lord Balfour of
     Burleigh, K.T., G.C.M.G.

     Methodism. By Rev. H.B. Workman, D.Lit.


EDUCATION

     Life in the Medieval University. By R.S. Rait, M.A.


LAW

     The Administration of Justice in Criminal Matters (in England and
     Wales). By G. Glover Alexander, M.A., LL.M.


BIOLOGY

     The Coming of Evolution. By Prof. J.W. Judd, C.B., F.R.S.

     Heredity in the Light of Recent Research. By L. Doncaster, M.A.

     Primitive Animals. By Geoffrey Smith, M.A.

     The Individual in the Animal Kingdom. By J.S. Huxley, B.A.

     Life in the Sea. By James Johnstone, B.Sc.

     The Migration of Birds. By T.A. Coward.

     Spiders. By C. Warburton, M.A.

     Bees and Wasps. By O.H. Latter, M.A.

     House Flies. By C.G. Hewitt, D.Sc.

     Earthworms and their Allies. By F.E. Beddard, F.R.S.

     The Wanderings of Animals. By H.F. Gadow, F.R.S.


ANTHROPOLOGY

     The Wanderings of Peoples. By Dr A.C. Haddon, F.R.S.

     Prehistoric Man. By Dr W.L.H. Duckworth.


GEOLOGY

     Rocks and their Origins. By Prof. Grenville A.J. Cole.

     The Work of Rain and Rivers. By T.G. Bonney, Sc.D.

     The Natural History of Coal. By Dr E.A. Newell Arber.

     The Natural History of Clay. By Alfred B. Searle.

     The Origin of Earthquakes. By C. Davison, Sc.D., F.G.S.

     Submerged Forests. By Clement Reid, F.R.S.


BOTANY

     Plant-Animals: a Study in Symbiosis. By Prof. F.W. Keeble.

     Plant-Life on Land. By Prof. F.O. Bower, Sc.D., F.R.S.

     Links with the Past in the Plant-World. By Prof. A.C. Seward.


PHYSICS

     The Earth. By Prof. J.H. Poynting, F.R.S.

     The Atmosphere. By A.J. Berry, M.A.

     Beyond the Atom. By John Cox, M.A.

     The Physical Basis of Music. By A. Wood, M.A.


PSYCHOLOGY

     An Introduction to Experimental Psychology. By Dr C.S. Myers.

     The Psychology of Insanity. By Bernard Hart, M.D.


INDUSTRIAL AND MECHANICAL SCIENCE

     The Modern Locomotive. By C. Edgar Allen, A.M.I.Mech.E.

     The Modern Warship. By E.L. Attwood.

     Aerial Locomotion. By E.H. Harper, M.A., and Allan E. Ferguson,
     B.Sc.

     Electricity in Locomotion. By A.G. Whyte, B.Sc.

     Wireless Telegraphy. By Prof. C.L. Fortescue, M.A.

     The Story of a Loaf of Bread. By Prof. T.B. Wood, M.A.

     Brewing. By A. Chaston Chapman, F.I.C.




SOME VOLUMES IN PREPARATION


HISTORY AND ARCHAEOLOGY

     The Aryans. By Prof. M. Winternitz.

     Ancient India. By Prof. E.J. Rapson, M.A.

     The Peoples of India. By J.D. Anderson, M.A.

     The Balkan Peoples. By J.D. Bourchier.

     Canada of the present day. By C.G. Hewitt, D.Sc.

     The Evolution of Japan. By Prof. J.H. Longford.

     The West Indies. By Sir Daniel Morris, K.C.M.G.

     The Royal Navy. By John Leyland.

     Gypsies. By John Sampson.

     A Grammar of Heraldry. By W.H. St John Hope, Litt.D.

     Celtic Art. By Joseph Anderson, LL.D.


ECONOMICS

     Women's Work. By Miss Constance Smith.


LITERARY HISTORY

     Early Indian Poetry. By A.A. Macdonell.

     The Book. By H.G. Aldis, M.A.

     Pantomime. By D.L. Murray.

     Folk Song and Dance. By Miss Neal and F. Kidson.


PHYSICS

     The Natural Sources of Energy. By Prof. A.H. Gibson, D.Sc.

     The Sun. By Prof. R.A. Sampson.

     Röntgen Rays. By Prof. W.H. Bragg, F.R.S.


BIOLOGY

     The Life-story of Insects. By Prof. G.H. Carpenter.

     The Flea. By H. Russell.

     Pearls. By Prof. W.J. Dakin.


GEOLOGY

     Soil Fertility. By E.J. Russell, D.Sc.

     Coast Erosion. By Prof. T.J. Jehu.


INDUSTRIAL AND MECHANICAL SCIENCE

     Coal Mining. By T.C. Cantrill.

     Leather. By Prof. H.R. Procter.




                      Cambridge University Press
                          C.F. Clay, Manager
                      London: Fetter Lane, E.C.
                    Edinburgh: 100, Princes Street





End of Project Gutenberg's The Life-Story of Insects, by Geo. H. Carpenter