Geo. H. Carpenter - The Life Story of Insects
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7 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withdrawn.)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 Reaumur (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.
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