The Life-Story of Insects Part 1

The Life-Story of Insects.

by Geo. H. Carpenter.

PREFACE

The object of this little book is to afford an outline sketch of the facts and meaning of insect-transformations. Considerations of s.p.a.ce 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.

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 b.u.t.terfly, 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.

[Ill.u.s.tration: 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 b.u.t.terfly. 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 b.u.t.terfly'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 h.o.r.n.y (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.

[Ill.u.s.tration: Fig. 2. A. Head of a typical Moth, showing proboscis formed by flexible maxillae (_g_) between the l.a.b.i.al 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 b.u.t.terfly 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 b.u.t.terfly, though well developed in c.o.c.kroaches, 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 a.s.sist 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 b.u.t.terfly 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 b.u.t.terfly, 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.

[Ill.u.s.tration: 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 _c.o.c.kroach_.]

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 b.u.t.terfly'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 pa.s.s 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 a.s.sociated 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 cla.s.s of animals, the Crustacea, nearly allied to insects, we find that its more lowly members, such as 'water-fleas' and barnacles, pa.s.s through far more striking changes than its higher groups, such as lobsters and woodlice. But among the Insects, a cla.s.s 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 b.u.t.terfly, 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 b.u.t.terfly 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 b.u.t.terflies 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 b.u.t.terfly. 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 gra.s.shopper 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 pa.s.ses 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 cla.s.s, 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 ca.n.a.l and the whole air-tube system are formed by inpus.h.i.+ngs 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 a.s.sociated 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--c.o.c.kroaches and gra.s.shoppers 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.

[Ill.u.s.tration: Fig. 4. Common c.o.c.kroach (_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 c.o.c.kroach (fig.

4 _d_) is known by its flattened body, rounded prothorax, and stiff, jointed tail-feelers or cercopods; the baby gra.s.shopper 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 a.s.sumed 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.

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

The changes through which these insects pa.s.s are therefore largely connected with the development of the wings. It is noteworthy that in an immature c.o.c.kroach 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 s.h.i.+eld 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 gra.s.shopper, 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 s.e.xual characters may also be noticed. For instance, in an immature c.o.c.kroach 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 s.e.xes 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 gra.s.shopper or c.o.c.kroach, 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 b.u.t.terfly.

One interesting and suggestive fact remains to be mentioned. There are gra.s.shoppers and c.o.c.kroaches 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 c.o.c.kroach, _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 s.e.x. It is generally a.s.sumed 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.

c.o.c.kroaches and gra.s.shoppers 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 c.o.c.kroach. It is the family of the plant-lice (Aphidae) that affords typical ill.u.s.trations of that alternation of generations to which reference has been made.

[5] See outline cla.s.sification 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 s.e.xes 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 c.o.c.kroach, 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.