David Miller

No doubt owing to the endless assortment of sizes, from mere specks to giants of a few inches, a widespread idea has arisen, particularly in regard to such insects as have a general resemblance to one another, that the smaller individuals are the younger stages of the larger. Though gradation in size may be a sign of successive ages in certain insects, the presence of functioning wings denotes that growth has ceased; in the case of wingless insects, the characters of maturity may be less conspicuous. Although there may be at times a fairly wide range in size among fully-grown individuals of the one species, such variation is not due to age, but to certain factors influencing the insect during growth, such as the abundance or scarcity of food supply, and favourable or unfavourable climatic conditions. On the other hand, the sex to which an individual belongs is often responsible for difference in size, males very frequently being smaller than females. Size, therefore, is by no means a sign of age, and the smaller winged insects must not be regarded as the young of the larger ones, no matter how close is the resemblance.

Insects, with the exception of certain species giving birth to living young, are reproduced from eggs laid by the females; with few exceptions, the latter take no further interest in the eggs beyond placing them in surroundings offering the most favourable conditions for their well-being, and a sufficient food supply for the forthcoming young; each egg is protected by a delicate shell, through which the young insect makes its way on hatching.

On emerging from the egg, the young insect commences to feed and grow in size, until very soon a stage is reached when the cuticle or shell becomes too small for the enclosed insect; a fluid then collects between the cuticle and the underlying skin, and a new and more roomy cuticle is secreted by the latter; on this process being completed, the old chitinous covering splits, and the insect withdraws itself. This moulting takes place several times, until the body is fully grown, when the cuticle formed at the last moult is retained by the now adult insect for the rest of its life.

The different stages through which an insect passes from egg to adult constitute its life history, or life cycle, and the relation of the latter to the seasons, its seasonal history. According to the species, a full twelve months or even more may be necessary for the complete life cycle, or the cycle may be repeated several times within the year; when the cycle occupies twelve months, the insect is single-brooded; but two, three, or four-brooded, etc., when the cycle is repeated two, three, or four times, respectively, in the year. Climatic and food-supply conditions have a distinct influence on the number of broods, the one species in many cases being single-brooded in colder, and two or three-brooded in warmer climates. During the winter, when the temperature is low enough, insects are more or less dormant in some stage of their life cycle; such a state is the period of hibernation.

FIGURE 4.

1, Silverfish. 2, Earwig; a, young larva; b–d, later stages; e, adult. 3, Cicada; f, young larva; g, resting stage prior to emergence of adult; h, adult. 4, Thrips; i and j, larvae; k, first stage pupa; l, second stage pupa; m, adult. 5, Aphis-lion; n, larva; o, pupa; p, adult. 6, Moth; q, egg; r–t, larvae; u, pupa; v, adult. 7, House-fly; w, egg; x–z, larvae; aa, puparium; bb, adult. NOTE: Developing wings shown in black.

All insects do not follow the same method of development from egg to adult, and the adaptations of structure and habit are many and varied as well as simple and complex. Species having a complex development, during which they pass through stages, each differing in form from its predecessor, undergo what is known as a metamorphosis; contrasted with such insects are those developing in a simple manner without pronounced differences in the form of successive stages, the young resembling the adult in most features except size and maturity—?these insects are without a metamorphosis. Intermediate between these two extremes are other insects with a partial metamorphosis.

A consideration of the life cycle of some common insects will serve to illustrate the principles of development discussed above. Firstly, will be taken examples of complex development or complete metamorphosis; secondly, examples of simple development or absence of metamorphosis, followed by a review of species having a partial metamorphosis, thus linking the first two types.

A convenient type of insect undergoing a complete metamorphosis is any common moth (Fig.4); one of the most suitable, most easily obtained in all stages and commonest in any part of the country from spring to autumn, is the magpie moth (Nyctemera annulata) and its caterpillar, the “woolly bear.” The moth, unlike most of its kind, is a day-flying species, and is very conspicuous owing to its black colour relieved by white wing spots, and orange-yellow bands on the abdomen; the equally conspicuous caterpillar, feeding on groundsel, ragwort and cineraria, is black, with a very hairy body marked with narrow brick-red lines.

The eggs are laid in clusters by the female moth on the under side of the leaves of the caterpillars’ food-plant; at first the eggs are of a pale green colour, but assume a darker yellowish tint within a few hours, and finally a leaden colour some time later. These colour changes are due to the developing embryo, and just before the young insect (the caterpillar in this case) hatches, its outline as it lies curled within the egg is easily seen through the transparent egg-shell; near the top of the egg is a black spot marking the position of the caterpillar’s head, while the numerous delicate black lines below the egg surface are the black hairs with which the caterpillar is clothed. According to temperature and humidity, the incubation period—?that is, the period between egg-laying and the hatching of the young caterpillar—?varies from eight days to three weeks. The process of hatching occupies about two hours, the young insect using its jaws to eat an exit hole through the egg. The caterpillar stage—?indeed, the first stage of all insects—?is known as the larva.

At first the larva of the magpie moth, measuring about one-sixteenth of an inch long, is pale yellow in colour, except for the black head and hairs clothing the body; very soon, however, the body becomes characteristically black, and develops the reddish lines. During growth the larva feeds continuously day and night, undergoing from five to ten moults before becoming fully grown. During a moult the cuticle of the head is cast separately from that of the body.

The body of the larva is worm-like, not only in general form, but also in its segmented appearance; it is, however, a very different animal from a worm. The larva has a distinct head, a pair of eyes, and short antennÆ, and a set of mouth parts, similar to those of the weta or grasshopper, well adapted for devouring foliage; the first three segments behind the head correspond to the thorax of the moth, and each bears a pair of short feet; the remaining segments are those of the abdomen, and have no true feet, but six pairs of sucker-like appendages called pro-legs. The number of pro-legs varies from four to six pairs, according to the species of moth, and are found only on the larva.

The time occupied by larval development of the magpie moth varies from forty to eighty days in summer and autumn; but if winter intervenes, causing the larvÆ to hibernate before completing their development, the larval period may be as long as two hundred and forty-eight days; normally this insect hibernates in the larval state, completing its development during the following spring. Throughout winter the larvÆ hibernate singly or in colonies under loose bark, in leaf axils, or any suitable crevice.

The fully-grown larva measures about one and a-half inches long. Prior to the final moult it ceases to feed, and wanders in search of a suitable place in which to undergo the next transformation, usually among stones, rubbish, or under loose bark, etc. There it spins a white silken cocoon, among the strands of which are entangled the long black body hairs; herein the larva undergoes the final moult, the cast cuticle being easily seen at one end inside the cocoon.

The insect, however, has now assumed a form quite different from that of the larva; this form is the chrysalis or pupa, and as such is incapable of locomotion and feeding. The pupa measures about three-quarters of an inch long, is yellowish at first, but soon becomes black with yellow markings, while the form of the future moth (head, antennÆ, thorax, legs, wings and abdomen) can be traced on the pupal cuticle. After from about two to five weeks, the pupa opens by a cross-shaped slit on the back just behind the head, and the moth draws itself out. At first the moth is comparatively helpless after having been confined within the limited space of the pupal cuticle; soon, however, the body hardens, the wings smooth out, and the insect is ready for flight.

Metamorphosis is carried to a much higher state of perfection in the case of such insects as blowflies and houseflies (Fig.4). The larva, or maggot, is without any external sign of head and legs, though these, together with the wings of the future fly, develop from rudiments within the body of the maggot. At the final moult the larval cuticle is not discarded, as in the case of the moth, but hardens to form a case—?the puparium—?within which the pupa lies.

The life-cycle of the magpie moth is illustrative of the principles of metamorphosis characterising the development of a great many insects, such as all moths and butterflies, beetles, flies, bees and wasps, etc.; but, although the general characters of the larva, pupa, and adult moth are common, with but slight variation, to corresponding stages of moths and butterflies as a whole, these stages in other insects, though readily recognised, have their own characteristics.

Outstanding features in a life-cycle involving metamorphosis are that growth takes place only in the larval state, and that the insect parades through life in different guises—?egg, larva, pupa, and adult—?each with its own peculiarity of habit and form, although the adult and pupa resemble one another much more than do the adult and larva; but no matter how dissimilar the larva, pupa, and adult may outwardly seem, structures common to them all may be traced throughout. Make, for example, a comparative study of the larva, pupa, and moth of the magpie moth; the head, thorax, and abdomen can be seen in each stage, while counterparts of the larval antennÆ, eyes, mouth-parts and feet persist in the moth, though more or less profoundly modified during pupal transformation. Although there are no external signs of wings in the larva, these appendages are developing, nevertheless, in concealed “pockets” within the larval thorax, and, at the time of pupal formation, become extruded and lie ensheathed with the legs and antennÆ in the pupal cuticle along the sides of the pupal body. Apart from these changes, the larval mouth parts undergo a most profound metamorphosis; apparently, though there is no similarity between the long “tongue” or proboscis of the moth and the jaws and accessory jaws of the caterpillar, the proboscis, adapted for sipping the nectar of flowers, is nothing but the accessory jaws of the leaf-chewing larva greatly elongated; with the exception of the palps of the accessory jaws, the other larval mouth parts are either absent in the moth or reduced to vestiges.

In the case of insects that develop without a metamorphosis, the life-cycle is one of comparative simplicity. An example of such an insect is the so-called “silverfish” (Lepisma saccharina), common in dwellings, especially in damp places, dark and dusty corners, flour and sugar bins, while not uncommonly it causes some considerable damage by devouring the paste and glaze from wallpapers and the binding and leaves of books.

The silverfish (Fig.4), wingless throughout life, measures about one-quarter of an inch long when full grown; it is silver-white in colour, due to a clothing of glistening scales that rub off as a silky powder when the insect is handled. It glides rapidly about, especially after dark, and is one of the most primitive insects, there being minute leg-like processes attached in pairs to the under side of the abdomen; the normal thoracic legs are well developed. The body is wedge-shaped, tapering to the posterior end, from which three tail-like appendages project, while anteriorly a pair of long, delicate antennÆ arises from the head.

All stages of the silverfish, from the minute, freshly-hatched individuals to fully-grown ones, may be found in the one place, the smaller ones being immature developing stages. In the case of another species allied to the common silverfish, the female lays from six to ten eggs at one time in sheltered crevices, and the young hatch forty-five to sixty days later, when the temperature ranges from 65 degrees to 68 degrees Fahrenheit.

Unlike the moth larva, that of the silverfish throughout its growth resembles the adult both in habit and form, the only marked differences being that of size and the absence of the abdominal leg-like appendages. During growth several moults take place, and at the final one the adult appears with all its characteristics. Some species take two years to reach maturity. In this type of insect there is, therefore, no pupal or resting stage, and the larval habits and food are the same as those of the adult insect, while there is but little difference in structure throughout all the stages.

There are many winged insects (e.g., cockroaches, crickets and earwigs) that show a slight advance toward a metamorphosis. Though their larvÆ differ from the adults principally in the absence of wings, there are stages between the younger larvÆ and the adults in which the wing rudiments appear. These rudiments first appear after one of the moults as small bud-like structures on each side of the thorax (earwig, Fig.4), becoming larger after each succeeding moult, when the developing wings may be seen enclosed in a sheath of the cuticle; at the final moult the wings, no longer enclosed in their coverings, straighten out and become functional. A very pronounced difference is here noted between the wing development of such insects and that of a moth, in that the wing rudiments of the former develop externally and those of the latter internally.

A decided advance toward a metamorphosis is exhibited by insects known as thrips (Fig.4). Though readily overlooked on account of their minute size (one-twenty-fourth of an inch and less), they are nevertheless conspicuous on green foliage and white flowers owing to their blackish or yellowish colour. Thrips, when magnified, are easily recognised by their peculiar wings; each is feather-like, being formed of a narrow rib-like membrane clothed along the margins with long and delicate stiff hairs. Thrips’ eggs are laid upon the plant surface or within the tissues, according to the species, and are very minute (about one-twenty-third of an inch long). The larvÆ puncture the plant tissues and feed upon the juices just as do the parents, which they resemble in general form, except that there are no wings and the antennÆ are very short and the eyes small. There are two or three larval moults, after which the insect is more like the adult, though still resembling the larva. It now differs from the latter, however, in the antennÆ being considerably shortened, and in the appearance of a pair of finger-like processes on each side of the body attached to the thorax and lying along the sides of the abdomen; these processes are the sheaths enclosing the wing rudiments of the future adult. The insect again moults, changing to a form resembling the preceding stage in many respects, but differing in the wing sheaths being much longer, and in having the antennÆ, enclosed in sheaths of cuticle, turned back over the head. Although during these two stages the insect is capable of moving about, it is nevertheless sluggish and does not feed; from this second semi-quiescent stage the adult emerges. In the thrip’s cycle, therefore, although the habits of the larva and adult are similar, the presence of the two intermediate semi-quiescent stages, during which feeding ceases, shows a decided advance toward a true metamorphosis and represents a pupal stage.

In the case of those insects not involved by a metamorphosis, as discussed above, the structure and habit of both adult and the immature stages differ but little, the development of wings being the principal change, except in the case of the thrips, where there is a definite tendency toward a pupa. However, passing on to a consideration of the common cicada (wrongly called a locust), a change in both structure and habit occurs during the life-cycle, the immature stages being adapted to a subterranean life, while the winged adult frequents the foliage of trees; all stages agree, however, in puncturing plant tissues with their proboscis and sucking up the nutrient juices from the roots by the larva and from the stems and leaves by the adult.

The female cicada (Fig.4) lays its eggs in colonies beneath the young bark of trees and shrubs; the larvÆ, on hatching, drop to the ground, into which they burrow; the antennÆ and soft body are comparatively long, while the fore legs are greatly modified for grasping plant roots and as digging tools. After a number of moults, the body shortens, the antennÆ come to resemble those of the adult, and the rudiments of the wings appear. Growth and the activities of the developing insect continue until finally the larva constructs an earthen underground chamber, in which it lies torpid until ready to undergo the final moult; in this inactive state, though still resembling the later larval stages, the insect corresponds to the pupa of the moth. For the final moult the pupa leaves the ground, crawls up some support (a tree trunk or post), where the winged adult emerges, leaving the empty pupal husk attached to the support. Besides the change in habit and the possession of functional wings, the adult cicada differs in many structural features from the immature stages. Outstanding differences are the normal fore legs, the development of a “voice-box” in the male, and an ovipositor in the female.

An insect that shows some linkage between those having a true metamorphosis and those having a partial metamorphosis is the aphis-lion (Micromus tasmaniÆ), though undergoing a true metamorphosis itself. The larvÆ are predaceous and feed upon aphids (Fig.4). Its larva, pupa, and adult are distinct forms, as in the moth, but the larva is not of the specialised caterpillar or grub type, rather resembling in general appearance the silverfish, or the type of young larva peculiar to such insects as the earwig or thrips before the wing rudiments develop. Furthermore, the pupa, though one in the strict sense, is capable of great freedom of movement, its head, mouth-parts, antennÆ, legs and wings, ensheathed by the cuticle, being freely movable, and not rigidly attached to the body.

A review of the early larval stages of the earwig, thrips and cicada, prior to wing development, and of the aphis-lion larva, shows a conformity to a generalised type exemplified by the primitive silverfish. On the other hand, the moth caterpillar exhibits another larval type more highly specialised, though still retaining a modified semblance to the silverfish type, while specialisation is carried to the highest degree in the blowfly maggot, where all outward sign of the primitive larval type is lost. Regarding the pupÆ, there are three types; the most simple is the free pupa, like that of the aphis-lion, and some moths, beetles, etc., where the appendages are freely movable. The most complex is the pupa of the blowfly, enclosed in its puparium, while intermediate between these two extremes are many moth pupÆ that have the appendages firmly attached to the body, but nevertheless visible.