Life Histories of Insects.
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.
[Illustration: 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.