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.