The Life-Story of Insects

ung 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 ev

he meaning of the more puzzling and complex cases in

rd 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

d to this layer is misleading. The layer is

rmal 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 cutic

ise 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

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

female; b, male; c, side view of female; d, young

h 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 nervure

insect during a stage of its life-story. Thus the creature as hatched from the egg is the first instar, after the first

mericana) with distinct wing-rudiments

ow 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

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 ca

e 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 ge