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 abdomin