The DEVELOPMENT of the WINGS of Lepidoptera has recently been much studied. It has been known since the time of Lyonnet, that the rudiments of the wings exist inside the body of the caterpillar when it is nearly adult. Verson considers that he has detected the rudiments in the silk-worm larva even before hatching, and he attributes their origin to a modification of form of those hypodermal cells that occupy the spots where the spiracles of the second and third thoracic segments might be looked for. (It will be recollected that there are no spiracles on these two thoracic segments in Lepidopterous larvae). Gonin has examined the wing-rudiments in the caterpillar, a few days old, of {329}_Pieris brassicae_,[193] and finds that the future wing is then indicated by a thickening and bagging inwards of the hypodermis, and by some embryonic cells and a trachea in close relation with this mass (Fig. 168, A). The structure grows so as to form a sac projecting to the interior of the body, connected with the body-wall by a pedicel, and penetrated by a trachea forming branches consisting of rolled and contorted small tracheae (Fig. 168, B). If the body-wall be dissected off the caterpillar immediately before pupation the wings appear in crumpled form, as shown in Fig. 169. This fact was known to the older entomologists, and gave rise to the idea that the butterfly could be detected in a caterpillar by merely stripping off the integument. The exact mode by which the wings become external at the time of appearance of the chrysalis is not ascertained; but it would appear from Gonin's observations that it is not by a process of evagination, but by destruction of the hypodermis lying outside the wing. However this may be, it is well known that, when the caterpillar's skin is finally shed and the chrysalis appears, the wings are free, external appendages, and soon become fastened down to the body by an exudation that hardens so as to form the shell of the chrysalis. [Illustration: Fig. 169—Anterior parts of a caterpillar of _P. brassicae_, the body-wall having been dissected off, immediately before pupation. _a_, _a′_, Anterior and posterior wings; _st I_, first spiracle; _p_, _p′_, second and third legs. (After Gonin.)] SCALES AND NERVURES.—Before tracing the further development it will be well to discuss the structure of the scales and nervures that form such important features in the Lepidopterous wing.