DEVELOPMENT WINGS NERVURES 



329 



Picris lirassicae} and finds that the future wing is then indi- 

 cated l)y a thickening and bagging inwards of the hypoderinis, 

 and by some embryonic cells and a trachea in close relation with 

 this mass (Fig. IGS, A). The structure grows so as to form a sac 

 projecting to the interior of the body, connected with tlie 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 crum])led 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 



si T 



pillar of P. brassirne, the body- 

 wall liaviug been dissected oti', 

 immediately before pupation. 

 a, «', Anterior and iiosterior 

 wings ; st I, first spiracle ; p, p', 

 second and third legs. (After 

 Gonin.) 



Gonin's observations that it is not Fig. 169.— Anterior parts of a cater- 

 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 liardens so as to form the shell of the chrysalis. 



Scales and nervures. — Before tracing the further develop- 

 ment it will be well to discuss the structure of the scales and 

 nervures that form such important features in the Lepidopterous 

 wing. 



If a section be made of the perfect wing of a Lepido})teron, 

 it is found that the two layers or walls of the wing are firmly 

 held together by material irregularly arranged, in a somewhat 

 columnar manner. The thickness of the wing is much greater 

 where the section cuts through a nervm-e (Fig. 170, A). The 

 nervures apparently differ as to the structures found in them. 

 Spuler observed in a nervure of TviiiliaoKi ■pronuha, a liody having 

 in section a considerable diameter, that he considered to l)e a 



1 Bull. Soc. Vaudoisc, xxx. 1894, No. 115. 



