vi DEVELOPMENT WINGS NERVURES 329 



1'icris brassicae, 1 and finds that the future wing is then indi- 

 cated 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 Fi. 169. This fact was known 



a o f 



to the older entomologists, and gave si I 

 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 FIG. 169,-Auterior parts of a cater- 

 by a process of evagination, but by pillar of P. ftrawicoe, the body- 

 . ... wall having been dissected ofl, 

 destruction Ot the hypodermis lying immediately before pupation. 



outside the wing. However this may ". ' Anterior and posterior 



wings ; st 7, first spiracle : p, p , 



be, it is well kuOWll that, when the second and third legs. (After 



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. 



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 Lepidopteron, 

 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 nervure (Fig. 170, A). The 

 nervures apparently differ as to the structures found in them. 

 Spuler observed in a nervure of Triphaena pronuba, a body having 

 in section a considerable diameter, that he considered to be a 

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



