﻿Z. P. METCALF 



WING-DEVELOPMENT 



333 



%> aa. 



the histories of the formation of the nervures and of their 

 relation to tracheae are different in various Lepidoptera. This 

 conclusion is rendered more probable by the statement of Corn- 

 stock and Needham, 1 that in some Insects the " peritracheal 

 spaces " that mark out the position of 

 the future nervures are destitute of 

 tracheae. Gonin thinks the nervures 

 are derived from the sheaths of the 

 peritracheal spaces, and a review of all 

 the facts suggests that the tracheae 

 have only a secondary relation to the 

 nervures, and that the view that a 

 study of the pupal tracheae may be 

 looked on as a study of the pre- 

 liminary state of the nervures is not 

 sufficiently exact. It is, however, 

 probable that in Lepidoptera the 

 pupal tracheae play an important 

 though not a primary part in the 

 formation of the nervures ; possibly 

 this may be by setting up changes in 

 the cells near them by means of the 

 air they supply. Semper long ago 

 discovered hypodermal cylinders tra- 

 versed by a string (Fig. 170, B), 

 placed near the tracheae in the 

 pupa.' 2 It appears probable that the "wing -ribs" found in 

 the nervures (Fig. 170, Afr and B) are the final state of these 

 cylinders, but the origin and import of the cylinders are still 

 unknown. 



The formation of the scales of the wing commences very 

 early — apparently soon after the casting of the larval skin — 

 though the completion of the scales and their pigmentation is 

 delayed to a late period of the pupal life. The scales are formed 

 by special cells of the hypodermis that are placed deeper in the 

 interior of the wing than the other hypodermal cells. Each 

 scale is formed by one cell, and protrudes through the over- 

 lying hypodermis; the membrane into which the scales are 

 inserted is a subsequently developed structure, and the beautiful 



Fio. 174. — Transverse section of 

 part of the newly disclosed 

 chrysalis of Pieris brassicae, 

 showing the position and struc- 

 ture of the wings, hanging 

 from one side of the body. 

 aa, Anterior wing ; ap, pos- 

 terior wing ; e, e, peritracheal 

 spaces ; t, t, tracheae. (After 

 Gonin.) 



1 Amer. Natural., xxxii. 189S. p. 256. 2 Zeitschr. wiss. Zool. viii. 1857, p. 326. 



