WING-DEVELOPMENT 



333 



the histories of the formation of the nervures and of their 

 relation to tracheae are different in various Lepidoptera. This 

 conclnsion is rendered more probable l)y the statement of Coni- 

 stock and I^eedham,^ that in some Insects the '■ peritracheal 

 spaces " that mark ont 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 

 liave 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 

 sufiriciently exact. It is, however, 

 probalde 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." It appears probable that the "wing -ribs" found in 

 the nervures (Fig. 170, A//' 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 

 1 Amcr. JVaturaL, xxxii. 1898, p. 256. - Zeitschr. wiss. Zool. viii. 1857, p. 326. 



Fig. 174. — Traiisver.se section of 

 part of the newly disclosed 

 chrysalis of Pieris hrassicae, 

 showing the position and struc- 

 tuve of the wings, hanging 

 from one side of the body. 

 aa, Anterior wing ; ajj, pos- 

 terior wing ; e, e, peritracheal 

 spaces ; f, t, tracheae. (After 

 Gonin.) 



