TEXT-BOOK OF ENTOMOLOGY 



" These two systems are absolutely independent of each other 

 within the wing. Their existence is simultaneous but not conjoint. 

 One is functionally active after the third moult ; the other waits the 

 final transformation before becoming active." 



Evagination of the wing outside of the body. We have seen that 

 the alary germs arise as imaginations of the hypodermis ; we will 

 now, with the aid of Gonin's account, briefly describe, so far as is 

 known, the mode of evagination of the wings. During the fourth 

 and last stage of the caterpillar of Pieris, the wings grow very 



undergo 

 important changes. 



Six or seven days 

 after the last larval 

 moult the chitinous 

 wall is formed, the 

 wing remaining trans- 

 parent. It grows 

 rapidly and its lower 

 edge extends near the 

 legs. It is now much 

 crumpled on the edge, 

 owing to its rapid 

 growth within the 

 limits of its own seg- 

 ment. Partly from 

 being somewhat re- 

 tracted, and partly 

 owing to the irregu- 

 larity of its surface, 

 the wing gradually 

 separates from its 

 envelope, and the 

 cavity of invagina- 

 tion (Fig. 145, c) 

 becomes more like a distinct or real space. The outer opening of the 

 alary sac enlarges quite plainly, though without reaching the level of 

 the edge of the wing. 



This condition of things does not still exactly explain how the 

 wing passes to the outside of the body. Gonin compares these con- 

 ditions to those exhibited by a series of sections of the larva, made 

 forty-eight hours later, on a caterpillar which had just spun its girdle 

 of silk. At this time the wings have become entirely external, but, 



in 



Fu;. 1-15. ,-1, section of wing-bud uf larva of I'ici-ix ///w.v.v/Vic 

 of stage I, in front of the invagination pit. JB, section p:iini;- 

 through the invagination pit. (', section of same in stage II. 

 through the invagination pit; />, behind it, making the bud 

 appear independent of the thoracic wall. L\ winy-bud at the 

 beginning of the 3d larval stage, section passing almost through 

 the pedicel or hypodermic insertion, the traces of which appear 

 &t/ii; h, hypodermis ; t or tr, trachea ; /, opening of in vagination : 

 ec, embryonic cells; I, external layer or envelope: ///. internal 

 wall of the wing; ex, external wall; , cell of a tactile hair; tc, 

 capillary tubes; c, cavity of invagination. After (Joiiin. 



