THE DEVELOPMENT OF WINGS 



115 



the histoblast that later becomes the wing. In the fifth stadium, the wing- 

 bud attains the form shown in Figure 115,^, which represents it dissected out 

 of the wing-pocket. At the close of the last lan^al stadium, the fifth, the 

 wing is pushed out from the 

 wing pocket, and lies under 

 the old larv^al cuticula dur- 

 ing the prepupal stadium. 

 It is then of the fomi shown 

 in Figure 115, /. The molt 

 that marks the beginning of 

 the pupal stadium exposes 

 the wing-buds, which in the 

 Lepidoptera become closely 

 soldered to the sides and 

 breast of the pupa. Imme- 

 diately after the last molt, 

 when the adult emerges, the 

 wings expand greatly and 

 assume their definitive form. 



The development of the 

 tracheation of the wings. — 

 In the earlier stages of its 

 development, the wing-bud 

 is not provided with special 

 organs of respiration, resem- 

 bling in this respect other 

 portions of the hypodermis 

 of which it is a part. It 

 should be noted, however, 

 that the histoblast is devel- 

 oped near a large trachea, a 

 cross-section of which is 

 shown in (Figure 115, a,b, 

 c, d), which represents sec- 

 tions of these parts of the 

 first, second, third and 

 fourth instars respectively. 



Diiring the fourth stadium, certain cells forming a part of the epithelium 

 of this trachea become greatly enlarged and project into the cavity of the 

 wing-bud (Fig. 115, d). Within each of these cells there is developed a 

 closely coiled tracheole. During this stadium the tracheoles have no 

 communication with the lumen of the large trachea; but as the new intima 

 of this trachea is formed it is not extended over the mouths of the tracheoles 



Fig. 115. — Several stages in the development of the 

 wings of a cabbage butterfly (After Mercer). 



