THE TRA CREATION OF WINGS 



23 



tracheae are retained, as in certain asilids, they have not retained their 

 primitive position. 



A similar condition exists in the Hymenoptera. In the more general- 

 ized Hymenoptera, as in Tretnex (Fig. 15), the main stems of the principal 



Fig. 15. — The wings of a pupa of Tremex (After C. & N.). 



tracheag are retained, and occupy very nearly the normal positions; but the 

 courses of the branches of these trachea? bear little if any relation to their 

 primitive courses. In the more specialized families, there is a greater 

 reduction of the tracheation and a wider deviation from the primitive type 

 in the branches of the trachea that are retained. The wings of a young 

 pupa of Apis (Fig. 16) illustrates this. In these wings the vein-cavities are 

 already fonned and the tracheae are beginning to push out into them, follow- 

 ing the most direct courses in the already formed vein-cavities. 



^J;- 



Fig. 16. — The wings of a pupa of Apis (After C. & X.). 



Eccentric tracheation of wings. — In the Ephemerida there are many 

 modifications of the tracheation of the wings that do not appear to result 

 from any definite course of specialization. In many cases a principal 

 trachea is reduced in length, so that it traverses only a part of the vein 

 with which it corresponds; frequently a trachea follows its vein for a dis- 



