THE EXTERNAL ANATOMY OF INSECTS 



63 



The wings of nymphs, naiads, and pupae are broad at the base, 

 and consequently the tracheag that precede the wing-veins are not 

 crowded together as are the wing-veins at the base of the wings of 



Fig- 75- — Jugum of a hepialid 



Fibula of Corydalus. 



adults. For this reason the identity of the wing- veins can be deter- 

 mined more surely in the wings of immature insects than they can be 

 in the wings of adults. This is especially true where two or more 

 veins coalesce in the adult wing while the tracheag that precede these 

 veins are distinctly separate in the immature wing. 



A study was made of the tracheation of the wings of immature 

 insects of representatives of most of the orders of insects, and, assum- 

 ing that those features that are possessed by all of them must have 

 been inherited from a common ancestor, a diagram was made repre- 

 senting the hypothetical tracheation of a nymph of the primitive 

 winged insect (Fig. 77). In this diagram the tracheag are lettered 



Fig. 77.— Hypothetical tracheation of a wing of the primitive nymph. 



with the abbreviations used in designating the veins that are formed 

 about them in the course of the development of the wing. The dia- 

 gram will serve, therefore, to indicate the typical venation of an insect 



