22 



THE TRACHEATION OF WINGS 



What may be regarded as the first step in the direction of an increased 

 tracheation of the wings is illustrated by the Neuroptera, of which Chaulio- 

 des (Fig. 13) will serve as an example. Here the primitive wing- veins and 

 the secondarily developed accessory veins are preceded by tracheae, while 

 the cross- veins are not. Although many fine tracheae branch from the 

 principal tracheae and ramify throughout the wing sac they do not follow 

 the courses of the cross-veins. 



The extreme limit of increased tracheation is reached by the Odonata, 

 where not only the principal veins and the intercalary veins are traversed 

 by tracheae, but there is also a distinct segregation of small tracheae and 

 tracheoles in the cross-veins; this is shown in Plate V. In this order the 

 study of the tracheation of the wings affords no help in distinguishing the 

 cross-veins from other veins. 



Illustrations of reduced tracheation of wings. — In several of the orders 

 of insects a remarkable reduction of the tracheation of the wings has taken 



place; and in some cases, 

 at least, associated with 

 this reduction is a retarda- 

 tion in the development of 

 those tracheae that are 

 retained. 



In what is regarded as 

 the more normal relation 

 between the tracheation and 

 the venation of wings, the 

 tracheas are developed early 

 in the growth of the wing, before the beginning of the development of the 

 wing- veins, which are later formed about them. Here the courses of the 

 tracheae determine to a great extent the courses of the wing-veins. 



But in those orders where there is a reduction of the tracheation of the 

 wings, the tracheae, in those cases that we have studied, do not penetrate 

 the wing-sac until after the vein cavities are foi-med, and then they follow 

 the most available channels, which in a wing where the venation is highly 

 specialized, as in the Hymenoptera, may be very different from the 

 primitive course of the tracheae. 



A great reduction of the tracheation of the wings has taken place in the 

 Trichoptera. If a wing of a pupa of a caddice-fiy be examined at that stage 

 when the forming wing-veins appear as pale bands (Fig. 14). it will be seen 

 that the tracheation bears but little relation to the wing-veins. Usually 

 only two or three main tracheae are present; and although these may 

 coincide with forming veins, their branches bear no relation to veins. 



In the Diptcra an equally great reduction of the tracheation of the 

 wings has taken place in most families; and even when most of the 



Fig. 14. 



-A wing of a pupa of a caddice-fly 

 (After C. & N.). 



