THE TRACHEA TION OF WINGS 



21 



mining the homologies of the wing-veins; in others, it is of little or no value 

 for this purpose. And, as a rule, there is throughout each order a marked 

 uniformity in this respect. 



The variations in the extent of the tracheation of developing wings can 

 be grouped under a few general heads : in certain orders the tracheation is 

 comparatively simple; in others, there has been developed a greatly 

 increased tracheation of the wings ; while in still others the tracheation has 

 been greatly reduced; and in one order at least, the tracheation is eccentric. 

 Examples of each of these classes are given below. 



Illustrations of the simpler type of the tracheation of the wings. — Excel- 

 lent illustrations of the simpler t}'pe of the tracheation of the wings of 

 n}TTiphs are presented by members of the Plecoptera, of which Nemoura 

 (Fig. 12) may be taken as an example. The figure represents the wings of a 



^a^ 



Fig. 13. — A wing of a pupa of Chaidiodes (After C. &' N.). 



grown nymph in which the developing veins appear as pale bands. In each 

 of the principal veins and in the branches of these there is a prominent 

 trachea; while the cross-veins are without tracheae.* 



Wings in which the tracheation is of this type present few if any difficul- 

 ties in the determination of the homologies of the wing- veins. This is 

 especially true as the cross-veins are sharply differentiated from the princi- 

 pal veins by the absence of tracheae in them. 



Of the other orders that exhibit a simple type of tracheation of the ^vings 

 there may be cited : the Corrodentia, of which Psociis (Fig. 5) wall serve 

 as an example; the Homoptera, illustrated by a cicada, the wings of which 

 are figured later; and the Lcpidoptera, which is discussed in detail in 

 another chapter. 



Illustrations of increased tracheation of wings. — In the simpler type of 

 tracheation of the wings only the primitive wing-veins, those represented 

 in our h^T^othetical primitive type, are represented by prominent tracheae. 



*In the specimen figured the costal trachea was not observed; but this trachea 

 was found in other,' closely allied, forms. 



