THE WINGS OF PLECOPTERA 



245 



The family Gripopterygidae is believed to include the most generalized 

 of living Plecoptera, on account of their strongly developed mandibles ; and 

 the Capniidas and Nemouridas are associated with this family because in 

 these families also the mandibles are well preser\'ed. On the other hand, 

 in the Pteronarcidae and Perlidae only vestiges of mandibles exist. 



It may be that farther studies will show that this is not a natural 

 division of the order; but with the data at hand it seems to be the best 

 grouping of the families yet suggested. 



The family Gripopterygidae is restricted to the tropics and to the 

 Southern Hemisphere; the other four families are well represented in the 

 United States and Canada. 



In m}' studies of the wings I have been unable to find any characters 

 indicating a dichotomous division of the order; each of the methods of 

 specialization observed exist in both the Holognatha and the Systellognatha. 



id) THE SPECI.\L FEATURES OF THE WINGS OF THE PLECOPTERA 



One of the more striking features of the venation of the wings of the 

 Plecoptera is a lack of uniformity in the number and courses of the sub- 



2d A 

 Fig. 245. — Wings of Isogenus sp. 



ordinate veins. Not only are striking differences in wing-venation to be 

 observed between different members of the same species, but frequently 

 the wings of the two sides of an individual will vary greatly in venation. 



