256 



THE WINGS OF PLECOPTERA 



The radial-sector. Accessory veins are frequently developed upon the 

 radial sector. In most cases when such veins are present they are borne by 

 vein R2+3; vein R4+5 is, as a rule, unbranched. Occasionally the arrange- 

 ment of the accessory veins is such as to give the radial sector the appear- 

 ance of preserving its primitive four-branched condition. But as already 

 stated this condition is merely fortuitous. In Isogenus (Fig. 245), vein 

 R2+3 bears two accessory veins in the fore wing and one in the hind wing. 

 In certain species of Acroneuria and in Pteronarcys several accessory veins 

 is borne by this vein. As a rule, these accessory veins extend in approx- 

 imately parallel courses except near the apex of the wing. But in certain 



Fig. 254.- — Wings of Pteronarcys dorsata, female (Drawn by 

 Miss L. W. Smith). 



forms, their courses are so irregular that in the adult wing the accessory 

 veins and the cross-veins extending between them form a chaotic network. 

 This is well-illustrated by the fore wing of Pteronarcys dorsata (Fig. 254); 

 in the hind wing the confusion is not quite so great. 



The media. — Accessory veins are comparatively rarely developed on the 

 branches of media, this vein remaining two-branched even when both the 

 radial and the cubital areas are expanded. This is wcll-sliown by Acro- 

 neuria. 



The cubitus. — Accessory veins are frequently developed on the distal 

 part of vein Cui. In Acroneuria for example, there are three such veins 

 that are preceded by tracheae beyond the ordinary cross veins extending 

 between veins Cui and Cu2, which are not preceded by tracheae. 



