PLECOPTERA OF NORTH AMERICA 



21 



variation in depth of forks: the mode is shown in our 

 figure of the normal wing. 



The variation in branches of the cubital vein was as 

 follows : 



No. of branches 12 3 4 



Times occurring, fore wing 42 7 1 



Times occurring, hind wing 46 4 



The point of origin of the radial sector is often of im- 

 portance in stonefly wing topography, but it varies con- 

 siderably in this species. Estimating in tenths of the 

 distance from the arculus to the cord outward along the 

 wing disc it varies as follows : 



Origin at 3 4 5 6 tenths of the disc 



Times, fore wing 10 40 



Times, hind wing 2 33 15 



Thus the radial sector in the fore wing normally 

 springs from vein Rl at two-fifths the length of the 

 wing disc, and in the hind wing it separates from vein M 

 halfway out. 



The matchings of the two anterior crossveins of the 

 cord, r and r-m, at their junction with the radial sector de- 

 termines the course of the cord. Sometimes these are op- 

 posite and erect. Oftener they are ajog either in or out. 

 The variation in this respect in these wings stated in 



7*7 r 



3dA^rr 



* dA IstA 



Cu, 



5thA 4thA 3dA 

 Fig. 6. Wings of Isoperla bilineata Say. 



