PLECOPTERA OF NORTH AMERICA 



23 



except in 1% of the specimens that had an extra fork on 

 one of the vein tips, but the depth of the main fork varied 

 as follows (expressed in tenths of the distance from the 

 cord to the wing tips) : 



Tenths of distance 2 3 4 5 6 



Times occurring, fore wing 3 9 21 15 



Times occurring, hind wing 2 2 13 22 9 



Thus the modal position for the fork is in the middle of 

 this space, 



The tip of the subcostal vein joins vein Rl modally a 

 distance before the cord about equal to the length of the 



C Sc R 



==— ; — * ^ / 7— 



arc / -^ ^^_ R s 



a -^^^"^ 



^dA* 



2dA ^M_Jt2^3 — -~^Cu 



5thA 



Fig. 7. Wings of Perlinella drymo Newm, 



crossvein r and the variants in the fore wing run gen- 

 erally to a little greater length of subcosta; in the hind 

 wing generally they run a little shorter. 



Variation in Perlinella drymo. — This was studied in fif- 

 teen specimens, 9 males and 6 females from Elkhart, 

 Ind. The crossveins in the space between the costal 

 and subcostal veins have so atrophied in this species that 

 they could not be counted with certainty but there ap- 



