18 THOMAS SAY FOUNDATION 



No. of extra erossveins 12 



Times occurring, fore wing 7 6 1 



Times occurring, hind wing 7 7 



Thus it will be seen that such a cord cell (inclosed ex- 

 ternally by the extra crossvein) was present in just half 

 of these wings. It was absent from all four wings of two 

 of the specimens and was present in all four in two other 

 specimens. 



Variation in Perla capitata. — This was studied in 18 

 specimens, 9 males and 9 females, all from Ithaca, N. Y. 

 Crossveins in the space between veins C and Sc varied as 

 follows : 



Sc 

 C ^-r-rrrr-T-, — _ R, 



i: i i 



** R s 



5thA 



^thA 3dA 2dA 



Cu 2 Cu > 



Fig. 4. Wings of Perla capitata Piet. 

 6 



8 9 10 11 12 13 14 15 16 17 

 ir> 74852301 

 5 1 



No. of erossveins 3 4 



Times occ, fore wing.... 



Times occ, hind wing....] 7 11 10 1 



showing the mode to be 12 in the fore wing and 5 in the 

 hind wing. 



The costal crossvein in the apical space varied as fol- 

 lows: 



No. of crossveins 2 3 4 5 6 7 8 



Times occurring 1 7 16 27 13 7 1 



Fore and hind wing alike. 



