PLECOPTERA OE NORTH AMERICA 43 



the scoop that projects forward and upward from the dor- 

 sum of the ninth segment in the male is very variable, and 

 in a series of specimens intergrades completely between 

 the two forms that she figured and described. The three- 

 lobed tip of the genital plate of the female in Pt. triloba 

 appears to be but another variant in the shape of this part 

 in this same variable species, Pt. badia. We are unable to 

 distinguish by reliable characters more than the two orig- 

 inal species that were described by Hagen in 1873. 



These stoneflies are superficially similar to Perla in ap- 

 pearance but distinguishable at a glance from Perla and 

 all its allies by the venation and by the greater length of 

 the first segment of the tarsus. The species look alike and 

 are only to be distinguished by careful microscopic ex- 

 amination of genital parts. 



The nymph is distinguished from that of other genera 

 by the presence of tufted gills on the ventral side of the 

 three basal abdominal segments. 



A western genus, common in mountain streams. 



Genotype, Pteronarcys regularis Hagen. 



Key to the Species of Pteronarcella. 



Males. 

 1. Recurved, scoop-shaped appendage on the dor- 

 sum of the ninth abdominal segment, acutely 

 pointed at the - apex and its side margins 

 straight, its posterior face lacking scurfy or 

 prickly pubescence except along a narrow me- 

 dian area regularis 



Recurved scoop-shaped appendage on the dorsum 

 of the ninth abdominal segment broadly 

 rounded at the apex, more elevated, blunter, 

 and its side margins sinuous, its posterior face 

 rather uniformly covered with scurfy or prick- 

 ly pubescence badia 



Females. 

 1. The small strongly chitinized subgenital plate in 

 the middle of the rear border of the eighth 

 sternite is M-shaped on its hind margin, with 

 distinct and acute median notch, its tips gen- 

 erally on a level with the sides of the segment 

 regularis 



