PLECOPTERA OE NORTH AMERICA 175 



Distribution. — This genus ranges over the entire area of 

 the United States and Canada. 



Acroneuria is closely allied to the genus Perla, from 

 which the males may be separated by the presence of the 

 hammer on the ninth sternite and the structure of the 

 subanal lobes which have been modified into recurved 

 genital hooks. There seems to be considerable variation 

 in the structure of individuals within single species of this 

 genus. Even the genital structures may show some varia- 

 tion. The eggs, however, have proved to be quite con- 

 stant within the species and often serve to identify the 

 females in doubtful cases. 



The genus Acroneuria, as the name implies, was origi- 

 nally based on the presence of crossveins in the outer sub- 

 marginal field of the wings, between the tips of the regu- 

 lar longitudinal veins and this character holds for most of 

 the species within the genus; but there is considerable 

 variation ; some species have many crossveins while oth- 

 ers have few or none. The number of crossveins varies 

 also within the same species, and even between the right 

 and left wings of the same individual. 



Genotype, Pcrla arcnosa Pictet. 



Among the numerous species described by the older 

 authors, A. abnormis appears to be the only one that has 

 been correctly identified by all those who have discussed 

 the genus. The others were found to be in a confusion 

 that seemed at first to be inextricable. But a restudy of 

 the types and topotypes in American collections, a com- 

 parison of notes made from types in European collections 

 by helpfully minded colleagues, and adequate collections 

 from typical localities, have enabled us to identify all the 

 species of the older authors with some assurance. 



Key to the Species of Acroneuria. 



1. Fore wings suddenly widened beyond origin of 



Rs. Color wholly yellow. Female subgenital 

 plate bilobed, the median notch rectangular. 

 Male genital hooks cylindrical, bearing at the 

 base a bunch of hairs ; hammer small, longi- 

 tudinally oval ; segments 9 and 10 above with 



short spines mnthenes 



Forewings gradually widened to stigma. Color 

 not wholly yellow 2 



2. Second anal vein of hind wing many (about 12) 



branched, the branches variously anastomos- 

 ing. Female subgenital plate produced over half 



