48 THOMAS SAY FOUNDATION 



two basal segments of the tarsi tend to be reduced to 

 small dimensions — characters that are distinctive of the 

 family. The venation and the male genitalia have been 

 rather fully discussed and illustrated in preceding pages 

 of the Introduction. 



Key to the Genera of Perlidae. 



1. Head as long as wide, much longer than prono- 



tum ; compound eyes situated far forward on 

 head so that the distance between the eyes and 

 the front margin of pronotum is at least twice 

 as great as the diameter of the eye—Kathroperla 

 Head usually wider than long, not longer than 

 pronotum ; the distance between the eyes and 

 front margin of pronotum usually less than di- 

 ameter of the eyes 2 



2. With two ocelli 3 



With three ocelli 5 



3. Prothorax much wider than head ; tails not 



longer than width of pronotum ; no intercu- 

 bital crossveins in hind wings; forewings with 



numerous heavy costal crossveins Peltoperla 



Prothorax little, if any, wider than head ; tails 

 much longer than width of pronotum ; intercu- 

 bital crossveins mostly present in hind wings 4 



4. Ocelli close together, usually on black spot ; anal 



field of f orewing without crossveins ; costal 

 crossveins present before end of Sc ; ninth 



sternite of male without a hammer Neoperla 



Ocelli two or more diameters apart (a trace of an 

 anterior ocellus sometimes noticeable) ; anal 

 field of f orewing with one or more crossveins ; 

 no costal crossveins before end of Sc ; ninth 

 sternite of male with a transverse oval ham- 

 mer A to perl a 



5. Anal area of forewing with a single row of cross- 



veins ; pronotum with a broad yellow or reddish 

 longitudinal band each side of the median dark 



band Perlinella 



Anal area of forewing without crossveins, except 

 those that form the anal cell ; markings of pro- 

 thorax not as in Perlinella 6 



