68 AN INTRODUCTION TO ENTOMOLOGY. 



TABLE OF GENERA OF THE EPHEMERID^ OF THE 

 UNITED STATES. 



'A. At the base of the fore wing the anal vein (a] meets the postbrachial (po}\ 

 hind tarsi, when not atrophied, have four distinct joints. 

 B. Female with the hind legs longer than the other pairs ; male with the 

 forceps-limbs sessile upon the border of the segment. 

 C. Includes males only. Two caudal setae in both genera. 



D. Hinder lateral angles of the 9th abdominal segment produced into a 

 short, tooth-like spine. JOLIA. 



DD. Hinder lateral angles of the abdominal segments subrectangular. 



POLYMITARCYS. 



CC. Includes females only. 



D. Two caudal setae. JOLIA. 



DD. Three caudal setae. POLYMITARCYS. 



BB. Fore legs of females at least as long as hind legs ; male with the forceps- 

 limbs inserted at the sides of the terminal border of a short transverse 

 laminar lobe prolonged from the segment. 

 C. Includes males only. 



D. Median seta extremely rudimentary. 



E. Fore leg nearly as long as body; the tibia about ii as long as 



femur. Segments of caudal setae well marked. External sexual 



organs strongly hooked. HEXAGENIA. 



EE. Fore leg about half as long as body; tibia about if as long as 



femur. Segments of caudal setae inconspicuous. External sexual 



organs nearly straight. PENTAGENIA. 



DD. Median seta about as long as the others. EPHEMERA. 



CC. Includes females only. 



D. Median seta extremely rudimentary. HEXAGENIA. 



DD. Median seta about as long as the others. 



E. Abdominal segments 6-10 together constituting about fof the ab- 

 domen ; segments of the caudal setae well marked. EPHEMERA. 

 EE. Abdominal segments 6-10 together constituting \ of the ab- 

 domen ; segments of the caudal setae inconspicuous. 



PENTAGENIA. 



AA. Anal vein (a) of fore wing does not directly meet the postbrachial vein 

 (po), but is connected with it by a more or less distinct channel of circula- 

 tion in the membrane. 



B. The channel of circulation connecting postbrachial and anal veins ob- 

 solete (except in Ccznis, a two winged genus). Hind tarsi with tour 

 distinct joints. 



C. Hind wings well developed ; with a sharply denned, almost right-an- 

 gled projection situated at about the first i of the costal margin. 



POTAMANTHUS. 



CC. Hind wings of small or moderate proportions. 



D. Hind wings either gently and on the whole continuously curved in 

 front, or else suddenly retracted in the middle of the fore margin. 

 E. Hind tibia usually longer than the femur, rarely subequal to it. 



