9l8 FRESH-WATER BIOLOGY 



KEY TO NORTH AMERICAN MAYFLIES 

 Imagos 



1 (13) The cubital and first anal veins strongly divergent at the base (Fig. 



1387). Venation never greatly reduced 2 



2 (3) The posterior fork of the median vein very deep, almost reaching the 



wing base; two long simple intercalaries between the first 

 and second anal veins Campsurus. 



3 (2) The posterior fork of the median vein (M3-M4) forked for not more than 



three-fourths of its length 4 



Fig. 1387. The wings of Ephemera. (Drawn by Dr. Anna H. Morgan.) 



4 (5) Between the first and second anal veins is a bunch of three or four long 



straight intercalaries, conjoined basally before their attach- 

 ment to the principal veins; the second anal vein nearly 

 straight and unbranched Polymitarcys. 



5 (4; Between the first and second anal veins are only shorter, sinuate, and 



sometimes forking intercalaries, attached directly to the 

 first anal; the second anal vein sinuate and often branched 

 (Fig. 1387) 6 



6 (7) The posterior fork of the median vein forked two- thirds to three-fourths 



its length; vein Cu2 not more strongly bent at base than 

 the first anal Euthyplocia. 



7 (6) This fork of the median vein occupying not more than half its length; 



vein Cu2 more strongly bent at base than is the first anal 

 (Fig. 1387) 8 



8 (12) The third anal vein simple, but attached to the hind margin by a 



number of cross veins; in the narrow posterior fork of the 

 median vein there are one or more cross veins before the origin 

 of the intercalary; male forceps four-jointed 9 



9 (10, 11) Caudal setae three in both male and female; fore tarsus of female 



imago three-fourths as long as the tibia. . . . Ephemera. 



10 (9, 11) Caudal setae two in the male and three in the female; fore tarsus 



of the female two-thirds as long as the tibia. . Pentagenia. 



11 (9, 10) Caudal setae two in male and female; fore tarsus of female as long 



as the tibia Hexagenia. 



