920 FRESH-WATER BIOLOGY 



25 (24) Third anal vein of the hind wing present, and often followed by one 



or two additional intercalaries; median caudal seta dis- 

 tinctly shorter than the others. ... Blasturus. 



26 (23) Hind wing with an angular lobe projecting forward from the middle 



of the costal margin; four longitudinal veins between Mi and 

 M2; wing margins free from veinlets, and few cross veins in 

 hind wing . . Hahrophlebia. 



27 (22) In the hind wing the subcostal vein terminates in the costa at hardly 



more than half the length of the wing, just beyond the 

 obtuse angulation having a thickened margin; forceps of 

 male more or less distinctly four-jointed. . Choroterpes. 



28 (21) Bisectors of the posterior fork of the median vein and of the cubital 



fork both tending to attach themselves to the posterior 

 branch of their respective forks; between the latter and vein 

 Cu2 are generally some short intercalaries (the cubital region 

 thus being better developed than in group 21); caudal setae 

 about as long as the body; penultimate segment of the male 

 forceps longer than the antepeniiltimate. . . .29 



29 (30) Veins Cu2 and ist ^ separate to base. . . . Ephemerella. 



30 (29) Veins Cu2 and ist A fused toward the base. . . . Drunella. 



31 (20) Hind wings absent. . . Caenis. 



32 (19) The intercalaries between the first and second anal veins represented 



by a series of veinlets, often sinuous or forking, extending 

 directly from the first anal to the wing margin; costal 

 angulation of hind wing close to the base; but two well- 

 developed caudal setae, the median one being rudimentary 

 or wanting; basal joint of hind tarsi evident but not well 

 developed. . . .... • • • • 33 



33 (36) Median caudal seta a distinctly segmented rudiment (Fig. 1354); for- 



ceps of male three-jointed; posterior prolongation of sternum 

 of ninth segment of abdomen of female bifid at tip. . . 34 



34 (35) Basal segment of fore tarsus of male shortest; claws of each tarsus 



unhke each to each; hind wing with the costal angulation 

 acute, and the posterior fork of the median vein occupying 

 two-thirds the length of that vein Coloburus. 



35 (34) Basal segment of fore tarsus of the male longest; claws of each 



tarsus alike; hind wing with the costal angulation obtuse, 

 and the posterior division of the median vein forked through 

 one-third its length Chirotonetes. 



36 (33) Median caudal seta more rudimentary or wanting; forceps of the 



male distinctly four-jointed; posterior prolongation of the 

 sternum of the ninth abdominal segment in the female entire 

 at tip 37 



37 (38) Claws of each tarsus alike; caudal setae at least one-half longer than 



the body Siphlurus. 



38 (37) Claws of each tarsus unlike; caudal setae about as long as the body 



in both sexes Ameletus. 



39 (18) Posterior fork of the median vein apparently simple, M4 being de- 



tached and appearing as an intercalary; hind wings when 

 present at least twice as long as wide, and provided with 

 but 1-3 longitudinal veins , 40 



