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 Habrophlebia. 



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 antepenultimate 29 



29 (30) Veins Cu2 and ist A 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 Colohurus. 



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 



