AQUATIC INSECTS 921 



40 (45) Hind wings present 4 1 



41 (42) Fore wings with numerous costal cross veins before the bulla; hind 



wings with a moderate number of cross veins . . Callibaetis. 



42 (41) Fore wings without costal cross veins before the bulla; hind wings 



without cross veins or with but 1-3 of them 43 



43 (44) Marginal intercalary veinlets in pairs; hind wings oblong, with a 



short costal angulation Baetis. 



44 (43) Marginal intercalary veinlets of the fore wing single; hind wings 



linear, with a spur-like costal angulation. . Centroptilum. 



45 (40) Hind wings absent Cloeon. 



NYMPHS 



1 (n) Mandibles with an external tusk-like ramus, visible from above; gills 



on abdominal segments 1-7 (often rudimentary on i), 

 double, flattened, linear, the margins fringed with respira- 

 tory filaments 2 



2 (9, 10) Mandibular tusks longer than the head (burrowing species) . . 3 



3 (6) With no frontal prominence. .'.-.'* 4 



4 (5) Legs increasing in length posteriorly; gills of the first abdominal seg- 



ment simple; labrum longer than wide; maxillary palpus 

 two- jointed Polymitarcys. 



5 (4) Legs decreasing in length posteriorly; labrum wider than long; maxil- 



lary palpus three-jointed Euthyplocia. 



6 (3) With a conspicuous frontal prominence 7 



7 (8) Frontal prominence rounded. ..-...* Hexagenia. 



8 (7) Frontal prominence bifid at tip Ephemera. 



9 (2, 10) Mandibular tusks shorter than the head, inconspicuous, only their 



tips visible from above Potamanthus. 



10 (2, 9) Unknown Campsurus and Pentagenia. 



11 (i) Mandibles without projecting tusk-like ramus; gills not as in i. 12 

 12(13) Eyes dorsal; body strongly depressed; tarsal claws with lateral teeth; 



dwellers in rapid streams and on wave-beaten shores adapted 

 to clinging to flat surfaces of rocks, timbers, etc. 



Heptagenia. 



13 (12) Eyes lateral; claws smooth or toothed below 14 



14 (15) Gills completely concealed under an enormously enlarged, four- 



spined dorsal thoracic shield Baetisca. 



15 (14) Gills exposed; thoracic dorsum normal 16 



16 (31) Outer caudal setae fringed on both sides .17 



17 (24) Gills on abdominal segments 1-7 double 18 



18 (21) Gills filamentous 19 



19 (20) Each a pair of simple filaments Leptophlebia. 



20 (19) Each a pair of clusters of slenderer filaments. . . . Habrophlebia. 



21 (i 8) Gills lamelliform, at least on the middle segments 22 



22 (23) Lamellae of each gill similar. , . Blasturus. 



23 (22) Lamellae of each gill markedly differing in form at tip. Choroterpes. 



