AQUATIC INSECTS 921 



40 (45) Hind wings present 41 



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



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



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 Bactis. 



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 Clocon. 



Nymphs 



1 (11) 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; lab rum longer than wide; maxillary palpus 

 two-jointed Polymitarcys. 



5 (4) Legs decreasing in length posteriorly; lab rum 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 Leptophlchia. 



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

 21(18) Gills lamelliform, at least on the middle segments 22 



22 (23) Lamellae of each gill similar Blast urns. 



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



