942 FRESH-WATER BIOLOGY 



8i (80, 82, 83) Surface of the elytra transversely aciculate. . . Colymhetes. 



82 (80, 81, 83) Surface of the elytra with eight to ten longitudinal striae. 



Copelatus. 



83 (80, 81, 82) Surface of the elytra otherwise sculptured or plain. . . 84 



84 (85) Prosternum plainly longitudinally sulcate; species reddish brown. 



Matus. 



85 (84) Prosternum not sulcate. . ... . . 86 



86 (87) Apical segment of each palpus distinctly emarginate. Coptotomus. 



87 (86) Apical segments of palpi obtuse or truncate. . . . .88 



88 (89) Claws of the hind tarsus equal . Agabetes. 



89 (88) Claws of the hind tarsus unequal . . Khantus. 



90 (49) Hind coxae free. . . 91 



91 (no) Claws large; the three basal ventral segments of the abdomen 



fused together, obhterating the sutures. 



Family Parnidae . . 92 



92 (93) Ventral abdominal segments (see Fig. 1392) seven in number. 



Psephenus. 



93 (92) Ventral abdominal segments five in number 94 



94 (103) Fore coxae transversely elongated. . ... .... 95 



95 (102) Sternum of the prothorax prolonged forward beneath the head in a 



flat rounded lobe; head retracted within the front of the 

 prothorax; the two basal segments of the antennae dis- 

 tinctly enlarged. . . . 96 



96 (97) Body rounded in outUne. ... Lutrochus. 



97 (96) Body oblong, elongate. . .... 98 



98 (99) Antennae approximate, the terminal segments pectinate. Pelonomus. 



99 (98). Antennae distant at base 100 



100 (loi) Antennae slender. . . Throscinus. 



loi (100) Antennae short and thick, the second joint triangularly dilated, the 

 close-set distal segments lamellate Dryops. 



102 (95) Sternum of prothorax not greatly produced forward; head free; 



anteimae long and serrate (California) Lara. 



103 (94) Fore coxae rounded. ... 104 



104 (109) Sternum of prothorax produced forward in a flat rounded lobe; 



head retracted within the front of the prothorax. . . 105 



105 (108) Antennae eleven-jointed 106 



106 (107) Fore tibiae pubescent internally Elmis. 



107 (106) Fore tibiae bare internally Stenelmis. 



108 (105) Antennae six-jointed Macronychus. 



109 (104) Sternum of prothorax not produced forward; head free. 



Ancyronyx. 

 no (91) Claws of moderate size; basal ventral segments free. 



Family Dascyllidae. 



