942 FRESH-WATER BIOLOGY 



81 (80, 82, 83) Surface of the elytra transversely aciculate. . . Colymbetes. 



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



Copelatus. 

 8s (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 Rhantus. 



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 outKne 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. 



101 (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; 



antennae 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 Sienelmis. 



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. 



