AQUATIC INSECTS 941 



57 (58) Hind margin of the posterior coxae grown solidly coherent with the 



first segment of the abdomen, which is considerably enlarged; 

 form round and very convex; shining; small. 



Desmopachria. 



58 (57) Hind margin of the posterior coxae overlapping but not coherent 



with the first segment of the abdomen, which is not espe- 

 cially enlarged. Form various, but never so round and 

 convex as Desmopachria, nor so small and elongate as 

 Bidessus 59 



59 (60) Hind coxal processes each divided by a deep posterior notch, the 



inner ramus appressed against the first abdominal segment. 



Hydrovatus. 



60 (59) Hind coxal process not so formed Hydroporus. 



(In the broader sense, as used here this genus includes 

 Coelambus, Deronectes.} 



61 (52) Scutellum visible ' 62 



62 (63) Clypeal suture entire Dytiscus. 



63 (62) Clypeal suture incomplete 64 



64 (65) More than 30 mm. long; inferior spur of hind tibiae much dilated, 



bifid, much broader than other spur Cybister. 



65 (64) Less than 20 mm. long; the two spurs of the hind tibiae of equal or 



nearly equal breadth 66 



66 (73) Distal margin of each segment of the hind tarsi beset with a trans- 



verse row of minute appressed bristles; anterior tarsi of 

 male with dilated segments forming a round disc. . . 67 



67 (68) Spurs of hind tibiae acute at tip; claws of hind tarsi unequal, the 



inner one sometimes obsolete Hydaticus. 



68 (67) Spurs of hind tibiae emarginate at tip; claws of hind tarsi equal or 



nearly so .". 69 



69 (70) Elytra closely punctate, usually four-sulcate; female. . . Acilius. 



70 (69) Elytra not punctate, partly aciculate in female 71 



71 (72) Middle femora beset with elongate setae; female. . Thermonectes. 



72 (71) Middle femora beset with short and stout setae. . . . Graphoderes. 



73 (66) Hind tarsi without such appressed bristles; anterior tarsi of female 



with dilated segments forming an oval disc 74 



74 (79) A linear group of minute setae present upon the postero-external angle 



of the hind femora 75 



75 (76) Claws of the hind tarsus unequal; the tarsal segments produced 



posteriorly in overlapping lobes Ilybius. 



76 (75) Claws of the hind tarsus equal and segments simple. 77 



77 (78) Wing of the metasternum very narrow and deflexed around the 



front border of the external lamina of the hind coxa. 



Ilybiosoma. 



78 (77) Wing of the metasternum wedge-shaped, less noticeably deflexed. 



Agabus. 



79 (74) No such linear group of setae on the postero-external angle of the 



hind femora 80 



So (8 1, 82, 83) Surface of the elytra reticulate Scutopterus. 



