HYDROrillLIDES. 127 



JR. Thorax narrower at l)ase tlian at apex. 



Labruni not hidden ; femora almost cylindrical ; antennae 

 witli nine (rarely seven) joints. Helophorides. 



HYnnOPHILIDES, 



A. Abdomen with five visil)le ventral segments. 

 (I. Antennae nine-jointed. 



I. Posterior tarsi strongly compressed, oarlike ; pro- 

 sternum and mesostermim ridged. 



1* Apex of metasternum reaching much beyond 

 posterior coxae. 



Labrum entire ; inner margin of mandil)les toothed. 

 Last joint of anterior tarsi dilated in male. 



Hydrophilus, Geoff. 



2» Apex of metasternum not reaching beyond pos- 

 terior coxae. 



Labrum emarginate ; inner margin of mandibles fringed. 

 Last joint of anterior tarsi dilated in male. 



Hydrous, I^rull. 



II. Posterior tarsi scarcely compressed, not oarlike ; 

 prosternum not ridged. 



1, Maxillary palpi long. 



A A, Tibiae strongly spined. 



Oblong oval. Maxillary palpi moderately long, their 

 third joint shorter than the second and fourth, wliich are 

 about equal in length ; labial palpi moderately long and 

 moderately stout, second joint longer than the third ; front 

 of clypeus straight or nearly so ; mesosternum ridged ; 

 head with two depressions near eyes. Hydrohius, Leach. 



J3 S, Tibiae feebly spined. 



Cl (I. Mesosternum simple. 



Oblong oval. Maxillary palpi very long, third joint not 

 so long as second, fourth not so long as third but more than 

 half as long as second; Ld)ial palpi short, third joint not 

 quite so long as second ; mentum notched in front. 



Helochares, Muls. 

 h If, Mesosternum ridged. 



