70 CULEOPTERA OF NORTH AMERICA. 



Mesosternutn moderate, frequently longitudinally ele- 

 vated ; side pieces not divided, extending to the^ coxae, 

 which are large, oblique, and flat, prominent only inside of 

 the insertion of the thigh. 



Metasternum large, frequently carinate, and produced into 

 a long spine behind; side pieces large, epimera not visible. 



Posterior coxae oblique, flat, extending to the sides of the 

 abdomen. 



Abdomen usually with tive ventral segments, in Limne- 

 bius -vVith seven, and in Cyllidium with but apparently four ; 

 segments not connate. 



Legs moderate; tibiee terminated by two large spurs; 

 tarsi live-jointed, the middle and posterior ones sometimes 

 compressed and fimbriate, for swimming. Trochanters not 

 prominent on the inner part of the thigh. 



This family contains insects wliicli live on decomposiiit!,- veg- 

 etable matter, thoug-h the larviB are carnivorous and quite vora- 

 cious; the majority of them are aquatic. Except those of the tribe 

 Helophorini, they are of an oval, convex form, sometimes lienii- 

 spherical; the elytra are sometimes striate, sometimes without 

 dorsal striae, but with a distinct sutural stria ; sometimes the 

 latter is also eifaced. In the species with smooth elytra three 

 irregular series of punctures may be seen on each elytron, as in 

 Dytiscidte. The scutellum is never wanting. The palpi in most 

 of the genera are very long, but always slender, whence the name 

 Palpicornes, given by Latreille to these insects. 



According to the proportions of the joints of the tarsi, four 

 tribes are apparent, which may be separated as follows: — 



Middl(i and hind tarsi witli the first joint short ; 



I'rotliorax narrowed behind, narrower than the elytra. Helophorini. 

 Prothorax at base as wide as the elytra ; 



Tarsi compressed ; metasternum jirolonged into a spine. 



Hydropuilini. 



Tarsi not compressed ; metasternnm not prolonged. Hydrobiini. 



Middle and hind tarsi with the first joint elongated. SpHiERiniiNi. 



Tribe I.— HELOPHORir¥I. 



In this tribe are small aquatic species, of an oblong or elongate 

 form, usually of a pale gray color, more or less tinged with bronze 

 or silver. They are found in small pools, and rise to the surface 

 when the water is made turbid. 



