44 COLEOPTERA OF NORTH AMERICA. 



Antennae inserted under the sides of the front, behind the 

 base of the mandibles, moderately short, having from six to 

 nine joints, the outer joints forming a sudden club, of which 

 all the joints except the first one are pubescent. 



Prothorax with the episterna and epimera not distinct; pro- 

 sternum very short; anterior coxae globose, conical, exserted. 



Mesosternum moderate, frequently longitudinally elevated; 

 side pieces not divided, extending to the coxae, which are 

 large, oblique, and flat, prominent only inside of the inser- 

 tion 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 five ventral segments, in the 

 foreign genus Limnebius with seven, and in Cyllidium with 

 but apparently four; segments not connate. 



Legs moderate ; tibia? terminated by two large spurs ; tarsi 

 five-jointed, the middle and posterior ones sometimes com- 

 pressed and fimbriate, for swimming. Trochanters not pro- 

 minent on the inner part of the thigh. 



This group contains insects which live on decomposing vegetable 

 matter, though the larva? are carnivorous and quite voracious ; the 

 majority of them are aquatic. Except those of the tribe Helopho- 

 ridae, they are of an oval, convex form, sometimes hemispherical ; 

 the elytra are sometimes striate, sometimes without dorsal stria?, 

 but with a distinct sutural stria; sometimes the latter is also effaced. 

 In the species with smooth elytra three faint series of punctures 

 may be seen on each elytron, as in Dytiscida?. The scutellum is 

 never wanting. The palpi in most of the genera are very long, 

 but always slender, whence the name Palpicomes, 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 : — 



Middle and hind tarsi with the first joint short; 



Prothorax narrowed behind, narrower than the elytra. Helophorini. 

 Prothorax at base as wide as the elytra ; 



Tarsi compressed ; metasternum prolonged into a spine. 



Hydrophilwi. 



Tarsi not compressed ; metasternum not prolonged. Hydrobiini. 



Middle and hind tarsi with the first joint elongated. Sph^ridiini. 



