Horn.] Ho [Feb. 21, 



REVISION OF THE GENERA AND SPECIES OF THE TRIBE 

 HYDROBIINI. 



By George H. Horn, M. D. 



{Read before the American Philosophical Society, February 21, 1873.) 



The tribe Hydrobiini comprises all those genera of Hydrophilidae in 

 which the middle and hind tarsi have the first joint very short, and all 

 the joints are of cylindrical form, or at least not compressed and oar-like. 

 The thorax at base is always as wide as the base of the elytra, and the 

 metasternum never prolonged into a spine behind the coxae. The genera 

 may thus be tabulated : 



Head in front of eyes narrow ; labrum visible HYDROBIINI. 



Fifth abdominal segment emarginate ; antenna} 7- 



jointed Berosus. 



Fifth segment entire. 



Fii-st two segments concealed by plates Ch^etarthria. 



Abdominal segments not concealed. 



Abdomens with more than six segments Limnochauis. 



Abdomen with five segments, tip of sixth fre- 

 quently visible. 



Antennae 8-jointed Laccorius. 



Antennae 9-jointed. 

 Maxillary palpi with fourth longer than third 



joint Hydrobius. 



Maxillary palpi with fourth joint shorter. . . . Philhydkus. 

 Clypeus broadly dilated at sides ; labrum concealed. . . .HELOPELT1NI 

 Margins of thorax and elytra broadly expanded ; 



maxillary palpi very long Helopeltis. 



In Berosus the sixth abdominal segment is always visible in the emar- 

 gination of the fifth, it has also seven-jointed antennae, an elongate 

 scutellum and ciliate tibia} and tarsi. Limnocharis and Laccobius have 

 eight-jointed antenna}, the latter has large trochanters to the hind femur 

 while the former has eight very distinct segments to the abdomen. All 

 the other genera have nine-jointed antenna}. 



Berosus. Leach. 

 From all the genera of the group Hydrobiini, Berosus may be known 

 by the seven-jointed antenna}, the elongate scutellum and the hind pair of 

 tibia} and tarsi pilose. Certain characters common to all the species 

 appear to have been entirely overlooked by previous authors, or at least 

 no mention made of them. The under side of the head behind the men- 

 turn is provided with a longitudinal carina, more or less evident accord- 

 ing to the species, and as this ridge is more or less developed, so also is 

 the middle of the anterior edge of the presternum notched for its recep- 

 tion when the head is in repose. The mesosternum is always more ox- 

 less cristate and the free angle of the crest with an acute prolongation 



