HYDROPHILIDAE. 43 



Middle and posterior legs short, broad, very much com- 

 pressed ; tibiae without spurs; tarsi 5-jointed ; first joint of 

 middle feet large, triangular; second and third very short; 

 fourth large, triangular; fifth triangular, with two approxi- 

 mate claws. Of the posterior feet of Diheutes the first joint 

 is very large; the others are small, and diminish gradually 

 in size, the last with two very small claws. In Gyrinus the 

 posterior and middle tarsi are nearly alike. 



This family is one of the best defined and most distinct of any 

 in the whole order of Coleoptera, and contains a moderate number 

 of species, of an oval form, somewhat attenuated at either end, 

 usually of a very brilliant bluish-black color above, with the punc- 

 tures reflecting a golden tint. 



Their habits are aquatic, but remarkably different from those of 

 the Dytiscidse; they are usually seen in large numbers on the sur- 

 face of the water, circling about in labyrinthine curves, and diving 

 but rarely, and only to escape from an immediate danger; wheu 

 caught, many exhale a milky fluid, having an odor of apples. 



The elytra are in two of our genera striate, with rows of punc- 

 tures; in Gyretes they are without stria?, smooth and shining on 

 the disk, finely punctured and pubescent on the sides. The species 

 of Dineutus and Gyrinus frequently resemble each other very 

 closely, and ours have not yet been investigated with success. Of 

 Gyretes one species is found in the Colorado Ptiver of California, 

 and another in Illinois. 



Our three genera are thus separated : — 



Last ventral segment of abdomen depressed, rounded at tip ; 



Scutellum distinct. Gyrinus. 



Scutellum wanting (labrum transverse). Dineutfs. 



Last ventral segment of abdomen elongated, conical (labrum prominent, 



scutellum wanting). Gyretes. 



Fam. VI.— HYDROPHILIDAE. 



Mentum large, quadrate; gular suture distinct. 



Ligula broad, very short, usually concealed, with labial 

 palpi very distant at base. 



M axil he with two lobes ciliated at the extremity. 



Eyes round in all of our genera (emarginate or even divided 

 by the side of the head in some foreign genera). 



