HYDROPIIILIDAE. 69 



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

 in the \uhole 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 DytiscidEe; they are usually seen in large numbers on the 

 surface of the water, circling about in labyrinthine curves, and 

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

 when 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 stritB, 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. 



Our three genera are thus separated: — 



Last ventral segment of abdomen depressed, rounded at tip : 



Scatellum distinct. Gyrinus. 



Scntellum wanting (labnim transverse). Dineutus. 



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

 scutellum wanting). Gyretes. 



Gyrinus is widely distributed; Dinexdus is found in the Atlan- 

 tic region; Gyretes, with but one representative, in Arizona, 

 Texas, and Illinois. 



Fam. VII.— kydrophilidae. 



Mcntnm large, quadrate ; gular suture distinct. 



Ligula broad, ver}^ short, usually concealed, with labial 

 palpi very distant at base. 



Maxillae with two lobes ciliated at the extremity. 



Eves round in all of our genera (emarginate or even 

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



Antennas 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 exce])t the first one are pubescent. 



Prothorax with the episterna and epimera not distinct; 

 prosternum very short; anterior coxte globose, conical, ex- 

 serted. 



