852 On Aquatic Carnivorous Coleoptera or Dytiscidce. 



These small insects arc readily distinguished from Pachyhydrus, by the minute 

 prosternal process, by the different shape of the metasternum, (its middle part being 

 more parallel sided owing to the greater development of the hind coxse), and by the 

 less widely separated hind coxal cavities. The species are either unicolorous, or are 

 yellow, with a few thick black marks on the elytra (H. minimus, Wehncke, Hydro- 

 porus dispersus, and H. latissimus, Crotch) ; it is probable however that the latter 

 group will prove to be a distinct genus, somewhat approximating to the Old World 

 Hyphydrus, to which the species are similar not only by their colour, but also by 

 the structure of their front and middle tarsi. External distinctions between the 

 sexes appear to be extremely slight; but I find that in one or two species the prosternal 

 process is of two forms, and this is probably a sexual character. One of the features 

 ot the Dytiscidae, viz., the tendency to reduction in size of the pieces of the presternum 

 is in Desmopachria carried to the most extreme point it has reached in the family. 



The genus is peculiar to the New "World, but is probably represented by numerous 

 species there. 



I. 22.— Genus BIDESSUS. {Vide p. 344.) 



This is a large aggregate comprising fully eighty species. They are insects of 

 small or minute size, the largest attaining only about 3 m.m. of length, they are 

 oblong, or oblong-oval, in form, (a few species however are of broad, short, form, 

 convex beneath) ; with a sort of plica or longitudinal fold or impression on the basal 

 portion ot the thorax, a little nearer to the outside than to the middle, this fold 

 very often is continued on the basal portion of the elytra. Head not, or only 

 indistinctly margined. Presternum small, front coxse very small, prosternal process, 

 moderately long, rather narrow^, and acuminate, its length quite as great as the inter- 

 coxal portion of the jirosternum. Middle coxse nearly contiguous ; when the pro- 

 thorax is taken away, it is seen that the apex of the intercoxal portion of the meta- 

 sternum does not reach to, or connect with, the middle furcate portion of the 

 mesosternum, but a space intervenes between them, and in this space the middle 

 coxae are seen to be absolutely contiguous. Metasternum elongate in middle, 

 excessively short at sides ; hind coxas very large ; posterior cavities not contiguous, 

 but not widely separated, the coxal processes being rather narrow, these are adpressed 

 to the level of the ventral segments, the articular cavities quite unprotected, the 

 coxal lines nearly straight and parallel, shghtly convergent towards the apex; first 

 ventral segment soldered to the hind coxae : hind legs slendei", their tibiae with a slender 

 basal portion, but from the middle to the apex gradually and distinctly thickened : 

 front and middle tarsi 4-jointed, the third joint scarcely bilobed, the fourth joint 

 exserted, quite as long as the third : no tongue on the inner face of elytra near the 

 apex. 



The genus as here limited seems to extend its variations of structure in the 

 direction of several other aggregates from which however it is really distant. The 



