852 On Aquatic Carnivorous Coleoptera or Dytiscide. 
These small insects are 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 coxee), 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 bea 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 
of the Dytiscide, viz., the tendency to reduction in size of the pieces of the prosternum 
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 of 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. Prosternum small, front coxee very small, prosternal process, 
moderately long, rather narrow, and acuminate, its length quite as great as the inter- 
coxal portion of the prosternum. Middle coxe 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 
cox are seen to be absolutely contiguous. Metasternum elongate in middle, 
excessively short at sides; hind cox 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, slightly convergent towards the apex; first 
ventral segment soldered to the hind coxe: hind legs slender, their tibize 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 
