862 On Aquatic Carnivorous Coleoptera or Dytiscide. 
(Dytiscus parallelogrammus) almost angular; in such species the metasternum is 
elongate in the middle, and although its wing is much abbreviated as its outer portion 
is reached, yet it remains comparatively broad till the culmen of the coxe is 
reached, and there is very abruptly deflexed outside of it as a very slender 
band. 
Although the species of this aggregate show much difference in various points 
of structure, yet they are constant in the possession of the elytral ligula, and the 
genus is thus absolutely distinguished from Deronectes. Although the head differs 
much in the extreme forms, yet as it is not exactly alike in any two species it does 
not justify the formation of two (or more) distinct aggregates ; in the shorter 
species the head is completely rounded in front, and bordered with a distinct raised 
margin, and the labrum is placed so much on the undersurface as to be greatly 
concealed,—the head in such species’ closely approaches that of Hyphydrus. In 
other cases however the labrum is brought forward to the front of the head, and 
quite exposed (Hydroporus enneagrammus, No. 419, Dytiscus confluens, No. 423,&c.) 
the front of the head being truncate-emarginate ; various species are more or less 
intermediate between these extremes. 
In the development of the hind coxze there exists also considerable difference 
between the extreme forms. The species of short form possess hind coxee with their 
antero-external portions less extended towards the front of the body, so that the 
shape of the culmen of the coxal arch is more rounded and less abrupt and angular 
than it is in the more elongate species. 
In other of the structural pots characteristic of the genus, numerous shades of 
variation may be detected; such is the case with the form of the tarsi, and the 
development of the genicular area of the epipleura. 
In all the species the epipleura is smail, and is much reduced in its posterior 
portion: Hydroporus enneagrammus shows us the extreme of this reduction: in 
this species the epipleura at the base is very small, and before the middle length 
of the wing-case is reached has altogether disappeared: this diminution of area is 
accompanied by some alteration in the form of the shoulder of the wing-case, and 
as a result of thisit seems at first sight as if the genicular area were altogether 
absent, nevertheless on a more careful examination not only is the area seen to be 
definitely present, but also the existence of a line marking it off externally is certified ; 
in agreement with this reduction of the epipleura, the ligula of the elytra is also 
extremely minute in this species, but yet it exhibits the form characteristic of the 
other species; in this peculiar insect there cannot be detected on the undersurface 
of the body any trace of the coarse punctuation that exists (though in certain 
species only in a comparatively slight degree) in all the other components of the - 
aggregate. 
The external sexual disparities are, as a rule, not strongly marked: scarcely to 
be detected in Dytiscus ineequalis (No. 381), they become so far as regards the legs 
