868 On Aquatic Carnivorous Coleoptera or Dytiscide. 
(in the length between the hind margin of the posterior cox, and the second 
ventral segment), so that their terminal portion is very narrow, although in some 
species 1t remains quite distinguishable till as far, at least, as the commencement 
of the last ventral segment, (Dytiscus depressus, No. 472, &c.); while in others it 
cannot be distinguished after the second ventral segment is passed (D. griseo-striatus, 
No. 493, &c.); the difference between these two forms is however not great, and 
intermediates occur. There is no ligula on the inner face of the elytra near the 
apex ; the genicular area of the epipleura is generally indistinct, and is never 
limited externally by a raised line ; the swimming legs are but slender, their femora 
are never incrassate, and their postero-external angle is never acute. The hind 
cox are not large, their front border having but little extension in the anterior 
direction, and the culmen of its arch is rather rounded and broad, thus contrasting 
with Coelambus: the hind articular cavities are contiguous or nearly so. 
The species are found in the European and Mediterranean regions, but one or two 
exist in North India and Abyssinia and Arabia, and one or two others in the 
northern portion of the New World. 
I. 41.—Genus HYDROPORUS. (Vide p. 435.) 
Group 1. 
The first group of species included in Hydroporus consists of certain insects dis- 
playing a slight peculiarity in the structure of the apices of the coxal processes ; 
the middle portion of the apex being slightly prolonged, so that the articular 
cavities are distinctly separated by this middle portion; which is closely adpressed 
to the plane of the ventral segments. Six species are included in the group; they 
all have the third joint of the front tarsi strongly bilobed, and the general 
characteristics are much those of the following group ; the punctuation of the under 
surface is sometimes but little developed, and never very coarse, the wing-cases are 
variegate in colour, and the undersurface is never black, though sometimes greatly 
infuscate. The species are all rare, and'have therefore been butimperfectly examined ; 
one of them, (Hydroporus vittatipennis, No. 500), is perhaps not very naturally 
associated with the other species, for the lateral margin of its prothorax is extremely 
slender, even in front, while in all the other species it is more or less flattened and 
thickened in front. 
Group 2. 
In this group the front tarsi have the third joint more deeply bilobed than is the 
case in the following group ; (in some species however the distinction is but slight 
in this respect, see Hydroporus dimidiatus, No. 517) ; the thoracic side margin is 
usually thickened and flattened in front, but to this also there are exceptions (see 
H. mellitus, No. 502, and H. hybridus, No. 519). The wing-cases are variegate but. 
occasionally only obscurely so; the undersurface is red or yellow ; the metasternum. 
