866 On Aquatic Carnivorous Coleoptera or Dytiscide. 
the prothorax may be remarkably narrowed behind and so leave the shoulders of the 
elytra quite free (D. longipes, &c.), or its base may be of equal width with the base. 
of the elytra (Hydroporus platynotus, No. 459), or even slightly broader than the 
elytra (H. semirufus, No. 458); in some species there is considerable difference 
between the sexes in this respect (Hydrop. lareynei, No. 454, and H. opatrinus, 
No. 455), which fact has led to the sexes in such cases being considered as distinct 
species, even by skilled coleopterists. Where such sexual difference exists, it is 
always the case that the female has the base of the thorax narrower than the male. 
The coxal lines show very important difterences in the group; thus in Hydroporus 
latus (No. 461), they are very approximate and quite parallel till near the extremity, 
when they gradually diverge; in H. opatrinus (No, 455) they are very different 
from this, for in front they are widely separated, and then converge till the extremity 
is reached and here their terminal portion is abruptly turned outwards. In H. 
opatrinus the articular cavities of the swimming legs are distinctly separated, the 
portion intervening between the two, projecting backwards so as to be very visible, 
while in H. latus the two cavities are contiguous, except that they are separated 
by a thin lamina, which does not project beyond them, and can only be seen by 
looking along the plane of the ventral segments. Also the prosternal process shows 
a good deal of variation in the group: in H. bombycinus it is shorter than im the 
other species, and its terminal portion is considerably decurved or bent downwards; 
in H. latus also it is short, but there is no trace of decurvation of the apical portion ; 
in H. opatrinus and H. mestus it is more elongate, and its lateral compression 
reaches its maximum. 
The sculpture of the upper surface in this group is peculiar ; it consists of a fine 
dense nearly evenly distributed punctuation, and in addition to this a coarser and 
more unevenly distributed punctuation; in the species with the thorax narrowed 
behind the coarse punctuation is but little developed, but in other species such as 
H. opatrinus (No. 455), and H. bicostatus, (No. 460), it becomes so extensive as to 
overpower or mask the fine punctuation. The undersurface also is covered with a 
dense, fine sculpture, evenly distributed over the middle and hind bodies, and in 
addition there may be present on the hind coxee a few coarse but subobsolete 
punctures. 
Deronectes longipes may be considered the lowest form of the group, and H. 
latus the highest. Were it not for the existence of the last named species the 
group would be much more isolated than it actually is; for the separation of the 
hind articular cavities would then be a character constant in the whole of the 
group; this separation of the articular cavities exists however in the following 
group; and on the other hand H. latus of the present group departs from its allies 
in this respect, to resemble the species of the fourth group; so that this character 
does not justify the establishment of a distinct genus. 
