866 On Aquatic Carnivorous Coleoptera or Dytiscidce. 



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 tliem, 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 in 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. moestus 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 coxfe a few coarse but subobsolete 

 punctui-es. 



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. 



