On Aquatic Carnicorous Coleoptera or Dytiscidce. 957 



shows at the front edge in the middle a distinct vertical face, placed at right angles to 

 the plane of the prosteruum; a superficial approximation to the definition of the pros- 

 ternal structure of the Hydroporides is thus suggested ; but a very slight examination 

 is sufficient to show that so far from this being the case, Coptotomus is of all the 

 Colymbetides the farthest removed by its prosternal structure from the Hydro- 

 porides. In Hydroporides it is the inter-coxal band of the prosternum that is placed 

 on a different i^lane to the prosternal process, the very small transverse band forming 

 the front of the prosternum being again nearly horizontal in plane like the prosternal 

 process ; in Coptotomus the horizontal plane is continuous from the apex of the 

 prosternal process to the front margin of the prosternum, and then there occurs the 

 vertical free face of the incrassate portion. Dytiscus fuscipennis (Agabus, group 

 21) makes however a genuine approach to Hydroporides in the j)ro thoracic structure, 

 the band between the front coxte being placed, in its relation to the prosternal process, 

 somewhat as in that group : this interesting similarity is not however accompanied 

 by any other approach in D. fuscipennis to the Hydroporides, so that we may say 

 that those influences that have caused this insect to approach Hydroporides have 

 been confined in their operation to its prosternal structure. 



The front and middle tarsi are always conspicuously five-jointed, the fourth joint 

 being as far as its length goes, as much developed as the basal ones, and thus a 

 conspicuous contrast with what is usual (but not absolute) in Hydroporides is 

 exhibited. In the male sex of Colymbetides there is always a greater or less 

 incrassation of the three basal joints of the tarsi, and we find in certain members 

 of the aggregate that this incrassation is displayed to a less extent by the fourth 

 joint (vide Colymbetides), a most unusual character in the Dytiscidee. Still how- 

 ever it is interesting to remark that in both Colymbetides and Hydrojjorides in 

 certain exceptional cases, the sexual development exhibited by the three basal 

 joints of the tarsi is accompanied by an unusual development of the fourth joint. 



Except in the case of sexual clothing ot the males, the front and middle 

 tarsi are bare beneath, or armed only with rigid cilise : their structure being thus 

 that characteristic of locomotion on rough surfaces, while in the Hydroporides 

 we have seen the structure of the tarsi indicates an adaptation for locomotion on 

 plants. 



The male tarsi do not show a great range of variation in the Colymbetides, 

 although very different grades of development of their structure are included in 

 the aggregate. As I have already remarked, the three basal joints of the male front 

 feet are more or less incrassate; in one or two cases (Metronectes, Agabinus), only two 

 basal joints are thus aftected, the third reinaining undeveloped. This increase is 

 in its rudimentary forms but slight, and is an incrassation rather than a dilatation, 

 the increase in the size of the sole for bearing the sexual clothing being obtained 

 by enlarging the diameter of the cylinder in both the horizontal and vertical 

 directions ; in the higher forms it becomes greater, and the incrassation is acconi- 



6 G 2 



