On Aquatic Carnivorous CoJeoptera or Dytiscldie. 217 



tance, and is more or less distinctly marked off into three jiarts, 1. The portion 

 behind the coxse, this is called the prosternal process ; 2. The portion lying between 

 the coxse, which may be called the intercoxal band ; and 3. The portion in front 

 of the coxse, which may be called the anterior band. This anterior band forms on 

 either side the front boundary of the coxal cavity, and at the outer side of the coxa 

 is prolonged backwards, and articulates by aver}' distinct suture with the posterior 

 side-piece or epimeron ; external to the coxa the anterior band is continued outwards, 

 forming the side wing of the j^rosternum proper ; this side wing is always very 

 short (in the longitudinal direction). The anterior band varies somewhat in length, 

 according to whether the front coxje reach very near to the anterior edge of the 

 prosternum or not ; it is extremely short in Hyphydrus but is considerably longer 

 in most of the Noterides, and in the Colymbetides, Dytiscini, Cybistrini and Hyda- 

 ticides it is of moderate length (except in Eretes where it is exceptionally short), 

 and in these groups, which might be called the Macro-Dytiscidae, in consequence of 

 their greater size contrasted with the other groups, it is thickened along the middle, 

 though to a variable extent ; this thickening catises the under surface of the prothorax 

 to be of a keel-like form, as may be well seen in the Cybistrini where it reaches 

 its greatest degree of development ; in the Colymbetides this keel-like 

 thickening is usually much less than it is in the Cybistrini and Hydaticides, 

 and in the lower forms of the Agabini such as Agabus cephalotes (No. G63) and 

 A. cordatus it may be seen in its rudimentary condition ; Coptotomus, however, is 

 an exception in this respect to its allies. Passing now to the intercoxal portion of 

 the prosteinum, we find that in the Hydroporides this part forms a slender band 

 between the coxae, and that it attains its greatest elongation in Hyphydrus ; in 

 ■some of the Bidessini (Pachydrus), a very peculiar condition of this baud exists, in 

 front its margins are a little raised so that it is somewhat depressed or sulcata 

 along the middle, but about halfway of its length it becomes suddenly very promi- 

 nent or protuberant, and convex instead of sulcate, giving rise to the suggestion 

 that the anterior and posterior portions are distinct pieces ; traces of this division 

 of the intercoxal band into two portions exist in ntxmerous members of the Hydro- 

 porides, inasmuch as the posterior portion is less flat transversely than the anterior, 

 and that there is a more or less distinct tubercular prominence at the junction of 

 the two parts {vide Dytiscus parallelogrammus. No. 416, and numerous other 

 species of Coelambus and the allied genera). In the Macro-Dytiscidse the condition 

 of the prosternum between the coxse is very different, for the thickening or incrass- 

 ation of the middle which I have described when speaking of the anterior band, is 

 continued between the coxse, so that here the prosternum projects beyond the level 

 of the coxse, or appears as it were to be arched over them, not merely produced 

 between them, as in the Hydroporides. Behind the coxse, the prosternum becomes 

 broader, though sometimes only slightly so (Cybister, D}'tiscus, and many 

 others), and is more or less prolonged backwards to form the prosternal 



