On Aquatic Carnivorous Coleoptera or Dytiscide. 951 
(Dytiscus dorsalis, Celina, Sternopriscus, Hydroporus oblitus, e.g.), or may be very 
small, as in Dytiscus ovatus, (Hyphydrus, No. 347), and numerous Hydropori. The 
reduction of the pieces of the prosternum seems to attain its maximum in Hyphydrus, 
where the anterior and inter-coxal bands form a mere slender frame for the setting 
of the coxee, which are longer than is usual in the other Hydroporides. The pro- 
sternal process is not only variable in size but is extremely so in form ; its junction 
with the inter-coxal band is frequently very conspicuous and it may project at the 
point of junction asa kind of angular or tubercle-like prominence ; this is especially 
the case when the prosternal process is thick and powerful as in Pachydrus, where 
the prominence alluded to is clearly seen to be the result of a thickening and growth 
of the prosternal process, while no corresponding development of the inter-coxal 
band has taken place ; hence at the poimt between the front coxe where occurs the 
junction of the thickened prosternal process with the feeble inter-coxal band, there 
is an angular prominence. It is worthy of remark that the New World seems to 
have been more favourable to the production of this thickening of the prosternal 
process than the Old World has been; not only does it reach its maximum in the 
New World Pachydrus, but it will be found to occur with great frequency in North 
American Hydropori, and indeed in certain cases (e.g., Nos. 492 and 493, Deronectes 
prosternalis, and Dytiscus griseostriatus) is almost the only character by which a 
New World species is distinguished from an Old World one. 
In short though the members of the Hydroporides agree among themselves in 
showing a feebleness of the anterior parts of the prosternum, yet that feebleness is 
accompanied by great variety of structure, and I do not see the least reason for 
considering it as evidence of genetic connection among the insects possessing it. 
Its functional value will probably be expressed in terms defining the nature of the 
movements of the head and front legs performed by the insects. 
The front and middle tarsi of the Hydroporides show great variety, although 
agreeing in the generalized and guarded definition given above. Moreover they 
show much difference in the sexes of the same species, and this renders their use 
as a means of definition somewhat more difficult. Thus though the three basal 
joints agree in being more or less flat, and in presenting beneath a sole clothed 
with pubescence, yet as will be seen by examining the sexes of Dytiscus ovatus 
(No. 347), this structure is much more conspicuous in the males than in the 
females, for in the former the three joints are comparatively broad and flat, and 
present a well marked sole beneath, while in the female they are compressed in such 
a way as to be extremely convex transversely on their upper face and to present 
but a small surface or sole beneath. It is the rule throughout the aggregate that 
the males have the front and iniddle tarsi broader, and so displaying the character- 
istic structure more markedly than the females. Although the male tarsi may be said 
to have the basal joints dilated, they never have any tendency to assume the beautiful 
patellate form occurring in the corresponding sex of the Hydaticides, Dytiscini and 
