230 On Aquatic Carnivorous Coleoptera or Di/tiscidfe. 



Hydrocanthini and the Suphisini, the intercoxal process is very large, being both 

 elongate and broad, and is nearly truncate in front, in adapatation to the broad 

 truncate extremity of the prosternal process, the two edges of these pieces fitting 

 together at their line of junction in a most perfect manner : usually the intercoxal 

 process is impressed to receive the extremity of the prosternal process, and the 

 metasternal groove thus formed, coincides in shape with that of the point of the 

 prostei'nal process ; when the intercoxal process is very broad, (Hydrocanthini, 

 Suphisini, Hydrovatini, Pachydrus) then it bears no groove, for the prosternal 

 process is brought very closely into adaptation with the intercoxal process ; and 

 there is in these cases excessively little mobility of the prothorax, these being the 

 forms in which this characteristic of the Dytiscidae has become almost absolute ; 

 when the intercoxal process is very naiTow, (as in many Agabi, especially group 

 12; Eretes, &c.,) then the groove is elongate, narrow, and indefinite, consisting of 

 a mere depression along the middle of the process ; and when the prosternal process 

 is short, feeble, and acuminate, which usually occurs in such forms as have the middle 

 coxse rather approximate, and the intercoxal process more or less curved upwards 

 at its extremity (as in Hyphydrus, Coelambus, and other Hydi-oporini in which the 

 mesosternal fork is not connected with the intercoxal process) then it can scarcely 

 be said that there is any metasternal groove present ; while its maximum of per- 

 fection is attained in the Cybistrini, where it is deep, with very definite sides, and 

 perfectly adapted to the prosternal process, but yet when the point of the process is 

 slightly lifted allows some movement of extension between the prothorax and after- 

 body. 



The central piece of the metasternum, is more or less produced behind between 

 the front parts of the hind coxee, its sides usually meeting to form a more or less 

 acute angle; in Hyphydrus however the posterior portion of the metasternum does 

 not form an angle, but has a comparatively broad rounded extremity. From the 

 front of the intercoxal process to its posterior angle the metasternum is usually 

 elongate, but varies much in this respect and in Hydro vatus it is short and conse- 

 quently its posterior angle is obtuse ; the greatest reduction of the middle of the 

 metasternum occurs in the short globose Dytiscidse, and is exhibited by Suphis and 

 Colpius, where its posterior part has scarcely any backward prolongation in the 

 middle. Nearly always there is a fine longitudinal channel running along the 

 middle of the metasternum, but this is frequently very obsolete. The most peculiar 

 character exhibited by the middle of the metasternum, is that in the Noterini and 

 Hydrocanthini, it is more or less raised or prominent, so as to form in conjunction 

 with the prosternal process, and the middle portions of the hind coxae, a kind of 

 broad flat keel, increasing in width as it progresses backwards; this peculiar structure 

 reaches its greatest development in Hydrocanthus, and is without any jaarallel 

 in other beetles. The laciniee or side wings of the metasternum are as it were 

 compressed between the posterior coxse, lying behind them, and the episternum lying 



