956 On Aquatic Carnivorous Coleoptera or Dytiscuhe. 



III. 3.— Tribe COLYMBETIDES. (F/c/e p. 490.) 



This tertiary aggregate comprises two secondary aggregates and seven unassociated 

 genera placed between them, in all twenty-one genera, with about three hundred and 

 twenty species ; so that it is, next to the Hydroporides, the most extensive of the four 

 tri bes of the family. 



The tarsi are all invariably quite clearly five-jointed, and the scutellum is alwa\ s 

 visible at the base of the elytra ; the inter-coxal process of the metasternum always 

 connects with the mesosternal fork, and the apex of the prosternal process always 

 reaches over or between the middle coxae, and rests on the apex of the inter-coxal 

 process of the metasternum which is more or less grooved or impi-essed for its 

 reception. A line drawn along the middle of the prosternum, from front to back, 

 is nearly or quite a straight line,"" but occasionally, although raiely, the front portion 

 of the prosternum is so much thickened in this middle line, that it presents in front 

 a distinct vertical edge. The hind coxae are very rarely less than of moderate size, 

 and are sometimes very large : the side wings of the metasternum are very variable 

 in size. The posterior coxal cavitiesare always absolutely, or very nearly, conjoined, 

 and are always protected by free, projecting coxal lobes or processes. 



The swimming legs, sometimes quite slender, are in the highest species well 

 developed, but are never extremely thick ; the hind margins of the joints of their 

 tarsi are never externally closely set with adpressed cilia;, and they are 

 always terminated by two visible claws, which however are of very variable develop- 

 ment, sometimes being very small and equal, while as the other extreme the inner 

 one is very elongate and the claws thus become very unequal ; the spurs of the hind 

 tibia a)"e never incrassate or bifid at their apex, and the hind tibias themselves never 

 show even the rudiments of the development of a patch of spur-Hke setse at the 

 outer and upper angle of the extremity of their inner face. 



The Colymbetides are thus most essentially separated from the Hydroporides, by 

 the prosternal sti'ucture, and in a rather more subordinate degree by the con- 

 spicuously five-jointed front and middle tarsi and the visible scutellum. 



Tn speaking of the Hydi'oporides I pointed out that that group never exhibited 

 any tendency to thickening of the prosternum along its middle line, and that the 

 anterior portions of the prosternum were always placed on a very different plane to 

 the posterior portions. In the Colymbetides the reverse occurs. 



The prosternum is always more or less thickened along the middle, and projects, 

 so that the front coxte are as it were embedded in it, and there is but little change 

 in its direction or plane from the point of the prosternal jjrocess to the front margin. 

 The thickening is found in all degrees of development, it is very slight in the lower 

 forms of Agabus, but in the higher species is very much more developed and 

 reaches its maximum in Coptotomus : in that genus as a consequence the prosternum 

 * That is to say, is a straight line drawn on a plane. 



