956 On Aquatic Carnivorous Coleoptera or Dytiscide. 
III. 3.—Tribe COLYMBETIDES. (Vide 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 itis, next to the Hydroporides, the most extensive of the four 
tribes of the family. 
The tarsi are all invariably quite clearly five-jointed, and the scutellum is always 
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 coxe, and rests on the apex of the inter-coxal 
process of the metasternum which is more or less grooved or impressed 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 rarely, 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 coxee 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 ciliz, 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 are never incrassate or bifid at their apex, and the hind tibize themselves never 
show even the rudiments of the development of a patch of spur-like setze 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 structure, and in a rather more subordinate degree by the con- 
spicuously five-jointed front and middle tarsi and the visible scutellum. 
In speaking of the Hydroporides 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 coxee are as it were embedded in it, and there is but little change 
in its direction or plane from the point of the prosternal process 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. 
