On Aquatic Carnivorous Coleoptera or Dytiscide. 215 
As regards the slenderness of the antennz in the Dytiscidze it may be remarked 
that the degree of tenuity may probably prove to be in direct ratio with the activity 
of the species: very slender antennze are found only in the best swimmers, and we 
-can readily understand that it is favourable to active locomotion that the antennze 
should be very slender and flexible, so as to stream back with facility along the 
under surface of the body during rapid motion, and thus offer no obstacle to 
progression. 
The Prornorax is at its base very intimately applied to the elytra and mesoster- 
num, and so accurately fitted therewith as to shut out the entry of water, although 
this great articulation still permits considerable movement of extension : in front it 
very closely and accurately clasps the head. It is always strongly transverse, its 
greatest length being, in consequence of the prolongation of the anterior angles, 
at the outside, but the breadth is usually twice, or more, its greatest length. The 
sides are generally gently curved, and the breadth increases from the front angles 
to the base or very near it, the sides therefore diverging from the front to the base ; 
rarely this divergence is absent, as in Vatellini, less rarely the greatest width is 
across the middle, the sides being slightly contracted from thence towards the base 
(Amphizoa, Tyndallhydrus, Andex, many Deronectes, and a few Agabi). When the 
prothorax is broad at the base, the hind angles are well marked, frequently acute, 
and in Neptosternus they are produced so far backwards as to be spinose ; in other 
ceases the hind angles are nearly rectangular ; in Deronectes frequently obtuse and 
rounded, and obtuse in a few Agabi: it is worthy of remark that the continuity of 
the outline of the thorax and elytra is very complete in the higher Dytiscide, and 
in the exceptions where it is very incomplete it is apparently always or nearly 
always an accompaniment of an imperfect articulation of the prosternal process 
with the mesosternum and metasternum. Thus in Andex, Tyndallhydrus, and the 
Vatellini, where the pronotum is narrow, and not at all continuous in outline with 
the wing cases, the prosternal process fails to articulate with the metasternum at 
all, and is abbreviate in front of the middle coxz instead of being prolonged 
between them ; while in the genera of Hydroporini in which the outline of thorax 
and elytra are very discontinuous, the mesosternal fork is disconnected with the 
metasternum, and in those Agabini where the hind angles of the thorax are obtuse 
or rectangular, the articulation between the prosternal process and the metasternum 
is very imperfect (vde Agabus wasastjernee, A. cephalotes, &c.), only when the 
prosternal process is very perfectly held or fixed do we find the base of the thorax 
become very broad so as to completely continue the outline of the afterpart of the 
body, and render perfect the form for motion through the water:it is in the 
Hydaticides and Cybistrini that we meet with the most perfect outlines, and these 
are the groups where the prosternal process is most perfectly articulated with the 
metasternum. On the other hand in these most perfected forms the capability of 
extension or mobility of the prothorax from the afterbody is nearly completely lost. 
