On Aquatic Carnivorous Coleoptera or DytiscidoB. 2.15 



As regards the slenderness of the anteniiEe in the Dytiscidse 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 antennoB are found only in the best swimmers, and we 

 can readily understand that it is favourable to active locomotion that the antennce 

 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 Pkothorax 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 clasj^s 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 bi'eadth 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 

 (Amphizo;), 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 

 cases 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 Dytiscidce, 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 coxge 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 foi'k 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 (vide Agabus wasastjernte, 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. 



