On Aquatic Carnivorous Coleoptera or Dytiscide. M33 
qd Pp y 
and hind coxa, while in Eretes where there exists considerable capacity for 
thoracic movement, and very little adaptation of the elytra to the sides of the 
body, the whole of the large epimeron is horny. 
The more important of the characteristics of the metasternum in the Dytiscide 
are, 1, the peculiarly formed side wings ; 2, the intercoxal process grooved for the 
reception of the prosternal process, and 3, the absence of a transverse suture on 
its posterior part. None of these characters however are constant enough to serve 
as marks of distinction between the Dytiscide and Carabide ; the first of them is 
excessively variable in the Dytiscide ; it is chiefly dependent on the great develop- 
ment of the hind coxze, and in Amphizoa and Pelobius, and even in Colpius and 
Suphis, the form of the metasternum is quite Carabideous. The groove of the 
intercoxal process too is far from being constantly present in the Dytiscide, it is 
absent as we have seen in all cases where the middle coxze are widely separated, 
and all that can be said is that it never occurs in the Carabidz, but is present in the 
great majority of the Dytiscide. As regards the transverse suture in the 
metasternum, it occurs in the Dytiscide only in Pelobius and Amphizoa, while on 
the other hand it becomes indistinct in some Carabide, although I have not succeeded 
in finding any in which it is effaced. 
The metasternum of the Haliplides has none of the peculiarities of the Dytiscidee ; 
the intercoxal process is not grooved ; the outline is completely Carabideous, and 
the transverse suture is present, though without dissection it is not easily 
appreciated owing to a series of very large punctures placed on it. 
The contiguity or amount of separation of the middle coxee in the carnivorous 
Coleoptera appears to me to be dependent rather on the metasternum than on the 
mesosternum. In the Carabidse these coxe are nearly always very considerably 
separated by the intercoxal process, so that the one group of that family—the 
subfamily Ozeenidee—in which the coxee are unusually approximate is, according to 
Leconte, well distinguished by this character from all the rest of the family. In 
the Dytiscide, greater variety in respect to the contiguity of the middle coxz is 
found, but still the character appears to me to be one of great importance ; indeed 
if I might exercise my imagination I would suppose that in the early history of the 
Dytiscidee the fact whether the middle coxee were well separated, or were approxi- 
mate, exercised a most important, or indeed predominant influence on the future 
mode of evolution. When they were distant this fact appears to me to have 
facilitated rapid co-adaptation of the contiguous parts for the purpose of protection 
and keeping out water from entering the body by the great fissure between 
the prothorax and the after-body by a process of mere placing together of accurately 
fitting surfaces ; and this involved complete disuse of any mobility or extension 
between these parts, and rendered sedentary habits of advantage, for in all 
movements there was the danger of these parts becoming separated, and it was 
necessary to keep the pieces of this great joint without any motion, and yet they 
