On Aquatic Carnivorous Coleoptera or Dytiscide. 915 
Some of the structural characters have undoubtedly a direct relation to pecu- 
liarities of respiration, the structure of a number of different pieces adjacent to, or 
surrounding the basal abdominal stigma being peculiar; thus the elevated line of 
the first ventral segment, the reduced epipleura adjacent to it, and the gap thus 
formed, appear to indicate that air is either admitted to, or escapes from the stigma 
at this point, and the undeveloped condition of the apical stigmata seems to confirm 
the fact that the respiratory power is here largely concentrated on the one or two 
basal stigmata: the peculiar articulation between the basal segment and epimeron 
is probably in more or less immediately direct relation with this respiration: it 
may be that in order to exclude water from the interior of the body in this changed 
condition of the proximate parts, a peculiar development of this articulation was 
necessary ; it may.to be to form a wall to prevent the escape of air after it has been 
admitted by a peculiar channel. 
The great development of the hind coxe, and the cilize of the legs, and the very 
large eyes, as well as the pallid colour, seem to point to great or constant activity 
under much exposure to light. 
The sculpture of the hind tarsi is of great interest. The ciliation of the hind 
margins of the tarsal joints found in the Hydaticides is peculiar to them, and the 
sculpture and scales we find existing in Eretes may hint to us how it has been 
developed. In all the other Hydaticides the face of the tarsus is highly polished 
except for the cilize placed at the hind margin of each joint, whereas in Eretes the 
face of the tarsus is punctured, but each puncture is filled by an adpressed squama, 
and it is such squame that project over the hind margins of the joints. I think 
then we may assume that the ancestors of the species of Hydaticides had hind 
tarsi punctured externally and bearing scales in the punctures, that these scales 
and punctures disappeared from the joints except at their hind margin, because the 
process of evolution in Dytiscidee is to produce smooth polished surfaces which 
shall move through the water with greatest facility ; but at the hind margins of 
the joints the cilize instead of disappearing become more highly developed because 
of the fact that by their perfect accuracy of adaptation they add greatly to the 
integrity of the surface of the tarsus, and to its rigidity, thus increasing its utility 
as an organ of aquatic locomotion. 
The geographical distribution of the genus is highly interesting, one species is 
found over a large portion of the warm parts of the world, except Australia, while 
the other (closely allied) species is peculiar to Australia. 
The widely distributed species is very abundant in the Indo-Malay region, and it 
has spread to a considerable number of islands in various parts of the world, 
including even the Galapagos 
TRANS. ROY. DUB. SOC., N S., VOL. II. 6B 
