On Aquatic Carnivoi ous Coleoptera or Dytiscidce. 915 



Some of the structural characters have undoubtedly a direct relation to pecu- 

 liarities of respiration, the structure of a number of dift'erent pieces adjacent to, or 

 surrounding the basal abdominal stigma being peculiar ; thvis 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 coxse, and the cilice 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 

 excfept for the cilise 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 squamas 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 Dytiscidse is to produce smooth polished surfaces which 

 shall move through the water with greatest facility ; but at the hind margins of 

 the joints the cilias 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. 



Tire 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 woiid, 

 including even the Galapagos 



TBANS. HOT. DUB. SOC, :( S , VOL. IT. 6 B 



