962 On Aquatic Carnivorous Coleoptera or Dytiscide. 
Prodaticus pictus, which is the most primitive of existing Hydaticini; in it the 
episternal suture makes a considerable approximation to the average Dytiscid 
type, as displayed by, say, Colymbetes pulverosus, but still it shows the peculiarity 
characteristic of the Hydaticini markedly displayed ; so if we go to the Thermo- 
nectini and take their most primitive existing form—Acilius—we find the departure 
from the average type of episternal suture perfectly marked in the opposite direc- 
tion from Hydaticini, though to a much less extent than it is in the higher 
Thermonectini—Thermonectes for example. Thus, the conformity of structure of 
the hind tarsi of the Hydaticides, though it justifies us in classifying them 
together, does not justify us in considering that they are descended from a 
common ancestor, qua episternal suture. If we now turn to the Hydaticides 
which have the episternal suture of similar structure, viz., EHretes and 
Thermonectini, we shall find that this similarity of structure does not warrant 
any belief in descent from a common ancestor, although at first such a descent is 
readily suggested, and appears to be confirmed by other peculiarities. Thus for 
instance, Thermonectes approximates Eretes, not only by the condition of the 
episternal suture, but by the diminished epipleurse, by the large eyes, by the 
exposed apex of the metathoracic epimeron, and the small terminal stigmata ; they 
thus appear to be bound together, and these points would be cited as evidence of 
descent from a common aucestor, and yet in other respects, Eretes and Thermonectes 
thus bound together must be widely separated, and placed on opposite sides of a 
hypothetical common ancestor ; for example, Eretes has a wonderfully developed 
band of pubescence on the upper face of the hind femur, and of this there is no 
trace in Thermonectes, although in the more distant Hydaticus the band is present 
in a rudimentary form ; the tibial spurs too are bifid in Thermonectes, simple in 
Eretes and Hydaticus. If we carry this analysis to other points of structure, a 
similar inconsistency is repeated, and the search for a common ancestor or the 
endeavour to construct such by imagination becomes more and more bewildering 
and unsuccessful, in proportion as the facts considered become more extensive and 
exhaustive. 
The tribe is widely distributed over the earth’s surface, but does not occur in 
New Zealand and the Pacific islands. 
