D.—ZOOLOGY 73 
regarded as the highest genus in the group, connection is complete, 
whereas in Deronectes, separated from it only on this distinction, the 
distance between the parts differs in different species and suggests that 
the genus is developing along the same lines as Hydroporus and will 
ultimately join up with it. In the genus Hygrotus, also, one species 
(decoratus) has reached the stage seen in Hydroporus although, as Sharp 
points out, ‘ the contact is of the most minute and imperfect character.’ 
In the land relations of the Dytiscids and in the majority of the members 
of this family, the anterior foot is composed of five easily recognisable 
segments. In the Hydroporine, however, only one section of about 
twenty species (Methlint) possess five normal segments, all others having 
the fourth either reduced in size or absent, and although this tendency 
to reduction has not appeared elsewhere in the family, it occurs in other 
families of the beetles. 
The Colymbetine are separated from the Dytiscine by the shape of the 
eye, a small projection of the fronto-epicranial plate having grown out 
upon one side and so indented the outline. This character is common 
to the Hydroporine and Colymbetine, but in the Dytiscine the projection 
passes outside the eye, which is smoothly rounded. We thus see that, 
although the Hydroporine and Colymbetine agree as to eye condition, the 
Dytiscine having, as it were, advanced farther, in the prosternal character 
the Colymbetine are in advance of the Hydroporine. 
An examination of the upper side of the abdomen of the Dytiscid 
shows eight pairs of abdominal spiracles, of which the first pair are 
elongated transversely. ‘The second pair are often somewhat enlarged 
but the next five pairs are more or less alike in size, the last or eighth pair 
being usually somewhat smaller, and all, except the first, are circular. 
In the Dytiscini, a small group including Dytiscus and one other genus, 
about thirty-two species in all, the last two pairs are greatly enlarged 
transversely and thus this small group differs from all other Dytiscids. 
But a careful examination of other Dytiscine shows a tendency to trans- 
verseness in the hind pairs of spiracles, suggesting that the change is 
going to spread through the whole tribe. The spiracles are, of course, 
_ the respiratory openings by which air is taken in and expelled from the 
body, so a change in relative size of spiracles probably indicates a change 
in air intake and expulsion. 
The wing-cases of the Dytiscida are so constructed as to cover the 
upper surface of the abdomen and they are perfectly fitted over the hard 
edges of the abdominal segments. Beneath them is the large air-reservoir 
which, of course, exists in the land beetles but assumes a special function 
in the sub-aquatic beetles. In the Hydroporine these wing-cases are 
usually punctate, as is the rest of the upper surface, whereas in the 
Colymbetine and the Dytiscine punctate elytra are much less common, 
ornamentation usually taking the form of reticulations or modifications of 
these. The punctures are small pits, often with a short projecting hair. 
They may be thickly or sparsely scattered over the wing-cases, and they 
are either all of one size or there may be two sizes, producing what is 
called ‘double punctuation.’ This latter type occurs in a number of 
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