192 On Aquatic Carnivorous Coleoptera or Dytiscide. 
always convex insects, the convexity is chiefly exhibited by the upper surface, the: 
lower surface being unusually flat; this peculiarity is however reproduced in the: 
Colymbetides by many species of the genus Tlybius. It will thus be seen that as 
regards form the Noterides are more peculiar and isolated than any of the other 
Dytiscidee they being peculiar in their outlines both in the perpendicular and 
horizontal directions. Although peculiarities of form are usually to a large extent 
constant in allied species and genera and are even found to be characteristic in 
some of the still larger aggregates, yet exceptions are of frequent occurrence ; thus 
although a certain peculiarity of form is very characteristic of the genus Tlybius,. 
yet there are certain species of Hybius in which this peculiarity is absent, and. 
which would therefore at first sight be considered to belong to the genus Agabus. 
In other cases variation in facies goes to a still greater extent; thus the species of 
Lacconectus resemble Laccophili in size, form, and colour (although they have no 
approximation to that genus in any natural classification) and would scarcely be 
considered to belong to the Colymbetides by any one who examined them for the 
first time ; so too in many species of the genus Copelatus we find in their appearance 
but little of the Colymbetides, they more resemble the Hydroporides ; while certain 
Noterides although they have no affinity with the Hydroporides have been 
actually up to the present time always classified with them, and indeed placed in 
the genus Hydroporus. 
In Cotovr the Dytiscidee show much less variety than do the beetles dwelling on 
land ; brilliancy of colour is indeed quite absent. The usual colour is black, or yellow, 
or a combination of the two, or else a colour intermediate as it were between the 
two ; faint greenish, olivaceous, and brassy tints appear in the Colymbetides, Dytis- 
cini and Cybistrini; the gayest and most variegate colouration is found in the tribe 
Hydaticides, where a mixture of black and yellow, in conjunction with a very shining” 
polished surface gives rise to a very agreeable appearance : the colouration of these: 
Hydaticides seems however to be in many cases so very inconstant that it can 
scarcely be of much service in defining the species, or in assisting in their recognition. 
In the Colymbetini, especially in the genus Rhantus, the elytra are frequently of a 
yellow colour, which is however greatly obscured by a vast number of minute black 
specks or dots, these become in some species so dense and numerous that the wing- 
cases appear nearly or quite black, though frequently their speckled nature may be 
detected on a closer examination, especially when the lateral portions are looked at, 
the specks being there always less dense than near the suture: Several species of 
Hydaticus have a similar system of colouration and indeed so much resemble these 
Colymbetini that the species of the two widely separated genera are still much con- 
founded together, even by entomologists of good repute. In the genus Hyphydrus 
and in many Hydroporini (¢.g., Coelambus end Necterosoma) the prevalent colour is 
yellow or yellowish with black marks on the upper surface forming a very irregular 
pattern ; the black marks being usually more or less elongate longitudinal lines, which 
