192 On Aquatic Carnivorous Coleoptera or DytiscidcE. 



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 Th'bius. It will thus be seen that as 

 regards foiTo the Noterides are more peculiar and isolated than any of the other 

 Dytiscidfe 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 Ilybius, 

 yet there are certain species of Ilybius 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 aflSnity with the Hydroporides have been 

 actually up to the present time always classified with them, and indeed placed in 

 the genus Hydroporus. 



In Colour the Dytiscidie 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 ColjTiibetini, especially in the genus Rhantus, the el}i;ra 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 {e.g., Coelambus and 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 



