180 On Aquatic Carnivorous Coleoptera or Dytiscide. 
I have not endeavoured to explore the internal anatomy of the multitude of 
species, for to do this in such a way as to be trustworthy for taxonomical purposes 
would be more than the work of a lifetime. 
Thus the memoir I offer to the Society is a mere imperfect sketch of the 
characters of the species of aquatic carnivorous beetles, and a contribution to a 
synthetical or natural classification thereof based on the external structures of the 
perfect sects. 
I have commenced with a comparative sketch of the variations of structure 
exhibited in the family, and at the same time have touched slightly on its 
similarities to, and differences from the terrestrial carnivorous Coleoptera. 
The next part is intended to characterize all the species I have examined, and 
to enable the name, and as far as possible the special characters of any species to 
be recognized with facility. For those using the work for these purposes it is an 
advantage to be as brief as possible, and I have consequently avoided lengthened 
descriptions of points common to a considerable number of species, and have 
endeavoured to limit the diagnoses and remarks to a combination of characters 
sufficient to distinguish a species from the allied ones known to me, ‘The 
Dytiscidee however offer in this respect a serious difficulty : owing possibly to the 
comparative want of variety in the external conditions in which they exist, the 
species resemble one another much more closely than is the case in other families 
of beetles; there is in fact less variety in such details of sculpture and colour as 
are found in other families to afford important assistance in the recognition of 
species; there is too a great monotony of shape or form; indeed we may say that 
just as the various ships and boats devised by man have a greater resemblance to 
one another than have the various carriages and machines for travelling on land, 
so do the species of water beetles show a less variety in these respects than do the 
dwellers on land. The discrimination of the species of Dytiscidee is therefore not 
an easy task, and recourse must be had for the purpose to differences in the minor 
structural peculiarities. I have found that as regards the structure of the feet 
and legs the distinctions between the sexes vary much from species to species, and 
I have accordingly given particular attention to these and other secondary sexual 
characters ; 1t has been the more advisable to do so because this has hitherto been 
much neglected, and because the sexes frequently differ so much in their characters 
that they cannot be recognized as one and the same species unless these 
distinctions are specially noted. 
The analytical tables are merely intended to facilitate the determination of the 
species by curtailing the number of descriptions to be perused, and with this 
object the characters used have been arbitrarily selected, and such as are easily 
observed have been preferred. I have not tabulated the species of each genus, 
because the attempts I made to draw up such tables satisfied me that in a great 
number of cases I could not do this in a manner likely to be practically useful. 
