180 On Aquatic Caraivorous Coleopfera or Dytisc'ahe. 



I haw; not endcavoureu to explore the internal anatomy ut" the multitude of 

 species, for to do this in such a way as to be trustworthy for taxouomical 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 tlie external structures of the 

 perfect insects. 



I have commenced with a compai'ative sketch of the variations of structure 

 exhibited in the family, and at the same time have touched slightly on its 

 similarities to, and difierences 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 anj^ species to 

 be recognized with facilit3\ For those using the work for these purposes it is an 

 advantage to l)e as brief as possible, and I have consequently avoided lengthened 

 descriptions of points common to a considei-able number of species, and have 

 endeavoured to limit the diajT^noses and remarks to a combination of characters 

 sufficient to distinguish a species from the allied ones known to nie. The 

 DytiscidiB however offer in this respect a serious difficulty : owing possibly to the 

 comparative want of variet}^ 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 carriaijes and machines for travellino- 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 Dytiscidoe is therefore not 

 an easy task, and recourse must be had for tlie purpose to differences in the minor 

 structural peculiarities. I have found that as I'egards 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 ; it 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. 



