900 On Aquatic C<trnivorous Coleoptera or DytiacidcB. 



commencement in this arrangement of the species. The hind tarsi have the joints 

 lobed beneath externally but to a variable extent ; their claws are often extremely- 

 unequal in length, but this is not constantly so, and in Col. pacificus they are very 

 nearly equal ; there is no file on the second ventral segment except in Colymbetes 

 nitidus, where there is an extremely fine one ; the penultimate abdominal stigma is 

 small in comparison with what it is in the following genus; the jjrothoracic side margin 

 is usually quite distinct, but it is very fine in Ehantus plantaris (No. 923) whereas in 

 Colymbetes capensis (No. 9 5 7), it is broad, but so obsolete that it is not easily detected, 

 and in Rhantus goudoti (No. 958) and R. validus it is quite absent. The elytra are 

 quite rounded at their apex. The male tarsi are variable ; they always bear palettes 

 beneath, which may be very minute, or comparatively large, they are more or less 

 evidently laterally compressed, (very little however in E,. plantaris) and the front 

 claws are often very elongate, and frequently unequal. 



It will be seen that there is much variation in structure among these species, 

 which to a superficial glance possess a monotonous similarity ; a natural classifica- 

 tion of the genus must be deferred till all the species are known ; the characters which 

 1 anticipate will prove most useful for the purpose of such grouping, are the size of 

 tlie side-wings of the metasternum, the lobing of the hind tarsi, and the clothing 

 of the male front feet, but at piesent no useful purpose would be served by carrying 

 out such a classification. The most aberrant species are Colymbetes pacificus, (No. 

 920) on account of its nearly equal hind claws ; Rhantus plantaris (No. 923), on 

 account of its uncompressed male tarsi, Colymbetes atricolor (No. 939), on account 

 of the broader posterior portion of the epipleurse, Cohmbetes nitidus, (No. 940), 

 because of its possessing a fine ventral file ; Colymbetes caj^ensis and Rhantus 

 goudoti (Nos. 957 and 958), because they have the coxal processes less cleft asunder 

 than usual ; and these two and Rhantus validus (No. 959), because they have the 

 prothoracic side margin obsolete. Dytiscus pustulatus (No. 945) may also probably 

 prove to be a distinct genus. 



The genus as a whole is almost or quite cosmopolitan ; species being found even 

 in the Pacific islands and New Zealand : and one of the species Colymbetes 

 pulverosus (No. 924), has a remarkably wide geographical distribution in the Eastern 

 hemisphere, and islands of the world, but is wanting in the New World. It is 

 probable that ultimately the species of the genus will prove to be inhabitants of 

 temperate regions, and when found in the tropics to be confined to a considerable 

 elevation. 



I. 63.— Genus COLYMBETES. {VideTp. 624.) 



This aggregate comprises eighteen species whose individuals are of comparatively 



laro-e size ; the wing-cases are of obscure colour with the margins more dilute and 



in nearly all the species are marked with transverse scratches or gi-ooves, which in 



some species form a very conspicuous and beautiful sculpture. The side pieces of 



