DYTISCIDAE. 41 



Tribe III.— COLYMBETIXI. 



The only character by which this tribe is distinguished from 

 the next is the form of the dilated anterior tarsi of the males. 

 The dilated joints are here oblong and of equal width, covered 

 beneath with cups of equal or nearly equal size. The prosternum, 

 as in it, is narrow, compressed, and pointed behind. The form of 

 body is oval, not very convex. The species are small or moderate 

 in size. 



Our genera may be thus tabulated: — 



Scutellum distinct. • 2 



Scutellum invisible. Laccophilus. 



2 Last joint of palpi truneate or rounded. 3 

 Last joint of palpi emarginate. Coptotomcs. 



3 Prosternum not sulcate. 4 

 Presternum with a deep groove. Matus. 



4 Claws of posterior tarsi unequal, the outer one fixed. Colymbetes. 

 Claws of posterior tarsi equal, movable. 5 



5 Elytra striate. Copelatus. 

 Elytra smooth. 6 



6 Thorax narrowed at the base. Anisomera. 

 Thorax as wide at the base as at the middle, or wider. Agabus. 



I have included in Colymbetes the genus Ilybius Er., and en- 

 tirely agree with Lacordaire that the difference is too slight to 

 enable them to be retained as distinct. 



I am also inclined to believe that Copelatus and Anisomera 

 should be united with Agabus; the difference of the first is merely 

 in the striate elytra; that of the second in the thorax, being nar- 

 rowed from the middle to the base. 



Tribe IV.— DYTISCEVI. 



This tribe contains in the genera Cybister and Dytiscus all the 

 large species, and in the other genera species of moderate size. 

 As before mentioned, it only differs from the last group in having 

 the joints 1 — 4 of the anterior tarsi of the male of unequal width, 

 so as to form a round disk, furnished beneath with cups of very 

 unequal size. The scutellum is always visible. The body is oval 

 not very convex, usually broader behind the middle, rarely ellip- 

 tical ; the elytra of females of certain Dytiscus and Acilius are 

 sulcate. All the genera of the tribe are represented in our fauna, 

 and may be distinguished thus: — 



