On Aquatic Carnivorous Coleoptera or Dtjtiscidie. 



605 



917. Rhantus marginatus, Steinh. Atti. Soc. Ital. Sci. Nat. XII, p. 250.— Ovalis, 

 colore variabilis, testaceus, pectore plus ininusve nigricante, abdoinine, vertice, 

 prothorace anterius et posterius elytrisque nigris, his margine externo lineisque 

 ssepius obsoletis testaceis, punctis seriatis j^arum conspicuis, ad apiceiu truncatis ; 

 prosteruo in medio valde compresso-elevato, anterius rotundato declivo. Long. 9, 

 lat. 4 m.m. 



The male has the basal joints of the front tarsi incrassate and compressed, and 

 furnished beneath with long hairs, and with minute palettes ; their claws are 

 short, and the front one is slender and nearly straight, while the hinder, though 

 about of similar length is thicker, and is strongly curved and armed with a large 

 tooth beneath near the base ; the middle tarsi have the basal joints incrassate and 

 greatly comjiressed and clothed beneath in a similar manner to the front feet ; 

 the apical ventral segment is almost without sinuation at the sides, and is longitu- 

 dinally strigose. The female seems to be considerably larger and broader than the 

 male, and has the hind coxte more yellow, and the elytra very finely and obsoletely 

 punctulate, while in the male they are nearly smooth. 



South America, (Monte Video). 868. 



II. 11. — Group COLYMBETINI. 



The semi-membranous side piece of the first segment of the hind body (interposed 

 between the stigma and the edge of the ventral segment) marked by transverse 

 rugce or furrows ; setigerous punctures of hind femur, wlien present, forming an 

 irregular patch at the extremity, widely separated from the posterior margin. 



The four genera may be thus distinguished : — 



Ventral side pieces broad (as tliis character cannot be seen without raising tlie ] 



wing-cases, it may be supplemented by these, viz. : — upper suiface con- ! 



spicuously reticulate ; wings of luetasternum rather shorty ; two species ,' 



peculiar to North America. J 



Side-piece of fourth ven- 

 tral segment narrow ; 

 upper surface not reti- 

 culate, but wing-cases 

 either nearly smooth, 

 transversely aciculate, 

 or quasi-imbrieate (sub- 

 tuberculate). 



Metastemal groove broad and definite ; thorax 

 usually margined at sides : size not more than 

 15 m.m. long ; last joint of hind tarsus sub- 

 equal to the preceding joint ; many sjjecies with 

 great distribution. 



Metasternal groove very narrow and indistinct ; 1 

 eighteen species, chiefly boreal. j 



Metasternal groove moderately distinct ; size ~| 

 large (20 m.m. long) ; swunming legs elongate, | 

 the last joint of their tai'si longer than the \ 

 preceding one. Two 3Iediterranean and At- | 

 lantic island species. J 



SCUTOPTEEUS, 

 ( Vide p. 606.) 



RHANTUS, 

 ( Vide p. 607.) 



COLYMBETES, 

 (Vide IX 624.) 



MELADE.VA, 

 (Videi>. 631.) 



