On Aquatic Carnivorous Coleoptera or Dytiscidoe. 9 09 



much smaller,) on the basal portion, and a dense glandular pubescence on the outer 

 portion. 



The male intermediate tarsi are almost, or quite undilated. 



The aggregate, although consisting at present of only six species is hetero- 

 geneous : and the species might be arranged into two groups as follows : — 



1. (Acilius auctorum). — Coxal border not very large ; male intermediate tarsi 

 bearing beneath on their inner edge, on each of the three basal joints a 

 tuft of elongate hairs (these however in one species — A. fraternus — are but little 

 developed) ; females with grooves on the elytra, but in one species a second 

 form of the female — destitute, like the male, of grooves — occurs. 



2. (Homoeolytrus ex parte). — Coxal border smaller than in Acilius, and coxal lines 

 more indistinct ; males with no tufts of hair on the middle tarsi ; females des- 

 titute of grooves on the wing-cases. 



This grouping however would still leave each of the two groups heterogeneous ; 

 and the most satisfactory arrangement is one based on the sexual peculiarities, 

 this would bring the species into four categories : — 



1. Male intermediate tarsi with tufts of hair on the inner edge, and with a few 

 small palettes along the outer margin of the three basal joints; female wing- 

 cases always sulcate. (Dytiscus sulcatus and canaliculatus.) 



2. Male intermediate tarsi with tufts of hair on the inner edge, but without 

 palettes ; female either sulcate or smooth. (Acilius semisulcatus and fra- 

 ternus.) 



3. Male without tufts of hair on the middle tarsi, but with the basal joints 

 bearing numerous very minute palettes; female wing-cases without grooves. 

 (Acilius duvergeri.) 



4. Male without either tufts or palettes on the middle tarsi, females not sulcate. 

 (Dytiscus mediatus, Say.) 



The species of Acilius are found only in the temperate northern regions of the 

 Old and New Worlds. The}' are still insufficiently known, and it is probable that 

 other species may be discovered in Eastern Asia, in Japan, or in North America 



I. 70.— Genus THERMONECTES. {Vide p. 677.) 



The fourteen species composing this aggregate consist of individuals of moderate 

 size, polished surface, rather convex form, and more or less elegantly variegated 

 upper surface. The thorax is without lateral margin, the prosternal process is 

 broad and short, rounded or obscurely angulate at the apex. The swimming legs 

 are highly developed, being short and thick. The hind coxse are extremely large, 

 and approach very near to the middle coxse ; the wing of the metasternum is a slender 

 band dilated towards the apex and is much deflexed outside the coxa. The coxal 

 lines are quite short, but although fine are distinct for all their course, and they 



6 A 3 



