On Aquatic Carnivorous Coleoptera or Dytiscide. 909 
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 jomts 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, (Homeolytrus 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 imner 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 cither 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. They 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 coxe are extremely large, 
and approach very near to the middle coxe ; 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 
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