On Aquatic Carnivorous Coleoptera or Dytiscide. 251 
its apex becomes even very acute (Cybister). The femur is short, rarely so long 
as to project at the sides beyond the edge of the wing-case, but it does so in 
Vatellini, Amphizoa and those species of Agabus at the commencement of the genus, 
and even in some of the species of Deronectes: it is more or less compressed or 
flattened go that it is elongate from front to hind edge, very short in the vertical 
direction, but in the lower forms it is much nearer to cylindrical in shape ; at the 
base its hind portion is cut away to admit the trochanter, to which the femur is 
attached only by a small piece at its base ; although frequently (especially in the 
Hydroporides and Cybistrini) the trochanter is very closely applied to the excava- 
tion in the femur, yet in other cases (Dytiscus) the fitting between the two is very 
imperfect. The front of the femur forms a sharp edge in the highly evoluted forms, 
but is blunter in the lower ones, where the femur is less blade-like, (Pelobius, 
Amphizoa, and Hydrovatini, &c.); its upper face is flat, smooth, but with a few 
wrinkles, and in Pelobius with an obsolete series of distant punctures placed 
parallel with its hind margin, in Hydaticus with a closely placed series of setiger- 
ous punctures, and in Eretes with a band of densely placed fine cilize ; quite in front 
it usually bears a series of very short sete or spines, which usually become longer 
on the basal portion ; from base to apex the femur is slightly curved, so that in this 
direction the upper face is a little hollowed, which permits it to be extended in 
the forward direction along the curved surface of the breast, with which it comes 
in close contact at the point of its greatest flexion ; the lower or free surface, is 
smooth in the higher forms, more or less roughened in the lower forms ; just where 
it is rounded off in front at the knee, there are frequently some coarse punctures 
bearing short thick spines (Dytiscus) ; sometimes a series of distant punctures each 
bearing a seta is placed along the lower face of the femur parallel with its hind 
margin (Noterides) ; and the hind margin itself bears in the Noterides a regular 
group of elongate sete, placed at the knee, and which attain a very remarkable 
development in Hydrocanthus; in the Colymbetini and Dytiscini there is usually 
some punctures forming an indefinite group, at the outer extremity of the femur, 
and in Agabus there is a group of sete arranged in a short row near the hind 
margin at the outer extremity, which vary much in their development according to the 
species. The posterior face of the femur is more or less hollowed for the reception of 
the tibia when this latter is flexed ; this hollow does not extend to the base, (where 
indeed the hind face of the femur is almost an edge), but is confined to a greater or 
smaller area at the extremity ; this hollow causes the upper and lower faces of the 
femur to terminate each as an edge, or lamina, of which the lower one is larger and 
projects farther back than the upper; the angle at the knee of the upper plate is 
always rounded, that on the lower plate frequently rounded, but in the higher forms 
more or less acute : in the lower forms of the family where the swimming legs are 
of inferior structure, these lamine are scarcely to be detected: in the higher 
Noterides, the lower lamina is excessively developed. 
TRANS. ROY. DUB, SOC, N.S, VOL. 21 
