On Aquatic Carnivorous Coleoptera or Dytiscidcp. 251 



its apex becomes even very acute (Cy bister). 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 so 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 cilipe ; quite in front 

 it usually bears a series of very short seta3 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 oft' 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 setse, 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 setse arranged in a short row near the hind 

 margin at the outer extremity, which varymuch 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 laminse are scarcely to be detected : in the higher 

 Noterides, the lower lamina is excessively developed. 



TKANS. ROY. DUB. SOC, N.S. VOL. U. 2 1" 



