246 On Aquatic Carnivorous Coleoptera or Dytiscde. 
present ina high degree this assumes the form of complex patches or patterns. The 
explanation of this condition will probably therefore prove to be a physiological 
one. 
The Front Luas.—The front legs in the Dytiscide are always short, so that when 
the insect is moving through the water, they, as well as the middle legs, are capable 
of being quite packed away into the large hollow on the undersurface of the body 
between the metasternum and prosternum, except in Amphizoa where the front 
legs are longer than in the other Dytiscide. The coxze are more or less conical, 
and are most elongate in Hyphydrus, but in the Noterides they are spherical ; 
owing to the peculiar form of the prosternum they are very much exposed, the 
cavities excavated for their accommodation on the sides of the projecting middle 
portion of the prosternum covering only the base and inside of the cone; the 
muscles pass into the coxa at the base of the cone, so that the limb is capable of 
very free rotation. The articulation with the trochanter is at the apex of the 
coxa, and takes place by means of a slender neck, permitting of a great range of 
motion ; the trochanter is nearly triangular in form, and its broad base is attached 
to the base of the femur, but the connexion between these two parts is not a firm 
one, and the suture is frequently quite open behind, and at the lower part even 
altogether yawning. In the males of the genus Hyphydrus the anterior trochanters 
are the seat of extraordinary and incomprehensible modifications of form. The 
femur is short, and is thickest at its point of junction with the trochanter ; usually 
it is not cylindrical, but more or less compressed from behind backwards, but in 
Colpius it is very nearly cylindrical; its upper margin is slightly curved, and its 
lower is more or less hollowed for the reception of the tibia, when completely 
flexed ; the hollow is greatest at the knee, and there, in consequence of it, both 
the front and hind faces of the femur present a free edge below, but towards the 
base of the femur, the hellow becomes less marked, and the front sharp edge 
quite disappears: this hollowing of the lower surface of the femur frequently 
differs much in the sexes of the same species, for in the males of many of the Macro- 
Dytiscide, the tibie are greatly modified in form, and the under portion of the 
femur is shaped in accordance with these modifications. In the Dytiscini, 
Cybistrini and Hydaticides, the underface of the femur usually bears one or two 
pencils of setze just at its junction with the trochanter; in Eretes it is fringed 
with long dense cilize, and in Thermonectes it bears a few isolated, rigid sete: the 
upper edge bears along its posterior margin a more or less conspicuous series of 
sete, of various lengths and degrees of coarseness. The tibia is about the same 
length as, or slightly shorter than, the femur, and is articulated with it in such a 
manner as usually to allow of very complete approximation of the two by flexion : 
the tibia is rarely nearly cylindrical (Hyphydrus), but usually compressed so as to 
show a flat face in front, and is broader at the apex than at the knee, so that it 
has thus the form of a more or less elongate triangle; the outer lower angle of 
