246 On Aquatic Carnivorous Coleoptera or DytiscuUe. 



present in a 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 Legs. — The front legs in the Dytiscidge 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 j^rosternum, except in Amphizoa whei'e the front 

 legs are longer than in the other Dytiscidse. The coxte 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 tlie 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 holloAV 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 hollow 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- 

 Dytiscidte, the tibite 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 setse just at its junction with the trochanter; in Eretes it is fringed 

 with long dense cilitc, and in Thermonectes it bears a few isolated, rigid setae : the 

 upper edge bears along its posterior margin a more or less conspicuous series of 

 setse, 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 



