On Aquatic Carnivorous Colcoptera or DytiscicUe. '2i*7 



this triangle is usually broadly rounded oft"; and as the inner edge of the tibia is 

 longer than the outer one, the apex is oblique in its direction : the articular cavity 

 for the tarsus cannot be seen from the front, being placed always on the posterior 

 face of the tibia, and as the result of this the extremity of the tibia in front is a 

 more or less chisel-like, or cutting, edge ; this edge being however always fringed 

 with cilise, seta?, or spines. The tibise are very greatly modified in form according 

 to the sex ; in the Dytiscini, Cybistrini and Hydaticides, the tibite are dilated, and 

 in addition a greater or less extent of the outer part of their lower face is cut away, 

 so as to create a hollow, which permits of the rotation of the dilated tarsus when 

 its convex upper face is turned back, or when the tibia is pressed down on the 

 tarsus : and the sexual change in form is accompanied by modifications in other 

 respects as will be presently mentioned. The front face of the tibia is punctate 

 and its posterior one smooth ; the outer edge bears setae, spines, or cilise of various 

 lengths, and degi-ees of coarseness, and the front of the inner face usually bears 

 a dense series of extremely short cilise. The extremity of the tibia is, as already 

 remarked, not a truncature, but an edge, owing to the lower portion of the back 

 of the tibia being more or less cut away, so that here there is seen a smooth 

 polished space, and in this space is placed the cavity for the articulation of the 

 tibia ; it is usually very near the extremity and quite close to the inner angle, but 

 when the tibia becomes much modified in form (as in the males of Cybister) the 

 cavity may be situated nearer to the outer than to the inner margin. Usually it 

 is a simple circular depression but in Cybister i the lips of the orifice become 

 prolonged into a circular neck, and a similar modification, though to a less extent, 

 occurs in other genera where the males have highly modified tarsi. The hollowed 

 space on the back of the tibia is an adaptation to the form of the tarsus and may 

 be called the tarsal area. This tarsal area varies in size, as the tarsus varies in 

 its development, as may be well seen by comparing a species of Cybister where 

 the male tarsus is of small size (No. 1122) with another where it is large (C. owas 

 No. 1135). The tibial spurs also vary greatly, they may be two in number, 

 but so small and inconspicuous as to be readily overlooked, this is the case in the 

 Hydrovatini ; and indeed in the whole family Hydroporides the spurs of the 

 front tibite remain small and in this case are inserted at the inner margin of the 

 tibia one at its front the other at its hind edge ; that in the latter situation 

 being longer than the other and placed a little higher up on the tibia. In the 

 larger Dytiscidae the spurs are much more developed and conspicuous, and in the 

 females are placed in a similar position to that they occupy in the Hydroporides, 

 but in the males where the tibia has much changed its form the spurs are 

 apparently much changed in their position; thus in Cybister $ the anterior 

 sptir is placed quite at the extremity of the tibia while the posterior one is far 

 up on the inner edge ; in Dytiscus i the anterior spur is altogether absent, and 

 the postei'ior one is placed on the inner edge a good distance above the extremity ; 



