On Aquatic Carnivorous Coleoptera or Dj/tiscidce. 255 



extremity is conspicuous even when the tarsus is looked at from the outside ; quite 



close to the lower edge theie are irequently placed some swimmino- hairs, which 



however, are sometimes present only in the male (most Cybistrini, Agabus), &c. ; in 



the Hydaticides, Laccoj^hilus and a few species of Cy bister they are present in both 



sexes, but are never quite so developed as those on tlie opposite edge of the tarsus. 



The outer face of the tarsus is that which is usually turned upwards ; its outer 



margin (forming the true lower edge of the tarsus) is densely set with more or less 



elongate stout spines, and these are continued for a little way round the corner of 



each joint along its hind margin. In the Hydaticides the hind portions of each 



joint are fringed with densely set, adpressed ciJiae which laji over the face of the 



following joint, and which exist also on the inner face of the tarsus ; and in Eretes 



the outer and innei- faces of the tarsal joints bear shallow punctures, each of 



which is filled with a transparent, adpressed cilia or scale : except for these 



peculiarities the outer face is bare and polished. The joints of the hind tarsus are, 



in the higher forms such as Cybister and the Hydaticides, shaped so that they 



shall form together a compact piece to press against the water ; each joint is 



about the same width at the base as the extremity of that preceding it, and the 



hind margins are the most prominent part of the joint so that ^the base of each 



joint is received into the concave extremity of that preceding it : and thus both 



edges of the tarsus show a continuous or little broken outline ; it is the rule 



however that the outer (or lower) edge of the tarsus is (as is the case in the 



Carabidfe) much more broken and interrupted in its outline than is the opposite 



edge ; and in the genus Methles the tarsal joints are as loosely articulated and 



the tarsus as discontinuous in outline as in the Carabidse. In Laccophilus the 



tarsal joints are peculiarly shaped, inasmuch as the lower part of each joint is- 



more elongate than the upper part, and so projects backwards over the following 



joint, so that when the foot is looked at from the outside the joints have a peculiar 



lobed apjjearance ; a somewhat similar formation is found in the genus Ilybius, 



and in the group Colymbetini there is frequently a greater or less prolongation 



backwards of the hind margin of the lower part of each joint; by this structure 



the tarsus is strengthened so as to offer a greater resistance when used as a 



pi'opeller. 



The claws of the hind tarsus are very variable in their condition ; in the lower 

 forms the}'- are two in number, equal in size, curved, small and moveable, and the 

 under surface of the apex of the last tarsal joint has a distinct triangular excision 

 for the accommodation of each claw. In proportion to the change and development 

 of the tarsus so do the claws become altered, till in Cybister we find the foot 

 terminated by a single, stout, straight, acuminate, fixed spine which has lost all 

 resemblance to a claw, and for whose accommodation there is a large triangular 

 excision in the material of the under (inner) face of the hind margin of the last tarsal 

 joint. Between these extremes of development there exist numerous transition 



