On Aquatic Carnivorous Coleoptera or Dytiscide. 255 
extremity is conspicuous even when the tarsus is looked at from the outside ; quite 
close to the lower edge there are frequently placed some swimming hairs, which, 
however, are sometimes present only in the male (most Cybistrini, Agabus), &e. ; in 
the Hydaticides, Laccophilus and a few species of Cybister they are present in both 
sexes, but are never quite so developed as those on the 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 cilize which lap over the face of the 
following joint, and which exist also on the inner face of the tarsus ; and in Eretes 
the outer and inner 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 
Carabidee) 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 Carabidae. 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 appearance ; 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 
propeller. 
The claws of the hind tarsus are very variable in their condition ; in the lower 
forms they 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 
