250 On Aquatic Carnivorous Coleoptera or Dytiscide. 
are set with spines of variable length, but which are absent from the hinder part of 
the truncature. The tarsi also agree usually in structure with those of the front 
feet, except that they are less modified in the males; the number of joints is the 
same as on the front feet ; where the middle tarsi are dilated (in the male sex of 
the Macro-Dytiscidze), the dilatation does not go to such a great extent, so that the 
three basal joints do not assume a disc-like form, but are always elongate ; they 
may be furnished beneath with a peculiar clothing of hairs or cups, but these are 
less developed than on the front feet: the claws are frequently elongate; in the 
males of some species of Cybister there is a peculiar patch of hair on the under 
surface of the basal joint in the males, and in Homeceodytes a similar peculiarity is 
found on the third joint. The remarkable Australian genus Sternopriscus differs 
from other Dytiscidze inasmuch as the males have the middle legs very much more 
modified than the front ones are. 
Hriyp or Swimmine Leas.—The posterior pair of legs in the Dytiscidze are used 
only for aquatic locomotion, and are profoundly modified in accordance with their 
unusual function : no part of the structure of these water-beetles however varies so 
much as do the swimming legs ; the difference between their feeble, and nearly useless 
condition in Hydrovatus and Methles, and their powerful and complex development 
in Cybister and Megadytes and their allies, being enormous; in fact so great is 
the variety in the development and details of structure of these parts, that a very large 
proportion of the species of the family could be distinguished from other species 
by the swimming leg alone. An examination of this organ, and a comparison of it 
with the leg of a Carabid are sufficient to make it evident that the latter might 
possibly be modified so as to form the swimming leg of a Dytiscid ; while the study 
of the details of structure of the part in this latter family reveals so many cases in 
which remnants of a Carabideous structure still exist as to make us believe that 
the swimming leg of the Dytiscidee may have previously been a leg for running 
like that of the Carabide. The metamorphosis consists in an increase in the 
transverse dimensions of the pieces, and in a compression or flattening of them, so 
that one aspect of the limb presents a large surface, while another is reduced to a 
mere edge: at the same time there is a large growth of elongate ciliz or swimming 
hairs, which are very readily spread out to form a very large surface to be opposed 
to the water, and just as easily depressed so as to offer no resistance whatever to 
the return of the leg to a position for making a fresh stroke. The most powerful 
swimming lee exists in the Cybistrini, the feeblest in Hydrovatini, and Suphisini, 
Pelobius and Amphizoa, and Celina and Methles. 
The coxa has already been described in speaking of the under part of the body 
(page 234). The trochanter is never very large, and is usually, closely applied to 
the base of the femur, so that the two almost appear to form one piece ; it does not 
vary greatly in size, and is not the seat of any remarkable developments, but is 
large and more globose and inflated in the Vatellini, while in the larger Dytiscidee 
