250 On Aquatic Carnivorous Coleoptera or Dytiscidce. 



are set with spines of variable length, but which are absent from the hinder part ot 

 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 fi-ont feet ; where the middle tarsi are dilated (in the male sex of 

 the Macro-Dytiscidte), 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 Homceodytes a similar peculiarity is 

 found on the third joint. The remarkable Australian genus Sternopriscus differs 

 from other Dytiscidce inasmuch as the males have the middle legs very much more 

 modified than the front ones are. 



Hind or Swimming Legs. — The posterior pair of legs in the Dytiscidte 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 difterence 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 Cai'abid are sufficient to make it evident that the latter might 

 possibl}' 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 

 Avhich remnants of a Carabideous structure still exist as to make us believe that 

 the swimming leg of the Dytiscidie may have previously been a leg for running 

 like that of the Carabidas. 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 cilia; 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 leg exists in the Cybistrmi, the feeblest in Hydrovatini, and Suphisini, 

 Pelobius and Amphizoa, and CeUna 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 aj^ijlied 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 Dytiscidae 



