872 On Aquatit Carnivorous Coleoptera or Dytiscidce. 



most extensive genus of the Dytiscidse. The insects are always of small size (2-6 

 m.m. in length), and show much variation in colour, punctuation, pubescence and 

 outline, but are distmguished from all the other Hydroporini by the tact that the 

 mesosternal fork is connected with the intercoxal process of the metasternum. The 

 head is never margined in front ; the epipleurse are always slender, and there is never 

 any definite genicular fossa at the shoulder ; the prosternal process varies a good 

 deal, but is never truncate behind. There is no ligula on the inner face of the 

 elytra ; the swimming legs are always feeble, and their femora are never thick, and 

 their postero-external angle is rounded or obtuse : their tarsi are slender and 

 feeble, and the joints are not co-adapted to one another so as to give rigidity ; they 

 are terminated by two small, slender, equal claws : the posterior coxal cavities are 

 quite approximate, or at any rate very nearly so, and the hind coxa is not very 

 large, so that it is largely separated by the metasternum from the middle coxal 

 cavity. The scutellum is quite concealed. The front and middle tarsi have only 

 four visible joints. The distinctive characters of the sexes are shght, and sometimes 

 external differences can scarcely be found. In a few species dimorphic females 

 occur. 



In distinguishing the species of Hydroporus, great importance has been attached 

 by describers to the continuity or discontinuity of the outline of the body at the 

 point of the junction of the thorax and elytra. This character is of real importance 

 for it is correlative with important structural changes in the mesosternum ; when 

 this part is larger, or takes a less completely vertical direction than usual, then 

 the outline of the body is much interrupted at the point of junction of the thorax 

 and elytra ; with this there is frequently associated an elongation of the prosternal 

 process. 



The continuity of outline depends also largely on the extent to which the humeral 

 angle of the wing-case is developed, and extended into a covering for the knee ; 

 comp. on these points Dytiscus palustris, (No. 612) Hydroporus modestus, (No. 

 627) and Hydroporus melanarius, (No. 555). 



The genus is especially characteristic of the northern portion of the two hemi- 

 spheres, with a few species in parts adjacent thereto, as in Mexico, Guatemala, 

 Persia &c.; but there is also a species in South Africa and two in southern South 

 America (Chili and Monte Video). A considerable number of additional species 

 will no doubt be discovered in the northern portions of the New World. The 

 absence of the genus from Japan is a fact well worthy of notice. 



I. 42.— Genus CELIXA. {Vide p. 487.) 

 Six species form this aggregate. They are insects of small size, (4-6 m.m. in 

 length), of narrow, parallel form, but little convex beneath, of unifoi-m, obscure 

 colour, destitute of pubescence, and with mucronate termination of the body. The 



