930 On Aquatic Carnivorous Coleoptera or Dytiscidte. 



any other genus, iind a fortiori, no group the ancestor of any other group ; neither 

 can I find it credible that the similarities visible are due to descent from a common 

 unknown ancestor. 



The geographical distribution of the tribe is extensive, but fails to reach the 

 cooler temperate regions, or New Zealand and the Pacific islands. 



III. 2.— Tribe HYDROPORIUES. {Vide p. 319.) 



About five hundred species, arranged in twenty-six genera, and forming five 

 secondary aggregates or groups (in addition to two isolated genera) form this tertiary 

 aggregate or tribe. It is therefore by far the most extensive of the tribes of 

 Dytiscidse. They are all small insects, the largest size attained being about 6 m.m. 

 of length, and the surface of the body is nearly always punctate. 



The tribe is specially defined by thu structure of the prosternum, which is 

 deflected or bent between the front cox?e so as to be very discontinuous in the 

 plane of its direction with that of the prosternal process. The anterior border of 

 the hind coxa is directed forwards as well as outwards, in such a manner that its 

 greatest extension in front is at a point nearer to the epipleura than to the middle 

 line (longitudinally) of tlie body. The metathoracic episternum penetrates to the 

 middle coxal cavity. The front and middle tarsi have the three basal joints formed 

 so as to show a flat sole clothed beneath with a kind of glandular pubescence : these 

 tarsi have usually only four visible joints, the joint between the third and (true) 

 fifth joints being reduced to a mere knot ; in the cases where this rudimentary 

 joint is quite conspicuous (Necterosoma, Sternopriscus) it usually remains 

 small in comparison with the adjacent joints. The scutellum is usually quite con- 

 cealed, but in Celina is large and conspicuous. 



Of the above characters, that drawn from the structure of the prosternum is the 

 one by which an insect may most certainly and readily be identified as a member 

 of the ti'ibe ; but the tarsal structure and other points must be also taken into 

 consideration, for if not, Sujshis (a jjrimitive form of Noterides in the Dytisci 

 fragmentati) might be supposed to belong to the Hydroporides, as the form of its 

 prosternum corresponds to the above definition. 



The Hydroporides show no tendency whatever to thickening or strengthening of 

 the anterior parts of the prosternum along the middle, as do all the higher and larger 

 forms of the Dytiscidaj : on the contrary, the anterior parts of the prosternum are 

 ahvays feeble and but little developed, and afibrd but slight protection to the front 

 coxae. The anterior transverse band of this part is always small, and has not the least 

 tendency to being arched in the transverse direction : the band between the front 

 coxa3, connecting this transvei'se part with the prosternal process, is never more 

 prominent than the coxae, and is often more or less depressed between them : the 

 prosternal process itself is much more variable and may attain a large development 



