232 On Aquatic Carnivorous Coleo'ptera or DytiscidcB. 



extremely the appearance of being a distinct piece of tlie sternum, whicli however 

 is not the case. The upper or outer portion of the episternum is, at any rate in 

 its anterior portion, covered by the wing-case, and when the part covered by the 

 wino--case is large, as in Hydrovatus, then the posterior extremity appears very 

 slender and acuminate, but on the elytra being taken off it will be seen that in 

 such cases the termination of the episternum is really obtuse, the greater portion 

 of its area being turned upwards at an angle with the exposed portion ; this 

 covered portion of the episternum may readily be mistaken for the epimeron, 

 more especially as it is of a different texture from the exposed part, and is 

 traversed by a sort ot line, having somewhat the appearance of a suture, between 

 the covered and exposed portions. The epimeron of the metathorax, unlike the 

 episternum — is very indistinct, and might on a careless inspection, be supposed to 

 be absent ; it is however always present and is placed on the upper posterior 

 portion of the episternum, but extends farther backwards, than this piece does ; 

 the suture between the episternum and epimeron is covered by the wing-case, and 

 is often very fine and indistinct, but is alwa3's conspicuous when the part is 

 properly cleaned and exposed ; the episternum towards its extremity becomes, as 

 above stated, narrower, and thus trenches away from the edge of the wing-case, 

 and an angle is thus formed between it and the upper and outer portion of the 

 hind coxae, and into this angle the epimeron protrudes, so that at this point it 

 approaches more to the under surface than it does elsewhere, and in many forms 

 (especially in Eretes, Thermonectini, and Cybistrini,) the epimeron becomes visible, 

 even when the wing-cases are closed, as an acutely angular projection at the upper 

 and hinder angle of the apex of the episternum. Behind this point the epimeron 

 again completely covered by the wing-case, proceeds farther backwards along the 

 external or upper part of the hind coxa, till it terminates behind by connecting 

 with the side of the basal segment of the abdomen or hind body; sometimes 

 the connexion between these two parts is effected not by actual contact, but by 

 means of an intervening membranous space (Acilius, Hydaticus, and many others, 

 but in other cases (Hyphydrus, Eretes) there is complete contact between the 

 horny portions of the two parts ; between the extreme degrees of separation (as 

 seen for instance in Coptotomus) and the absolute contact of Hyphydrus and 

 Eretes, there exist every grade of connexion ; thus in Agabus it would be almost 

 equally correct to describe the two parts as separated by a membrane or as in 

 contact. The upper portion of the epimeron becomes membranous, and connects 

 with the pieces of the metanotum. In those cases where the middle coxae are 

 widely separated, and there is no mobility of the pieces of the thorax (Hydrovatus, 

 Hydrocanthus, Pachydrus) there is a most extreme and perfect adaptation ot the 

 inflexed edge of the epipleura, to the side of the body, and in these cases the 

 epimeron remains nearly entirely membranous, a mere strip of semicorneous matter, 

 along its lower edge and its extremity, serving for the suture with the episternum 



