On Aquatic Carnivorous Coleoptera or DytiscidcB. 229 



lower level than it, it extends completely across the body as a horny band con- 

 necting by its extremity with the metathoracic epimeron ; to its hind margin is 

 attached the basal abdominal segment. In Hydrocanthus australasise this band is 

 very peculiar, for instead of passing across the body in a straight line, it diverges 

 backwards in the middle, so that between the scutellum and post scutellum there 

 exists a large membranous space. 



The metasternum consists of two side pieces on each side, and a middle piece — 

 the metasternum proper ; this last is of very peculiar form, its hind margin is very 

 firmly connected with the front of the hind coxa3, so that these are frequently 

 supposed to be a portion of the metasternum ; in the middle in front this latter 

 stretches forward and extends between the middle coxse, so forming an intercoxal 

 process ; on each side of this are the middle coxae ; an inflexed portion of the 

 metasternum forms in fact the posterior half of the coxal cavity ; outside the coxae 

 the metasternum comes in contact with its episternum, and forms a curved or oblique 

 suture in adaptation with this part ; its hind border is encroached on by the exten- 

 sion forward of the hind coxse, and thus between the episternum and the coxa, the 

 metasternum is more or less — frequently excessively abbreviated — so as to form a 

 wing, called the lacinia or wing of the metasternum ; in the posterior direction the 

 metasternum projects farther back at the expense of the coxae, so forming in the 

 middle behind a more or less acute angle ; thus the metasternum is elongate along 

 its middle, but extremely short externally on each side. 



The intercoxal j^rocess in the majority of the Dytiscidis is connected in front with 

 the fork of the metasternum, and when so connected it is marked in the prominent 

 middle part with a groove or depression — the metasternal groove — for the reception 

 of the apex of the prosternal process, which is usually lodged in it : the intercoxal 

 process differs much in shape and in its various details, and is one of the best means 

 of recognizing genera and species : when the mesosternal fork is not sufficiently 

 elongate to reach the same plane as the front of the metasternum, then the apex of 

 the metasternal process is curved or bent up, so as to enter the mesosternal fork 

 (Dytiscus) or at least to touch it (Hyphydrus, Eretes) ; the nature of the intercoxal 

 process seems chiefly to have been determined by the distance separating the 

 middle coxae ; when the middle coxae are quite contiguous, and project from their 

 cavities, then there is properly speaking no intercoxal process (Vatellini, Sterno- 

 priscus) the middle of the metasternum showing merely a prominent angle behind, 

 and in adaptation to the coxae ; where the middle cox^ are broadly separated (as 

 in (Hydrovatini, Pachydrus) then the intercoxal process is correspondingly broad 

 and short, and its connexion with the mesosternal fork is very intimate or exact, 

 thus in Pachydrus each of its lateral angles projects and articulates with the raised 

 angle of the extremity of the mesosternal fork, which in these cases is reduced, in 

 the longitudinal direction, to a mere lamina, while the raised margin round the 

 coxal cavity is continued inwards along the front of the intercoxal process ; in the 



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