Oil Aquatic Carnivorous Coleoptera or DytiscidcB. 925 



"In short the broad coxal processes are each separated into two distinct parts by 

 the coxal notch — an inner adpressed part, and an outer free part. 



This structure is essentially the same as exists in Pelobius ; and in certain Hydro- 

 porini the structure is approached in one or more particulars, but in none of the 

 Hydroporini do we find a distinct coxal notch, coexisting with a broad intra-rimal 

 and a large extra-rimal part of the coxal pi'ocess. 



The approximations of other secondary aggregates to the Hydrovatini are impor- 

 tant ; the peculiar prosternal process is repeated in some Bidessini and Hyphidrini ; 

 and the general form, and the shape of the hind coxse (minus the coxal processes), 

 and the swimming legs, is repeated in some Hydi'oporini, (in certain species ot 

 Ccelambus) ; the peculiar acuminate hind extremity of the body is reproduced in 

 Celina and Methles, two isolated primary aggregates of Dytisci complicati ; while 

 Hydrocoptus and Pronoterus, two primitive forms of Noterides in the Dytisci 

 fragmentati, not only resemble Hydrovatus to an extraordinary degree in their 

 facies or general appearance, but show a very similar structure of the coxal processes. 

 These facts render it probable that the Hydrovatini will be found to be creatures 

 that have retained to a large extent their primitive characters, while becoming 

 highly specialized for a life of very small locomotor activity. If we recall the fact 

 that in the Hydrovatini the middle and hind coxEe are unusually distant from one 

 another, it seems probable that the primitive ancestors of the group, at the period 

 when they began to be modified for an aquatic life, were beetles whose coxal cavities 

 were separated by a considerably greater interval from one another than were 

 those of the ancestors of the other (now existing) Hydroporides. 



The group is widely distributed over the globe, but wanting in the Pacific 

 Islands and New Zealand, and does not extend to the cooler parts of the world ; the 

 extreme south of England being, so far as is yet known, its farthest extension into 

 the temperate regions. 



II. 7. — Group Bidessini. {Vide p. 336.) 



Rather more than one hundred species arranged in six primary aggregates (of 

 which however no less than three are autogenera) form this secondary aggregate. 

 It is distinguished by a single character not only from other water beetles, but 

 from all other beetles, viz., that the hind coxa is soldered Completely to the ventral 

 segments, so that the under surface of the body from the front of the metasternum 

 to the hind margin of the third ventral segment consists of one rigid piece. 



A second important character of the Bidessini is that the posterior articular 

 cavities are not contiguous, and are not protected by the coxal processes ; they 

 open directly on the under surface of the cox83 and are separated from one another 

 by the coxal processes which are completely adpressed to the level of the ventral 

 segments ; in this portion of their structure they are closely approximated by the 

 Hyphydrini. 



The hind coxse are always large, with greatly arched anterior border : this 



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