954 071 Aquatic Carnivorous Coleoptera or Di/tiscidce. 



front and behind, and its posterior part projects as a fine edge, which laps over the 

 basal sutural angles of the wing-cases when they are closed. 



In Hydroporus nigriceps (Hydroporus, No. 576) the anterior pieces of the 

 mesonotum are smaller than in Dytiscus duodecimpustulatus ; the scutellum is 

 however similar in form, but it is received into an angular depression at the base 

 of the elvtra, for the reception of the middle of the base of the prothorax, whereas 

 in D. duodecimpustulatus, the elytra are truncate at the base. 



Tn all the above insects the scutellum is completely concealed when the prolhorax 

 is in its natural position. In Hydroporus oblitus, however, a small portion of its 

 apex remains uncovered, and in Celina it is largely visible. I have not been able 

 to obtain a specimen of either of these insects for more detailed examination. 



Thus the definition "scutellum invisible" applied to tlie Hydroporides covers 

 and includes a series of diverse and interesting modifications. 



The functional value of these parts is con.siderable. Nothing more perfect can 

 be imagined than the manner in which the elytra when closed are locked together 

 by different portions of their structure and by projections of the mesosterna and 

 mesonota ; a complicated set of surfaces and edges is so beautifully coadapted that 

 there cannot be the slightest admission of fluid from without, and great strength is 

 attained although the material used is very small. 



Almost everytbing remains to be done in the study of these pai-ts, for they have 

 hitherto been but little examined. 



The Hydroporides are found in all parts of the world, except some of the Pacific 

 islands. The Hydrovatini and Bidessini are widely distributed in the waimer p.arts 

 of the world ; the Hypliidrini are more specially African ; and the Hydroporini 

 Palajarctic and Nearctic ; while Sternopriscus is peculiar to Australia, and Celina 

 to the warmer parts of the New World. 



There exist in the Hydroporides numerous grades of development of various 

 organs, and this instability of the structure, combined with the fact that some of 

 the gradations can clearly be distinguished as higher or lower than others, suggests 

 the idea that some of the species or genera may be placed in an ancestral relation 

 to others. Hydrovatus, having very poorly developed locomotive organs, possesses 

 a great resemblance to some of the species of Coelambus, such as Dytiscus 

 inaequalis, and this latter form in the structure of its swimming legs might be con- 

 sidered to be an advanced Hydrovatus, the apparent connection between the 

 two being greatly increased by their possessing in common a margined head, and a 

 genicular impression on the epipleura, as well as by the general form, by the 

 sculpture, and by the structure of the antenna. Thus a truly genetic relationship 

 seems to be suggested. But on continuing the comparison the probability of any 

 such relationship quite disappears, for while in Hydrovatus the middle and hind 

 coxse are widely separated and the prosternal process of remarkable width and 

 peculiar form, and the posterior coxal cavities covered and protected, in Dytiscus 



