On Aquatic Carnivorovs Coleoptera or Dytiscidce, 21ft 



the special characters of the family. So that the epoch to which we must remount 

 before we can imagine a common ancestor for these two allied genera is enormously- 

 remote : Avhile to believe that there may have been a common ancestor for the 

 whole of the Dytiscidse becomes almost impossible. 



The anterior coxal cavities are placed as a kind of excavation, one on each side of 

 the middle piece of the prosternum, and vary in shape in confomiity with the coxoe ; 

 they are imperfect inasmuch as they only very partially enclose the coxce, especially 

 in those forms where the coxse are elongate as in Hyphydrus ; owing to their 

 peculiar form and position, the cavities appear to be open behind, but this is not 

 really the case, and in fact all the Dytiscidse have the coxal cavities closed behind ; 

 the closing of the cavities is effected, as usual, by an expansion given off from either 

 side of the posterior aspect or edge of the middle piece of the prosternum ; this 

 expansion is in the Dytiscidfe nearly colouidess, and might be supposed to be 

 membranous, or semimembranous in consistency, but it is really quite corneous : 

 moreover it does not connect together the posterior extremities of the side pieces as 

 is usual in the Coleoptera with closed coxal cavities, but is placed as it were somewhat 

 in the interior of the prothorax, and so leaves the angles of the side pieces free and 

 prominent ; this mechanism is highly important as it is, in conjunction with the 

 articulation between the prosternal process and the afterbody, the means of 

 securing a perfect articulation between the prothorax and the afterbody, and 

 consequently of excluding water from the interior of the insect ; the prominent 

 transverse portion of the bridge closing the coxal cavities, actually enters into 

 the interior of the mesosternum (of which it has exactly the width) while on each 

 side there is left a groove or depression into which the front edge of the 

 mesosternum fits, the free edge of the prothoracic side piece playing over that of 

 the mesothoracic piece. The coadaptation of the various parts of the posterior 

 aspect of the prothorax, to corresponding parts of the afterbody and base of the 

 wing-cases is extremely perfect and \exy complicated; proceeding from below 

 upwards we have, first, the prosternal process stretching beyond the mesosternum 

 to be received in a metasternal groove ; directly above the prosternal process we 

 see a considerable protuberance or prominence which fits into the fork of the 

 mesosternum ; then come the posterior aspects of the cox^ which fit into facetts 

 on the face of the mesosternum, and on a still higher level we have the transverse 

 bridge closinsf the coxal cavities which fits into the interior of the mesosternum, 

 while on the upper surface we find that the base of the mesothorax and scutellum 

 are shaped so as to allow the hind margin of the pronotum to overlap and 

 accurately fit them, while the shoulders of the wing cases are prominent, and rest 

 on an expansion of the posterior face of the pronotum which ^ beautifully sinuate 

 and emarginate to facilitate the coadaptation. This jointing is so perfect in the 

 higher forms such as Cybister, that if after the prothorax has been detached from 



the afterbody an attempt be made to replace it in its natural position, this 



a G 



TEAKS. EOT. DUB. SOC. , N.S., YOL. II. , 



