On Aquatic Carnivorous Coleoptera or Dytiscidw. 883 



large ; front border of hind coxte little arched ; swimming legs feeble, and elongate; 

 male front tarsi with the basal joints but little incrassate, the fifth joint angularly 

 dilated in the middle beneath, claws elongate. 



The unique species of this group forms the genus Arctodytes, Thorns, ; it may be 

 readily distinguished by the above combination of characters. 



Group 16. 



Prosternal process compressed on its apical half or through its whole length, 

 finely but quite distinctly margined ; wings of metasternum moderately large, or 

 large ; swimming legs moderately stout ; reticulation of surface often very conspi- 

 cuous ; male front tarsi much incrassate and dilated, their claws variable, either 

 short and dentate beneath, or elongate, or very elongate. 



The group approximates closely to the tenth group, but differs from it by the 

 broad male tarsi : and the prosternal process is usually broader. The species 

 forming the group difler a good deal, however, in the various peculiarities of the 

 male tarsi, and in the prosternal process, and as they are not bound together by 

 the possession of any positive common character, the group cannot be considered a 

 natural one. Agabus lecontei has the prosternal process resembling almost 

 exactly that of Dytiscus congener, of group 10 ; A. reticulatus has the process 

 broader, only compressed on its apical half, while Colymbetes erythropterus has it 

 compressed for its whole length so greatly as to appear carinate along the middle. 

 In this latter very remarkable species, the front claws of the male are excessively 

 elongate, while in A. griseipennis, they are remarkably (one might say abnormally) 

 short. The male front tarsi indeed of this latter species are highly peculiar, they 

 are dilated and comparatively little compressed, and the fifth joint is densely set 

 beneath with fine pubescence ; A. lecontei agrees with it in these peculiarities, 

 although they are not quite so highly developed in it. The group also shows 

 much variation in the development of the sexual clothing of the undersurface of 

 the male feet ; thus in A. reticulatus, the palettes are but inconspicuous, while in 

 A. obsoletus they are highly developed. 



Group 17. 



Prosternal process flat, very finely or indistinctly margined ; male front tarsi 

 much incrassate, and furnished beneath with remarkably large palettes ; wings of 

 metasternum only moderately large ; swimming legs moderately long and stout ; 

 surface conspicuously reticulate. 



In these two species the combination of fiat, almost unmargined, prosternal 

 process, with large palettes on the feet of the male is diagnostic ; the prosternal 

 process is not very different from that of Dytiscus elongatus, (group 15), while the 

 large palettes are approximated by certain species of group 16. 



TRANS. ROT. DUB. SOC, N.S., VOL. II. * X 



