878 On Aquatic Carnivorous Coleoptcra or DijtiscidcB. 



Group 1. 



Outline of thorax discontinuous witli elytra ; prosternum but little thickened in 

 the middle in front; swimming legs elongate and slender, the femora feeble, with 

 rounded postero-external angle. Mesosternal cavity ill developed. 



The insects associated by the above characters may be looked on as little deve- 

 loped forms, as the characters used in defining them indicate. They show no close 

 relationship inter se : thus Agabus cordatus has the middle coxae very approximate, 

 while in Agabus caraboides they are much more separated ; A. cephalotes is in this 

 character intermediate between the two. Agabus abnormicollis seems to approximate 

 to Dytiscus biguttatus (No. 676), the prosternal process and metasternal cavity being 

 very conform in the two. A. cephalotes seems to have only the two basal joints of 

 the male tarsi clothed with sexual pubescence beneath, while in Agabus cordatus 

 there are distinctly three joints so clothed. A. maderensis is a specis of imperfect 

 form, but its thorax is very nearly as broad as the base of the elytra. 



The prosternal process and metasternal cavity in the group are comparatively 

 imperfect, and in Agabus intersectus the former terminates in front of the middle 

 coxse and is not received into the metasternal cavity. 



It is probable that the species will ultimately be separated to form as many dis- 

 tinct groups or genera; all the species (with the exception perhaps of A. cephalotes) 

 seem to be excessively rare, so that I have had a good deal of difficulty in ascer- 

 taining their characters, and have as yet only very imperfectly succeeded. 



Geoup 2. 



Outline of thorax slightly discontinuous, or continuous, with that of elytra ; 

 prosternal process comparatively broad, nearly flat, evenly and distinctly margined, 

 shining and impunctate ; metasternal groove moderately broad. Hind coxae never 

 large; wings of the metasternum large; coxal lines deeply impressed in their upper 

 part, much prolonged in the anterior and outward direction ; hind legs slender or 

 moderate (never thick) the femora little thickened, their postero-external angle little 

 obtuse, sometimes rectangular. Male front feet but little developed, clothed beneath 

 with very short "glandular" pubescence ; sexual differences of sculpture slight. 



The species comprised in this group show a good deal of difference in various 

 points. Dytiscus guttatus, appears to approach the first group by the comparatively 

 ill-fitting base of the thorax and elytra ; while the latter species, by their more 

 perfect form, shorter swimming legs, polished surface and more highly developed 

 metasternal groove, approach the two following (3rd and 4th) groups. Agabus 

 hypomelas is a somewhat exceptional species, for although greatly resembling D. 

 guttatus, it has the prosternal process slightly raised along the middle, and feebly 

 p unctate. 



