862 On Aquatic Carnivorous Coleoptera or Dytiscidce. 



(Dytiscus parallelogrammus) almost angular ; in such species the metasternura is 

 elongate in the middle, and although its wing is much abbreviated as its outer portion 

 is reached, yet it remains comparatively broad till the culmen of the coxae is 

 reached, and there is very abruptly deflexed outside of it as a veiy slender 

 band. 



Altliouo'h the species of this aggregate show much diffei-ence in various points 

 of structure, yet they are constant in the possession of the elytral ligula, and the 

 genus is thus absolutel}' distinguished from Deronectes. Although the head differs 

 much in the extreme forms, yet as it is not exactly alike in any two species it does 

 not justify the formation of two (or more) distinct aggregates; in the shorter 

 species the head is completelj^ rounded in front, and bordered with a distinct raised 

 margin, and the labrum is placed so much on the undersurface as to be greatly 

 concealed, — the head in such species closely approaches that of Hyphydrus. In 

 other cases however the labrum is brought forward to the front of the head, and 

 quite exposed (Hydroj)orus enneagrammus, No. 419, Dytiscus confluens, No. 423, &c.) 

 the front of the head being truncate-emarginate ; various species ai'e more or less 

 intermediate between these extremes. 



In the development of the hind coxae there exists also considerable difference 

 between the extreme forms. The species of short form possess hind coxa3 with their 

 antero-external portions less extended towards the front of the body, so that the 

 shape of the culmen of the coxal arch is more rounded and less abrupt and angular 

 than it is in the more elongate species. 



In other of the structural points characteristic of the genus, numerous shades of 

 variation may be detected ; such is the case with the form of the tarsi, and the 

 development of the genicular area of the epipleura. 



In all the species the epipleura is small, and is much reduced in its posterior 

 portion : Hydroporus enneagrammus shows us the extreme of this reduction : in 

 this species the epipleura at the base is very small, and before the middle length 

 of the wing-case is reached has altogether disappeared : this diminution of area is 

 accompanied by some alteration in the form of the shoulder of the wing-case, and 

 as a result of this it seems at first sight as if the genicular area were altogether 

 absent, nevertheless on a more careful examination not only is the area seen to be 

 definitely present, but also the existence of a line marking it oflF externally is certified ; 

 in agreement with this reduction of the epipleura, the ligula of the elytra is also 

 extremely minute in this species, but yet it exhibits the form characteristic of the 

 other species ; in this peculiar insect there cannot be detected on the undersurface 

 of the body any trace of the coarse punctuation that exists (though in certain 

 species only in a comparatively slight degree) in all the other components of the 

 aggregate. 



The external sexual disparities are, as a rule, not strongly marked : scarcely to 

 be detected in Dytiscus insequalis (No. 381), they become so far as regards the legs 



