On Aquatic Carnivorous Coleoptera or Dytiscide. 877 
able as to this are really by no means naturally allied-by their other characters 
(c.f, D. abbreviatus, and D. femoralis, Nos. 727, and 726). 
The peculiar form of the wings of the species, for which I have established group 
23 (viz. Colymbetes bifarius, Kirby) is worthy of special attention : ample wings are 
so characteristic of the Dytiscide, and evidently so valuable to insects whose power 
of walking is nearly destroyed by the transformation of their hind legs into swim- 
ing organs, as to render of great interest such a reduction of the means of flight as 
we find here exhibited. 
The genus shows species of various grades of development. In some, notably 
those of the first group, the swimming legs are very poorly adapted to aquatic 
locomotion, and the general structure of the external parts is imperfect, less compact. 
and not closely fitted together. Other species such as Dytiscus brunneus (No. 688), 
D. didymus (No. 705), and Agabus conspicuus (No. 738), are of perfect and con- 
tinuous form, with really powerful swimming legs. 
A genus comprising such a variety of forms is pretty certain to have numerous 
points of approach to other genera. The great deviation from the rectilinear direction 
of a line drawn from the front to the back of the prosternum shown by Dytiscus 
fuscipennis (No. 752), is an approach to the structure of that part as existing in 
the Hydroporides, but the species alluded to does not possess any other points of 
resemblance with the Hydroporides. Group 19 of the genus seems however to 
make a real approximation to Ilybius ; and various of the allied smaller genera like- 
wise approximate to some or other of the genus. 
A point of special interest and requiring very careful investigation, is the sexual 
sculpture. It is very common in the genus that there should be some slight 
difference between the two sexes of a species in the sculpture of the upper surface, 
and in certain cases this becomes very marked, and then it is common that the 
females (the sex displaying the peculiar sculpture) should vary very much in this. 
respect. The variation is often, perhaps generally, geographical: Dytiscus con- 
spersus (No. 714), and D. congener (No. 706), are notable instances of this. These 
cases require careful investigation to ascertain whether they are instances of dimor- 
phism or polymorphism, or whether the variations are connected intricately 
together by intermediate forms. My own impression is that the latter will prove 
to be the case. It is possible, too, that some of the peculiarities considered by me 
to be sexual variations may ultimately prove to be specific ones ; for this is a genus 
in which a minute examination of specimens in a good state of preservation is 
indispensable for deciding on specific characters, 
The geographical distribution of the species is confined to the northern portions 
of the two hemispheres ; and the richly watered and extensive northern portions of 
of the New World, are probably the most rich in species of Agabus, and it is 
there no doubt that numerous others yet unknown will be discovered. 
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