On Aquatic Carnworous Coleoptera or Dytiscide. 939 
extremely different as to the form assumed by the peculiar females,—so distinct 
indeed, that their development may be said to have been in opposite directions ; 
nothing could be more different than the deep regular grooves of the sculptured 
females of Dytiscus, and the dense irregular tubercles of the corresponding females 
of Hyderodes; this fact would suggest the view that the resemblance or approxi- 
mation between Dytiscus and Hyderodes does not arise from genetic connection (or 
common origin), but is due to a parallelism in the environment of the organisms 
during their evolution. Much has been written as to these dimorphic feraales of 
Dytiscus, but little or no light has at present been thrown on the subject ; and it 
will require a very extensive accumulation of observations and recorded facts, 
before successful generalization can be accomplished. It is worthy of note that 
the curious difference between the sculptured females of Dytiscus and of Hyderodes 
is completely paralleled in the Hydaticides, where the females of certain Acilii are 
sulcate like the females of Dytisci, while the females of certain Graphoderes are 
tuberculate like Hyderodes. 
The geographical distribution of the group is very peculiar. Dytiscus with its 
twenty-three species is confined to the northern hemisphere, where it is specially 
at home in the northern parts of the Old and New Worlds; while Hyderodes is 
peculiar to Tasmania and Australia, and seems especially characteristic of the 
former; the large intermediate tropical zone comprising the parts of the earth 
richest in insects is absolutely destitute of Dytiscini, so far as we at present know. 
This peculiar restriction of a small group to the two most widely separated 
zoological districts of the world is remarkable, and is similar to that of the genus 
Pelobius, except that this latter does not occur in the New World. 
II. 18.—Group Hyparicini. (Vide p. 647.) 
This aggregate of the second degree is formed by the extensive genus Hydaticus 
(forty-five species), and the autogenus Prodaticus. ‘The characters are similar to 
those of the more extensive group Thermonectini, except as regards posterior legs, 
and the metasternum. The hind coxa is not enormous, its length never being quite so 
great as its width, and frequently considerably less, so that the front border of the 
coxa is considerably separated from the middle coxa; the suture between the 
wing of the metasternum and the episternum is a straight line (or nearly so) drawn 
obliquely from the front of the breast outside the middle coxa, to the epipleura. 
On the upper face of the hind tibia, parallel or sub-parallel to its outer margin is 
an elongate series of punctures, each bearing a stout furcate seta; the spurs of 
the hind tibia are quite acuminate at the apex. 
The species are probably sparingly distributed over all the warmer and 
temperate parts of both hemispheres, but are absent from New Zealand, and 
probably from most of the Pacific islands, 
TRANS ROY, DU. SOC. N.S., VOL. IT, ara 6E 
