On Aquatic Carnivorous Coleoptera or Dytiscidw. 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 j)arallelisni in the environment of the organisms 

 during their evolution. Much has been written as to these dimorphic females 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 tjie northern hemisphere, whei'e it is specially 

 at home in the northern jsarts 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 w^idely 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. 



n. 13. — Group Htdaticini. {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 nOY. DUl.. SOC. N.S., VOL. 11. g E 



