952 On Aquatic Carnivorous Coleojytera or Dytiscidce. 



Cybistrini, where not only is the dilatation of the three joints in question on the 

 front feet enormous, but it is accompanied by a shortening and coadaptation which 

 render them in fact a sinsrle orofan or saucer. Neither do the males show an 

 incrassation of the basal joints of the tarsi such as exists in Agabiniand Laccophilini, 

 where an enlarged sole is gained by an increase in the whole circumfei'ence of the 

 joints, the increase being in fact as great in the vertical as in the horizontal direction 

 of each incrassate joint. 



Putting aside these sexual differences in the tarsi, other interestingf variations 

 in the basal joints, especially in the outer of them — the third joint — may be noted ; 

 this is almost subquadrate or truncate-cordate in certain forms (H}'phydrus, Macro- 

 vatellus, e.g.) with the upjier surface bearing a short groove at the extremity for 

 the insertion of the basal joint; while in others (many North American species of 

 Hydroporus of groups 1 and 2), it is deeply grooved, or indeed cleft nearly to the 

 base, so as to exhibit elongate lobes, the terminal joint being inserted near to its 

 base. In this case the structure is essentially that of the tetramerous Coleoptei'a, 

 as represented by the Curculionidfe, Phytophaga, Longicornes, &c. : and there is everj^ 

 reason to believe that this modification of structure is essentially similar in its 

 functional value in the two cases. The Coleoptera which walk or run on the 

 surface of the earth ha,ve slender cylindric tarsi, destitute of clothing beneath or 

 bearing only a few rather rigid setaB, while those groups which live much on foliage 

 have the three basal joints of the tarsi dilated to form a sole, which is clothed 

 beneath with a peculiar pubescence, while the terminal joint is not used in walking 

 but is inserted so that the claws terminating it are held up from the surface ; so in 

 the Hydropori mentioned we have good grounds for supposing that the species are 

 much in the habit of frequenting and walking on aquatic plants, while in the 

 other tribes of Dytiscidfe the front and middle tarsi (when not modified for sexual 

 purposes) have essentially the structure of the Carabidse. As another striking 

 instance of the relation between the form of the tarsi and these habits of the species, 

 I may be allowed to mention the genus Stenus of the Staphylinidse, some species 

 of which run swiftly on earth and mud and have filiform elongate feet, while others 

 which live nuich on plants have the tarsi broad with a lobed penultimate joint and 

 the surface clothed beneath with fine pubescence. In these Steni the tarsi however 

 remain five-jointed and have not as in these specialized Hydropori acquired the 

 fully characteristic structure of the tetramerous Coleoptera. In the fact therefore 

 that the structure of the tarsi of some Dytiscidte diflers fundamentally but little, 

 scarcely at all, from that of some Curculionidfe, we have clearly a clue to decij^hering 

 certain of the environments under which they have been developed. 



It is worthy of note that where the true fourth joint of the tarsus is conspicuous, 

 as in Sternopriscus and Necterosoma, it is accompanied by an elongation of the 

 fifth joint, and this elongation is different sexually, being always somewhat, ofteji 

 greatly, more developed in the males. A striking example of this may be pointed 



