960 On Aquatic Carnivorous Coleoptera or Dytiscidce. 



isolated form, and of all the Hydaticides is I tliink the one that is most removed 

 from the Colymbetides. 



The question whether the tetramerous aggregates (Hydroporides) of the Dytisci 

 complicati, should be classed as higher or lower than the aggregates possessing 

 five joints to the tarsi, although a complicated and difficult one, can I believe, be 

 answered with some certainty. There can be no doubt, but that the families of 

 Coleoptera possessing five-jointed tarsi, are — other things equal — higher than those 

 possessing only four, or a less number of joints in the feet. But on the other 

 hand, there are in the higher of the pentamerous families — the Carabidfe and 

 Cicindelidse and Dytiscidse — certain members which depart in a greater or less 

 degree from the type of foot, structure usually found in the Pentamera, and 

 approach in this respect to the tetramerous or vegetable-feeding Coleoptera : now 

 in Cicindelidse and Carabidse those aberrant-footed forms which frequent trees 

 are, I believe, higher than their purely terrestrial-footed allies ; and we might 

 therefore suppose that also in the Dytiscidas this should be the case, and that those 

 members which have feet of the phytoi)hagous type are higher than those having 

 feet of the more purely predaceous type. But it is clear, ou further consideration, 

 that this would be incorrect. The plant-frequenting Cicindelidee and Carabidse 

 only attain the vegetable-frequenting tarsal structure in some of the details (those 

 being variable according to the genus), but retain always the number of joints 

 characteristic of the carnivori, and as is clear from their general structure frequent 

 plants only for the sake of carrying out their predaceous activities thereupon, 

 instead of on the surface of the earth : they are thus extreme differentiations of 

 the predaceous type. In the Dytiscidae, on the contrary, those members which 

 possess the plant-lrequenting foot do so to a very much greater extent — to such an 

 extent that as regards both the number of the tai'sal joints and the details of their 

 structure they are truly tetramerous. Taking it then as probable, that the order 

 Coleoptera at a very early period of their evolution were divided into (I), plant- 

 eaters and frequenters, and {-), carnivorous and predaceous creatures, operating 

 more particularly on the earth, and that the tarsal structure has been developed 

 in conformity with these habits, we are led to conclude that the plant-frequenting 

 Carabidae and Cicindelidte have only assumed these habits at a comparatively late 

 period of their evolutionary record, but that the phytophagous-footed Dytiscidae 

 have been so over an enormously longer period of their history, and probably 

 have many of them never been purely pentamerous, predaceous, terrestrial 

 Coleoptera : and we are fully justified by the tarsal structure as well as by other 

 points in classing them as less advanced than the Colymbetides and the following 

 still higher group. 1 cannot forbear here from pointing out that these phytopha- 

 gous-footed Dytiscidae show us how very diffident we ought to be in interpreting 

 similarity of even important structures as evidence of community of descent ; thus 

 although the tarsal structure is one of the most important and ti'ustworthy of 



