966 On Aquatic Carmvorous Coleoptera or Dytiscide. 
isolated form, and of all the Hydaticides is I think the one that is most removed 
from the Colymbetides. 
The question whether the tetramerous aggregates (Hydroporides) of the Dytise1 
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 Carabide and 
Cicindelidee and Dytiscidee—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 Cicindelidee and Carabidee 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 Dytiscidee this should be the case, and that those 
members which have feet of the phytophagous type are higher than those having 
feet of the more purely predaceous type. But it is clear, on further consideration, 
that this would be incorrect. The plant-frequenting Cicindelidee and Carabidee 
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 Dytiscidee, on the contrary, those members which 
possess the plant-frequenting foot do so to a very much greater extent—to such an 
extent that as regards both the number of the tarsal 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 (1), plant- 
eaters and frequenters, and (2), 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 
Carabidxe and Cicindelidee have only assumed these habits at a comparatively late 
period of their evolutionary record, but that the phytophagous-footed Dytiscidee 
have been so over an enormously longer period of their history, and probably 
have many of them never been purely pentamerous, predaceous, terrestrial 
Joleoptera: 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. I cannot forbear here from pointing out that these phytopha- 
gous-footed lytiscidee 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 trustworthy of 
