192 SHARP: HYDRADEPHAGA OF LANCASHIRE AND CHESHIRE. 
We have already seen that the five-jointed tarsi is a feature 
common to both these groups, and we may now add that both are 
distinguished by moniliform antennz, slightly tapered at the extremity, 
and never thickened, clubbed, or foliated, and by a maxillary palpus 
having a jointed inner lobe, with very few exceptions. These are 
points of similarity which undoubtedly denote a common origin, but 
if we turn to the table of the families of the Adephaga, given at the 
end of this article, we shall see a group called Gyrinide, which is 
placed after all the other families, and if this group or family be 
included among the Adephaga, as it usually is, then these remarks 
about similarities in structure will not apply. For these Gyrinide 
are quite an aberrant group, they are indeed one of the most singular 
and exceptional in the whole order of Coleoptera, and we can hardly 
understand why they should come under the heading of Hydradephaga 
at all, except perhaps because of their purely aquatic habits, 
Morphologically they will not come into line at all with the other 
families of the section. They represent a much greater advance in 
development, a much more elaborated adjustment of structure to 
function, than do any other members of the group with which they 
are usually associated. If we consider more in detail the consecutive 
differentiation of organs in the other members of the Hydradephaga, 
we shall the more readily see how widely divergent these Gyrinidz are 
from the eine Dytiscus after which they are usually placed. 
we must bear in mind that the great idea expressed by this 
eimeees the ultimate aim of this modification of structure, is simply 
this—from perfect adaptation to the needs of a terrestrial life to 
perfect adaptation to the needs of an aquatic life—that is a transition 
actually bridged in the course of ages, and the links in the chain of 
that transition are still stamped in the physiology of the various 
genera from Ha/liplus to Gyrinus. 
From this point of view let us, then, consider these external 
features, viz., antenne (a), legs anterior and posterior (B), a0 
general contour (Cc). We need say nothing of the mouth organs, 
because they are adapted not so much to the environing medium 
as to the food, and the food is similar all along the line. From a like 
reason the internal organisation is undifferentiated, the disparity in 
habitat affects neither the nervous, digestive, nor ‘respiratory systems. 
A). If you examine the antennz of any Geodephagous beetle 
you will notice how the joints are set angularly on one another, 
and furnished with bristles or pubescence. In the Ha/iplus type 
the angles are bevelled down, the joints being set close one against 
another, very much narrowed, and quite glabrous. This type obtains, 
without much modification, all through the true Hydradephaga, 206 and 
wararalt, 
Fy 
