194 SHARP: HYDRADEPHAGA OF LANCASHIRE AND CHESHIRE. 
The tarsal joints also on these legs are much compressed in length 
(although enlarged in diameter), until in Dy#iscus the first three joints 
are together almost spherical. In Gyrinide again on the contrary 
these napaighinn! ai ms very much lengthened (for ep tae purposes), 
but they can p very neatly is in rapid motion, 
so as not to impede its course. The third direction of leg modifica- 
tion we have to notice, is that peculiar enlargement of the anterior tarsal 
joints of the male. » Now this is a characteristic which is found more or 
less all through the Coleoptera, and its use has generally been assumed 
to lie in increased facility in pairing. It is not quite certain, however, 
that the structure was originated solely to serve such a purpose as this. 
In the Geodephaga it is by no means uniform as one would expect so 
so purely sexual an apparatus to be, and in certain genera the second 
as well as the first pair of legs have the tarsal joints enlarged. The © 
theory, however, derives some support from the extraordinary develop- 
inent of these joints in Dytiscus and the allied genera, because it is 
obvious that if union in pairing requires any adventitious assistance at 
all, it will require it more in the denser medium of water than in the 
less resisting one of air. However that may be, we can trace a distinct 
increase in the enlargement of these joints correlated with more 
perfect adaptive modification. In any species of Harfa/us, you see the 
normal enlarged tarsus of a Geodephaga. In Haliplus they are 
hardly enlarged at all, but in Deronectes and Hydroporus we notice the 
first three joints swollen and globular, tending to the still fuller 
development of Dytiscus in which the first three joints are not only 
much swollen, coalescent and almost spherical, but furnished with 
small cup-suckers, after the manner of the arms of a cuttle-fish, 
which appears to be a structure quite unique among Coleoptera. 
So far then for adaptive modification of antenne and legs among 
Hydradephaga. We have next to notice the considerable alteration 
in shape, lending itself more and more to the physical needs of the 
environment. 
(c). Now, if we examine any typical terrestrial form, we notice 
a generally angular contour, protuberant eyes, and a very distinct 
angle between thorax and elytra. In Aadipflus, the lines have 
become more rounded, and the eyes more deeply sunk in the head. 
In WDeronectes, on the other hand, the general contour seems 
hardly so well disposed as in H@lif/us. This form marks in this 
respect the least differentiated among the Dytiscidz ; you notice the 
distinct head, the rounded sides of the thorax, and the wide angle 
between thorax and elytra; but now, on turning to Agadus or Dytstcus, 
= arg rg a distinct improvement. - Their lines compared with the 
like those of the s.s. ‘Teutonic,’ by the side of a Dutch Dutch 
Naturalist, 
