‘SHARP: HYDRADEPHAGA OF LANCASHIRE AND CHESHIRE. 195 
‘brig. If we saw the insects in profile, the singular adaptation of form 
to planes of least resistance would be still more striking. Here we 
-have certainly the perfection of aquatic form. We see it exemplified 
in the genera Agadus, Jlybius, and Colymbetes, as well as in Dytiscus 
_or Acilius, and perhaps nothing could more graphically exhibit the 
modifying power of environment than the contrast, in outline alone, 
between one of these Dytisci, and such a form as Cicinde/a, perfectly 
fitted as ## is to a terrestrial life. 
In this slight survey of the morphology of the group, we ought not 
to omit a very important feature. I refer to the wings, which are 
ample and well developed in all the Hydradephaga. The purpose is 
‘obvious. Perfect adaptation to a life in the water means perfect 
helplessness on dry land. But in summer the pools and marshy 
places, in which so many of these Hydradephaga live, Hae sendy 
‘become dry, and this would involve the utter extermination of those 
“inhabitants unless they had some other means of (iceaagsi to rely 
on than that of their legs. Hence, while in most Geodephagous 
forms, the wings are aborted and useless, in all the Hydradephaga 
they are perfectly developed and constantly. used. 
There is one other point to which I must allude, as the feature is 
very characteristic of this group. Their females, in many cases, 
‘appear under two forms, dimorphic as it is called; this difference is 
only an external one, the dissimilarity is restricted to striation and 
punctuation, one form of the female being like the male, the other 
quite distinct, elytra deeply sulcate, whereas the male and first 
female form, are quite smooth: or dull, whereas the others are polished 
and soon. It must be admitted that we are really quite ignorant of 
_ the origin or utility of this female dimorphism ; some have assigned 
to it a sexual purpose analogous to the masculine enlarged tarsi, but 
the real puzzle is not female differentiation, but, female dimorphism ; 
and sexual purposes which might explain the first class of phenomena, 
are utterly inadequate to throw any light on the second. The females 
of the Gyrinide do not seem to share this peculiarity at all. 
I trust I have not out-worn the patience of my readers in dwelling 
- thus at some length on these structural details. To summarise them, 
we find in the Hydradephaga, from Haliplus to Dytiscus, a gradual 
process of adaptation from a typical Geodephagous to a typical Hydra- 
dephagous form. Of course the small points of differentiation, which 
, distinguish and divide genus from genus and species from species, are 
innumerable ; but, broadly speaking, we find :-— 
1.—Slight shortening, tapering, and complete nakedness of 
antenne, cylindrical joints, and effacement of angular 
apertures between joints. 
July 1892. 
