OF THE ANTENNÆ OF INSECTS. 389 
that part of the body. If this insect employed its antennæ to scent its food, which is 
aquatic, does it not seem very improbable that the olfactory organs should be carefully 
shut off by a peculiar provision, from the medium in which both the inseet and the food 
are, and which medium is known to be well fitted for the diffusion of odorous particles ? 
Now in Dyticus marginalis we shall see the reason that its antennæ are bathed with the 
water, —not because it uses its antennal organs for scent, but because it uses them as in- 
struments of touch, and has them provided with special organs for that purpose. 
The antenna of Dyticus marginalis (L.), very similar in general appearance to the 
palpi of Hydrophilus piceus, consists of nine joints, all nearly alike, except the terminal 
one, which is pointed. The antenna is almost universally smooth: the only representa- 
tives of the hairs are on the prominent parts of the joints and apex, which I will describe 
shortly. But on one side of the upper part of each of the joints are a number of 
circular depressions (L. fig. 1552, fig. 3.), of the diameter of 3455 inch, more numerous 
towards the apex of the antenna; in the centre of each circular depression is a short 
canal, which enters a sac having its interior aspect in communication with the interior of 
the antenna; whether closed or not it is difficult to determine, but it appears to be so by 
a firm substance frequently found on the inner surface of the antenna where these organs 
are present. The nerve is to be seen very plainly giving off branches as it passes up the 
antenna to each of these groups, and finally expends itself in those of the apex, sending 
off a branch to the roots of the hairs. 
To these hairs, of which I have given a drawing (L. figs. 2, 4), I would also request atten- 
. tion, because of their peculiar form, and because such a form is not uncommon in insects, 
both on the antennæ and palpi, when those parts are used as organs of touch. That all the 
hairs, long as well as short, possess to some extent the faculty called in the higher animals 
“ general feeling,” I think cannot be doubted for a moment; but hairs of the form I am 
about to describe seem, from their position and shape, to be specially adapted as instruments 
of that more refined tactile sense called “touch.” Perhaps the most marked example 
is to be found in the antenna of Dyticus marginalis. The organ in question consists of 
a widened follicle (L. fig. 1 a a a a, figs. 2, 4), becoming gradually narrowed to the centre, 
from which a delicate membrane, in the form of a depressed cone, stretches across; from 
the centre of this membrane springs the modified hair, something like a ninepin, its apex - 
reaching a little above the general surface of the antenna. There are three on the apex 
of the antenna, three on each side of the widest part, and two or three on cach — 
the upper end of each joint at its widest part. Their situation on the prominent parts 
will show their use for touch; and the habits of the insect also show how valuable as a 
taetile organ is its antenna. : pens 
The whole of the antenna of this insect is wetted by the water, in which it passes the 
. greater part of its life, and where it finds its sustenance. In form the organs on p 
are not essentially different from those before described in other insects; but it is diffi- 
cult to suppose that odorous particles could pass through a membrane, then through a 
i i Josed, before they could affect the nerve placed 
very small tube into a sac, itself probably closed, be y eg IRON 
at the inner surface; while it is easy to understand how suc 
i ; al them 
for hearing in water. In the larval antennæ no such organs are found: the hairs on 
