DR. J. BRAXTON HICKS ON UNDESCRIBED SENSORY ORGANS IN INSECTS. 143 
the depth of which varies extremely—from a mere depression, as in the Lacquey-moth 
(Clmocampa Nemtria), to a tnbe extending inwards to half, or more, of the length of the 
joint. Sometimes the tnbe is of the same diameter throughout, or even dilated at the 
mnermost end. What the nature of the inner surface of this tubular depression is, it is 
very difficult to ascertain from its position. It certainly possesses numerous hairs, and 
apparently there are some circular markings between them at the deepest part. How- 
ever, upon one point there is no doubt, viz. that a nerve proceeds distinctly to it alone, 
and spreads out upon the apex of this cavity (PL XIX. fig. C, 1 b, 2 b). It is held in its 
position by two or three bands which are attached to the interior of the wall of the palpus. 
The aperture of this structure when tubular (which is its most frequent form) is shown 
at fig. 0, 1 a, 2 a. I have given figures of it in Argynnis Faphia and in a Geometer- 
moth. In a large number of different species examined, I have never found this struc- 
ture absent. 
In Acherontia Atropos (Death's-head Moth) there is no depression, the part being merely 
marked by a circular yellow spot. 
I have found something similar to this in the first joint of the palpus in Bibio Marci 
(Diptera), which I have drawn at PI. XIX. fig. D, 1. There are about twenty-four 
irregularly-shaped hollow cells or compartments on the floor of the cavity, each of which 
is divided from its neighbours by a rather stout septum; and a number of hairs 
spring up from the floor of each cell, and from the sides of the walls dividing them 
ee PL XIX. fig. D, 2). I have not as yet met with this condition in any other 
dipterous insect. 
I shall now pass to a class of structures quite dissimilar to those already mentioned. 
They are to be found on the palpi and those parts about the mouth whose function is 
that of touch, as the apex of the palpi, proboscis of Lepidoptera and of Diptera, and also on 
the apex of the antennae, being peculiar modifications of true hairs, and not processes of 
the cuticle. It is requisite to bear in mind the true nature of the hair in Insects, — 
namely, that it is situated in a depression caused by the absence of the inner layers of the 
integument, into which the cuticle is continued. In the centre of this cuticular depression 
a small elevation or papilla arises, which is the true root of the hair, and which rises from 
it of various length. By means of this arrangement, the hair itself is capable of some 
degree of motion. The interior of the root of each hair is in connexion with the 
internal parts of the member on which it is situated ; commonly fibres run to it, probably 
always including a branch of a nerve ; and this is decidedly the case in those hairs situated 
near the prominent parts and extremities of the various members, as, for instance, the tips 
of the antennae, the palpi, pads of tarsi, &c. ; and this branch of the nerve does not run into 
the interior of the hair, but only to the inner aspect of the root, which separates it entirely 
from the interior. The difference between this structure (true hairs) and cuticular pro- 
cesses must be particularly borne in mind : the latter, having no root, and not being situated 
in a depression, evidently only spring from the surface. The spine must also be distinctly 
separated from the true hair, being a tapering process of the whole integument, into the 
interior of which the contents of the body can freely pass. 
The palpi have been considered by most naturalists as the organs of touch, both from 
