144 DR. J. BRAXTON HICKS ON UNDESCRIBED SENSORY ORGANS IN INSECTS. 
observations upon the habits of insects, and also from the palpi having, in those instances 
where they are particularly used in the investigation of food, &c, an apparently thin 
delicate, bladder-like membrane at their extremities. Now, although I shall be able to 
show that they are endowed with a very delicate sense of touch, yet that function depends 
on a condition very different from that simply of a delicate membrane ; for, in the numerous 
palpi which I have examined, the thin bladder-like membrane, instead of being perfectly 
smooth, is in most, if not in all instances, furnished with numerous bodies, which I shall 
be able to show are but modified hairs, sometimes of extreme minuteness and delicacy. 
But even the palpi of the Orthoptera (Gryllus, Zocmta, Tetrix, for instance) bristle all 
over with hairs, as will be shown below. Proceeding to each of these modified hairs, in 
every instance, can be plainly observed a branch of the palpal nerve ; indeed, in some 
Beetles, which have the extremities of the palpi dilated, the palpal nerve seems to undergo 
an extraordinary development, which perhaps may be only from the multitudinous sub- 
division to which it is subjected in order to supply each hair, or from the formation 
of a ganglionic enlargement of the nerve itself. This latter supposition does not seem to 
me improbable, as I think I have seen such a condition on the antennal nerve. The 
subject, however, is one of difficulty. The palpus of Timarchus (PI. XIX. fig. H) is 
well-fitted for examination. 
The palpi of many Insects are covered with large stiff hairs, such as the palpi of most 
of the Diptera, Hymenoptera, &c, which evidently can possess only a general sense of 
feeling ; but in those Insects which are remarkable for the use they make of their palpi, as 
the Coleoptera, Orthoptera, &c, the integument becomes more delicate towards the apex, 
and the large scanty hairs found on the shaft become much smaller, and altered in general 
appearance near the apex, while on the delicate tip itself they are further modified, and 
are sometimes exceedingly minute and very numerous. Sometimes, however, the shaft of 
the palpus is almost destitute of hairs, while those on the apex are largely developed, as in 
the larva of Melolontha. The antennas themselves are frequently used as supplemental 
organs of touch, and in those Insects which use them largely for that purpose, a certain 
modification of the hairs takes place, but not so completely as in the palpus ; also the 
antenna-wall undergoes considerable reduction in thickness gradually as it approaches 
the apex, which is well observed in the antennae of Myrmica, Formica, Vespa, Apis 
mellifica, and Sirex gigas ; but in no instance have I found the change of hair so well 
marked, and so easily observed, as in Dyticus marginalia both in the antenna and 
palpus ; and by watching the habits of this insect we see that it uses the antennas 
in the same manner as Hydrophilus piceus does its palpi, which organs they much re- 
semble. The form and position of these modified hairs, to which I propose to give the 
name « tactile hairs," will be best understood by the investigation of the different 
drawings here given ; and first I will call attention to the palpi of Dyticus marginal!* 
(PL XIX. fig. E). 
On the prominent portions near the joints the largest tactile hairs are to be found, as 
hown in PI. XIX. fig. E, 4, which may be described as follows : a tube passes through 
the wall, becom 
g narrower in the centre, across which stretches the cuticle, which 
dips down the tube as far as this point and forms a conical septum ; in the centre of this 
