144 DR. J. BRAXTON HICKS ON UNDESCRIBED SENSORY ORGANS IN INSECTS. 



observations upon the habits of insects, and also from the palpi haAdng, in those instances 

 where they are particularly used in the investigation of food, &c., an apparently thin, 

 delicate, bladder-like membrane at their extremities. Noay, 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-lite 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 deKcacy. 

 But even the palpi of the Orthoptera ( Gryllus, Locusta, Tetrix, for instance) bristle aU 

 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 shatt of 

 the palpus is almost destitute of hairs, while those on the apex are largely developed, as in 

 the larva of Ilelolontha. The antennae 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 weU 

 marked, and so easily observed, as in Dyticus marginalis both in the antenna and 

 palpus ; and by watching the habits of this insect we see that it uses the antennae 

 in the same manner as Hydropliilits p)ioeus 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 fii'st I wiU call attention to the palpi of Dyticus marginalis 

 (PL XIX. fig. E). 



On the prominent portions near the joints the largest tactile hairs are to be found, as 

 shown in PI. XIX. fig. E, 4, which may be described as foUows : a tube passes through 

 the wall, becoming 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 



