OF THE ANTENNA OF INSECTS. 389 



that part of the body. If this insect employed its antennae 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 insect and the food 

 are, and which mediimi is known to be well fitted for the diffusion of odorous particles ? 

 Now in Di/ticua marginalis we shall see the reason that its antennae 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 Dyticits marginalis (L.), very similar in general appearance to the 

 palpi of Hydrophilus jnceus, 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 haii-s are on the prominent parts of the joints and apex, which I will descrilje 

 shortly. But on one side of the upper part of each of the joints are a number of 

 circular depressions (L. fig. Ibbb, fig. 3.), of the diameter of 3 crtro inch, more numerous 

 towai'ds the apex of the antenna ; in the centre of each circular depression is a sliort 

 canal, which enters a sac having its interior aspect in communication with the interior of 

 the antenna ; whether closed or not it is difiicult 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 hau's. 



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 antennae 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 oi 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, sti'etches 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 each side of 

 the upper end of each joint at its mdest part. Their situation on the prominent jiarts 

 \vill show their use for touch ; and the habits of the insect also show how valuable as a 

 tactile organ is its antenna. 



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 them 

 are not essentially different from those before descrilicd in other insects ; but it is diffi- 

 cult to suppose that odorous particles could pass through a membrane, then througli a 

 very small tube into a sac, itself probably closed, before they could affect the nerve placed 

 at the inner siu'face ; while it is easy to understand haw such a structure is well adajjtcd 

 for hearing in water. In tlie larval antennae no such organs are found : the hairs on tliem 



