DR. J. BRAXTON HICKS ON UNDESCRIBED SENSORY ORGANS IN INSECTS. 145 



septum is a point or papilla, wliich is the root of the hair, which springs from it, not long 

 and tapering, but oval like a ninepin, its apex scarcely reaching beyond the general level 

 of the exterior. To the inner surface of the root a nerve passes. 



If we look with a higher power {\ or ^ inch objective) at the delicate membrane at the 

 extremity of the apex of this palpus, it will seem to be studded all over with minute bodies, 

 as shown in PL XIX. fig. 'E, 1 b b. These upon careful examination will be seen to be 

 but modified hair-follicles, the space between each being depressed (fig. E, 2). The prin- 

 cipal part of the palpal nerve proceeds to these, sending a branch to each. Compare this 

 palpus with that of another Water-Beetle, SydrophUus jnceus (PI. XIX. fig. F, 1, 2), 

 where, in the centre of the bladder-like apex, a small depression occurs, in which are seen 

 two shortened hairs, whose apices just appear above the general level (fig. F, 3 a). Be- 

 sides these is a multitude of very delicate hairs (true) which seem to be collected into two 

 grou-ps, those towards the middle of each group being the tallest. To these, as in all 

 others, the nerve is plainly seen proceeding. 



The labial palpi of Ilelanotus (PI. XIX. fig. G) present a similar structure, the smaller 

 hairs being shorter and more distinct, as shown at fig. G, 2. Their diameter is yoVo ii^ch. 

 The surface of the maxillary palpi has nothing but the short sunken hairs over the whole 

 surface of the bladder-like membrane (fig. G, 1 & 3). 



In Timarchiis, the last segment of whose palpus is much dilated, the apex is covered by 

 a very thin membrane, which is thickly covered by a more elongated form of hair, as 

 shown in PL XIX. fig. H, 1, 2. The nerve (1 a), as before remarked, becomes so enor- 

 mously enlarged or expanded on reaching the roots of these hairs, that it seems not im- 

 probable that it is increased by the addition of some ganglionic matter ; certainly among 

 the expanded nerve-threads globular bodies very much like ganglion-cells may be noticed. 



In PL XIX. fig. I, 1, 2, I have shown another slightly different form of tactile hairs 

 covering the ^^iiole membrane of the palpi of a Carabiis. Some of them are similar to 

 those of Dyticus marginalis. 



In another species of Carahis may be seen (at PL XIX. fig. J, 1 & 1 a) only a few tactile 

 hairs ; these, however, are long and well-marked ; while in a third species they are 

 reduced to a very small size (fig. J, 2). A similar condition is observable in the maxil- 

 lary palpus of the Glow-worm {Lampyris noctiluca) (PL XIX. fig. K, 1 & 2), the 

 numerous fine delicate hairs being seen at the edge. 



But sometimes the termination of the nerves in the surface is so delicate and minute 

 as to have the appearance of mere points even under a |^-inch objective. This is shown in 

 the palpi of Cyclirus rostratus (PL XIX. fig. L, 1 & 2). Some of them, however, are a 

 little larger ; but it is impossible to decide if they possess any hair-like structure, such 

 as appears in Goerius olens (PL XIX. fig. L, 3), where they are shown as roujided 

 elevations of the membrane. However, between these delicate tactile hairs and the large 

 long hairs on some palpi, there are various grades more or less approaching to the con- 

 dition of tactile hairs. For instance, on the apex of the antennse and palpi of the larva 

 of Melolontha vulgaris (PL XIX. fig. M, 1, 2, 3) are found from fifteen to eighteen stiff 

 hairs, which diverge from each other at the apex, and have their extremities roixnded 

 off, being especially blunted on the antennae. In the perfect insect these are altogether 



VOIi. XXIII. u 



