334 ARACHNIDA- ARANEAE 



touch. Whether or not they have a true auditory sense is still a 

 matter of doubt. Since sounds are conveyed by vibrations of the 

 air, it is never very easy to determine whether responses to sounds 

 produced near the animal experimented upon are proofs of the 

 existence of an auditory organ, or. whether they are only per- 

 ceived through the ordinary channels of touch. In any case, the 

 organs of hearing and of smell have not yet been located in the 

 Spider. M'Cook considers various hairs scattered over the body 

 of the spider to be olfactory, but from Gaskell's researches upon 

 allied Arachnid groups it would seem that the true smelling organ 

 is to be sought for in the rostrum. 



Eyes. Spiders possess from two to eight simple eyes, the 

 external appearance and arrangement of which have already been 

 briefly explained. They are sessile and immovable, though often 

 so placed as to command a view in several directions. In structure 

 they are essentially like the ocelli of Insects. Externally there, is 

 a lens, succeeded by a mass of transparent cells, behind which is 

 a layer of pigment. Then come the rods and cones of the retina, 

 to which the optic nerve is distributed. A comparison of this 

 with the arrangement in the Vertebrate eye will show a reversal 

 of the positions of the retina and the pigment-layer. The lens 

 is part of the outside covering of the animal, and is cast at the 

 time of moulting, when the spider is temporarily blind. It is 

 stated, however, that the eyes do not all moult simultaneously. 

 There is often a considerable difference between the various eyes 

 of the same spider, especially with regard to the convexity of the 

 lens and the number of rods and cones. 



Though most spiders possess eight eyes, the number is some- 

 times smaller, and in some groups of eight-eyed spiders two of 

 the eyes are sometimes so reduced and degenerate as to be prac- 

 tically rudimentary. As might be expected, Cave-spiders (e.g. 

 Anthrobia mammouthia) may be entirely sightless. 



Touch. The sense of touch would appear to be extremely 

 well developed in some spiders, and there is reason for believing 

 that the Orb-weavers, at all events, depend far more upon it than 

 upon that of sight. 



Among the hairs which are distributed over the spider's body 

 and limbs, several different forms may be distinguished, and some 

 of them are undoubtedly very delicate sense-organs of probably 

 tactile function. 



