PART IV -THE SENSES OF SPIDERS AND THEIR 

 RELATIONS TO HABIT. 



CHAPTER X. 

 THE SENSES OF SPIDERS, AND THEIR ORGANS. 



I. 



A SPIDER'S eye is a globose object or capsule, lined internally with pig- 

 ment and having the outer surface transparent and convex, constituting 

 the cornea or corneal lens. Behind the lens is the optic cham- 

 .p pl ' ber, filled with a semiliquid and lined on the hinder part with 

 the retina, which receives and reverses any image, as in the 

 human eye. 



The eye of a spider corresponds with the ocellus of insects, and speak- 

 ing generally, the ocellus may be regarded as consisting, first, of a lens, 

 forming part of the general body covering; second, of a layer of trans- 

 parent cells; third, of a retina or second layer of deeper lying cells, each 

 of which bears a rod in front, while their inner ends pass into the fila- 

 ments of the optic nerve; fourth, of the pigment. From the convexity of 

 the lens it would have a short focus, and the comparatively small number 

 of rods would give but an imperfect image, except of very near objects. 

 Though these eyes agree so far with ours, there is an essential difference 

 between them. It will at once be seen that the pigment is differently 

 placed, being in front of the rods, while in the vertebrate eye it is behind 

 them. Again, the position of the rods themselves is reversed in the two 

 cases. 1 



In details, the structure of fully formed ocelli presents many differences ; 

 and it is remarkable that in some species this is the case even with the 

 eyes of the same individual, as in those of the well known En- 

 glish Orb weaver, Epeira diademata. (Fig. 282). The figure, 

 which is taken from the admirable work of Grenadier, 2 represents a section 

 through the front (A) and hinder (B) dorsal eyes. In this case the eye B 

 would receive more light and the image therefore would be brighter, but on 

 the other hand the image would be pictured in greater detail by the eye, A. 



1 Sir John Lubbock, "On the Senses, Instincts, and Intelligence of Animals," page 147. 



2 H. Grenacher, "Untersuchungen iiber das Sehorgan der Arthropoden," Gottingen, 1879. 

 I have made the cut from Lubbock' s book. 



(283) 



