284 



AMERICAN SPIDERS AND THEIR SPINNINGWORK. 



Fig. 2S3 is a drawing of a section through an anterior median eye of 

 Agalena naivia eight days after hatching multiplied about three hundred 

 and fifty times. The retinal portion has not reached its full development, but 

 on the whole in this stage the essential features of the eye are established. ^ 



It will be seen that in this species, according to Locy, the eye consists, 

 -first, of the lens, which a few days after hatching assumes the form of the 

 cuticular lens of tlie adult ; second, the vitreous body, which is the 

 Structure „^,,gj^it^y,i portion of the hypodermis, with which it has never 

 ° ^^^' ceased to be continuous; third, of the hypodermis; and, finally, of 

 the retina, consisting of the first or inverted layer of optic invagination, 

 and the second or non inverted layer of optic invagination. 



Speaking roughly, the eyes of spiders and ocelli of insects may be said 

 to see as our eyes do ; that is to say, the lens throws on the retina an 



image, wliich is per- 

 ^^^^-^^^^^ ^ ceived by the fine 



; j ~*^^'" J^ ^ ^.£ ,..•''%' of them when alive 



'^' "*■ "'^ .,„ n. . r ap].eared to satisfy 



Fig. 282. Long section Uirough the front (A) and hinder (B) dorsal eyes of -J. ,, ,,., ,, 



Epeira diademata. (After Grenacher.) A, anterior eye ; B, posterior eye ; Mr. Oamjibell- that 



Hp, hypoderm ; Ct, cuticle ; ct, boundary membrane ; K, nuclei of the cells ^nidei'S UOt Onlv lulVC 



of the retina; M, muscular fibres; M, M', cross sections of ditto; St, rods; 1 '^ '_ J 



Pg, P', pigment cells; L, lens; Gk, vitreous body ; Kt, crystalline cones; jvU adjusting pOWCr 



Bt,retina; Nop, optic nerve. ^^^^j. ^^^^ lenSCS, but 



that they also can move the eye itself within the cavity covered by the 

 transparent cuticle. This appeared to the author to be the only way to 

 account for the fretpient changes of color, as well as of the form and posi- 

 tion of the color, which take place in spiders' eyes, and which resemble 

 that of a moving liquid globule. This opinion seems to mc unfounded. 



n. 



How far Orbweavers may be guided hy siglit in making their webs, 

 and how far by touch, is an open question. The organs of sight do not 

 seem to be as highly organized in these and other Sedentary spiders as 

 in the Wanderers. 



' William A. Locy, Observations on the Development of Agalena njevia, Bulletin IMus. 

 Ctompar. Zool. Harv. Coll., Vol. XII., No. 3, plate x., Fig. G'J. 

 ^ Observations on Spiders, page 42. 



