284 



AMERICAN SPIDERS AND THEIR SPINNINGWORK. 



Ok' *C 



Fig. 283 is a drawing of a section through an anterior median eye of 



Agalena naevia eight days after hatching multiphed about three hundred 



and fifty limes. Tlie retinal portion has not readied its full development, but 



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



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



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



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



- _ magnified portion of the hypodcrmis, with which it has never 



ceased to be continuous; third, of the hy[)odermis; and, linally, 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, which is per- 

 ceived by the fine 

 terminations of the 

 optic nerve. 



From the nature 

 of the external in- 

 tegument, the eyes of 

 spiders would seem 

 to be fixed in one di- 

 rection. Yet micro- 

 scopic examination 

 of them when alive 

 apj^eared to satisfy 

 Mr. Campbell'^ that 

 spiders not only have 

 an adjusting power 

 over the lenses, but 

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

 transparent cuticle. This api)eared to the author to be the only way to 

 account for the frequent clianges 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 me unfounded. 



<k lip. ■"■ K'oj, 



Fig. 282. Long section through the front (A) and hinder (B) dorsal eyes of 

 Epeira diadeniata. (After Grenacher.) A, anterior eye ; B, posterior eye ; 

 Hp, hypoderm ; Ct, cuticle ; ct, boundary membrane ; K, nuclei of the cells 

 of the retina ; M. muscular fibres ; M, M ' , cross sections of ditto ; St, rods ; 

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

 Rt, retina ; Nop, optic nerve. 



II. 



How far Orbweavers may be guided by sight 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 nsevia, Bulletin Mub. 

 Compar. Zool. Harv. Coll., Vol. XII., No. 3, plate x.. Fig. 69. 

 * Observations on Spidcif, page 42. 



