EYES 



983 



cornea; 6, transparent pyramids 

 surrounded with pigment ; c, fibres 

 of the optic nerve ; d, trunk of the 

 optic nerve. 



or squares; each facet is the ; corneule ' of a separate ocellite, and 

 has a convexity of its own; hence, by counting the facets, we 

 can ascertain the number of ocelli 

 in each ; compound eye.' In the two 

 eyes of the common fly there are 

 as many as 4,000 ; in those of the 

 cabbage-butterfly there are about 

 17,000; in the dragon-fly 24,000; 

 and in the Mordella beetle 25,000. 

 The structure of the arthropod eye 

 is best explained by a comparative 

 account of the various stages 'of 

 complication which it presents. 

 In various larvse the cuticular 

 layer is modified to form a single 

 lens, behind which are simple, sepa- FlG . 73 2.-Diagram of a section of the 

 rate, elongated hypodermic cells, composite eye of Melolontha vul- 

 some of which are continuous with 9 aris (cockchafer) : a, facets of the 

 fine branches of the optic nerve ; 

 these may be called retinal 'cells. 

 The next stage in complication is 

 seen when these last combine to form 

 groups, * retinulse ; ' the sensitive 

 cells may become divided into tw r o 

 regions, an outer one, which is 

 ' vitreous ' and refractive in function, 

 while the inner part remains sensi- 

 tive ; the cornea! surface may be- 

 come broken up into a number of 

 facets, each of wilich corresponds to 

 one of the ' pyramids ' so formed, 

 and within the retinula there may 

 be differentiated a rhabdom (see fig. 

 733) formed by the nerve-rod. 



After traversing the pyramids 

 the rays reach the extremities of 

 the fibres of the optic nerve, which 

 are surrounded, like the pyramid, 

 by pigmentary substance. Thus the 

 rays which have passed through the 

 several ' corneules ' are prevented 

 from mixing with each other; and 

 110 rays, save those which pass ill FlG - 733. Part of the compound eye 

 , , J ' f , i . , of Phniqanea ; the retinal cells are 



the axes of the pyramids, can reach seen to be united into a retinula (r) 



the fibres of the optic nerve. Hence, 



it is evident that, as no two ocelli 



011 the same side (fig. 731) have 



exactly the same axis, no two can 



receive their rays from the same point of an object ; and thus, 



as each compound ' eye is immovably fixed upon the head, the 



combined action of the entire aggregate will probably afford but 



which is differentiated into a rhab- 

 dom (m) posteriorly ; cc, crystalline 

 cone ; f, facet of compound eye ; 

 pg, pigment. (After Grenacher.) 



