EYES 



983 



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 A 



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 25,001). 



The structure of the arthropod eye 



is best explained by a comparative 



account of the various stages of 



complication which it presents. 



In various larva? the cuticular 



layer is modified to form a single 



lens, behind which are simple, sepa- FIG. 732. Diagram of a section of the 



rate, elongated hypodermic cells, 

 some of which are continuous with 

 fine branches of the optic nerve ; 

 these may be called retinal cells. 

 The next stage in complication is 

 seen when the.sc last combine to form 

 groups, ' retinuhe ; ' the sensitive 

 cells may become divided into two 

 regions, an outer one, which is 

 ' vitreous ' and refractive in functi< in, 

 while the inner part remains sensi- 

 tive ; the corneal surface may In- 

 come broken up into a number of 

 facets, each of which 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 

 no rays, save those which pass in 



composite eye of Melolontha vul- 

 t/iiriij (cockchafer) : a, facets of the 

 cornea; I, transparent pyramids 

 surrounded with pigment ; c, fibres 



of the optic nerve ; 

 optic nerve. 



trunk of the 



the axes of the pyramids, can reach 



the fibres of the optic nerve. Hence, 



it is evident that, as no two ocelli 



on 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 



Fit.. 7o8. Part of the compound eye 

 of Ph )//'/"'"" ; the retinal cells are 

 seen to be united into a retinula (/) 

 which is differentiated into a rhab- 

 dom (m) posteriorly; cc, crystalline 

 cone; /, facet of compound eye : 

 2->g, pigment. (After Grenadier.) 



