ANATOMY AND PHYSIOLOGY 



III 



FIG. 141. 



and retinal), (5) fenestrate membrane, (6) fibers of the optic 

 nerve, (7) trachea. 



The cornea (Fig. 142) is a biconvex transparent portion 

 of the external chitinous cuticula. Immediately beneath it are 

 the cone cells, which may contain a 

 clear fluid or else, as in most insects, 

 solid transparent cones. The rhab- 

 dom is a transparent chitinous rod 

 or a group of rods (rhabdomeres) 

 situated in the long axis of the 

 ommatidium and surrounded by 

 greatly elongated cells, which 

 constitute the retinula. T w o 

 zones of pigment are present : an 

 outer zone, of iris pigment, in 

 which the pigment in the form of 

 fine black granules is contained 

 chiefly in short cells that surround 

 the retinula distally; and an inner 

 zone of retinal pigment, in which 

 the pigment cells are long and 

 slender, and enclose the retinula 

 proximally. All these parts are hypodermal in origin, as is also 

 the fenestrate basement membrane, through which pass tracheae 

 and nerve fibers. The nerve * fibrillse, which are ultimate 

 branches of the optic nerve, pass into the retinal cells the end- 

 organs of vision. Under the basement membrane is a fibrous 

 optic tract of complex structure. 



Physiology. After much experimentation and discussion 

 upon the physiology of the compound eye the subject of the 

 monumental works of Grenacher and Exner Miiller's " mo- 

 saic " theory is still generally accepted, though it was proposed 

 early in the last century. It is thought that an image is 

 formed by thousands of separate points of light, each of which 

 corresponds to a distinct field of vision in the external world. 



nc 



Portion of compound eye of 

 fly, Calliphora vomitoria, radial 

 section, c, cornea; i, iris pig- 

 ment; n, nerve fibers; nc, nerve 

 cells; r, retinal pigment; t, tra- 

 chea. After HICKSON. 



