140 



INTRODUCTION 



of vision, must fall upon one or several of them in the direction of their axes, 

 A faceted eye, the surface of which frequently occupies more than a hemisphere, 



consists firstly of a number of hexagonal, 

 transparent conieae, or facets, which, while 

 moderately flat externally, frequently 

 present lenticular prominences internally, 

 and are separated from one another by 

 shallow CTOoves. Behind each cornea 



Fig. 57 a. Head of a Drone seen from the front, 

 showing the compound eyes, the three ocelli, and the 

 antennae. (After Swammerdam.) 



there is a transparent refractive organ 

 the ayslallme cone surrounded by a dark, 

 funnel-like, pigment sheath. Internal to 

 these two zones is the third and last 

 layer, that of the visual rods. The narrow, 

 stalk-like, internal end of the crystalline 

 cone is enclosed by a funnel-shaped de- 

 pression at the external end of the visual 

 rod, so that the two are directly con- 

 nected. The large compound visual rods 

 make up a hemispherical retina convex 

 externally, and this extends to the bulbous 

 expansion of the optic nerve, which 

 receives external impressions and trans- 

 mits them to a ganglion corresponding 

 to the brain of higher animals, where 

 they result in sensations. 



Each facet, therefore, with its visual 

 rod forms an independent eye, connected 

 with others only by means of the common 

 nerve-trunk. If, now, a reversed and 

 reduced image of the surroundings is 

 formed behind each facet (Gottsche), 

 which is convex internally, this image is 

 remote from the irritable part of the 

 visual rod, and only its vertically incident 

 axial ray (strengthened by refraction) can 

 be perceived, as all the lateral rays are 

 absorbed by the pigment. The im- 

 pressions produced by such axial rays, 

 the number of which corresponds to that 

 of the individual nerve-rods, consequently 

 form a kind of mosaic, which repeats 

 upon the retina the arrangement of the 

 points of the external object from which 

 light is received. The picture thus pro- 

 jected, however, is deficient in brightness and detail (Glaus, ' Lehrbuch der Zoologie,' 

 5th ed., 1 89 1, p. 84). 



-K 



r 



-B 



'I 



Fig. 57 b. Three facets with retinulae from the 

 compound eye of the Cockchafer : the pigment has 

 been removed from two of them. F, corneal 

 facet ; K, crystalline cone ; P, pigment sheath ; 

 /", pigment cells; /"', pigment cells of the second 

 order ; R, retinulae. (After Grenacher.) 



