292 ANATOMY. 



spending with the entire circumference of the eye, but proportionately 

 less convex. Both hemispherical divisions of the glassy body are of a 

 different convexity, and, indeed, the upper is the flatter, and the lower 

 the most convex side. The rete or superior bowl-shaped distended 

 end of the optic nerve spreads itself at the posterior margin of the 

 glassy body. It closely embraces this body, which lies in it as in a 

 shell. It is again exteriorly covered by the pigment. This bends 

 itself in the entire circumference of the eye, up to the horny tunic, and 

 forms around the lens a small iris beneath that tunic. Where the 

 optic nerve spreads into the rete, the pigment covers it, but thus far it 

 comes entirely free from the cerebrum, as was shown above. The 

 pigment varies much in colour: in the majority of cases it is of a brown 

 red or dark cherry brown, sometimes black, or of a bright blood red. 

 In this case, or, rather, in general, the margin lying next to the horny 

 integument shines through it, and thus forms in the circumference of 

 the lens a beautifully coloured iris. It is more evident in the large 

 eyes of the scorpions and of the Solpugce, but even the small eyes of 

 insects exhibit an annular iris. 



194. 



The presence of compound eyes is shown by the above Table. 

 Regaining their structure, the horny integument consists of many small 

 hexagonal surfaces, which correspond exactly with each other, and 

 cause the hemispherical, or, at least, convex figure of the superior 

 surface of the eye. Each of these hexagonal facets, the number of 

 which varies, and is sometimes very considerable, as the following list 

 of them shows, 



Mordella .... 25,088 



LibeUula .... 12,544 



Papilio 17,355 



Sphinx Convolvulus . 1,300 



Cossus ligniperda . . 11,300 



(Estrns 7,000 



Liparis Mori . . . b',236 



Musca Domestica . . 4,000 



Formica 50 



forms a distinct lens, convex on both sides, varying in thickness. The 

 proportion of its thickness to its transverse diameter is, for example, in 

 a sphinx, 1 : 2 ; in others, this lens is still thicker, which is especially 



