188 Memoir upon the compound 



form : it is not composed, like that of the compound ej'es, of an 

 infinite number of facets, but rather of a membrane of a single 

 piece, anrl on which no divisions are perceptible. We may con- 

 sider this first membrane as a cornea, on acconnt of its trans- 

 parency, a transparency which we observe even at the exterior 

 of the eye, on account of the little colour of the tarnish applied 

 on tlie membranes situated under it. 



After having removed the cornea, we find a vii^cous tunic more 

 or less tliick, and of vvliich the colour also undergoes consider- 

 able variations. Thus this tunic or varnish presents the most 

 opposite colours: it is ahnost always black in tiie hymen op terce, 

 whereas it is whitish in tlie orthopterce. !n the caterpillar, this 

 varnish is frequently black, yellow, or red, and sometimes it is 

 of the most beautiful brilliant green. This viscou'? tunic has 

 been regarded by Swammerdam as a kind of uvea, although it 

 seems very like the tunic which covers the cornea in compound 

 eyes. 



We ought not, however, to decide that the tunic of the cornea 

 of the simple eyes is distinct from the varnish of the choroid. 

 This appears probable notwithstanding, since in certain species 

 the external colour of the simple eyes is not the same with that 

 of the varnish with which the choroid is covered. Thus much 

 is certain, that the cornea of the simple eyes is always coated 

 internally with a kind of varnish, the colour of which appears 

 at the exterior of the eye on account of the transparency of the 

 cornea. This coating surrounds the optic nerve, which proceeds 

 into the concavity of the cornea after having passed through the 

 choroid and its varnish. Swammerdam very properly remarks, 

 that the smooth eyes of insects receive nerves which are ftir- 

 nished to them by the cerebriform ganglion lodged in the head. 

 Lyonnet, whose researches announce so much sagacity, has also 

 observed that the simple eyes of the willow caterpillar are com- 

 posed of a cornea and a choroid traversed by the extremity of 

 tlie optic nerve*. 



The optic nerves of the simple eyes issue immediately from 

 the brain, when these eyes are removed far enough from each 

 other; but when, on the contrary, they are close together, as 

 observed in the caterijillar and a great number of larvae, the 

 optic nerves are only divisions of a larger nerve which issues di- 

 rectly from the brain. 



Lyonnet has described this arrangement accurately, and the 

 figure which he gives of it, Plate XVIII. No. 6, is very correct. 

 Then there exists a peculiar membrane in the form of a funnel, 

 to which the six branches of the optic nerve are attached, and 



* Truilc Auutomique de la Chenille du Suule,j)a7' Lr/onnet, p. 570. 



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