^.90 Memoir upon the compound and simple Eyes of Insects. 



die of the eve. It does not appear that this optic nerve has 



any swelling at its base, or that a kind of retina is formed ana- 

 logous to that of the compound eyes. In some species it ap- 

 pears to me that this nerve becomes broader towards its ex- 

 tremity, i. e. at the point where it corresponds to the cornea. 

 I shall not, however, assert this positively, for it is possible that 

 this dilatation may depend on the contraction produced by the 

 section of the nerve. As I have not dissected these organs in the 

 living subject, (which ought always to be done for the sake of 

 precision,) I have not been able to verity, whether this dilatation 

 depended on the sensibility of the nervous organ. However the 

 case may be, the optic nerve before reaching the cornea tra- 

 verses the muscles of the various parts of the head, sometimes 

 enters a trachea, penetrates afterwards through the choroid and 

 its varnish, and afterwards, when surrounded with the tunic of 

 the cornea, corresponds to the internal surface of this membrane, 

 on which it seems to lose itself. 



As the cornea of the simple eyes is not, like that of the com- 

 pound eyes, divided by nvmierous iacets, it has not been necessary 

 that the small nerves should give it a great number of small 

 threads, but only that they should proceed to the poin.t where the 

 divergency of the rays of light should be the lea'^t considerable. 



The small optic nerves which proceed to the simple eyes issue 

 from the brain. As their position varies in regard to the other 

 parts, they are sometimes the second, third, fourth, or fifth pair 

 of nerves : this depends on their situation with respect to the 

 various organs of the head, a situation which determines by what 

 pair they are furnished. These nerves are directed always to- 

 wards the simple eyes, being retained in their position either by 

 pneumatic pouches, or by tracheae, proceeding more or less 

 obliquely according to their porition with respect to the brain. 

 They always terminate under the cornea, forming a kind of re- 

 tina. 



It residts from this description, that the simple eyes are formed 

 quite differently from the compound eyes. Thus the cornea of 

 the former is all of one piece, whereas in the latter it is formed 

 bv the union of a great number of hexagonal facets. As to the 

 tunic of the cornea, it does not exhibit any difference in the two 

 kinds of eyes, except perhaps in point of thickness; but with re- 

 spect to the varnish of the choroid, it is always less distinct in 

 the simple than in the compound eyes. Finally, the choroid is 

 not alwavs black in the simple eyes, whereas it is uniformly in 

 the compound. Sometimes this membrane, instead of being 

 black and opaque, is of a peculiar whiteness and lustre,. To con- 

 clude : this membrane and its tmiic are placed in the same way 

 with respect to the other parts in both kinds of eyes, and their 



uses 



