and smooth or simple Eyes of Insects. 117 



the fourth, or the fifth pair of nerves furnished by this organ. 

 The optic nerve at its origin is a little cylindrical, and, directing 

 itself laterally, enters soon after its origin into the small circular 

 trachea when it exists; and, when it does not exist, between the 

 filaments of the adductor muscle of the mandible, which forms 

 a kind of circular aperture for its passage. Gradually this nerve 

 expands, and forms a cone, which has its base on the cornea 

 and its summit on the brain. This expansion is greater or less 

 according to circumstances. The libellidoe, the lamellicorn 

 coleopterce, most of the Upidopterce, as well as the gryllus, the 

 inixalis, and the mantis, present it almost the whole length of 

 the cornea ; whereas in the greater number of species in which 

 the cornea is not spherical, this breadth is in general much less, 

 and that in a very decided manner. It is from this expansion 

 that a very considerable number of nervous filaments issue, which ■ 

 proceeding between the trachea; fuinished by tlie large circular 

 trachea, form the network which we have already mentioned. 

 These are the filaments which, traversing the choroid and its 

 varnish, as well as the tunic of the cornea, go to form the par- 

 ticular retina of each facet, each penetrating into the hollow 

 of one of these facets, in order to receive the impression of the 

 light which they transmit to the brain. We see at the exterior 

 of the compound eyes a black point which seems moveable, and 

 the apparent mobility of which is owing to a cause which we 

 shall explain when speaking of vision in general. In order to 

 resume this description, we may observe that, according to the 

 conformation of the compound eyes, the union of the small 

 facets forms altogether the first membrane or transparent 

 cornea, and that, besides, each of these facets may be consi- 

 dered as itself a cornea. The nervous filaments which pass 

 through the tunic of the cornea are probably the particular re- 

 tina of each facet. As to the tunic with which they are as it were 

 surrounded, and which everywhere fringe the coinea, its chief 

 use must be to diminish the impression of the luminous rays, 

 an impression the stronger because it takes place in an imme- 

 diate manner. The blackish varnish which covers the opaque 

 membrane, situated under the tunic of the cornea, may with 

 great probability be assimilated to the varnish of the choroid, 

 as the membrane itself may be to the choroid. Finally, the ex- 

 pansion of the optic nerve applied under the choroid, must be 

 considered with M. Cuvier as a true nervous membrane perfectly 

 similar to that of the red-blooded animals. 



After having given a general description of the compound 

 eyes of insects, it only remains for us to describe some pecu- 

 liarities of organization which different species present. 



H3 If 



