and smooth or simple Eyes of Insects. 189 



this membrane ends at the place where the nerve itself divides 

 into these six branche.?. We do not know positively, iF this ar- 

 rangement exists in the spiders: v,-e only know that it does not 

 hold in the scorpions. Besides, externally the eves of the spiders 

 and of the scorpions have the same structure and figure with 

 the simple eyes of insects. 



Immediately after the optic nerve and the tunic of the cornea, 

 we observe a peculiar membrane which we shall consider as a 

 kind of choroid. It exhibits, however, this difference from the 

 choroid of tbe compound eyes, tliat it has no varnish so distinct; 

 and finally, its breadth is always greater than the circumference 

 of the cornea itself. This membrane, coloured frequently in red 

 or black, is also som.etimes colourless ; and lastly, its whiteness 

 is so dull that it is easy to distinguish it from the tracheae. The 

 thickness of this membrane is great enough to make it resist 

 even a long maceration. 



In the species in which the simple eves are whitish, we see this 

 membrane clothed with a coating of colours varying with the 

 colours of the tunic which covers the choroid. This arrange- 

 ment is even very perceptible at the exterior of the eye. 



It would seem that this choroid is particularly \ cry thick and 

 white in such species as have their cornea concave and as if 

 sunk. This arrangement must have taken place on account of 

 the divergency undeigone by the rays of light in arriving on a 

 concave and transparent surface, their direction even being pa- 

 rallel, a divergency which is such that we may very well con- 

 ceive that the eye would be too much affected by it, if a black 

 membrane had also absorbed a certain number of Ii.nninous rays. 

 A white membrane, sending back on the contrarv all the rays 

 which it receives, may augment the excitability of the optic nerve, 

 and thus contribute to render the vision more distinct. Al- 

 though this species of choroid presents in certain circumstances 

 a very great whiteness, it is nevertheless always easy to distiji- 

 guish it from the tracheae, the only vessels with which it can be 

 confounded. For, as little as we are in the habit of seeing the 

 organs of insects, we can easily ascertain the tracheae from their 

 azure colour; and besides, by dissecting them in water (the most 

 advantageous way of acting in these experiments) we see the 

 tracheae rise above the surface of the liquid ; and from that mo- 

 ment we can no longer have the smallest doubt, for the tracheae 

 are the only organs which have so little specific gravity. 



This choroid, the membranes of which are close and very 

 thick, seems formed by a cellular texture of very close meshes, 

 over which a heap of trachea; are distributed. It is therefore 

 underneath the cornea that the optic nerve is situated, which 

 Msuing from the anterior faces of the brain proceeds to the mid- 

 dle 



