a?id smooth or simple Ej/es of Insects. 115 



the colour of this varnish be always the same with that of the 

 choroid ? This single fact demonstrates, in my opinion, an 

 analogy between the choroid and its varnish : it would even seem 

 that the latter is produced by a kind of transudation which takes 

 place through the meshes of this membrane itself. 



If the varnish of the choroid be formed l)y a transudation of 

 this membrane, it is evident that this varnish ought always to 

 rover it, and this is precisely what is observed. But this varnish, 

 as well as tlie choroid itself, does not ahvays exist: it would 

 veem even that all the species which shun the light are totally 

 deprived of it. At least I have not observed it in the /laps, the 

 pedinus, and most of the ieneluios: it is wanting beyond doubt 

 in the blatl(e, which, as is vrell known, are stupefied with the 

 light of tiie sun. When the varnish and the choroid exist, it is 

 not very ditiicult to separate this membrane from the varnish. 

 It seems then to be most generally cellular; and in certain species^ 

 the longitudinal fibres of which it is composed are sufficiently 

 decided to make it appear slightly streaked, on account of the 

 tracheae which are distributed over it : tliis is distinctly observa^ 

 ble in the traxalcp. This arrangement has been well described 

 by M. Cuvicr in his Memoir on the Nutrition of Insects, p. 42, 

 tab. i. fig. .3. This organisation might induce a belief that the 

 choroid is formed by the prolongation or union of the small 

 tracheae furnished by the large circular trachea: at least, all the 

 tradiese. which proceed to this membrane are totally lost in it; 

 and as we cannot recover them, after they reach it, we ought 

 to regard them as contributing to form the choroid. This 

 memijrane is therefore composed of a cellular texture tolerably 

 close, on which there exists a collection of tracheae furnished by 

 the large circular trachea, and which are imbued with, or rather 

 deeply penetrated bv, a blackish varnish. The choroid is more 

 or less black, but always opaque ; and its sombre tir't is as con- 

 stant as its opacity. Fi:ially, the longest macerations cannot 

 make it lose its colour or opacity, as we have already observed. 



The choroid of th.- compound eyes of insects has been i-e- 

 garded bv Swammordavu as an uvea. It would seem, however, 

 that it cannot be called an uvea, as it does not occupy the bot- 

 tom of the eye of insects. However this may be, this mem- 

 brane is attached by its circumference to the whole of the edge 

 of the cornea, and conse([Uently follows the contours of this 

 same membrane. It is smrounded by a large circular trachea, 

 in general furnished by the tracheal artery of Swammerdam, 

 but varying as to its arrangement in the different genc-a. For 

 ijwtam'e, in the gnjllui and truralis, it is the third principal 

 division of this same trachea situated in the head, which, whea 

 it arxives at tlie eye, becomes l)ifurcated, and the two tracheae 



II 2 which 



