1 1 8 Memoir vpon the compound and smooth Eyes of Insects. 



If we study the compound eye of certain lamelllcorn coleop" 

 terce, as, for examjile, that of the nasicorn geotnipce awAsilevce, 

 we observe that its form is like a heart, and that it is divided into 

 two in its upper part, by the portion of the cranium which sup- 

 ports the horn. The cornea is tolerably thick, and divided, as 

 usual, into hexagonal facets : under this membrane we ob- 

 serve the mucous tunic, the optic filaments, the varnishing of 

 the choroid, and that membrane itself, black like the tunic of 

 the cornea. 



The circular trachea exists, and the great optic nerve receives, 

 besides, an infinity of other tracheas, several of which accompany 

 the small optic nerves, and reach the cornea by some subtile 

 ramifications. The roots or principal trunks of the trache<£ are 

 placed under the optic nerve, and are attached in the first in- 

 stance to the tunic -vvith v/hich it is covered. They derive their 

 origin from a considerable branch adhering below the principal 

 trachea, and surrounded with other small similar tracheae. The 

 geotnipce, flying only at sunset, present the same organization 

 in their optic nerve with most of the Ivc'ifugce. Their principal 

 retina or great optic nerve ajiproaches nearer to the cornea than 

 in the species wliich fly about in open day. Thus the eyes of 

 the giant scarite become after the death of the insect of a 

 reddish white, and yet they are black when the insect is alive. 

 This whitish colour is owing to an alteration in the tunic of the 

 corner; but as this alteration does not act upon the mucous 

 varnish of the choroid, the latter remains black, of which we 

 may be convinced on removing the cornea with its tiuiic. This 

 appearance is also very common in an infinite number of cole- 

 opterce, and even in several other families. In general the cornea 

 is very thick in the eyes of the colcopterce : a few only of tlie 

 hymenopterce (and in particular the apis violacea) present this 

 membrane of an equal thickness. When the coleopterce have 

 vesicular tracheae only, which takes place in almost all the la~ 

 mellicorn insects, they want the circular trachea which generally 

 surrounds the optic nerve. To conclude : the optic nerve is 

 always surrounded with tracheae even in a considerable number, 

 and these tracheai form most frequently several vesicles, which 

 leave some interval between them, an interval generally very 

 small. Nevertheless, in the species having the tunic of the 

 cornea of a clear colour, instead of being black, we see on the 

 exterior part of the eye a single point of the same colour cor^ 

 responding to the aperture through which the optic nerve passes; 

 or rather we distinguish several points, as v\e shall mention 

 when we come to speak of the butterfly. 



[To be continued.] '' 



XXI. Re-^ 



