110 Memoir upon the eompouni 



Placed in orbitary cavities, they are protected not only by the 

 sides oi these cavities, sufficiently hard to oppose the impression 

 of external bodies, and by their external membranes, which are 

 almost as hard as the slselly envelope which covers the bodv, but 

 also internally by soft parts which seem to consist of tracheae 

 oidv. In the animals with vertebroe, the eves are besides pro- 

 tected l)y several soft parts like the cartilages, the membranes of 

 the eyei>rows and eyelids, which conceal thorn under a tliifk veil, 

 and defend them from external objects. These parts, called by 

 Haller feidamina oadi, are totally wanting in insects ; and l>c- 

 sides, after l;no.ving the conformation of their e\^s, of what use 

 could they be ? 



We must however observe that some insects present in their 

 cornea, ai.d in general or the furrows which separate the hexa- 

 gonal facets of this membrane, fine hairs, more or less long, 

 and more or less thick. These have been regarded by Sv/am- 

 mcrdam and other anatomists, as the eyelashes of the eyes of 

 insects, although they do not seem to perform their office : in 

 fact, they appear to be so little essential that they are wanting 

 in the greater number; and when they are obseived, they are 

 always arranged on the lower y>art of the eye, a position which 

 is far from being the most favourable to guard it against the ad- 

 mission of foreign bodies. Besides, scarcely auv but the most 

 glutinous bodies can adhere to the convex or jjolished surface of 

 the eyes of insects; and when they do, the animal can easily remove 

 them with its fore legs. This is frequently observable in bees 

 and flies, which take a pleasure in repeating the operation. 



The situation of the eyes, or rather their position, is very va- 

 riable in the different classes. As this position has a great in- 

 fluence in vision, we shall describe it in the chief families. 



The most external membrane of the compound eyes is hard 

 and transparent : it might therefore either he called the sclero- 

 tica on account of its hardness, or travsptmnt cornea from its 

 transparency. This last denomination agrees with it perfectly ; 

 for,as Swammerdara remarks, it has the flexibility, the firmness, 

 and transparency of horn. This cornea or sclerotica, comex 

 externally and concave internally, is formed by on infinity of 

 hexagonal facets arranged with the greatest regularity along- 

 side of each other. These facets, divided or separated by 

 furrows whicii always follow the direction of the cornea, exhibit 

 in some insects, the hiimenoptarce in particular, hairs which 

 resemble down. As to the lines or furrows of the facets of 

 the cornea, they are curved and a little folded on account of the 

 spherical convexity of the cornea, which interweaves in difi'erent 

 places the hexagonal facets with the lines which separate 



them. 



