and, smooth or simple Eyes of Insects.- 281 



•nly receive the impression from it by the nervous filaments fur- 

 nished by its spreading. 



From what has been said, it is evident that the eye in insecti 

 does not present so complex a structure as the same organ in ani- 

 mals of a superior order. In the eyes of insects we do not observe 

 different humours which by their density can change the direc- 

 tion of the rays of light, by operating a refraction, always in 

 proportion to the nature of the mediums which they pass through. 

 Nevertheless, if the structure of the eye is very simple in this 

 order of animals, it seems to accommodate itself very well with 

 the properties and laws of light, as well as with the object which 

 this organ ought to attain. In fact, the cornea, by its transpa- 

 rency, its convexity, and its more consideraljle density than thai 

 of the air, gives in the first place, on account of its transparency, 

 a passage to the rays of light, while, as a consequence of these 

 two other properties, the ravs undergo a refraction, which tends 

 to render convergent tlie broken fasciculi, and to approximate 

 them to the perpendicular. As the rays which traverse the 

 cornea strike at the same time the nervous filaments, or the 

 retina of the facets of the eye and the tunic of the cornea, they 

 may be perceived by the nerves, and tlie latter may transmit the 

 impression which they experience to the expansion of the optic 

 nerve. It would, it seems, be in this point that all the rays 

 should be concentrated, in order to form the image ; but as, in 

 order to do this, they would have to pass through an opaque mem- 

 brane, it is difficult to admit it. \^'e must, therefore, suppose 

 that the sensation is there concentred; and probably this species 

 of retina has no other use than that of centralizing and bringing 

 to one point all the impressions perceived by the optic filaments. 



The protuberance of the cornea seems to be frequently in in- 

 sects in the ratio of the smallness of their eyes : and this ratio 

 is any thing but indifferent. In short, the more protuberant the 

 .cornea, the more numerous are the incident rays; consequently, 

 the rays which reach the nervous filament being in greater num- 

 ber, vision is produced with a force similar to that which a 

 greater dimension in the eyes could excite ; with this difference, 

 however, that insects in this case do not see so far. We may 

 cite as a proof the locnsta lilifolia and acuminata of Fabricius. 



Lastly, it remains to make known the singular arrangement 

 observed in the compound eves of the liieUida vulgaris, and of 

 the locui(a lilifolia, as well as to say a few words on the struc- 

 ture of the eyes of the bicifugce species. The compound eyes 

 of the libellula vulgaris and of the Locusta idifoiia present a 

 singular arrangement on account of the tunic of their cornea, 

 which is red in the upjier and posterior parts, and of a clear 

 green in the inferior and anterior parts. These two colours 



are 



