12 INSECTS. 



concluded from the appearance of a lateral section 

 of the cornea, in which the convexities of the 

 sides of each lens have appeared similar*. The 

 celebrated Swammerdam is of opinion that vision 

 is not performed by each lens as a separate eye, 

 as in quadrupeds, &c. but that it is excited, as he 

 expresses himself, by the mere appulse of the rays 

 of light on the lenses, which thus convey the idea 

 of the object. He also observes, that under every 

 lens is a hexagonal cone of fibres, the base of the 

 cone corresponding to the size of each lens. Dr. 

 Hook maintains that each lens is convex on the 

 external, and concave on the internal side. 



Besides the eyes just described, there are on 

 the heads of many Insects three small, smooth, 

 lucid globules resembling so many separate eyes, 

 placed on the top of the head, between or above 

 the lateral ones. These parts Linnasus distin- 

 guishes by the title of Stemmata. Their real na- 

 ture is not yet very clearly understood. 



The existence of the Brain in insects is denied 

 by Linnreus, but by this he can only be supposed 

 to mean that it does not bear much resemblance 

 to that of the larger animals. 



* Some insist that they are so many magnifying' Menisci of 

 unequal spheres, the exterior or convex part of each being a 

 portion of a smaller sphere than the concave or interior part. The 

 accurate Roesel represents the supposed convexity of both sides, 

 but candidly confesses that he cannot absolutely determine the 

 point. The late eminent optician Mr. Benjamin Martin, in an 

 ingenious treatise on the nature of vision in Insects, insists 

 on their being double convex lenses, and has represented the 

 proportional convexity in the eye of the Dragon-Fly. 



