EYES OF INSECTS. 195 



to prove, their eyes are of two kinds, the two large ones being 

 adapted for distant and horizontal vision ; the three smaller, at 

 the top of the head, serving for that which is near and upwards. 



One of the most curious peculiarities observable in insect 

 eyes, in those of them, at least which are large and conspi- 

 cuous, is their compound construction. Their cornea (or outer 

 coat) instead of being smooth, is numerously divided into 

 what are called facets, each of itself a little perfect eye.* Of 

 these, a Butterfly has been assigned in each compound organ 

 17,335 a Dragon Fly 12,544 a House Fly 7000. 



Possessing such a multitude of eyes or eyelets under one, it 

 might seem that of every single object Insects must be pre- 

 sented with a multitude of images. This, however, we have 

 no reason to suppose, inasmuch as we, with our pair of single 

 eyes, are not in the habit of seeing double, and as according 

 to Muller) "each individual facet of an insect's compound eye 

 can survey but a small space in the field of vision, each only 

 contributes to the perception of all things within it. Each 

 separate one does not at the same time see all such objects, 

 but only conveys its impression to the nervous filament with 

 which it is supplied, and the latter being united in the great 

 optic nerve, a common and distinct image is ultimately 

 produced." 



The compound eye of an insect would not seem, therefore, 



* So it is said by Dr. Hooke and others ; but Swammerdam did not consider as 

 such the numerous hexagonal divisions in the eye of the Bee. 



