PHYSIOLOGY OF THE BEE. 85 



be easily seen in flies, bluebottles, moths, butterflies, 

 &c. The numbers of the facets vary greatly in 

 different families of the Articulata. In the common 

 house-fly there are, it is stated, about 4,000 ; in the 

 white cabbage-butterfly, 17,000; in the dragon-fly, 

 24,000. It has been computed that in each com- 

 pound eye of the bee there are about 3,500 of them. 



Fig. 25. — Eyes of a BeEj greatly magnified. s^. 



Behind the horny covering, or cornea, which con- 

 sists of two plano-convex lenses, is a layer of dark 

 pigment, which gives the characteristic colour to 

 these eyes. This is pointed like the neck of a vase, 

 and serves the purpose of the iris in the higher 

 animals. This is traversed by a minute aperture or 

 pupil, through which the rays pass by a longer 

 conical lens to the optic nerve. A vertical section 

 shows that each ocellus (or little eye) is the frustum of 

 a pyramid, the large end or base of which is bounded 

 by the cornea, while the other and pointed end 



