COMPOUND EYES OP ARTICULATA. 441 



eyes of a kind entirely different from those which, have been 

 previously described. In most Insects we notice a large black 

 or dark-brown hemispherical body, situated on either side of 

 the head (fig. 212) ; and in Crabs, Lobsters, &c., we find 

 spherical bodies of similar appearance mounted on short 

 footstalks, which are capable of some degree of motion. 

 When these are examined with the microscope, their surface 



Fig. 212. HEAD AND EYES OF THE BEE, SHOWING THE DIVISION INTO FACETS. 

 a, a, antennae; A, facets enlarged ; B, the same with hairs growing between them. 



is seen to be divided into a vast multitude of hexagonal (six- 

 sided) facets. In a species of Beetle (Mordella) upwards of 

 25,000 of these have been counted ; in a Butterfly, above 

 17,000; in a Dragon-fly, more than 12,500; and in the 

 common House-fly, 4,000. Every one of these facets may be 

 regarded as the front of a distinct eye, which, however, instead 

 of being globular, is conical in its form ; the front being the 

 base of the cone, and the apex or point being directed towards 

 the optic nerve, which swells-out into a bulbous expansion 

 that fills a large part of the interior of the hemisphere. Each 

 individual eye consists, therefore, of its facet, which (being- 

 convex on both surfaces) acts as a lens ; of the transparent 

 cone behind this, which may be compared to the vitreous 

 humour; and of the fibre which passes from the bulbous 

 expansion of the optic nerve to the point of this cone. The 

 several fibres are separated from one another by a considerable 

 quantity of black pigment, which also fills up the spaces 

 between the cones ; and it is to this that the black appearance 

 of the whole compound eye is due. 



574. We must thus regard each of the cones, which, united 

 together, constitute the hemispherical or globular mass, in 

 the light of a distinct eye ; but the entire aggregate seems to 



