COMPOUND EYES OF 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 
