COMPOUND EYES OF ARTICULATA. 44] 
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.—Heap anp Exes or rue BEE, sHOWING THE DIVISION INTO FACETS. 
a, a, antennz; 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. Ina species of Beetle (Mordella) upwards of 
_ 25,000 of these have been counted; in a Butterfly, above 
— se 
—— 
17,000 ; in a Dragon-fiy, more than 12,500; and in the 
common House-fiy, 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 
_ one 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 
