60 Histology of Eyes. 
branes. This serves like the black lining of our own eye, 
and of optical instruments, to limit the rays of light. At the 
base of the rods is spread the nervous termination of the 
great optic nerves (Fig. 17), which extend from the brain, 
and which hefore reaching the eye passes through a gan- 
glionic enlargement. Whether the true retina exist in the 
Fic. 14. 
Rods much magnified, Retinu’a Eye. 
columns between the rods, or at the base of the columns, 
is a disputed question. The old ideaof Miiller, that the 
image of each eye is a distinct portion of a large compound 
whole—a mosaic—is now abandoned. The philosophy of 
sight in insects is rather like that of higher animals, except 
thousands of eyes instead of two are used as one. 
Delicate trachee pass into the eyes between the rods. 
The color of eyes varies very much, owing to pigment. In 
some of the bees, wasps, and Diptera or two-winged flies, 
the coloration is exceedingly beautiful. Often the irides- 
cence, or play of colors, as the angle of vision changes, is 
wonderfully rich. 
The form, size and position of eyes vary much, as seen 
by noticing the eyes of drones and workers. Sometimes 
as in bees (Fig. 12), the eyes are hairy, the hairs arising 
from between the facets. Usually the eyes are naked. 
The number of facets, or simple eyes which form the 
compound eye, is often prodigious. There may be 17,00c 
in a single compound eye. , 
