6o 



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 bram, 

 and which before reaching the eye passes through a gan- 

 glionic enlargement. Whether the true retina exist m the 



Fig 



Rods much magnified. Retlml'a Eye. 



columns between the rods, or at the base of the columns, 

 is a disputed question. The old idea of 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 tracheae 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. 



