76 the; bke-kbkpbr'S GUIDE ; 



are enclosed by pigment membranes. This serves in the black 

 lining of our own eyes and of optical instruments, to limit or 

 absorb the rays of light. At the base of the rods is spread the 

 nervous termination of the great optic nerves (Pig. 21), which 

 extend from very near the brain, and which, before reaching 

 the ,eye, passes through the three ganglionic enlargements 

 (Fig. 22). Unlike the same in vertebrate eyes, the rods point 

 forward. 



It is thought that the optic nerve is very short, and that 

 the retina of other higher animals is represented by the three 

 enlargements (Fig. 22), which, as in higher animals, are fibrous 

 cellular and ganglionic, and by the central rods of the reti- 

 nulas. The sensitive portion is doubtless the end of these rods. 

 Insects, like bees, have a well-developed crystalline cone (Fig. 

 23), and such eyes are called eucone ; others have this less de- 

 veloped, and their eyes are called pseudocone. 



The old theory of L,eeuwenhoek, Gottsche, and Platean, 

 that each of the parts of a compound eye, each ommatidium, 

 forms a distinct image, and these together make a compound 

 whole, as, do our two eyes, the images overlapping, is now 

 abandoned for the mosaic theory of Muller. Lubbock argues 

 strongly for this view, and nearly all now accept it as true. 

 Each of the ommatidia give a direct, not reverse, image, as do 

 the ocelli, and each an image of only a point. Thus, the image 

 is a true mosaic, as Muller called it. The crystalline cone 

 covered with black pigment permits only a point to be imag- 

 ined, and so each of the separate eyes or ommatidia images a 

 separate point of the object seen, and all the entire object. 

 LfUbbock argues that the compound eyes do not determine form, 

 but only motion, and that is what would be useful to protect the 

 insect. Delicate tracheas pass into the eyes between the rods. 



The color of eyes varies very much, owing to the pigment. 

 In some of the bees, wasps and Diptera, or two-winged flies, 

 the coloration is exceedingly beautiful. Girschner thinks that 

 insects with highly colored eyes do not see as well as others. 

 Often the irridescence 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 (Fig. 3, 4) of drones and workers. Some- 



