SENSE ORGANS. 



165 



Fig. 104. — Anterior part 

 of a dragon fly, show- 

 ing the compound eyes. 



Compound Eye. — The compound eye of insects and 

 crustaceans is very different. If we examine the head 

 of any insect, such as a fly, a dragon fly, a butterfly, or 

 a beetle, we find that it consists 

 largely of two great hemispherical 

 masses, often of brilliant metallic 

 luster, green, or purple, or yellow. 

 These are the two compound eyes 

 (Fig. 104). If their surface be ex- 

 amined with a hand lens, or, better, 

 if the outer transparent corneal por- 

 tion be removed and placed under 

 a microscope, we see that it consists 

 of thousands (twenty-eight thou- 

 sand in the dragon fly) of transparent hexagonal plates 

 nicely fitted together (Fig. 105). Each plate covers a 

 hexagonal prism, which runs back to abut against the 

 convex surface of the optic ganglion, which acts as the 

 retina and connects in its turn through the optic nerve 

 with the cephalic ganglion. Each tube 

 is lined with pigment, which may be 

 likened to a choroid, and filled with a 

 transparent substance, which may be 

 likened to the vitreous humor. The 

 whole is covered with a hexagonal cor- 

 neal plate, which is thickened into a 

 kind of lens over each prism. One 

 prismatic element is called an ommatidium (Fig. 106). 



Now see in a general way (for it is not well under- 

 stood) how vision is accomplished by this instrument. 

 Remember, the condition of distinct image is that each 

 radiant should impress its own focal point on the ret- 

 ina. Rays from several points must not mix (page 

 104). Now if an object, A B (Fig. 106), be piaced before 

 such an eye, the central ray from each point, ABC, 



Fig. 105. — A portion 

 of corneal surface 

 of the compound 

 eye magnified. 



