AND PHYSIOLOGY. 



387 



magnifying glass, is found to be made up of a vast number 

 of facets, which are generally hexagonal. In the common 

 house fly there are 4,000 in each eye, in the dragon fly 

 24,000, and in one species of the beetle 25,000. Each one 

 of these facets is found to be the end of a little eye, the frus- 

 trum of a slender pyramid standing by its apex on a bulbous 

 expansion of the optic nerve. The interior of this pyramid 

 i3 filled with a trans- Fig. 356. 



parent substance which 

 represents the vitreous 

 humor, while between 

 the frustra is found the 

 black pigment. Both 



Surfaces Of these facets Eye of Fogsil Tribolite constructed on the same 



are found to be convex, P lan - 



and it has been calculated that the focus of these lenses would 

 be at a point just at the extremity of the pyramid where it 

 joins the optic nerve. And since the rays of light entering 

 one of these facets can not enter the other on account of the 

 black pigment intervening between them, the range of vision 

 to the insect would necessarily be very limited were it not for 

 their enormous multiplication, by which in reality a separate 

 eye is provided for every point to be viewed, thus giving to 

 the insect as perfect an apparatus of vision as we have, al- 

 though immovably fixed on the body. 



710. Stemmata. — Besides the composite eyes, insects usu- 

 ally have a small number of simple eyes situated upon the top 

 of the head, called Stemmata. If they are covered with paint, 

 the movements of the insect are constantly upward. 



711. Other Articulates. — A few insects that live in dark 

 places have no eyes. Some Crustaceans have compound eyes 

 without facets, and others with them. Some Annelids have 

 eyes, others none. The Rotatoria have vision, but the Hel- 



What number of facets are there in the house fly and in the dragon fly ? Give the an- 

 atomy of each facet or pyramid. 710. What other organs of vision besides compound 

 eyes do insects have ? If the stemmata are removed what is the result ? Til. What 

 of the eyes of articulates ? 



