THE HEAD, 43 



Their strncture resembles that of a lens of a compound 

 eye, and will be given in that connection. 



Their color is usually black and shining, though many 

 have the centre beautifully marked with red, sapphire, 

 or crystal. 



Where there are more than two they vary in magni- 

 tude and in shape. 



The situation and arrangement of simple eyes may be 

 readily studied with an ordinary magnifying glass, on 

 the head of any insect, and it cannot be too strongly 

 and repeatedly urged on the learner that these details 

 be observed in the specimens themselves, and not merely 

 taken and remembered from books. 



Conglo7nerate eyes differ in nothing from simple eyes 

 except that instead of being dispersed they are collected 

 into a body, so as at first sight to resemble the appear- 

 ance of a compound eye. 



Compound eyes are the most usual form in the adult 

 insect. They also may be observed in relation to their 

 structure^ number^ situation^ figure^ clotJmig^ color and 

 size. 



The compound eye, when seen under the miscroscope, 

 displays a great number of convex, six-sided pieces; 

 each of these is a distinct lens and has on the inner side 

 of it a cone-shaped retina at the very point or apex of 

 which alone is the optic nerve, or nerve of sight. These 

 cones are often beautifully colored and placed side by 

 side as the lobes of a section of an orange. 



The number of these lenses varies from about 50 to 

 over 30,000. 



Hooke computed those in the eye of a horse-fly to 

 amount to nearly V,000; Leeuwencock found more than 

 12,000 in that of a dragon-fly; and 17,325 have 

 been counted in that of a butterfly, and, according to 



