32 The Structure and Special Physiology of Insects 



vision to the particular kind of seeing accomplished by the compound 



eye. 



The character or degree of excellence of sight by the two kinds of 

 eyes obviously varies much. The fixed focus of the ocelli is extremely short, 



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Fig. 62. 



Fig. 64. 



Fig. 6.{. 



Fig. 62. — Longitudinal sections through outer part of eye-elements (ommatidia) of com- 

 pound eyes of Lasiocampia quercijoliti; ommatidia at left showing disposition of 

 pigment in eyes in the light, at right, in the dark. (After Exner; greatly magnified.) 



Fig. 63. — Longitudinal section through a few eye-elements of the compound eye of Calo- 

 cola niipla; left ommatidia taken from an insect killed in the dark, right ommatidium 

 taken from insect killed in the light. (.-Vfter Exner; greatly magnified.) 



Fig. 64. — Section through the compound eyes of a male May-fly, showing division of 

 each compound eye into two parts, an upper part containing large eye-elements 

 (ommatidia), and a lower part containing small eye-elements (ommatidia). (After 

 Zimmerman; greatly magnified.) 



and probably the range of vision of these eyes is restricted to an inch or 

 two in front of the insect's head. Indeed entomologists commonly believe 

 that the ocelli avail little beyond distinguishing between light and darkness. 

 With the compound eyes the focus is also fixed, but is longer and the range 

 of vision must extend to two or three yards. It is obvious that the larger 



