I . 
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, 
Fig. 64. Fig. 63. 
Fig. 62.—Longitudinal sections through outer part of eye-elements (ommatidia) of com¬ 
pound eyes of Lasiocampia quercijolia; 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 Cato- 
cola nupta; left ommatidia taken from an insect killed in the dark, right ommatidium 
taken from insect killed in the light. (After 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 
