The Structure and Special Physiology of Insects 33 
and more convex the eyes the wider will be the extent of the visual field, 
while the smaller and more abundant the facets the sharper and more dis¬ 
tinct will be the image. Although no change in focus can be effected, cer¬ 
tain accommodation or flexibility of the seeing function is obtained by the 
movements of the pigment (Figs. 62 and 63) tending to regulate the amount 
of light admitted into the eye (as shown by Exner), and by a difference in size 
and pigmental character of the ommatidia (Fig. 64) composing the com¬ 
pound eyes of certain insects tending to make part of the eye especially 
Fig. 65.—A section through the compound eye, in late pupal stage, of a blow-fly, Calli- 
phora sarracenice. In the center is the brain with optic lobe, and on the right-hand 
margin are the many eye-elements (ommatidia) in longitudinal section. (Photomi¬ 
crograph by George O. Mitchell; greatly magnified.) 
adapted for seeing objects in motion or in poor light, and another part for 
seeing in bright light and for making a sharper image (as shown by Zim¬ 
merman for male May-flies, and by myself for certain true flies (see p. 318)). 
Our careful studies of the structure of the insect eye, and the experimentation 
which we have been able to carry on, indicate that, at best, the sight of 
insects cannot be exact or of much range. 
The psychology of insects, that is, their activities and behavior as deter¬ 
mined by their reflexes, instincts, and intelligence, is a subject of great inter¬ 
est and attractiveness, but obviously one difficult to study exactly. The 
