Numerous experiments have shown that in some cases animals orient so that the critical stimulus registers equally on their left and right sides. Consider grayling butterflies, which escape from predators by flying toward the sun, thereby causing the predators to be partly blinded. A grayling orients to the sun by turning to that position in which both eyes are equally stimulated. If one eye is experimentally blinded, the butterfly flies in circles; it cannot orient toward the sun because it cannot achieve equal stimulation of the two eyes
Keeton: Biological Science