112 HELIOTROPIC MECHANISM 



proportional to the area blackened. Thus a dot of black centrally 

 located on one eye may produce no appreciable effect upon the 

 position of the legs of Proctacanthus, although by increasing the area 

 circumferentially until half of the eye area is blackened, postural 

 changes are produced. But even before enough of the eye has been 

 blackened to show the resting postural changes, circus motions 

 toward the normal eye take place. 



While it is true that this result attends blackening of an equal area 

 on any part of the eye, there is nevertheless a marked difference in 

 the relation of different parts of the eye to the different muscles; for 

 example, the radius of the circus movement is less when the outer 

 half of one eye is blackened than when the inner half is thus treated. 

 This was put to a simple test. The outer half of the right eye was 

 blackened and the robber fly circled to the left; the inner half of the 

 left eye was now blackened but the fly still circled to the left, although 

 in a much larger circle than formerly, showing only a partial neutrali- 

 zation of the effect. By extending the line of blackening farther out 

 on the left eye or decreasing the area of black on the outer side of the 

 right eye, exact neutralization could be obtained, and by continuing 

 this process Proctacanthus could be made to circle to the right. The 

 reversal of the direction of circling is thus brought about in this fly 

 by simply altering the relative amount of light entering the two eyes 

 although the space relation of the light field to the dark field does not 

 change. This reaction is wholly incompatible with the idea of an 

 "avoiding reaction." 



An experiment, performed on Eris talis, gave results confirming the 

 contention that in this form, each eye controls the muscles of both 

 sides of the body. Blackening the outer half of one eye may not pro- 

 duce a strong tendency to circle to the opposite side. The additional 

 bldckening of the inner half of the other eye, so that corresponding 

 visual fields of both eyes are obliterated, produces marked circus 

 motions — much more marked than result after blackening all of one 

 eye. Still more marked is the effect of blackening all of one eye and 

 the inner half of the other; the circles made by Eristalis then have a 

 minimal diameter, the flies often simply rotating about the vertical 

 axis of the body if the light is intense. The explanation of this 

 phenomenon has already been considered. 



