122 



HELIOTROPIC MECHANISM 



the light, the greater the number of spiral circuits required to make the 

 ascent. Tahanus and Eristalis are excellent flies for these experiments. 

 They may be dealated or the wings glued together to prevent flying. 

 It is convenient to place the flies inside a graduated glass cylinder to 

 prevent escape. Surround this cylinder with a much larger one of 

 white mat-paper, or blotting paper, and illuminate by hght reflected 

 from above. In one such experiment, which is quite typical, an 

 Eristalis made the ascent of the inner wall of the graduate in two 

 spirals, when illuminated by dim artificial light placed above the 



Fig. 8. Position of a robber fly on a vertical spindle after blackening the right 

 eye. It creeps up the spindle in a spiral path to the left. 



cylinder in the dark room. In the laboratory, with the window shades 

 down, the fly made four spiral turns. With the shades up, twelve 

 spiral circuits were completed in making the ascent of 12 inches. 

 When illuminated by sunhght, the fly simply pivoted on the wall of 

 the cylinder and was entirely unable to make the ascent. In the 

 l^atter instance the marked difference in the tonus of the muscles of the 

 two sides, caused by the intense light, produced forced motions 

 about the fly's dorsoventral axis. 



If an opaque spindle is arranged vertically so that one side is in the 

 bright light, the other in the shade, insects with one blackened eye 



