124 HELIOTROPIC MECHANISM 



equally on all sides. The ascent of the wall was made by the normal 

 fly in a vertical line when the cylinder was stationary. When the 

 cyhnder was slowly rotated the fly circled in the opposite direction as 

 if to maintain a vertical line of ascent; the result of the rotation how- 

 ever is to turn the body axis to an angle with the vertical, and the fly 

 traverses a spiral path on the wall of the revolving cylinder. In- 

 creasing the speed of rotation (within limits) increases the number of 

 spirals and may even cause the fly to walk horizontally or simply 

 circle on the wall, ascent thus being rendered impossible. When this 

 experiment is performed after blackening one eye, the fly ascends the 

 stationary cylinder in a spiral path as described previously. Slow 

 rotation which carries the fly toward the side of the blackened eye 

 intensifies this forced motion, the number of spirals is increased, or 

 the vertical component of the path is entirely nullified with a much 

 slower rotation than in the normal fly. On the other hand, if the 

 rotation is toward the unblackened eye, the tendency to circle in the 

 opposite direction, which was noted in the normal fly, is still evident 

 in the fly with the blackened eye, for it neutralizes the circus motions 

 in part or whofly depending upon the speed of rotation. An appro- 

 priate speed may be found at which the fly no longer moves in a spiral 

 path but ascends a vertical line on the wall of the cylinder. Faster 

 rotation will cause ascent in a spiral in the opposite direction — a 

 forced circus motion toward the blackened eye. 



The intensity of the illumination has a decided effect on the turn- 

 table reactions of these flies. It requires a greater speed of rotation 

 in the bright hght to cause the change from a spiral to a vertical path, 

 and conversely, if the spiral path has been changed to a vertical one 

 by rotation toward the unblackened eye, an increase in the illumina- 

 tion will restore motion in a spiral path toward the good eye. This is 

 of course due only to a reestablishment of a difference in the muscle 

 tonus of the two sides by increase in the illumination of the un- 

 blackened eye. 



Concordant results are obtained on the horizontal surface of a turn- 

 table. A fly, blinded in one eye, circles in a given radius; this radius 

 is decreased and the fly simply turns about a vertical axis in one spot 

 if rotated toward the blind eye. The circles are widened, or circling 

 in the opposite direction results by rotating toward the gooci eye 



