WALTER E. GARREY 123 



move in very different paths on the two sides. On the shaded side 

 the spirals are parallel and the pitch is acute, but in the bright light 

 of the other side the fly's path is more nearly horizontal; it may 

 become horizontal and then the fly moves in a small circle downward 

 and may thus be trapped on the bright side of the spindle. It resumes 

 its upward path only if, when executing the smaller circles, it is carried 

 to the shaded side of the spindle. The different character of the 

 paths on the two sides is only the expression of forced movements 

 due to the different conditions of muscle tonus in light of different 

 intensities. The facts are significant in that they explain the move- 

 ment of positively heliotropic insects to a light even when one eye is 

 blackened. They do so, as the records of many observers attest, in a 

 succession of large and small circles. In circling, the good eye is 

 successively illuminated by, or shaded from, the luminous source. 

 The large arcs are executed when the good eye is shaded, and the 

 smaller arc is traversed when the good eye is coming into bright light. 

 The effect is due only to the effects of the varying illumination upon 

 the muscle tonus. The movements are "forced motions" — thus 

 vanishes the mystery of ''trial and error" in these instances. The 

 mechanism of the "attraction" is the same as that in true heliotropism 

 of flies with both eyes normal. 



A study of the behavior of heliotropic insects on a turntable has 

 yielded results confirming the conclusions drawn from the preceding 

 experiments. These tests included observations on positively heho- 

 tropic flies, butterflies, and beetles, slow walking forms being best 

 adapted to the end; Tabanus and Eristalis gave excellent results. 

 Loeb^ (1890) first described the fact that flies when rotated on a 

 turntable, showed compensatory circus motions in a direction opposite 

 to that of rotation. Lyon* showed that these disappear when the 

 eyes are blackened, and RadP^ believed them to be the result of a 

 visual fixation. These reactions, as well as the nystactic movements 

 of the head of insects, are so suggestive of those of vertebrates, where 

 they are due to the internal ear, that they strongly emphasize the 

 fact that the muscle tonus of the insecta is controlled by reflexes 

 from the eyes. 



Experiments on the Turntable. — ^A normal fly {Tabanus for example) 

 was placed in a cylinder at the center of a turntable, illuminated 



