170 THE AMERICAN NATURALIST. (VoL. XXXIX. 
the center of the earth, and, responding to the kinetic influence 
of mechanical stimulation, moves upward. The general result 
is a progression in a definite direction, precisely such as is 
obtained when a light is placed at one end of a horizontal glass 
cylinder containing the insect. 
Summary of  Results.— 1. Mechanical stimulation has a 
kinetic effect upon Drosophila, since it induces locomotion. 
2. Gravity has a directive effect upon the active insect, which 
is negatively geotropic, that is, the insect moves away from the 
center of the earth. 
3. Light has both a kinetic and a directive effect. The insect 
moves toward the source of light, being positively phototropic. 
The directive effect is apparent only when the kinetic stimulus is 
sufficient to induce locomotion. 
4. The exposure of Drosophila to light of high intensity is 
accompanied by an increase in the kinetic effect. Under the 
influence of the highest intensity used, that of a 250 c. p. arc 
light at 40 cm., the muscle reflexes of an insect become very 
rapid and violent, and the directive influence of the light seems 
inhibited. There is no indication of a reversal of the directive 
influence from positive to negative. ^ 
5. After continued exposure to direct sunlight in a large 
cylindrical glass vessel many insects come to rest in the least 
brightly illuminated regions and with their heads away from the 
source of light. This is not an indication of negative pho- 
totropism. The fatigued insects remain quiet in this position 
because it is the one in which the least light enters the eyes, 
and in which, as a consequence, the kinetic stimulus is least. 
