164 THE AMERICAN NATURALIST. | (Vor. XXXIX. 
In all, four flies, two males and two females, were used in this 
set of experiments. Each fly was sent five times in each direc- 
tion. The average time for twenty creeping excursions toward 
the light from left to right, a distance of 10 cm., was 7 65 866; 
from right to left, 7.71 sec. The average rate in either direc- 
tion was accordingly about the same, 1.3 cm. per sec. That 
Drosophila possesses the character of positive phototropism is 
obvious. 
Directive Effect of Gravity.— By arranging the apparatus used 
in the phototropic experiment just described so that the glass 
cylinder had a vertical position, it was possible to test the 
directive effect of gravity on flies stimulated to action by light. 
The incandescent lamps were now situated one at the upper and 
one at the lower end of the cylinder, and alternately turned on 
and off. By means of a stop watch, time records of excursions 
along the vessel were taken as before. Not all the flies tested 
responded readily to the light, and preliminary mechanical stim- 
ulation produced by shaking the cylinder was sometimes resorted 
to. Finally two active male insects were found, for each of 
which five readings were obtained for each direction. The aver- 
age time for ten creeping excursions toward the light from the 
bottom to the top of the cylinder through a distance of 10 cm. 
was 6.2 sec., or at a rate of 1.61 cm. per sec. ; from the top to 
the bottom, 44 sec. or at a rate of 0.23 cm. per sec. The flies 
crept upward quickly and continuously, and at a rate slightly 
more rapid than that of flies in the cylinder placed horizontally. 
In their course downward they frequently stopped for short 
intervals. 
The response to gravity accordingly appears to be a negative 
one, as was indicated by the behavior of the insects when 
mechanically stimulated in the dark. In the rapid upward 
excursions positive phototropism and negative geotropism coin-. 
cided. In the slow downward ones these two influences were 
opposed, and although the directive influence of the light proved 
the stronger, the retarding effect of the opposed influence of 
gravity was apparent in the strikingly reduced rate of progres- 
sion. 
It happened occasionally that an insect would fly through a 
