No. 459.) REACTIONS OF POMACE FLY. 163 
have been scarcely necessary since under natural conditions 
Drosophila gives such unquestionable evidence of positive pho- 
‘totropism. However, in order to obtain a more detailed ex- 
pression of this, and to make the observations more complete, 
the glass cylinder, again containing a single fly, was placed hori- 
zontally in the dark box, and the lights on the right and left 
were alternately turned on and off. By this arrangement the 
influence of the light was brought to bear on the insect at right 
angles to the influence of gravity, which could not, therefore, 
affect the results. The window in the side of the dark box 
was left open in order that the movements of the fly might be 
watched. At the beginning of the experiment, when the insect 
was in the section of the cylinder the farthest to the left, the 
right light only was turned on. The fly would then creep 
toward the right along the sides of the cylinder. The time of 
the excursion from the line bounding the last section on the left 
to the one bounding the last section on the right was taken with 
a stop watch. As soon as the fly had crossed the last line on 
the right, the light at that end of the box was turned off, and 
the left light turned on. This change in the direction of the 
illumination was followed by a progression to the left on the part 
of the fly, an active insect setting out on the return excursion 
within an average time of about 10 seconds. The progress of 
the fly toward the left was also timed by the watch. Thus, by 
reversing the lights, the animal could be made to travel back 
and forth in accordance with the changes in the direction of 
the illumination. Flying responses occasionally took place, but, 
since these were difficult to time, they were not taken into 
account. Moreover, by neglecting these the records were kept 
homogeneous in that they were made for creeping excursions 
only. Individual insects differed considerably in the degree of 
activity they displayed, and often the same insect would respond 
less promptly at one time than at another. On those occasions 
when the kinetic influence of the light seemed for. some Teason 
to be partially inhibited, recourse was had to mechanical stim- 
ulation to bring the animal into a more active state. This was 
effected by removing the glass cylinder from the dark box and 
shaking it. Flies thus treated usually reacted more readily to 
the light rays for some time after. 
