REACTIONS TO LIGHT AND GRAVITY IN DROSOPHILA 105 
the relative difference in energy content, that difference remained 
constant for all the eye colors tested in my experiments. 
Lastly, it may be well to emphasize the peculiar relation 
which exists in Drosophila between general activity and photo- 
tropism This phenomenon has been clearly recognized by 
Carpenter and in general I agree with this author’s conclusions. 
The fact seems to be that this insect is not phototropic unless 
it is in a certain physiological state brought on by, or at least 
accompanied by, activity. When the fly reaches a certain de- 
gree of activity, induced by various means, it suddenly becomes 
phototropic. When it quiets down, however, it may still crawl 
about but ceases to be phototropic. Thus, when an insect has 
been exposed to constant illumination for some time, it no 
longer orients to light but wanders aimlessly up and down the 
tube. Eventually such an animal may even come to rest with 
its head away from the source of light. This phenomenon, Car- 
penter suggests, is probably due to slight fatigue. However 
this may be, it is certain that without a continuance of the me- 
chanical agitation or sudden increases in light intensity, the ani- 
mal’s general activity soon falls to the point where phototropic 
response ceases. 
