1921] Riley: Responses of Water-Strider 213 
of illumination, the majority of the water-striders changed their 
direction of movement. Instead of continuing in lines prac- 
tically parallel to the sides of the aquarium, they assumed 
positions with the long axes of their bodies oblique to the 
longitudinal axis of the receptacle. While their heads con- 
tinued to be directed toward the general source of illumination, 
that is, toward the end of the aquarium where the light was 
placed, they now strode along the surface-film in oblique 
directions. Some of them turned their heads to the right and 
others turned them to the left. The directions in which their 
heads were pointed apparently depended on the swinging of 
the light—that is to say, it depended on the point that the 
light had reached in its oscillation. There appeared to be a 
certain amount of ‘‘indecision’”’ as to the directions that the 
gerrids would take in their locomotor movements. As the 
light swung to the right, they tended to turn to the right and 
often moved for a short distance in that direction; then, as the 
light swung back again to the left, there was a tendency for 
them also to move back again in that direction. Whether the 
gerrids moved to the left or to the right, with respect to the 
oscillating light—while there were some slight movements 
toward it—the bulb swung back toward them so quickly that, 
in many instances, there were not many actual locomotor 
movements in its direction. So that the insects, in some cases 
at least, remained, approximately, in the same locations, 
although there were certain slight changes in positions. 
Some of the gerrids congregated at the end of the aquarium 
nearest to the oscillating light, but they were found at any point 
along this end. Evidently, there was not much additional 
stimulation, in so far as the oscillating light was concerned, 
for they did not stride after it, but rather they moved about 
anywhere close to the lighted end of the aquarium. A few 
remained more or less stationary with respect to locomotion, 
some of them being actively engaged in cleaning their legs and 
antenne. Other gerrids—as they strode across the water-film 
from the negative toward the positive end of the aquarium— 
on reaching the region where their paths became oblique, 
due to the influence of the oscillating light, moved close up to 
the glass in the end of the vessel. After the light swung past 
them, they moved, for a very short distance, along paths 
parallel to the end of the aquarium, but in the direction that 
