1921] Riley: Responses of Water-Strider 2H 
fifteen water-striders which had responded by moving toward 
the light, assembled at the positive end of the vessel. After 
an interval of a few minutes, the light was removed to the 
opposite end of the receptacle and the gerrids responded to 
the photic stimulation in a similar manner as before. By 
placing the oscillating bulb first at one extremity of the vessel 
and then, after a short interval of time, at the other extremity 
of the aquarium, the greater number of the water-striders 
always moved toward the source of illumination. 
As the hemipterons strode along the water-film toward the 
light, the long axes of their bodies were approximately parallel 
with the sides of the aquarium and also with many of the 
entering rays of light, but by no means were their bodies parallel 
with all the incoming rays. However, when the animals reached 
points roughly six or eight inches from the positive end of the 
aquarium, their directions of locomotion became oblique with 
respect to the paths that they so far had travelled. Evidently 
these changes of directions of movement were due to the 
directive influence of the swinging light as it assumed various 
positions along the plane of its oscillation. 
The further responses of the gerrids were much like those 
described in the experiments with the swinging 44 ca. m. 
electric light. The locomotor movements, in connection with 
the experiments with the light of weaker intensity, in a general 
way, were very similar to those evinced in the experiments with 
the stronger light of 44 ca. m. in the field of experimentation. 
There was, perhaps, a little less promptness in orienting to 
the oscillating light and also a little less definiteness in moving 
toward it than was the case when the light of greater intensity 
was employed as the source of illumination. 
7. RESPONSES TO MOVING BUT NON-OSCILLATING ARTIFICIAL 
LIGHT OF MODERATE INTENSITY. 
It was found that the gerrids responded readily to a moving 
incandescent electric light that was not oscillating. The light 
that was employed in the experiments was approximately 44 
ca. m. in the experimental field. At the beginning of the 
experimental work, the light was placed directly in front of the 
aquarium containing the water-striders and at one end. Water 
was poured into the vessel to a depth of about four inches. 
