1921] Riley: Responses of Water-Strider 277 
regularity about this. In the manner described, many of the 
water-striders followed the light entirely around the aquarium, 
until the starting point was reached. There appeared to be 
no difference in the responses whether the light was moved 
round the aquarium from left to right or vice versa. When 
the light was moved about above the gerrids, there was a 
tendency for them to keep in the lighted area. In general, the 
water-striders displayed considerable promptness in responding 
to the moving light, for they followed it with great readiness. 
8. RESPONSES TO MOVING BUT NON-OSCILLATING ARTIFICIAL 
LIGHT OF WEAKER INTENSITY. 
Some experiments were performed in which a moving but 
non-oscillating electric light was employed, in the field of 
experimentation the illumination being about 22 ca. m. The 
responses of the gerrids were of much the same character as 
those that have been described in connection with their behavior 
toward a moving light of 44 ca. m. When the light was placed 
at one end of the aquarium, the insects responded by striding 
across the surface-film toward the source of illumination. 
Then the light was moved slowly around the outside of the 
aquarium and the gerrids responded by following after it. 
There were no marked differences in their behavior to light 
of this intensity from that which they evinced in their responses 
to the 44 ca. m. moving light, except that it fairly may be said 
that they did not respond with quite the equal readiness to the 
photic stimuli of weaker intensity. 
9. RECORDS OF OTHER OBSERVERS ON THE PHOTOTAXIS OF 
GERRIDZ AND RELATED AQUATIC HETEROPTERA. 
Other observers have noticed that certain Heteroptera 
evince behavior of a definite character with respect to the 
photic stimuli of a moving light. Holmes (1905), in an excellent 
paper, has proved that Ranatra fusca exhibits decided positive 
phototaxis to a moving light and also to a light that often is 
changed in its position. He (1907, pp. 160-161) further has 
demonstrated that, in water,’ Ranatra quadridentata responds 
promptly to the change in position of the light. Individuals 
of this species respond to a moving light by exhibiting move- 
ments of the head toward it. Gerris orba, according to Essen- 
