1921] Riley: Responses of Water-Strider 269 
in the aquarium next to the window. However, the behavior 
of the insects in the farther receptacle seemed to be the result 
of a negative response to sunlight, for it is evident that the 
water in the aquarium near the window must have modified to a 
considerable degree the temperature of the sunlight as it passed 
through the farther receptacle. 
3. RESPONSES TO ARTIFICIAL LIGHT OF MODERATE 
INTENSITY. 
In order to discover whether the gerrids would respond to 
artificial light of a weaker intensity than sunlight, the animals 
were subjected to electric light. They were placed in an 
aquarium—all aquaria used in this work being of the dimensions 
previously given—with parallel glass sides and ends, containing 
a few inches of water. The bulb of an incandescent light was 
hung in front of one end of the aquarium and about four inches 
away from it, the illumination within the field of experimenta- 
tion being approximately 44 ca. m. All experiments with 
artificial light were performed in a dark room. 
When the light first was placed near the aquarium, the 
insects, about fifty of them, were found to be scattered about on 
the surface-film. There were movements of the antenne, legs 
and bodies as soon as the animals were subjected to the influence 
of the light. In a few seconds, perhaps half of the gerrids so 
oriented themselves that their heads were directed toward the 
source of illumination. This placed the longitudinal axes of 
their bodies parallel with the sides of the aquarium and also 
parallel with many of the rays of light passing through the bulb. 
It can not be said, however, that the long axes of the bodies of 
the insects were definitely parallel with all the rays of lght 
passing into the aquarium from the source of illumination. 
Directly after orientation, the gerrids strode along the surface- 
film to the end of the vessel next to the light. The majority 
of them remained there, with their heads pointing toward the 
light. ; 
By the time that these twenty-five water-striders reached the 
positive end of the aquarium, all but four of the remaining 
gerrids were oriented, with their heads directed toward the 
light and the long axes of their bodies placed parallel with 
many of the incoming rays. They then moved over the 
