1921] Riley: Responses of Water-Strider 287 
phase of their responses. They readily follow the light, 
whether it is moved around the aquarium from left to right, or 
vice versa. If the light is moved about above the gerrids, 
they tend to keep in the illuminated region. They display 
much promptness in their responses to the moving light and 
these apparently are always positive. 
The gerrids respond to a moving electric light of half the 
intensity of the one recently mentioned. Their behavior is 
very similar to that described when the source of illumination 
is 44 ca.m. The chief difference is that the responses are not 
quite so prompt as in the case with the light of greater intensity. 
3. CONCLUSION. 
It is evident that responses to contact and to photic stimula- 
tion play an important réle in the daily lives of these water- 
striders. Such responses appear to form very definite elements 
in their general behavior. Many of the normal habitat activi- 
ties, undoubtedly, are due to the thigmotactic and phototactic 
propensities of these animals. If such forms of stimulation 
were eliminated from their environment, the result would be a 
marked modification in the whole general behavior of the 
species. It is not.improbable that the responses of Gerris 
remigis to contact and to light stimuli may have had some 
influence on the development of certain of the phases of behavior 
discussed in this paper, perhaps in a manner somewhat 
analogous to that pointed out by Holmes (1903) in his discussion 
of the relation of thigmotaxis to the evolution of the death- 
feigning instinct. 
V. ACKNOWLEDGMENTS. 
Herewith certain acknowledgments are made to various 
persons who have rendered assistance to me, either directly 
or indirectly, while accumulating the information necessary 
for the preparation of this paper. 
Thanks are due to the following: Mr. T. L. Hankinson, 
ichthyologist of the Roosevelt Wild Life Forest Experiment 
Station, the New York State College of Forestry at Syracuse 
University, for the loan of the photograph from which Fig. 4 
was made and also for general suggestions; Mrs. E. L. 
