bo 
QO 
Or 
1921] Riley: Responses of Water-Strider 
the case of the other gerrids. Contact stimulation plays a 
definite part in such responses, exerting an influence to bring 
the gerrids to rest against solid bodies and also in keeping them 
in such situations for considerable periods of time. 
Gerris remigis readily responds to photic stimuli of various 
intensities. Frequently, on cloudless days, at a temperature 
of 100° F., they are not found on the open, unshaded surfaces 
of streams. They congregate in the shade of the banks, 
rocks, trees, and other vegetation. However, this may be 
partially due to responses to temperature and may not be due 
entirely to negative responses to strong sunlight. 
In laboratory aquaria, they are found to be, on certain 
occasions at least, negatively phototactic to sunlight. They 
move, either to the ends of the aquaria farthest from the beams 
of sunlight, or else they simply stride out of the sunlight into 
the more shaded regions of the aquaria. 
The majority of these gerrids are positively phototactic 
to electric light of 44 ca. m. within the field of experimentation. 
In an aquarium, when fifty individuals were used, they orient 
with their heads pointing toward the source of illumination, 
thus having the longitudinal axes of their bodies parallel with 
many of the entering rays. Then, they stride to the positive 
end of the vessel. When the light is changed to the opposite 
end of the aquarium, the gerrids respond again by moving 
toward the source of illumination. All of them do not orient 
to the light with equal promptness. Some delay for several 
seconds before completing this response. None appeared to 
be entirely indifferent to the light. A few individuals moved 
only two or three inches toward the source of illumination. <A 
few others evinced movements of the antenne, legs and bodies. 
sometimes, there were one or two alate individuals among those 
subjected to photic stimulation, but none of them flew toward 
the light. 
Water-striders respond positively to the stimuli of an 
electric light of 22 ca. m. in the field of experimentation. Their 
behavior with respect to light of this intensity is much the same 
as that described for a light of 44 ca. m. in the field of experi- 
mentation. The chief differences of importance are that the 
gerrids neither orient quite so promptly to, nor do they move so 
readily toward the light of lesser intensity. 
