1921] Riley: Responses of Water-Strider 255 
ay 
periods of severe and extended droughts. It was evident, 
if they reached another stream or pool, that the result was 
accomplished by means of trial and error. During the course 
of their wanderings, they frequently came in contact with 
various objects in their path, such as lumps of dry mud, drift- 
wood, clumps of dead leaves, and stones (Figs. 8, 12). Often, 
the forward movement of the gerrids ceased at such contact. 
Stimuli of this character inhibited locomotion and the water- 
striders became stationary, their bodies being applied closely 
to such objects as have been enumerated. Apparently, as a 
result of such contact stimuli, sometimes they crawled under- 
neath these obstructions (Figs. 8, 12). In both cases, whether 
their bodies were in close contact with the objects, or whether 
they crawled beneath them, they remained stationary for 
various periods of time. Frequently, these extended from a 
few minutes to an hour or more. Sometimes, after such 
inhibition of locomotor activities, there was a change in the 
direction of their movement, when their journey was renewed. 
Occasionally, they fell or jumped into large cracks which had 
been formed in the mud of the dry bed of the stream and they 
remained there during intervals of time extending from a 
few minutes to several hours, on one occasion at least for three 
hours. During such wanderings as occur after their habitats 
become dry (Figs. 8, 12), responses to contact appear to be a 
common feature of their behavior. Contact stimuli, frequently, 
result in inhibiting locomotor activities and cause the insects 
to become quiet, so that they remain motionless in one position 
for various periods of time. 
7. CONTACT RESPONSES AND LABORATORY OBSERVATIONS. 
While conducting certain experiments in the laboratory on 
Gerris remigis, some phases of the behavior of these water- 
striders indicated that they responded to contact stimuli. 
After collecting the gerrids in the field, they were placed in 
large, glass aquaria—dimensions 36 x 18 x 20 inches—in the 
laboratory. These aquaria contained a few inches of water 
and several large stones, the upper surfaces of which pro- 
truded an inch or so above the surface of the water. At first 
the insects moved about freely on the surface of the water, but, 
frequently, after a few minutes of time, some of them left the 
