No. 630] HABITAT RESPONSES OF WATER STRIDER 75 



There appeared to be a tendency on the part of the 

 water-striders to keep moving along the path already 

 taken, unless some other stimulus diverted them. This 

 frequently occurred, and contact proved to be the com- 

 monest form of stimulus that brought about such diver- 

 sion. They wandered along a certain path until some 

 stimulus acted upon them. Then they changed their path 

 and tried another direction. There were times when it 

 was difficult to observe what was the stimulus causing the 

 change in direction. In fact on certain occasions there 

 appeared to be no new external stimulus, no change in the 

 external environment, and yet there occurred a change in 

 direction. Therefore the change in direction probably 

 was due to some disturbance of the physiological condi- 

 tion of the animal brought about by some internal 

 stimulation. 



Fifty per cent, of the total number of gerrids entrapped 

 on the surface of the stream pool were successful in 

 reaching water elsewhere. In this instance the water was 

 ten yards away from the site of the pool on which the 

 insects were trapped. So large a number, I am con- 

 fident, is very unusual, for several other observations 

 of a similar character show that a very much smaller 

 percentage were able to find water after the pools on 

 which they were confined had become dry. In some 

 cases the water was at distances of less than ten yards, 

 while in other cases it was eleven, twelve, and fourteen 

 yards distant. I believe that large numbers of apterous 

 individuals die during periods of long and severe 

 droughts. I have some evidence of this from out-of-door 

 observations. Further, I have found that water-striders, 

 frequently, soon die in the laboratory, when the water in 

 aquaria was permitted to evaporate to dryness. This 

 was true even if the temperature was not higher than 

 85° F. 



Experiments were carried on near Urbana for the pur- 

 pose of observing with what promptness and directness 

 water-striders, Gerris rcmigis, returned to their habitat 



