No. 630] HABITAT RESPONSES OF WATER STRIDER 73 



For an entire day particular attention was directed to one 

 small pool, dimensions 12 X 5 X i inches, which became 

 dry at that time. There were twenty gerrids on its sur- 

 face and they made no attempts to escape, as the pool 

 rapidly became reduced in size. 



After the pool had become entirely dry, the water- 

 striders did not move away for a period of ten minutes. 

 The initial locomotor responses were due, primarily, to 

 the drying up of the water. This was the only change in 

 external conditions and there was no other evident stim- 

 ulus. Similarly, when gerrids were removed from 

 aquaria, where they had been kept in captivity, they be- 

 came very active when placed upon a solid surface away 

 from the water. This was true even if previously they 

 had been inactive. These water-striders moved with an 

 awkward stumbling gait on land, but they made fairly 

 rapid progress. Their methods of locomotion were by 

 walking and jumping. Not infrequently, when jumping, 

 the gerrids seemed to lose control of the orientation of 

 the body, and sometimes made a turn of 180 degrees. 



The gerrids responded readily to contact stimuli, which 

 usually was evinced by them in coming to rest against 

 pieces of dry mud, driftwood, stones, and clumps of dead 

 leaves. They occasionally crawled underneath objects of 

 the character that have been mentioned. They did not re- 

 main there permanently, for even after carefully marking 

 the exact place, I never have been able to find them the fol- 

 lowing day. Shade and a lower temperature, combined 

 with contact, probably were the factors which influenced 

 the water-striders to stay quietly in such places. They 

 did not burrow into the mud, nor into the banks of the 

 brook for the purpose of activating until the drought had 

 passed. So far as I was able to observe, the gerrids did 

 not aestivate. 



Ten out of the twenty gerrids, or 50 per cent., reached 

 the nearest pool— dimensions 3 yds. X 2 yds. X 5 in.— 

 which was ten yards down the dry bed of the brook away 

 from the site of the former pool where the twenty water- 



