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THE AMERICAN NATURALIST [Vol. LIV 



were made of the length of time that was necessary to 

 find the water. Forty gerrids were employed in each of 

 the experiments, which well might be considered as mass 

 experiments. They were placed on the ground one yard 

 away from the water in a number of the experiments, and 

 in the case of the other experiments, the water-striders 

 were placed on the ground three yards away from the pool. 



The results of the different experiments one yard away 

 from the water were very similar in many instances. 

 Therefore the data will be given of only one experiment. 

 A great majority of the gerrids were back on the surface 

 of the pool within fifteen seconds from the time they were 

 placed on the ground. In thirty-five seconds all but two 

 individuals had reached the water, and in one minute of 

 time all the gerrids were striding back and forth on the 

 surface-film. The water-striders jumped toward the pool 

 with considerable promptness. They made comparatively 

 few errors in direction and few random movements. 

 Sometimes there was a gerrid that did not reach the pool. 



In the experiments three yards distant from the pool 

 of water, a great majority of the gerrids were back on the 

 surface-film within forty seconds after they were placed 

 on the ground. In the majority of these experiments, all 

 the water-striders had returned to the water within 2 

 minutes and 5 seconds. A very few gerrids were not suc- 

 cessful in reaching the water. A fair degree of prompt- 

 ness and directness were evinced by the water-striders 

 in jumping toward the pool. There were, perhaps, more 

 errors made in direction of movement than was the case 

 with the gerrids in the experiments one yard away from 

 the water. 



In all these experiments conducted at the brook near 

 Syracuse, it seemed probable that the sense of sight was 

 the most important factor in directing the gerrids to the 

 water, although moisture also may have exerted an influ- 

 ence on their responses. 



