70 THE AMERICAN NATURALIST [Vol. LIV 



the sunlit areas in the woods. The gerrids go toward 

 the water— not quite so directly as Vanessa antiopa 

 moves toward areas of sunlight— with but few prelimi- 

 nary random movements, except in the case of those 

 having their heads directed away from the brook, in 

 much the same fashion and probably for much the same 

 reason that Vanessa antiopa goes toward sunlit spots in 

 the woods. 



I observed, as early as the summer of 1911, that these 

 water-striders respond to moving objects and shadows 

 more promptly than they do to stationary ones. In the 

 early fall of 1918, I discovered that individuals of Gerris 

 remigis, confined in an aquarium, respond definitely and 

 in a pronounced manner to a moving incandescent electric 

 light and also to frequent changes in the position of such 

 a light. Essenberg (1915, p. 402) states that in Gerris 



The sense of sight is keenly developed, the insects detecting a moving 

 object or a shadow very quickly. 



The responses of Ranatra fusca to a moving light and 

 also to a light frequently changed as to position are well 

 known through the admirable work of Holmes (1905). 

 He (1907, pp. 160, 161) has also pointed out that the 

 young of Ranatra quadridentata respond to changes in 

 position of a light. Therefore, a brook with a current of 

 moderate velocity is more likely to be seen by the gerrids 

 than is still water. The rimes and small waves serve as 

 additional reflecting surfaces for diffuse daylight and sun- 

 light, which facts aid in making the position of the brook 

 still more noticeable to the water-striders. 



2. Role Played by Moisture.— It is possible that mois- 

 ture from the brook diffusing through the atmosphere 

 may serve, to a certain extent, as a stimulus which may 

 produce a positive response when the water-striders are 

 in such close proximity to the stream as previously has 

 been indicated. However, I am much in doubt of such an 

 explanation. The plan to prevent the gerrids from seeing 



