No. 630] HABITAT RESPONSES OF WATER STRIDER 



to discriminate between two luminous areas of different 

 size, even though they are of the same intensity. Water- 

 striders may possess this sort of discrimination. 



At this point I desire to direct attention to certain in- 

 teresting experiments of Parker (1903) on the butterfly, 

 Vanessa antiopa Linnaeus, because of their probable bear- 

 ing on some of the responses of Gerris rernigis. Accord- 

 ing to this writer (1903, p. 467), 



Vanessa antiopa ... [is able to] discriminate between light derived 



from these two sources is of equal intensity as it falls on the animal. 

 These butterflies usually fly toward the larger areas of light. 

 He (1903, p. 465) remarks that in the sunlit spots in the 

 woods, this butterfly responds to the large areas of sun- 

 light rather than to the smaller ones. This form of re- 

 sponse applies also to the sun, although 



the retinal image of the sun must be vastly brighter than those of all 

 other spots [of sunlight]. 



Furthermore, writing of the way in which Vanessa an- 

 tiopa finds a patch of sunlight, he (1903, p. 464) makes 

 the following statement : 



This patch [of sunlight] is found not through the accidental wander- 

 ing of the butterfly into it, but by the butterfly's taking a direct course 

 to it, precisely as the insect finds a single light window in an otherwise 

 dark room. The directive influence, then, is not the intense sunlight that 

 makes the patch, but the much less intense reflected light radiating from 

 the patch. This must form a localized spot on each retina of the butter- 

 fly, and it is the position of these spots that determines the direction of 

 flight. 



The surface of the water in a brook forms an excellent 

 reflecting surface, either for moonlight or for sunlight, 

 and it is probable that the gerrids respond to such re- 

 flected light much in the same way that they do to arti- 

 ficial light. Or they may respond to water, or rather to 

 the reflections from its surface, according to the same gen- 

 eral principle that Vanessa antiopa responds to patches 

 of sunlight. These areas of sunlighted water in the 

 brook must have much the same appearance to insects 

 with image-forming eyes, such as water-striders, as do 



