74 THOS. H. MONTGOMERY, JR. 



This result seems at first sight to be contradictory to the pre- 

 ceding experiments. But I believe it is explainable by the nearly 

 vertical position of the sun's rays; for on the beach, at noon when 

 the sun is nearly vertical, the spiders do not run in any regular 

 direction. 



It thus seems that these spiders are decidedly negatively 

 phototropic to lamplight and to diffuse daylight, when these 

 impinge upon them from an angle; but that when sunlight falls 

 upon them nearly vertically they do not orient themselves to it. 

 How is it then on the beach that the spiders run so regularly 

 landward whenever the sun is shining from an angle, irrespective 

 of the position of the sun, which may be in front or behind or to 

 the right or the left of them? One would not suppose it to be a 

 case of negative phototropism when they run straight towards 

 the sun. Yet I believe it is this nevertheless, in that the spiders 

 may orient themselves not to the sun directly but to the light 

 from the water. For the area of light in the sky and clouds 

 above the water is certainly greater, and the light more intense, 

 than that to landward, owing to the great amount of reflection 

 from the water. It is well known that a man becomes more 

 sunburnt upon the water, a test that the sunlight is more effective 

 there. It would seem that only in this way can we correlate 

 the experiments made with the oil lamp and with diffuse sunlight, 

 with the observations recorded upon the sea beach. That animals 

 react to area as well as to intensity of light is now well known. 

 Thus G. H. Parker 1 showed that Vanessa, when in the open, 

 always comes to rest with its head away from the sun, and that 

 it thereby "reacts positively to large patches of bright sunlight 

 rather than to small ones, even though the latter, as in the 

 case of the sun, may be much more intense." Cole 2 has con- 

 firmed Parker's results, and concluded from his own pains- 

 taking observations on several forms, that animals with direc- 

 tion eyes, as those without eyes, respond to light intensity only; 

 and that animals with image-forming eyes when positively 



Phototropism of the Mourning Cloak Butterfly, Vanessa antiopa Linn.," 

 Mark. Anniv. Vol., 1903. 



2 "An Experimental Study of the Image-forming Powers of Various Types of 

 Eyes," Proc. Amer. Acad. Arts and Sci., 42, 1907. 



