REACTIONS OF ARTHROPODS TO LIGHTS 509 



are used the larvae proceed at an angle to the direction of the 

 rays, as they do when they are stimulated by opposed white 

 lights of unequal intensity; but if the efficiency of one of the 

 lights is much greater than that of the opposing light, as, for 

 example, blue against red, the larva is oriented directly towards 

 the latter by the dominating stimulus of the more potent color.' 

 The larvae of Zeuzera pyrina, which are positively phototrophic, 

 never crawl at an angle to the direction of the rays, but, after 

 once having been oriented, proceed directly toward the source 

 of light. When paired lights of the same wave length are used 

 the larvae instead of crawling at right angles to the direction 

 of the rays, as the Calliphora larvae do, go directly to one or 

 the other of the two sources of light. When the Ughts are of 

 different wave lengths, the larvae are oriented by the more potent 

 color and proceed in a direction approximating that of the rays. 

 The reactions of Zeuzera agree with the general statement made 

 by Loeb ('05, p. 82) concerning the reactions of animals to white 

 light from two sources: ''If there are two sources of light of 

 different intensities, the animal is oriented by the stronger of 

 the two lights." Loeb's statement applies to the reactions of 

 Zeuzera larvae but it certainly does not hold for the reactions 

 of Calliphora larvae nor for that of many other animals, a criti- 

 cism of Loeb's work which has also been expressed by others. 

 Adults of Drosophila and Calliphora, insects with image-forming 

 eyes, creep or fly toward the lights and, like Zeuzera larvae, 

 when balanced lights of different wave lengths are used, are 

 oriented by the more potent color. In all of these tests with 

 adult flies, however, a certain percentage of the indi\dduals go 

 away from the source of the light which to the majority of the 

 flies is the most effective stimulus. It is difficult to explain this 

 difference in organisms of the same species, reared under identi- 

 cal conditions and tested with, the same lights. To say it is 

 due to a difference in physiological condition brings us no nearer 

 to the solution. It is evident from the results shown in table 8 

 that the flies which go away from the more effective hght at the 

 first trial do so not because of a permanent difference in their 

 phototropism, but because of mere chance or some unknown 



