INSECT BEHAVIOR 309 



mrce of light. In consequence of this orientation its progressive 

 movements must lead it to the source of light." 



Factors Influencing Phototropism. The response of an organism 

 to light is influenced by previous exposure to light, by temperature, 

 moisture, nutrition and other factors, all of which have to be taken into 

 account in experiments on phototropism. 



Loeb found that larvae of the brown-tail moth, Euproctis chrysor- 

 rhcea, driven by the warm sunshine out of the nest in which they have 

 hibernated, crawl upward to the tips of branches and feed upon the 

 buds and new leaves. This self -preservative "instinct" is purely a 

 response to light. The caterpillars are positively photo tropic, and as 

 the horizontal components of the surrounding light neutralize each 

 other, only the light from above is effective as a stimulus to orientation. 

 After feeding, however, the larvae are no longer positively phototropic 

 and crawl downward; in other words, they are positively phototropic 

 only so long as they are unfed. Here the kind of phototropism is 

 dependent upon nutrition. 



Phototropism may be overruled by chemotropism and influenced by 

 conditions of metabolism, as Parker found for the butterfly Vanessa 

 antiopa. In his words: Vanessa antiopa, in bright sunlight, comes to 

 rest with the head away from the source of light, that is, it is negatively 

 phototropic, when the surface on which it settles is not perpendicular 

 or very nearly perpendicular to the direction of the sun's rays. When, 

 however, this surface is perpendicular to the sun's rays the insect settles 

 without reference to the direction of the rays. When feeding or near 

 food [such as running sap] the butterflies do not respond phototropically. 



This negative phototropism is seen only in intense sunlight and after 

 the butterfly has been on the wing, i.e., after a certain state of metab- 

 olism has been established. 



V. antiopa creeps and flies toward a source of light, that is, it is 

 positively phototropic in its locomotor responses. Positive photo- 

 tropism also occurs in intense sunlight, and is not dependent upon any 

 particular phase of metabolism. 



Both negative and positive phototropism in this species are inde- 

 pendent of the "heat rays" of sunlight. 



The position assumed in negative phototropism exposes the color 

 patterns of the wings to fullest illumination, and probably has to do 

 with bringing the sexes together during the breeding season. 



To these may be added other important conclusions of Parker's: 



