Ill II \\ I 

 source of li^hi. In consequence of tail oriehtatiob its progressive 



movements must lead it to the BOUTCe of hVht." 



Factors Influencing Phototropism. The 1 nism 



to li.^ht is influenced by previon ire to li^lit, by t rmperat urc, 



moisture, nutrition and other fat tOTS, all of \vhi< h have to be taken into 

 account in experiment^ on photot ropi>m. 



Loeb found that larva- of the brown tail moth. I'.ufiroctis chrysor- 

 rha-a, driven by the warm sunshine out of the neat 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 phototropic, and as 

 the horizontal components of the surrounding light neutralize eaqh 

 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: 



