35 2 ENTOMOLOGY 



the light become more active than those of the opposite side, 

 and correspondingly the head of the animal is turned toward 

 the source of light. As soon as the head of the animal has 

 this orientation and the median-plane (or plane of symmetry) 

 comes into the direction of the rays of light, the symmetrical 

 points of the surface of the body are struck by the rays of light 

 at the same angle. The intensity of light is the same on both 

 sides, and there is no reason why the animal should turn to 

 the right or left, away from the direction of the rays of light. 

 Thus it is led to the source of the light. Animals that move 

 rapidly (like the moth) get into the flame before the heat of 

 the flame has time to check them in their flight. Animals that 

 move slowly are affected by the increasing heat as they ap- 

 proach the flame; the high temperature checks their progres- 

 sive movement and they walk or fly slowly about the flame." 

 As Loeb insists, the moth " does not fly into the flame out of 

 ' curiosity,' neither is it ' attracted ' by the light ; it is only 

 oriented by it and in such a manner that its median-plane is 

 brought into the direction of the rays and its head directed 

 toward the source of light. In consequence of this orienta- 

 tion 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 photo- 

 tropism. 



Loeb found that larvae of the moth Euproctis chrysorrhoea, 

 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 "in- 

 stinct " is purely a response to light. The caterpillars are 

 positively phototropic, 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 



