352 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 larve of the moth Euproctis chrysorrhea, 
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 ght neutralize each other, only the light from 
above is effective as a stimulus to orientation. After feeding, 
however, the larve are no longer positively phototropic and 
