64 STUDIES IN GENERAL PHYSIOLOGY 



under blue glass just as rapidly and in the same way as 

 in mixed daylight since in mixed daylight it is chiefly 

 the more refrangible rays which are heliotropically effect- 

 ive the ventral orientation of fly larvae which has just 

 been described occurs neither under blue nor under red 

 glass. In direct sunlight it took one to one and a half 

 minutes before the animals were densely gathered on the 

 upper side of the horizontally lying test-tube. Not one of 

 these animals moved to the upper side of the tube in less 

 than twenty-five minutes under red glass, or in less than 

 five minutes under blue glass. 



The ventral orientation of the Musca larvae toward a 

 source of light can be observed most distinctly in freshly 

 hatched larvae. As the animal grows larger, the phenomenon 

 becomes less marked. The lump of eggs laid by a fly was 

 distributed among three tubes. In all three tubes the animals 

 immediately after hatching oriented themselves ventrally 

 toward the diffuse light. I then fed meat to the animals 

 in one tube and left the animals in the other two tubes 

 unfed. On the next day the unfed animals were oriented 

 ventrally toward the daylight, while this was not the case in 

 the rapidly growing larvae which had been fed. I have ob- 

 tained the same result by feeding the larvae of one lot of 

 eggs with fat, while another lot was given lean meat. The 

 latter grew more rapidly than the former. While those fed 

 on fat were oriented ventrally in diffuse daylight, direct sun- 

 light was necessary to bring about this effect in those fed on 

 meat. 



I might have doubted that this was the effect of light, 

 had I not been able to prove that with a decrease in the in- 

 tensity of the light the phenomenon becomes less distinct, 

 and finally disappears entirely. I do not think that the 

 ventral orientation could have been the effect of heat, as the 

 animals move away from a non-luminous source of heat, as 



