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 larvee 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 larve toward a 
source of light can be observed most distinctly in freshly 
hatched larvee. 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 larve which had been fed. I have ob- 
tained the same result by feeding the larve 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 
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