274 STUDIES IN GENERAL PHYSIOLOGY 
different animals. When other external conditions are the 
same, we deal in these cases with different degrees or inten- 
sities of heliotropic irritability. We therefore measure the 
intensity of the irritability by the (reciprocal) value of the 
oscillating deviations of the animal from the direction of the 
rays of light. It will therefore be understood what is meant 
when I speak of an increase or a decrease in the positive- 
ness or negativeness of the heliotropism. 
2. I succeeded regularly in making the larvee of Poly- 
gordius negatively helotropic through an increase of tem- 
perature, and positively heliotropic through cooling. 
A large number of freshly caught larvee were distributed 
into seven glass dishes. Hach dish contained thousands of 
larve, and they were all without exception negatively helio- 
tropic. I chose a vessel with such negative animals, and set 
it into a larger vessel containing ice and salt in order to cool 
the water containing the animals. The experiment was 
made before a window facing the north. At the beginning 
of the experiment, at 2:05 Pp. M., the temperature in all the 
vessels was about 16.5° C. In the course of the next seven 
minutes the temperature in the dish surrounded by the mix- 
ture of ice and salt fell to 11° C., without a change occur- 
ring in the behavior of the animals. No matter how often I 
changed the orientation of the dish toward the window, the 
animals went in a straight line back to the room side of the 
vessel. At 2:15 p.m. the temperature had fallen to 8° C. 
A few of the animals then left the negative side of the dish 
and moved to the window side. The temperature fell to 
6° C., and the larvee went in swarms to the opposite side. At 
2:33 the temperature of the dish was 5° C., and only a small 
proportion of the animals were negatively heliotropic. At 
2:30 at a temperature of 4° C. only about ten animals 
remained at the negative side; while the remainder, practi- 
cally thousands, were collected at the positive side. In the 
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