36 STUDIES IN GENERAL PHYSIOLOGY 
6. The effect of temperature on the caterpillars of Por- 
thesia chrysorrheea.—The caterpillars of Porthesia chry- 
sorrhoea behave toward a source of heat in a manner opposite 
to that in which they behave toward light; they move away 
from the source of heat. If the animals contained in an 
opaque vessel are brought in the neighborhood of a hot 
stove, they leave the side of the vessel which is nearest the 
stove. Yet the heat does not compel the animals to move 
in a straight line, as they do when struck by the more 
refrangible rays of light. This directing effect of the more 
refrangible rays of the visible spectrum is greater than that 
of the dark heat rays. In this way it is possible for the 
same animal which flees from the source of the-dark rays of 
heat nevertheless to move in the direction of the sun’s rays 
to the sunny side of a vessel. 
It is a well-known fact that irritability in a tissue is a 
function of the temperature. I have already mentioned that 
at a temperature of less than 13° C. the animals are no longer 
affected by light. It can be shown that heliotropic irrita- 
bility increases with an increase in temperature. If the 
animals are kept during the day in a room having a tem- 
perature of about 18°, it is found that they no longer respond 
to light when beyond a certain distance from the window. 
If, however, the temperature of the test-tube is increased a 
few degrees, the animals move the more quickly to the win- 
dow side of the tube the higher the temperature. It can 
easily be demonstrated that the orientation takes place more 
rapidly, and that the direction of the progressive movements 
coincides more nearly with the direction of the rays of light, 
whenever the temperature is raised. If, however, the tem- 
perature is increased to 30° or over, the animals become very 
restless; they raise the anterior ends of their bodies higher 
than is usual in their movement, and so decrease the velocity 
of their progressive movements. The most suitable tem- 
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