36 STUDIES IN GENERAL PHYSIOLOGY 



6. The effect of temperature on the caterpillars of Por- 

 thesia chrysorrhoca. 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 tern- 



