24 STUDIES IN GENERAL PHYSIOLOGY 



V. THE POSITIVE HELIOTROPISM OF THE CATERPILLARS OF 

 PORTHESIA CHRYSORRHGEA 



I will enumerate the observations which show the identity 

 of animal and plant heliotropism in the caterpillars of Por- 

 thesia chrysorrhoea. I shall mention only such experiments 

 as in my experience were always successful under the given 

 conditions, and which may be taken as the prototype of the 

 experiments made upon all the animals treated of in this 

 discussion. 



1. The direction of the progressive movement in animals 

 is determined by the direction of the rays of light. I placed 

 a large number about a hundred specimens of the small 

 gregarious caterpillars of Porthesia chrysorrhoea which had 

 just crept out of the web in which they had passed the win- 

 ter into a test-tube. They had not fed as yet, and in this 

 hungry condition they were exposed to the light. The tem- 

 perature of the room was necessarily more than 12-15 C., 

 as otherwise they would have crowded together and fallen 

 asleep again a state in which they react neither to light 

 nor to gravity. 



Experiment 1. If the test-tube is laid on a dark table, 

 so that the longitudinal axis of the tube is perpendicular to 

 the plane of the window, the animals, which are at first scat- 

 tered about irregularly, all assume the same orientation. 

 They creep to the upper portion of the test-tube, turn their 

 heads toward the window, and with their ventral surfaces and 

 their heads turned toward the light creep in a straight line 

 toward the window side of the test-tube. The process 

 requires from one to five minutes, according to the tempera- 

 ture and the condition of the hibernated animals. All with- 

 out exception, provided they are not sickly, move in the 

 direction of the rays of light to the window side of the test- 

 tube. If the tube is turned about an angle of 180, the 



