HELIOTROPISM OF ANIMALS 33 



in a test-tube open on the room side ; but when I turned the 

 vessel through an angle of 180 in the daytime, hardly two 

 minutes elapsed before all the animals had moved to the 

 open end of the vessel which was now turned toward the 

 window. Under these conditions they of course escaped 

 from the test-tube. A position which the animals have 

 assumed under the influence of light is usually not changed 

 when the light is removed, unless some other stimulus comes 

 into play. 



5. On negative geotropism and contact-irritability in the 

 caterpillars of Porthesia chrysorrhcea. The reader may 

 perhaps have noticed that in all of these experiments on 

 caterpillars the test-tubes were always placed with their 

 longitudinal axes horizontal. This was due to the fact that 

 the animals behave like plant structures, not only in regard 

 to their heliotropic, but also in regard to their geotropic, 

 irritability. Just as is frequently the case in positively 

 heliotropic plants, we find that the caterpillars are also nega- 

 tively geotropic ; that is, they are compelled by gravity to 

 creep vertically upward until they come to rest in the highest 

 part of the test-tube. These experiments were made in a 

 dark room, with the long axis of the test-tube in a verti- 

 cal direction. If the test-tube is inverted, the animals again 

 creep to the top ; if left undisturbed, the animals remain in 

 the uppermost regions of the test-tube. It is necessary in 

 these experiments, as in those on heliotropism, to have the 

 temperature of the room at least 15, preferably as high as 

 20-22 . It is simplest to put the test-tube in one's pocket 

 with its longitudinal axis vertical. In a few minutes the 

 animals are found at the highest point in the tube. An 

 increase in temperature increases the geotropic irritability 

 of the animals. 



It must now seem questionable whether in our former 

 discussion of the heliotropism of these animals we were 



