HELIOTROPISM OF ANIMALS 65 



we shall see presently. Because of the preponderance of 

 this ventral orientation, it is no easy matter to demonstrate 

 the negative heliotropism of the young larvaB in a test-tube ; 

 they assume the ventral orientation, and no longer trouble 

 themselves about the direction of the rays of light. 



The orientation of the larvae of Musca toward a source of 

 heat. If a Musca larva in its movements comes to a spot where 

 the temperature is only one degree higher than in the sur- 

 rounding area, it stops and turns its head laterally. If in 

 so doing its head encounters a spot with a lower temperature, 

 it turns thither and continues to move in this direction. 

 One can easily convince oneself of this by laying the tip of a 

 finger on a spot on the outside of a test-tube containing the 

 larvse. The increase of temperature of the spot touched can 

 be ascertained by a sensitive and finely graduated thermom- 

 eter. As soon as the animal comes to the spot touched by 

 the finger, it turns its head. If it does not turn far enough 

 to touch a cooler spot, it continues in the old direction to the 

 region of higher temperature. According to this, the stimuli 

 which reach the oral pole determine the orientation of the 

 animal toward a source of heat also, just as in the case of 

 light. If the experimenter puts a test-tube containing a 

 large number of Musca larvse into his pocket, where no light 

 reaches them, the animals collect in a few minutes densely 

 on that side of the tube which is turned away from his body. 

 The same thing happens when the tube is exposed to the 

 rays of a non-luminous source of .heat. 



If one-half of a tube is surrounded by a water jacket of a 

 higher temperature, and the other half by a water jacket of 

 room temperature, the animals in the warmer part become 

 restless or perish ; they are not oriented, however, and conse- 

 quently cannot save themselves by moving into that portion 

 of the tube having a lower temperature. 



In these experiments the animals were contained in a 



