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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 larve 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 larvee 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 
larve. 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 larve 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. 
Tf 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 consc- 
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 
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