266 Animal Instincts and Tropisms 



TABLE VIII 



Percentage Difference in the 

 Intensity of the Two Lights 



Average Angular Deflection of the 



Two Paths of the Larva towards 



the Weaker Light 



Per Cent. 

 o 



25 



50 



66% 



IOO 



Degrees 



0.09 



- 2.77 



- 5-75 



- 8.86 



11.92 

 20.28 



30-90 



46.81 



-77.56 



Let us assume that the negatively heliotropic animal 

 is at an equal distance from the two unequal lights and 

 placed so that at the beginning of the experiment its 

 median plane is at right angles to the line connecting 

 the two lights, but with its head turned away from 

 them. In that case the velocity of reaction in the 

 symmetrical photosensitive elements of the eyeless 

 larvae is greater on the side of the stronger light. Since 

 the animal is negatively heliotropic this will result in a 

 greater relaxation or a diminution of the energy pro- 

 duction of the muscles turning the head of the animal 

 towards the side of the stronger light. Hence the 

 animal will automatically deviate from the straight 

 line towards the side of the weaker light. By the 

 alteration of the position of its body the photosensitive 

 elements exposed to the stronger of the two lights 



