54 



TROPISMS 



these motions of the positively heliotropic animal will be 

 facilitated when the light is in front and will be rendered 

 impossible when the light is behind. 



The writer had observed long ago that when the con- 

 vexity of one eye is cut off in the housefly it will no longer 

 go in a straight line but will make circus movements, the 

 normal eye being directed toward the center of the 

 circle. 280 It was shown by Parker that blackening of one 



Fig. 18. — The lower figure represents the position of Ranatra when the light is behind 

 the body. The upper figure represents the position assumed when the light i^n j front of 

 the animal. (After Holmes.) 



i^n front 



eye of the positively heliotropic butterfly Vanessa antiopa 

 calls forth circus movements, with the unblackened eye 

 toward the center of the circle. 398 Holmes, Radl, 447 

 Axenfeld, Garrey, 17T and many other authors have since 

 made similar observations. In the positively heliotropic 

 Ranatra, Holmes described the effect of blackening one 

 eye as follows : 



If one eye of Ranatra is blackened over or destroyed the insect 

 in most cases no longer walks in a straight line but performs more or 

 less decided circus movements toward the normal side. Under the 

 stimulus of light the insect assumes a peculiar attitude; the body leans 

 over toward the normal side and the head is tilted over in the same 

 direction. 228 



