HELIOTROPISM 



55 



This is the combination of circus movements with roll- 

 ing movements familiar to those who have experimented 

 on the brain of fish, where a destruction of one side of the 

 midbrain calls forth rolling motions as well as circus 

 motions toward the same side. Holmes 's observations 



Fio. 19. Robber fly under normal conditions seen from above. (After Garrey.) 



were extended by Garrey 's experiments on a large num- 

 ber of insects. Garrey found that the robber fly (Procta- 

 canthus) (Fig. 19), which is positively heliotropic, is an 

 unusually good object for the demonstration that the 

 heliotropic reactions of animals are of the type of forced 

 movements. When one eye of this fly is blackened the 

 legs on the side of the unblackened eye are flexed and the 



