64 Studies in Animal Behavior 



of the insect's body. If the insect is placed in a 

 cylinder on a stationary center and the cylinder ro- 

 tated, the insect tends to walk around in the direc- 

 tion of rotation. A frog under the same circum- 

 stances will do the same thing. In these cases the 

 animal reacts so as to keep, so far as possible, in 

 statu quo with the visual field. 



A beautiful illustration of this is afforded by the 

 so-called rheotropism of fishes. Many fishes have 

 the instinct to head up stream against the current. 

 This trait has been shown by Lyon to be dependent 

 upon a visual reflex. He placed fish in an aquarium 

 with the lower side made of glass below which could 

 be drawn a long piece of cloth with alternate black 

 and white stripes on it, giving the appearance of a 

 moving bottom. As the strip was pulled along the 

 fish swam in the direction of movement. Reversing 

 the motion caused the fish to turn about and swim 

 to 'the other end of the aquarium. In another ex- 

 periment fishes were placed in a long bottle. When 

 this floated down stream the fishes all swam to the 

 up stream end. When it was pulled up stream the 

 fishes all swam to the opposite end. Fishes in a 

 stream, passively carried along, have no means of 

 becoming aware of their movements except by means 

 of objects in their field of vision any more than a 

 man in a balloon who is carried along in a current 

 of air. This automatic tendency to keep in constant 

 relations with the objects in their field of vision 

 keeps them from being passively carried down 



