38 THE TROPISMS 



In another experiment by Lyon fish were placed in a 

 long bottle which was corked. When the bottle was pulled 

 through the water the fish immediately swam opposite the 

 direction of movement. If the bottle was allowed to flow 

 down stream the fish would swim to the up stream end; 

 if it was pulled up stream the fish would swim to the down 

 stream end. It is evident that rheotaxis takes place through 

 orientation to objects in the field of vision. In young 

 lobsters Hadley has shown that rheotaxis is a sight response 

 much as in fishes. In later stages when the lobsters keep 

 closer to the bottom rheotaxis is gradually lost. 



Blinded fish may orient themselves to currents so long as 

 they are in contact with the bottom of the stream, but when 

 they leave the bottom they lose entirely their rheotactic 

 response, unless different parts of the stream with which 

 they come in contact have different velocities. 



Many insects tend to fly against the wind (anemotropism). 

 This also is probably a sight reflex, since the insect receives 

 pressure from the air only when flying against it. May 

 flies, according to Radl, often hover over one spot, slowly 

 rising and sinking, but keeping their bodies facing the wind 

 and their long fore-legs stretched forward. The same trait 

 is shown by many insects, especially flies, even when there is 

 no perceptible breeze. Often a group of flies will keep 

 hovering near one, and will move as one's body moves always 

 keeping away at about the same distance. Apparently 

 we have to do here with an effort to maintain a certain 

 relation to the visual field which so largely determines the 

 rheotactic responses of fishes. 



It is obvious that under the head of rheotaxis phenomena 

 have been included which are quite unlike in their causation, 

 some of them being reactions to pressure differences, others 

 presenting interesting points of similarity to phototaxis. 



