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INSECT BEHAVIOR 305 



bottle. It is obviously the motion of the retina images of the objects 

 on the bank of the brook which causes the "rheotropic" orientation of 

 fishes. When driven backward by the current or when dragged back- 

 ward in a bottle through the water, the objects on the bank of the river 

 seem to move in the opposite direction. The animal being compelled 

 to keep the same object fixed, an apparent forward motion of the fixed 

 object changes the muscles of the fins in such a sense as to cause the 

 animal to follow the fixed object automatically. When such rheotropic 

 fishes were kept in an aquarium and a white sheet of paper with black 

 stripes was moved constantly in front of the aquarium the fishes ori- 

 ented themselves against the direction in which the paper and its 

 stripes moved. The phenomenon was more marked in young than in 

 older specimens. All the phenomena of rheotropism ceased in the dark 

 or when the fishes were blind. (J. Loeb.) 



Anemotropism.- Various flies orient the body with reference to the 

 direction of the wind. Wheeler observed swarms of the male of Bibio 

 albipennis poising in the air, with all the flies headed directly toward the 

 gentle wind that was blowing. If the wind shifted, the insects at once 

 changed their position so as again to face to windward; a strong wind, 

 however, blew them to the ground. The males of an anthomyiid 

 (Ophyra leucostoma), according to the same naturalist, hover in swarms 

 in the shade for hours at a time; if the breeze subsides they lose their 

 definite orientation, but if it is renewed they face the wind with military 

 precision. In Syrphidae, he finds, either males or females are positively 

 anemotropic. Midges of the genus Chironomus, which on summer days 

 dance in swarms for hours over the same spot, orient themselves to 

 every passing breeze. So also in the case of Empididae, which Wheeler 

 has observed swarming in one spot every day for no less than two weeks, 

 possibly on account of "some odor emanating from the soil and attract- 

 ing and arresting the flies as they emerged from their pupae." 



The Rocky Mountain locusts "move with the wind and when the 

 air-current is feeble are headed away from its source;" when the wind is 

 strong, however, they turn their heads toward it. 



Anemotropism and rheotropism are closely allied phenomena. As 

 Wheeler says, "The poising fly orients itself to the wind in the same way 

 as the swimming fish heads upstream," adjusting itself to a gaseous 

 instead of a liquid current. "In both cases the organism naturally 

 assumes the position in which the pressure exerted on its surface is sym- 

 metrically distributed and can be overcome by a perfectly symmetrical 

 action of the musculature of the right and left halves of the body." 



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