34 8 ENTOMOLOGY 



ence 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. The midges of 

 the genus Chironomus, which on summer days dance in swarms 

 for hours over the same spot, orient themselves to every pass- 

 ing 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 attracting 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 phenom- 

 ena. 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 symmetrically dis- 

 tributed and can be overcome by a perfectly symmetrical action 

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



Geotropism. Gravity frequently determines the orienta- 

 tion and direction of locomotion of an animal. A freshly 

 emerged moth hangs with the abdomen downward and re- 

 mains in this position until the wings have expanded. Certain 

 dolichopodid flies found on the bark of trees " rest or walk 

 with the long axis of the body perpendicular to the earth and 



