348 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 Syrphidee, 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 Empididee, which Wheeler 
has observed swarming in one spot every day for no less than 
oe 
two weeks, possibly on account of ‘* some odor emanating from 
the soil and attracting and arresting the flies as they emerged 
from their pup.” 
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 
