368 ENTOMOLOGY 



MOVEMENT 



The movement of air is physiologically important in affecting the 

 rate of evaporation from the bodies of animals. With other conditions 

 constant, the rate of evaporation is proportional to the strength of 

 the air-current. 



The directive effect of currents of air (anemotropism) has been dis- 

 cussed (page 305). Some^insects turn away from currents of air be- 

 cause of increased evaporation. (Shelford.) 



Winds are highly effective agents in the distribution of insects. 

 To what has been said on this subject (page 323) these remarks may be 

 added. 



In the case of the cotton boll weevil, "prevailing winds frequently 

 cause the majority of the insects to follow one course." (Hunter and 

 Pierce.) 



The natural spread of the gipsy moth is accomplished chiefly by 

 means of winds, acting on the hairy first-stage larvae. (A. F. Burgess.) 



Hessian flies are often carried two miles, in an uninjured condition, 

 by strong winds. One female must have been carried five miles. (J. W. 

 McColloch.) These flies may be borne by winds with a velocity of 

 twenty-five miles or more per hour; mosquitoes, on the other hand, 

 cling to herbage near the ground during strong winds, but are conveyed 

 many miles by gentle breezes. 



The green-bug, Toxoptera graminum, and many other plant lice 

 are widely distributed, as winged viviparous females, by the wind. 

 "If the temperature be below the point of activity for the species, 

 it is very clear that the velocity of the wind would have no effect what- 

 ever upon the diffusion of the insect. The conditions necessary, then, 

 for the wind to exert its greatest influence will be a decreasing food 

 supply for the insect under a temperature considerably above that 

 actually necessary for its activity, with numbers not seriously reduced 

 by parasites; under these conditions, many species of aphids are known 

 to be carried about in immense numbers by the winds. " (Webster and 

 Phillips.) 



ELECTRICITY 



Electric currents have a directive effect on animals (electrotropism, 

 gahanotropism) but the conditions under which this effect is obtained 

 are artificial, and may or may not be paralleled in nature. 



Atmospheric electricity, the effects of which vary with variations 



