PLANT LICE 



supply, and the duration ot light. At a temperature 

 around sixty-five degrees few winged individuals ever 

 appear, but they are produced at temperatures either 

 below or above this point. Likewise it has been found 

 that when the food supply gives out through the drying 



Fig. 97. The green apple aphis, Aphis pami. A, B, adult stem mothers. 



C, a newly-born young of the summer forms. D, a wingless summer 



form. E, a winged summer form 



of the leaves or by the crowding of the aphids on them, 

 winged forms appear, thus making possible a migration 

 to fresh feeding grounds. Then, too, certain chemical 

 substances, particularly salts of magnesium, added to 

 the water or wet sand in which are growing cuttings 

 of plants infested with aphids, will cause an increase of 

 winged forms in the insects subsequently born. This 

 does not happen if the plants are rooted, but it shows 

 that a change in the food cmi have an effect on wing 

 production. 



[165] 



