PLANT LICE
supply, and the duration of 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 pomi. 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 can have an effect on wing production.
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