INSECTS
the tree depends for a stunted and yellowed. Now is the time for the orchardist to spray if he has not already done so.
The entomologist, however, takes note that all the young aphids on the apple trees are not alike; perhaps there are three kinds of them in the orchard (Fig. 95), differing slightly, but enough to show that each belongs to a separate species. When the first buds infested are
Fig. 95. Three species of young aphids round on apples in the spring
A, the apple-grain aphis, Rhopalosiphum prunifoliae. B, the green apple aphis, Aphis pomi. C, the rosy apple aphis, Anuraphis roseus
exhausted, the insects migrate to others, and later they spread to the larger leaves, the blossoms, and the young fruit. The aphids all grow rapidly, and in the course of two or three weeks they reach maturity.
The full-grown insects of this first generation, those produced from the winter eggs, are entirely wingless, and they are all females. But this state of affairs in no wise hinders the multiplication of the species, for these remarkable females are able of themselves to produce offspring (a faculty known as parthenogenesis), and furthermore, they do not lay eggs, but give birth to active young. Since they are destined to give rise to a long line of summer generations, they are known as the stem mothers.
One of the three aphid species of the apple buds is known as the green apple aphis (Fig. 95 B). During the
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