PLANT LICE
no bustle, no commotion, for each insect has its sucking bristles buried in the leaf, and its pump is busy keeping the stomach supplied with liquid food. The aphis crowds are mere herds, not communities or social groups as in the case of the termites, ants, or bees.
Wherever there are aphids there are ants, and in contrast to the aphids, the ants are always rushing about all over the place as if they were looking for something and each wanted to be the first to find it. Suddenly one spies a droplet of some clear liquid lying on the leaf and gobbles it up, swallowing it so quickly that the spherule seems to vanish by magic, and then the ant is off again in the same excited manner. The explanation of the presence and the actions of the ants among the aphids is this: the sap of the plants furnishes an unbalanced diet, the sugar content being far too great in proportion to the protein. Consequently the aphids eject from their bodies drops of sweet liquid, and it is this liquid, called "honey dew," that the ants search out so eagerly. Some of the ants induce the aphids to give up the honey dew by stroking the bodies of the latter. The glistening coat often seen on the leaves of city shade trees and the shiny liquid that bespatters the sidewalks beneath is honey dew discharged from innumerable aphids infesting the under surfaces of the leaves.
In studying the termites, we learned that it is possible for a single pair of insects to produce regularly several kinds of offspring differing in other ways than those of sex. In the aphids, a somewhat similar thing occurs in that each species may be represented by a number of forms; but with the aphids these different forms constitute successive generations. If events took place in a human family as they do in an aphid family, children born of normal parents would grow up to be quite different from either their father or their mother; the children of these children would again be different from their parents and also from their grandparents, and when mature they
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