[XXXV]
GREEN-FLIES, PLANT-LICE, AND PARTHENOGENESIS
The minute “green-flies” which attack all kinds of plants, and among which are ranked the hop-louse or hop-blight, the rose aphis or green-fly of rose trees, the woolly blight or aphis of apple trees and pear trees, and the terrible vine-killer—the Phylloxera vastatrix—form a special group of bug-like insects known as the Aphides. They have soft cylindrical bodies, six legs, sometimes two pairs of transparent wings, sometimes none, and a sharp beak (in some kinds this is one and a half times as long as the body), with which they prick the soft parts of plants, when they suck up the juices which issue from the wound ([Fig. 59]). There is in the temperate regions of the world a special kind of aphis or plant-louse peculiar to each of many kinds of flowering plants, including most trees. A very complete, illustrated account of the kinds or species of British aphides, amounting to some two hundred, was produced by the late Mr. Buckton, F.R.S., and published by the Ray Society.
There are many facts of extraordinary interest about these tiny swarming insects. In the first place, they are closely related to the minute scale-insects or Coccidæ, several species of which produce the celebrated lac of lacquer-work and the dyes known as lake, cochineal, and kermes, the latter a dye manufactured in South Europe and used to colour wool and cloth crimson before cochineal reached us from Mexico. The Coccidæ include also the “mussel-scale” and other destructive diseases of fruit trees. A beautiful purple colour can be extracted from crushed masses of some kinds of aphides (as well as from Coccidæ), and has been used as a dye. The aphides have very generally a green colour, like many insects (caterpillars and leaf insects) which pass their lives upon green leaves and feed on them. It is often supposed that this green colour is merely the green colouring matter (so-called chlorophyll) of the leaf, taken up by the insects in feeding on the leaf. But this is not so; it is a peculiar substance derived in a crude state from the plant-juice, but digested in the stomach and completed in the insects’ blood and tissues. Then, again, the aphides produce curious secretions, often in great abundance, which surround them as the lac surrounds the lac-insect. The threads which are produced in such abundance, by the woolly aphis of apple trees, as to look like masses of cotton wool adhering to the twigs of the tree, are of this nature.
Fig. 58.—Foundress or stock-mother of the hop-louse: the individual hatched from a winter egg, laid on the bark of a plum tree, who produces viviparously a wingless virgin brood. That brood produces wing-bearing young, which fly off to the hop-plants.
Fig. 59.—Side view of winged viviparous female of the hop-louse, b, the stabbing beak.