“The gardener pulls it all up and throws it on the dung-hill. His care and vigilance have been unable to arrest the invasion. In vain he crushed legions at a time under his angry heel: in a few days the half-dozen survivors had propagated a larger colony than ever. Man is hardly in a position to contend successfully against this lowly vermin which braves extinction by virtue of its countless numbers. “As I told you, the plant-louse does not like to change its place. It plants its sucker on the very spot where it has just been born, and thenceforth sticks to that spot, filling its stomach with sap and surrounding itself with a family. This love of repose explains to us very well how the twig of a rosebush or the top of a beanstalk undergoes a progressive colonization; but it does not account for the distant propagation of the species. “With its home-keeping habits the insect ought to be confined within narrow limits, on a single leaf and not on all leaves, on one rosebush and not on the neighboring rosebushes. But as a matter of fact it is disseminated everywhere. When one patch of beans becomes infested, those in the neighborhood are equally unfortunate; when one rosebush shows a colony of plant-lice, all those around it are similarly visited. No vegetable growth can defend itself from the pest. How, then, is it that this obese animalcule, which totters with fatigue after one step forward, succeeds in passing from rosebush to rosebush, from garden to garden? By what means is it able to spread in all directions without limit? “Let us examine a number of rosebushes, and we shall have a prompt answer to our question. In addition to the wingless plant-lice, big of belly and all grouped on the tender twigs, we shall see others, green like the first ones, but more elegant in form, of greater freedom of movement, and provided with four wings, very beautiful wings too, diaphanous and gleaming with rainbow tints. These creatures are no lazy sap-bibbers forever squatting over the well their sucker has bored. They are seen to come and go, circulating briskly among the stationary herd, inspecting the foliage, passing from branch to branch, and even taking flight for some distant goal. They are the travelers of the family. Their function is to propagate the race in the surrounding district, with the aid of their wings, and even at considerable distances when a puff of wind carries them thus far.