The honeydew is excreted in such quantities that often the pavement beneath trees may be seen to be spattered by the drops of this sweet rain. It seems to be excreted solely for attracting the ants. In return for this, the ants give care and protection to their herds. They sometimes take them into their nests and care for them. In one case, at least, one species of ant builds for one species of aphid (which lives upon dogwood) a little mud stable which protects the aphids from all enemies. This stable is neatly placed at the fork of the twigs and has a little circular door by which the ants may enter (Fig. 158). The lady-bug larvæ and the ant-lions both feed voraciously on the aphids; an ant will attack single-handed one of these depredators, although it be much larger than herself, and will drive it away or perish in the attempt. Some so-called practical people say, "Let us study only those things in Nature that affect our pocketbook, and not waste our time studying irrelevant things." If this spirit had animated scientists from the first, many of the most important economic discoveries would never have been made. This relation of ants to aphids is an example to the point. For a hundred years has the fact been known that ants use the aphids for their cows, and the practical men said, "This is a very pretty story, but what we want is some method of killing the aphids." It remained for Professor Forbes, of Illinois, to show the practical application of this "pretty story" in the life history of the corn-root plant-louse, which did great damage to the corn crop of the West. These plant-lice winter in the ground wherever they chance to be left by the dying roots of the last year's crop, and with their soft bodies could never work their way in the hard earth and to the roots of the newly-planted corn in the spring. Professor Forbes discovered that the ants in these infested fields make mines along the principal roots of the new corn; and that they then go out and collect the plant-lice, and place them in these burrows, and there watch over them and protect them. OBSERVATION LESSON ON THE RELATION OF ANTS TO PLANT-LICE. _A reading-glass or lens may be used to advantage in making these observations._ _Find some plant near at hand that is infested by aphids in order to note from time to time the relation of ants to these little creatures. Some aphids on the petiole and leaves of the Virginia Creeper on our piazza once afforded me a convenient field for daily observation._ 1. How does the ant approach the aphid and ask for honeydew? 2. Does she wait long if there is no response? 3. Does the ant step on the aphids as she runs about among them? 4. What are the colors of the aphids you have observed? 5. On what plants were they feeding? 6. What sort of mouth parts have the aphids? 7. What part of the plant is their food, and how do they get it? 8. Why does not Paris green applied to the leaves on which aphids are feeding kill them?