"Yes, please, if it isn't too long," says Mary.
Mary and I are on perfectly frank terms. We are polite, but also inclined to be honest. And Mary is not going to be an unresisting victim of a garrulous old professor. But Mary need not be afraid that I sha'n't know when I am boring her. We have wireless communication, Mary and I. That's one, probably the principal, reason why we are such good companions. No true companionship can possibly persist without wireless and wordless communication.
"All right," I answer, "here goes, Mary. Say when!"
"I forget how many millions of bushels of corn were raised in the state of Illinois last year, but they were very many. And that means thousands and thousands of acres of corn-fields. Now in all these corn-fields there live certain tiny soft-bodied insects called corn-root aphids. Their food is the sap of the growing corn-plants which they suck from the roots. Although each corn-root aphid is only about one-twentieth of an inch long and one-twenty-fifth of an inch wide and has a sucking-beak simply microscopic in size, yet there are so many millions of these little insects all with their microscopic little beaks stuck into the corn-roots and all the time drinking, drinking the sap which is the life-blood of the corn-plants that they do a great deal of injury to the corn-fields of Illinois and cause a great loss in money to the farmers.
"So the wise men have studied the ways and life of these little aphids to see if some way can be devised to keep them in check. The aphids live only two or three weeks, but each one before it dies gives birth to about twelve young aphids. Now this is a very rapid rate of increase. If all the young which are born live their allotted two or three weeks and produce in their turn twelve new aphids, we should have about ten trillion descendants in a year from a single mother aphid. Ten trillion corn-root aphids, tiny as they are, would make a strip or belt ten feet wide and two hundred and thirty miles long!
"Some other kinds of aphids multiply themselves even more rapidly. An English naturalist has figured out that a single-stem mother of the common aphis, or 'greenfly' of the rose, would give origin, at its regular rate of multiplication and provided each individual born lived out its natural life, which is only a few days at best, to over thirty-three quintrillions of rose aphids in a single season, equal in weight to more than a billion and a half of men. Of course such a thing never happens, because so many of the young aphids get eaten by lady-bird beetles and flower-fly larvæ and other enemies before they come to be old enough to produce young.
"However, besides this rapid increase of the corn-root aphids, there is something else that helps them to be so formidable a pest. And this is that they find very good and zealous friends in the millions of little brown ants that also live in the Illinois corn-fields. These swift, strong, brave little ants make their runways and nests all through the corn-fields, and are very devoted helpers of the soft-bodied helpless aphids. For the aphids pay for this help by acting as 'cattle' for the ants.
"This is what Professor Forbes, a very careful and a very honest naturalist, found out about the ants and the aphids. The eggs of the aphids, hosts of shining black, round, little seed-like eggs, are laid late in the autumn. These eggs are gathered by the ants and heaped up in piles in the galleries of their nests, or sometimes in special chambers made by widening the runways here and there. All through the winter these eggs are cared for by the ants, being carried down into the deeper and warmer chambers in the coldest weather, and brought up nearer the surface when it is warm. When the sunny days of spring begin to come, the eggs are even brought up above ground and scattered about in the sunshine, then carried down again at night. The little ants may be seen sometimes turning the eggs over and over and carefully licking them as if to clean them of dust-particles.
"In the late spring the aphid eggs hatch, and the young must have sap to drink right away. Their little beaks are thirsty for the plant-juices that are their only food. But there are no tender corn-roots ready for them in the fields because the corn has not yet been planted. What, then, shall the hungering baby aphids and their foster-mothers, the little brown ants, do?