What excatly is a aphid

The aphid, or plant louse, is an insect that feeds on the green parts of plants. It is so small that it often takes fifteen or twenty aphids in a row to measure an inch. The various kinds of aphid differ in color but are most often greenish. They are soft-bodied and sluggish. Some aphids have wings, others do not. Aphids suck the sap or juice of a plant with mouths that look like tubes.

This destroys many farm crops and vegetables. Since aphids breed in tremendous numbers, they would destroy much more if there were not many insects that eat aphids. Great numbers of aphids are also eaten by birds. Farmers protect their crops against aphids by spraying plants with a poison called nicotine sulfate. Some aphids give off a sweet liquid called honeydew, which ants like to lick off their backs. Many ants keep "herds" of aphids, and feed them, in order to get this honeydew, just as farmers keep cows to get milk. For this reason aphids are sometimes called "ant cows."