Ants and aphids share a well-documented relationship of mutualism. Ants feed on the sugary honeydew left behind by aphids. In exchange, the ants protect the aphids from predators and parasites. In fact, honey ants will go to unusual lengths to ensure the health of the aphids in their care.
Aphids suck the sugar-rich fluids from their host plants. Because these liquids are low in nitrogen, the aphids must consume large quantities of them to gain adequate nutrition. The aphids then excrete equally large quantities of waste, called honeydew, which is high in sugar content.
Where there's sugar, there's bound to be ants. Some ants are so hungry for the honeydew, they'll actually "milk" the aphids to make them excrete it. The ants use their antennae to stroke the aphids, stimulating them to release the honeydew. Some aphid species have lost the ability to poop on their own, and now depend on their caretaker ants to milk them.
Aphid-herding ants make sure their "cattle" stay well-fed and safe. When the host plant is depleted of nutrients, the ants carry their aphids to a new food source. If predatory insects or parasites attempt to harm their wards, the ants will defend them aggressively. Some honey ants even go so far as to destroy the eggs of known aphid predators like lady beetles.
Some species of honey ants continue to care for their aphids during winter. The ants carry the aphid eggs home, and tuck them away in their nests for the winter months. They store the precious aphids where temperatures and humidity are optimal, and move them as needed when conditions in the nest change. In spring, when the aphids hatch, the ants carry them to a host plant to feed.
While it appears the ants are generous caretakers of their aphid charges, they've really got their own interests in mind. Aphids are almost always wingless, but certain environmental conditions will trigger them to develop wings. If the aphid population becomes too dense, or food sources decline, the winged aphids can fly to a new location. Not wanting to lose their food source, honey ants may prevent aphids from dispersing.
Ants have been observed tearing the wings from aphids before they can become airborne. A recent study has also shown that ants can use semiochemicals to stop the aphids from developing wings, and to impede their ability to walk away.