Several species of ants have a special symbiotic relationship with aphids- they farm them! Aphids feed primarily on the sap from plants and secrete a liquid called honeydew. This secretion is very sugar-rich, and quite favoured by ants as a food source. As a result, a system has been hashed out by these insects wherein the ants herd the aphids around to the juiciest parts of plants, protect them from predators, and carry them into their nests at night and for winter. In return they are allowed to ‘milk’ the aphids- stroking the aphids with their antennae, coaxing them to secrete their honeydew which is then lapped up by the ant. In every species of ants, workers will specialize in different roles such as nursing or foraging to fulfill the needs of the colony - in farming ants, some workers will specialize just in shepherding and caring for the aphids! There's even some evidence that ants build pastures of a sort, to keep their herded aphids in. When the colony departs one nest site to form another at a new location, they will carry an aphid egg with them, to establish a new herd and maintain their resources.
If you are wondering what all the fuss for honeydew is about, you should know that many areas of the world, most notably Germany’s Black Forest, actually tend bee colonies that collect and make honey from honeydew, resulting in a stronger tasting, darker product. Ants certainly are the world’s oldest, and smallest, farmers.
Click here to watch some footage of this process.