Leaf-cutter ants live in colonies that can be huge, up to three or four million individuals, occupying as many as three thousand underground rooms.
These ants live in a partnership with a fungus that grows nowhere else in the world except in leaf-cutter ant colonies. The ants depend on the fungus and the fungus depends on the ants. The fungus has lost the capacity to reproduce on it's own. The ants grow the fungus, feeding it pieces of leaves and flowers that they bring back, often from high up on rainforest trees. They destroy more leaves than any other insect in the rainforest.
The ants chew up the leaves for the fungus, and fertilize it, and weed out any other plants that start growing in the fungus garden. It is also the ants that start the new gardens (the fungus has lost the ability to do so by itself). When a young leaf-cutter ant queen flies off to start a new ant colony, she takes with her in her mouth a small piece of the fungus to begin a new garden.
The ants also depend on parts of the fungus to digest their food; they feed the fungus the part of the plant they were unable to digest (the celulose). The ants, in turn, then feed on the fungus; the leaf-cutter ant larvae eat nothing but the fungus.
You can see ants carrying pieces of leaf down a tree in the rainforest. Many carry pieces so large that it would be as if you were carrying a dinning room table down a tree.
And so many of these leaves, a smaller ant is perched. Why? It acts as a guard, protecting the larger ant against a parasitic fly. This kind of fly tries to lay its eggs on an ant's head. The larvae that hatch from these eggs would kill the ant. The ant carrying the leaf can't protect itself without dropping the leaf, so the smaller ant rides along to act as guard.