Leaf cutter ants:

Leaf cutter ants are a superb example of an obligate mutualism. The ants, which live in colonies of up to five million individuals, depend on a species of fungus that is found nowhere but in leaf cutter nests. The ants cut and bring huge amounts of leaf matter into their nests, which serves as a substrate for the fungus. The ants feed bits of the fungus to their developing larvae.