Leaf-cutter ants (Atta and Acromyrmex) feed exclusively on a fungus that grows only within their colonies. They continually collect leaves which are taken to the colony, cut into tiny pieces and placed in fungal gardens. Workers specialize in related tasks according to their sizes. The largest ants cut stalks, smaller workers chew the leaves and the smallest tend the fungus. Hyphal swellings of the fungus called gongylidia are fed to the developing larvae and distributed throughout the colony to feed workers, soldiers, and the queen. The ellipsoid gongylidia are about 30-50 micrometers in diameter and are rich in lipids and carbohydtrates. [Some references say that the ant larvae feed on the fungus and adults feed on leaf sap].

The symbiotic basidiomycete fungus living inside colonies of Atta cephalotes and Atta sexdens has been identified as Leucoagaricus gongylophorus = Leucocoprinus gongylophorus. Interestingly enough, fruiting bodies (basiocarps) of Leucoagaricus species can be found in urbanized areas of North America, including L. americanus and L. naucinus. The two latter fungi have also been listed in the genus Lepiota.

The tropical leaf-cutter colony (Atta cephalotes) is composed of several castes, including the queen, workers and large soldiers who often stand guard at the entrance of the nest, or even go on scouting missions to protect the colony from predators. Like the queen, the males are winged, and their only role is to inseminate the virgin queen. Workers include larger "media workers" who cut and carry leaf sections back to the nest, and "minima workers" who cut the leaves into minute pieces for the fungus garden. They also cover the leaf fragments with their antibacterial saliva which retards the growth of competing fungi, thus protecting their symbiotic fungus that is vital for their survival. They also feed the entire colony of ants.

From Wikipedia (5 February 2017). Of all the ant species, the leaf-cutter ants have the most complex caste system. Next to humans, leafcutter ants form the largest and most complex societies on Earth. Each queen mates with multiple males and stores sperm to start her colony. She carries bits of the fungus garden mycelium in her oral cavity (infrabuccal pocket).

There are 4 main castes:

1. Minims: The smallest workers that tend the brood and care for the fungus gardens. Their head width is less than one millimeter.

2. Minors: Present in large numbers around foraging columns and are the first line of defense.

3. Mediae: Generalized foragers that cut leaves and bring leaf fragments back to the nest.

4. Majors (Soldiers): Largest ants that defend nest from intruders.

Another symbiotic partner, the actinobacterium Pseudonocardia, grows on the ants and secretes antibiotic chemicals. Actinobacteria are responsible for producing the majority of the world's antibiotics.