Leafcutter Ants 

Leafcutter ants are social insects found in warmer regions of Central and South America. These remarkable ants have evolved an advanced agricultural system. They feed on a specialized fungus that grows only in the underground chambers of the ants' nest. The ants actively cultivate their fungus, feeding it with freshly cut vegetation and maintaining it free from pests and weeds. This is done by a symbiotic relationship with a colony of bacteria that grows on the ants that protect the fungus. The bacteria evolve along with the mold that would feed on the fungus - essentially the ants have their own portable antibiotics. 

Leafcutter ants comprise 2 genera — Atta and Acromyrmex — with a total of about 50 species, some of which are major agricultural pests. Some Atta species, for example, are capable of defoliating an entire citrus tree in less than 24 hours. 

A mature leafcutter colony can contain more than 2 million ants, mostly sterile female workers. They are divided into castes, based mostly on size, that perform different functions involving gathering leaves outside the nest, turning plant material into paste for the fungus to digest, and harvesting the fungus for others to eat. Some are soldiers who defend the nest from intruders. 

Atta is one of the most spectacular of the attines, and colonies can comprise in excess of one million individuals. Atta exhibits a high degree of polymorphism, five castes being present in established colonies - minims (or 'garden ants'), minors, mediae and majors (also called soldiers or dinergates) being present. 

The high degree of polymorphism in this genus is also suggestive of its high degree of advancement. Every caste has a specific function, and some remarkably advanced phenomena have been observed in respect of Atta species. An example of such is the behavior of older minor workers: young minors work within the nest, tending the fungus gardens, but older ants perform a different duty - they climb on the cut sections of leaf whilst they are carried back to the nest by the media workers solely to protect the latter from a particular species of phorid fly that parasitizes the leaf-carrying caste. When a media is maneuvering a leaf section back to the nest, it cannot protect itself from this fly, and that the minors behave in this way demonstrates the apex of evolutionary advancement that this species embodies. 

Like Acromyrmex, Atta subsists entirely on a particular species of fungus that it cultivates on a medium of masticated leaf tissue. This is the sole food of the queen and other colony members that remain in the nest. The media workers also gain subsistence from plant sap they ingest whilst physically cutting out sections of leaf from a variety of plants. Before leaving their parent colonies, female alates take a small section of fungus into their bucchal pouches and it is with this that the subsequently dilated queens 'seed' the fungus gardens of incipient colonies, cutting and collecting the first few sections of leaf themselves. 

Atta has evolved to constantly change food plant, preventing a colony from completely stripping of leaves and thereby killing trees, thus avoiding negative biological feedback on account of their sheer numbers. However, this does not diminish the huge quantities of foliage they harvest - Atta is estimated for being responsible for the decomposition of 20% of all leaves in South America. Consequently, the genus is considered a major agricultural pest species in areas where its range coincides with arable farming activity.