Atta (genus) - Overview

Overview

Ants of the genus Atta are leafcutter ants that comprise one of the two genera of leafcutting ants within the tribe Attini, along with Acromyrmex.

Atta is one of the most spectacular of the attines, with colonies that can comprise in excess of one million individuals.

Atta exhibits a high degree of polymorphism, with four castes being present in established colonies: minims (or 'garden ants'), minors, mediae, and majors (also called soldiers or dinergates).

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 Atta species. An example of such is the behaviour of the minim ants, which climb on the cut sections of leaf whilst they are carried back to the nest by the media workers to protect the latter from a particular species of phorid fly that parasitises the leaf-carrying caste. While hitchhiking, the minims also work to decontaminate the fragment before it arrives at the nest, and feed on the sap of the leaf. That the minims behave in this way demonstrates the highly derived character of the species.

Like Acromyrmex, Atta subsists mostly on a particular species of fungus which 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, winged females take a small section of fungus into their bucchal pouches, and with this, the subsequently wingless queens 'seed' the fungus gardens of incipient colonies, cutting and collecting the first few sections of leaf themselves.

Atta has evolved to change food plants constantly, preventing a colony from completely stripping off 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 farming activity.