Atta (genus)
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 queen of A. columbica
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Atta is a genus of New World ants of the subfamily Myrmicinae.
[edit] Overview
Commonly known as "leaf-cutter ants" they comprise one of the two genera of advanced attines within the tribe Attini, along with Acromyrmex.
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 behaviour 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 parasitises 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 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, female alates take a small section of fungus into their bucchal pouches and it is with this that the subsequently deälated queens 'seed' the fungus gardens of incipient colonies, cutting and collecting the first few sections of leaf themselves.
Atta has evolved to change foodplant 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 arable farming activity.
[edit] See also
