Leafcutter ant

Leafcutter ants are social insects found in warmer regions of Central and South America. These unique ants have evolved an advanced agricultural system based on ant-fungus mutualism. They feed on special structures called gongylidia produced by a specialized fungus that grows only in the underground chambers of the ants' nest.

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Different species of leafcutter ants use different species of fungus, but all of the fungi the ants use are members of the Lepiotaceae family. The ants actively cultivate their fungus, feeding it with freshly-cut plant material and maintaining it free from pests and molds. This mutualist relationship is further augmented by another symbiotic partner, a bacterium that grows on the ants and secretes chemicals, or secondary metabolites, which protect the fungus from molds that would feed on the fungus - essentially the ants use portable antimicrobials. Leaf cutter ants are sensitive enough to adapt to the fungi's reaction to different plant material, apparently detecting chemical signals from the fungus. If a particular type of leaf is toxic to the fungus the colony will no longer collect it.

  Leafcutter ants comprise two genera — Atta and Acromyrmex — with a total of 39 species^[1] (15 in Atta and 24 in Acromyrmex), some of which are major agricultural pests. For example, some Atta species are capable of defoliating an entire citrus tree in less than 24 hours.

The Acromyrmex and Atta ants have much in common anatomically; however, the two can be identified by their external differences. Atta ants have 3 pairs of spines and a smooth exoskeleton on the upper surface of the thorax while Acromyrmex have 4 pairs and a rough exoskeleton. [1]

A mature leafcutter colony can contain more than 8 million ants, mostly sterile female workers. They are divided into castes, based mostly on size, that perform different functions. Acromyrmex and Atta exhibit a high degree of biological polymorphism, four castes being present in established colonies - minims, minors, mediae and majors. Majors are also known as soldiers or dinergates. Atta ants are more polymorphic than Acromyrmex, meaning that there is comparatively less differential in size from the smallest to largest types of Acromymex.

• Minims are the smallest workers, and tend to the growing brood or care for the fungus gardens. Head width is less than 1 mm.

• Minors are slightly larger minima workers and are present in large numbers in and around foraging columns. These ants are the first line of defense and continuously patrol the surrounding terrain and vigorously attack any enemies that threaten the foraging lines. Headwidth are around 1.8-2.2 mm

• Mediae are the generalized foragers, who cut leaves and bring the leaf fragments back to the nest.

• Majors are the largest worker ants and act as soldiers, defending the nest from intruders, although there is recent evidence that majors participate in other activities, such as clearing the main foraging trails of large debris and carrying bulky items back to the nest. The largest soldiers (Atta laevigata) may have total body lengths up to 16 mm and head widths of 7 mm.

When the ants are out collecting leaves, they are at risk of being attacked by the phorid fly, a parasitic pest which lay eggs into the crevices of the worker ants head. Often a minim will sit on the worker ant and ward off any attack. [2]

In Central America, leafcutter ants are referred to as "Wee Wee" ants, though not based on their size. They are one of the largest ants in Central America; the queen may be larger than a mouse.

See also

• Atta sexdens
• Lepiotaceae

References