Atta mexicana major worker in Mazatlan Mexico.

Though this ant is found mainly in Mexico, the southern part of its range extends down into El Salvador and Guatemala. Surprisingly, at its extreme northern range limit, A. mexicana (the Mexican leaf cutting ant) can also be seen in the U.S. - in and around Organ Pipe Cactus National Monument (extreme southern Arizona).

A. mexicana nests and lives in a wide variety of habitats within its range, from elevations of sea level to over 2100m/7000 ft.

These ants are well known for their leaf-cutting, and fungus-growing activities.

Ants of the genus Atta (at least 15 species, all in the New World) can have enormous colonies, often with several million inhabitants. A variety of differently sized worker subcastes comprise "the most complex systems of division of labor known in ants" [Hölldobler & Wilson 2011]. Mid-sized (media) workers are the ones most often seen cutting pieces of leaves from plants and trees, and carrying them back to the nest in long lines (this activity usually takes place in the daytime during the winter months, and nocturnally during summer). Inside, smaller workers (minors) chop up the plant matter into tiny fragments. Yet still smaller nestmates pulverize the material further, then add fecal droplets. Tiny strands of fungus are then added (by even smaller workers), and the pulverized plant material now acts as a growing medium for the fungus. Ultimately, the very smallest workers tend the fungus gardens, keeping them free of mold and other contaminants. All of the workers are non-reproductive females - winged (reproductive) females and males are produced, and take part in occasional mating flights. The males die soon after mating, and make no other contributions to the colony.

The cultivated fungus serves as food for the ants, especially the larvae. Adult leaf cutters probably have much of their nutritional requirements met by consuming plant sap directly, as they cut fresh vegetation.

The very largest colony members are the majors (also referred to as soldiers). These giants are much larger than the previously mentioned media and minor workers. Fewer in number than the other physical subcastes, the soldiers serve mainly as defenders, protecting the colony - especially from vertebrate threats, and in some species, from raiding army ants. They can also be seen 'patrolling' the long foraging columns of media workers. Their extremely powerful mandibles are their defensive weapons - these ants do not sting. At times, these specialized defenders also help with moving larger pieces of vegetation, slicing fruit into manageable pieces, and possibly with the removal of soil and other materials from the nest interior.

The nest structures of Atta ants can be truly colossal. The visible, above-ground portions often consist of numerous volcano-like soil craters with an entrance/exit in the center of each. These structures are made of the excavated soil brought up from below by workers, as new tunnels and galleries are excavated. In many cases, an older, more established colony will lack these prominent surface features (because the rapid rate of tunnel/gallery excavation seen in a new expanding colony has slowed or stopped).

In one study of Atta sexdens (a species closely related to Atta mexicana), a nest was found to contain 1,920 chambers, with 248 being occupied by the ants and their fungus gardens. A measured 22.72 cubic meters (40,000 kilograms / 88,184 lbs) of soil had been excavated by the ants. The researchers estimated that during the 77 months that this colony had existed, the workers had harvested 5,892 kilograms / 12,989 lbs of leaves [Hölldobler & Wilson 1990].

Some Atta species discard the refuse from their fungus gardens in underground chambers - but others, including A. mexicana, dump this material outside of the nest, often creating large piles of organic material (fungal dumps).

These ants will construct long lateral tunnels from the central nest that emerge close to reliable sources of leaves. This minimizes the time that workers must spend on the surface and exposed to the elements. These tunnels can extend for 60 meters or more, and sometimes pass beneath roads and other obstacles.

Leafcutters are very conspicuous as they engage in their foraging and nest-maintenance behaviors. Seeing long lines of workers bobbing along the ground (or a branch), carrying green leaves over their heads, is truly a delightful sight.

"Because they posses one of the most complex communication systems known in animals, as well as the most elaborate caste systems, air-conditioned nest architecture, and populations in the millions, they deserve recognition as Earth's ultimate superorganisms" - Hölldobler, B. & E.O. Wilson, The Leafcutter Ants; Civilization by Instinct

[IMAGE: Atta mexicana major worker in Mazatlan, Mexico] [scroll down for additional notes/references]

ADDITIONAL NOTES/REFERENCES:

·Hölldobler, B. & E.O. Wilson. 1990. The Ants. Cambridge, MA, Belknap/Harvard Press
·Hölldobler, B. & E.O. Wilson. 2011. The Leafcutter Ants; Civilization by Instinct. W.W. Norton & Company, New York & London
·Mintzer, A.C. Changes Over 20 Years in Populations of the Mexican Leafcutting Ant, Atta mexicana, at Organ Pipe Cactus National Monument, Arizona. National Park Service and Western National Parks Association
·Mintzer, A.C. 1979. Foraging Activity of the Mexican Leafcutting Ant Atta mexicana (F. Smith), in a Sonoran Desert Habitat (Hymonoptera, Formicidae). Insectes Sociaux, Paris, Volume 26, No. 4, pp. 364-372
·Rico-Gray, V. & Oliveira, P.S. 2007. The Ecology and Evolution of Ant-Plant Interactions. University of Chicago Press, Interspecific Interactions Series, Chicago and London
·Smith, M.R. 1963. Notes on the Leaf Cutting Ants, Atta Spp. of the United States and Mexico. Proc Entomol Soc Washington 65:299-302