Atta (ant)

Atta
Queen of Atta colombica tending her fungus
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Insecta
Order:	Hymenoptera
Family:	Formicidae
Subfamily:	Myrmicinae
Tribe:	Attini
Genus:	Atta Fabricius, 1805
Type species
Atta cephalotes (Linnaeus, 1758)
Diversity
17 species
Synonyms
Archeatta Gonçalves, 1942 Epiatta BorgmeAier, 1950 Neoatta Gonçalves, 1942 Oecodoma Latreille, 1818 Palaeatta Borgmeier, 1950

Atta is a genus of ants of the Neotropical realm belonging to the ant subfamily Myrmicinae. In English often the species are known as leafcutter ants, though that name is shared with members of the genus Acromyrmex.

Workers of Atta ants gather plant material from the colony's local area, carry the plant parts into the colony's underground chambers, and there fungus grows on it. All the nest's ants feed on that fungus, not the plant material they collect. Leafcutters don't sting, thus inject no venom, although they are known as strong biters.

Life Cycle

It's unclear how many species the genus Atta comprises. GBIF lists over 60 -- though many of those name are regarded as "doubtful" -- while the Catalogue of Life lists only 16.

As with all ants, Atta species undergo complete metamorphosis in which each individual ant hatches from an egg as a larva, the larva grows until it forms a pupa, from which later emerges an adult ant.

Among adults leafcutter ants, forms -- each distinctly unlike the others in appearance and duties -- are recognized. These different adult forms are referred to as castes.

Beyond that, among the worker and larva castes, physically distinct forms also have been recognized. For example, a study of Atta sexdens found that among larvae there are two distinct forms: 1) gardeners and nurses, and 2) within-nest generalists.

Atta cephalotes, Wilhelma Zoo, Stuttgart, Germany

A special behavior of certain workers is that they'll climb onto cut sections of leaf and ride the leaf back to the nest as another worker carries the leaf and its passenger. Such leaf riders protect the other workers from a particular species of phorid fly that parasitises the leaf-carriers. While hitchhiking, the ones on the leaf also work to decontaminate the leaf fragment before it arrives at the nest.

In fact, in any healthy ant community, there's such a complex division of labor among highly specialized forms that often the community itself is regarded as a superorganism.

Workers of [[Atta colombica]] cutting tree leaves

The Worker Caste

By far the greatest number of ants living in a colony belong to the worker caste. All workers are wingless females who can't reproduce sexually. Workers, besides gathering food, also excavate the nest and care for the queen and brood. Mature colonies of Atta ants can consist of millions of workers. Below, you can see a line of leafcutter workers carrying cut pieces of leaves to their colony's subterranean tunnels:

animation courtesy of "Filo gèn"; licensed under
Cc-by-sa-4.0

The Soldier Caste

Often the leafcutter soldier caste is considered to constitute a sub-cast among workers, though others recognize a separate soldier caste. Soldiers appear among most lines of workers carrying their leaf fragments, and are recognizable by their conspicuously larger size, more formidable spines, and their very large heads. Big heads are needed for housing the soldiers' powerful muscles used for closing their powerful pincer-like mandibles on the enemy. Soldiers shouldn't be confused with army ants. Like leafcutter workers, soldiers are wingless and sexless; they are in charge of the colony's defense. Someone who bothers a busy worker with a finger may end up with a soldier clamped onto the finger.

The Winged Male Caste

In any leafcutter colony, a third caste present is that of the males, often referred to as drones, or winged males. Drones are larger than workers and soldiers. They are the only males inhabiting the colony, and their sole purpose is to mate with the queen during her mating or nuptial flight. Drones are fed by workers and do little or no work in the nest. Commonly among ant species, queens can live for up to 15 years and workers about seven years, while drones survive only one to two weeks. That's because once they mate with the queen, the colony no longer needs them, and they die.

The Queen Caste

At the right, a Texas Leafcutter (Atta texana) drone climbs atop a queen, the queen constituting the fourth caste. The queen has shed her wings after her nuptial flight. When a queen is ready to mate she leaves the nest and flies into the air with many drones following her. One or more may mate with her, the number depending on the species and circumstances. Once mating is accomplished, the queen finds somewhere to begin digging a nest. Inside her nest she lays a few eggs, which hatch into workers who will expand the nest. After several years, a typical queen may average laying more than 25,000 eggs per day, and over her lifetime she may produce as many as 150 million daughters.

The Leafcutters' Fungus

Leafcutter workers carry fresh plant material into their subterranean nests, where a specialized species of fungus, Leucoagaricus gongylophorus grows on the plant material, and all the ants feed on that fungus. Larvae, pupae, and adults feed on liquids produced in the fungus' bulbous tip structures, the gongylidia.

The ants totally depend on their fungus, and provide all the fungus's needs for thriving, so both species benefit in this classic ecologically mutualistic relationship. Also the relationship can be described as symbiotic, but that's a more general term.

Atta leafcutter queens, before leaving their parent colonies, store a small part of fungus from their home colony into what's sometimes called their fungus pockets or infrabuccal pocket. Basically, the pocket is a depression below the head.

With this sample of fungus, which stays with her during her nuptial flight and while mating, when the queen loses her wings and has dug out ground to begin her future colony, she "seeds" her underground space. She doesn't eat from the fungus initially; she gives it time to grow by fertilizing it with her fecal matter. While waiting for the fungus to grow, she survives on her body fat reserves, by eating 90% of the eggs she lays, and by using nutrients made available by the degeneration, or catabolism, of her now unneeded wing muscles.

Besides their fungus, workers acquire water and nutrients from plant sap they ingest as they cut plant leaves.

Special Behaviors

Often workers are seen harvesting leaf parts well away from their nest, while the same plant species with similarly available leaves grows much closer to the nest. Also it's observed that a nest's workers may harvest leaf sections from a certain species for some time, then change to another species.

Studies suggest that many factors explain these behaviors. One reason they might harvest from a distant tree instead of a closer one is that ants can't see where the trees are the way humans do, and while scouting for food sources they do so in a rather random fashion, so they may miss something good nearby.

Also, the ants avoid plant material containing chemical compounds which are fungicidal (dangerous for the fungus), repellent, and/or toxic.

If Atta ants become trapped in a collapsed tunnel, they make a tapping sound, and their sisters will rescue them.

Predators and Disease

Army ants prey on leafcutter ant larvae. Birds, bats, and ground mammals feed on Atta queens as they're searching for a nest site after mating. Armadillos are well adapted to feed on all ant species; In Texas, Nine-banded Armadillos often feed on Texas leafcutting ants.

Evolution

The leafcutter ant genera Atta and Acromyrmex split from a common ancestral species about 10 million years ago (Mya). The Trachymyrmex group and Sericomyrmex are the closest relatives to the leafcutters; they split off about 17 Mya.

Beginning around 50 Mya, the evolutionary history of various leafcutter ant taxa has been much affected by ants coevolving with their fungus. The fungus eventually lost its ability to produce spores and the ants capitalized on that by making the fungus its main food source. Leafcutter ants aren't native to the Old World because, before this coevolutionary event began, about 66 Mya, the ants' home area in South America was isolated from other land masses. Leafcutter ants are thought to have propagated the same fungal lineage for 25 million years, during which time the ants took over the fungus's reproduction process.

Ecological effects

Atta ants are so important to the environments in which they occur that often they're referred to as ecosystem engineers, meaning that they create and modify habitats. Among their most important effects are that they transfer organic matter underground, enhance soil aeration, and increase soil nutrient availability and nitrogen fixation rates.

These effects normally are regarded as beneficial to the soil. However, a study found that emissions of carbon dioxide, or CO₂, over soil in which leafcutter nests were present, were 15 to 60% more than over nearby soil having no leafcutter nest below it. At the ecosystem scale, this amounted to leafcutter nests causing an increase of 0.2 to 0.7% more CO₂ added to the atmosphere. The emitted CO₂ was produced by the large amounts of decomposing plant material stored in the leafcutters' undergroun nests.

As human behavior and climate change increasingly fragments the tropical and subtropical parts of the Americas where leafcutters occur, leafcutter ants are becoming more abundant and their impact on soil carbon dynamics is expected to increase.

Leafcutter ants also can create gaps in forests with an otherwise closed or unbroken canopy by trimming the leaves of plants in the understory; this allow more light to hit the forest floor. Also they can alter the types of trees and other plants in their area by selectively bringing seeds into the underground chambers. If the chambers aren't too deep, seed can send shoots upward which reach light, and grow into established plants.

When Leafcutters Live among Humans

Defoliation of Desired Plants

Often people consider leafcutters to be major pests. They can entirely defoliate a wide range of ornamental, garden and agricultural plants. Because leafcutters only eat the fungus they cultivate, they may not be much affected by most commonly used insecticides.

As food

Atta winged males ("Chicatanas") for sale in Oaxaca, Mexico

Atta flying males, or drones, are a popular ingredient in Mexican cuisine, particularly in the southern states such as Chiapas, Guerrero, Hidalgo, Puebla, Veracruz, and Oaxaca. It is considered a delicacy, as well as a food of high protein content, so often it's served as a main dish, not as garnish. They may be eaten as the sole filling in tacos. A fan of chicatana dishes in Mexico describes them as "smoky and earthy, with a crunchy texture -- ahumado y terroso, con textura crujiente".

In Brazil, the flying adults of leafcutter ants (locally known as tanajuras) are highly appreciated as delicacies in several regions. The techniques involving their capture and cooking are considered an part of the cultural heritage of people of the Tianguá municipality, in Ceará.

The native Guanes people of central Colombia first began cultivating and cooking the insects in the 7th Century. Also they used the ants' sharp pincers as stitches to heal wounds. Later the marauding Spanish conquistadors adopted the habit.

Leafcutter Ants in Popular Culture

Princess Atta from A Bug's Life was named after the leafcutter genus Atta.
In Atta (novel), a man is shrunk by a bolt of lightning and befriends an ant named Atta.

Species

• Atta bisphaerica Forel, 1908
• Atta capiguara Gonçalves, 1944
• Atta cephalotes (Linnaeus, 1758) -- central Mexico south throughout most of northern South America
• Atta colombica Guérin-Méneville, 1844
• Atta cubana Fontenla Rizo, 1995 -- Cuba
• Atta goiana Gonçalves, 1942
• Atta insularis Guérin-Méneville, 1844
• Atta laevigata (F. Smith, 1858) (Colombia south to Paraguay)
• Atta mexicana (Smith, 1858) -- Arizona and Texas in the US south into Nicaragua
• Atta opaciceps Borgmeier, 1939
• Atta pilosa (Buckley, 1866) -- AntWiki considers this taxon to be unidentifiable and its identity uncertain
• Atta robusta Borgmeier, 1939
• Atta saltensis Forel, 1913
• Atta sexdens (Linnaeus, 1758)
• Atta tardigrada (Buckley, 1866)
• Atta texana (Buckley, 1860) - Texas leafcutter ant (East Texas and western Louisiana in the United States, northeastern Mexico)
• Atta vollenweideri Forel, 1893

See also

• List of leafcutter ants

External links

• BBC archive video of Atta Leaf-cutter ants
• Bristol Zoo factsheet