Atta (genus)

Atta
Queen of A. colombica
Scientific classification
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[1]
17 species
Synonyms
Archeatta Gonçalves, 1942 Epiatta Borgmeier, 1950 Neoatta Gonçalves, 1942 Oecodoma Latreille, 1818 Palaeatta Borgmeier, 1950

Atta is a genus of New World ants of the subfamily Myrmicinae. It contains at least 17 known species.

Leaf-cutter ants are relatively large, rusty red or brown in colour, and have a spiny body and long legs. The three main castes within a nest are the queen, worker and soldier.^[2] Only the queens and males have wings, and these ants are also known as 'reproductives' or 'swarmers'. Although most of the ants in the nest are female, only the queens produce eggs. Queens are usually over 20 mm long.

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).

Workers of A. colombica cutting all the leaves from a young tree

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 while 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.^[3] That the minims behave in this way demonstrates the highly derived character of the species.

A. cephalotes at work

Like Acromyrmex, Atta subsists mostly on a particular species of fungus that 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 while physically cutting out sections of leaf from a variety of plants.^[4]

A. cephalotes castes

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.

Biology

The leafcutter ant is morphologically similar to all other Hymenoptera with a body made up of three segments: the head, thorax, and abdomen (gastor). The thorax can be broken down into two major parts: the alitrunk, which contains the legs and wings, and the petiole, which is found directly anterior to the abdomen.^[5] Leafcutter ants can range from 0.1 - 0.65 inches in size. On the head of the leafcutter ant is the antennae. Their antennae are usually fairly long and generally have 10 or more segments. They are the main sensory organs of the insect; the others are the eyes. Leafcutter ants have two kinds of eyes: the 1) compound, lateral eyes, two in number and placed on the sides of the head, and the 2) simple, median eyes, ocelli, or stemmata, of which there are three on the vertex. Both kinds are best developed in the males, less in the females and least in the workers, which often lack the stemmata altogether.^[6] Atta ants have variable morphology in the sizes of their mouth parts and heads across different caste groups. The soldiers have the largest and strongest mandibles, which they use for defense and chewing. In addition, they also have the largest heads. In contrast, the worker ants use their jaws for cutting plant material, including leaves and grass, for use to harvest fungus. Atta leafcutter ants have some of the most morphologically diverse workers of any ant. The smallest workers (the minims) mainly work in the nest, caring for the brood and tending the fungus garden. Medium-sized workers (medias) include foragers, who cut the leaves and bring them back to the nest for processing. The largest workers (majors) are usually associated with colony defense. Their suitability as defenders is readily apparent in their large heads, with impressive mandibles powered by large muscles. Their giant, armored heads and sharp, powerful mandibles are formidable weapons. Although they do little else, the soldiers are always ready to rush out of the nest when disturbed to aggressively defend the colony.

Social organization

Atta ants are eusocial, meaning they exhibit cooperative care for immature individuals, have an overlap of at least two generations in same society, and exist as both reproductive and non-reproductive individuals.

Reproductives

Once a year, a colony, consisting of one queen and many thousands of workers, produces reproductive individuals called alates, which have a different morphology, including wings for flight. After these individuals leave the nest of the parent colony, mating occurs high in the air with each female mating with between three and eight males.^[7] Colonies in close proximity conduct nuptial flights at close to the same time, increasing outbreeding. Males die after the mating flight. The queens then store the sperm acquired from the males in spermathecae, which they will use to found a new colony. Mortality for queens during mating is estimated to be as high as 52%.^[7] Queens begin a new colony starting by excavation of a nest chamber 20–30 centimetres (7.9–11.8 in) below the surface; the cavity is about 6 cm long. After excavating the cavity, the queen begins a new fungus garden by spitting out a mycelial wad harvested from the fungus of her parent colony. By the third day, the mycelia has started to grow new fungus and the queen has begun to lay eggs, usually between three and six. The queen and the first larvae do not feed from the garden; rather, they eat 90% of the eggs laid by queen. During this initial stage of nest development, the queen is the sole cultivator of the new fungus garden. If she does not maintain the garden well, the colony will not survive.^[7] ·

Division of labor

Head width is an important determining factor in the division of labor for these ants. In a new colony, head width sizes only vary from 0.8-1.6 mm, because workers that tend the gardens must have a head size of less than 1 mm, while 1.6 mm is the smallest head size that can facilitate the cutting of average-toughness vegetation. As a colony grows in size, the queen begins producing individuals with a wider range of head sizes, from 0.7–5 millimetres (0.028–0.197 in). The older the colony becomes, the higher frequency of larger individuals there.^[7] This variation in size allows the ants to divide the labor into two main groups; the larger ants are foragers and the smaller ants gardeners.

The most common size group for foragers is from 2.0–2.2 millimetres (0.079–0.087 in). These ants bring in foraged leaves and drop them on the floor of a nest chamber. Then, a smaller-sized ant trims the leaves to a size of 1–2 mm across. An even smaller ant balls up the pieces of leaves to cultivate fungus on them. Finally, the smallest ants plant the fungus on the leaves, tend the garden and remove any spores from other species of fungus.^[7] Their legs are prehensile, a quality that enables efficient cutting and processing leaves.^[8][9]

The defense of the colony is also divided. When threatened by something such as a large vertebrate animal, the colony deploys its largest workers to defend the colony; however, if defending against something such as other ants, they deploy many smaller workers to repel the threat. In Atta ants, an estimated seven castes split up a total of 20-30 different tasks within the colony.^[7]

Diet

The ants of the Atta group monoculture fungus for food by removing competing fungi from their gardens. To do this, they harvest many different types of plants and plant materials, including leaves, flowers, seed husks, and stems. They use this material as a substrate for the fungus. The ants also harvest and grow bacteria of the genus Streptomyces to produce antibiotics to help fight off unwanted fungi and parasites.^[10] To grow these bacteria, the ants must first harvest the leaves. They use chemical as well as vibrational communication to uniformly dismember a branch of its leaves and return home with their harvest. While transporting the leaves, the mandibles of the workers are full and unable to be used for protection from predators. One of the most common and deadly predators to the leaf cutter ants is the parasitic phorid fly. The phorid fly will lay its eggs inside of an unexpecting ant while it is carrying its load back to the colony. The leafcutters evolved a look-out system where the smallest ants will ride on top of the leaf being carried by the worker and keep watch for any phorid flies looking to parasitize the workers. When the leaves finally reach the nest, other individuals lick the waxy cuticle off the leaves and chew it up into small pieces. Next, they inoculate the leaves with a fecal cocktail of enzymes from their hindgut. This initiates the digestion of the newly chewed leaves. The plant material will be transported to the garden, where it will be used to grow fungus on which the ants will feed. Leafcutter ants tend to dominate the ecosystems they inhabit. Some grassland Atta species colonies consume as much vegetation per hectare as domestic cattle. Certain rainforest species are estimated to cause up to 80% of all leaf damage and to consume up to 17% of all leaf production. Generally, Atta species are not generalists and have been shown to use between 50 and 75% of all Neotropic rainforest plant species.

Mounds

An ant nest is used as a place to shelter the colony, for the queen to rear her brood, to store and cultivate fungi, and to exchange food among the workers. The location of the nest is crucial and is ideally situated for an optimal balance of protection and food abundance. Atta nests are below ground, can contain multiple chambers, and can be very complex. Usually, the size of the nest is a good indicator of how old the colony is, and the size of the nests differs among species. Atta cephalotes nests are about 30 m², and A. vollenweideri has an average nest size of 34.6 m². A. colombica has a much smaller average size, which may be due to a small worker population and high nest relocation rate.^[7] Among the Atta species, A. laevigata have nests that are the deepest and contain the most chambers.^[11] These nests can have between 1,100 and nearly 8,000 chambers, can be 7 metres (23 ft) deep, and foraging tunnels can extend as far as 70 metres (230 ft).^[12] The ants control the climate within underground chambers by building them at certain soil depths and near structures (e.g., rocks or mounds) that can collect heat. They can also add decaying plant material to the chambers to warm them. This is especially important in the brooding chambers, because the developing ants need to be kept at optimal temperatures for growth. Atta ants dispose of waste from the fungus garden and other parts of the nest into special waste chambers. Usually, older ants are responsible for this because they are generally less useful to the colony.^[11] Once abandoned, the empty nests can then be utilized as nests for Brazilian stingless bees species, including Paratrigona Subnuda.^[13]

Evolution

Leafcutter ants are very specialized organisms in that they coevolved with another organism through symbiosis. This process took millions of years to occur, about 50 million years ago, which is when these ants began their relationship with plants.^[14] The fungus eventually lost the ability to produce spores and the ants capitalized on that by making the fungus its main food source. About 66 million years ago, South America was isolated from other land masses, and this is when gardening ants started their relationship with a fungus.^[14] It has been hypothesized that leafcutter ants propagated the same fungal lineage for 25 million years, which means they caused the fungus to reproduce itself.^[14] The leafcutter ants are different from other ants by their underground fungi cultivation; they have not been thought to be derived from another ant, but they bear a resemblance to the harvester ant, Pogonomyrmex.

Ecological effects

Leafcutter ants can create bottom-up gaps by forming their large nests. The ants excavate soil rich in organic matter, and store additional organic matter in their underground chambers. This creates rich soils that promote plant growth.^[15] The ants can also trim the leaves of plants in the understory, allowing for more light to hit the forest floor. They can also control the types of trees and other plants by selectively bringing seeds into the underground chambers. Depending on the location of the chamber, a seed can grow by reaching light.^[15]

Species

In popular culture

In Pixar's 1998 film A Bug's Life, the colony's princess is named Atta.

Uses

As food

Atta species are a popular ingredient in Mexican cuisine, particularly in the southern states such as Chiapas, Guerrero, Hidalgo, Puebla and Veracruz. It is considered a delicacy, as well as a food of high protein content, therefore it is often served as a main dish, not as garnish, despite its small portions. They may be eaten as the sole filling in tacos. The Atta, the type of ant most eaten in Mexico, has a nutty flavor.

Atta is also eaten by the Guanes tribe.

See also

• Acromyrmex, the other genus of leafcutter ants
• List of leafcutter ants

References

External links

Wikimedia Commons has media related to Atta.

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