Atta sexdens

Species of ant

Atta sexdens is a species of leafcutter ant belonging to the tribe Attini, native to America, from the southern United States (Texas) to northern Argentina. They are absent from Chile. They cut leaves to provide a substrate for the fungus farms which are their principal source of food. Their societies are among the most complex found in social insects. A. sexdens is an ecologically important species, but also an agricultural pest. Other Atta species, such as Atta texana, Atta cephalotes and others, have similar behavior and ecology.

A. sexdens colonies are primarily subterranean with a mound of excavated material on the surface. The diameter of the colony may reach 10 m with a depth of 6 m. The colonies contain up to 2000 chambers with a combined volume of more than 20 m3. The two basic types of chambers are fungus farms and waste chambers. The farms contain the fungal culture which sustains the colony and hosts larvae and pupae. The waste chambers are located at the rim of the colony and are significantly larger. They are used to dispose used fungal culture and dead ants. A mature colony contains 5–8 million workers. The colony often forms the centerpiece of a large jungle clearing. At the top of the mound are structures resembling sand castles surrounding hundreds of openings to the colony. Positioning the openings on top of these structures minimises the amount of rainwater flowing into the colony. The openings also have an important role in air conditioning. As the ant activity and fungal metabolism heat up the colony, hot air rises through the central passageways. Simultaneously, fresh air is drawn in from the openings at the rim of the colony.

A. sexdens workers spend a considerable time cleaning each other and the queen. They also spread antibiotic secretions from their metapleural glands on their bodies. Dirt collected by licking is stored in the workers' infrabuccal cavities, which are special recesses in their mouths. The purpose of these activities is to avoid infections, especially by parasitic fungi. Several fungus species, such as Cordyceps, specially infect and kill ants. All waste in the nest, including infrabuccal pellets, spent substrate from the gardens, and dead ants are carried in the waste chambers to avoid contamination. Feces, however, are not carried away, but used to fertilise the fungal gardens. The waste chambers are larger than the human head and located at the rim of the colony. There, the waste can decompose without endangering the ants. Due to the high concentration of various nutrients in the decomposing waste, a thick net of plant roots usually permeates the compost.

From late October to mid December, the A. sexdens colonies produce winged virgin queens and males. Before the nuptial flight, young queens visit the colony's fungal gardens and place a small piece of fungal mycelium in their infrabuccal cavities. During their nuptial flights, the queens mate with several males, which die shortly after. The queen retains the sperm in a special organ for the rest of her life. It is estimated that each queen is inseminated by 3 8 males and that the number of fathers per colony is between 1 and 5. The mated queen lands on the ground and tears off her now unnecessary wings. Then she digs a vertical tunnel to a depth of about 30 cm. At the end of the tunnel, she excavates a small chamber. The queen then starts a fungal garden using the piece brought from her birth colony. Usually, the garden is fertilized only with feces, but sometimes it is necessary for the queen to forage a small quantity of plant material. The queen also lays a few eggs. If A. sexdens were to spread into tropical Africa, the results are speculated to be devastating. As the local plants have not developed defensive compounds against leafcutters and Africa does not have parasites evolved to infect them, the results for both the ecosystem and agriculture would be disastrous.