Atta sexdens

Atta sexdens is a species of leafcutter ant belonging to the tribe Attini, native to the New World, 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 follows the basic body plan of ants fairly closely. They have sharp spikes or hooks rising from their heads and midsections to deter predators. Another distinguishing feature is a relatively large, two-lobed head. Its purpose is to accommodate the large muscles moving the well-developed mandibles. In addition to the queen, the colony contains four castes of sterile female workers: Considerable variation in size occurs within the castes, some of which may be related to the division of labour. Some researchers separate seven castes, while others dismiss the idea of caste altogether. The division of labour also depends on the age of the worker. Foraging expeditions are hazardous, so are handled by older, thus more expendable, members of the caste. A. sexdens, like all ants, has four stages of development: egg, larva, pupa, and adult. Their developmental larval stages are observed associated with fungal hyphae, and possess unique morphological features, compared to other ants. They are, like all hymenopterans, haplodiploid, meaning males are haploid (one of each chromosome) and females, including workers, are diploid (a pair of each chromosome). Sex is determined by the type of egg laid. Unfertilized eggs will turn out to be males, which are strictly used for mating and are short-lived. Fertilized eggs produce females. The caste of the resulting adult depends on environmental cues. Three to four weeks after being laid, the egg hatches and a larva emerges. In Atta ants, the larvae are fed by secretions from gardener-nurse ants and trophic eggs (unfertilized eggs used as food). The larvae, after an additional three to four weeks pupate. Their pupae are cocoonless, as are all of the subfamily Myrmicinae. After three to four more weeks, the pupae hatch into adult workers. A. sexdens larvae grow embedded in the fungal garden. Despite being surrounded by food, they are incapable of feeding themselves. Their adult sisters constantly feed and clean them. This is not a primitive feature. On the contrary, only the most advanced ant societies can expend such lavish care to their offspring as required by Atta larvae.