Atta sexdens

Appearance

''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:

Soldiers are the physically largest caste, with a head width of 3 mm and well-developed sharp mandibles. They are completely dedicated to defence, and do not participate in the running of the colony. In the event of attack by a predator, the soldiers storm out of the nest and attempt to overpower the aggressor. Their bite can easily penetrate human skin. The soldier caste is expensive to maintain, and they are absent from young colonies with less than 100,000 workers.

Forager-excavators are the second-largest caste, with an average head width of 2.2 mm. They are responsible for venturing out to cut pieces of leaves and carrying them back to the colony to feed the fungus. They also excavate new chambers to the colony.

Within-nest specialists have an average head width of 1.4 mm. They usually remain inside the colony where they process the leaf material brought in by the foragers to a smaller form. They also dispose waste, help the gardener-nurse caste to take care of larger larvae, and attend to the queen.

Gardener-nurses have an average head width of 1 mm. They are the physically smallest caste, being several hundred times less massive than the larger, more robust soldiers. Their tasks are to attend to the fungal culture and to the developing eggs, larvae and pupae.

The queen is the largest ant in the colony, outweighing the smaller worker by a factor of 700. She is also the only member of the colony capable of reproducing. The queen resides in a special chamber where she is continually fed, cleaned and protected by workers. The queens have a life span of 10–20 years.

There is considerable variation in size 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 and therefore handled by older, and thus more expendable, members of the caste.

Reproduction

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 flight the queens mate with several males, who die shortly after. The queen retains the sperm in a special organ for the rest of her life.

The mated queen lands on the ground and tears off her now-unnecessary wings. Then she digs a vertical tunnel to a depth of approximately 30 centimeters. 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 faeces, but sometimes it is necessary for the queen to forage a small quantity of plant material. The queen also lays a few eggs.

The queen then tends to the fungal garden and the developing larvae and pupae, feeding them fungus and trophic eggs. The queen herself eats nothing during this period, sustaining herself on fat deposits and her shrinking flight muscles. After the first brood of workers is ready, they take over the running of the fledgling colony and the queen becomes strictly an egg-laying machine. This process takes 40–60 days.

At first the colony grows slowly, but after 2–3 years the growth becomes faster and faster. One possible reason for this is that before the colony is established enough to sustain any large soldier-caste workers, it is useful to remain unnoticed. Soldiers usually appear when the colony has a population of about 100 000. After the number of workers reaches 5–8 million, the colony stops expanding and diverts resources into producing winged virgin queens and males.

The virgin queens are very rarely successful. Assuming that the number of colonies in an area remains constant over time, on average only one queen among the many thousands sent out by a colony successfully founds a new one. However, a mature colony of several million workers faces very few dangers. No known predator, except the mostly subterranean army ant Nomamyrmex esenbeckii, actively attacks the nests and even other highly aggressive army ants show a healthy respect for an ''A. sexdens'' colony. If spared from floods and human activity, the colony is usually destroyed only when the queen dies of old age, giving a successful colony a lifespan of 10–20 years. In that time the colony will have sent out numerous males and virgin queens to found new colonies.