Invertebrates
Army Ant
Eciton burchellii · also called army ant, New World army ant, legionary ant
An army ant colony is less an insect nest than a moving predator the size of a household. Up to a million ants march through the rainforest in a raiding swarm that strips the leaf litter of anything living, then, at nightfall, hook their legs together and become a nest — with walls, rooms, and a nursery, all made of ant.
Overview
Army ants of the genus Eciton live in lowland rainforest from Mexico to Brazil (related groups — the driver ants — live in Africa and Asia). A colony of the swarm-raiding species Eciton burchellii holds 150,000–700,000 workers and a single enormous queen, and it has no permanent nest.
A nest made of ants
Each night the colony forms a bivouac — a mass of workers linked by their own legs and jaws, about the size of a basketball, with internal chambers, a temperature-controlled brood core, and doorways. The living walls adjust to keep conditions stable. In the nomadic phase the colony takes this apart and rebuilds it in a new place every night, driven by the appetite of its huge batch of larvae; in the stationary phase it stays put while the queen lays up to 300,000 eggs.
The raid
By day, a fan-shaped swarm front up to 15 metres wide advances through the forest at about 15 metres an hour, flushing and overwhelming insects, spiders, and scorpions. A raid can take tens of thousands of prey — nearly all of it carried back and fed to the larvae, not eaten by the workers.
A travelling ecosystem
A raid drags a crowd with it: antbirds (300+ species follow Eciton, ~18 depend on it) that snap up fleeing insects, butterflies that feed on the birds’ droppings, parasitic flies, and hundreds of specialist beetles and mites that live only with army ants.
Status
Army ants have not been assessed for conservation. They are abundant in intact forest but, as large-colony predators needing big unbroken tracts, they decline in forest fragments.
Army Ant: common questions
Do army ants really have no nest?
They have no permanent, dug nest. Instead, each night the colony builds a temporary living nest called a bivouac entirely out of the workers' own bodies — hundreds of thousands of ants hooking their legs and jaws together into a dense mass about the size of a basketball, usually hanging in a sheltered spot like a hollow log or under a fallen tree. Inside are chambers and passages, with the queen and the developing brood in a protected, temperature-buffered core, and the outer layers of ants acting as walls and adjusting to keep conditions stable. In the nomadic phase the colony dismantles this structure and rebuilds it somewhere new every single night.
Are army ant raids dangerous to people or livestock?
A New World army ant swarm (Eciton) is not a serious danger to a person or a large animal, which can simply walk away — the front advances at only about 15 metres an hour. The ants deliver painful bites and, in some species, stings, and getting caught standing in a raid is very unpleasant, but healthy mobile animals are not at risk. The African driver ants (Dorylus) are more formidable, with larger colonies and huge soldiers, and there are credible accounts of them killing animals that could not escape — tethered livestock, caged animals, or a person unable to move — but these cases are rare and involve immobility, not the ants hunting people.
What is the point of the swarm raid if the ants barely eat the prey?
The raid is about feeding the brood, not the adult workers. Army ant colonies produce enormous batches of larvae — hundreds of thousands at a time — and those larvae are ravenous, growing fast and needing a constant supply of insect protein. The swarm raid is essentially a mobile harvesting operation — the fan of workers flushes and overwhelms prey across a wide front, other workers carry the pieces back along chemical trails to the bivouac, and it is fed almost entirely to the larvae. When the larvae pupate and stop demanding food, the colony's raiding intensity drops and it enters its stationary phase.
Why do birds follow army ants?
More than 300 bird species in the Neotropics regularly follow army ant swarms, and about 18 (the "professional" ant-followers, mainly antbirds) depend on them. The birds do not eat the ants — they eat the katydids, cockroaches, spiders, and other prey that flee upward and outward ahead of the advancing swarm, which would otherwise be almost impossible to catch. It is a form of commensal foraging. A whole travelling ecosystem forms around a raid, including butterflies that follow the antbirds to feed on their droppings, and parasitic flies that dart in to lay eggs on the fleeing insects.