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Army ant headquarters: the bivouac

When army ants swarm, you’d better watch out. And if you’re an insect, you’d better flee, far and wide. Army ants—as their name suggests—are raiders. During the day they fan out as the swarm advances through the rainforest, pinning down, dismembering, and taking any living creature they can subdue back to the nest to feed the queen and the rest of the growing colony.

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Soldier ants are endowed with powerful, oversized mandibles used to defend the colony from attack.

Eciton burchellii, from the rainforests of Central and South America, is one of the most common army ant species in this part of the world. Colonies can be enormous, with some estimated at 2,000,000 individual ants or more. As you might imagine, a colony of voracious raiders this large can consume quite a large amount of food—the ants eat their way through any insect, spider, or other small animal not quick enough to escape their marauding hordes. How, then, do the ants ensure they do not exhaust their food supply?

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Eciton burchelli ants on a raid in the Costa Rican rainforest.

When local resources are spent, the Eciton colony simply picks up and moves. They do this approximately every 35 days or so—the time it takes for the youngest ants in the colony to grow from egg to pupa, a period during which they must be fed regularly. When the brood is in pupal form, a stage in which the young ants change from larval to adult form and do not feed, it can be safely moved, but not before.

Unlike most other ants, Eciton’s nomadic lifestyle makes it impractical to build a permanent nest, potentially leaving the ants vulnerable to predators. It is, after all, strength in numbers that makes ants so successful at the game of survival. Instead of a permanent nest, Eciton come together nightly in the bivouac—a living mass made of the ants themselves, surrounding the queen and brood at its center. The ants hold the bivouac together with tiny hooks found on the tips of their feet, which they interconnect in a vast matrix. Eciton typically form the bivouac in semi-concealed places—under a fallen log, say, or between the buttressed roots of a rainforest tree—and although fairly open to attack by would-be predators, very few creatures dare hassle the pulsing mass of angry, biting ants.

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What the bivouac lacks in reinforced, physical structure it makes up for with mobility. Like a great army on the move, the Eciton colony is large yet agile and ready to pack it up when the time comes, after the voracious ants eat through the local resources. All other insects be warned when the bivouac moves in to the neighborhood!

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