Army ants are coordinated insects found predominantly in the tropical and subtropical regions of Central and South America, Africa, and Asia. They are recognized for their massive, synchronized movements across the forest floor, driven by the colony’s large appetite. The collective action of a colony allows them to operate as a single, overwhelming predatory unit.
What Defines an Army Ant
The classification of army ants centers on the “army ant syndrome,” which includes obligate group predation, nomadism, and specialized queen morphology. While not a single taxonomic group, the majority of true army ants belong to the subfamily Dorylinae, encompassing genera like Eciton (New World) and Dorylus (Old World). Their highly specialized caste system enables their unique predatory lifestyle through a division of labor.
Colonies feature polymorphic workers, meaning individuals vary significantly in size and function, from small nurses to large soldiers. Soldier ants often possess proportionally large heads and specialized, hooked mandibles used for defense and dismembering larger prey. Many worker castes exhibit reduced eyesight or are completely blind, relying instead on chemical signals for navigation and communication.
The Nomadic Cycle and the Living Nest
The existence of an army ant colony is governed by the nomadic cycle, which dictates their movement and reproduction. This cycle alternates between a stationary phase and a nomadic phase, each lasting approximately two to three weeks. The stationary phase begins when the queen lays a large clutch of eggs, and the colony remains in one location while the larvae from the previous cycle enter the pupal stage.
During this resting period, the colony forms a temporary structure called a bivouac, a living nest constructed entirely from the ants’ own bodies. Thousands of workers interlock their legs and mandibles to create a protective shell around the queen, the brood, and the food stores. The bivouac can be quite large, often hanging from a log or nestled in a tree hollow.
The transition to the nomadic phase is triggered by the hatching of the new brood, which requires high caloric demands. The colony must move daily to new foraging grounds to meet these needs. The bivouac is disassembled before dawn, and the entire colony follows chemical trails to a new resting location before sunset, maximizing their access to fresh prey resources.
The Swarm: Hunting Strategies and Scale
The synchronized foraging effort known as the swarm raid is the primary behavior of army ants. In New World species like Eciton burchellii, the raid front spreads out from the bivouac in a broad, advancing fan shape. This massive wall of ants moves across the forest floor, overwhelming any prey in its path.
The ants rely on numerical superiority rather than individual strength to secure food. The raid moves at a pace of up to 20 meters per hour, with smaller worker ants scouting ahead and larger soldiers positioned along the flanks. As prey is encountered, it is immediately subdued, dismembered, and passed back toward the central column.
Their diet consists mostly of invertebrates, including spiders, scorpions, and wasps. Army ants are specialized predators of social insects; they systematically raid the nests of other ant species and termites, carrying off their brood and food supplies. This provides sustenance for the millions of larvae back at the bivouac.
Dispelling the Myths
The reputation of the army ant has led to many exaggerations, particularly the myth that they pose a threat to humans or large livestock. Army ants do not target large vertebrates, as their primary diet consists of small invertebrates. While their bites can be painful, the colonies avoid contact with large animals, moving around obstacles rather than attacking them.
Army ants are integrated into the forest ecosystem and form ecological relationships. One notable example involves “antbirds,” which have evolved to specialize in following the swarms. These birds do not attack the ants but instead feed on the insects, spiders, and other small creatures that are flushed out of the leaf litter by the advancing raid front. This interaction demonstrates the ecological role these nomadic predators play in the tropical environment.