Army ant (legionary ant)

Key characteristics

• Nomadic lifestyle alternates with short stationary phases when brood is reared.
• Large, coordinated raids that can strip areas of arthropods and small animals.
• Distinct castes including small workers and large soldier morphs with reinforced mandibles.
• A wingless, highly fecund queen often central to colony reproduction and movement.

Behavior and colony structure

Army ant colonies operate as tightly integrated units. During nomadic periods the colony travels in columns or fan-shaped swarms searching for food; these raids are efficient at capturing insects, other arthropods and occasionally small vertebrates. When the colony finds a suitable site to rear young it forms a bivouac by clustering workers around the queen and brood. Communication during raids and nest movements relies on pheromones and tactile signals.

Life cycle and reproduction

Many species exhibit cyclical behavior with alternating statary (stationary) and nomadic phases. The statary phase allows for intensive feeding of developing larvae and colony growth, while the nomadic phase follows when larvae require frequent feeding and the colony must move to new foraging areas. Reproduction depends on the queen's egg-laying; some species produce new queens and males seasonally, and others reproduce by budding of the colony.

Distribution, convergent evolution and taxonomy

Army ants are largely tropical and occur in the New World (e.g., genus Eciton) and Old World (e.g., driver ants of the genus Dorylus). The similar suite of behaviors across unrelated lineages is an example of convergent evolution, sometimes called legionary behavior, in which different ant groups independently evolved mass raiding and nomadism. For lists of species and taxonomic details see species accounts and family summaries such as Formicidae overviews.

Ecological role and notable interactions

Army ants act as keystone predators in many tropical ecosystems. Their raids flush out prey and provide feeding opportunities for a variety of other animals; specialist birds, known as ant-followers, track swarms to catch insects and small animals fleeing the ants. Human observers have long been fascinated by the spectacle of mass raids and the living bivouac, which illustrate complex collective behavior without central planning. For more on raid dynamics see resources about ant raids and studies of convergent evolution.

Distinctions: Not all nomadic or aggressive ants are true army ants; the term refers to species that combine obligate group raiding, a nomadic colony cycle, and formation of bivouacs rather than fixed nests.