Army ants are a diverse collection of highly predatory, social insects known for their nomadic lifestyle and massive cooperative hunting raids. The name “army ant” refers not to a single species but to hundreds of species across numerous genera, making a single, definitive size impossible. These ants exhibit a dramatic range in body dimensions, a variation closely tied to their specific roles within the colony. Understanding army ant size requires looking closely at the different castes and the distinct evolutionary paths taken by species in the Old World and the New World.
General Size Parameters
The most commonly observed army ant is the minor worker, the baseline individual that makes up the bulk of the colony’s foraging force. These workers are generally small, with body lengths typically falling within the range of 3 to 12 millimeters, depending on the species and genus. For the New World species Eciton burchellii, minor workers average 3 to 7 millimeters long, roughly the size of a grain of rice. These smaller workers are responsible for foraging and tending to the brood within the temporary nest.
These ants are built for speed and high maneuverability, necessary for moving rapidly across the forest floor during their frequent, massive raids. Their small dimensions allow them to navigate the leaf litter and dense tropical undergrowth efficiently. Though individually small, their sheer numbers—often exceeding a million individuals—create the immense collective biomass that defines the army ant phenomenon.
Variation by Caste and Species
The true extremes of army ant size are found in the specialized castes, reflecting polymorphism where individuals within the same colony have distinct body shapes and sizes. Major workers, often called soldiers, are significantly larger than their minor counterparts and feature specialized physical adaptations. In species like Eciton burchellii, soldiers can reach 10 to 12 millimeters and possess enormous, hooked mandibles and disproportionately large heads. These oversized heads house the powerful muscles required to operate the mandibles, used primarily for defense and subduing larger prey items.
The reproductive castes show significant size disparity, particularly the queen of the Old World Dorylus genus (driver ants). While the New World Eciton queen is large, the Dorylus queen can measure between 40 and 63 millimeters in total body length. This colossal size is due to physogastry—a massively enlarged abdomen—which accommodates her high egg-laying capacity. She is wingless and remains within the nest, relying entirely on her workers for movement and defense.
The African and Asian Dorylus species contain individuals, particularly the queens and large-form males (drones), that dwarf their American relatives. The males, sometimes referred to as “sausage ants” due to their elongated, cylindrical bodies, are also notably large. They serve as the sole dispersal unit when they fly out to mate with new queens.
Size and Ecological Role
The varied body sizes within an army ant colony are directly linked to the colony’s unique ecological functions, especially its nomadic and predatory behaviors. The large size and powerful mandibles of the major workers are necessary for their role in colony defense and hunting raids. These soldiers position themselves along the flanks of the foraging column, where their robust bodies and strong jaws deter vertebrate predators and help capture or dismember large arthropod prey.
The collective size and physical robustness of the workers are also fundamental to the construction of the bivouac, the temporary nest formed entirely from the ants’ own bodies. Millions of ants link their legs and bodies together, forming a living, cohesive structure that houses the queen and the developing brood. Research on bivouac construction shows that the ants must be able to carry a load of several times their own body weight to maintain the nest’s stability. Their collective mass allows the bivouac to maintain a thick, protective outer shell, insulating the queen and larvae at the hollow center.