There are about 150 species of army ants in the New World (i.e. North, South and central America). They are all classified in the ant sub family Ecitonini. Although army ant species are found from Kansas to Argentina, few people in North America realize that there are plenty of army ants living in the US because most army ants only come out at night and many live underground.
Eciton burchelli and Eciton hamatum are the most visible and best studied of the New World army ants because they forage above ground and during the day. Their range stretches from southern Mexico to the northern part of South America.
In Africa, thrushes of the genus Alethes follow army-ant swarms, as do some bulbuls in both Africa and Asia.
New World army ants belong to the subfamily Ecitoninae. This subfamily is further broken into two groups, Cheliomyrmex and the Ecitonini. The most predominant species of Eciton is Eciton burchelli, whose common name is army ant and which is considered to be the archetypal species.
Army ant taxonomy remains ever-changing, and genetic analysis will continue to provide more information about the relatedness of the various species.
Eciton army ants have a bi-phasic lifestyle where they alternate between a nomadic phase and a stationary stage. In the stationary or statary phase ('statary' is an old English word meaning "to stand in place"), which lasts about three weeks, the ants remain in the same location every night. They make a nest out of their own bodies, protecting the queen and her eggs in the middle. This temporary home is known as a bivouac. In the nomadic phase the ants move their entire colony to a new location nearly every night for two weeks.
When the ants first enter the statary phase, the queen's body swells massively and she lays as many as 250,000 eggs in less than a week. While the eggs mature, the ants swarm with less frequency and intensity. When the eggs hatch, the excitement caused by the increased activity of the larvae causes the colony to enter the nomadic phase. The colony swarms much more intensely and nearly every day, and the ants move to a new location every night. After two weeks, around the time when the larvae begin to pupate, the colony again enters the statary phase, and the cycle begins anew.
Because of the regularity and intensity of E. burchelli and E. hamatum swarms, many insect, bird and butterfly species have evolved complex relationships with these ants. There are butterflies that lay their eggs on insects disabled by the ants. There are ant-mimicking staphylinid beetles, shaped like the ants they follow, that run with the swarm preying on stragglers or other insects injured or flushed by army ant activity; and there are some insects that spend their entire lives hidden in Eciton colonies mimicking ant-larvae. There are also more than 10 species 'ant-birds' that rely on the ants partially or completely for their food. Some of these birds actively check army-ant bivouacs each morning, follow the foraging trail to the swarm front, and prey on insects fleeing the ants.
Watching an E. burchelli swarm move through the forest is an amazing experience. The swarm pours forward amoebically, scouring the leaf-litter, bushes, and climbing high into trees. Despite being almost blind, E. burchelli are highly effective foragers, catching and killing nearly every small creature they find in their path.
The first sign of the approaching swarm is quiet a jumping and scurrying in the leaf-litter as insects begin fleeing for their lives. The first army ants arrive as a small trickle. Individual workers run forward, extending the pheromone trail very slightly before turning quickly back the way they came. As each ant turns, another passes it extending the trail a little further. Small insects begin to appear, flushed by the arriving invaders. A burst of red leaf-litter ants, larvae clutched in their mandibles, run for their lives up the nearest plant stem.
The steady trickle of army-ant scouts grows and thickens to a flood and suddenly the leaf-litter boils with terrifying ferocity as tens of thousands of ants cover the ground. A spider, flushed from its home, flees frantically, finding no escape. Jaws clamp down on every limb and soon the only sign of life is a tight, dark pile of stinging ants. An earthworm spasms and jumps, flinging ants in all directions, but its movements only attract more attackers until it is quickly stung to death and cut into pieces. A cockroach staggers from a hole, ants clinging to all sides and then collapses, swiftly becoming another thick pile of busily working ants.
As the front slowly passes, a maze-like network of trails develops behind it. Ants carrying small pieces of prey enter into the steady stream of workers running down the long trail home.
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