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The naturalist and author William Beebe once observed a bivouac of army ants that had taken temporary residence in the outhouse near his laboratory in Guyana. Transfixed by the sight, he determined to observe the insects as they set up their encampment. He first noted the odor of the group, which was "sometimes subtle, again wafted in strong successive waves. It was musty, like something sweet which had begun to mold; not unpleasant, but very difficult to describe." He was deterred from further rumination by "a dozen ants [that] had lost no time in ascending my shoes, and, as if at a preconcerted signal, all simultaneously sank their jaws into my person." Beebe proceeded to take a chair into the outhouse and use the traditional technique of placing each of its legs into a can of disinfectant; he then rushed over to the chair, hung a bag of equipment over the back, and pulled his legs onto the seat. "Close to my face were the lines ascending and descending, while just above me were hundreds of thousands, a bushel-basket of army ants, with only the strength of their thread-like legs as suspension cables. It took some time to get used to my environment, and from first to last I was never wholly relaxed, or quite unconscious of what would happen if a chair-leg broke, or a bamboo fell across the outhouse."

This rhythm of activity, with the ants alternating between going out on raids and forming bivouacs, continues for months. In the Central American army ants that Schneirla studied, the ants sometimes will form a new bivouac every evening and sometimes settle in their self-manufactured housing for a few weeks at a time. Because army ants have no permanent nest site, they do not reproduce as many other ants do, with the release of winged males and females that mate in flying swarms before the newly inseminated queens found new colonies. Instead, at least in the species of army ants that have been the best studied, although both fertile males and females are produced at a certain time of year, only the males can fly. They attempt to join the bivouacs of another colony. At the next raiding period, a group of workers stays with the old queen and moves to a new bivouac, while one of the virgin queens is surrounded by another set of workers and travels to a different site, where she mates with one of the males that had flown into the colony. In some species of army ants the young queen mates with several males in succession, in others with only one. The male ants, like the drones of honeybees and many other social insects, die soon after mating, assuming they get a chance to mate at all. The rest of the handful of reproductive females are abandoned to the company of a small group of workers, but they do not hunt for food, and eventually all die, leaving their mother and sister to carry on in their place.

Army ant queens themselves have dramatically episodic reproductive lives; instead of monotonously laying egg after egg, day after day, for their entire adult life span, the queen's ovaries will develop only while the colony is in their more long-term bivouac. At that point, rapidly making up for lost time, her abdomen distends and she lays up to three hundred thousand eggs in one fell swoop. When the workers that are the product of her labor appear, they seem to perk up the energy levels in the group, and after a while the entire colony starts the migratory phase again, as the queen's labor subsides and she is shielded from harm as the columns of scissor-jawed daughters resume their activity.

The term army ant is not a truly scientific designation; it is used to describe those species of ants that exhibit both incessant migration of the entire colony and coordinated group hunting, including the raids by large numbers of individuals and the carrying of prey back to the nest. Sometimes terms such as legionary or driver ants are used, but virtually everyone who has written about them describes the ants' behavior in the most aggressive terms. While Hölldobler and Wilson, in their monumental tome