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The army ant Eciton burchellii (treated for years as Eciton burchelli, although originally described as Eciton burchellii) is a keystone predator in the leaf litter of many Neotropical forests (keystone species are species whose ecological importance is disproportionately great relative to their biomass). There are many species of army ants (not all related) in both the New World and Old World (mainly, though not only, in the tropics), but the most extensively studied species is E. burchellii, with much of the work on this species undertaken over many decades on Barro Colorado Island in Panama. (O'Donnell et al. 2007 and references therein).
Eciton burchellii live in colonies that can exceed half a million individuals. In the course of a day, a single colony may capture some 30,000 prey items. Their activity also flushes out larger arthropods, some of which are quickly devoured by ant-following birds (in some regions, there are bird species that depend on following army ants for nearly all their food). Eciton burchellii colonies typically exhibit a 35-day activity cycle. For 20 days, a colony resides in a fixed bivouac (a temporary nest, see below) from which raids emerge nearly every other day. During this phase, the colony's single queen may lay as many as 100,000 eggs. At the end of the 20-day period, these eggs hatch into larvae and with the increased demand for food the raiding becomes more frequent and intense. The colony now enters the nomadic phase, during which a new bivouac is formed at the end of each day's raiding. This behaviour lasts for about 15 days, until the larvae pupate, when a new statary (i.e., non-nomadic) phase begins. At the end of each statary phase, the pupae become new callow (i.e., newly emerged) workers. Such 35-day cycles continue throughout the year. Colonies may go extinct when the queen dies or the colony becomes too small. The largest colonies reproduce by rearing a sexual brood (males and a small number of queens) and then splitting. The daughter colonies are then headed either by the existing queen or by one of the new queens reared prior to the fission of the original colony. (Boswell et al. 1998 and references therein)
Eciton burchellii do not build nests like most other ants. Instead, they form temporary above-ground nests known as "bivouacs". Most of the cover in these partly sheltered locations is provided by the bodies of the workers themselves, which link their legs and bodies together with strong hooked claws at the tips of their feet. Together the workers--perhaps a half million, with a mass of about a kilogram-- form a solid cylidrical or ellipsoidal mass around a meter across. Toward the center of the mass are thousands of larvae and the single queen. (Holldöbler and Wilson 1994) For a broad and detailed overview of army ant biology (and ant biology in general) see the extraordinary volume by Holldöbler and Wilson, The Ants (1990), as well as early publications they cite by T.C. Schneirla and later ones by C.W. Rettenmeyer, both of whom were pioneers in understanding the biology of army ants.