The name army ant (or legionary ant or "Marabunta") is applied to over 200 ant species, in different lineages, due to their aggressive predatory foraging groups, known as "raids", in which huge numbers of ants forage simultaneously over a certain area, attacking prey en masse.
Another shared feature is that, unlike most ant species, army ants do not construct permanent nests; an army ant colony moves almost incessantly over the time it exists. All species are members of the true ant family, Formicidae, but there are several groups that have independently evolved the same basic behavioral and ecological syndrome. This syndrome is often referred to as "legionary behavior", and is an example of convergent evolution.[1]
Most New World army ants belong to the subfamily Ecitoninae which contains the two groups, the Cheliomyrmecini and Ecitonini. The former contains only the genus Cheliomyrmex whereas the latter Ecitonini contains four genera, Neivamyrmex, Nomamyrmex, Labidus, and Eciton.[2] The largest genus is Neivamyrmex which contains more than 120 species. But the most predominant species is Eciton burchellii of the genus Eciton; its common name "army ant" is considered to be the archetype of the species. Old World army ants are divided between Aenictini and Dorylini. The Aenictini contains more than 50 species of army ant in the single genus, Aenictus. However the Dorylini contains the Dorylus, these are the most aggressive species of driver ants, there are 60 species known.
Originally the Old World and New World lineages of Army Ant were thought to have evolved independently, an example of convergent evolution. However in 2003, genetic analysis of various species suggest that they all evolved from a single common ancestor which lived approximately 100 million years ago at the time of the separation of the continents of Africa and America.[2] Army ant taxonomy remains ever-changing, and genetic analysis will continue to provide more information about the relatedness of the various species.
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Army ants have two different phases of activity: a nomadic (wandering) phase and a stationary phase.
During the nomadic phase the ants move during the day, capturing insects, spiders, and small vertebrates. At dusk they form their nest, which they change almost daily. Some species protect their paths with soldiers. During their hunt they are accompanied by various birds, such as antbirds, thrushes, and wrens, which devour the insects that are flushed out by the ants. Among the army ants there are also species that only venture out at night. However, there have been no adequate studies of their activities. Of the army ants which are active during the day, the species Eciton burchelli and Eciton hamatum are the most studied.
The stationary phase, which lasts approximately two to three weeks, begins when the larvae pupate. From this point on, the prey that were previously fed to the larvae are now fed exclusively to the queen. The abdomen (gaster) of the queen swells significantly, and she lays her eggs. At the end of the stationary phase, the pupae emerge from their cocoons (eclosion). After this, the ants resume the nomadic phase.
Army ants do not build a nest like most other ants. Instead, they build a living nest with their bodies, known as a bivouac. Bivouacs tend to be found in tree trunks or in burrows that are dug by the ants. The members of the bivouac hold onto each other's legs and so build a sort of ball, which may look unstructured to a layman's eyes but is actually a well-organized structure. The older female workers are located on the exterior; in the interior are the younger female workers. At the smallest disturbance, soldiers gather on the top surface of the bivouac, ready to defend the nest with powerful pincers and (in the case of the Aenictinae and Ecitoninae) stingers. The interior of the nest is filled with numerous passages and contains many chambers with food, the queen, the larvae, and the eggs.
Army ants can consume up to 100,000 prey animals each day and thus can have a significant influence on the population, diversity, and behavior of their prey. The prey selection differs with the species. Underground species prey primarily on ground-dwelling arthropods and their larvae, earthworms, and occasionally also the young of vertebrates, turtle eggs, or oily seeds. A majority of the species, the "colony robbers," specialize in the offspring of other ants and wasps. Only a few species seem to have the very broad spectrum of prey seen in the raiding species. Even these species do not eat every kind of animal. Although small vertebrates that get caught in the raid will be killed, the jaws of the American Eciton are not suited to this type of prey, in contrast to the African Dorylus. These undesired prey are simply left behind and consumed by scavengers or by the flies that accompany the ant swarm. Only a few species hunt primarily on the surface of the earth; they seek their prey mainly in leaf litter and in low vegetation. There are about five species that hunt in higher trees where they can attack birds and their eggs, although they focus on hunting other social insects along with their eggs and larvae.
In their raids, army ants follow two patterns: column raids and swarm attacks. The species Eciton hamatum is a typical example of the column raider. In this type, the swarm members separate to the sides of the main route and make small foraging groups, similar to a tree with its branches. The individual side paths can be widely separated from one another. The tropical army ants such as Eciton burchelli opt for the swarm attack. They, too, have a main route in the beginning which is then separated out into many branches in a form like an umbel, but their side paths are close together and may cross each other many times, so that the individual teams effectively cover a large area. In this way the column can fan itself out to a width of up to 20 metres.
Historically, "army ant" referred, in the broad sense, to various members of 5 different ant subfamilies: in two of these cases, the Ponerinae and Myrmicinae, it is only a few species and genera that exhibit legionary behavior; in the other three lineages, Ecitoninae, Dorylinae, and Leptanillinae, all of the constituent species are legionary. More recently, ant classifications now recognize an additional New World subfamily, Leptanilloidinae, which also consists of obligate legionary species, and thus is another group now included among the army ants.
A 2003 study of thirty species (by Sean Brady of Cornell University) indicates that the ecitonine and doryline army ants together formed a monophyletic group: all shared identical genetic markers that suggest a common ancestor. Brady concluded that these two groups are therefore a single lineage that evolved in the mid-Cretaceous period in Gondwana,[3] and so the two subfamilies are now generally united into a single subfamily Ecitoninae, though this is still not universally recognized.[4]
Accordingly, the army ants as presently recognized consist of the following genera: