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.
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 several groups 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.
Most New World army ants belong to the subfamily Ecitoninae, which contains two tribes: Cheliomyrmecini and Ecitonini. The former only contains the genus Cheliomyrmex, whereas the latter contains four genera: Neivamyrmex, Nomamyrmex, Labidus, and Eciton. The largest genus is Neivamyrmex, which contains more than 120 species; the most predominant species is Eciton burchellii; its common name "army ant" is considered to be the archetype of the species. Old World army ants are divided between the Aenictini and Dorylini tribes. Aenictini contains more than 50 species of army ants in the single genus, Aenictus. However, the Dorylini contain the genus Dorylus, the most aggressive group of driver ants; 60 species are known.
Originally, the Old World and New World lineages of army ants were thought to have evolved independently, in an example of convergent evolution. In 2003, though, genetic analysis of various species suggests 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. Army ant taxonomy remains in flux, and genetic analysis will likely continue to provide more information about the relatedness of the various taxa.
The workers of army ants are usually blind or can have compound eyes that are reduced to a single lens. There are species of army ants where the worker caste may show polymorphism based on physical differences and job allocations; however, there are also species that show no polymorphism at all. The worker caste is usually composed of sterile female worker ants.
Colonies of real army ants always have only one queen, while some other ant species can have several queens. The queen is dichthadiigyne (a blind ant with large gaster) but may sometimes possess vestigial eyes. The queens of army ants are unique in that they do not have wings, have an enlarged gaster size and an extended cylindrical abdomen. They are significantly larger than worker army ants and possess 10–12 segments on their antennae. Queens will mate with multiple males and because of their enlarged gaster, can produce 3 to 4 million eggs a month, resulting in synchronized brood cycles and colonies composed of millions of individuals all related to a single queen.
Males are large in size and have a large cylindrical abdomen, highly modified mandibles and uncommon genitalia not seen in other ants. They have 13 segments on their antennae, are alated (have wings) and therefore can resemble wasps. Males are born as part of a sexual brood. As soon as they are born, they will fly off in search of a queen to mate with. The wings will fall off after entering a new colony and mating. Because of their size, males are sometimes called "sausage flies" or "sausage ants."
Army ant syndrome
The army ant syndrome refers to behavioral and reproductive traits such as obligate collective foraging, nomadism and high specialized queens that allow these organisms to become the most ferocious social hunters. 
Most ant species will send individual scouts to find food sources and later recruit others from the colony to help; however, army ants dispatch a cooperative, leaderless group of foragers to detect and overwhelm the prey at once. Army ants do not have a permanent nest but instead form many bivouacs as they travel. The constant traveling is due to the need to hunt large amounts of prey to feed its enormous colony population. Their queens are wingless and have abdomens that expand significantly during egg production. This allows for the production of 3-4 million eggs every month and often results in synchronized brood cycles, thus each colony will be formed of millions of individuals all descendant form a single queen. All three of these traits are found to be in all army ant species and the defining traits of army ants. 
Nomadic and stationary phase
Army ants have two phases of activity -- a nomadic (wandering) phase and a stationary phase -- that constantly cycle, and can be found throughout all army ants species.
The nomadic phase begins around 10 days after the queen lays her eggs. This phase will last approximately 15 days to let the larvae development. The ants move during the day, capturing insects, spiders, and small vertebrates to feed its voracious brood. At dusk, they will form their nests or bivouac, which they change almost daily. 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 are some species that only venture out at night, but no adequate studies of their activities have been made. At the end of the nomadic phase, the larvae will spin pupal cases and no longer require food. The colony can then live in the same bivouac site for around 20 days, foraging only on approximately two-thirds of these days. 
The stationary phase, which lasts about 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, both the pupae emerge from their cocoons (eclosion) and the next generation of eggs hatch so the colony has a new group of workers and larvae. After this, the ants resume the nomadic phase.
Army ants will split into groups when the size of the colony has reached a size threshold which happens approximately every three years.  Wingless virgin queens will hatch among a male sexual brood that will hatch at a later date. When the colony fissions, there are two ways new queens are decided. A possible outcome is a new queen will stay at the original nest with a portion of the workers and the male brood while the old queen will leave with the other portion of workers and find a new nest. Another possibility is that the workers will reject the old queen and new queens will head the two new colonies.  The workers will affiliate with individual queens based on the pheromone cues that are unique to each queen. When new bivouacs are formed, communication between the original colony and the new bivouacs will cease to exist. 
Being the largest ants on Earth, army ants, such as African Dorylus queens hold the world record in reproductive potential among the insects, with an egg-laying capacity of several millions per month. Army ant queens never have to leave the protection of the colony, where they mate with foreign incoming males which disperse on nuptial flights. The exact mating behavior of the army ant queen is still unknown, but observations seem to imply that queens may be fertilized by multiple males. Due to the queens large reproductive potential, one colony of army ants can be related to a single queen.
When the queen ant passes away, there is no replacement and army ants cannot rear emergency queens. Most of the time, if the queen passes away, the colony will most likely die too. Queen loss can occur due to accidents during emigrations, predator attack, old age or illness.  However, there are possibilities to avoid colony death. When a colony loses its queen, the worker ants will usually fuse with another colony that has a queen in a few days., Sometimes, the workers will backtrack along the paths of prior emigrations to search for a queen that has been lost or merge with a sister colony. By merging with a related colony, the workers would increase their overall inclusive fitness. The workers that merge into a new colony may cause the a colony to increase in size by 50%.
Sexual selection by workers
Workers in army ant species have a unique role in selecting both the queen and the male mate.
When the queens emerge, the workers in the colony will form two ‘systems’ or arms in opposite directions. These queens that are hatched will move down either the arms and only two queens will succeed, one for each branch. The remaining new queens will be left in the middle and are abandoned to die. Two new bivouacs will be formed and break off into different directions. The workers will surround the two to-be queens and ensure they reach the arms and survive. These workers that surround the queens are affected by the CHC (pheromone) profile emitted from the new queen.
When males hatch from their brood, they will fly off to find a mate. For males to access the queen and mate, they must run through the workers in the colony. Males that are favored are superficially similar in size and shape to the queen. The males also produce large quantities of pheromones to pacify the worker ants.
In the colony, instead of having both queen and workers producing off spring, the queen is the sole reproductive in the colony. With queens mating with multiple males, workers are on average more closely related to the offspring of the queen that to the offspring of other workers. Both the queen and the daughter workers would benefit from producing sons, because worker reproduction has a colony cost or each worker is more related to the queen’s son than to other worker’s sons, the worker caste will oppose worker reproduction and pull their resources to only raise the queen’s offspring. Three factors have been suggested to rationalize the loss of worker reproduction in the presence of a queen. First, if the worker reproduces, it lowers the general performance of the colony because it is not working. Second, workers increase their inclusive fitness by policing other workers because they themselves are more related to the queen’s offspring than other worker’s offspring. Lastly, male larvae have a larger body size relative to workers, the larvae become too large to be transported so colonies with a sexual brood must remain in a nest for a period of 41-56 days as compared to non-reproductive colonies that remain in the nest a average of 17 days. This suggests that if workers produced sons, it would be produced asynchronously with the queen's sexual brood and not likely to be reared to adulthood as it would be hard for the worker to predict when the queen would reproduce.
The whole colony of army ants can consume up to 500,000 prey animals each day, so 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. About five species 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. Colonies of army ants are large compared to the colonies of other Formicidae. Colonies can have over 15 million workers and can transport 3000 prey (items) per hour during the raid period.
When army ants forage, the trails that are formed can be over 20m wide and over 100m long. They stay on the path through the use of a concentration gradient of pheromones. The concentration of pheromone is highest in the middle of the trail, splitting the trail into two distinct regions: area with high concentration and two areas with low concentrations of pheromones. The outbound ants will occupy the outer two lanes and the returning ants will occupy the central lane. The returning worker ants have also to be found to emit more pheromones than those leaving the nest, causing the difference in concentration of pheromone in the trails. The pheromones will allow foraging to be much more efficient by allowing the army ants to avoid their own former paths and those of their conspecifics. 
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 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 Pincer (biology) 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.
Nest building activities are classified as:
- excavation of material to form cavities (shafts, tunnels and chambers)
- deposition of materials to fill or enclose cavities (mounds, chambers, craters)
- import of materials from outside the nest or special purposes
- compounding of materials to form special substances, with particular physical properties 
The materials that are imported into the nest can be for food, decorations that mark a defended area or even items such as stones with multiple purposes. Stones will be used to close nest entrances before a rainstorm and to protect the surface of the nest from erosion. Furthermore, the stone's smooth surface will prevent laminar flow to increase heat loss to the air and even collect dew from the surrounding air, which can then be absorbed by the underlying soil. 
Historically, "army ant" in the broad sense, referred to various members of five different ant subfamilies: in two of these cases, the Ponerinae and Myrmicinae, only a few species and genera 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, so is another group now included among the army ants.
A 2003 study of 30 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, suggesting a common ancestor. Brady concluded that these two groups are, therefore, a single lineage that evolved in the mid-Cretaceous period in Gondwana, so the two subfamilies are now generally united into a single subfamily Ecitoninae, though this is still not universally recognized.
Accordingly, the army ants as presently recognized consist of these genera:
- Subfamily Aenictinae
- Subfamily Dorylinae
- Subfamily Ecitoninae
- Subfamily Leptanillinae
- Subfamily Leptanilloidinae
- Subfamily Myrmicinae
- Subfamily Ponerinae
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