The name army ant is applied to over 200 ant

Ant

Ants are social insects of the family Formicidae and, along with the related wasps and bees, belong to the order Hymenoptera. Ants evolved from wasp-like ancestors in the mid-Cretaceous period between 110 and 130 million years ago and diversified after the rise of flowering plants. More than...

 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

Convergent evolution

Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...

.

Most New World

New World

The New World is one of the names used for the Western Hemisphere, specifically America and sometimes Oceania . The term originated in the late 15th century, when America had been recently discovered by European explorers, expanding the geographical horizon of the people of the European middle...

 army ants belong to the subfamily Ecitoninae

Ecitoninae

Most New World army ants belong to the subfamily Ecitoninae. This subfamily is further broken into two groups in the New World, the tribes Cheliomyrmecini and Ecitonini. The former contains only the genus Cheliomyrmex, and the tribe Ecitonini contains four genera, Neivamyrmex, Nomamyrmex, Labidus,...

 which contains the two groups, the Cheliomyrmecini and Ecitonini

Ecitonini

The New World army ant tribe Ecitonini contains most of the familiar species of army ants. The genus Neivamyrmex is the largest of all army ant genera, containing some 120 species, most in the Neotropics but some with ranges in the United States....

. The former contains only the genus Cheliomyrmex whereas the latter Ecitonini contains four genera, Neivamyrmex, Nomamyrmex, Labidus, and Eciton

Eciton

The New World army ant genus Eciton contains the most familiar species of army ants. The most predominant and well-known species is Eciton burchellii, whose common name is "army ant" and which is considered to be the archetypal species....

. The largest genus is Neivamyrmex which contains more than 120 species. But the most predominant species is Eciton burchellii

Eciton burchellii

Eciton burchellii is the predominant species of the genus Eciton and a type of New World army ant. Distinct in its expansive, highly organized swarm raids, it is often considered the archetypal species of "army ant" — so much that the term has become its common name — and remains one of...

of the genus Eciton; its common name "army ant" is considered to be the archetype of the species. Old World

Old World

The Old World consists of those parts of the world known to classical antiquity and the European Middle Ages. It is used in the context of, and contrast with, the "New World" ....

 army ants are divided between Aenictini and Dorylini

Dorylini

Most Old World army ants belong to the tribe Dorylini. This tribe was formerly classified as the subfamily Dorylinae, and is composed of two genera, one of which was formerly placed in its own tribe, Aenictini. The other genus, Dorylus, contains the aggressive "driver ants", of which there are...

. The Aenictini contains more than 50 species of army ant in the single genus, Aenictus

Aenictus

The army ant genus Aenictus is an enigmatic group known from Africa, tropical Asia, and Queensland. There are some 100 species presently recognized, though many other names are applied at the rank of subspecies...

. However the Dorylini contains the Dorylus

Dorylus

The army ant genus Dorylus, also known as driver ants, safari ants, or siafu, is found primarily in central and east Africa, although the range also extends to tropical Asia. The term siafu is a loanword from Swahili, and is one of numerous similar words from regional Bantu languages used by...

, 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

Convergent evolution

Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...

. 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

Gondwana

In paleogeography, Gondwana , originally Gondwanaland, was the southernmost of two supercontinents that later became parts of the Pangaea supercontinent. It existed from approximately 510 to 180 million years ago . Gondwana is believed to have sutured between ca. 570 and 510 Mya,...

. Army ant taxonomy remains ever-changing, and genetic analysis will continue to provide more information about the relatedness of the various species.

Nomadic and stationary phase

Army ants have two different phases of activity: a nomadic (wandering) phase and a stationary phase.

Nomadic 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.

Stationary phase

The stationary phase, which lasts approximately two to three weeks, begins when the larvae

Larvae

In Roman mythology, lemures were shades or spirits of the restless or malignant dead, and are probably cognate with an extended sense of larvae as disturbing or frightening...

 pupate. From this point on, the prey that were previously fed to the larvae are now fed exclusively to the queen. The abdomen (gaster

Gaster

The gaster is the bulbous posterior portion of the metasoma found in Apocrita Hymenoptera . This begins with abdominal segment III on most ants, but some make a postpetiole out of segment III, in which case the gaster begins with abdominal segment IV....

) 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.

Nesting

Army ants do not build a nest like most other ants. Instead, they build a living nest with their bodies, known as a bivouac

Bivouac (ants)

A bivouac is a structure formed by migratory army ant and driver ant colonies, where a nest is constructed out of the living ant worker's own bodies to protect the queen and larvae, and is later deconstructed as the ants move on....

. 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

Ecitoninae

Most New World army ants belong to the subfamily Ecitoninae. This subfamily is further broken into two groups in the New World, the tribes Cheliomyrmecini and Ecitonini. The former contains only the genus Cheliomyrmex, and the tribe Ecitonini contains four genera, Neivamyrmex, Nomamyrmex, Labidus,...

) stingers. The interior of the nest is filled with numerous passages and contains many chambers with food, the queen, the larvae, and the eggs.

Food

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

Eciton

The New World army ant genus Eciton contains the most familiar species of army ants. The most predominant and well-known species is Eciton burchellii, whose common name is "army ant" and which is considered to be the archetypal species....

are not suited to this type of prey, in contrast to the African Dorylus

Dorylus

The army ant genus Dorylus, also known as driver ants, safari ants, or siafu, is found primarily in central and east Africa, although the range also extends to tropical Asia. The term siafu is a loanword from Swahili, and is one of numerous similar words from regional Bantu languages used by...

. 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.

Raids

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

Umbel

An umbel is an inflorescence which consists of a number of short flower stalks which are equal in length and spread from a common point, somewhat like umbrella ribs....

, 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.

Usage and circumscription

Historically, "army ant" referred, in the broad sense, to various members of 5 different ant subfamilies: in two of these cases, the Ponerinae

Ponerinae

Ponerinae is a subfamily of ants in the Poneromorph subfamilies group, with about 1,600 species in 28 extant genera, including Dinoponera gigantea - one of the world's largest species of ant....

 and Myrmicinae

Myrmicinae

Myrmicinae is a subfamily of ants. There are about 140 genera within the group, with the family being cosmopolitan. The pupae lack cocoons. Some species retain a functional sting. The petioles of Myrmicinae consist of two nodes...

, it is only a few species and genera that exhibit legionary behavior; in the other three lineages, Ecitoninae

Ecitoninae

Most New World army ants belong to the subfamily Ecitoninae. This subfamily is further broken into two groups in the New World, the tribes Cheliomyrmecini and Ecitonini. The former contains only the genus Cheliomyrmex, and the tribe Ecitonini contains four genera, Neivamyrmex, Nomamyrmex, Labidus,...

, Dorylinae, and Leptanillinae

Leptanillinae

Leptanillinae is a subfamily of ants. They are further divided into the tribes Anomalomyrmini and Leptanillini.In all Leptanillini the larva feed their hemolymph to the queen through specialized processes on their prothorax and third abdominal segment...

, 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

Cornell University

Cornell University is an Ivy League university located in Ithaca, New York, United States. It is a private land-grant university, receiving annual funding from the State of New York for certain educational missions...

) 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

Gondwana

In paleogeography, Gondwana , originally Gondwanaland, was the southernmost of two supercontinents that later became parts of the Pangaea supercontinent. It existed from approximately 510 to 180 million years ago . Gondwana is believed to have sutured between ca. 570 and 510 Mya,...

, and so the two subfamilies are now generally united into a single subfamily Ecitoninae

Ecitoninae

Most New World army ants belong to the subfamily Ecitoninae. This subfamily is further broken into two groups in the New World, the tribes Cheliomyrmecini and Ecitonini. The former contains only the genus Cheliomyrmex, and the tribe Ecitonini contains four genera, Neivamyrmex, Nomamyrmex, Labidus,...

, though this is still not universally recognized.

Accordingly, the army ants as presently recognized consist of the following genera:
Subfamily Ponerinae:

• Leptogenys (some species)
• Onychomyrmex
• Simopelta
  Simopelta
  Simopelta is a genus of ants, containing the following species:*Simopelta andersoni MacKay & MacKay, 2008*Simopelta bicolor Borgmeier, 1950*Simopelta breviscapa MacKay & MacKay, 2008*Simopelta curvata...
  
  

Subfamily Myrmicinae:

• Pheidolegeton

Subfamily Leptanilloidinae:

• Asphinctanilloides
• Leptanilloides

Subfamily Leptanillinae:

• Anomalomyrma
• Leptanilla
  Leptanilla
  Leptanilla is a genus of ant in the subfamily Leptanillinae. Like other genera in this subfamily, the queen is fed by the hemolymph of their own larvae, which have specialized processes for this purpose.-Biology:...
  
  
• Phaulomyrma
• Protanilla
• Yavnella

Subfamily Ecitoninae:

• Aenictus
  Aenictus
  The army ant genus Aenictus is an enigmatic group known from Africa, tropical Asia, and Queensland. There are some 100 species presently recognized, though many other names are applied at the rank of subspecies...
  
  
• Cheliomyrmex
• Dorylus
  Dorylus
  The army ant genus Dorylus, also known as driver ants, safari ants, or siafu, is found primarily in central and east Africa, although the range also extends to tropical Asia. The term siafu is a loanword from Swahili, and is one of numerous similar words from regional Bantu languages used by...
  
  
• Eciton
  Eciton
  The New World army ant genus Eciton contains the most familiar species of army ants. The most predominant and well-known species is Eciton burchellii, whose common name is "army ant" and which is considered to be the archetypal species....
  
  
• Labidus
• Neivamyrmex
• Nomamyrmex

External links

• A Comprehensive Ant Website
• Army Ants Plug Potholes, March 2009.
• Army ant research page: personal website on research and computer modeling of army ants
• Instinct to swarm
• Army ants are creators ...(BBC)