National Geographic - Army Ants - BBC Wildlife Documentary

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.[1]
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 may be an example of convergent evolution.[2][n 1]
Most New World army ants belong to the subfamily Ecitoninae, which contains two tribes: Cheliomyrmecini and Ecitonini. The former contains only the genus Cheliomyrmex, whereas the latter contains four genera: Neivamyrmex, Nomamyrmex, Labidus, and Eciton.[2] 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.[citation needed]
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 South America.[2] Army ant taxonomy remains in flux, and genetic analysis will likely continue to provide more information about the relatedness of the various taxa.