Individually blind, this 500,000-strong colony works together in chemical harmony to feed and defend its queen
There are roughly 14,000 species of ants currently known to science, but some estimate there may be as many as 8,000 undiscovered species, scuttling through the undergrowth in some of Earth’s more inaccessible areas.
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Of those species identified so far, 200 are referred to as ‘army ants’, though this doesn’t constitute an official, scientific group. Rather, ‘army ant’ is a common name applied to species from several different lineages, each of which has independently evolved the same bizarre behaviours.
What differentiates army ants from other ants?
Unlike most species of ants, army ants don’t construct permanent nests. They spend most of their time on the move, only stopping at night to assemble a temporary, bivouac-like shelter. These structures resemble typical ant nests, with internal chambers and tunnels, but instead of being constructed from a mixture of soil, sand, and leaves, they’re built entirely out of the workers’ own bodies.
These bivouacs measure a metre across and can be composed of hundreds of thousands to over a million individuals. The queen and her brood sit near the centre while workers buzz around them, bringing them food and carrying larvae to new, splinter bivouacs. They’re typically found in dark and humid areas, such as under fallen logs, inside hollow trees, and between large surface roots.
Army ants aren’t the only types of ants known to make ‘living’ structures out of their own bodies. During times of flood, fire ants (Solenopsis invicta) come together in their hundreds of thousands, interlocking their legs and jaws to construct makeshift rafts that can stay afloat for several weeks.
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As dawn breaks, army ant bivouacs dissolve into a series of columns, forming a fan-shaped front that’s ready to head off in several directions in search of food. These columns are led by smaller workers, while larger soldiers with jaws almost half the size of their bodies protect the flanks. The queen joins her subjects as they march, though she leads from the rear and is almost always protected by a smaller bivouac of her strongest soldiers.
How do army ants hunt their prey?
Most workers are functionally blind, so they rely on chemical trails (pheromones) left by those in front to direct them. While unable to see, army ants are incredibly well-drilled and will retain their strict formation as they seek out potential prey. Together, an army ant colony can forage an area roughly the size of six tennis courts in a single day!
Not much is off the menu for these primarily carnivorous, nomadic hunters. They’ll predominantly target other insects, such as beetles, cockroaches, and grasshoppers, as well as scorpions and spiders, but on occasion they’ve been documented preying on lizards, snakes, and even baby birds.
Devoid of sight, army ants rely on their sense of touch to find prey. As long as prey remains completely still, a passing ant won’t register its presence - just one small twitch, however, is enough to give it away. Once an ant senses movement, it releases a chemical that attracts nearby ants, forming a swarm that, within minutes, can disembody an unlucky insect. In just one day of foraging, a single colony can consume as many as 500,000 prey items.
Eciton burchelli is one of the most populous and well-known species of army ant, living in the lowland forests that span from southern Mexico to northern Argentina. This species, like several other army ants, displays a high degree of polymorphism, meaning individuals differ in size depending on their specialised tasks. The workers at the bottom of the colony’s hierarchy measure roughly 3mm in length, while the higher ranking soldiers can grow to lengths of up to 12mm.
Do army ants go to ‘war’?
Like other army and non-army ants, Eciton burchelli are known to prey on members of their own species and even wage war on rival colonies. Some colonies form alliances with others, creating ‘supercolonies’ that stamp out uprisings from smaller colonies. These wars often involve entire colonies and can result in the complete destruction of one faction - or, intriguingly, the establishment of peaceful relations between former rivals.
The largest known ant war is still ongoing and, ironically, involves a type of ant not considered an army ant. This war is being waged by the invasive Argentine ant (Linepithema humile) - a species that forms massive, interconnected colonies spanning continents - against many native species. The peak of the fighting is currently taking place in Southern California, where as many as 30 million ants are killed annually.
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