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A swarm of army ants (Eciton burchellii) on the march. Photo by Alex Wild.

An entire world follows the march of the army ants

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The army ant E. burchellii nests in temporary encampments called bivouacs, whose walls are formed by the living bodies of the workers. Photo by Stefanie Berghoff.

Army ants have a reputation as destroyers. As they march through the jungle in battalions several thousand-strong, they supposedly kill all in their path. But this infamy is overblown. There’s no doubting their success as predators, but army ants also bring life wherever they march. They have an entourage of over 550 species that hang around their legions, of which 300 or so depend on the ants for their survival.

Carl and Marian Rettenmeyer spent much of their lives studying army ants. Carl passed away in 2009, but this year, Marian has completed the couple’s masterpiece – a comprehensive catalogue of the animals associated with a single species of army ant Eciton burchellii. Their record is incredible – a menagerie of animals running alongside the columns, tracking them by air, living within the nests and garbage dumps, and even riding on the ants themselves.

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The intimidating mandibles of an Eciton burchellii soldier defend the colony from larger predators. Photo by Alex Wild.

Think about army ants and you probably picture a large swarm of individuals crawling over the ground. In truth, only two of the 150 or so species in the Americas actually do this. E.burchellii is one of them. It forms armies of half a million ants that march from temporary nests or ‘bivouacs’. For three weeks, their legions issue forth from, and return to, the same bivouac, while they wait for their eggs to develop. When the larvae hatch, the whole army moves to a new encampment on a nightly basis. Two weeks later, the larvae pupate and the colony stalls again.

On the screen – from Indiana Jones to MacGyver – a marching column of army ants is a threat to all life. Even the naturalist William Mann wrote in National Geographic that “Even men flee as the mighty column writhes through the jungle, wiping out all insect and animal life in its path.” But these are bold exaggerations. E.burchelli mainly attacks the denizens of the undergrowth – insects, spiders and other arthropods. While it can kill small back-boned animals, its jaws can’t cut skin or flay flesh. Humans aren’t in any danger, nor are a whole host of creatures that accompany the army on its manoeuvres.

As the army marches, it flushes out thousands of animals from the leaf litter, and this attracts birds. Over 200 species track the ants and pick off the morsels that flee from the army. They almost never touch the ants themselves, except by accident, when a worker happens to be clinging onto another tasty insect.

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An ocellated antbird, one of several species of birds that follow army ant swarms, looking for prey that are flushed out by the legions. Photo by Mdf.

Several of these birds – the antbirds, in particular – get around half of their food by following the swarm. They fly to different bivouacs to check for colonies that are about to march, and they compete with one another for the best thieving spots, just in front of the ants. The birds themselves attract hangers-on – around 239 species of skipper butterflies follow them to feed off their droppings.

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A parasitic Stylogaster fly hovers over the army ant swarm, ready to shoot their harpoon-like eggs at fleeing cockroaches. Photo by Daniel Kronauer.
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A Calodexia fly sits by an army ant swarm (a), waiting for cockroaches and crickets. It lays its eggs on a cricket, which dies a day later with a maggot visible behind its head (b). Three days later, the cricket is mostly eaten with maggots jutting out of its corpse (c). Four days later still, and the maggots have pupated. Photos by Stefanie Berghoff.

Birds aren’t the only airborne followers with an interest in the animals flushed out by the ants. Parasitic wasps and flies are on the lookout too, ready to dart in and lay their eggs in a scurrying cockroach or grasshopper. When the eggs hatch, the unfortunate victims will be devoured from the inside out. These parasites have many different strategies. Caledoxia flies lays live larvae directly onto their victims. Stylogaster flies shoot harpoon-like eggs at fleeing cockroaches. And flesh flies lay their eggs in the open wounds of animals that have been injured but not dismembered by the ants.

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Spot the Ecitophya beetle. Got it? It’s in the middle, the one just below the most obvious ant soldier. Photo by Daniel Kronauer.
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A Tetradonia beetle tries to drag away an army ant for the kill. Photo by Daniel Kronauer.

Some associates risk death by joining the march, living inside the bivouacs, or even riding on the ants themselves. Unsurprisingly, they need special adaptations to avoid being eaten. Springtails are probably too fast and agile to be caught. Resident beetles mimic the ants’ appearance, or have streamlined bodies to deflect snapping jaws. Some use the ants as mobile restaurants, jumping onto workers that are carrying food, and eating their booty right under (or over) their very jaws. Tetradonia even kills the ants themselves, dragging workers away from the main column to dismember elsewhere.

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A Circocylliba mite sits on an army ant’s head, using a dome-like shell to create tight seal that prevents it from being dislodged. Photo by Eberhard Wurst.

Many mites have specialised at hitching aboard the ants, and some are found nowhere else. They too have many adaptations to avoid being found or dislodged. Larvamima, as the name suggests, looks like an ant larva. Planodicus has hairs that match those on the ant’s legs. Circocylliba has a dome-like shell that forms a tight seal with the ant’s body. Rettenmeyerius carli – named after the late naturalist himself – sucks the ant’s blood from probably the safest location of all: the very bottom of its jaws. And Pinoglyphus has only ever been found attached to the eye of an army ant worker.

All in all, the Rettenmeyers counted 557 species that associate with the ants and that’s probably just the tip of the iceberg. In their 55 years of research, the duo have collected thousands of specimens of species found near or within the marching armies. Many of which haven’t been described yet and they represent the followers of just one species of army ant.

For more on army ant associates, you can buy two DVDs – “Associates of Eciton burchellii” and ‘‘Astonishing Army” from the Rettenmeyers’ website, containing video shot by the duo themselves. They cost $25 a copy, which goes towards the Carl and Marian Rettenmeyer Ant-Guest Endowment.

Thanks to Alex Wild at Myrmecos for drawing my attention to the story.

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