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Ants in Their Plants
12 MAY 2005

The Trap's The Thing
Gruesome, but ingenious, the use of a trap as a predatory strategy has never been seen before in antsGoliath never knew what hit him when David nailed him with a rock. In the jungles of the Amazon, hapless insects of all kinds are meeting a similar fate as they wander into traps set by much smaller creatures -- ants. To accomplish this feat, Amazonian ants have become engineers of the most murderous kind.

Faced with the imposing task of capturing prey several times their size, this species of tropical ant has devised a gruesome trap to ensnare and dismember any large insect that may mosey their way. The ants have co-evolved with a plant and a fungus to become mechanized wunderkinds of the jungle.

1. Green plant stem covered in prickly thorns
Not just a stem - Hundreds of ants wait below these holes for a tasty treat to unknowingly fall into their trap.
All photographs this page courtesy Alain Dejean and Jérôme Orivel/Nature

The trap is a cleverly devised platform constructed of plant fibers and fungus. The platform is pitted with dozens of holes in which the Allomerus decemarticulatus lurk, mouths open just below the surface, to grab any unsuspecting passer-by. As an insect, like a locust, travels across the trap, the ants spring their trap as legs, antennae or wings slip through the holes into waiting open jaws. The locust then meets a grisly death as the ants stretch the body, sting it to induce paralysis and finally, swarm for the kill. The ants then pass the carcass up the length of the stem towards their nest where it's carved up like a Thanksgiving turkey.

Not the most pleasant way to go.

2. Cricket  gingerly traverses plant stem
An unfortunate locust has really stepped in it this time. The ants start to swarm as he tries to escape, no doubt.

This trap marks the first known example of tools used by ants. Which we think this is a good thing, especially if the treacherous little fellas are going to use them this way. Picnicers everywhere would be in peril.

Great and Small
Researchers first noticed the traps, or galleries as they like to call them, while studying ant and plant interactions in the tropical rain forests of South America. The ants construct their galleries on a host plant, Hirtella physophora. The ants resorted to building death traps because the plant alone does not meet the ants' nitrogen needs.

3. Cricket swarmed by small, stinging ants
Now completely covered in ants, the locust stands no chance of escape. They've begun to carve him up, after subduing him with stings.

The plant usually houses colonies of the small orange-yellow ant in leaf pouches. When the researchers noticed that insects, apparently captured by the Allomerus, could be found on the stems of the plant, they began to think that the ants could be creating traps, according to Jerome Orivel, a biologist at Toulouse University and co-investigator of the study.

The trap-setting Allomerus can only be found on this plant. They "are specific to the association with Hirtella physophora and the plant is not inhabited by other ant species," says Orivel.

Good for the Allomerus; bad for any big bugs with skinny legs that happen to be out for a stroll on the stem of the Hirtella.

4. Dead cricket is held to stem by ants
That's one dead locust.

Antsy Engineering
To build their devilish snare, worker ants cut plant hairs, clearing paths along the plant stem. The ants leave the uncut hairs to act as pillars, supporting the deadly platform. The platform itself is woven from the cut hairs and reinforced with a special type of fungus. Orivel opines "the fungus is specific to this ant-plant association because we do not find it in empty plants and when the ants are removed the fungus degenerates after a while."

Once the platform and pillars are complete, creating a chamber-like structure, the ants puncture holes in the surface that they can reach their heads through, allowing them to enter and exit. Then, hundreds of ants climb into their (torture) chamber and wait for the carnage to begin.

-- Megan Andersonlittle ant illustration

Arboreal Ants Build Traps To Capture Prey, by A. Dejean and J. Orivel, et al. , Nature, Apr. 21, 2005.

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