"Torture Racks" Are First Known Traps Made by Ants

April 20, 2005

A tiny tropical ant has developed a gruesome way to catch prey much bigger than itself.

Scientists say it uses traps reinforced with fungus to ambush large insects, stretching them out like victims on a medieval torture rack as nest mates swarm in for the kill.

Even for creatures as ingenious as ants, it's a device to marvel at. Using cut plant fibers glued together by a specially cultivated fungus, the tree- dwelling ants construct a spongy platform pitted with holes, or galleries.

Lurking in these holes, the ants grab the legs and antennae of unsuspecting insects. The ants then stretch their prey out flat and sting them to death before they are carved up by a swarm of nest mates.

The trap allows the ant to subdue prey massively bigger than itself.

The grisly technique was discovered by French scientists in the tropical rain forests of South America. They say it's the first known example of a trap constructed collectively by ants.

Details of the find are published in tomorrow's issue of the science journal Nature.

While working in French Guiana on relationships between ants and plants, the research team found the fungus-coated, galleried structures on the Amazonian plant Hirtella physophora. The plant has evolved a close association with Allomerus decemarticulatus, a tiny orange-yellow ant that's less than a tenth of an inch (two millimeters) long.

The fungus-coated galleries had been noted before, says co-investigator Jérôme Orivel, but their purpose was unknown.

"When we saw insects much bigger than the ants stretched on the galleries, we immediately thought they could be traps," said the Toulouse University biologist. "Our observations revealed that Allomerus workers hide in the galleries with their heads just under the holes, mandibles wide open," waiting to seize any prey that enters the trap.

Rapidly Dismembered

Orivel says any insect that walks or lands on the trap is caught immediately, though smaller bugs such as mosquitoes and flies are difficult to see because they are dismembered so rapidly. It is larger prey like locusts, caterpillars, and spiders that get the full "torture rack" treatment.

"Almost all the biggest insects can be caught, as long as their appendages can enter the holes of the traps," Orivel added.

Traps are built on the stems of the plant, which has specialized structures called leaf pouches that the ants live in. The plant has a symbiotic relationship with Allomerus," Orivel explained. "By hosting ant colonies, it is protected against its enemies."

Many ants have such an association with plants, says David Jones, an entomologist at the Natural History Museum in London.

"Ants help to remove and deter herbivores from the plant. In return, it provides them a habitat. For example, some plants have large, hollow thorns in which the ants keep their brood. Other species have enlarged stems where ants construct their home. Ants have co-evolved with a whole range of plants," Jones said.

Yet Allomerus has developed a second mutually beneficial partnership—with a fungus.

The ants cultivate the fungus, turning it into a sooty mold that acts as living cement, reinforcing the structure of their traps. In return, the fungus gains from having the ants promote its growth and protect it from predators and parasites.

Fungus farming is well known among tropical ants, though usually it's a way to break down plant cellulose that the ants can't digest themselves. Leaf-cutter ants cultivate fungal gardens underground, harvesting fungus grown on composted leaves.

Ant Protein

In the case of Allomerus, however, the research team believes the fungus's main role is to help provide the ants with protein in its six-legged form. Insects are a good source of protein which in turn is a major source of nitrogen—important to the ants for producing venom, nourishing larvae, and strengthening the cuticle (the ant's outer covering).

"Nitrogen is one of the main limiting factors [for ants], especially in the arboreal [tree-rich] environment, where prey are randomly distributed," Orivel said.

Other arboreal, or tree-living, ants are known to use group ambush to catch prey before it can hop or fly away.

"There's a whole range of ambush techniques," Jones said. "For example, one strategy is to hide on the underside of a leaf. You might get large parties of ants waiting just under the leaf's edge. If an insect lands on top, they all come charging out at once and jump on it."

However, the newly described ambush strategy is believed to be the first where ants employ a collectively made trap. "Apart from social spiders that build mutual webs, I can't think of any other examples" of such a collective trap strategy among insects or arachnids, Jones added.

And while a spider's silk is produced internally, Allomerus uses stem fibers together with a compound it regurgitates before adding the sooty mold. The fungus starts growing around the trap's holes before spreading to the rest of the structure, reinforcing the whole trap.

Armed with such a cunning contraption, this tiny ant becomes seriously deadly. Even the biggest bugs in the rain forest must tread warily—or risk ending up on the rack.

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