Ambush ants: beware the moldy patch on that branch.

On the plants where they live, tiny tropical ants build fungus platforms and then hide underneath, ready to ambush insects that may be far larger than themselves.

The ants use the plant's natural hairs as pillars supporting a hole-riddled roof of fungus and other tidbits, says Jerome Orivel of Toulouse University. in France. Lurking beneath this structure, the ants reach through the holes to capture prey ranging from mosquitoes to grasshoppers, he and his colleagues report in the April 21 Nature.

"The observation of an ambush trap built by ants is very exciting," comments Joachim Offenberg of Aarhus University in Denmark. The trap gives a new twist to familiar ideas about how ants can protect the plant they live on from other insects.

Orivel's research group was studying ant-plant relationships in French Guiana. The small tree Hirtella physophora attracts the ant Allomerus decematiculatus by growing hollow pouches at its leaf bases that make perfect ant apartments and by releasing nectar in ant-snack stands apart from the flowers. Yet the ants still need to find a source of protein.

Orivel found a big, dead insect stretched out on one of the moldy patches along the tree's branches, which often swarmed with ants. He says that he began to wonder whether these patches were ant-constructed traps.

He and his colleagues observed that the fungus coverings typically had ants lurking underneath with their mouthparts open. When the researchers placed insects such as grasshoppers on the structures, the ants poked through the holes, grabbed at legs and antennae, and within seconds, had immobilized their catch. Ants then swarmed over it, killing it with multiple stings.

The researchers also suggest that the ants bring in or encourage the growth of the sooty, mold that knits together a trap. Stems of several dozen plants too young to have leaf pouches and attract ants had no mold, the researchers report. Plants grown in an antfree greenhouse grew pouches but never bore the mold. However, the mold showed up in saplings that the researchers provided with ants.

In a working trap, the mold surrounds the holes, but when researchers removed the ants, the mold expanded into a shaggy tangle.

The possibility that three species--plant, ant, and fungus--have intertwined lives sounds quite plausible to Doyle MeKey of the Center for Functional and Evolutionary Ecology in Montpellier, France. Other ants live as favored guests on particular plants where they tend herds of aphids, which in turn rely on bacteria to digest the plant sap. That's a four-way arrangement, he notes.