Ants lurk for bees, but bees see ambush. (Zoology).

A tropical ant hunts bees by setting ambushes. However, the bees have developed a trick or two of their own.

The New World ant Ectatomma ruidum waits outside the tiny holes in the ground that lead to nests of the sweat bee Lasioglossum umbripenne, explains William T. Wcislo of the Smithsonian Tropical Research Institute headquartered in Balboa, Panama. A bee flying home typically pauses at the entrance while a guard bee checks her chemical credentials as a nest mate. During this brief delay, the ant lunges, grabs the bee in her mouthparts, and then stings the captive to death.

Wcislo and Bertrand Schatz of Centre d'Ecologie Fonctionnelle et Evolutive in Montpellier, France, described such ambushes in 1999. Now, in the February Behavioral Ecology and Sociobiology, the researchers report bee countermeasures.

When an ant is hanging around the nest, 97 percent of returning bees interrupt their first swoop to the nest and veer away. Nearly half make a second approach, trying to slip in from the far side. Others land at a distance and walk home. This can save the bee if the ant keeps scanning the sky or moves on.

The warning for bees seems to be visual, say the researchers. Bees shied away from a dead ant beside the nest, even a dead ant that researchers had washed in solvent to remove body odors. A little black square or rectangle, however, didn't alarm the bees.

Once a bee falls into an ant's fatal grasp, it doesn't get a chance to learn from its mistake. So just how bees have come to recognize the ant dangers remains a puzzle, says Wcislo.