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By Root 453 0

out from it at almost the full ant-running speed of 3 meters per minute. When I scratched the dust in front of one arm, the workers pooled there, confused. That told me the panicked ants were making their escape by backtracking along the base trail of that day’s raid as well as all its abandoned subsidiary trails that the ants could still detect by scent. Thick ribbons of ants flowed on a dozen routes spread over an area the size of a one-bedroom apartment. The army ants were obeying their life’s simple rule “follow a trail.” In applying this rule now, there was a danger: when the path to their nest is cut off, backtracking on old pheromone trails can eventually lead army ants in a circle many meters across. The throngs starve to death as they follow each other round and round, stuck in the social equivalent of the endless loop of a defective computer program or unbalanced brain. This colony was fortunate: by chance one ribbon extended on a path that linked back to the driver ants’ original trail on the other side of the weaver ant nest, enabling the driver ants to retreat to their own nest while giving the weaver ants a wide berth. Soon, half a million ants had drained from the system of emergency trails.

It’s common for ants to react explosively to serious threats. Marauder ants carrying food flee when their trail is interrupted, and carpenter ants rush up grass blades during a driver ant raid. But the scale of this driver ant retreat made “overkill” an understatement, especially for an army ant, whose mode of life is to eat other ants.3 Strangely, though, not a single worker on either side had come into conflict, let alone been killed. Clearly the withdrawal began with the detection not of the weaver ants themselves but of their nest. What about Oecophylla merited such an extreme response?

Each weaver ant nest can hold thousands of workers. A colony can occupy hundreds of these nests in one or two dozen full-grown trees spread over 1,600 square meters of ground, with a total population of half a million, the same size as many driver ant colonies. Weaver ants don’t have stingers; like driver ants, they overwhelm their enemies with their superior numbers and sheer mandible power.

And quite a bite they have, too. Javanese children are warned that they will be tied to trees, to be overrun by biting weavers, if they misbehave. People around the world have learned from experience to back off when they notice the spastic, open-jawed motions of alert weaver ants on leaf and twig outposts in their nest trees. Though driver ants are more powerful, weaver ant bites are intensified by acidic secretions, which feel like lemon juice rubbed into abraded skin. (That said, some cultures have learned how to benefit from these ants’ defensive chemistry. The best chicken dish I ever ate was served in Cambodia with a tangy weaver tapenade. And in Australia I joined an Aborigine suffering from nasal congestion in sniffing a cake of fresh-crushed weaver ants; the fumes had the sinus-clearing effect of the mentholated gel Vicks VapoRub.)

I retrieved the nest I had dropped on the trail. It tore like paper and revealed a mere fifty workers. From the driver ants’ reaction to this little nest, I predict that if there ever were a battle between weaver and driver ants, the ordinarily unstoppable driver ants, normally the terrors of the jungle, would be unlikely to come out on top.4

One basic difference between army ants and weaver ants is this: army ants strip away the workers’ autonomy to the point where few signals are needed to coordinate their troops, whereas weaver ants move with more freedom—even getting lost can be an asset if a stray ant blunders into an overlooked meal.5 But this freedom comes at the cost of added logistics, because a worker must persuade others to come together to help her perform tasks as varied as making a leaf nest and killing driver ants. Perhaps the endless looping that can happen to an army ant column shows that their colonies can at times be too much like an organism for their own good—no such catastrophic failures