Adventures Among Ants - Mark W. Moffett [60]
One hour later, the column was succeeded by the full-bore swarm raid of driver ants, which swept through, swamping everything. The disoriented scramble that ensued reminded me that, compared to Pachycondyla’s methodical predation on termites, army ant raids seem based less on finesse than on brute force. I’m convinced driver ants have little success with prey smaller than themselves not because such prey isn’t worth the effort—since so few ants take food most of the time, this argument hardly seems viable—but because the sightless rampagers are individually clumsy.1 This again is what we expect with ants with a large colony size: faster movements and more inefficiency.
Even so, there were signs of order. As I watched the raiding ants struggle with a scarab beetle, others walked over or alongside them, delivering food to the nest. The frenzied workers on the beetle never seemed to confuse the movements of their prey with those of the dead insects being convoyed past them. Meanwhile, their food-hefting sisters managed to stick with their job and ignore the fighting, even though the ants embroiled with the beetle must have been discharging a powerful alarm pheromone. I couldn’t imagine how they kept it all straight.
I turned my attention back to the entrance where the driver ants had poured into the Pachy nest. By now the swarm had departed, leaving the hole deserted, in ominous stillness. There was not one Pachy to be seen.
But in the morning, I saw the Pachys marching out once again, with no dead of their species in evidence. Remarkably, they had survived the rubellus attack. In fact, the Pachys must have been picking off the driver ants one by one and (I imagine) eating them underground the whole time the multitudes were passing overhead. Instead of facing their demise, the Pachys had beaten the odds with a classic maneuver: by taking advantage of the choke point at the nest entrance, they had greatly decreased their adversary’s access and combat power. They had been in control of the situation the whole time, transforming the driver ant raid into the ultimate stay-at-home feast.
How could they take such a pounding? I found out by dropping a Pachy onto a driver ant trail. In one fell swoop, she was buried from view by a mass of driver ants. That was the sort of brutality I expected from an army ant! Convinced she was done for, I returned twenty minutes later to find driver ants still laying siege. I extracted the hapless Pachy with a pair of forceps and, shaking off all but two of her attackers, put her down for a look. She lay motionless yet looked intact. When I picked off the last two ants, she roused herself and ran off. She had been playing dead.
I surmised that the Pachys were too well armored to be killed—the driver ants’ mandibles slip right off their exoskeletons. Because army ant raids pass by within ten or twenty minutes, a victim need only stay immobile and wait for her assailants to give up. On several subsequent occasions, I saw a Pachy escape after driver ants had restrained her.
Fighting back is rarely an effective way to survive an army ant raid. No matter how many of its workers die while catching prey, the raid never appears to retreat; the attackers just keep piling on—an advantage to having a humongous army. Some beetles and millipedes avoid death by exuding noxious chemicals; driver ants respond by burying them with soil and abandoning them with no harm done. Spiders and praying mantises avoid capture by New World army ants by freezing in position; unable to detect prey except by their escape response, the ants leave them unscathed.
Many ant species have evolved other defenses to give their colonies