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By Root 615 0

near the base of a Brazil nut tree, was a throng of ants—shimmering with the movements of thousands in the cropped grass. I’d caught glimpses of such mobs in my Indian plantation’s understory brush, but here was one open to scrutiny, a band of ants 2 meters wide and over 7 centimeters from front to back. At the back of this band was a V-shaped network of columns 3 meters long that resembled the web of veins in a human hand and, judging by the slaughtered prey being carried along, served the same purpose of conveying nourishment. The web converged into a single column that was the colony’s aorta to the nest. Workers laden with plunder marched along this route all the way home.

Paddy came over and gave a whistle of astonishment. My hand sped across a waterproof notebook as I penciled a sketch of the action. Afterward, examining my drawing, I realized how closely it resembled illustrations of army ant raids. In fact, the swarm compared point for point with descriptions of the most extreme form of army ant attack, the swarm raid.1

In the terminology of army ant researchers, the advancing margin, where the workers meandered ahead of their sisters, is the swarm front. The swarm is the band of ants behind the front, and the fan is the network of columns farther back still, which converge to form the single base column that extends to the nest. Among army ants, swarm raids are peculiar to some New World Eciton and Labidus ants and to a few African Dorylus species known as driver ants. Skirmishes within a raid appear chaotic when viewed in isolation; but when a raid is seen as a whole, a sense of order and even aesthetic beauty emerges.

The phalanx of ants stayed in tight formation. This made the raid’s anatomy easy to pick out—a boon to humans, whose noses are too poor to register the pheromone scents that the ants prefer to use for communication and that bind the raid together. A century ago, Herbert Spencer saw a “closeness of parts” of this kind as strengthening a society’s similarity to an organism. After all, we recognize a dove or rice grain by its boundaries: each has an inside and an outside. The workers that form a marauder ant or army ant raid may be separate creatures, but they do not drift apart, and therefore they form an entity that is not only cohesive but also distinct and well bounded.

The same was true outside the raids, throughout the colony. Over the next weeks I would learn that while trunk trails and their temporary offshoots could extend for a hundred yards, individual marauder ants stay on these roads and seldom travel more than a few centimeters from their sisters.2 All foraging, I determined, is done in a group: my observations revealed no rogue hunters. (I did come upon strays, though. Some were stragglers, sick or lame, on paths all but abandoned. Then there was the occasional isolated worker that was just plain lost. I spent hours watching these individuals stumble around. But even after I gave one lost marauder a bit of my lunch, she had no idea where to go with it. Presumably these forlorn souls wander until they die.)

Certain things became clear to me as I sketched the raid that afternoon in the Botanic Gardens. Within the raiding horde, there’s little appreciable movement of any ant at the swarm front beyond the ground covered by her nestmates—no exploration of fresh terrain except for a stint at the front of the raid, which is the one time in a marauder ant’s life that can be unambiguously described as foraging. The trailblazers at the front (too temporary and plentiful to be considered scouts, they are appropriately called pioneers, as they are in army ants) cross onto new soil. Pioneers don’t appear to be specialists at this task; whoever reaches the front does the job. Nor do they press ahead and fall back with the precision seen in movies depicting Roman soldiers massed against the Gauls. Sometimes they wander a bit. In any case, their actions are restricted to the vicinity of their neighbors, and the raids as a whole have no ultimate destination.

Marauder and army ant raids differ only