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 in degree. One obvious difference is their speed: marauder raids move at a measly 2 meters an hour, maximum, while army ants can travel ten times that fast, the record being 25 meters in an hour. Scale the ants up to human sizes, and that would be over 800 meters an hour for the marauders, versus up to 8 kilometers an hour for an army ant raid. As a result of their slow speeds, marauder ant raids, which last a few hours, cover only 20 meters at most, while over the same time some army ant raids can traverse 100 meters or more.3
A marauder ant swarm raid, based on my original drawing, advancing toward the top of the page.
Workers of Proatta butteli seizing a wasp at the Singapore Botanic Gardens.
In Malayan rainforests to the north of Singapore I would later find a second swarm-raiding Pheidologeton species called silenus, closely related to P. diversus but with raids twice as fast, matching the raid speeds of a slow army ant.4 Sluggish or not, the painstaking searches don’t hinder the hunting prowess of these Pheidologeton species, particularly in the diversus marauder ant, which usually takes food in abundance.
I came to believe that there was simply no need for the mass of marauder ants to move along any faster. In fact, in the Botanic Gardens I came upon a different kind of ant nesting at the base of a withered tree that taught me that mass foraging might not require the group to move at all.5 Lumpy beasts with unimpressive jaws, Proatta look incapable of doing anyone harm. Yet that day I saw three workers grab a wasp that must have outweighed them by a factor of fifty; trying to escape, it nearly lifted them all from the ground in an attempt to take flight. Nearby comrades, attracted by the commotion, seized the quarry by the hind legs. Then more nestmates, perhaps drawn to the site from a distance by pheromones, helped to pull the wasp into their nest.
Groups of the same Proatta ants also killed marauder ants that lagged behind on the base trail after a raid. What accounted for their success? While much of their effort is spent scavenging by themselves for all kinds of tidbits, the Proatta workers accumulate in such numbers within inches of their nest entrances that when an insect walks by, several ants are often close enough to pin it down. Proatta essentially stay in place and let prey come to them, using a group version of the ambush tactic employed by a human duck hunter hidden in a blind, or by solitary-living species such as the snapping turtle, which lies in wait for fish to pass by.6
As we have seen, proximity is not essential for ants to act as a group. But the Proatta behavior demonstrates that, as in the packed raids of the marauder and army ants, a high density of participants increases the likelihood that encountered prey will be caught. When their workers are close together, some ant species are even able to avoid active foraging almost entirely. Ants squeeze together inside their nests, and a Mexican Leptogenys species takes advantage of this density by giving their living quarters a scent that attracts the pill bugs on which they feed. The ants jointly kill and feast on these little crustaceans without leaving home.7
But by keeping on the move in a crowded mass, the marauder and army ants accelerate the odds of encountering dinner, compared to these sit-and-wait strategists. It is the difference between dragging a net through water and leaving it fixed in place. Both methods work, but a motile net almost certainly catches more fish during a given period.
ANATOMY OF A RAID
One morning as I watched the foremost workers in a marauder raid nose their way forward through the grass, Paddy leapt up behind me with an insect net. Suddenly the 12-centimeterlong praying mantis he was chasing landed with a shudder within my swarm. Paddy backed away with a mild expletive as the ants overcame the mantis. Some grabbed the wings by their edges and spread them out to their full green glory, while others took its head between their jaws until it cracked open like a nut seized by pliers. Soon the marauders were slicing and dicing the mantis with the cold efficiency of slaughterhouse employees.
Army ants, and particularly swarm-raiding army ants, are exceptional for their ability to consistently trap difficult, even dangerous, prey. I now saw that this attribute applied to swarms of marauders as well. To uncover the secrets of the marauders’ predatory success, I began to study the moment-by-moment organization of their raids. By watching where the ants first advanced at the front and then doing a slow scan
