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By Root 458 0

I envisioned her ants racing out from these chambers along trunk trails that resemble subway tubes crammed with pedestrians, and wondered how they conducted their raids underground, an operation impossible to observe. In my imagination, a small worker pushed into a crack in the tube wall, to be followed by others. From this humble beginning, columns of workers soon spread through the soil matrix. Looking closely at one of the columns, I could see the ants shove aside or remove objects from their path, creating a route through the porous matter. As they reached a crack, they roused an earthworm and drove it ahead of them. (Although the surface-swarming driver ants earn their common name from this herding behavior, there is no reason to suppose the same thing doesn’t occur in species raiding underground.) The worm crawled into a labyrinth of abandoned root channels. The ants cornered it in a cul-de-sac, much as the rubellus forced the carpenter ants up grass stalks until they could go no farther. Their exertions focused thus far on advancing, the small workers had cleared a corridor just wide enough for themselves. While some of them restrained the earthworm, others went back over the raid path, enlarging it to make room for the larger workers who would be recruited to assist in the kill.

Barring cave-ins, such a passage will be available for later use. Because of the laevigatus ants’ constant raiding, commonly foraged areas will eventually fill with their tunnels “until they look like Swiss cheese,” Stefanie told me. By clearing their abandoned passageways during occasional raids, the army ant subs provide attractive living space—a limited resource belowground—for their future prey.

Everyday ants, not just army ants, build tunnels when constructing their nests, and these can yield “homegrown” food as well. As a child I had an ant farm, which the manufacturer provided with Formica “wood ant” workers and pupae. The earth I gave them for digging had prey in it. One day, I noticed a worm nose through the soil and enter one of the ant galleries. The ants normally came to the surface to eat bits of my dinner dropped on the area at the top of the farm, but they were able to grab the worm and consume it without leaving their chambers. I suspect this kind of thing happens all the time, even with species that search for food on the surface.

How does raiding underground compare to raiding on the surface? The capacity of subterranean army ants to swarm out in three dimensions vastly increases their prospects for locating prey, though the constant need to excavate raid pathways may slow the search. Yet because army ants often take prey larger than they are, the subs are unlikely to lose what they track. Their quarry, especially their larger quarry, is unlikely to find an escape route that the ants can’t follow.

Driver ants such as rubellus are conspicuous for raiding in swarms on the surface, but that doesn’t stop them from searching underground, too. Raiders investigate every cranny, occasionally dredging up earthworms, and as Caspar and I had discovered, they will also go underground to demolish immense termite nests embedded deep beneath their mounds. Even the trail leading from the raid to the nest may be replaced by alternative, subterranean routes. The first time I sat down to watch a rubellus trail, the ants had all disappeared by the time I looked up again from my notes. Then I noticed that they were avoiding me by going into and out of holes in the ground on either side of me. My meddling had set the ants to patrolling, and apparently they’d probed around for a preexisting tunnel that now served as a substitute route to bypass me. What a sneaky superorganism!

The difference in habitat between “sub” army ants and surface-foraging driver ants such as rubellus must therefore be one of degree. After all, driver ants nest underground. And since they migrate frequently to new sites, they must be skilled at exploring belowground to evaluate possible homesteads.

To determine if a cavity is suitable for a new nest, honeybees fly