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By Root 506 0

ants concentrate their foraging on a specific stratum on or under the ground; yet within their preferred layer, most species are consummate generalists, scouring the raid front for any prey they can detect and catch, in whatever habitat they happen to be passing through, and with, as far as we know, little regard for costs and benefits. Their polymorphism, however, offers the possibility of specialization within a colony, because the workers can act almost like different species in their contributions to hunting and gathering. One way for this to occur is through size matching, in which, for example, small workers find or kill and harvest smaller prey than do larger workers. While there is no evidence that an army ant worker will pass up prey in favor of another more suited to her size, small workers are able to search tunnels too narrow for their larger sisters, which could open up unique opportunities for them.10 Meanwhile, bigger ants can carry bigger items to the nest, though rather than selecting their booty by size, it’s more likely each ant cuts a chunk appropriate for her to lift.

SUBTERRANEAN RAIDERS

Back at the Gashaka field station there was a species of ant that didn’t put on anywhere near as obvious a show as rubellus. This ant, which belongs to one of the primarily subterranean groups of the genus Dorylus, was in an easily overlooked colony that I was lucky enough to locate and study. In the lingo of army ant biologists—of whom there are perhaps a dozen—these subterranean driver ants are simply “subs.” Compared to intermediate army ants like the congolensis-kohli complex species, with their moderately long legs, and the even more lissome surface-active driver ants such as rubellus, which use their spindly legs to run fast on open ground while holding large objects beneath them, subterranean army ants have narrow heads and short extremities.11 These are sensible adaptations. Long limbs get in the way in cramped quarters, whereas short, stocky legs are more suitable as digging tools, giving better leverage for moving soil. Compared to driver ants on the surface, which can be spaced several body lengths apart as they move along, the subs are often piled on top of each other within their narrow passages.

Subs live in terra incognita. Stefanie Berghoff, then a doctoral student at the University of Würzburg in Germany, has made the only attempt to date to study them in depth.12 She went after an Asian species, Dorylus laevigatus, one I have from time to time seen crossing trails and moving under logs, typically massed as thick as porridge. By placing bait in buckets full of holes, Stefanie showed that the ants employ stable underground trunk trails to continuously access the same foraging areas for two months or even longer—a pattern previously unheard of among the army ants.

As with marauder ant trails, the laevigatus trunk trails are an expanded base from which to hunt en masse. Shifting networks of raiding columns extend from the trunk trails to catch invertebrates. At the front of each column, Stefanie found, pioneer ants led the advance in classic army ant style, with each ant replacing and surpassing others while presumably laying a short exploratory trail. Columns stayed beneath the surface, but on occasion they would emerge aboveground. Usually the column raids retreated after ten or twenty minutes, but when the ants in the column contacted a termite mound or palm oil in one of Stefanie’s buckets, the route they were taking was reinforced by the workers exploiting the food bonanza until it transformed into a branch of the trunk trail, remaining active for twenty-seven days or longer. The workers harvested few termites at a time from the mounds, but they also attacked several ant species for their brood, along with worms and a wide spectrum of insect larvae.

Although laevigatus is essentially a column raider, Stefanie also recorded three full-bore swarm raids on the surface, all of them small for any of the army ants that swarm-raid full time, the widest being only 3.5 meters at the front. Each swarm