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By Root 620 0

rather, it sets the leaf itself vibrating. The better the leaf and the hungrier the cutter, the more often she chirps, which suggests the vibrations communicate the location of a good leaf to nearby workers, encouraging them to follow recruitment trails in her direction.73 Stridulation is a modulatory signal, meaning chirp intensity motivates workers to respond, much as we take cues from how feverishly a dog wags its tail.74 The vibrations become especially urgent during the worker’s final moments of sawing and her initial maneuvers to carry the leaf, at which point small workers react to her signals by climbing on the fragment as she carries it away.75 The function of these feisty hitchhikers has been the subject of much research, but primarily they serve as shields to keep the leaf carriers from losing their heads.76 The shotgun riders thrust their legs or snap their mandibles when a fly comes close. Too small for decapitating flies to target, the bodyguards become more numerous whenever the flies are abundant.

Even when a leaf carrier lacks a protective force, she can summon one quickly by stridulating.77 That’s because the small workers that ride on leaves are the same ones that reinforce trails with pheromones, during which time they patrol for threats and respond to any sign of trouble. They are also the workers that, in preparing the leaf fragments inside the nest, lick them to ingest a meal of oozing sap and, more important, to scrub off any contaminating microorganisms. It makes sense to get started on this essential task before the foliage reaches the delicate gardens—and in wide-open spaces rather than the cramped quarters of the nest.

Workers of the leafcutter ant Atta cephalotes toting leaf fragments on Barro Colorado Island, Panama.

The flies, and the occasional raid by predatory army ants, are among the biggest problems the leafcutters face. Gardening frees them from competition over food with other kinds of ants, though leafcutter colonies have been known to fight with each other along contact zones that shift back and forth like the battle lines at Gettysburg.78 Otherwise their chief competitors are solitary plant-feeding insects, few species of which have anywhere near the ant aptitude for search and seizure. Little has been written about whether leafcutters ignore, scare off, or kill caterpillars, bugs, and beetles, but in all likelihood these leaf eaters are inconsequential to them.

THE EMBRYONIC EMPIRE

Perhaps the biggest challenge any colony faces is getting started. The process is much the same for all ants. Leafcutters add a wrinkle to the story with their fungus, which is an essential part from the beginning. New queens, larger than the workers of any ant species and each the size of an unshelled peanut, tuck a wad of it into their mouths when they leave their birth nest. To pursue the superorganism metaphor, they are like so many eggs cast out by a fish, and are similarly fertilized as they disperse. (As in all ant species, they often have several midair consorts, who die immediately. Most queens don’t last long, either, but soon become food for animals and humans. Eating fried queens is like eating crunchy nuts.) Afterward, each queen will search for a place to rear her new colony. Once she digs the first chamber of her new nest, which she will probably never leave, she spits out the fungus and defecates to fertilize it. This moment must be as delicate as blowing on sparks to start a fire, for if her fungus dies, she will, too.79

With luck, in a couple of days she will have a small, robust garden. Meanwhile, she lays two kinds of eggs: small ones that develop into larvae, and large, infertile ones that serve as food for her developing brood. (Later the queen will eat similar eggs, laid by a clustering retinue of young mid-sized workers born with rudimentary ovaries that shrivel away as they get older.)80 From now on the queen serves as the ovaries for the whole—the superorganism she has created, which has, like a fish or a person, a life cycle of its own. Her workers take over the other