A scanning electron micrograph of the marauder ant Pheidologeton diversus depicting the normal behavior of a minor worker riding on the head of a major. There’s a 500-fold difference in body weight between these two workers.

      One day I spent hours rummaging through hundreds of the naphthalene-scented cabinets searching for the least-understood specimens. From childhood, I have had an eye for all that is quirky in the natural world. In those cabinets, accordingly, I was drawn to the ants with oddball heads and mandibles, curious body shapes and hairs. I wondered what their bodies said about their lives and habits.

      Continuing my search the next day, I came upon three drawers labeled Pheidologeton, a name I had never heard before. The glass tops of the drawers were dusty, and their contents were in disarray. The dried specimens, glued to small wedges of white cardboard that in turn were affixed with insect pins to foam trays, had obviously not been looked at for many years.

      I was struck at once by the ants’ polymorphism—that is, how different they were from one another in size and physical appearance. As in most ant species, the queen was distinctive, a heavy-bodied individual up to an inch long. But it was the workers that gave me an adrenaline rush. While the workers of many species are uniform in appearance, in Pheidologeton the smallest workers, or minors, were slender with smooth, rounded heads and wide eyes. The intermediate-sized workers, or medias, had larger, mostly smooth heads, and the large workers, known as majors, were robust, with relatively small eyes and cheeks covered with thin parallel ridges. The wide, boxy heads of the majors were massive in relation to their bodies, housing enormous adductor muscles that powered formidable mandibles.

      I had never seen anything like this. The minor, media, and major workers didn’t look like they belonged to the same species. The heads of the largest workers were ten times wider than those of the smallest. The biggest majors, which I came to call giants, weighed as much as five hundred minors. The energy and expense required to produce these giants—and to keep them fed and housed—must, I thought, be immense, which meant they must be of extraordinary value to their colonies. I left the collection that day certain I had found something special: few ants display anything close to the extreme polymorphism of Pheidologeton.

      As a student I knew that the best-studied polymorphic ants were ones I’d seen on my first trip to Costa Rica—certain Atta, or leafcutter ants, and New World army ants such as Eciton burchellii. These ants have some of the most complex societies known for any animal, giving them an exceptional influence over their environment. Their social complexity is due in part to the division of labor made possible by their varied workers, which, with their differing physical characteristics and behavior, can serve different roles in their societies. Called castes, these classes of labor specialists focus variously on foraging, food processing or storage, child rearing, or defense, such as when large individuals serve as soldiers. Given its minor, media, and major castes, I suspected that Pheidologeton would be a treasure trove of social complexity.

      From reading the books of Jane Goodall and other modern naturalists, I had developed the view that the best path to a career in biology was to find a little-known group of organisms and claim it, at least temporarily, as my own. I could then, like an old-fashioned explorer studying a map in preparation for a voyage, pinpoint those regions most likely to yield rich scientific rewards. Buoyed by this belief, I decided Pheidologeton would be my version of Jane Goodall’s chimpanzee.

      I soon found that my point of view was outdated. All around me, starry-eyed students who had come to biology because they loved nature were becoming lab hermits, indentured to high technology. Watching my fellow students, I realized that too much of modern biology represents a triumph of mathematical precision over insight. Sure, laboratory techniques allow for unprecedented measurements, but what good are those streams of numbers if it is unclear how they apply to nature? One thing I’d already absorbed from Ed Wilson was that much could still be done with a simple hand lens and paper and pencil. I was determined to spend my life in the field.

      In the fall of 1980, I proposed to Professor Wilson that I would journey across Asia to investigate Pheidologeton—which I confidently proclaimed would be among the world’s premier social species. My enthusiasm, if not my charts and graphs describing the species’ polymorphism, won him over. I received his blessing and, within days of passing my oral exams, boarded a plane bound for India. Over nearly two and a half years I would visit a dozen countries without a break, vagabonding through Sri Lanka, Nepal, New Guinea, Hong Kong, and more.

      Since then, ants have led me to all the places I dreamed of as a child. That’s far more than can be described in a book, and so I focus my narrative on a few remarkable ants. I start with the marauder ant, taking my time both because this species was my own introduction to ants and because it exemplifies behaviors that come up repeatedly, such as foraging and division of labor. Thereafter, subjects are organized, in a crude way at least, by the ant approximations of human societies throughout history—from the earliest hunter-gatherer bands and nomadic meat eaters (army ants), to pastoralists (weaver ants), slave societies (Amazon ants), and farmers (leafcutter ants)—ending up, at last, with the world-conquering Argentine ant, with its hordes of trillions now sweeping across California.²

      In this book, I will consider what it means to be an individual, an organism, and part of a society. Ants and humans share features of social organization because their societies and ours need to solve similar problems. There are parallels as well between an ant colony and an organism, such as a human body. How do ant colonies—sometimes described as “superorganisms” because of this resemblance—reconcile their complexities to function as integrated wholes? Whose job is it to provide food, dispose of waste, and raise the next generation—and what can ants teach us about performing these tasks?

      To find out, let’s begin our adventures among the ants.

      a brief primer on ants

      Anatomically, ants are like other insects in having three primary body sections: head, thorax, and abdomen—though the addition of a narrow waist gives ant abdomens extra mobility, enabling a worker to, for instance, aim a stinger or repellant spray from her rear end.¹ Almost every ant has pores near the rear of the thorax through which two metapleural glands discharge phenyl acetic acid and other fungicides and bactericides, required for a healthy life in the soil.

      Ant antennae are elbowed at the midpoint so they can be manipulated like arms, though unlike the individual’s jaws, often called mandibles, they can’t grip. Ants keep their antennae moving for the same reason that we scan with our eyes: to monitor the environment. Beyond their elbows, antennae are flexible and endowed with sensors for touch and smell, senses more valuable for most ants than sight. An ant’s compound eyes use many adjacent facets to produce images that are put together by the brain into a mosaic view. The eyes of most ants have little resolving power, though there are certain exceptions: inch-long Australian bulldog ants are so visual that I’ve watched them station themselves near flowers and seize bees out of the air.²

      Mandibles are the prototypical tools used by ants to manipulate objects, and they are toothed in different ways to serve the needs of different species. Many ants can also grip eggs with spurs on the foreleg above the foot, in much the way that squirrels hold acorns with their paws.³ Each foot, called a tarsus, is flexible and multisegmented and clings to surfaces not with toes but with two terminal claws and cushiony adhesive pads.

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