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of probabilities: while workers focus their efforts in areas that are likely to yield food, or even certain kinds of food, they can take on other functions as the need arises, as if a switch has been flipped in their head. For example, in northern Argentina, David Holway, Edward LeBrun, and I observed Ectatomma workers after their nest entrance was inundated: the first workers out of the nest ignored prey insects for several minutes while they cleared the entrance. I have observed Amazon ants switch from “move away from the nest” behavior, in which they ignore food, to a “foraging phase,” in which they search for an ant nest to raid the brood (see chapter 13). Deborah Gordon describes how the first ants to leave a harvester nest each day assessed the local conditions but didn’t return with seeds; the foragers emerged only later (Deborah Gordon, Ants at Work: How an Insect Society Is Organized [New York: Free Press, 1999], p. 34).

11. Webster’s Unabridged defines “army ant” as “any species of ant that goes out in search of food in companies” (Webster’s Unabridged International Dictionary, 2d ed. [New York, 1939]). Foraging “in companies” is the unique and archetypal trait of the three army ant subfamilies, though there are ancillary characteristics widely associated with an army ant “syndrome,” such as group predation (the catching of prey in a group), group retrieval (used to describe both group transport—see chapter 5—and the retrieval of food along a common path), nomadism, queen morphology, and mode of colony foundation (see chapter 4), as originally discussed by EO Wilson 1958, The beginnings of nomadic and group-predatory behavior in ponerine ants, Evolution 12 (1958): 24–36.

12. Group foraging and group hunting are the most common terms, though group too easily conjures the discrete packs (i.e., with circumscribed memberships) of recruited workers characteristic of some “raiding” ant species that shouldn’t be confused with army ants. Certain ants display intermediate strategies between solitary and group foraging, such as the Argentine ant (see chapter 16).

13. KG Facurel, AA Giarettal 2009, Semi-terrestrial tadpoles as vertebrate prey of trap-jaw ants (Odontomachus, Formicidae), Herpetol. Notes 2: 63–66.

14. Several categories of forager (scout) and recruit have been described, such as recruits that find food other than that to which they were recruited, but most of the distinctions seem limited in value. See, e.g., JC Biesmeijer, H de Vries 2001, Exploration and exploitation of food sources by social insect colonies, Behav. Ecol. Sociobiol. 49: 89–99.

15. Admittedly, distinguishing these situations is more easily said than done. Rather than choose their routes at random, for example, solitary foragers may favor sites where they found food before; see, e.g., JFA Traniello, V Fourcassié, TP Graham 1991, Search behavior and foraging ecology of the ant Formica schaufussi: Colony-level and individual patterns, Ethol. Ecol. Evol. 3: 35–47. As a result, multiple workers may come to one place through individual choice, rather than as a result of a coordinated action. It’s also unlikely that foragers take completely independent courses, entirely ignoring any nestmates encountered in their travels; see, e.g., DM Gordon 1995, The expandable network of ant exploration, Anim. Behav. 50: 995–1007. To my mind, as long as workers don’t strongly constrain or guide each other throughout the food search, for all intents and purposes they are acting solitarily.

16. More accurately, no worker travels without guidance from other workers for more than a minute fraction of the span explored by all the participants during the course of a raid.

2. The Perfect Swarm

1. M Moffett 1984, Swarm raiding in a myrmicine ant, Naturwissenschaften 71: 588–590.

2. Many details of this chapter are discussed in MW Moffett 1988, Foraging dynamics in the group-hunting ant, Pheidologeton diversus, J. Insect Behav. 1: 309–331.

3. Chapter 8 will describe the subterranean army ant Dorylus laevigatus, studied since my work on the marauder ant; this