An army ant is any ant species in which the workers hunt in groups (classically, for animal prey).
This sounds simple but even some of the experts are confused about what this means, so let me go into the technical details of how the phrase has been used by others.
Hunting in groups is often called group predation, or my preference, mass foraging. (The problem is that "group predation" has often been applied loosely to any situation in which more than one worker catches, cuts up, or carries prey—actions that are widespread in many ants other than army ants; nor do army ants necessarily catch or carry prey in groups.)
Indeed, the term "army ant" brings to mind a concentrated (and often huge) mass of predatory hunting ants, and it was in fact used specifically to denote such mass foraging species until Edward O. Wilson showed (in a 1958 article in Evolution) that these species also tend to be nomadic, that is, change their nest locations frequently (and sometimes regularly). Since then, other authors have pointed out even more traits that could also be useful in designating which ants might be called “army ants,” notably, the queens are wingless and physogastric (bloated with eggs); food and brood are carried slung under the worker's body; "nests" are produced by workers linking their bodies together, in some cases yielding an exposed mass of resting ants called a bivouac; and the societies reproduce by fission—a young queen starts a new colony assisted by part her mother’s worker population rather than on her own.
Varied combinations of these traits have been treated as a “syndrome.” While this army ant syndrome can be a valuable idea, when it comes to the usage of the term "army ant" each additional trait (nomadism included) should be regarded as a proprium (a term philosophers employ to describe a nonessential property common to examples of a thing, but which is nonetheless not defining or essential). This is so for three reasons: 1) there are examples of undoubted army ants (species in the groups Dorylinae and Ecitoninae) that show these traits at best weakly (as AAA describes, some army ants can stay at one nest site for months if not years and so appear to be no less sedentary as many other ants; for example, from what I have seen and what Stefanie Berghoff describes, undisturbed colonies of the Asian Dorylus laevigatus may well stay in one place almost indefinitely); 2) none of these additional traits are unique to army ants (for an example, see the description of the image below); and 3) the one characteristic that is unambiguously unique to army ants is as clear today as it was for early explorers: masses of foraging ants advance over long distances as a group in which food can be collected every step of the way.
An unfortunate consequence of complexly overdefining "army ants" in terms of a suite of often-connected traits (the army ant syndrome) has been that mass foraging itself has been neglected, to the point that it is often unclear from reading the literature what is truly distinct about army ant foraging. (Indeed, the origin and evolution of mass foraging are almost untouched as topics: how have army ants, the marauder ants, and a few ponerine ants come to search blindly as a group, without the aid of scouts?) As a result of this muddle, certain species in which multiple ants sometimes happen to catch or carry prey together but which forage in a manner antithetical to army ants are still mistakenly called army ants. (In such species prey search is conducted by long-distance scouts acting alone rather than by advancing throngs.)
For this reason, it is my hope that some of what I have written about group (mass) foraging in the opening chapters of AAA will serve to focus future researchers on army ant style raids, as far as I am concerned the pinnacle of foraging behavior in the animal kingdom. I also hope that the book's arguments lead us to simplify the definition of "army ant" to focus it as it was originally on the truly diagnostic trait of mass foraging. This would disengage the term from the useful idea of an "army ant syndrome," which I propose is best relabeled the "group predatory ant syndrome" to include (as it now already often does) those nomadic ants that employ large-scale recruitment to capture or retrieve live prey, among them species that do not show true group foraging, such as Leptanilla, Onychomyrmex, Pachycondyla analis, and certain Leptogenys. The only writer who I have found to have made a clear statement of this critical distinction to date is Sean Brady (in his 2002 PNAS article).
Three questions that often come up:
Does mass foraging necessarily involve capturing live prey? Not at all. Certain army ants find and eat some vegetable matter during raids, and marauder ants consume as much vegetation as animal flesh. It is entirely conceivable, though perhaps unlikely, that a completely vegetarian species could employ this hunting strategy, for example to drive off competitors from its food finds.
Does mass foraging necessarily involve a large mass of ants? Not at all. Though we expect this strategy to have the most benefits when the number of participants is large, and certainly the largest swarms get most of the attention, the raids of some species can be in the hundreds. We can even imagine two workers hunting as a team. As long as they stick together and one isn't simply leading (recruiting) the other to a food item, the pair would be "mass" foraging.
Could any ants forage both in groups and solitarily? While such a mixed strategy should be possible, no examples are known to date; as AAA describes, the marauder ant forage entirely in raids (mass foraging), as do driver ants and the other "true" army ants discovered to date.
Definitions of a few terms.
Foraging. The search for food (all the behaviors that lead up to first locating a food item), as distinguished from harvesting (see). An individual searching for food is a forager.
Group foraging. See mass foraging.
Group predation. I follow what I interpret to be the usage of most researchers, and define the term broadly to describe any situation where multiple individuals work together to forage, harvest, or retrieve live prey (the term comes up most often for prey that are that are difficult to catch because of their size or defenses).
Group transport. Carrying of a food item by multiple individuals.
Harvesting. In ants, describes all the steps that ensue out side the next after a successful bout of foraging (that is, after food is found), including such behaviors as recruitment, killing prey, dissecting the food, and transporting it to the nest (whether by single individuals or by group transport).
Mass foraging. Multiple individuals search for food together. (Whether they harvest or retrieve that food together after they find it is not relevant: see "group predation"). In army ants and the marauder ant, there is a continuous stream of participating workers rather than any specific group of individuals (which among ants is often an indicator of a group of workers recruited to harvest food rather than of a food search--or foraging--strategy). This is why I prefer the term "mass foraging" over the more widespread but often misleading phrase "group foraging."
Mass recruitment. A food harvesting strategy where a stream of many individuals is recruited from a trail or the nest to a food item (or to a food-rich area).
Solitary foraging. Single individuals depart (for example, from the nest or from a trail such as a trunk trail) to search for food alone.
