What Are "Army Ants", Anyhow?

Film still from "The Creeping Terror"; the flocculent matter might be the monster, or just some hay

Most apex predators are faster than we humans, and stronger to boot—so there is something inexplicably unsettling about a multi-kilogram carnivorous mass that could devour a human, yet can be escaped by strolling in the other direction. Our fear of slothful super-predators may inspire horror, but as a rule, such creatures are rightly unable to terrify us: and any attempt to make something that is obviously benign frightening devolves into, well, silliness. I am principally thinking of the "Z-movie" "The Creeping Terror" (1964), a film that failed in every respect cinema can, but not least in its premise: the eponymous gigantic, amorphous extraterrestrial's lethargy forces its human "victims" to actively climb into its mouth (whilst screaming in what the pitiable actors hope is a convincing manner): see the downright shabby film still above. (The Creeping Terror must be unaware of the shortage of great legs on our planet, and that it would therefore be a pity to digest the ones shown.)

Col. Duvchenko devoured by what appears to be a seething mass of Boston baked beans

It so happens that we are faced with huge, amorphous carnivores in real life: namely, colonies of army ants. Since these million-mouthed, sometimes 20-kilogram super-organisms issue forth to the hunt at roughly the same speed as molasses, their prowess is usually exaggerated in film. A case in point would be the siafu that eat a Soviet Special Forces colonel alive in "Indiana Jones & the Kingdom of the Crystal Skull" (2008). Disregarding whether this movie really "nuked the fridge" of its franchise, I will merely address the inaccuracies bearing on its treatment of the siafu, as Indy himself calls them; when asked to elaborate, he adds: "[They're] !@#$%^&* big ants. Run!" The usage of the word "siafu" constitutes the film's first error: the word (Swahili in origin) refers only to the driver ants of the Old World (Dorylus sp., particularly the subgenus Anomma; Savage, 1847)—but the so-called siafu of "Indiana Jones & the Kingdom of the Crystal Skull" are conspicuously inhabitants of South America (New World and Old World army/driver ants are distinct, as we shall soon see).

A soldier of Dorylus (=Anomma) nigricans (note scale bar) photographed by April Nobile

Furthermore, the "Dorylus" jonesi sp. nov. look nothing like genuine Anomma: they're at least 2 inches long (driver ant workers aren't particularly outsized, as far as ants go) and—in my private opinion—resemble the members of the subfamily Formicinae: Dorylus, conversely, is the sole constituent of the Dorylinae (Bolton, 1990a; Dlussky & Wedmann, 2012). Finally (paying no heed to the fact that Dorylus and suchlike are incapable of bringing down a healthy adult human male—although unattended infants might be on the menu; Gotwald, 1984-85), the ants are provoked to attack by a Jeep being inadvertently dropped on their mound of residence. Not that their reaction wasn't justified; but real army ants do not reside in one place long enough to construct such massive formicaria.

The approximate range of army ants in North America, according to Alex Wild

Clearly, the screenwriters of "Indiana Jones & the Kingdom of the Crystal Skull" (like most who inhabit the First World) knew little of these ants, despite at least 182 years of we Westerners' interest in them (Lund, 1831); most people hereabouts are content to cite the lethargic super-predators as a reason they're glad they don't live in the tropics (under a false impression that these insects do not inhabit temperate regions—far from it). And so, after spending so much time discussing what army ants are not, I must spend even more time explaining what they truly are.

Snapshot by Alex Wild of major and minor workers of Eciton rapax (Ecitoninae)

Before I enumerate the ethological formula that defines "legionary behavior" or "the army ant syndrome", as myrmecologists refer to it (Gotwald, 1995), I must address the essential issue of army ant taxonomy and zoogeography, not to mention their sometimes thorny terminology. The term "army ant" strictly refers to all species belonging to three subfamilies of the Formicidae: Dorylinae (Afrotropical and Oriental ecozones*), Aenictinae (the same as Dorylinae but extending into northern Meganesia), and Ecitoninae (Brady, 2003). "Legionary ant" is usually applied only to ecitonines, which are restricted to the Americas (south to Argentina and as far north as Iowa and Virginia), whereas "driver ant" exclusively denotes a doryline. ("Legionary behavior" has been used synonymously with "army ant syndrome" on some occasions, which has probably stretched the former phrase to the breaking point.) Despite their many differences, these taxa are so often considered collectively that their trinity will be termed henceforth as AenEcDo army ants (Kronauer, 2009)—this being more convenient than the synonymous "classic army ant" (Berghoff et al., 2003) and more accurate than "true army ant" (Wilson, 1964).

Workers of Dorylus (=Rhogmus) fimbriatus attacking a grub (Scarabaeidae), photographed by Daniel Kronauer

Army ant biodiversity is considerable, with any given locality in the Neotropics* inhabited by at least 10 ecitonine species alone; every square meter of rainforest floor in the same region experiences the depredations of one army ant species or another once a day on average (O'Donnell et al., 2007). This sympatric diversity is owed to ecological partitioning (duh). Eciton, undoubtedly the best-studied of all army ant genera, is very much a huntress of the ground's surface (hence epigaeic); some Dorylus (all of them in Anomma) are in the open air at least partially (Kronauer et al., 2007a), as are those aenictines whose biology is known (Schneirla, 1965): but these are atypical. Most army ants are diminutive and hypogaeic†, making their natural history difficult to study; some may rise from their earthen habitat after heavy rains (as have the Kenyan D. fimbriatus shown above): but not frequently. Neivamyrmex (Ecitoninae) can be found throughout the southern United States in colonies hundreds of thousands strong; but their subterranean ways and nocturnal raids mean that most humans living within their range are unaware that there may very well be army ants beneath their feet.

A collected drone of Aenictogiton sp. (Aenictogitoninae), photograph by April Nobile

Retiring, hypogaeic habits limit our understanding of three further non-AenEcDo subfamilies that qualify as ants of a martial persuasion: the Leptanillinae, Leptanilloidinae, and Aenictogitoninae. No workers or queens of Aenictogiton (the sole member of its subfamily; Baroni Urbani et al., 1992) have ever been collected; that the aenictogitonines (found only in central Africa) are army ants can be inferred only from their morphology and phylogeny (Brady et al., 2006), and so remains as yet unconfirmed. Our study of the Leptanilloidinae is little better off, with Asphinctanilloides anae and Leptanilloides nomada the only species of this Neotropical* subfamily to have been observed alive (engaging in seemingly legionary behavior; Brandão et al., 1999; Donoso et al., 2006); of the Leptanillinae, only colonies of Leptanilla japonica (Masuko, 1987) have been located: and leptanillines reside throughout the subtropics of the Old World from Spain (Barandica et al., 1994) all the way to southern Japan (Baroni Urbani, 1977) and Australia (Wheeler, 1932). Lastly, two genera (and one species) of army ant are placed in taxa that are otherwise non-legionary: the inhabitant of Latin American cloud forests, Simopelta (Ponerinae) (Longino, 2009); Onychomyrmex (Amblyoponinae), found only in a corner of northeastern Queensland (Miyata et al., 2009); and the Malayan Leptogenys distinguenda (Ponerinae) (Maschwitz et al., 1989).

And now for E. burchellii's impression of Lower Egypt (Rettenmeyer, 1963)

The first criterion (first due to narrative convenience, not preeminence) that forms a diagnosis of the army ant syndrome is group predation. Ant collaboration in foraging (either for vegetable or animal victuals) is nothing uniquely legionary: but AenEcDo ants take it to an extreme. There are no loner scouts: the task of seeking prey is performed en masse in a great column—punctiliously dendritic in most army ants, with a booty cache at each trail crux; in Eciton burchellii, famously terminating in a swarming disorganized front of carnage (see schematic at left); whereas driver ants sally forth in a globular pseudopod-like mass (Raignier and van Boven, 1955). Workers cleave to the throng with an alacrity that may have untoward results, such as the infamous "spiral of death" first reported as occurring in Labidus praedator (Ecitoninae): accidental disconnection from the odor trail causes a loss of direction, with every ant following her sister's guidance in what is literally an example of the blind leading the blind; this behavior results in the huntresses' centrifugal milling, which continues until they die of exhaustion (Schneirla, 1971). Seldom going awry so spectacularly, swarm raids are an adaptation for swiftly reaping all the bounty of the rainforest that a colony can, up to and including small vertebrates in the cases of Anomma and Cheliomyrmex andicola (Ecitoninae) (O'Donnell et al., 2005)—this is true at least among those army ants with notoriously heterodox diets (Eciton, most Dorylus, L. coecus, and Leptogenys distinguenda): the latter ecitonine has even been seen to viciously attack grains of rice (Longino, 2007), and the remarkable swarm-raiding marauder ant (Pheidologeton sp.: Myrmicinae) eats plenty of fruit—but most of the AenEcDo and company are strictly specialist predators (Rettenmeyer, 1963).

Two Atta sp. workers preemptively attack an N. esenbeckii in Panama; photographed by Alex Wild

Significantly for our study of the army ant syndrome's origins, in their cases group hunting is necessitated by complete dependence upon formidably defended prey. Leptanilla japonica is an exemplar of this: these half-centimeter-long ants feed solely upon centipedes that are nearly as massive as the entire cumulative colony (Masuko, 1990); Onychomyrmex are also obligate centipede-slayers (Hölldobler & Wilson, 1990); but most specialist army ants have developed their massive raids in order to manage assaults on other eusocial insects. Dorylus (=Typhlopone) may even feed upon their congeners (Leroux, 1979), but usually invade the great fortresses of the fungus-farming termites (Termitidae: Macrotermitinae), which Typhlopone's epigaeic cousins generally avoid (Schöning & Moffett, 2007). Both Aenictus and Neivamyrmex live chiefly on other ants' brood, as do Simopelta (Mackay & Mackay, 2008); Nomamyrmex esenbeckii (Ecitoninae) dares to attack leaf-cutter ants (Myrmicinae: Attini: Atta) (Swartz, 1998). In one observed raid, the N. esenbeckii were estimated to carry off 60,568 A. cephalotes larvae and pupae (Powell & Clark, 2004). Interestingly, group predation has also been observed in some termite-hunting Pachycondyla (Ponerinae) and ant-hunting Cerapachys (Cerapachyinae): but their raids are still initiated by solitary scouts (Longhurst & Howse, 1979; Hölldobler, 1982; Leal & Olivieira, 1995).

A queen of Simopelta pergandei on the move with attendant workers; photographed (again) by Alex Wild

The next of the interrelated criteria that define the army ant syndrome is nomadism, which is a rather inaccurate term: "nomad"—already suspect, as it has anthropological origins—implies emigration for husbandry. Among ants, certain Southeast Asian mealybug-herding Dolichoderus spp. (Dolichoderinae) are the only genuine nomads (Dill et al., 2002). Most ant colonies will evacuate their nests if necessary: some even do so with little provocation. All army ants, however, migrate cyclically, with alternating sedentary and migratory phases. Many of the AenEcDo cohort (the Aenictinae and ecitonines aside from Labidus) as well as the Leptanilloidinae, Leptanilla japonica and Simopelta are phasic (Schneirla, 1971; Donoso et al., 2006; Masuko, 1987; Gotwald & Brown, 1966), with emigrations occurring on a regular basis; in E. burchellii, the colony is on the move with frequent raids (in the nomadic phase) for 14.3 days on average, and in the statary phase (with infrequent foraging) for 20.4 days (Teles da Silva, 1977); in Aenictus laeviceps, the statary phase is 8 days longer, with the nomadic one of about the same duration (Gotwald, 1978).

Cerapachys biroi workers on the march; note the horizontal position of the pupa underneath its bearer

Each night during the migration, the E. burchellii mass together in a bivouac (often between the buttressed roots of some tree), at the center of which lies the brood and queen; a more sheltered bivouac (from which raids emanate in a predetermined pattern in order to avoid covering the same ground; Franks & Fletcher, 1983)—usually within a hollow log—is formed during the statary phase (Schneirla, 1933). Not all phasic army ants operate on such a clockwork-like basis as Eciton, but all are driven by an internal stimulus (Schneirla, 1971) despite exterior factors (an experimentally overfed colony of Neivamyrmex dispensed with nomadism; Topoff et al., 1981): to be specific, the development of the brood, which is rhythmically synchronized such that at any given time all larvae or pupae are in the same stage of development. Synchronous brood development is also observed in some Cerapachys and Sphinctomyrmex spp. (Cerapachyinae) (Wilson, 1958; Buschinger et al., 1989). The nomadic phase is characterized by intensive foraging to accommodate the growing larvae; come pupation, the colony settles down and the queen commences egg-laying. In Eciton, the appearance of callow workers theoretically stimulates the colony's emigration (Schneirla, 1971; Topoff & Mirenda, 1980); but in the proto-legionary Cerapachys biroi, teneral‡ workers' effect is transient compared to that of the larvae, and this may be true of most phasic army ants (Ravary et al., 2006). C. biroi is aberrant at any rate, being parthenogenetic§ and thelytokous (queenless: all workers reproduce) (Ravary & Jaisson, 2004).

"Mine tailings" of a statary Kenyan D. wilverthi colony located by Daniel Kronauer

By contrast, some army ants (Labidus, Dorylinae, and Leptogenys distinguenda) are non-phasic and without synchronous broods. The duration of their phases is variable: particularly so in the driver ants; the statary phase of Dorylus (=Anomma) molestus may last anywhere from 3 to 111 days. Correspondingly, Dorylus settle much more permanently than do Eciton, excavating prodigious tunnel systems 3-4 meters deep for their (all 10 million or more of them; Leroux, 1982) accommodation in a matter of days. Non-phasic AenEcDo ants' nomadism hence has a much more straightforward cause than their phasic cousins': the reduction of food attendant on the presence of any army ant colony. On the basis of this simplicity, it was once assumed that the non-phasic lifestyle was the less "derived" of the two schools of nomadism (fallaciously supposing that simple=primitive) (Gotwald, 1988); but it is now hypothesized that non-phasic migration is actually a secondary adaptation resulting from more efficient foraging strategies (Kronauer, 2009). Nomadism (of the non-phasic type) has evolved independently in the Malaysian ant Euprenolepis procera (Formicinae), which feeds exclusively on sporocarps (i.e., mushrooms). The commonality between army ants and E. procera, evidently, would be specialization upon an already-ephemeral food source that is swiftly consumed: the said fungivorous ants can completely harvest a 40-gram Pleurotus mushroom in a little over three hours (Witte & Maschwitz, 2008).

Queen of Dorylus (=Anomma) molestus

Naturally, you may wonder (as many myrmecologists have) what drives the brood cycle: and thus we must turn to army ant queens, which at least among the "AenEcDo" appear to drive said cycle in reciprocal oscillations with the brood (Schneirla, 1949). Their habitus is unmistakable: blind, or nearly so; bearing a robust petiole and legs; wingless throughout adulthood; and with the abdomen disproportionately large, bearing an excess of exocrine glands (Whelden, 1963). The "dichthadiiform ergatogyne||"—as she is dubbed—is perhaps the only diagnostic trait that all legionary ants share (and all non-legionary ants lack; Hölldobler & Wilson, 1990). Queens in phasic army ants lay immense quantities of eggs during the statary phase: from a comparatively piddling 30,000 per batch in Aenictus gracilis to 100,000-300,000 in E. burchellii, abdomens profoundly bloated (physogastric) in order to accommodate her vast load of developing eggs; during the migratory phase, the queens deflate and refrain from labor. Queens of Leptanilla japonica fuel their statary pregnancies with the blood of their larvae, which is imbibed (Masuko, 1989) through a surreal faucet-like process on the larval abdomen (Wheeler & Wheeler, 1965). Non-phasic dichthadiigynes are more or less permanently physogastric (see above D. molestus queen's gratuitous length), with the result that (in driver ants at least) they can be behemoths (which is to be expected with 15,000 ovarioles): at 53 mm. long, the queen of D. wilverthi is the largest living ant. Such gigantism is necessary in order to support a mature colonial population of 22 million (Raignier & van Boven, 1955) (also the greatest of any ant), with the queen laying approximately 200,000,000 eggs throughout her lifetime.

These Ugandan Leptanilla sp. are blue. No—wait—YELLOW! ...

Additional dichthadiiform traits are adaptations for a life of vigorous exercise (muscular legs and enlarged petiole), while the exocrine array is irresistible to the workers (Hölldobler et al., 1989), causing them (in Eciton) to form a retinue surrounding their gyne, which is especially useful as protection during emigrations. But the most important dichthadiiform idiosyncrasy by far would be the permanent lack of wings, which has great reproductive and ecological repercussions on legionary ants: it means that they cannot disperse to new territories on the wing, unlike most formicids. Not only does this delimit their range to continental landmasses—Madagascar, New Caledonia, and the Greater Antilles are all large low-latitude army-ant-lacking islands that have no shortage of non-legionary ants (Corsica and Sardinia are a notable exception to this, as they are inhabited by Leptanilla; Emery, 1916)—but it also makes them vulnerable to habitat fragmentation (Berghoff et al., 2008), which does not bode well for some army ants in the Anthropocene. N. esenbeckii has probably been extirpated from the Rio Grande Valley (the northernmost tier of its natural range), depriving the region of what could be an effective biological control of those epic defoliators, leaf-cutter ants (Sánchez-Peña & Müller, 2002); and Simopelta minima was designated extinct due to deforestation at its type locality: it has since been found alive and well elsewhere (Brandão et al., 2008; for more about the issue of insects that are only seemingly rare, see "Mormotomyiids' Terrible Hairiness").

E. burchellii drone and queen caught in the act by Daniel Kronauer

The dichthadiiform syndrome's consequences in the matter of reproduction are sundry. One is that all army ants studied thus far found their colonies by fission: one portion (nearly half) remains with the senior queen, the other departs with her daughter (Gotwald, 1995). This mode of colonial reproduction is rare among ants, but it may sound familiar to any apiculturalists out there: the honeybee super-organism reproduces in much the same way. Army ants' peculiarly flightless queens also pose a challenge for their corresponding drones, which are contrastingly alate (winged): if the virginal queen cannot come to the drone, then the drone must come to her. In order to do so, the said drone must first locate a colony with callow queens, and then must penetrate such a colony in order to fulfill his duty: and thus, in a unique situation among social insects, the sterile workers play a decisive role in their reproductive sister's choice of mate (Franks & Hölldobler, 1987). The need to pass this gauntlet's muster is the adaptive driver behind army ant drones' bulky wiener-like physique—"hunky" males are likely more virile—along with the need to accommodate huge quantities of seminal vesicles (Gotwald & Burdette, 1981) and a worker-appeasing glandular complement convergent with a dichthadiigyne's (Hölldobler & Engel-Siegel, 1982); this physique causes them to be sometimes mistaken for wasps (Fabricius, 1793) and receive the colloquial name "sausage flies" (only applicable to driver ant drones). Inter-male competition is another adaptive factor at play: in E. hamatum at least, it is inevitable, as there are 1,500 males to 6 virginal gynes in a reproductive brood (Schneirla & Brown, 1952). This highly skewed sex ratio results in polyandry (queens mating with multiple drones), which appears to be the ancestral condition for the AenEcDo army ants. Among the few army ants that have more than one queen (are polygynous), both within and without the AenEcDo, monandry¶ prevails (Kronauer et al., 2007b; Kronauer & Boomsma, 2007; Kronauer et al., 2011).

Holotype of Anomalomyrma taylori, photographed by April Nobile

My usage of the phrase "ancestral condition" would imply that the AenEcDo ants share a common ancestor exclusive of other formicid subfamilies (is monophyletic). This was a given for taxonomists throughout the nineteenth and early twentieth centuries, with all three subfamilies placed together in the Dorylinae (often ranked as a family—Formicidae once had a much narrower definition than it does now) (Ashmead, 1905), the Leptanillinae regarded as either that subfamily's sister-group (Wheeler, 1923) or included as dorylines themselves (Emery, 1870). Implicitly, an ancestor that was itself an army ant gave rise to the two taxa. Legionary behavior was the only convincing characteristic linking the pair: thus, the description of the non-dichthadiiform queens of the leptanilline tribe Anomalomyrmini (which suggest that anomalomyrmines are not army ants) ruled out their supposed kinship (Bolton, 1990b). Nowadays, the leptanillines are often placed as close cousins of the Amblyoponinae, which they resemble in several respects (the oddball amblyoponine Apomyrma stygia has even been classified as a leptanilline from time to time; Kugler, 1992), since queens (and workers) of some amblyoponines are known to be vampiric (Wheeler & Wheeler, 1988; Saux, Fisher & Spicer, 2004): it's even possible that leptanillines themselves are highly modified members of the Amblyoponinae (Ward, 2007). Some placed the Leptanillinae as sister-group to all remaining ants (no insignificant distinction), but the myrmecological-headline-making "ant from Mars", Martialis heureka, usurped that phylogenetic position (Rabeling et al., 2008).

Mass of Sphinctomyrmex froggatti (Cerapachyinae) spotted by Alex Wild

The AenEcDo, on the other hand, are now consistently placed in the "doryline section", along with the Cerapachyinae, Leptanilloidinae, and Aenictogitoninae (Ward, 2007); however, the dorylomorphs' exact cladistic arrangement is still in flux. Although the ant-brood-hunting cerapachyines' position has varied, it is now increasingly clear that the subfamily is paraphyletic (that is, ancestral to the remaining dorylomorphs), with the Leptanilloidinae and the AenEcDo subfamilies arising from among separate cerapachyine lineages (Brady & Ward, 2005): but the latter groups' monophyly (along with the Aenictogitoninae) remains contentious (Kronauer, 2009), if not downright dubious (Grimaldi et al., 1997). DNA sequencing has suggested that the putative clade originated in a primeval army ant 100 million years ago in Atlantica, with the subfamilies diverging as that chunk of Gondwana broke into South America and Africa (Brady, 2003): but a more recent analysis of the "molecular clock" suggested that the last common ancestor of the dorylomorphs as a whole lived 20 million years later, long after the South Atlantic opened; this would suggest that the Aenictinae, Ecitoninae, and Dorylinae each independently converged upon the army ant syndrome (Brady et al., 2006). A further enigma lies in Cerapachys sexspinus: initially classified as a doryline (as its own genus, Yunodorylus; Xu, 2000), then synonymized with Cerapachys (Bolton, 2003), placed in the Cerapachyinae as Yunodorylus (Moreau et al., 2006), or determined to be phylogenetically closer to the Aenictinae and Ecitoninae than to the Dorylinae (Brady et al., 2006).

Well, spring break beckons, and that about does it as far as Earth's native "creeping terror" is concerned. Now if this dratted computer will only let me publish...                   

 





*The Afrotropical ecozone includes southern Arabia, Madagascar and sub-Saharan Africa; the Oriental one consists of southeastern Asia from Sindh to the Moluccas and north to China and Tibet; and the Neotropical ecozone comprises South America, in addition to North America south of the Trans-Mexican Volcanic Belt. The nomenclature is rather misleading: the "Neotropics" extend as far south as the frigid wastes of Patagonia.
†Primarily subterranean in habit and therefore often blind.
‡Just eclosed.

§Practicing conception without fertilization.  
||Or simply "dichthadiigyne" (from Dichthadia, a subgenus of Dorylus; and gyne, sociobiological jargon for what laymen call a "queen").
¶For crying out loud, don't you know an antonym when you see one?

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