Eciton

Eciton comprises the most conspicuous army ants in the New World. The huge colony size combined with epigaeic nesting and foraging habits makes these ants major invertebrate predators and key species of the tropical ecosystems.

Identification
Borowiec (2016) - Worker Eciton is recognized by a combination of 12-segmented antennae, propodeal spiracle high on the propodeum, propodeal declivity armed with cuticular tubercles or lamellae, binodal waist, pretarsal claws armed with a tooth and presence of a prominent metatibial gland visible as an elongate patch of whitish or yellowish cuticle on the flexor (inner) surface of tibia. Among New World army ants, Eciton is similar to its closest relative Nomamyrmex, with which it shares propodeal armament, but workers of all sizes are easily separated by a conspicuous white stripe on inner hind tibiae that is absent in Nomamyrmex. Labidus species can be distinguished from Eciton by their smooth, unarmed propodeum.

Male The males of Eciton possess wing venation characteristic of all the New World army ants (also see under Cheliomyrmex male diagnosis). A combination of absence of very long setae approaching femur length on abdomen, apices of penisvalvae without setae, gradually tapering volsellae, and deeply concave dorsal surface of the petiole will distinguish Eciton males from all other army ant genera in the New World. The dense tufts of long setae on abdomen are characteristic of Nomamyrmex, although Eciton setigaster also has long setae abdominal setae; those are not quite as long and abundant as in Nomamyrmex, however, not approaching fore femur length. The penisvalvae without setae are also found in Neivamyrmex but in that genus the volsellae taper to a sharp point and often turn downwards towards the apex or are forked, not simply gradually narrowing to a blunt apex as in Eciton. In addition, Eciton males have a very conspicuously excavated dorsal surface of the petiole, which is usually more flattened in Neivamyrmex.

Distribution
Northern Mexico to northern Argentina (Borowiec 2016).

Biology
Borowiec (2016) - Eciton is the best studied lineage of the dorylines, owing to the lifetime efforts by pioneers of army ant biology, including Thomas Schneirla, Thomas Borgmeier and Carl Rettenmeyer.

Among the twelve described species, Eciton burchellii has attracted the most attention (see the biology section of the burchellii species page), followed by Eciton hamatum, although most species have been at least briefly observed in the field. Most accounts of Eciton biology are based on the two well-known species.

The literature on Eciton is vast, and it is impossible to cite all of the even more significant original contributions. Good overviews of Eciton biology can be found in Rettenmeyer (1963), Schneirla (1971), Telles Da Silva (1977a, 1977b), Rettenmeyer et al. (1983) and Gotwald (1982, 1995). The account below is based on these sources, unless noted otherwise.

The life of an Eciton colony can be summarized as follows. The colony alternates between the so-called statary and nomadic phases. The cycles are understood to be regulated by brood development rather than an endogenous rhythm in adult ants. During the statary phase a single queen is laying eggs and the brood inside the nest consists of pupae and eggs; foraging does not happen every day and raids are relatively much less intensive. There are no emigrations to new nesting sites. In the nomadic phase, the queen stops producing new eggs and her abdomen contracts; the colony contains many developing larvae that need nutrition. Raids and emigrations usually occur every day. In Eciton burchellii, the statary phase lasts on average 20 days and the nomadic phase is 14 days long.

A mature colony containing a single mated queen will eventually produce up to six virgin queens and hundreds to thousands of males, depending on the species. Usually the queen that emerges first leaves the colony with workers clustered around her. She has the best chance to survive and lead the fissioning part of the nest. About half of the workers eventually leave with the virgin queen. Because the colony is divided into approximately equal halves, the workers represent a substantial part of the reproductive investment. This explains the highly male-biased sex ratio, also typical of other social insects with colony fission (Pamilo 1991). The older, mated queen emigrates together with brood while the virgin queen disperses with the remaining workers. Shortly after the fission, the colony will accept multiple males that enter the bivouac. The males must first be accepted by the workers and they lose their wings before mating. Each male can mate only once, but E. burchellii queens are known to mate with a dozen males on average, this mating frequency being among the highest in eusocial Hymenoptera (Kronauer et al. 2006).

Although mature colonies have been observed to occasionally admit new males, there is strong evidence that all of the mating occurs when the queen is young (Kronauer and Boomsma 2007a). A fertilized queen can produce up to 225,000 eggs per 35-day cycle and 14 million eggs during her lifetime (Schneirla 1971, Kronauer and Boomsma 2007a).

Colony structure and nesting behavior has been studied in some detail in several species. Temporary nests are made up of bodies of workers, hanging together by their legs from a supporting structure. These bivouacs can be found in a variety of microhabitats, but common nesting sites include hollow logs, spaces between buttresses of large trees, and empty soil cavities such as abandoned mammal burrows. Eciton species vary in their preferences for bivouac sites, with E. burchellii and E. hamatum nesting in exposed sites, the former often hanging above ground without touching the surface. Eciton dulcium and Eciton mexicanum are known to nest only in underground cavities, and Eciton vagans is intermediate, sometimes found in relatively exposed sites, but often nesting under logs and in rock crevices.

Colony size estimates vary widely and reliable data exists only for E. burchellii and E. hamatum. Rettenmeyer estimated that mature colonies of E. burchellii contain from 300,000 to 700,000 worker ants before fission and 100,000 to 500,000 for E. hamatum. Colony densities have been estimated in several localities for Eciton burchellii, ranging from 3.5 colonies per 100 ha on Barro Colorado Island, Panama, to 11 colonies in Corcovado, Costa Rica (Franks 1982, Vidal-Riggs and Chaves-Campos 2008).

Foraging behavior in Eciton has been studied extensively. Workers forage either mostly above ground (E. burchellii, E. hamatum, E. rapax) or with some part of the raid unfolding underground. The latter mode has been reported for most other species, but the paucity of data precludes comparisons. The surface foragers also ascend vegetation and are capable of foraging arboreally. Ant brood constitutes a major portion of Eciton prey, although other arthropods, especially other social insects, are often targeted. Eciton burchellii is the most generalist predator, still hunting ants, but also actively preying on a variety of other arthropods and even opportunistically killing small vertebrates.

At the beginning of a raid, foragers emerge from the nest and gradually assemble into narrow trails that often branch and extend for up to 100 m (200 m in Eciton rapax) from the bivouac. These columns are typical of most species except for Eciton burchellii where the front of each raid progresses as a ‘swarm’, a continuous front up to 10 m wide. Group foraging is a self-organizing process with no scouts to guide the ants to a particular source of food, but the workers do follow trail pheromones produced by sternal glands (Billen and Gobin 1996). The progress of an advancing ant column can be rapid, and was estimated at up to 20 m per hour in E. hamatum. A remarkable adaptation for improving the efficiency of foraging is found in Eciton burchellii. Workers of this species have the ability to form living plugs over gaps in the substrate, significantly smoothening the surface and allowing faster movement of fellow foragers (Powell and Franks 2007). Eciton foragers are also extremely efficient at cooperative transport of prey. As a group they are able to carry more than a combined mass of what they could transport individually. Although cooperative transport has been documented for many ant species, this type of ‘superefficient’ transport is rare (Czaczkes and Ratnieks 2013, McCreery and Breed 2014). Eciton raids also establish caches for temporary storage of prey along the trail. In the species foraging in columns there can be more than one trail radiating from a bivouac at any given time, whereas an E. burchellii colony conducts one swarm raid at a time. The direction of raids of E. burchellii during the statary phase has been also shown to systematically change each day, apparently minimizing the overlap of foraging area (Willson et al. 2011).

During the nomadic phase, Eciton conducts raids every day and at some point these raids transition into an exodus of workers and finally an emigration of the entire colony. The emigration does not always follow the same route as the day’s raid and can be sustained by agitated returning foragers carrying booty past the bivouac. Other workers follow these foragers and eventually start to carry brood away from the bivouac. When the transport of brood is well advanced, myrmecophiles appear in the emigration column and the queen passes, surrounded by an entourage of workers. The duration of emigration is dependent on the colony size and species, and distances covered vary greatly as well; Schneirla (1971) reported emigration trail lengths from 100 to 450 m in E. hamatum.

Eciton colonies have an extraordinarily rich associate fauna and over 300 species, from mites to birds, have been recorded to depend on E. burchellii (Kistner 1982, Rettenmeyer et al. 2011). Remarkably, as many as 29 species are birds that rely almost exclusively on insect prey flushed out of the leaf litter by Eciton raids. This behavior evolved multiple times, and obligate ‘antbirds’ are found in the families Thamnophilidae, Formicariidae, and Furnariidae (Willis and Oniki 1978, Rettenmeyer et al. 2011). The bird droppings in turn attract many butterflies, especially skippers (family Hesperiidae; DeVries et al. 2009). A multitude of fly, wasp, beetle, and other arthropod species are found preying on the insects fleeing from a raid or scavenging in the refuse piles of Eciton bivouacs. It seems that relatively very few of these are predators or parasites of the ants themselves, although rove beetles in the genus Tetradonia are known to kill and feed on injured workers. Within the colony, some mites are known to suck on the ant hemolymph. Macrocheles rettenmeyeri is a parasitic mite found with E. dulcium. It is remarkable because it functionally replaces the ant’s distal tarsal segment. The mite attaches itself to the membrane of hind leg pulvilli and its curved hind legs serve as the ant’s claws without affecting the host’s behavior. As documented for the staphylinid genus Vatesus, some myrmecophiles synchronize their life cycle with the nomadic and statary phases of their host Eciton colonies (von Beeren et al. 2016).

Eciton species are important predators of ants and other social insects and elicit a wide range of responses from its prey. Chadab-Crepet and Rettenmeyer (1982) studied behavior of social wasps affected by army ant raids and found that many species exhibit coordinated alarm response allowing the adult wasps to survive and reestablish the nest later. Dejean et al. (2013) review the antipredatory behaviors of ants to army ants in general and to E. burchellii and E. hamatum in particular. They show that many species evacuate the nest in the face of an Eciton raid. This behavior ranges from well-organized evacuations starting in advance of the attack and resulting in no casualties on either side to cases where a substantial portion of brood is lost by the defending species. Paratrechina longicornis is an example of the former, while the less efficient Pachycondyla harpax represents the latter. Some species of ants are ignored by Eciton, particularly the enormous colonies of leaf-cutting Atta, and some can have a repellent effect, like the antplant-associated Pseudomyrmex ferrugineus and Azteca alfari. A few species, such as the arboreal Azteca chartifex and Dolichoderus bispinosus, manage to resist Eciton raids by attacking the raiding army ants (Dejean et al. 2013).

Nomenclature

 *  ECITON [Ecitoninae: Ecitonini]
 * Eciton Latreille, 1804: 179. Type-species: Formica hamata, by subsequent designation of Shuckard, in Swainson & Shuckard, 1840: 173.
 * Eciton senior synonym of Camptognatha, and material of Ancylognathus (nomen nudum) referred here: Smith, F. 1855c: 160.
 * Eciton senior synonym of Mayromyrmex: Emery, 1906a: 718.
 * Eciton senior synonym of Holopone: Borgmeier, 1936: 55.
 *  ANCYLOGNATHUS [Nomen nudum]
 * Ancylognathus Lund, 1831a: 121, 135. Type-species: Ancylognathus lugubris. Nomen nudum.
 * Ancylognathus material referred to Eciton: Smith, F. 1855c: 160.
 * CAMPTOGNATHA [junior synonym of Eciton]
 * Camptognatha Gray, G.R. 1832: 516. Type-species: Camptognatha testacea (junior synonym of Formica hamata), by monotypy.
 * Camptognatha junior synonym of Eciton: Smith, F. 1855c: 160.
 * HOLOPONE [junior synonym of Eciton]
 * Holopone Santschi, 1925b: 11 [as subgenus of Eciton]. Type-species: Eciton rapax, by original designation.
 * Holopone junior synonym of Eciton: Borgmeier, 1936: 55.
 * MAYROMYRMEX [junior synonym of Eciton]
 * Mayromyrmex Ashmead, 1905b: 381. Type-species: Labidus fargeavii (junior synonym of Atta quadriglumis), by original designation.
 * Mayromyrmex junior synonym of Eciton: Emery, 1906a: 718.

Borowiec (2016) - Eciton is the sister lineage to Nomamyrmex (Brady et al. 2014, Borowiec, in prep.). An effort to infer the internal phylogeny is currently under way (Daniel Kronauer, Max Winston pers. comm.).

Worker
Borowiec (2016) - Head: Antennae with 12 segments. Apical antennal segment not enlarged, not broader and longer than two preceding segments combined. Clypeus with cuticular apron. Lateroclypeal teeth absent. Parafrontal ridges reduced. Torulo-posttorular complex vertical. Antennal scrobes absent. Labrum with median notch or concavity. Proximal face of stipes projecting beyond inner margin of sclerite, concealing prementum when mouthparts fully closed. Maxillary palps 2-segmented. Labial palps 3-segmented. Mandibles polymorphic, from triangular with teeth through falcate with teeth on masticatory margin, to falcate without teeth on elongated masticatory margin. Eyes present, appearing as single large and convex ommatidium, in reality composed from fused ommatidia. Ocelli absent. Head capsule with differentiated vertical posterior surface above occipital foramen. Ventrolateral margins of head without lamella or ridge extending towards mandibles and beyond carina surrounding occipital foramen. Posterior head corners dorsolaterally immarginate. Carina surrounding occipital foramen ventrally absent. Mesosoma: Pronotal flange not separated from collar by distinct ridge. Promesonotal connection with suture completely fused. Pronotomesopleural suture completely fused. Mesometapleural groove not impressed. Transverse groove dividing mesopleuron absent. Pleural endophragmal pit concavity present. Mesosoma dorsolaterally immarginate. Metanotal depression or groove on mesosoma present. Propodeal spiracle situated high on sclerite. Propodeal declivity with distinct dorsal edge or margin and in form of narrow strip. Metapleural gland with bulla visible through cuticle. Propodeal lobes present, short. Metasoma: Petiole anterodorsally immarginate or marginate, dorsolaterally immarginate, and laterally above spiracle immarginate. Helcium in relation to tergosternal suture placed at posttergite and axial. Prora narrowed into anteriorly directed spine. Spiracle openings of abdominal segments IV–VI slit-shaped or oval in small workers. Abdominal segment III anterodorsally immarginate and dorsolaterally immarginate. Abdominal segment III about half size of succeeding segment IV, which is strongly constricted at presegmental portion (binodal waist). Girdling constriction of segment IV present, i.e. pre- and postsclerites distinct. Cinctus of abdominal segment IV a gradual concavity, not gutter-like. Abdominal segment IV conspicuously largest segment. Abdominal tergite IV not folding over sternite, and anterior portions of sternite and tergite equally well visible in lateral view. Girdling constriction between pre- and posttergites of abdominal segments V and VI absent. Girdling constriction between pre- and poststernites of abdominal segments V and VI absent. Pygidium small, reduced to narrow strip, without impressed medial field and simple, not armed with cuticular spines or modified setae. Hypopygium unarmed. Legs: Mid tibia with single pectinate spur. Hind tibia with single pectinate spur. Hind basitarsus not widening distally, circular in cross-section. Posterior flange of hind coxa not produced as raised lamella. Metatibial gland present as patch of whitish cuticle occupying at least half of tibia length. Metabasitarsal gland absent. Hind pretarsal claws each armed with a tooth. Polymorphism: Highly polymorphic.

Queen
Borowiec (2016) - Dichthadiiform, with eyes but no ocelli (see e.g. Wheeler 1921, 1925b, Borgmeier 1958). See Hölldobler (2016) for a description of queen exocrine glands in Eciton.

Male
Borowiec (2016) - Head: Antennae with 13 segments. Clypeus without cuticular apron. Parafrontal ridges absent. Torulo-posttorular complex vertical. Maxillary palps 2-segmented. Labial palps 2-segmented. Mandibles falcate. Ventrolateral margins of head without lamella or ridge extending towards mandibles and beyond carina surrounding occipital foramen. Carina surrounding occipital foramen ventrally absent. Mesosoma: Pronotal flange not separated from collar by distinct ridge. Notauli absent. Transverse groove dividing mesopleuron absent. Propodeal declivity reduced, without distinct dorsal edge or margin. Metapleural gland opening absent. Propodeal lobes present. Metasoma: Petiole anterodorsally immarginate, dorsolaterally immarginate, and laterally above spiracle immarginate. Helcium in relation to tergosternal suture placed at suture and axial. Prora forming a simple, wide U-shaped margin not delimited by ridge. Spiracle openings of abdominal segments IV–VI slit-shaped. Abdominal segment III more than half size of succeeding segment IV; latter weakly constricted at presegmental portion (uninodal waist). Girdling constriction of segment IV absent, i.e. pre- and postsclerites indistinct. Cinctus of abdominal segment IV absent, not impressed. Girdling constriction between pre- and postsclerites of abdominal segments V and VI absent. Abdominal segment IV not conspicuously largest segment. Abdominal sternite VII simple. Abdominal sternite IX distally armed with two spines, with lateral apodemes longer than much reduced medial apodeme, directed anteriorly (towards head). Genitalia: Cupula very long, nearing or surpassing length of rest of genital capsule and of approximately equal length on both dorsal and ventral surfaces. Basimere narrowly fused to telomere, with suture modified into membrane at junction, and ventrally with left and right arms abutting. Telomere expanded at apex. Volsella laterally flattened, narrow and tapered towards tip. Penisvalva hook-like, strongly curved ventrally. Legs: Mid tibia with single pectinate spur. Hind tibia with single pectinate spur. Posterior flange of hind coxa not produced as raised lamella. Metatibial gland absent. Metabasitarsal glands absent. Hind pretarsal claws each armed with a tooth. Wings: Tegula present, broad, demiovate in shape. Vein C in fore wing present. Pterostigma narrow. Abscissa R·f3 present, running toward distal wing margin and enclosing cell with Rs·f5. Abscissae Rs·f2–3 present, connecting with Rs+M&M·f2. Cross-vein 2r-rs present, differentiated from Rs·f4 by presence of Rs·f2–3. Abscissae Rs·f4–5 differentiated into Rs·f4 and Rs·f5 by 2rs-m. Abscissa M·f2 in fore wing present, separated from Rs+M by Rs·f2. Abscissa M·f4 in fore wing present, reaching wing margin. Cross-vein 1m-cu in fore wing present. Cross-vein cu-a in fore wing present, arising from Cu and distal to, at or near M·f1. Vein Cu in fore wing present, with both branches Cu1 and Cu2. Vein A in fore wing with abscissae A·f1 and A·f2 present. Vein C in hind wing absent. Vein R in hind wing present, reaching distal wing margin. Vein Sc+R in hind wing present. Abscissa Rs·f1 in hind wing present, shorter than 1rs-m. Abscissa Rs·f2 in hind wing present, reaching wing margin. Cross-vein 1rs-m in hind wing fused with M·f1. Vein M+Cu in hind wing present. Abscissa M·f1 in hind wing present. Abscissa M·f2 in hind wing present. Cross-vein cu-a in hind wing present. Vein Cu in hind wing present. Vein A in hind wing with abscissae A·f1 and A·f2 present.

Larvae
Borowiec (2016) - Larvae of several Eciton species have been described by Wheeler (1943) and Wheeler and Wheeler (1964b, 1984). Cocoons are present.