Labidus

With seven described species, Labidus is a relatively small but widely distributed genus. Its members are more generalized predators than most other New World army ants and may have the greatest overall ecological impact due to high densities.

Identification
Borowiec (2016) - Worker Labidus workers are easily recognized by a combination of spiracle positioned high on the propodeum, 12-segmented antennae, propodeum not armed with spines or cuticular lamellae, short propodeal lobes, two-segmented waist, metatibial gland present, and pretarsal claws with a tooth. Labidus belongs to New World army ants with an unarmed propodeum and it could only be confused with Cheliomyrmex and certain larger species of Neivamyrmex. The former have one-segmented waist, and the latter always lack teeth on pretarsal claws.

Male Labidus males have the army ant habitus with abdominal segment III much larger than the preceding segment II (petiole), and head small relative to mesosoma. See discussion under Cheliomyrmex for characters differentiating New World army ant males from those of Old World Aenictus, Aenictogiton, and Dorylus. Among New World army ants, Labidus possesses the following unique character combination: no conspicuous tufts of long setae on gaster, apices of penisvalvae with setae, abdominal sternite IX (subgenital plate) with two spines, and hind basitarsus with a groove that accommodates the tibial spur. The lack of long gastral setae differentiates Labidus from Nomamyrmex, the apices of penisvalvae are hairy in Eciton and Neivamyrmex, and in Cheliomyrmex there are four spines on the abdominal sternite IX and hind basitarsus has no oblique grooves.

Distribution
Borowiec (2016) - South Central United States to northern Argentina.

Biology
Labidus are often the most common army ants throughout their range, with up to three species occurring in a given area (do Nascimiento et al. 2004, author’s personal observations). They nest mostly underground (Fowler 1979) and forage in swarm raids. It is unclear whether brood production is synchronized; colonies appear to emigrate infrequently, their bivouacs staying in place for prolonged periods of time (Fowler 1979).

Rettenmeyer (1963) and Fowler (1979) detailed the biology of Labidus praedator. The bivouacs are found in rotten logs or are subterranean, occupying preformed cavities such as abandoned nest chambers of Atta leaf-cutting ants (Rettenmeyer 1963, Monteiro et al. 2008). Mature colonies have been estimated to contain up to a million individuals (Fowler 1979).

Labidus forages in swarm raids similar to those of Eciton burchellii (Rettenmeyer 1963) and its species are even more generalized predators that in addition to ant brood will take a variety of other arthropods, sugar, and plant parts, including flowers, seeds, fruit, and even processed food such as boiled rice (Borgmeier 1955, Monteiro et al. 2008). The two best-studies species, Labidus coecus and Labidus praedator, are similar in this respect and data on other species is lacking. Henry Walter Bates (1863) described L. coecus constructing soil tunnels over its raiding columns. Monteiro et al. (2008) studied L. praedator in agricultural lands in Brazil, finding that Lepidoptera caterpillars were the most common type of prey, followed by arils of many plant species and various non-Lepidopteran arthropods, both in adult and larval stages. Fowler (1979) observed the same species in Paraguay and reported that it frequently raided other ant colonies. The raids occur mostly during the day, although nocturnal activity is also substantial (O’Donnell et al. 2009). Perfecto (1992) observed an underground raid of L. coecus on several ant species.

The reproductive biology of Labidus is poorly known. There is conflicting evidence as to whether brood production is synchronized or not, with available brood samples consisting of immatures at one or multiple stages of development and queen specimens with either extended or contracted gasters (Rettenmeyer 1963). Given the rarity of emigrations and confirmed existence of long-term bivouac sites, lasting up to eight months (Fowler 1979), it is possible that Labidus queens retain the ability to lay eggs in pulses but do not cease brood production long enough for non-overlapping brood cohorts to emerge and for colonies to exhibit the nomadic-statary cycle characteristic of Eciton.

Nomenclature

 *  LABIDUS [Ecitoninae: Ecitonini]
 * Labidus Jurine, 1807: 282. Type-species: Labidus latreillii (junior synonym of Formica coeca), by monotypy.
 * Labidus subgenus of Mutilla: Blanchard, in Cuvier, 1846: pl. 118, fig. 2.
 * Labidus junior synonym of Eciton: Dalla Torre, 1893: 1.
 * Labidus revived from synonymy as subgenus of Eciton: Emery, 1910b: 21.
 * Labidus revived status as genus: Borgmeier, 1953: 4.
 * Labidus senior synonym of Nycteresia, Pseudodichthadia: Emery, 1910b: 21; Borgmeier, 1955: 80.
 * NYCTERESIA [junior synonym of Labidus]
 * Nycteresia Roger, 1861a: 21. Type-species: Formica coeca, by monotypy.
 * Nycteresia junior synonym of Eciton: Mayr, 1865: 76.
 * Nycteresia junior synonym of Labidus: Emery, 1910b: 21; Borgmeier, 1955: 80.
 * PSEUDODICHTHADIA [junior synonym of Labidus]
 * Pseudodichthadia André, 1885: 838. Type-species: Pseudodichthadia incerta (junior synonym of Formica coeca), by monotypy.
 * Pseudodichthadia junior synonym of Eciton: André, in Forel, 1899c: 160 (footnote).
 * Pseudodichthadia junior synonym of Labidus: Emery, 1910b: 21.

Taxonomic Notes
Borowiec (2016) - The species-level taxonomy of Labidus requires revision. There are currently seven valid species names and three of those are based only on males. In addition, morphology and preliminary molecular analyses suggest that the widely distributed Labidus praedator may be in fact a complex of reproductively isolated species (Barth et al. 2015). The phylogenetic position of Labidus is well-established as the sister group to the Eciton plus Nomamyrmex clade (Brady et al. 2014, Borowiec, in prep.).

Worker
Borowiec (2016) - Head: Antennae with 12 segments. Apical antennal segment not enlarged, not broader and longer than two preceding segments combined. Clypeus with or without 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 to falcate with teeth on elongated masticatory margin. Eyes present, composed of seemingly single large ommatidium, in reality composed from multiple 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 without distinct dorsal edge or margin and rectangular in posterior view. Metapleural gland with bulla visible through cuticle. Propodeal lobes absent or very short. Metasoma: Petiole anterodorsally marginate with carina low on anterior face, dorsolaterally immarginate, and laterally above spiracle immarginate. Helcium in relation to tergosternal suture placed at posttergite and axial. Prora forming a V-shaped protrusion. Spiracle openings of abdominal segments IV–VI oval. 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 gutter-like and sculptured but not cross-ribbed. 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 armed with 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 minute eyes and no ocelli. The queen is known for Labidus coecus and Labidus praedator. For more details and a description of the former see Weber (1941) and Borgmeier (1958) for a description of L. praedator queen.

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 U-shaped margin or a broad cuticular lip, not delimited by carina; central protuberance may be present. 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 sulcus discernable at junction, and ventrally with left and right arms abutting. Telomere expanded at apex. Volsella laterally flattened, narrow and tapered towards tip. Penisvalva not flattened at apex, expanded. 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 present. 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.

Shuckard (1840) - Char. Body elongate, cylindrical.

Head small, short, transverse, flat. Antennae varying in length, usually setaceous, curved and inserted within two facial projections (forming vertical carinae) upon the anterior margin of the nearly obsolete clypeus, the scape never more than one-fourth the length of the flagellum, the apex of which frequently extends as far back as the insertion of the superior wings. Eyes large, lateral, subglobose, and very prominent. Ocelli large and very prominent, and placed in a curve upon the vertex. Mandibles elongate, slender, arcuate, and forcipate, always leaving an open space usually semicircular between them and the clypeus. Labrum triangular, the apex rounded, and in repose shutting down upon and inclosing the internal trophi. Maxillary palpi two-jointed, shorter than the labial? Labial palpi two-jointed, slender, the basal joint the longest. Labium triangular.

Thorax ovate, gibbous: prothorax extending laterally to the insertion of the wings, which is at about half the length of the thorax: scutellum transverse: metathorax perpendicular and abruptly truncated.

Superior wings usually as long or longer than the abdomen, rarely shorter, with one marginal and three submarginal cells, which vary in form in the species, and one recurrent nervure, which is inserted about the middle of the second submarginal cell.

Legs varying in length in the species: coxae large, not deeply excavated above: trochanters small, triangular: femora and tibiae cylindrical, all the latter with a single calcar at their apex, which is usually dilated at the base: tarsi long and slender, the basal joint the most robust and the longest, the remainder decreasing in length, excepting the terminal one, which is a little longer than the penultimate: claws armed with a minute tooth just within the apex, and furnished with a small pulvillus within their fork.

Abdomen cylindrical, slightly curved, the segments frequently slightly constricted, the basal one forming a variously constructed peduncle, occasionally either flat or concave above, but most frequently transversely convex, and ... always separated from the following by a deep incisure. Penultimate and antepenultimate segments subequal, and the terminal one strictly compressed vertically at its apex, where it is profoundly emarginate. The male sexual organ usually protruding in the form of a deeply canaliculated and ernarginated plate or two acuminated compressed and curved spines.

Larva
Borowiec (2016) - Larvae of Labidus have been described in Wheeler (1943) and Wheeler and Wheeler (1964b, 1984). Cocoons present.