Megaponera

Megaponera is a monotypic genus, with an additional five subspecies, widespread in Sub-Saharan Africa. It is notable for its ergatoid queens, polymorphic workers, obligate group foraging, and specialized termite predation.

Identification
Schmidt and Shattuck (2014) - Diagnostic morphological apomorphies of Megaponera workers include the presence of preocular carinae and size polymorphism, which do not occur in combination in any other ponerine. Preocular carinae occur in Odontoponera and many Neoponera species, but Megaponera lacks Odontoponera’s striate sculpturing and denticulate clypeus, and Neoponera’s prominent white projecting arolia, simple tarsal claws and U-shaped cuticular lip posterior to the metapleural gland orifice.

Distribution
Megaponera is widespread in tropical Sub-Saharan Africa. Wheeler (1922b) shows the range as encompassing the African continent from roughly 10° N latitude to 30° S latitude.

Biology
Megaponera analis has an unusual suite of ecological, behavioral, and social adaptations relative to most ponerines. The typical ponerine species has small colonies (usually fewer than 100 individuals), alate queens, and monomorphic workers which forage singly. Megaponera has diverged significantly from this basic pattern, having relatively large colonies, flightless queens, and polymorphic workers which are specialized mass raiders of termites.

Megaponera nests in the ground, in deserted termite mounds, or even partially in rotting logs (Lèvieux, 1976B; Longhurst & Howse, 1979a), and has relatively large colonies by ponerine standards, with from 500 to 2,000 workers (Peeters, 1991b; Hölldobler et al., 1994). The nests are host to a diverse assemblage of myrmecophiles (Arnold, reprinted in Wheeler, 1922b). Colonies have a single highly fecund ergatoid queen, which in most respects resembles a large major worker but is endowed with a rich array of glands for chemical communication with the workers (Peeters, 1991b; Hölldobler et al., 1994). Males locate virgin queens by following the recruitment trails of workers back to their nests (Longhurst & Howse, 1979b). Workers retain spermathecae and ovarioles, but do not appear to mate and do not produce viable haploid eggs (Villet, 1990a; Villet & Duncan, 1992). Workers are continuously polymorphic for size (Longhurst & Howse, 1979a; Crewe et al., 1984), though they are often referred to as “majors”, “medias” and “minors”. They are known to emit a strong odor, to stridulate loudly when disturbed, and to have a very painful sting (Arnold, 1915).

Megaponera are specialized mass raiders of termites (Wheeler, 1922b, 1936; Weber, 1964; Lévieux, 1966). Longhurst & Howse (1979a) studied their foraging behavior in detail and described the sequence of events in a typical raid (see also the account by Weber, 1964). Raids begin when a solitary major worker locates foraging termites. This scout returns directly to its nest, laying a trail of poison gland-derived pheromones exuded through its sting (Longhurst et al., 1979A), and recruits up to several hundred of its nestmates (Corbara & Déjean, 2000). They proceed as a column to the termites’ protected foraging tunnels, which the major workers tear open. Minor workers then haul out termites. When this process is complete, the major workers stack the termites in their mandibles and the ants return as a column to their nest. Corbara & Déjean (2000) compared the behavior of minor and major workers during prey capture and found that though they are generally similar, major workers are more likely to attack termite soldiers than are minor workers. Longhurst & Howse (1979a) reviewed the literature on Megaponera foraging behavior and found significant regional variation in how raids are conducted. Taylor (2008) hints that this variation supports his hypothesis of multiple species within Megaponera.

Hölldobler & Wilson (1990; see also Wilson, 1958a) hypothesized that the Megaponera style of foraging, in which scouts lead columns of workers on raids of other social insects, could be the first step in the evolution of true legionary behavior, such as that of the dorylines (see also Oster & Wilson, 1978). Of the hallmarks of legionary behavior identified by Brady (2003), Megaponera exhibits obligate collective foraging and a weak form of nomadism, by which colonies frequently emigrate to new nest sites. This latter behavior was described in detail by Arnold (reprinted in Wheeler, 1922b) and Longhurst & Howse (1979a). Wheeler (1922b) also discussed the adaptations of Megaponera brood to emigration. The reasons for these frequent emigrations are unknown, but Longhurst & Howse (1979a) argue that they are not likely caused by prey shortages and may be a response to predation by Dorylus driver ants. In an interesting observation, Beck & Kunz (2007) found cooperative altruistic defensive actions among Megaponera workers under attack by Dorylus.

Castes
Queens are ergatoid (primarily wingless) and have 64 ovarioles in the ovaries, which is exceptionally high among Ponerinae. Workers are polymorphic in size, and the largest workers have 24-30 ovarioles (Peeters 1991).

Nomenclature

 *  MEGAPONERA  [Ponerinae: Ponerini]
 * Megaponera Mayr, 1862: 734. Type-species: Formica foetens (junior primary homonym in Formica, replaced by Formica analis), by monotypy.
 * Megaponera junior synonym of Pachycondyla: Brown, in Bolton, 1994: 164.
 * Megaponera revived from synonymy: Schmidt & Shattuck, 2014: 104.
 * [ Megaloponera : incorrect subsequent spelling by Roger, 1863b: 17; repeated by several authors, for example Emery, 1877b: 368; Forel, 1917: 237.]

Description
Schmidt and Shattuck (2014):

Worker
Large to very large (TL 9–18 mm) ants with the standard characters of Ponerini. Workers polymorphic, varying principally in size and pubescence. Mandibles triangular. Eyes large, located near head midline, with distinct preocular carinae. Frontal lobes widely separated anteriorly and appearing flattened in frontal view. Scapes flattened, with distinct anterior margins. Metanotal groove shallow. Propodeum moderately narrowed dorsally. Propodeal spiracles slit-shaped. Tarsal claws with a preapical tooth. Metatibial spur formula (1s, 1p). Petiole nodiform. Constriction between pre- and postsclerites of A4 indistinct. Stridulitrum present on pretergite of A4. Head and body finely punctate, with scattered pilosity and dense pubescence (sparse in minor workers). Color black.

Queen
Wingless, similar to a major worker but larger (TL 18.5 mm), with more extensive sculpturing, denser pilosity, a deeper metanotal groove, an anteriorly slanting squamiform petiole, and a more expansive gaster. (Adapted from Arnold, 1915.)

Male
See descriptions in Emery (1897b), Arnold (1915), and Wheeler (1922b).

Larva
Described by Wheeler & Wheeler (1952).