Nomamyrmex

Nomamyrmex is a relatively commonly observed genus with only two species and two additional subspecies recognized. It is the only army ant genus that has been reported to successfully attack well-defended and often enormous colonies of Atta leaf cutter ants.

Identification
Diagnosis. Worker. The workers of Nomamyrmex are easily recognized by a combination of highly positioned spiracle and lack of pronounced propodeal lobes, propodeum armed with cuticular projections, two-segmented waist, armed pretarsal claws, and absence of metatibial gland. The lack of conspicuous lighter area of cuticle on the inner side of hind tibia (the metatibial gland) distinguishes this genus from all other Eciton genus-group ants except for some Neivamyrmex, but those always have simple pretarsal claws.

Watkins (1977) - Workers: postpetiole distinct from gaster; antennal insertions not covered by frontal carinae; each eye composed of a single ocellus-like unit; each tarsal claw with a small inner tooth; scape thick (apical width greater than one-third its length). Males: length 15-23 mm; gaster cylindrical with conspicuous tufts of long dense setae.

Species richness
Species richness by country based on regional taxon lists (countries with darker colours are more species-rich). View Data



Biology
Henry Walter Bates was perhaps the first to report on the habits of Nomamyrmex in his famous narrative (Bates 1863), describing a ‘(...) very stout-limbed Eciton, the E. crassicornis, whose eyes are sunk in rather deep sockets’ that ‘(...) goes on foraging expeditions like the rest of its tribe, and attacks even the nests of other stinging species (Myrmica), but it avoids the light, moving always in concealment under leaves and fallen branches’. Borgmeier (1955) and Rettenmeyer (1963) summarize what was known about Nomamyrmex to date, most observations being on N. esenbeckii. The summary below regarding raids and emigrations is based on these resources unless stated otherwise. Nomamyrmex presumably forms bivouacs which are always subterranean and have never been directly observed. Based on the durations of emigrations observed, Rettenmeyer (1963) estimated that the colonies must be enormous, perhaps in the excess of a million workers. The diet of these army ants consists mostly of immatures of multiple species of other ants, although they have been observed raiding nests of other social insects, including termites and bees (see also Souza and Moura 2008). It appears that raids are primarily subterranean, although columns of these ants are also observed above ground. The raid columns are narrow, not forming swarms. The raids have been observed both at night and during the day and often last throughout the day. Rettenmeyer reports that N. esenbeckii on Barro Colorado, Panama conducted raids mostly during the day but there are reports of the same species raiding at night and being strongly photophobic (Sánchez-Peña and Mueller 2002). Given the large size of the colonies, raids and emigrations can take a very long time and last well over 24 hours (Rettenmeyer 1963, Powell and Clark 2004). Numerous myrmecophiles have been observed in emigration columns, including multiple limulodid beetles riding the emigrating queen. The brood is synchronized. A remarkable aspect of Nomamyrmex biology is the capability to successfully raid the huge colonies of leaf-cutting ants in the genus Atta, otherwise mostly ignored by army ants. Most published records of Nomamyrmex foraging contain observations of raids on leaf cutters (Swartz 1998, Sánchez-Peña and Mueller 2002 and references therein) and Powell and Clark (2004) conducted the most comprehensive study of interactions between these ants to date. They show that Nomamyrmex is capable of successfully raiding both young and mature colonies of Atta and that the latter respond in a specific manner to the presence of workers of Nomamyrmex but not Eciton. The leafcutters defend their nests by mobilizing large numbers of major workers and plugging nest entrances with cut leaves. Nomamyrmex and Atta workers that directly engage in combat are most often the largest ants in the colonies of both species and the encounters usually result in the ants becoming locked head-to-head. Furthermore, slightly smaller workers of both species also participate in combat but in a slightly different way. On the Atta side, they assist in spread-eagling the attacking army ants while the ‘primary combatants’ are locked with their mandibles. On the Nomamyrmex side they overrun and sting the leaf-cutter majors to death. Nomamyrmex is capable of inflicting significant damage on a raided Atta colony. A subterranean raid on a partially excavated Atta mexicana colony was observed where the army ants killed a large proportion of adult Atta, including the queen (Rettenmeyer et al. 1983). Swartz (1998) reported that an army ant raid on a young A. cephalotes colony extirpated the leaf-cutters and eventually turned into an emigration, the Nomamyrmex colony relocating into the abandoned nest. Powell and Clark (2004) estimated that during one nearly 36-hour raid the Nomamyrmex removed over 60,000 brood items from an A. cephalotes colony, possibly over a half of all the brood present in the nest'

Nomenclature

 *  NOMAMYRMEX [Ecitoninae: Ecitonini]
 * Nomamyrmex Borgmeier, 1936: 55 [as subgenus of Eciton]. Type-species: Eciton crassicornis, by original designation.
 * Nomamyrmex subgenus of Eciton: Kusnezov, 1956: 9.
 * Nomamyrmex raised to genus: Borgmeier, 1953: 4; Borgmeier, 1955: 135; Smith, M.R. 1958c: 108.