Neivamyrmex swainsonii

Due to its large size and relative abundance N. swainsonii is easily one of North America’s most conspicuous army ants.

Distribution
United States: Kansas, Louisiana and Texas, west to California; Mexico: border states south to Chiapas and Yucatán; south to Argentina.

Distribution based on Regional Taxon Lists
Nearctic Region: United States. Neotropical Region: Argentina, Brazil, Colombia, Costa Rica, Ecuador, Guatemala, Mexico, Panama, Paraguay, Trinidad and Tobago, Venezuela.

Biology
The following notes are provided by Hill (2007) under the name N. fallax (a junior synonym of N. swainsonii):

On 27 June 2006, while on a collecting trip in west Texas, a colony of N. fallax was observed raiding a colony of Solenopsis xyloni. These observations were made just outside of Alpine, in Brewster County, Texas (30°20’46”N 103°41’39”W) at 1,548 m, behind a pavilion along a fencerow separating a hotel parking lot and a pasture. The activity occurred in an area measuring approximately 1x1.5 m that consisted of mostly bare soil and gravel with some forbs and Cynodon dactylon (L,) Pers (Poaceae) (Bermuda grass). The observations were made between 7:55 P.M.-9:10 P.M., and the temperature was 28.8°C.

While collections of ants were being made in the area, a large number of S. xyloni were observed, apparently relocating their colony from an old nest site to a new one approximately one meter away. Many N. fallax workers were emerging from three holes in the ground between these two locations, whereupon they attacked the S. xyloni workers and took their brood (eggs and pupa). In most cases, the S. xyloni workers only minimally defended their brood, before dropping it and running away. In other cases, the N. fallax took the brood from the mandibles of the S. xyloni workers after a brief skirmish. Several S. xyloni workers carrying brood apparently tried to evade the onslaught of their attackers by climbing onto a small forb. When a N. fallax worker ventured up the forb, the S. xyloni moved further up the plant until they were at the top. When the N. fallax neared them, the Solenopsis dropped their brood, and fell to the ground. The N. fallax workers also were observed also attacking male and female S. xyloni alates and carrying them underground after they were subdued. One S. xyloni worker also was observed being carried underground. Additionally, several N. fallax workers were seen entering and exiting the S. xyloni colony, but none of those exiting were observed carrying anything.

During the course of these observations, the movement trail of the Solenopsis became more obtuse as the Neivamyrmex pushed further into their ranks. A couple of workers of two other ant species, Pogonomyrmex rugosus and Novomessor cockerelli, were also moving throughout the area. The S. xyloni attacked both of these larger species when encountering them with seemingly greater aggressiveness than they exhibited for the more similar sized N. fallax, and in one case were able to kill one of the P. rugosus workers.

The observations ceased near sundown. The next morning the site was visited again, but there was no sign of either the Neivamyrmex or the Solenopsis. It would be interesting to know whether the Solenopsis were already moving their colony at the time and the Neivamyrmex took advantage of their vulnerability, or if the Solenopsis were moving because the Neivamyrmex already had attacked their original colony.

Nomenclature

 *  swainsonii. Labidus swainsonii Shuckard, 1840a: 201 (m.) BRAZIL. Combination in Eciton: Forel, 1895b: 121; in E. (Acamatus): Emery, 1900a: 187; in E. (Neivamyrmex): Borgmeier, 1948b: 462; in Neivamyrmex: Borgmeier, 1953: 15. Senior synonym of arizonense: Borgmeier, 1955: 454; of fallax, mexicanus: Snelling, G.C. & Snelling, R.R., 2007: 490..
 * mexicanus. Labidus mexicanus Smith, F. 1859b: 7 (m.) MEXICO. Forel, 1899c: 27 (w.); Reichensperger, 1939: 297 (q.). Combination in Eciton: Dalla Torre, 1893: 4; in E. (Labidus): Emery, 1895c: 260; in E. (Acamatus): Emery, 1900a: 187; in E. (Neivamyrmex): Smith, M.R. 1942c: 544; in Neivamyrmex: Borgmeier, 1953: 8. Subspecies of pilosus: Borgmeier, 1936: 59. Junior synonym of pilosus: Smith, M.R. 1942c: 544. Revived from synonymy as subspecies of pilosus: Borgmeier, 1953: 14; Borgmeier, 1955: 361. Senior synonym of clavicornis: Mayr, 1886b: 120; of subsulcatum: Dalla Torre, 1893: 4; of aztecum: Borgmeier, 1936: 60; of militarium: Borgmeier, 1953: 14. Junior synonym of swainsonii: Snelling, G.C. & Snelling, R.R., 2007: 490. See also: Borgmeier, 1955: 361.
 * aztecum. Eciton aztecum Forel, 1901h: 49 (m.) GUATEMALA. Subspecies of mexicanus: Emery, 1910b: 26; of pilosus: Wheeler, W.M. 1921d: 314. Junior synonym of mexicanus: Borgmeier, 1936: 60.
 * arizonense. Eciton (Acamatus) arizonense Wheeler, W.M. 1908e: 414, pl. 26, fig. 5 (m.) U.S.A. Combination in E. (Neivamyrmex): Smith, M.R. 1942c: 581; in Neivamyrmex: Borgmeier, 1953: 18. Subspecies of swainsonii: Borgmeier, 1953: 19. Junior synonym of swainsonii: Borgmeier, 1955: 454.
 * fallax. Neivamyrmex fallax Borgmeier, 1953: 48, figs. 31, 33 (w.) U.S.A. See also: Borgmeier, 1955: 425. Junior synonym of swainsonii: Snelling, G.C. & Snelling, 2007: 490.

Snelling and Snelling (2007) - We have determined that N. fallax is the worker of N. swainsonii. The evidence for this association is scanty: it is based on a worker of N. fallax found attached to the leg of a male collected in Arizona. Although throughout the United States and Mexico the ranges of these two taxa overlap nicely, N. fallax is unknown south of Guatemala.

Male
Smith (1942), for arizonense - Length 12-13 mm.

Head approximately twice as broad as long. Eye large, strongly convex, protuberant. Ocelli large, placed on high protuberance above general surface of head; from above, appearing as if on a distinctly elevated, transverse ridge; lateral ocellus less than its greatest diameter from inner margin of eye. Frontal carinae elevated, sharply margined, slightly converging posteriorly, with deep groove between them. Antenna short; scape robust, very noticeably wider than base of funiculus, but slightly shorter than combined length of first 4 funicular segments; funiculus distinctly tapering from base to apex. Clypeus excised. Mandible flattened dorsoventrally, very long, strongly curved, especially toward apex, and tapering to form an extremely acute point. Head, from above, remarkably broad and short, not prolonged behind eyes. Thorax very robust, strongly protrudirrg above dorsal surface of head. Mesonotum with distinct anteromedian and parapsidal lines. Epinotum, in profile, concave. Tarsal claws toothed. Petiole with a protuberance beneath. Apex of seventh gastric sternum with 3 teeth; a short, somewhat blunt, median tooth, and 2 rather acute lateral teeth. Paramere, in profile, abruptly enlarged toward apex, and with a dorsal emargination which varies considerably with regard to depth and breadth.

Head and anterior border of each gastric segment smooth and shining; remainder of body more opaque. In some lights, various parts of the body have a glabrous appearance in spite of the dense hairs covering the surface. Thorax, petiole, and gaster very finely punctulate.

Hairs yellowish, rather dense and somewhat appressed; usually longer on lower surface of body, epinotum, petiole, and tip of gaster.

Yellowish brown to reddish brown, with darker head and seventh gastric sternum. Wings distinctly yellowish, pilose, with light-brown or yellowish-brown veins and stigma.

Type Material
Smith (1942)  arizonense: Nogales, Ariz.; E. J. Oslar. Male cotypes