Polyergus mexicanus

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Biology
Trager (2013): In more open habitats (fields, prairie reconstructions) in eastern Missouri, I have often found colonies of P. mexicanus-Formica subsericea interspersed with, and within raiding distance of colonies of P. lucidus-Formica incerta, and it is perhaps more often found among Formica rubicunda-F. subsericea and/or Formica subintegra-subsericea colonies in open woodland habitats. I have directly observed and seen indirect evidence of F. subintegra attacking and even bringing home pupae of P. mexicanus, as follows: One ''F. subintegra-F. subsericea colony observed in Missouri contained a contingent of a few dozen P. mexicanus workers that raided separately and later in the afternoon than the dulotic Formica'' whose nest they inhabited.

On two occasions in Missouri, I have observed mexicanus raid colonies of F. pallidefulva-group species. In contrast to their typically non-lethal interactions with their normal hosts, many workers and the queen of these nonhost species were killed, and both killed adults and some live brood were carried home. The brood of these non-host Formica never developed to adulthood in the mexicanus nests. In effect, the usually strictly dulotic Polyergus became predators.

Forel (1899) described this species from specimens collected in “Mexico”. The specimens were collected by Brinkmann, who made other collections in the mountains of Durango that were passed on to Forel (P. S. Ward, pers. comm.), so it seems very likely the series was collected from a montane conifer forest there, similar to the podzolic soil conifer forests it inhabits in western USA. A sample collected by Creighton (at LACM) near el Salto, Durango, Mexico, has workers very similar to the MHNG types, and with Formica cf. occulta host workers, as with southern Arizona samples. Southern samples are from relatively high altitude, with Chiricahua Mts. samples all from above 2200 m, a Vergel, Chihuahua, MEX sample came from about 2800 m (in an open conifer forest), and one from Nevada at a surprising 3200 m. To the north, this protean species is found at lower elevations and in various, mesic, open woodland or grassland-woodland mosaic habitats, but not in moist, closed-canopy forest (though it may occur in natural gaps and in clear-cuts within these), and only occasionally in open prairie. This is characteristically a species of woodlands with little or no shrub layer, usually of oak, oak-pine, or pine in the US Midwest, Ozark Hills and West Coast, and of airy conifer woodlands in the western mountains. It is also found in grassland-woodland ecotones, and prairie groves. Some Illinois and Wisconsin samples came from sandy prairie openings among sandy-soil black oak savannas. It occasionally nests in windbreak plantings of trees, and in less tended areas of parks, cemeteries, gardens and tree-studded lawns, where these are not heavily treated with pesticides. Forel reported that the host was unknown, but the samples I have seen reveal that this species has a wide variety of hosts in the Formica fusca and neogagates groups. In the southwestern USA and northern Mexico, the host is usually Formica cf. occulta, a conifer woodland inhabitant that is larger and darker-colored than typical Formica occulta. From elsewhere, I have studied samples with Formica argentea, Formica podzolica, Formica subsericea, Formica fusca (marcida & subaenescens), Formica accreta, Formica microphthalma, true Formica occulta, Formica neoclara, Formica pacifica, Formica neorufibarbis, Formica hewitti, Formica neogagates, Formica manni, and Formica vinculans. Moffett (2010) vividly describes and illustrates raids of this species (as P. breviceps) on Formica argentea in eastern California.

Polyergus mexicanus and Formica subsericea in an oak woodland in eastern Missouri, USA (James Trager).

A Raid on a Non-Host Species
As Trager (2013) reported, P. mexicanus, most often a parasite of members of the Formica fusca and subsericea complexes, occasionally raids non-host species, and unlike when raiding their proper host, kills them. In the photos below, what looked at first like a normal raid on their host species turned out to be a raid on Formica incerta (of the pallidefulva group), as evidenced by the Polyergus carrying home dead F. incerta adults, which will be be eaten, as will their brood. They also left quite a few dead F. incerta workers lying about, some of which were harvested by other ants, including a yet-to-be described Myrmica species.

Fungi
This species is a host for the ectoparastic fungus Laboulbenia formicarum (Espadaler & Santamaria, 2012). Note that the ants involved were identified as P. breviceps and predate Trager's (2013) revision. It is likely that these are actually P. mexicanus but this has yet to be confirmed.

Nomenclature

 *  mexicanus. Polyergus rufescens r. mexicanus Forel, 1899c: 129 (w.) MEXICO. Raised to species: Trager, 2013: 515.
 * laeviceps. Polyergus rufescens subsp. laeviceps Wheeler, W.M. 1915b: 420 (w.) U.S.A. Junior synonym of mexicanus: Trager, 2013: 515. See also: Creighton, 1950a: 559.
 * umbratus. Polyergus rufescens subsp. umbratus Creighton, 1950a: 560 (w.) U.S.A. [First available use of Polyergus rufescens subsp. breviceps var. umbratus Wheeler, W.M. 1915b: 419; unavailable name.] Junior synonym of breviceps: Wheeler, J. 1968: 163; of mexicanus: Trager, 2013: 515.

Worker
Trager (2013) - Syntypes (N=6 on 3 pins) [MHNG] HL 1.52–1.62 (1.57), HW 1.45–1.60 (1.54), SL 1.08–1.13 (1.09), ½ VeM 0–1 (0.33), ½ PnM 5–6 (5.60), WL 2.20–2.40 (2.29), GL 2.04–2.40 (2.26), HFL 1.58–1.66 (1.62), CI 95–99 (98), SI 68–74 (71), HFI 104–112 (106), FSI 143–154 (148), LI 3.70–4.00 (3.86), TL 5.76–6.34 (6.12).

N=122) HL 1.28–1.96 (1.57), HW 1.24–1.92 (1.53), SL 0.94–1.36 (1.11), ½ VeM 0–3 (rarely, 5+) (1.20), ½ PnM 3–9 (6.77), WL 1.92–2.80 (2.31), GL 1.34–2.68 (2.17), HFL 1.32–1.96 (1.64), CI 91–103 (99), SI 65–81 (73), HFI 95–121 (107), FSI 134–161 (146), LI 3.24–4.76 (3.85), TL 5.01–7.36 (6.02).

This is the most widely distributed and most variable North American Polyergus, and accounts for most literature records of “breviceps,” other than the cited works of Howard Topoff and his students (regarding Polyergus topoffi). Head variable in shape by region, but locally less so, subquadrate with nearly straight sides curving-convergent toward the vertex, to round-sided and convergent toward the mandibles (pomoid) or occasionally nearly suborbicular in outline, HL usually slightly greater than HW, to HW very slightly broader than HL, the latter corresponding with more rounded sides; vertex pilosity of 0–10 macrosetae (> 5 is uncommon); scapes not reaching vertex corners by about 2X their maximum width, curved, clavate in the apical third; HFL roughly equal to HL to slightly longer (rarely up to 1.2X, especially on West Coast); vertex weakly concave in full face view, corners often without erect setae, or each vertex corner may have 1–3 (up to 5) erect setae; pronotum with 4–10 (rarely up to 18) dorsal erect setae; mesonotum with profile flat or at most weakly convex for most of its length, but often convex and bulging in samples from along the West Coast and southwestern Canada (“umbratus” form), and occasionally inland samples; propodeum evenly rounded; petiole with sub-parallel, straight to slightly rounded sides; petiole about as broad as propodeum (above metapleura) in postero-dorsal view; petiolar dorsal margin nearly flat, or faintly convex and medially flattened, occasionally shallowly emarginate; first tergite densely pubescent; first tergite pilosity in 3 or 4 transverse arrays but concentrated in the anterior third; first tergite pilosity flexuous, suberect to subdecumbent.

Head glossy in many specimens from California, Arizona, and Mexico, decreasingly so eastward and northward, thus over most of the population weakly shining, even becoming matte in Canada and the Dakotas; mesonotum matte dorsally and somewhat to notably shining laterally, rarely entirely matte; gaster somewhat shining beneath pubescence and shinier laterally, where pubescence is dilute.

Color mostly red with infuscation of posterior portions of tergites (sometimes entire tergites, especially Canadian provinces and the Dakotas), and with slightly darker legs; pilosity browner than prevailing body color; pubescence gray (never yellowish as in P. breviceps).

Forel erred when he characterized the types of this species as lacking pilosity. The five specimens of the type series, though their pronota and gastral tergites appear hairless, bear the darkly pigmented impressions of macrosetal bases typical in this genus. These impressions come in an array and in numbers within the range of macrosetal counts of other specimens belonging to this species that are in full possession of their macrosetae.