Subfamily FORMICINAE
Tribe FORMICINI

Polyergus mexicanus Forel 1899

by Joe A. MacGown, uploaded 21 July 2009, updated 23 July 2014

Polyergus mexicanus, full face view of a worker (photo by James Lewis and Joe MacGown)
Polyergus mexicanus, profile view of a worker (photo by James Lewis and Joe MacGown)
Polyergus mexicanus, dorsal view of a worker (photo by James Lewis and Joe MacGown)
Polyergus mexicanus, full face view of a queen (photo by James Lewis and Joe MacGown)
Polyergus mexicanus, profile view of a queen (photo by James Lewis and Joe MacGown)
Polyergus mexicanus, dorsal view of a queen (photo by James Lewis and Joe MacGown)
Polyergus mexicanus, full face view of a male (photo by James Lewis and Joe MacGown)
Polyergus mexicanus, profile view of a male (photo by James Lewis and Joe MacGown)
Polyergus mexicanus, dorsal view of a male (photo by James Lewis and Joe MacGown)

Overview
Polyergus workers can be easily recognized by their large size (approximately 4.0 - 7.7 mm long), yellowish-red to dark reddish-brown coloration, distinct promesonotal suture, three small ocelli, and sickle-shaped (falcate) mandibles with minute serrations on the inner borders.

Polyergus species are obligatory or true slave-making ants. In nest founding, the female enters a nest of the host species, eventually kills the rightful queen, and uses the host workers to tend her brood. Polyergus colonies conduct slave raids on nests of various species of Formica, and workers of the host are taken and used by the Polyergus colony to feed and rear the brood and excavate the nest. Polyergus workers are incapable of surviving without slaves. In laboratory colonies, a colony without slaves will starve to death even when plentiful food is available. (Hedlund, 2007; King and Trager, 2007).

Polyergus mexicanus is a reddish-brown species that is widely distributed in the western US and east to Indiana and Arkansas,ranging south into Durango and Chihuahua Mexico and north to Alberta, Canada. This common species has a fairly wide variety of hosts in the F. fusca and neogagates groups.

Taxonomic History (from Trager 2013, Bolton 2014)
Described as Polyergus rufescens r. mexicanus by Forel in 1899. Raised to species by Trager, 2013. Senior synonym of Polyergus laeviceps, Polyergus rufescens umbratus: Trager, 2013.

Identification
Worker Measurements (in mm) from Trager 2013 (for more comprehensive measurements, see Trager's paper). (N=122) HL 1.28–1.96 (1.57), HW 1.24–1.92 (1.53), SL 0.94–1.36 (1.11), WL 1.92–2.80 (2.31), TL 5.01–7.36 (6.02). Color: mostly reddish with posterior parts of gastral tergites infuscated to entire gaster with darker infuscation, legs slightly darker than body, pilosity dark brown, pubescence gray. Head shiny in western populations to weakly shiny or matte eastward and northward; mesonotum matte dorsally, shiny laterally (occasionally matte); gaster somewhat shiny dorsally beneath pubescence, and shinier laterally (due to sparcer pubescence). Head subquadrate to slightly suborbicular; vertex with 1–10 macrosetae (usually < 5); scapes not reaching corners of vertex by approximately twice their maximum width, curved, and clavate for the apical third; vertex feebly concave (full face view), with 0–5 erect setae. Pronotum typically with 4–10 (with as many as 18 rarely) erect setae present dorsally. Mesonotum flat or weakly convex in profile view. Propodeum evenly rounded. Petiole about as broad as propodeum, with straight to slightly rounded subparallel sides; flat to slightly convex dorsally, occasionally shallowly emarginate. First tergite of gaster with dense pubescence; flexuous, suberect to sumdecumbent pilosity present in 3 or 4 rows mostly in the anterior third of first tergite; pubescence and pilosity more scattered on remainder of gaster.

This species can be separated from other Polyergus species in the southeastern United States by its shorter antennal scapes, which are shorter than the interocular distance, clubbed appearance of antennal scape, and the more densely pubescent gaster.

Biology and Economic Importance
Trager notes that P. mexicanus nests in a variety of habitats depending on the the region. Specimens from southern areas tend to be from altitudes above 2200m in open montane conifer forests. However, northern records include collections from lower elevations in mesic open hardwood, pine-hardwood, and conifer forests with grassy understories; sandy prairies; oak savannas; and occasionally in slightly untended parks, cemeteries, gardens, and lawns with scattered trees. Indeed, this was the case for collection we made in Arkansas at Petit State Park, with a colony being found in an open grassy area with scattered trees present.

Trager (2013) reported that species has a wide variety of hosts in the F. fusca and neogagates groups. In the southwestern USA and northern Mexico, the host is usually F. cf. occulta. Hosts from other regions include F. argentea, F. podzolica, F. subsericea, F. fusca (marcida & subaenescens), F. accreta, F. microphthalma, true F. occulta, F. neoclara, F. pacifica, F. neorufibarbis, F. hewitti, F. neogagates, F. manni, and F. vinculans. In Arkansas, we observed this species raiding F. subsericea at approximately 5:00 PM from 5-7 August 2008.

Alates of this specie have been collected in the summer.

This species poses little in the way of economic importance, as is rarely encountered. However, if provoked or handled worker can inflict a somewhat painful bite.

Distribution (from Trager 2013)

CANADA: Alberta, British Columbia, Manitoba.

MEXICO: Chihuahua, Durango.

USA: Arizona, California, Colorado, Idaho, Iowa, Minnesota, Missouri, Montana, Nevada, New Mexico, North Dakota, South Dakota, Utah, Washington, and Wyoming. The MEM has also collected this species in Conway County, Arkansas.

Literature Cited
Forel, A. 1899. Formicidae. Biol. Cent.-Am. Hym. 3: 1-160.

Hedlund, K. S. 2007. The Ants: North America Catalog: Genus Polyergus. http://www.cs.unc.edu/~hedlund/playpen/dev/ants/catalog/ (accessed 6 June 2008).

King, J. R. and J. C. Trager. 2007. Natural history of the slave making ant, Polyergus lucidus, sensu lato in northern Florida and its three Formica pallidefulva group hosts. 14 pp. Journal of Insect Science 7: 42, available online: insectscience.org/7.42

Smith, D. R. 1979.  In Catalog of Hymenoptera in America north of Mexico. Smithsonian Institution Press, Washington D. C.  Vol. 2, pp. 1323-1427. 

Smith, M. R. 1947. A Study of Polyergus in the United States, based on the workers (Hymenoptera: Formicidae). The American Midland Naturalist 38: 150-161.

Trager, J. C. 2013. Global revision of the dulotic ant genus Polyergus (Hymenoptera: Formicidae, Formicinae). Zootaxa 3722 (4): 501–548.

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