Polyergus bicolor

This species lives in rotting logs, possibly preferring pine, with its host Formica subaenescens (James Trager).

Identification
P. bicolor is usually easily distinguished from other Nearctic species by its distribution, distinctive two-tone coloring and sparse pilosity. Northern (North Dakota, USA to British Columbia, Canada) populations of Polyergus mexicanus may exhibit similar coloration, but mexicanus is always more pilose. Bicoloration and reduced pilosity occur commonly in the small workers from young colonies of mexicanus, causing possible confusion. Nanitics of mexicanus are usually recognizable by their slender heads with a rounded vertex and longish appendages, and at least a pair of erect pronotal setae. Large workers of other breviceps-complex species also may appear somewhat bicolored, but only rarely is the bicoloration so neatly defined by a nearly completely dark gaster as in bicolor, and these others have more pilosity on the head and mesosoma.

Identification Keys including this Taxon

 * Key to Polyergus species
 * Key to US Polyergus species

Distribution based on Regional Taxon Lists
Nearctic Region: Canada, United States.

This species is apparently endemic to the upper Mississippi Valley, from the western Great Lakes region west to the Dakotas and southern Manitoba. In the past it was found in southern Wisconsin and as far south as Rockford, Illinois, but it has not been seen in this area in recent decades.



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Biology
Trager (2013): P. bicolor normally parasitizes Formica subaenescens, and normally nests with it in rotten stumps or fallen limbs in forests. Wheeler (1910) described raids occurring in early afternoon in a mature mesic forest in northern Illinois that originated from nests in stumps. Two samples examined in this study had Formica neorufibarbis hosts, also a denizen of moist woods, especially tamarack bogs in the eastern part of its range. Through the course of this study I was not able to obtain or study any specimens of bicolor (nor of its host F. subaenescens) collected within about the last 50 years, from bicolor’s historic range, and I am led to wonder if they have contracted northward due to climatic warming or other causes. Just before submitting this manuscript, in July 2013, I collected a sample in northern Wisconsin. As in the published records, this sample occupied a rotting log with F. subaenescens. The log was about 35 cm in diameter, with bark beginning to loosen and wood in transition from white to red rot. At first glance, the mixed colony bore a striking resemblance to a young colony of the locally common Formica aserva.

Nomenclature

 *  bicolor. Polyergus rufescens subsp. bicolor Wasmann, 1901: 369 (w.q.m.) U.S.A. Junior synonym of breviceps: Wheeler, J. 1968: 163. Raised to species: Trager, 2013: 514.

Type Material


Polyergus rufescens subsp. breviceps var. fusciventris Wheeler 1917: 555 (part). Unavailable name; following material referred here: CANADA, MANITOBA, South Cypress RM. Treesbank. (Wheeler) [USNM #59925, USNM ENT 00529453] (image examined by Trager, 2013).

Worker
Trager (2013) - Syntype [LACM, red “type series” label, top specimen] HL 1.30, HW 1.28, SL 1.04, ½ VeM 0, ½ PnM 0, WL 1.96, GL 1.88, HFL 1.48, CI 98, SI 81, HFI 116, FSI 142, LI 3.26, TL 5.14.

(N=44) HL 1.24–1.66 (1.40), HW 1.24–1.74 (1.41), SL 0.92–1.16 (105), ½ VeM 0, ½ PnM 0–2 (0.58), WL 1.88–2.32 (2.08), GL 1.60–2.80 (2.07), HFL 1.40–1.68 (1.55), CI 97–105 (100), SI 64–81 (75), HFI 97–120 (110), FSI 137–158 (147), LI 3.12–3.98 (3.48), TL 4.83–6.58 (5.56).

Head subquadrate to suborbicular, its length and breadth about equal, sides often quite rounded; vertex concave, the concavity about half the head width in breadth, completely lacking vertex pilosity; scapes short, not reaching vertex corners, notably clavate in the apical third; pronotum lacking pilosity, or rarely with 1–2 dorsal erect setae; mesonotum profile flat or very weakly convex for most of its length; propodeum profile a rounded weakly obtuse angle; petiole with rounded sides, petiolar dorsum rounded, not at all or only feebly emarginate; first tergite moderately pubescent, with pilosity much like that of breviceps, in 4–5 transverse arrays; first tergite pilosity flexuous, subdecumbent.

Head matte; mesonotum matte; gaster matte to weakly shining.

Color of head, mesonotum and often petiole dull red, gaster very dusky red (nearly black); forelegs often redder than middle and hind legs; pilosity reddish brown, pubescence fine and grayish.

Etymology
Trager (2013) - Wasmann coined this name from the Latin nominal adjective “bicolor” meaning two-colored.