Key to Polyergus Species

The following key to Polyergus species is based on Trager (2013).

1

Entire body dark brown to blackish; pilosity of anterior first tergite gently curved to nearly straight; eastern temperate or desert central Asia (samurai group) => 2
Body rich orange to brownish red, with varying amounts of dark brown or blackish coloring on the legs, lower mesosoma, and gaster; pilosity of anterior first tergite conspicuously bent or strongly flexuous in the one Eurasian species; European Atlantic Coast east to western Asia (about 88°E), otherwise North American => 3

Browner, matte, larger species—LI of most workers > 375 (usually > 390); with longer scapes—SI of most workers 81–85; dry, open habitats, but of humid temperate Japan, Korea, China, eastern Russia; hosts Formica japonica, Formica glabridorsis, rarely others => Polyergus samurai
Blacker, somewhat shining, smaller species—LI < 375 (usually < 370); with shorter scapes—SI 74–81; steppe deserts of Mongolia, Tuva, adjacent south-central Russia; with Formica candida or Formica kozlovi as hosts => Polyergus nigerrimus

Gastral tergites with pubescence very sparse or lacking, smooth and often strongly shining; North American, most abundant from Atlantic Coast states west to Mississippi Valley (one species extends west to southern Rocky Mountains) (lucidus group) => 4
Gastral tergites with dense pubescence yielding a silky sheen, obscuring the surface of the integument beneath; Eurasian and North American, in North America from western Great Lakes, prairie and Rocky Mountain states and provinces, to Pacific Coast and Mexican mountains (rufescens group) => 9

Vertex pilose, ½ VeM usually > 10; head and mesosomal dorsum matte and sides at most feebly shining; host Formica dolosa => 5
Vertex less pilose, ½ VeM usually < 8; head and mesosomal dorsum matte or shiny; but if matte, then sparsely pilose; host is never Formica dolosa => 6

Scapes and legs shorter, SI < 105, usually < 100, FI < 140; northern species, New England west to the Dakotas => Polyergus sanwaldi
Scapes and legs longer, SI > 100, but usually > 110, FI > 140; southern species, North Carolina to Florida, west to Mississippi and southern Missouri => Polyergus longicornis

Rear portion of head less pilose, ½ VeM usually < 5; head and mesosoma matte or weakly shining on sides; host Formica pallidefulva or Formica archboldi => 7
Rear portion of head pilose, ½ VeM usually > 5; all tagmata at least partly shiny; host Formica incerta or Formica biophilica => 8

Larger and less pilose species, LI of most specimens > 390; ½ VeM 0–1 (very rarely more); eastern Canada to northern Florida, west to Wisconsin, Colorado, New Mexico; host Formica pallidefulva => Polyergus montivagus
Smaller and slightly more pilose, LI of most specimens < 390; ½ VeM 1–5; Florida; host Formica archboldi => Polyergus oligergus

Smaller, shorter-limbed, SL 1.19–1.36, HFL 1.72–2.04; northeastern, Midwestern, and higher altitude Appalachian distribution; host Formica incerta => Polyergus lucidus
Larger, longer-limbed, SL 1.68–1.79, HFL 2.08–2.44; southern and south central USA and foothill Appalachian distribution; host Formica biophilica => Polyergus ruber

Pilosity of anterior first tergite usually gently curved to nearly straight; North American species => 10
Pilosity of anterior first tergite mostly strongly curved or conspicuously bent; continental Europe east to mountains of Kyrgyzstan, Kazakhstan, western China (about 88°E) => Polyergus rufescens

Vertex lacking pilosity or with ½ VeM < 8, usually < 4; pronotal erect pilosity usually all dorsal; hosts in the Formica fusca group, or less often, in the Formica neogagates group, but exclusive of the Formica cinerea complex => 11
Vertex notably pilose, ½ VeM > 6, usually 10–30; pronotum always with some erect setae on sides near lower edges; hosts usually (at least predominantly) Formica montana, Formica canadensis, and/or Formica altipetens => Polyergus breviceps

Head and pronotum moderately pilose, ½ VeM < 6 (rarely more), ½ PnM usually > 4; sometimes with infuscated posterior tergites, if gaster appears entirely brown or blackish, ½ PnM > 4 => 12
Head and pronotum non-pilose or sparsely pilose, ½ VeM 0, ½ PnM 0–2; distinctly bicolored, head and mesosoma red, gaster entirely dark brown to blackish; western Great Lakes states to Dakotas, Manitoba, usually in moist or bog forests; host Formica subaenescens (less often Formica neorufibarbis) => Polyergus bicolor

Scapes fall short of vertex corners by a scape width or more, SI < 85; legs shorter, HFI < 120, usually < 110; hosts various, but apparently never Formica moki => 13
Scapes nearly reaching to slightly surpassing vertex corners, SI > 85, usually > 90; legs longer, HFI > 120; coastal hills of southern CA and BC, MEX; host Formica moki => Polyergus vinosus

Head often and especially sides of mesosoma weakly to conspicuously shining; scapes not reaching vertex corners by about 1.5–2 scape widths, scapes distally clavate, SI 65–82, but usually < 77; legs shorter, HFI 99–120 (but usually < 113 and always < 110 in southern Arizona and Mexico); upper elevation conifer forests of Mexican mountains, north to British Columbia, Rocky Mountain and Plains states, east to Mississippi Valley; hosts are a wide variety of high elevation or northern Formica fusca and Formica neogagates group species => Polyergus mexicanus
Head and mesosoma typically entirely matte, or at most weakly shining; scapes longer, not reaching vertex corners by 1.5 scape widths or often less, scapes thickened distally, but at most weakly clavate, SI 75–86, usually > 79; legs longer, HFI 108–121, usually > 115; HD, Mexico north to lower elevations of southern Arizona mountains in oak or mesquite woodlands; known hosts are various Mexican, hot climate fusca group species, including Formica gnava, Formica foreliana, Formica subcyanea => Polyergus topoffi