Key to Nearctic species in the Formica rufa group

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This key to species in the Formica rufa group has been provided by James Trager. Formica reflexa and Formica dakotensis, traditionally placed in the rufa complex, are excluded.


  • Erect setae of pro-mesonotum widen from base to a flattened apex broader than the shaft of the seta; major workers small, outstretched length from clypeal margin to acidopore rarely exceeding 6 mm, their head capsule usually longer than broad => microgyna complex
  • Erect setae of pro-mesonotum tapering or if blunt-tipped, not broader apically nor spatulate; major workers larger, outstretched length exceeding 6 mm, often up to 8-9 mm; head of largest workers most often broader than long - rufa group sensu stricto => 2


  • Antennal scapes with <5 or most often without erect or suberect hairs except for a few at the distal end, those few present on the shaft occurring singly and scattered => 3
  • Antennal scapes with >12, often many more erect hairs on all surfaces, these not uncommonly appearing paired => Formica oreas
Formica oreas casent0005390 head 1.jpg
Formica oreas casent0005390 profile 1.jpg


  • Median portion of clypeus box-like, in dorsal view with steeply descending sides appearing pinched in from the mid-sides; clypeal fossae deep, pit-like => 4
  • Median portion of clypeus descending to clypeal fossae through an even curve, which begins at median carina, clypeal fossae shallow, not pit-like => 6


  • Worker with fewer erect or suberect setae visible in profile, patchy or few or no erect setae on mesonotum and propodeum => 5
  • Worker with numerous erect or suberect setae about evenly distributed over entire dorsal profile, including the mesonotum and propodeum => Formica obscuriventris


  • Dorsal profile with scattered, moderately abundant pilosity; western mountain states and provinces => Formica laeviceps
Formica laeviceps casent0103372 head 1.jpg
Formica laeviceps casent0103372 profile 1.jpg
  • Dorsal profile lacking or with very few erect setae; prairie states east to New England and Canadian Maritime provinces => Formica fossaceps
Formica fossaceps casent0103366 head 1.jpg
Formica fossaceps casent0103366 profile 1.jpg


  • Worker with numerous erect or suberect setae about evenly distributed over entire dorsal profile, including the mesonotum and propodeum => 7
  • Worker with fewer erect or suberect setae visible in profile, at least with patchy or few or no erect setae on mesonotum and propodeum => 8


  • Legs red, matching or at most only slightly browner than mesosoma; all sizes of workers bicolored, without infuscation; dorsal surfaces of middle and hind tibiae with abundant, suberect pilosity => Formica coloradensis
Formica coloradensis casent0104875 head 1.jpg
Formica coloradensis casent0104875 profile 1.jpg
  • Legs blackish; workers infuscated to varying degrees ranging from symmetrical dark dorsal blotches to blackening of all tagmata; dorsal surfaces of middle and hind tibiae with variable short, suberect pilosity, sometimes lacking => Formica obscuripes
and Formica planipilis


  • Erect hairs absent or at most very few and irregularly scattered on upper surface of head, mesosoma, and petiole => 9
  • Erect hairs present and approximately evenly spaced on at least dorsum of pronotum and propodeum => 11


  • Non-shiny species, gaster densely pubescent and grayish => 10
  • Shinier species, at least sides and venter of head and gaster shiny => Formica subnitens
Formica subnitens casent0005394 head 1.jpg
Formica subnitens casent0005394 profile 1.jpg


  • Compound eyes with a few, short, fine erect setae arising among the facets; western North America (as far east as the Dakotas and New Mexico) => Formica ravida
MCZ ENT Formica ravida 01 hef.jpg
MCZ ENT Formica ravida 01hal.jpg
  • Compound eyes without erect setae; eastern North America (as far west as Wisconsin and Mississippi) => Formica integra
Formica integra casent0105554 head 1.jpg
Formica integra casent0105554 profile 1.jpg


  • Gaster clothed with close-set, short erect setae that give the appearance in profile of a loose, plush-like investiture => 12
  • Erect setae on gaster more widely spaced and not forming an even investiture => 14


  • Underside of head with numerous erect setae, usually >10 => 13
  • Underside of head (gula) usually devoid of erect setae, at most with <4 => Formica ciliata
Formica ciliata casent0104876 head 1.jpg
Formica ciliata casent0104876 profile 1.jpg


  • Erect setae of gaster longer, >0.10 mm; smaller species, major workers usually < 7.0 mm total length => Formica mucescens
Formica mucescens casent0103491 head 1.jpg
Formica mucescens casent0103491 profile 1.jpg
  • Erect setae of gaster shorter, <0.06 mm; larger species, major workers > 8.0 mm total length => Formica comata


  • Tergites lacking or with very few erect setae (sometimes a few near the anterior face of first tergite => 15
  • Tergites with evenly distributed, moderately abundant erect setae present => 17


  • Numerous erect setae on profile of pronotum and on the angle of the propodeum; gastral dorsum lacking erect pilosity, even at the rear margins of tergites, but venter with abundant, flexuous pilosity; grassland and open oak woodland species, Ohio west to Iowa => 16
  • Promesonotum and propodeum lacking pilosity; gastral dorsum with a few erect setae, especially at the rear margins of tergites, and a similar array ventrally; grassland from western Dakotas to base of Rocky Mts. => Formica criniventris
Formica criniventris casent0104874 head 1.jpg
Formica criniventris casent0104874 profile 1.jpg


Formica prociliata hef.jpg
Formica prociliata hal.jpg
  • Clypeal margin and mandible with only a few, short inconspicuous, setae => Formica ferocula
Formica ferocula casent0105598 head 1.jpg
Formica ferocula casent0105598 profile 1.jpg


  • Erect setae of tergites tapering, pointed at tip => 18
  • Erect setae of tergites flattened, blunt at tip => Formica calviceps
Formica calviceps casent0103235 head 1.jpg
Formica calviceps casent0103235 profile 1.jpg


Formica propinqua casent0005391 head 1.jpg
Formica propinqua casent0005391 profile 1.jpg
  • Rear margin and sides of head to at least the level of the eyes with numerous erect hairs => Formica integroides
Formica integroides MCZ001H.jpg
Formica integroides MCZ001L.jpg