Key to Nearctic species in the Formica rufa group

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.

1

 * 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

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

3

 * 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

4

 * 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

5

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

6

 * 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

7

 * 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
 * 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

8

 * 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

9

 * Non-shiny species, gaster densely pubescent and grayish => 10
 * Shinier species, at least sides and venter of head and gaster shiny => Formica subnitens

10

 * 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
 * Compound eyes without erect setae; eastern North America (as far west as Wisconsin and Mississippi) => Formica integra

11

 * 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

12

 * Underside of head with numerous erect setae, usually >10 => 13
 * Underside of head (gula) usually devoid of erect setae, at most with Formica ciliata

13

 * Erect setae of gaster longer, >0.10 mm; smaller species, major workers usually < 7.0 mm total length => Formica mucescens
 * Erect setae of gaster shorter, <0.06 mm; larger species, major workers > 8.0 mm total length => Formica comata

14

 * 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

15

 * 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

16

 * Clypeal margin and mandible with numerous coarse, flexuous, setae => Formica prociliata
 * Clypeal margin and mandible with only a few, short inconspicuous, setae => Formica ferocula

17

 * Erect setae of tergites tapering, pointed at tip => 18
 * Erect setae of tergites flattened, blunt at tip => Formica calviceps

18

 * Rear margin and sides of head lacking erect hairs => Formica propinqua
 * Rear margin and sides of head to at least the level of the eyes with numerous erect hairs => Formica integroides