Key to Nearctic species in the Formica rufa group

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This worker key is based on: Stockan, J., E. Robinson, J. Trager, I. Yao, and B. Seifert, B. 2016. Introducing wood ants: Evolution, phylogeny, identification and distribution. In J. Stockan and E. Robinson (Eds.), Wood Ant Ecology and Conservation. pp. 1-36. Cambridge: Cambridge University Press. doi:10.1017/CBO9781107261402.002

In the following key, Formica reflexa and Formica dakotensis, which are traditionally placed in the rufa complex, and the introduced Formica paralugubris, have been excluded.

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1

  • Erect setae of pronotum 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 in full face view usually longer than broad . . . . . microgyna complex (not further keyed)
  • Erect setae of pronotum 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 capsule of largest workers in full face view most often broader than long . . . . . rufa group sensu stricto . . . . . 2

2

return to couplet #1

  • Antennal scapes with <10 setae, or most often without erect or suberect setae except for a few near the distal end (Pacific Northwest F. obscuripes may have more, but these are very short and decumbent), when present on the shaft, setae occur singly and are scattered . . . . . 3
  • Antennal scapes with >12 setae, often many more erect setae on all surfaces, these not uncommonly appearing paired . . . . . Formica oreas
Formica oreas casent0005390 head 1.jpg
Formica oreas casent0005390 profile 1.jpg

3

return to couplet #2

  • Cross section of median portion of clypeus box-like, dorsal surface meeting steeply descending sides through a distinct angle, dorsal view of clypeus appearing pinched in from the sides and broadly hourglass shaped; clypeal fossae deep, broad (Figure 1.8) . . . . . 4
Figure 1.8.
  • Cross section of median portion of clypeus parabolic, descending to clypeal fossae through an even curve or broadly rounded angle which begins at median carina, dorsal view of median clypeus not pinched in, rather, roughly quadrate, trapezoidal or heptagonal in shape; clypeal fossae shallow, narrower (Figure 1.9) . . . . . 6
Figure 1.9.

4

return to couplet #3

  • Worker with few erect or suberect setae visible in profile; mesonotum and propodeum with few to no standing setae, erect setae patchy when present . . . . . 5
  • Worker with numerous erect or suberect setae about evenly distributed over entire dorsal profile, including the mesonotum and propodeum . . . . . Formica obscuriventris
Formica-obscuriventris-MCZ001H.jpg
Formica-obscuriventris-MCZ001L.jpg

5

return to couplet #4

  • Dorsal profile with scattered, moderately abundant pilosity; western mountain states and Canadian provinces . . . . . Formica laeviceps
Formica laeviceps casent0103372 head 1.jpg
Formica laeviceps casent0103372 profile 1.jpg
  • Dorsal profile lacking or with very few erect setae (Figure 1.10); prairie states north to Alberta and east to New England and Canadian Maritime provinces . . . . . Formica fossaceps
Figure 1.10.
Formica fossaceps casent0103366 head 1.jpg
Formica fossaceps casent0103366 profile 1.jpg

6

return to couplet #3

  • Dorsal profile with numerous erect or suberect setae about evenly distributed on head vertex, pronotum, mesonotum and propodeum . . . . . 7
  • Dorsal profile with few erect or suberect setae on vertex, and especially, with patchy, few and scattered, or no erect setae on mesonotum and propodeum . . . . . 8

7

return to couplet #6

  • Legs red, matching or at most only slightly browner than mesosoma; all sizes of workers bicoloured, without infuscation; dorsal surfaces of middle and hind tibiae with abundant, long, 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 but short, suberect pilosity, or this sometimes almost lacking . . . . . Formica obscuripes (as conceived here, obscuripes includes questionably distinct taxon planipilis)
Formica-obscuripes-MCZ001H.jpg
Formica-obscuripes-MCZ001L.jpg

and Formica planipilis

Formica-planipilis-MCZ001H.jpg
Formica-planipilis-MCZ001L.jpg

8

return to couplet #6

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

9

return to couplet #8

  • Completely or largely non-shiny species; sides and venter of head and dorsum of gaster opaque to subopaque; gaster densely pubescent and greyish . . . . . 10
  • Shinier species: at least sides and venter of head and dorsum of gaster shiny; gaster pubescence sparser, not completely obscuring view of integument . . . . . Formica subnitens
Formica subnitens casent0005394 head 1.jpg
Formica subnitens casent0005394 profile 1.jpg

10

return to couplet #9

  • 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) (Figure 1.11) . . . . . Formica ravida
Figure 1.11
MCZ ENT Formica ravida 01 hef.jpg
MCZ ENT Formica ravida 01hal.jpg
  • Compound eyes without erect setae; eastern United States (as far west as Wisconsin and Mississippi) . . . . . Formica integra
Formica integra casent0105554 head 1.jpg
Formica integra casent0105554 profile 1.jpg

11

return to couplet #8

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

12

return to couplet #11

  • Underside of head with numerous erect setae, usually >10 (Figure 1.13) . . . . . 13
Figure 1.13
  • Underside of head usually devoid of erect setae, usually <4 (Figure 1.12) . . . . . Formica ciliata
Figure 1.12
Formica ciliata casent0104876 head 1.jpg
Formica ciliata casent0104876 profile 1.jpg

13

return to couplet #12

  • Standing setae of gaster longer, >0.10 mm; smaller species, major workers usually <7.0 mm total length (Figure 1.14) . . . . . Formica mucescens
Figure 1.14
Formica mucescens casent0103491 head 1.jpg
Formica mucescens casent0103491 profile 1.jpg
  • Standing setae of gaster shorter, <0.06 mm; larger species, major workers >8.0 mm total length (Figure 1.15) . . . . . Formica comata
Figure 1.15
Formica-comata-MCZ001H.jpg
Formica-comata-MCZ001L.jpg

14

return to couplet #11

  • Gastral tergites lacking or with very few erect setae (sometimes a few near the anterior face of first tergite) . . . . . 15
  • Gastral tergites with erect setae more abundant and usually more evenly distributed . . . . . 17

15

return to couplet #14

  • Pronotum and angle of propodeum with numerous erect setae; gastral dorsum lacking erect pilosity, even at the rear margins of tergites, but venter with abundant, flexuous pilosity; Midwestern US grassland and open oak woodland species . . . . . 16
  • Pronotum 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 Mountains . . . . . Formica criniventris
Formica criniventris casent0104874 head 1.jpg
Formica criniventris casent0104874 profile 1.jpg

16

return to couplet #15

  • Anterior clypeal margin and mandibles with numerous coarse, flexuous, setae, Ohio west to Wisconsin and south western Iowa . . . . . Formica prociliata
Formica prociliata hef.jpg
Formica prociliata hal.jpg
  • Anterior clypeal margin and mandibles with only a few, short inconspicuous, setae, known only from northern Illinois . . . . . Formica ferocula
Formica ferocula casent0105598 head 1.jpg
Formica ferocula casent0105598 profile 1.jpg

17

return to couplet #14

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

18

return to couplet #17

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