Key to Nearctic Formica fusca group workers

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This worker key is based on: Francoeur, A. 1973. Révision taxonomique des espèces néarctiques du groupe fusca, genre Formica (Formicidae, Hymenoptera). Mémoires de la Société entomologique du Québec. 3:1-316. PDF

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1

  • Metasternum with two distinctly pilose processes, arising on each side of the spinasternal cavity (fig. 15, 42); mesometasternal profile composed of a convexity followed by an inverted triangle (fig. 33) . . . . . 2
  • Metasternum without such processes (fig. 195); mesometasternal profile composed of a convexity followed by a straight or concave line (fig. 34-35) . . . . . 10

2

return to couplet #1

  • Eyes smaller, ocular index usually less than 30; anterior border of median clypeal lobe distinctly angulate, i.e. formed of two straight sides meeting at midline to form an obtuse angle; standing hairs of gastric dorsum long and flexuous, tapering from base to apex; gastric pubescence very dilute; surface strongly shining; West of the continent . . . . . Formica subpolita
{
Formica subpolita casent0005377 head 1.jpg
Formica subpolita casent0005377 profile 1.jpg
  • Eyes larger, ocular index more than 30; anterior border of median clypeal lobe weakly angulate, i.e. formed of two convex sides meeting at midline to form a wide angle, or completely convex or truncate; standing hairs of gastric dorsum short and bristle-like, of the same diameter for greater part of length, then either truncate or abruptly tapered (except for pilicornis, but in this case erect hairs present on the eyes); gastric pubescence normal or dense; surface feebly shining or opaque . . . . . 3

3

return to couplet #2

  • Body pilosity more abundant: in particular, erect hairs present on ventral surface of head, occiput and propodeum (sometimes absent on ventral surface of head of altipetens); present on genre, except for altipetens and usually except for montana; cinerea complex . . . . . 4
  • Body pilosity less abundant: in particular, erect hairs absent on genre, ventral surface of head, occiput (in most cases) and propodeum; neoclara complex . . . . . 8

4

return to couplet #3

  • Eyes and scapes with standing hairs; erect hairs on gastric tergites long and slender . . . . . Formica pilicornis (=Formica francoeuri)
Formica francoeuri casent0005363 head 1.jpg
Formica francoeuri casent0005363 profile 1.jpg
  • Eyes and scapes without standing hairs; erect hairs on gastric tergites short and stiff . . . . . 5

5

return to couplet #4

  • Episterna without erect or suberect hairs; if a few (1 to 3) are present, these limited to peripheral margin of mesothoracic katepisterna; genre and usually occipital angles without erect hairs; sometimes erect hairs absent on ventral surface of head and crest of petiole; West of the continent . . . . . Formica altipetens
Formica altipetens casent0005360 head 1.jpg
Formica altipetens casent0005360 profile 1.jpg
  • Episterna with erect or suberect hairs (except for montana in some cases); genre with erect hairs, frequently absent on montana; occipital angles usually pilose . . . . . 6

6

return to couplet #5

  • Sides of head less pilose; head in full face view, erect hairs usually limited to occipital angles, but may be present down to the ventral border of eyes; genre normally without erect hairs, sometimes 1-2 near the tentorial fossa; occipital angles becoming glabrous in the western part of its range; eastern center of the continent . . . . . Formica montana
Formica montana casent0102154 head 1.jpg
Formica montana casent0102154 profile 1.jpg
  • Sides of head more pilose; head in full face view, with erect hairs present below the ventral border of eyes; genre normally with more than 2 erect hairs . . . . . 7

7

return to couplet #6

  • Body brownish or blackish brown; density of pubescence normal to feeble; integument slightly shining, particularly on head and gaster; ocular halo distinct, but weakly shining; scapes shorter, scape length minus head length varying from - 0.20 to - 0.03 mm; West and western center of the continent . . . . . Formica canadensis
Formica canadensis casent0005362 head 1.jpg
Formica canadensis casent0005362 profile 1.jpg
  • Body black; pubescence very dense; integument opaque, appearing granulose; without ocular halo; scapes longer, scape length minus head length varying from - 0.03 to + 0.08 mm; Mexico . . . . . Formica pulla

8

return to couplet #3

  • Body concolorous black or brownish black; dorsal margin of petiole low and convex or angulo-convex (fig. 25, 28), median portion entire or weakly emarginate on the largest workers; propodeal angle distinct; South-west of the continent . . . . . Formica occulta
Formica occulta casent0102798 head 1.jpg
Formica occulta casent0102798 profile 1.jpg
  • Body brownish to yellowish, usually appearing bicolored; dorsal margin of petiole and propodeal angle having a different combination of characters . . . . . 9

9

return to couplet #8

  • Dorsal margin of petiole high and angulo-convex, rarely with a median notch; propodeal angle distinct; usually more than 5 erect hairs on promesonotum; Mexico . . . . . Formica propatula
Formica propatula casent0280352 h 1 high.jpg
Formica propatula casent0280352 p 1 high.jpg
  • Dorsal margin of petiole low and convex, usually with a median notch; propodeal angle reduced to an even convexity; without erect hairs on promesonotum (except in Idaho), sometimes 1 to 3 small hairs on pronotum; from Yukon to Texas . . . . . Formica neoclara
Formica neoclara casent0005370 head 1.jpg
Formica neoclara casent0005370 profile 1.jpg

10

return to couplet #1

  • Body concolorous: black, blackish or yellowish brown; if appearing bicolored, alitrunk yellow and very pilose or alitrunk reddish and the upper half of head black . . . . . 11
  • Body bicolored: head, alitrunk and petiole reddish or reddish brown, gaster black or blackish brown; surface opaque and granulose (except in F. gnava and F. foreliana, but then the propodeum is depressed compared to the promesonotum); southern half of the continent; rufibarbis and occidua complexes . . . . . 28

11

return to couplet #10

  • The portion of the gena lying between the eye and the insertion of the mandible covered with coarse, elongate punctures, widely spaced in the posterior half of the gena (fig. 37); the surface between them with a microreticulum composed of coarse lines readily seen under low magnification; on gastric dorsum pilosity very short and pubescence dilute at least on posterior half; metasternal pilosity abundant and surrounding the spinasternal cavity; neorufibarbis complex . . . . . 12
  • Genae without coarse, elongate punctures or if present, these punctures are mostly concentrated in the posterior half of the genae, where they are mixed with fine circular punctures and are closely spaced (fig. 36); the surface between them with a microreticulum composed of delicate lines hardly seen under low magnification; pilosity and pubescence variously combined . . . . . 13

12

return to couplet #11

  • Body pilosity abundant: in particular, erect hairs numerous on ventral surface of head and metafemora, occiput, prosternum, alitrunk, dorsal margin of petiole and gastric dorsum; varying from 8 to 27 (mean = 17) on the first gastric segment not counting the posterior row; sometimes erect hairs absent on the ventral surface of head and metafemora, and dorsal margin of petiole among a few workers of a colony; body brownish black or black . . . . . Formica hewitti
Formica-hewitti-MCZ001H.jpg
Formica-hewitti-MCZ001L.jpg
  • Body pilosity reduced: in particular, erect hairs absent on ventral surface of head, occiput, prosternum, alitrunk and dorsal margin of petiole; in toundra form specimens, 1 to 5 erect hairs may be present on mentioned structures except petiole; gastric dorsum pilosity reduced; erect hairs from 0 to 13 (mean = 2) on the first gastric segment not counting the posterior row; posterior half of head and gaster black, anterior half of head and alitrunk usually reddish . . . . . Formica neorufibarbis
Formica neorufibarbis casent0005371 head 1.jpg
Formica neorufibarbis casent0005371 profile 1.jpg

13

return to couplet #11

  • Body pilosity abundant: in particular, erect hairs present on ventral surface of head and metafemora, occiput, promesonotum, dorsal margin of petiole and propodeum (except in F. subcyanea); West and South of the continent . . . . . 14
  • Body pilosity reduced: in particular, erect hairs absent on propodeum, dorsal margin of petiole and at least on one or two or three of other mentioned structures . . . . . 20

14

return to couplet #13

  • Body concolorous black; surface opaque and finely granulose; erect hairs absent on propodeum, very short and large on gastric dorsum. Mexico . . . . . Formica subcyanea
Formica subcyanea casent0903287 h 1 high.jpg
Formica subcyanea casent0903287 p 1 high.jpg
  • Body yellowish to blackish brown; alitrunk often lighter in color than head and gaster; surface feebly shining and shagreened; erect hairs present on propodeum; on gastric dorsum erect hairs longer and finer; West and South of the continent; lepida complex . . . . . 15

15

return to couplet #14

  • Erect or suberect hairs present on the four faces of femora (fig. 6, 206); petiolar scale seen in profile thick and rounded at summit; California . . . . . Formica lepida
Formica lepida casent0005366 head 1.jpg
Formica lepida casent0005366 profile 1.jpg
  • Erect or suberect hairs limited to ventral and lateral faces of femora (fig. 7); petiolar scale seen in profile thinner and angulate at summit . . . . . 16

16

return to couplet #15

  • Episterna, occipital angles and genre with erect hairs . . . . . 17
  • Episterna, occipital angles and geme without erect hairs . . . . . 19

17

return to couplet #16

  • Dorsal margin of petiole high and narrow, angulate or angulo-convex, entire (fig. 28-29); scapes shorter than head length, scape index 108-118; Mexico . . . . . Formica pachucana
  • Dorsal margin of petiole low and broad, convex; scape length varying . . . . . 18

18

return to couplet #17

  • Head more elongate, cephalic index 82-87; scapes longer than head, scape index 116-130; crest of petiole with a median notch; length of longest pronotal hairs less than the height of frontal triangle; from British Columbia to California . . . . . Formica transmontanis
Formica transmontanis casent0005378 head 1.jpg
Formica transmontanis casent0005378 profile 1.jpg
  • Head less elongate, cephalic index 87-91; scapes shorter than head, scape index 97-109; crest of petiole entire; length of longest pronotal hairs more than the height of frontal triangle; California . . . . . Formica longipilosa
Formica longipilosa casent0005367 h 1 high.jpg
Formica longipilosa casent0005367 p 1 high.jpg

19

return to couplet #16

  • Head rounded and broader or trapezoidal, with convex sides, cephalic index 85-96; scapes shorter (fig. 198), scape length minus head length varying from - 0.14 to - 0.05 mm, scape index 96-117; anterior margin of clypeus convex; crest of petiole usually entire . . . . . Formica aerata
Formica aerata casent0005359 head 1.jpg
Formica aerata casent0005359 profile 1.jpg
  • Head rectangular and narrower, with straight sides, cephalic index 72-88; scapes longer, scape length minus head length varying from - 0.04 to + 0.08 mm, scape index 114-129; anterior margin of clypeus angulate and prominent in the middle; crest of petiole usually with a median notch . . . . . Formica pacifica
Formica pacifica casent0005372 head 1.jpg
Formica pacifica casent0005372 profile 1.jpg

20

return to couplet #13

  • Upper surface of body shagreened, shining at least on some parts of head, and never granulose; punctures of frons coarser, readily seen under low magnification, interspaces large and shining (less evident on F. argentea because the pubescence is very dense); big elongate punctures beneath and behind the eyes (except on F. argentea); propodeum higher, with the basal and declivitous faces distinct and forming in general a well marked angle . . . . . 21
  • Upper surface of body opaque and granulose; punctures of frons finer, difficult to locate under high magnification, interspaces very reduced; without big elongate punctures beneath and behind the eyes; propodeum depressed, without a distinct angle, basal and declivitous faces forming a single even convexity; West of the continent; sibylla complex . . . . . 27

21

return to couplet #20

  • Pilosity abundant on the first gastric tergite: rarely less than 10 erect hairs (mean = 20) exclusive of the posterior row (fig. 250, 290), the distance between the hairs of this row usually inferior or equal to their length; metasternum less pilose, hairs not surrounding the spinasternal cavity and generally restricted behind it; mesothoracic katepisterna of female completely covered with normal or dense pubescence; subsericea complex . . . . . 22
  • Pilosity reduced on the first gastric tergite: rarely more than 10 erect hairs (mean = 4) exclusive of the posterior row (fig. 290), the distance between hairs of this row usually superior to their length; metasternum more pilose, hairs surrounding the spinasternal cavity; mesothoracic katepisterna of female with very dilute pubescence at least in the center; fusca complex . . . . . 25

22

return to couplet #21

  • Pubescence dense to very dense on genre and the first four gastric tergites; big elongate punctures absent on genre, beneath and behind the eyes (fig. 36); pronotum usually with some short erect hairs; gastric dorsum with numerous shorts and somewhat swollen erect hairs (fig. 16c); from 10 to 43 (mean = 23) erect hairs on the first tergite exclusive of

the posterior row (fig. 250); blackish brown to brown . . . . . Formica argentea

Formica argentea casent0005361 head 1.jpg
Formica argentea casent0005361 profile 1.jpg
  • Pubescence dilute to normal on the fourth gastric tergite and on genre at least on the posterior half; big, elongate punctures present on genre, beneath and behind the eyes (fig. 37); black to piceous black . . . . . 23

23

return to couplet #22

  • Scapes not shorter than head length (fig. 289); head more rounded; occipital margin distinctly convex, becoming straight medially on the largest workers; frontal carinae shorter, usually bent mesad at the posterior ends; pubescence normal on the first three gastric tergites; without erect hairs on pronotum, sometimes 1 to 3 very short; in most cases from 10 to 25 (mean = 18) erect hairs on the first gastric tergite exclusive of the posterior row (fig. 250); larger species, alitrunk length 1.80 - 2.95 mm (mean = 2.37 mm); East of the continent . . . . . Formica subsericea
Formica subsericea casent0103942 head 1.jpg
Formica subsericea casent0103942 profile 1.jpg
  • Scapes often shorter than head length (fig. 269, 289); head more trapezoidal, occipital margin straight becoming slightly convex on smaller workers; frontal carinae longer, bent laterad at the posterior ends or straight; pubescence and pilosity differently combined; smaller species, alitrunk length 1.55- 2.60 mm (mean = 2.08 mm) . . . . . 24

24

return to couplet #23

  • Promesonotum normally without erect hairs, exceptionally 1 to 6 on pronotum; gastric pubescence normal on the first and second tergites, rarely dilute on the posterior half of the second, dilute on the others; the first gastric tergite with 8 to 32 (mean = 17) erect hairs exclusive of the posterior row; North-east of the continent . . . . . Formica glacialis
Formica glacialis casent0104762 head 1.jpg
Formica glacialis casent0104762 profile 1.jpg
  • Promesonotum normally with erect hairs, sometimes reduced in number (mean = 6) or abraded on some workers of a colony, sometimes 1 to 2 on the ventral surface of head; gastric pubescence usually moderately dense or normal on the first three tergites, dilute on the others; the first gastric tergite with 15 to 45 (mean = 26) erect hairs exclusive of the posterior row (fig. 290) . . . . . Formica podzolica
Formica podzolica casent0005373 head 1.jpg
Formica podzolica casent0005373 profile 1.jpg

25

return to couplet #21

  • Eyes smaller, ocular index 31-35 (fig. 360); scapes longer than head length; antero-paracarinal setae more than 0.45 X the length of clypeus; head more quadrate, convergence index 12-19 (mean = 16); California . . . . . Formica microphthalma
Formica microphthalma casent0005368 head 1.jpg
Formica microphthalma casent0005368 profile 1.jpg
  • Eyes larger, ocular index 34-43; scapes length varying; antero-paracarinal setae less than 0.42 X the length of clypeus; head more trapezoidal, convergence index 16-28 (mean = 22) . . . . . 26

26

return to couplet #25

  • Scapes longer than head length (fig. 324); anterior margin of clypeus usually angulate; antero-paracarinal setae of clypeus longer than or equal to the height of frontal triangle in the majority of workers; gastric pubescence normal on the first three tergites, sometimes partly dilute on the second and the third; larger species, alitrunk length 1.75 - 2.85 mm; diameter of eye 0.43- 0.54 mm (fig. 360); West of the continent . . . . . Formica accreta
Formica accreta casent0005358 head 1.jpg
Formica accreta casent0005358 profile 1.jpg
  • Scapes often shorter than head length (fig. 324); anterior margin of clypeus broadly convex, rarely angular in the middle; antero-paracarinal setae usually shorter than the height of frontal triangle; gastric pubescence dilute or normal (see VARIATIONS GEOGRAPHIQUES); smaller species, alitrunk length 1.35 - 2.36 mm; diameter of eye 0.35 - 0.46 mm (fig. 360) . . . . . Formica fusca
Formica-fusca-MCZ001H.jpg
Formica-fusca-MCZ001L.jpg

27

return to couplet #20

  • Erect hairs present on the ventral surface of head and prosternum; head rectangular, convergence index less than 16; antero-paracarinal setae of clypeus longer than 1.6 X the height of frontal triangle; eyes smaller, ocular index less than 36; anterior margin of clypeus strongly angulate in the middle, formed by straight lines . . . . . Formica sibylla
Formica sibylla casent0005374 head 1.jpg
Formica sibylla casent0005374 profile 1.jpg
  • Erect hairs absent on the ventral surface of head and prosternum; head trapezoidal, convergence index more than 17; antero-paracarinal setae of clypeus shorter than 1.2 X the height of frontal triangle; eyes larger, ocular index more than 36; anterior margin of clypeus convex or anguloconvex, formed by curved lines . . . . . Formica subelongata
Formica subelongata casent0005376 head 1.jpg
Formica subelongata casent0005376 profile 1.jpg

28

return to couplet #10

  • Surface shining and shagreened or opaque and granulose, microreticulum of type 2 or 3; gastric pubescence dense or normal or partly dilute on the first three tergites, dilute at least in part on the fourth; propodeum high and short; if low and somewhat elongate, then gastric dorsum with dilute pubescence; mesometasternal profile usually compressed and showing an angle (fig. 34); metasternal pilosity surrounding in part the spinasternal cavity; rufibarbis complex . . . . . 29
  • Surface opaque and granulose, microreticulum of type 3; gastric pubescence dense on tergites l to 4; propodeum low and elongate; mesometasternal profile elongate, without angle (fig. 3 5); metasternal pilosity restricted behind the spinasternal cavity; occidua complex . . . . . 32

29

return to couplet #28

  • Surface opaque and granulose with microreticulum of type 3; pubescence normal on genre, dense on gastric tergites l to 3; propodeum high compared to promesothorax. Mexico . . . . . 30
  • Surface shining and shagreened with microreticulum of type 2; pubescence dilute on genre, normal or partly dilute on gastric tergites 1 to 3; prododeum lower compared to promesothorax. Mexico and southern U.S . . . . . 31

30

return to couplet #29

  • Pilosity more abundant: in particular, erect hairs present on ventral surface of head, occiput and alitrunk . . . . . Formica browni
Formica browni casent0280368 h 1 high.jpg
Formica browni casent0280368 p 1 high.jpg
  • Pilosity less abundant: in particular, erect hairs present on ventral surface of head, occiput and alitrunk . . . . . Formica retecta
Formica retecta casent0614913 h 1 high.jpg
Formica retecta casent0614913 p 1 high.jpg

31

return to couplet #29

  • Gastric dorsum with long, fine, flexuous, erect hairs (fig. 16a); length of longest hairs on the first tergite more than 1.20 X the height of frontal triangle; numerous on that tergite, 24 to 43 (mean = 33) hairs exclusive of the posterior row (fig. 441); usually numerous on pronotum, 7 to 30 (mean = 16) hairs . . . . . Formica foreliana
Formica foreliana casent0064830 head 1.jpg
Formica foreliana casent0064830 profile 1.jpg
  • Gastric dorsum with shorter, stiff, erect hairs; length of the longest hairs on the first tergite less than 1.00 X the height of frontal triangle; less numerous on that tergite, 1 to 24 (mean = 12) hairs exclusive of the posterior row (fig. 441); usually few on pronotum, 0 to 16 (mean = 3) hairs . . . . . Formica gnava
Formica gnava casent0005365 head 1.jpg
Formica gnava casent0005365 profile 1.jpg

32

return to couplet #28

  • Scale seen in profile high, thin, angulate at summit; eyes projecting beyond the margin of head only on sma11er workers; scapes shorter, scape length minus head length from 0.02 to 0.25 mm (fig. 466) . . . . . Formica occulta
Formica occulta casent0102798 head 1.jpg
Formica occulta casent0102798 profile 1.jpg
  • Scale seen in profile low, thick, rounded at summit; eyes projecting beyond the margin of head even on the largest workers; scapes longer, scape length minus head length from 0.20 to 0.3 5 mm (fig. 466) . . . . . Formica xerophila
Formica xerophila casent0005379 head 1.jpg
Formica xerophila casent0005379 profile 1.jpg