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Introduction to the Genus

Ants in the genus Camponotus are collectively known as carpenter ants because some species nest in wood, including man-made structures. This genus includes some of the largest and most common ants in the world, and they are found in all biogeographical regions (Bolton, 1995).  More than 900 species of Camponotus are known worldwide, with 50 species reported from the United States (Hanson and Klotz, 2005), and 17 species found east of the Mississippi River (Deyrup, 2003; Smith, 1979). 

Species in this genus are variable in size with workers ranging in size from 3 to 15 mm or more in length and queens (also referred to as females) of some species attaining a length of 19 mm or more. Many species are polymorphic. Workers have a 12-segmented antenna that lacks an apical club. Antennal fossae do not touch the posterior border of the clypeus. Ocelli are not present on the heads of workers. The workers of most species have an indistinct metanotal suture between the promesonotum and the propodeum, although this suture is present in C. sexguttatus.

Key to Camponotus species in the southeastern US

(from Creighton 1950, Snelling 1988; Mark Deyrup, pers. comm., William MacKay-Camponotus website)
1 The scapes and legs with long, coarse, brown or golden, erect hairs on all surfaces
...2
  Scapes and legs without long erect hairs
...4
2(1)  Larger size, total length of major about 10 mm, TL of minors about 7 mm, TL of queen about 14 mm; mesosoma arched over entire length
....3 (subgenus Myrmothrix)
  Smaller, TL of major about 5 mm, TL of minor about 4 mm, TL of queen about 8 mm; mesosoma deeply impressed at metanotal suture
...sexguttatus (Fabricius) (subgenus Myrmosphincta)
3(1)  Head and mesosoma reddish colored with gaster dark brownish to blackish colored (found on the coast)
 

Head brownish red and the rest of the body a concolorous golden brown (Introduced ant, probably not established. I am not exactly sure what is going on with this group, there seems to be some confusion, but for now I'm going with this)
4(1)   The thorax stout, no longer than the head excluding mandible; the pronotal humeral angles are well marked
  The thorax longer and much longer than head; humeral angles are rounded
...5
5(4)  The anterior margin of the clypeus feebly projecting and depressed in the middle with a narrow, median notch, behind which is a short, triangular impression; the total length of the major does not exceed 8 mm
...6 (subgenus Myrmentoma)
  The anterior margin of the clypeus not projecting and usually without notch medially, but if there is a notch present then there is no triangular impression behind it; the length of the major rarely less than 8 mm and usually much more
....11
6(5)  The malar area with conspicuous suberect to erect hairs that arise from coarse and somewhat elongate foveae
...7
  Malar area without suberect or erect hairs (sometimes a few near the base of mandibles) and malar area punctate, but without elongate foveae
...9
7(6) Clypeus with usually less than 12 long, erect hairs which are found along and adjacent to margins of clypeus (none of these hairs as short as those found on malar area) with few on clypeal disc (0-3); the gaster usually with transverse light and dark brown stripes (in woods under rotting logs, etc.)
...subbarbatus Emery
  The clypeus with more than 20 (usually) erect hairs, many of these (especially those found in mid-clypeal region) as long as erect hairs on malar area; gaster usually colored dark brown to black
...8
8(7) Erect hairs on clypeus of varying lengths with the shortest hairs being about the same length as those on the malar region; color light to dark brown
  Erect hairs on the clypeus of two distinct lengths, long and short, with the short hairs being shorter than those on the malar region; the head, mesosoma, petiole, and appendages red, and the gaster blackish
Note:  Some consider C. caryae and C. discolor to be a single species, C. caryae. 
9(6) Color either completely black or brownish-black, or head, mesosoma, petiole, and first to second gastral tergites yellowish to yellowish-red with the rest of the gaster blackish
...10
  Head, mesosoma, and petiole reddish colored, gaster black
10(9) The head and mesosoma dark brownish-black to black
  The head, mesosoma, appendages, and the first 1 or 2 gastral tergites yellowish to yellowish-red
Note:  Some taxonomists consider C. decipiens, nearcticus, and snellingi to be a single species, C. nearcticus.
11(5) The occipital corners (posterior edge of head) with numerous erect hairs; gaster opaque or shining
...12
  Occipital corners lacking or with few erect hairs; gaster usually shining (at least feebly) to opaque
...13
12(11) Color reddish-brown with gaster sometimes darker and usually with transverse dark bands; gaster completely opaque
...socius Roger (subgenus Tanaemyrmex)
  Bicolored with head and mesosoma orangish-brown and gaster blackish-brown; gaster shining
...tortuganus Emery (subgenus Tanaemyrmex)
13(12) mesosoma black or with some reddish color, gaster either entirely black or black with some reddish color near base where it joins with the petiole; gaster with dense pubescence, not shining in appearance
...14
  Both mesosoma and gaster uniformly orangish-brown or yellowish-brown colored; gaster with sparse pubescence, shining
...15
14(13) Body usually black, but occasionally some reddish color at base of legs or legs may be mostly reddish-brown; the short appressed hairs (pubescence) on the gaster are less numerous and barely overlap the posterior edges of the gastral tergites, usually much less than a third of the total hair length extends past the edge, hairs more silvery-white in color (common)
...pennsylvanicus (DeGeer) (subgenus Camponotus)
  The posterior part of propodeum, the petiole, the base of gaster, and legs usually reddish, the rest of the body black (color is variable, sometimes there is less red present); the appressed hairs (pubescence) found on gaster are more numerous and long, they overlap the posterior edges of the gastral tergites (best seen in dorsal view) by at least half of their lengths and are golden in color (common)
...chromaiodes Bolton (subgenus Camponotus )
15(13) Color a uniform oranish-red, occasionally the head is a darker shade than the rest of body, but not black; cheeks with few or no erect hairs (if present then usually restricted to the area just above the mandible)
...castaneus (Latreille) (subgenus Tanaemyrmex)
  Head black to brownish-black, mesosoma and gaster yellow-brown; cheeks with or without erect hairs
...16
16(15) Cheeks with numerous erect hairs (especially in majors); scape extends beyond pastor edge of head by more than diameter of scape, at least the distance of the first funicular segment or more; scape not flattened at base; head of major longer than wide; dorsum of mesosoma often with brown infuscation
...americanus Mayr (subgenus Camponotus)
  Cheeks lacking or with few erect hairs; scape flattened at base and often lobe-like; scape barely reaching posterior edge of head or only by the length of less than one funicular segment (majors); head of major wider than long; mesosoma dorsum usually without infuscation (AR, LA)
....sansabeanus (Buckley) (subgenus Tanaemyrmex)

Literature Cited

Bolton, B. 1995. A new general catalogue of the ants of the world. Harvard University Press: Cambridge, Massachusetts, USA.

Creighton, W.S. 1950.  The Ants of North America. Bulletin: Museum of Comparative Zoology 104:  1-585. 

Hansen, L. D., and J. H. Klotz. 2005.  Carpenter ants of the United States and Canada.  Cornell University Press.  Ithaca, N. Y.  i-xii+204 pp.

Deyrup, M. 2003. An updated list of Florida ants (Hymenoptera: Formicidae). Florida Entomologist 86: 43-48. 

Smith, D. R. 1979.  In Catalog of Hymenoptera in America north of Mexico. Smithsonian Institution Press, Washington D. C.  Vol. 2, pp. 1323-1427. 

Snelling, R. R. 1988.  Taxonomic notes on Nearctic species of Camponotus, Subgenus Myrmentoma(Hymenoptera:  Formicidae).  pp. 55-78 in J.C. Trager (ed). Advances in Myrmecology. E.J. Brill: New York, New York, USA.