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Key to Solenopsis species in the southeastern United States

(from Creighton 1950, Trager 1991, Thompson 1989, Pacheco 2007, and William MacKay web site at: http://www.utep.edu/leb/antgenera.htm)
1  The second and usually the third segment of the funiculus at least one and one-half times as long as broad
...2
 

The second and third segments of the funiculus usually broader than long, at most only very slightly longer than broad
...5
Couplets 2-5 are for major workers
2(1)   In full face view, the clypeus lacks a median tooth with at most a small blunt protuberance
...3
  In full face view, the clypeus has a conspicuous median tooth
...4
3(2)  The sides of the head sub parallel; the emargination of the posterior border on the head deep and extending toward the frons as a median rugose furrow; a distinct carinae present on propodeum originating near the junction of the propodeal dorsum and the declivity and extending forward toward the anterior edge of the propodeum; subpetiolar process small and rarely flange-like
  The sides of the head divergent toward the occiput; the occipital furrow shallower and not rugose; propodeum lacking carinae or only developed at junction of propodeal dorsum and declivity (rarely a lobe- or tooth-like flange present here); subpetiolar process developed as a flange or lobe
4a(2)  Head and scapes brownish black, as dark as gaster or only slightly lighter; elongate triangular mark on frons barely to not at all visible; a yellowish spot present on first tergite of gaster and spot usually with a definite posterior border; head is sub elliptical to weakly ovate and relatively narrow in full face view; the dorsum of the pronotum is medially concave; pronotum has humeral bosses (north central to north MS)
4b.  

Head, scapes, and thorax reddish brown, distinctly lighter than gaster; the elongate triangular mark on frons is conspicuous and dark brown to black in color; the first gastral tergite either lacking a spot, or if present, it is dusky reddish and grades into a darker posterior band; head ovate to weakly cordate and broader in full face view; humeral bosses are lacking or indistinct (southern to central MS)
4c.         Intermediate between some of the characters of 4a and 4b; usually the basic color looks like a washed out S. richteri, but with the head and thorax more brownish or mottled than gaster; the gastral "spot" is dusky with the posterior margin indistinct; elongate triangular spot on frons visible and head shape is ovate to weakly cordate (central to northeast MS, these hybrids are hard to identify definitively without cuticular hydrocarbon methods)
5(1)  Postpetiole greatly dilated, more than half as wide as the gaster; the propodeum is finely and densely sculptured (coastal, on beaches)
  Postpetiole is not dilated and barely more than one-third as wide as the gaster; propodeum smooth and without sculpture
...6
The following species are all minute and mostly very difficult to identify. Solenopsis carolinensis, S. molesta, and S. texana are very similar to one another. Identification is somewhat simplified if queens are present, as there appears to be greater differences between queens than workers. As more information becomes available, the keys will be updated. A high power stereoscope (at least 100X) with a micrometer is needed to identify members of this group. For now, good luck, you will need it!
6(5)

Node of petiole is placed anterior to the petiolar-postpetiolar juncture giving the petiole a distinct slender posterior peduncle; color dark brown with a lighter reddish brown head and/or thorax to black; metanotal groove deeply impressed distinctly setting off the promesonotum from the propodeum; arboreal species (found in twigs). Queens dark brown; alitrunk slender; petiole robust, somewhat widened (profile view); small eyes

  Node of petiole placed near the petiolar-postpetiolar juncture (in S. molesta the node is slightly anterior to the juncture, but it does not have quite so slender a peduncle and the color is yellow); color of body usually pale yellow, but if dark, then appendages pale; metanotal groove not usually so impressed; not arboreal. Queen-color variable, if dark brown, then alitrunk more robust
....7
7(6)   Large species, reddish brown in color (HL 0.56-0.58 mm; HW 0.50-0.52 mm; WL 0.68-0.70 mm (considered by MacKay to be a synonym of S. validiscula Emery, a western species.). Queen-dark brown (?); alitrunk less narrowed, more robust [according to Pacheco, 2007, this species was originally misidentified and is now a synonym of S. carolinensis)
  Size distinctly smaller, color yellow or  darker (HL and HW is 0.48 mm or less; WL 0.32-0.68 mm)
...8
8(7) 

Eyes extremely small, nearly absent and with very little or no obvious pigmentation (much of the eye the same color as the rest of the head); head elongate and with coarse punctures; in frontal view, the head with a median strip free of punctures and hairs and forming a weakly impressed longitudinal furrow; in dorsal view, the postpetiole is circular in shape

....9
  Eyes small, but obviously visible, black and with several ommatidia; head not overly elongate, punctures either coarse or not; head without median strip free of punctures and hairs; postpetiole usually oval in shape (except in S. pergandei)
..11
9(8)  Erect hairs on pronotum of various lengths and not as dense as on the head;  erect hairs on gaster somewhat sparse and of various lengths; head narrow, elongate, and flat; very small species (the head and thorax length 0.61-0.69 mm). Queen blackish; large, TL 3.5 - 5.0mm; dark wings; petiole robust, somewhat widened (profile view). Male dark brown; TL about 3.0 mm; dark, dusky brown wings.
  Pronotum and gaster with erect hairs all of approximately the same length;  medium sized species (the head and thorax length 0.76-0.83 mm); densely covered with hairs
...10
10(9)  Head narrow from dorsal to ventral areas, as seen in profile view
  Head convex ventrally; a medium sized species (the head and thorax length 0.76-0.83 mm); densely covered with hairs. Queen blackish; large, TL>5.0mm; hyaline wings. Male dark reddish brown; wings hyaline.
11(8)   Length of segments 3-8 of the funiculus longer than the distance between the frontal carinae (length greater than 0.1 mm and usually greater than 0.12 mm); subpetiolar tooth obvious; females usually more than 4.0 mm in total length
...12
  Length of segments 3-8 of the funiculus shorter or about equal to the distance between the frontal carinae (length less than 0.1 mm, rarely longer); subpetiolar tooth variable or lacking; females usually less than 4.0 mm in total length
...13
The following species in the molesta group, with the exception of S. pergandei, are often identified as S. molesta group, especially when queens are not in the series. This is the safest course, especially if one does not have identified reference material to compare specimens with.
12(11)   Punctures on the head are small and not much greater in diameter than the hairs which arise from them; postpetiole, as seen from above, somewhat oval shaped; color clear golden yellow to light brownish yellow; queens yellowish-orange, head and gaster slightly darker brownish orange, shiny in appearance. Queens-yellow; large (TL about 4-5.5 mm); eyes large (EL about 0.24 mm). Males-dark grayish brown to blackish with pale antennae and legs; 3.48-4.32 mm in total length. (This species looks a lot like S. carolinensis, which differs by having the shorter length of funicular segments 3-8, though this is not always obvious; by usually having erect hairs on hind tibia; and by the queens being more consistently yellow over the entire body).
  Punctures on the head are coarse and larger than the diameter of hairs which arise from them; postpetiole, as seen from above, circular in shape; color pale yellow to milky white.  Queens yellowish-orange, opaque in appearance; much larger (TL greater than 5 mm). Males yellowish-orange, opaque, large (approximately 4 mm in TL).
13(11) Worker mostly brown (may be bicolored with gaster dark brown)
...14
  Worker color mostly yellow
...15
14(13) Light to medium brown color, legs concolorous with alitrunk; head of worker relatively short (CI 82- - 91); lateral clypeal teeth poorly developed and reduced to angles, not tooth-like; eye elongate. Queen-yellowish; hyaline wings; large, HL 0.66 mm, WL 1.32 mm, MEL 0.18 mm; alitrunk narrowed; eyes relatively small
  Head, alitrunk and gaster dark brown and appendages (especially the legs) pale yellow; head relatively elongate (CI 77 - 80); lateral clypeal teeth moderately well developed; eye not elongate (?). Queen-dark brown; smaller, HL 0.44, WL 0.70, MEL 0.13; alitrunk more robust; eyes small
15(13) Scape with several erect and suberect hairs, most over 0.03 mm and up to 0.05 mm in length; mid and hind tibiae often with numerous erect and suberect hairs; queen mostly yellow or medium brown. Queen-large eyes, about half the length of head.
...16
  Scape with few or no erect hairs, most hairs appressed and less than 0.03 mm in length; mid and hind tibia with few or no erect hairs, most hairs appressed. Queen-small, medium to dark brown; alitrunk robust; relatively small eyes (EL 0.174 - 0.180), much less than half the length of head; wings hyaline. Male- small, medium to dark brown; wings hyaline.
16(15) Most of the hairs on the posterior tibia are suberect or erect; color usually yellow. Queen-yellow; alitrunk somewhat narrowed; large eyes, about half the length of head (about 0.25 mm in diameter); 3.5-4.20 mm TL; EL about 0.24 mm. Male-bicolored with head and gaster brown and alitrunk, waist, and antennae yellow; 2.88- 3.0 mm in total length.
  Most of the hairs on the posterior tibia are appressed; color yellow to brownish. Queen with a small eye (about 0.18 mm in diameter) or if larger, than color brown.
...17
17(16)   Punctures on head fine, not much larger than the diameter of the hairs arising from them; clypeal teeth straight, thick, and blunt. Queen-light to medium brown with head and gaster usually slightly darker than rest of body, which may be yellowish to entirely light to medium brown; small, less than 3.5 mm in TL; EL 0.17-0.20 mm; extralateral teeth of queen poorly developed. Male: head brown, alitrunk and petiole yellowish to brownish, postpetiole and gaster brownish, and appendages pale; less than 3.0 mm in total length.
  Punctures on head moderately coarse, much larger than the hairs arising from them; clypeal teeth bent inward. Queen medium brown, head darker; larger, total length usually more than 4.5 mm; EL about 0.24 mm. [Male unknown].