Key to Myrmecocystus species

The following keys are modified from Snelling 1976 and 1982.

Snelling, R. R. 1976. A revision of the honey ants, genus Myrmecocystus (Hymenoptera: Formicidae). Nat. Hist. Mus. Los Angel. Cty. Sci. Bull. 24: 1-163.

Snelling, R. R. 1982. A revision of the honey ants, genus Myrmecocystus, first supplement (Hymenoptera: Formicidae). Bull. South. Calif. Acad. Sci. 81: 69-86.

The Myrmecocystus species can be separated into three distinct subgenera based on morphology that also, in part, corresponds to their life history and behavior. These short keys, one for each caste, are an important first step in species identification. Following the subgeneric keys are three species keys for each subgenera. These species specific keys are also in three parts for each subgenera with a key for each caste.

More information about each subgenera can be found here: Myrmecocystus subgenus descriptions.

Snelling also provided the following guidance for keying out species: "Worker keys for the subgenera Myrmecocystus and Eremnocystus should present few problems. Workers in the subgenus Endiodioctes will often prove more troublesome and isolated strays may not key at all well, especially if they are minor workers. Ideally, one should have a nest series reflecting a full gamut of size variation; the key to Endiodioctes is constructed assuming that this is the case."

1

 * Integument either: (a) bicolorous, head and thorax ferruginous, gaster black; (b) unicolorous ferruginous or orange; (c) unicolorous blackish or dark brown; mandibles six or seven toothed; eye small, maximum diameter less than to slightly exceeding length of first flagellomere; ocelli present, sometimes obscure in small individuals; diurnal, matinal or crepuscular . . . . . 2


 * Integument light yellow or brownish yellow; mandibles eight to ten toothed; eye large, maximum diameter much exceeding length of first flagellomere; ocelli absent or much reduced; nocturnal . . . . . Myrmecocystus

2

 * Small (HL not exceeding 1.26 mrn), uniformly blackish or dark brown, usually with anterior one-third of head paler; erect hairs very reduced on head and thorax (two exceptions); pubescence sparse on head; usually crepuscular or matinal . . . . . Eremnocystus


 * Larger (HL exceeding 0.83 mm), either bicolorous or ferruginous; usually abundantly hairy, erect hairs numerous on head and thorax, appressed pubescence usually dense on third tergum; diurnal . . . . . Endiodioctes

1

 * Integument either (a) uniformly blackish or dark brown or (b) mainly ferruginous on head and thorax, latter often with black markings, gaster ferruginous or black; mandible seven-toothed . . . . . 2


 * Uniformly yellowish, thorax and gater often with brownish infuscations; mandible eight- or nine-toothed; wing membranes without short, erect hairs, marginal fringe present/absent . . . . . Myrmecocystus

2

 * Integument uniformly blackish or dark brown, anterior one-third of head sometimes lighter; small species, HL not exceeding 1.63 mm; wing membrane with numerous short, erect, white hairs . . . . . Eremnocystus


 * Integument of head and thorax largely ferruginous, gaster blackish (except in M. wheeleri); larger species, HL exceeding 1.43 mrn; wing membrane without short, erect, white hairs . . . . . Endiodioctes

1

 * Integument either (a) uniformly blackish or dark brown or (b) mainly ferruginous on head and thorax, latter often with black markings, gaster ferruginous or black; mandible seven-toothed . . . . . 2


 * Uniformly yellowish, thorax and gaster often with brownish infuscations; mandible eight- or nine-toothed; wing membranes without short, erect hairs, marginal fringe present/absent . . . . . Myrmecocystus

2

 * Integument uniformly blackish or dark brown, anterior one-third of head sometimes lighter; small species, HL not exceeding 1.63 mm; wing membrane with numerous short, erect, white hairs . . . . . Eremnocystus


 * Integument of head and thorax largely ferruginous, gaster blackish (except in M. wheeleri); larger species, HL exceeding 1.43 mm; wing membrane without short, erect, white hairs . . . . . Endiodioctes

1

 * Abundantly hairy species: 20+ erect hairs on malar area in frontal view; scape, femora and tibiae with numerous suberect to erect hairs on all surfaces; first three (usually four) terga with dense pubescence (except Myrmecocystus intonsus); HW of major usually in excess of 1.7 mm . . . . . 2


 * Less hairy species: fewer than 20 erect hairs on malar area in frontal view, usually fewer than 6; scape and femora often sparsely hairy; third tergum often sparsely pubescent; HW of major less than 1.7 mm . . . . . 6

2
return to couplet #1
 * Longest occipital hairs usually shorter than EL; longest pronotal hairs as long as or (usually) shorter than EL; longest discal hairs of second tergum less than EL; head of major often strongly orbiculate . . . . . 3


 * Longest occipital, pronotal and discal hairs of second tergum exceeding EL; head of major not orbiculate . . . . . Myrmecocystus melliger

3
return to couplet #2
 * Third tergum, and usually fourth as well, densely pubescent; pronotal hairs variable, but often more than 0.5 x MOD; HW of major in excess of 1.7 mm; head of major orbiculate or not . . . . . 4


 * Third and fourth terga with scattered pubescence only; pronotal hairs stiff, uniform, less than 0.5 x MOD; HW of major less than 1.7 mm; head of major orbiculate . . . . . Myrmecocystus intonsus

4
return to couplet #3
 * Longest hairs on pronotal dorsum and disc of second tergum no more than 0.5 x MOD, usually less; head of major orbiculate or not . . . . . 5


 * Longest hairs on pronotal dorsum and disc of second tergum at least 0.6 x MOD, usually about 0.75 x MOD; head of major with margins, in frontal view, nearly parallel . . . . . Myrmecocystus mendax

5
return to couplet #4
 * Frontal lobes shiny, closely, sharply punctate, punctures round or weakly ovoid; frons usually with distinct, coarse punctures greater in diameter than hairs arising from them; head of major not, or weakly, orbiculate (Calif., Baja Calif.) . . . . . Myrmecocystus semirufus


 * Frontal lobes dull, irregularly, weakly punctate, punctures often elongate; frons weakly punctate, if at all, punctures, when present not exceeding diameter of their hairs; head of major strongly orbiculate (Tex. To Ariz.) . . . . . Myrmecocystus placodops

6
return to couplet #1
 * Malar area, in frontal view, with six or more erect hairs evenly distributed between eye and base of mandible . . . . . 7


 * Malar area, in frontal view, with not more than four erect hairs, these confined to lower half, near base of mandible . . . . . 11

7
return to couplet #6
 * First three terga uniformly, densely pubescent; clypeus moderately shiny to polished, lightly tessellate to smooth; frontal lobes shiny, sparsely to closely punctate; HW variable . . . . . 8


 * Third tergum apubescent or at least conspicuously less pubescent than first two; clypeus slightly shiny, distinctly tessellate; frontal lobes slightly shiny, distinctly tessellate, with scattered coarse punctures and dense micropunctures; HW of majors 1.03-1.63 mm . . . . . Myrmecocystus intonsus

8
return to couplet #7
 * Tergum IV conspicuously pubescent in specimens with PW in excess of 1.0 mm; erect hairs present over at least half of distance between inner eye margin and antennal socket; punctures of frontal lobes and frons of variable size and irregularly distributed . . . . . 10


 * Tergum IV with pubescence, when present, very widely scattered; erect hairs present only adjacent to inner eye margin, not extending more than 0.25 of distance between eye and antennal socket; punctures of frontal lobes and frons uniformly minute, evenly distributed . . . . . 9

9
return to couplet #8
 * Longest pronotal hairs more than 0.5 x MOD; malar area usually with 12-16 erect hairs; vertex usually conspicuously, though finely, punctate toward sides . . . . . Myrmecocystus romainei


 * Longest pronotal hairs less than 0.5 x MOD; malar area usually with fewer than 12 erect hairs; vertex without punctures at sides . . . . . Myrmecocystus flaviceps

10
return to couplet #8
 * Longest occipital hairs 0.75-0.96 x MOD; longest pronotal hairs at least 0.6 x MOD; petiole, in profile, thickly cuneate, length, at lower margin of spiracle, exceeding height from spiracle to summit . . . . . Myrmecocystus koso


 * Longest occipital hairs 0.59-0.65 x MOD; longest pronotal hairs less than 0.5 x MOD; petiole, in profile, higher than thick, measured as above . . . . . Myrmecocystus semirufus

11
return to couplet #6
 * Longest occipital hairs at least equal to MOD in majors . . . . . 12


 * Longest occipital hairs no more than 0.6 x MOD usually about 0.5 . . . . . 15

12
return to couplet #11
 * Longest pronotal hairs about equal to EL, always exceeding MOD; hind femur with numerous suberect hairs on dorsal surface . . . . . 13


 * Longest pronotal hairs less than MOD; hind femur with only very short subappressed hairs on dorsal surface . . . . . 14

13
return to couplet #12
 * Hairs on dorsal face of hind femur less than half as long as longest hairs on disc of second tergum; CI commonly over 89; head, thorax and legs distinctly brown . . . . . Myrmecocystus depilis


 * Hairs on dorsal face of hind femur at least half as long as longest hairs on disc of second tergum; CI less than 89 in 90% of specimens; head, thorax and legs ferruginous . . . . . Myrmecocystus nequazcatl

14
return to couplet #12
 * Hairs on dorsal face pf hind femur appressed to subappressed; frontal lobes and frons finely, closely and uniformly punctate; mandible septemdentate; abdomen usually wholly orange-ferruginous . . . . . Myrmecocystus wheeleri


 * Hairs on dorsal face of hind femur suberect, conspicuous; frontal lobes finely and very irregularly punctate, with large impunctate areas; mandible octodentate; abdomen usually black, often with basal two or three terga extensively yellowish . . . . . Myrmecocystus kathjuli

15
return to couplet #11
 * Tergum III with little or no pubescence '. . . . . 16


 * Tergum III with dense pubescence . . . . . Myrmecocystus flaviceps

16
return to couplet #15
 * Pubescence very sparse on face; head often subpolished and shiny; side of head of large worker often convex; head, thorax and legs brownish . . . . . Myrmecocystus mimicus


 * Pubescence abundant on face; head usually distinctly tessellate, not notably shiny; side of head straight in frontal view; head, thorax and legs ferruginous (top of head may be slightly brownish) . . . . . Myrmecocystus kennedyi

1

 * Penultimate segment of maxillary palp more than twice wider in basal third as at apex, the remaining segments very broad; gaster ferruginous . . . . . 2


 * Penultimate segment of maxillary palp slender, more or less parallel-sided, never more than 1.5 x wider in basal third than at apex, remaining segments slender; gaster brownish or blackish . . . . . 3

2
return to couplet #1
 * Punctures of frontal lobes separated by a puncture diameter or more; scutellum with median area punctate, interspaces mostly about one to two times a puncture diameter; longest scutal hairs exceeding EL . . . . . Myrmecocystus kathjuli


 * Punctures of frontal lobes uniformly dense, separated by less than a puncture diameter; scutellum, along midline, impunctate or very sparsely punctate, interspaces mostly three to five times a puncture diameter; longest scutal hairs less than EL . . . . . Myrmecocystus wheeleri

3
return to couplet #1
 * Parapsis closely, uniformly punctate; malar area with abundant standing hairs; HW usually in excess of 2.0 mm . . . . . 4


 * Parapsis with punctures usually of two sizes and variably spaced, never uniformly dense; malar area usually with few or no standing hairs; HW usually less than 1.9 mm . . . . . 8

4
return to couplet #3
 * Smaller species, HW less than 1.9 mm; if more, malar area with fewer than 10 erect hairs; disc of mesoscutum finely, sparsely punctate, usually broadly impunctate in center . . . . . 5


 * Larger species, HW more than 2.0 mm; if as little as 1.9, malar area with at least 15 erect hairs; disc of mesoscutum closely, coarsely punctate, often with median impunctate area . . . . . 6

5
return to couplet #4
 * Malar area with fewer than 10 erect hairs, those present usually restricted to area near mandibular base; frons, on either side of midline, convexly raised, impunctate or nearly so . . . . . Myrmecocystus romainei


 * Malar area with more than 10 erect hairs, evenly distributed along entire length; frons not notably raised on either side of midline, uniformly, though sparsely, punctate . . . . . Myrmecocystus depilis

6
return to couplet #4
 * Longest occipital hairs 0.5-1.0 x MOD, usually in excess of 0.7; longest mesoscutal hairs at least 0.5 x MOD, usually more; longest scutellar hairs at least 0.9 x MOD; scutal punctation variable; HW less than 2.3 mm . . . . . 7


 * Longest occipital and scutal hairs 0.6, or less, x MOD; longest scutellar hairs 0.8, or less, x MOD; scutum with large anteromedian impunctate area; HW 2.3 mm . . . . . Myrmecocystus placodops

7
return to couplet #6
 * Mesoscutum uniformly punctate, sometimes with small anteromedian impunctate area . . . . . Myrmecocystus mendax


 * Mesoscutum with median impunctate area extending nearly entire length of disc . . . . . Myrmecocystus melliger

8
return to couplet #3
 * Median area of first two terga very sparsely punctate . . . . . 9


 * Median area of first two terga uniformly, densely micropunctate . . . . . 11

9
return to couplet #8
 * Median area of third and fourth terga nearly impunctate, similar to first two terga; scutellum very sparsely punctate; HW less than 1.9 mm . . . . . 10


 * Median area of third and fourth terga closely and uniformly micropunctate; scutellum abundantly, finely punctate; HW 2.0 mm or more . . . . . Myrmecocystus semirufus

10
return to couplet #9
 * Frontal lobes closely micropunctate and with scattered coarse punctures; hairs along inner eye margin about as long as on scape; inner face of fore femur with numerous erect hairs along entire length, hairs about as long as those of scape . . . . . Myrmecocystus koso


 * Frontal lobes sparsely, coarsely punctate; hairs inconspicuous or absent along inner eye margin; when present, shorter than those of scape; inner face of fore femur with few erect hairs, and these on apical half and much shorter than those of scape . . . . . Myrmecocystus flaviceps

11
return to couplet #8
 * Face, between eye and frontal lobe, with punctures irregularly distributed, with large impunctate areas; vertex usually with conspicuous areas of sparse punctation between ocellus and eye . . . . . 12


 * Face, between eye and frontal lobe, uniformly punctate, interspaces usually no more than twice a puncture diameter; vertex with numerous evenly distributed punctures . . . . . Myrmecocystus kennedyi

12
return to couplet #11
 * Longest hairs of mesoscutum, pleura and second tergum less than MOD, often less than 0.5 x MOD . . . . . Myrmecocystus mimicus


 * Longest hairs of mesoscutum equal to EL, longest of pleura slightly exceeding MOD, longest of second tergum about equal to MOD . . . . . Myrmecocystus nequazcatl

1

 * Lower margin of ventral lobe of aedeagus strongly convex in profile, teeth small . . . . . 2


 * Lower margin of ventral lobe of aedeagus concave or straight in profile, teeth coarse (Fig. 195) . . . . . Myrmecocystus wheeleri

2
return to couplet #1
 * Disc of second tergum moderately to slightly shiny, uniformly micropunctate and pubescent, first and third terga usually similar; mesoscutum dull, densely, uniformly tessellate . . . . . 3


 * Discs of first two or three terga polished, impunctate and apubescent, at least in middle; mesoscutum wholly, or in part, polished and shiny . . . . . 5

3
return to couplet #2
 * Posterior margin of hind wing with fringe hairs along basal half, at least; if fewer, longest occipital hairs less than 0.5 x MOD; HW often less than 0.90 mm . . . . . 4


 * Posterior margin of hind wing without fringe hairs; longest occipital hairs at least 0.60, usually 0.75 or more, x MOD; HW in excess of 0.90 mm . . . . . Myrmecocystus melliger, . . . . . Myrmecocystus mendax, . . . . . Myrmecocystus placodops

4
return to couplet #3
 * Longest occipital and mesoscutal hairs exceeding MOD; frontal lobes shiny; side of dorsum of first tergum smooth, shiny, with scattered coarse punctures; HW 0.80-0.87 mm . . . . . Myrmecocystus kathjuli


 * Longest occipital and mesoscutal hairs less than MOD, usually less than 0.5 x MOD; entire face dull, closely tessellate; dorsum of first tergum uniformly, closely micropunctate; HW 0.83-0.97 mm . . . . . Myrmecocystus semirufus

5
return to couplet #2
 * Posterior margin of hind wing without fringe hairs . . . . . 6


 * Posterior margin of hind wing with fringe hairs . . . . . 8

6
return to couplet #5
 * Longest occipital hairs not less than 0.75 x MOD; second and third terga with or without conspicuous pubescence laterad . . . . . 7


 * Longest occipital hairs not exceeding 0.50 x MOD; second and third terga at sides apubescent or very sparsely pubescent . . . . . Myrmecocystus kennedyi

7
return to couplet #6
 * Longest occipital and mesoscutal hairs about equal to MOD; first three terga with little or no pubescence laterad . . . . . Myrmecocystus nequazcatl


 * Longest occipital and mesoscutal hairs no more than 0.75 x MOD; first three terga often conspicuously pubescent laterad (especially Mexican populations) . . . . . Myrmecocystus depilis

8
return to couplet #5
 * Longest occipital and mesoscutal hairs no more than 0.5 x MOD . . . . . 9


 * Longest occipital and mesoscutal hairs at least 0.75 x MOD, often equal to MOD . . . . . Myrmecocystus koso

9
return to couplet #8
 * First three terga without conspicuous appressed pubescence or with apubescent median areas of first and second segments larger than lateral, pubescent areas . . . . . Myrmecocystus mimicus, . . . . . Myrmecocystus romainei


 * First tergum largely apubescent, second and third with numerous appressed hairs over a larger area than median, apubescent area . . . . . Myrmecocystus flaviceps

1

 * Antennal scape and dorsum of propodeum without fully erect hairs . . . . . 2


 * Antennal scape, usually, and dorsum of propodeum, always, with some fully erect hairs (propodeal hairs may be short and inconspicuous in Myrmecocystus perimeces; this species has an unusually elongate head, CI 70-81) . . . . . 3

2
return to couplet #1
 * Pronotum and mesonotum with at least eight conspicuous, fully erect white hairs; petiolar scale strongly compressed in profile, crest distinctly notched . . . . . Myrmecocystus lugubris


 * Pronotum and mesonotum each with no more than two fully erect hairs; petiolar scale not notably compressed, crest slightly concave or flat, but not conspicuously notched . . . . . Myrmecocystus creightoni

3
return to couplet #1
 * Scape and/or tibiae, usually both, with conspicuous fully erect hairs; or, head moderately broad, CI 89 or more . . . . . 4


 * Scape and tibia without erect hairs and head unusually elongate, CI 70-81 . . . . . Myrmecocystus perimeces

4
return to couplet #3
 * Scape with conspicuous erect or semierect hairs; femora and tibiae with abundant erect hairs, some present on dorsal femoral surfaces; petiolar scale variable, but often compressed with summit distinctly notched . . . . . 5


 * Scape with no erect hairs, except at apex; femora and tibiae with few erect hairs, none on dorsal femoral surface; petiolar scale thick in profile, crest flat or slightly convex . . . . . Myrmecocystus yuma

5
return to couplet #4
 * Scape with scattered erect and numerous suberect to subdecumbent hairs; malar area with fewer than 6 erect hairs in frontal view; if more, pubescence of second tergum much less dense than that of first . . . . . 6


 * Scape with numerous fully erect hairs; malar area with 10+ erect hairs in frontal view and second tergum fully as densely pubescent as first . . . . . Myrmecocystus hammettensis

6
return to couplet #5
 * Pubescence of second tergum as dense as that of first . . . . . 7


 * Pubescence of second tergum much sparser than that of first . . . . . Myrmecocystus arenarius

7
return to couplet #6
 * Appressed pubescence abundant on frons and third tergum; petiolar scale thick, at level of spiracle distinctly more than twice the length of the spiracle, crest flat or convex . . . . . Myrmecocystus colei


 * Appressed pubescence sparse on frons and third tergum; petiolar scale thin, at spiracle less than twice the length of spiracle, crest conspicuously notched . . . . . Myrmecocystus tenuinodis

l

 * Mesoscutum, finely, densely and uniformly punctate . . . . . 2


 * Mesoscutal punctation not uniform, punctures often of two sizes, with a large median impunctate area or with widely scattered punctures or with impunctate disc and median longitudinal punctate zone . . . . . 3

2
return to couplet #1
 * Punctures subcontiguous on side of clypeus, very coarse, distinctly coarser than those of upper half of mesopleuron; occiput distinctly shiny, with scattered fine punctures; mid and hind femora without fully erect hairs except a few along lower side . . . . . Myrmecocystus creightoni


 * Punctures on side of clypeus distinctly separated, no coarser than on upper half of mesopleuron; occiput slightly shiny, densely shagreened, with fine obscure punctures; mid and hind femora with conspicuous fully erect hairs on all surfaces . . . . . Myrmecocystus colei

3
return to couplet #1
 * Fourth segment of maxillary palp elongate-clavate, conspicuously broader at apex than at base; parapsis sparsely punctate, punctures irregular in size and spacing, but mostly separated by two or more puncture diameters . . . . . 4


 * Fourth segment of maxillary palp not clavate, about as broad in basal third as in apical third; parapsis with punctures uniform in size and spacing . . . . . 6

4
return to couplet #3
 * Summit of first gastric tergum and much of discs of second and third shiny and polished, very nearly impunctate; scutellum with extensive nearly impunctate areas on either side of middle . . . . . 5


 * Summit of first gastric tergum and entire discs of second and third terga shiny between fine and evenly spaced piligerous punctures; scutellum with sparse, evenly spaced minute punctures . . . . . Myrmecocystus tenuinodis

5
return to couplet #4
 * Fifth segment of maxillary palp uniformly broad, not conspicuously narrowed over apical one-half; forewing with numerous fringe hairs along costal margin . . . . . Myrmecocystus hammettensis


 * Fifth segment of maxillary palp conspicuously narrowed over apical one-half (Fig. 4); forewing without fringe hairs on costal margin . . . . . Myrmecocystus arenarius

6
return to couplet #3
 * Face with abundant fine punctures between eyes and frontal lobes; clypeus shiny, polished; appressed gastric pubescence whitish; mesopleural hairs about as long as those of mesoscutum . . . . . Myrmecocystus yuma


 * Face sparsely punctate between eye and frontal lobe; clypeus dull, densely sculptured; appressed gastric pubescence brownish; mesopleural hairs conspicuously longer than those of mesoscutum, little shorter than those of scutellum . . . . . Myrmecocystus perimeces

1

 * Dorsal surface of propodeum strongly, angularly projected upward over posterior two-thirds; erect hairs very sparse, with few or none on outer face of hind tibia; upper eye margin little below upper margin of head . . . . . 2


 * Dorsal surface of propodeum either flat or evenly convex; body often abundantly hairy; upper eye margin usually well below upper margin of head . . . . . 3

2
return to couplet #1
 * With at least two erect pronotal hairs as long as apical breadth of scape; first tergum with a few erect hairs on disc; hind tibia with a few erect hairs on outer face beyond basal third . . . . . Myrmecocystus ewarti


 * Erect pronotal hairs, when present, shorter than apical breadth of scape; first tergum without erect discal hairs; hind tibia without erect hairs on outer face beyond basal third . . . . . Myrmecocystus pyramicus

3
return to couplet #1
 * Fully erect hairs conspicuous on thoracic dorsum, discs of first and second terga and on scape and tibiae . . . . . 4


 * No erect hairs on thoracic dorsum, discs of first and second terga, scape or tibiae . . . . . Myrmecocystus christineae

4
return to couplet #3
 * Head, pronotum and gaster with abundant appressed pubescence; mid and hind tibiae usually with numerous erect hairs along apical half of outer face; upper eye margin often distinctly below occipital comer; if metanotal suture impressed, HL exceeds 1.3 mm . . . . . 5


 * Head, pronotum and gaster shiny, with little or no appressed pubescence; mid and hind tibiae with not more than 3 or 4 erect hairs beyond basal third of outer face, usually none; upper eye margin coincident with occipital comer; metanotal suture deeply impressed and dorsal face of propodeum convex . . . . . Myrmecocystus navajo

5
return to couplet #4
 * Large, highly polymorphic species, HL 1.0-2.0 mm or more, usually in excess of 1.3 mm; metanotal suture usually impressed and propodeum as long as high or longer, juncture of dorsal and posterior faces broadly rounded . . . . . 6


 * Smaller, moderately polymorphic species, HL 0.8-1.4 mm; metanotal suture not impressed; propodeum higher than long, juncture of dorsal and posterior faces abruptly rounded, often subangulate . . . . . Myrmecocystus testaceus

6
return to couplet #5
 * Eye with numerous erect hairs which are longer than diameter of ocular facets; mandible with 8 or 9 teeth; color uniformly brownish, gaster a little darker . . . . . Myrmecocystus melanoticus


 * Eye with erect hairs, when present, very diffuse, length less than diameter of ocular facets; mandible with 9 or 10 teeth; color usually distinctly yellow, but may be extensively brownish in southern populations . . . . . Myrmecocystus mexicanus

1

 * Fifth segment of maxillary palp broadest in middle, narrowed basally and apically; hind femur without erect hairs on dorsal surface; hind tibia with hairs decumbent, never fully erect . . . . . 2


 * Fifth segment of maxillary palp broadest well basad of middle, gradually narrowed toward apex or parallel sided; hind femur with erect hairs on dorsal surface; hind tibia with abundant fully erect and suberect hairs . . . . . 5

2
return to couplet #1
 * Antennal scape with decumbent or appressed hairs only except a few erect hairs at tip; ocelli normal, OD less than 0.12 mm . . . . . 3


 * Antennal scape with conspicuous coarse erect and suberect hairs; ocelli large, OD greater than 0.15 mm . . . . . Myrmecocystus navajo

3
return to couplet #2
 * Fourth segment of maxillary palp gradually widened from base to apex; fine punctures of parapsis abundant and uniformly spaced (Figs. 17, 18); side of frontal lobe shiny between close, fine punctures . . . . . 4


 * Fourth segment of maxillary palp distinctly clavate (Fig. 9); fine punctures of parapsis very irregularly spaced (Fig. 16); side of frontal lobe dull and roughened between fine punctures which are at least partially obscured . . . . . Myrmecocystus christineae

4
return to couplet #3
 * Fine punctures of parapsis conspicuously coarser than those of disc of mesoscutum (Fig. 17); first tergum with fewer than 6 erect hairs on disc, often none; interocellar distance about equal to ocellocular distance . . . . . Myrmecocystus pyramicus


 * Fine punctures of parapsis not obviously coarser than those of disc of mesoscutum (Fig. 18); first tergum with at least 12 erect to suberect hairs on disc; interocellar distance less than ocellocular distance . . . . . Myrmecocystus ewarti

5
return to couplet #1
 * Thorax and gaster brown, legs usually yellowish; HW usually less than 1.85 mm; penultimate segment of maxillary palp slender, parallel-sided . . . . . Myrmecocystus testaceus


 * Thorax and gaster light brownish yellow to yellow, legs concolorous with thorax; HW 1.90 mm or more; usually more than 2.0 mm; penultimate segment of maxillary palp distinctly broadest basad . . . . . Myrmecocystus mexicanus