WORKER- BASED KEY TO SPECIES OF CREMATOGASTER IN AMERICA NORTH OF MEXICO

This key is broadly based on Buren (1968), but updated to incorporate recent discoveries and more quantitative measures of species differences. The user is cautioned that the species-level taxonomy of these ants has not been fully resolved, especially among the complex of species related to Crematogaster californica and C. depilis, and among taxa found in Mexico (the latter not treated here). In some couplets we have been forced to characterize species based on several features, no single one of which is diagnostic by itself. The second lug of such couplets is often of the form ‘without this combination of characters’, followed by a series of conditional statements. This makes the key somewhat cumbersome but we can find no other way to accurately account for the variation observed within and among species.

Features of pilosity assume importance in many couplets, and they will be difficult or impossible to assess accurately in specimens that are badly worn or covered in glue. Use of soft (fluorescent) light and an effective specimen manipulator that permits viewing from all angles and with varying degrees of backlighting will greatly assist in discerning differences in pilosity and sculpture. Accurate linear measurements are also critical for identification of many of these species. Differences among species are sometimes less pronounced in the smallest workers, so if the choice exists, it is advisable to use the larger workers of a series for identification.

To assist the user we provide images of some of the more salient morphological features that are cited in the key (Figs 3–12), as well as standard views of the worker caste of each species in the C. scutellaris group (Figs 13–39). However, images of single specimens do not provide information about the variability of each species. Linear measurements, ratios of measurements and setal counts more accurately capture such variation. Bivariate plots of these measurements can sometimes be helpful for distinguishing species (Figs 40–47).

Following Buren (1968), the nominate subgenus of Crematogaster, represented in the Nearctic region by the Crematogaster scutellaris group, is divided by geography into those species occurring in eastern and western North America, here defined arbitrarily as east and west of 102°W longitude. Only two species ( C. cerasi and C. punctulata) occur in both regions. The last portion of the key (couplets 31–35) covers the six species of Crematogaster (Orthocrema) occurring in the United States and is based on Longino (2003) and Shattuck & Cover (2016). Two other Neotropical species of the subgenus Orthocrema, C. sotobosque Longino, 2003 and C. curvispinosa Mayr, 1862, have been reported from the United States (Morgan & Mackay, 2017), but we do not consider these records credible.

1. Petiole as wide as, or wider than, long and broadened anteriorly (Fig. 3A); postpetiole bilobed, wider than long and with distinct median sulcus; Crematogaster ( Crematogaster) .........................................................2

- Petiole rectangular or ovorectangular, longer than wide and not widened anteriorly (Fig. 3B); postpetiole subglobular and lacking a distinct median sulcus, except in one species ( C. corvina); Crematogaster (Orthocrema) ....................................................................................................................................................31

2. Species occurring in the western United States, west of 102°W .................................................................... 3

- Species occurring east of 102°W in the United States and Canada .............................................................22

3. Mesosoma dorsum densely reticulate-foveolate and opaque, with some overlying rugulae in larger workers; declivitous face of propodeum completely covered with reticulate-foveolate sculpture (Fig. 4A); seta-bearing posterolateral extremity of petiole in form of small acute tooth; standing pilosity short and stout, relatively sparse on mesosoma (MSC 4–12), but abundant on gaster (A4SC 48–80) (Fig. 13A); relatively small species (HW 0.73–1.02), with broad petiole (PTW /HW 0.37–0.41) (Fig. 13C); Arizona, New Mexico and Mexico ........................................................................................................................................................ C. dentinodis

- Mesosoma sculpture variable, but declivitous face of propodeum usually weakly reticulate and sublucid, or smooth and shining (Fig. 4B); seta-bearing posterolateral extremity of petiole usually blunter and not in the form of a small acute tooth; standing pilosity almost never in the combination of sparse on the mesosoma and abundant on the gaster, but if this condition applies (see couplet 4 below) then larger, on average (HW 0.96–1.27), with narrow petiole (PTW /HW 0.32–0.34) ............................................................. 4

4. Mesosoma predominantly reticulate-foveolate and opaque, with overlying rugulae (Fig. 14C); promesonotum with short, standing hairs that grade into, and are sometimes difficult to distinguish from, uplifted pubescence (MSC 6–24); gaster with abundant, short, standing pilosity (A4SC 38–70) (Fig. 14A); larger species (HW 0.96–1.27) with moderately long scapes (SI 0.74–0.83, SL/HL 0.82–0.92) and propodeal spines (SPL/HW 0.20–0.23) and narrow postpetiole (PPW /HW 0.27–0.30) (Fig. 14C); southern Arizona, Sonora .............................................................................................................................................................. C. vetusta

- Sculpture and pilosity variable but not in the above combination; if standing pilosity abundant on gaster (A4SC> 35) then species smaller, on average (HW 0.71–1.10), and either with more conspicuous mesosomal pilosity (MSC 16–30) and broader postpetiole (PPW /HW 0.29–0.35) ( C. navajoa, C. punctulata) or mesosoma with reticulate-striate sculpture and scapes shorter (SL/HL 0.73–0.81) ( C. marioni)............ 5

5. Standing pilosity abundant, present on most of the dorsal surface of the mesosoma and gaster (MSC 16–30, A4SC 20–55) (Figs 5A, 15A); scape and legs of moderate length relative to mesosoma length (SL/ WL 0.69–0.82, MtFL /WL 0.77–0.85) ................................................................................................................6

- Standing pilosity varying from sparse to common, but not present on the entire dorsal surface of the mesosoma and gaster (MSC 0–15, A4SC 0–38), if present on mesosoma dorsum then usually confined to the pronotal humeri and the mesonotal declivity (Fig. 5B); rarely (some workers of C. marioni) one or two standing hairs on middle of promesonotum but in that case scape and legs short relative to mesosoma length (SL/WL 0.64–0.69, MtFL /WL 0.72–0.76) ..............................................................................................7

6. Standing pilosity on head and mesosoma relatively long and slender, giving worker a shaggy appearance (Fig. 15A, B); PP-SL /HW 0.17–0.25; promesonotum with weak reticulate-foveolate to reticulostriate sculpture, the surface sublucid; legs longer (MtFL /HW 0.89–0.92); southern Utah, northern Arizona, New Mexico (with a divergent, outlying population in southern California) ....................................... C. navajoa

- Standing pilosity on head and mesosoma relatively short and stout (Fig. 16A, B); PP-SL /HW 0.13–0.18; promesonotum reticulate-foveolate, subopaque; legs shorter (MtFL /HW 0.82–0.87); Great Plains south to northern Mexico, west to Arizona.............................................................................................. C. punctulata

7. Sides of propodeum convex in dorsal view, the bases of the propodeal spines not inserted at the widest portion (Fig. 6A); propodeal spines short (SPL/HW 0.15–0.19, SPL/WL 0.13–0.17); standing pilosity sparse, each pronotal humerus with one or no seta (MSC 0–3), and abdominal tergite 4 with none to several short setae, along or near the posterior margin (A4SC 0–4) (Fig. 17A, C); arboreal species occurring in Arizona, Utah, New Mexico, west Texas and northern Mexico ...................................................................... C. isolata

- Propodeum not configured as described above, bases of propodeal spines inserted at widest portion (as in Fig. 6B); either propodeal spines longer and/or standing pilosity more common .......................................8

8. Dorsum of head (except small shiny median patch, of variable size) and mesosoma densely reticulate-striate to reticulate-foveolate and opaque (Fig. 18B, C); eye relatively small (OI 0.23–0.25, ED/HL 0.24– 0.27); propodeal spines and legs relatively short (SPL/HW 0.18–0.22, MtFL /HW 0.87–0.93); standing pilosity sparse on mesosoma, restricted to 1–3 setae on each pronotal humerus (MSC 2–5), better developed on gaster (A4SC 10–18); southern Arizona, Mexico; part of an unresolved complex of forms related to C. opaca Mayr, 1870 .................................................................................................. C. sp. cf. opaca

- Head sculpture less densely developed, not rendering head fully opaque; if promesonotum predominantly reticulate-foveolate and opaque, then either propodeum with contrastingly coarse longitudinal rugulae and/or eye larger (OI 0.25–0.31, ED/HL 0.27–0.32) and/or standing pilosity sparser on gaster (A4SC 0–6)...................................................................................................................................................................9

9. Petiole broad (PTW /HW 0.36–0.42) (Figs 7, 19C) with prominent anteroventral tooth (Fig. 8); promesonotum predominantly reticulate-foveolate and opaque, variably overlain by weak striae; dorsal face of propodeum with contrastingly coarse longitudinal rugulae, on a reticulate-foveolate background; declivitous face of propodeum with moderate to weak reticulate sculpture, subopaque to sublucid; scape short (SI 0.75–0.81, SL/WL 0.69–0.75); standing pilosity sparse on mesosoma, with one or no seta on each pronotal humerus (MSC 0–2) and sparse but long on gaster (A4SC 0–6) (Fig. 19A); Oregon, Nevada and California, suspected of being a temporary social parasite of other Crematogaster species ........ C. mutans

- Without the above combination of characters; petiole usually less broad and with anteroventral tooth normal in size or absent; mesosoma usually without the contrasting pattern of sculpture described above, but if present (some workers of C. californica, C. opuntiae and C. depilis) then petiole generally narrower (PTW /HW 0.31–0.37); declivitous face of propodeum mostly unsculptured and shiny; scape length and standing pilosity variable .............................................................................................................................10

10. Promesonotum predominantly reticulate-foveolate and subopaque (Fig. 20C); scapes, propodeal spines and legs relatively long (SI 0.82–0.92, SPL/HW 0.21–0.26, MtFL /HW 0.94–1.00) compared to related species (Fig. 42); standing pilosity sparse (MSC 0–2, A4SC 0–11); paired postpetiolar setae usually lacking, if present then short (PP-SL 0.12–0.15); hemilobes of postpetiole sharply angulate in profile (Fig. 20A); southern Utah, Arizona, New Mexico and west Texas...................................................... C. colei

- Without the above combination of characters; if promesonotum predominantly reticulate-foveolate then scapes and legs shorter (SI 0.72–0.84, MtFL /HW 0.80–0.91) ....................................................................11

11. Promesonotum predominantly reticulate-foveolate and generally opaque, sometimes with weak overlying rugulae or striae anteriorly (Fig. 9A); standing pilosity sparse on mesosoma, usually restricted to a single hair (rarely two) on each pronotal humerus, or entirely absent (MSC 0–4); eye large and legs relatively short, such that ED/ MtFL 0.29–0.37 ...........................................................................................................12

- Promesonotum reticulate-striate, often with smooth, shiny interspaces (Fig. 9B), or predominantly smooth and shiny; standing pilosity more common on mesosoma, usually with two or more (rarely one) hairs on each pronotal humerus (MSC 1–15); eye size and leg length variable........................................17

12. Standing pilosity absent from mesosoma dorsum and sparse on the gaster (MSC 0, A4SC 0–4), if present on abdominal tergite 4 then short (~ 0.08 mm long or less) and confined to posterior border (Figs 21A, 22A, 23A); postpetiolar seta either lacking or present but short (PP-SL /HW 0.10–0.13) .........................13

- Standing pilosity usually present on mesosoma dorsum as a single hair on each pronotal humerus (MSC 1–4) and more common on gaster (A4SC 5–25) (Figs 14A, 15A); postpetiolar seta present and usually longer (PP-SL /HW 0.12–0.21 .......................................................................................................................15

13. Pubescence on scape relatively conspicuous, decumbent to suberect (Fig. 21B); in larger workers (HW> 1.05) legs longer (MtFL /HW 0.87–0.91, MtTL /HW 0.77–0.79) (Figs 43, 44), eye tending to be smaller (ED/ MtFL 0.29–0.32) and body more slender (WL/HW 1.08–1.17) than in related species (see remarks under ‘Species accounts’) (Fig. 21); south-eastern California, southern Nevada and southern Utah ................................................................................................................................................... C. detecta

- Pubescence on scape variable, usually less conspicuous, predominantly decumbent to appressed; legs shorter (MtFL /HW 0.80–0.87, MtTL /HW 0.71–0.77), eye larger (ED/ MtFL 0.32–0.37) and body more robust (WL/HW 0.97–1.09); widespread ......................................................................................................14

14. Head and mesosoma dark brown, gaster blackish-brown (Fig. 22); queen: posterior quarter of pronotum smooth and shiny; male: small (3.0 to 3.2 mm long), head largely smooth and shining, suberect pubescence sparse on mesosoma; west Texas to eastern California, nesting among lower stems and roots of Larrea divaricata Cav. (creosote bush); workers not always separable from those of C. depilis (see remarks under ‘Species accounts’)........................................................................................................................................ C. larreae

- Head and mesosoma yellowish-brown to reddish-brown (head may be darker than mesosoma), gaster medium to dark brown (Fig. 23); queen: pronotum largely or entirely reticulate-foveolate; male: larger (5.5 to 6.0 mm long), head reticulate-foveolate, suberect pubescence conspicuous on mesosoma; south-western United States and northern Mexico; ground-nesting species, in desert and semi-desert regions, not specifically associated with Larrea divaricata ......................................................................... C. depilis

15. Mesosoma sublucid, the reticulate-foveolate sculpture weakened, except on lower mesopleuron; dark reddish-brown with black gaster; described from males and queens from northern Arizona; putative workers from southern Utah have not been definitively linked to this species (see Buren 1968: 114) ......................................................................................................................................................... C. nocturna

- Mesosoma opaque, with dense reticulate-foveolate sculpture, overlain by longitudinal rugae on dorsal face of propodeum; variable in colour; widespread in the south-western United States and north-western Mexico ............................................................................................................................................................16

16. Pubescent hairs suberect on scapes and head (Fig. 24B), long on body (Fig. 24A); central-southern California and Baja California ................................................................................................... C. californica

- Pubescent hairs appressed on scapes and head (Fig. 25B) and shorter and less noticeable on all surfaces (Fig. 25A); desert locations in Arizona and adjacent regions; possibly conspecific with C. californica (see under ‘Species accounts’) ............................................................................................................... C. opuntiae

17. Small species (HW 0.72–1.05) with short scapes (SL/WL 0.64–0.71) (Fig. 26B) and legs (MtFL /WL 0.72– 0.78); side of pronotum usually reticulate and lower mesopleuron usually longitudinally striate; standing pilosity present on pronotum and mesonotal declivity (MSC 4–15) and common on gaster (A4SC 19–38); most standing pilosity short, grading into (and sometimes difficult to distinguish from) subdecumbent pubescence (Fig. 26A); PP-SL /HW 0.08–0.14; California, Baja California ................................... C. marioni

- Scapes and legs generally longer (SL/WL 0.69–0.83, MtFL /WL 0.75–0.92); either side of pronotum and lower mesopleuron with weaker sculpture and/or pattern of standing pilosity different from that described above; PP-SL /HW 0.12–0.25 ........................................................................................................18

18. Lower mesopleuron reticulate-foveolate, overlain with conspicuous longitudinal striae (Fig. 27A); scape and legs relatively long in relation to head length (SL/HL 0.87–0.96, MtFL /HL 0.97–1.07); Utah, Nevada, Oregon, California, Baja California ............................................................................................. C. coarctata

- Lower mesopleuron reticulate-foveolate, usually lacking conspicuous longitudinal striae; scape and legs shorter (SL/HL 0.78–0.90, MtFL /HL 0.84–0.98) .........................................................................................19

19. Side of pronotum predominantly smooth and shiny, the dorsum of promesonotum similar but overlain with fine longitudinal striae (Fig. 28A, C); in profile promesonotum evenly and notably convex, lacking a prominent mesonotal declivity posteriorly (Fig. 28A); often bicoloured, with dark gaster contrasting with reddish-brown head, mesosoma, petiole and postpetiole (dark unicoloured populations occur on the Pacific Coast); west Texas and Chihuahua west to California and Baja California ..................... C. hespera

- Promesonotum, including side of pronotum, more strongly sculptured, reticulate or reticulate-striate; in profile promesonotum somewhat flattened with a notable mesonotal declivity (Fig. 29A) or gently convex (Fig. 30A); unicoloured to weakly bicoloured...............................................................................................20

20. Upper-third of head predominantly smooth and shiny, with relatively coarse punctures (> 0.01 mm diameter); standing pilosity long and abundant, especially on the gaster (A4SC 14–30) (Fig. 29A); longest postpetiolar seta about two-thirds of postpetiole width (PP-SL / PPW 0.59–0.75) (Fig. 10A; see also Fig. 40); west Texas, New Mexico, Arizona and adjacent Mexico ........................................................... C. browni

- Upper-third of head with smaller punctures (<0.01 mm diameter) and often with greater incursion of finely reticulate surface sculpture; standing pilosity shorter and less abundant on gaster (A4SC 4–16) (Fig. 30A); longest postpetiolar seta about one-half of postpetiole width or less (PP-SL / PPW 0.38–0.54) (Fig. 10B).....21

21. Standing pilosity present on pronotal humeri and on mesonotal declivity (MSC 6–13) (Figs 5B, 30A); scape relatively long (SI 0.82–0.89); propodeal spines short (SPL/HW 0.18–0.22); Colorado and Utah to northern Mexico ........................................................................................................................... C. emeryana

- Standing pilosity on mesosoma restricted to pronotal humeri (MSC 2–7) (Fig. 31A); scapes shorter (SI 0.72–0.81); propodeal spines longer (SPL/HW 0.22–0.25); eastern Canada, south to Florida and west to the Great Plains and Rocky Mountain foothills................................................................................ C. cerasi

22. Promesonotum with distinct rugulose sculpture, imparting a vermiculate (worm-like) appearance (Fig. 32C); small species (HW 0.67–0.85), with short scapes and legs (SI 0.72–0.78, MtFL /HW 0.79–0.84) (Fig. 32B); propodeal spines short and stout (SPL/WL 0.15–0.19), inwardly curved and directed posterad (SPTD/HW 0.39–0.48) (Figs 32A, 32C); swamp-inhabiting species from south-eastern United States .................................................................................................................................................... C. vermiculata

- Promesonotum lacking distinct rugulose-vermiculate sculpture; other characters variable ...................23

23. Pronotum with smooth or weakly sculptured median impression, flanked on each side by ruguloreticulate sculpture; in anterior view median impression of pronotum enhancing the distinctness of the median carina of the mesonotum (likened to a gun sight); small, dark-coloured arboreal species (HW 0.88–1.06) with conspicuous standing pilosity on mesosoma and gaster (MSC 12–17, A4SC 20–40) (Fig. 33A, C); south-eastern Texas, north-eastern Mexico ..................................................................................... C. rifelna

- Pronotum without a smooth median depression, flanked by ruguloreticulate sculpture; size, pilosity and nesting habits variable .................................................................................................................................24

24. Propodeal spines short and distinctly incurved (SPL/HW 0.14–0.18; SPTD/HW 0.39–0.47) (Fig. 6B); mesosoma smooth to weakly foveolate; standing pilosity relatively sparse on mesosoma and gaster (MSC 2–6, A4SC 5–13); small, arboreal species (HW 0.74–0.92)..........................................................................25

- Either propodeal spines longer and less incurved, or mesosoma more densely sculptured; standing pilosity and size variable..............................................................................................................................26

25. Uniformly dark brown (fresh specimens) (Fig. 34); arboreal species, with variable nesting habits, south-eastern United States .................................................................................................................. C. ashmeadi

- Bicoloured, gaster black and other body parts ferrugineus-red (Fig. 35); nesting in pine branches and under pine bark in open habitats; Florida and adjacent states ................................................... C. pinicola

26. Standing pilosity conspicuous on head (Fig. 36B), with numerous short subdecumbent and suberect hairs (uplifted pubescence), in addition to several pairs of longer setae flanking the midline; standing pilosity also present as numerous fine hairs of varying degrees of inclination and length on mesosoma and gaster (MSC 10–30, A4SC 20–60) (Fig. 36A); propodeal spines long (SPL/HW 0.22–0.29) (Fig. 36C); south-eastern United States ............................................................................................................... C. pilosa

- Standing pilosity generally less common and more distinct from the appressed pubescence (at least on the mesosoma and gaster), or present as short bristle-like hairs; propodeal spine length variable........27

27. Propodeal spines long and divergent (SPL/HW 0.26–0.34, usually> 0.27) (Fig. 37A, C); dorsal face of propodeum smooth or weakly sculptured; standing pilosity sparse (MSC 0–8, A4SC 11–24) (Fig. 37A); marsh-inhabiting species from south-eastern United States, often building carton nests..... C. parapilosa

- Propodeal spines shorter (SPL/HW 0.19–0.27); dorsal face of propodeum usually distinctly sculptured; standing pilosity variable; carton nests rarely or never constructed.........................................................28

28. Standing pilosity relatively sparse on mesosoma dorsum, confined to a clump of flexuous setae on the pronotal humeri and occasionally one or two setae on the mesonotal declivity, always lacking on propodeum (MSC 2–14) (Figs 31A, 38A)......................................................................................................29

- Standing pilosity common and well distributed across the mesosoma dorsum, often including the propodeum (MSC 15–28) (Figs 5A, 39A) .....................................................................................................30

29. Side of pronotum and dorsum of promesonotum smooth and shining (Fig. 38A, C); midwestern species, occurring from Kansas and Missouri to north-eastern Mexico C. laeviuscula

- Side of pronotum usually distinctly foveolate-striate, dorsum of mesosoma striate (Fig. 31A, C); widely distributed, from eastern Canada, south to Florida and west to the Great Plains and Rocky Mountain foothills................................................................................................................................................ C. cerasi

30. Mesosoma dorsum reticulate-foveolate and opaque, lacking striae or rugulae (Fig. 16C); propodeal spines directed posteriorly, such that SPTD/HW 0.39–0.46 and SPTD/ PPW 1.11–1.36 (Fig. 47); Great Plains south to northern Mexico, west to Arizona; see remarks under ‘Species accounts’ ................. C. punctulata

- Mesosoma dorsum usually with longitudinal rugulae or striae, on a weak reticulate or reticulate-foveolate sublucid background (Fig. 39C); propodeal spines directed more posterolaterally such that SPTD/HW 0.46–0.63 and SPTD/ PPW 1.38–1.76 (Fig. 47); eastern Canada to southern United States and west to the Great Plains ................................................................................................................................... C. lineolata

31. Dorsum of head with dilute appressed pubescence and six or fewer standing hairs (Fig. 11A ................ 32

- Dorsum of head with abundant standing pilosity and no underlying short, appressed pubescence (Fig. 11B) ......................................................................................................................................................34

32. Postpetiole bilobed, much wider than long and with a distinct median sulcus; head reticulate-foveolate and opaque ..................................................................................... C. corvina (southern Texas to Honduras)

- Postpetiole globular to subquadrate, slightly broader than long, and lacking a distinct median sulcus; head less densely sculptured, mostly smooth and shiny, at least medially ............................................... 33

33. In profile promesonotum usually broadly convex (Fig. 12A); anteroventral petiolar tooth relatively well-developed and sharp; gaster with abundant, evenly distributed standing pilosity; southern Texas to Argentina ........................................................................................................................................... C. crinosa

- In profile promesonotum tending to be somewhat flattened (Fig. 12B); anteroventral petiolar tooth usually less well-developed, forming a right or weakly acute angle; standing pilosity moderately common on gaster, denser anterolaterally and often leaving a median strip relatively clear of setae; southern United States to South America ........................................................................................................ C. torosa

34. Standing pilosity on head short and bluntly clavate; body dark brown; Florida and Texas, south to Venezuela ....................................................................................................................................... C. obscurata

- Standing pilosity on head long and flexuous; body yellow to orange-brown, sometimes with darker head 35

35. Queen small (4–4.8 mm long); worker with short propodeal spines (SPL/HW 0.10–0.19); promesonotum with scattered longitudinal carinulae; mesopleuron predominantly smooth and shiny; eastern United States, west to Texas, south to Costa Rica .............................................................................. C. minutissima

- Queen larger (5–6 mm long); where sympatric with C. minutissima in eastern United States, worker with longer propodeal spines (SPL/HW 0.20–0.24), promesonotum tending to have fewer carinulae, mostly on sides, and mesopleuron often reticulate-foveolate and opaque (in allopatry these worker differences do not apply; further study required); eastern United States, west to Utah, Arizona and northern Mexico ................................................................................................................................................ C. missouriensis