Stenamma crypticum

Stenamma crypticum is a rather common component of the leaf litter in mid- to high-elevation mesic forest habitats in Central America. It has been collected from 900–2800 m, but is most common from 1500–2500 m. Habitat types include cloud forest, mesophyll forest, oak forest, and mixed hardwood forest. Most collections come from samples of sifted leaf litter collected from the forest floor, but some specimens are also known from cookie baits. Nests have never been found, but dealate queens, as well as workers, are common in the leaf litter, suggesting that nests might be in this stratum.

Identification
Branstetter (2013) - Integument dark red-brown to brown; small-sized species (see HL, ML, PrW below); basal margin of mandible sinuous, usually with a small basal depression; anterior clypeal margin undulating, with two small blunt teeth bordering midline; eye of moderate size (EL 0.07–0.11, REL 15–20), oval-shaped, with 4–5 ommatidia at greatest diameter; face mostly rugoreticulate; mesosoma often mostly sculptured, but pronotum variable, usually rugose with smooth patches on dorsum and side, but sometimes mostly rugose or mostly smooth; propodeal spines tuberculate to short (PSL 0.07–0.10, PSI 1.2–1.8); gastral pilosity usually short, dense and clearly bilayered, with a layer of suberect setae and a denser underlying layer of subdecumbent setae, but sometimes setae more uniformly subdecumbent, or suberect setae thickened; geography useful in species determination. Similar species: Stenamma connectum, Stenamma huachucanum, Stenamma ignotum, Stenamma picopicucha.

Stenamma crypticum should be distinguished easily from S. ignotum and S. picopicucha using the diagnostic characters given above. Separating S. crypticum from S. connectum and S. huachucanum is more challenging. This is because each species comprises a complex of multiple divergent populations, with no clear evidence of sympatry among distinct forms. This species is quite variable. Using morphology alone, it might be best to name a single species; however, molecular phylogenetic data strongly suggest the existence of at least three species (Branstetter unpublished data). Even though some populations/specimens may prove difficult to identify, Branstetter (2013) delimits these species as best as possible. There are a few key morphological characters that separate the type populations of each species from one another, but when considering all populations, geography is useful. Stenamma connectum occurs in the Mexican states of Veracruz and Oaxaca only. Within Oaxaca, it is found along the Caribbean slope in wet forest habitats. Stenamma huachucanum is distributed from the southwestern U.S.A to Oaxaca. Within Oaxaca, it occurs only in the drier habitats in the central and western parts of the state. Specimens from Hidalgo and San Luis Potosí states are considered to be S. huachucanum, but some of these specimens are hard to place. Stenamma crypticum occurs mainly from Chiapas, Mexico to Nicaragua; however, a couple of specimens known from one sample collected in Veracruz at 1600 m (11km N San Andrés Tuxtla) appear most like S. crypticum. These specimens lack the broadly rounded propodeal lobes (in profile) and have the promesonotum mostly smooth. Until more material is collected, Branstetter (2013) treats these as S. crypticum, but notes that it is possible that they are actually abberant specimens of S. connectum. One specimen that clearly has the characteristics of S. connectum was collected at nearly the same locality, only slightly lower at 1400 m elevation.

Distribution
Southern Mexico (Chiapas, Veracruz?) to Nicaragua.

Distribution based on Regional Taxon Lists
Neotropical Region: Guatemala, Honduras, Mexico.

Castes
Males have yet to be collected.

Nomenclature

 *  crypticum. Stenamma crypticum Branstetter, 2013: 80, figs. 68-71 (w.q.) MEXICO.

Worker
(21 measured) HL 0.54–0.65 (0.59), HW 0.45–0.57 (0.51), FLD 0.12–0.16 (0.14), PCW 0.02–0.04 (0.03), SL 0.39–0.51 (0.42), EL 0.07–0.11 (0.08), ACL 0.39–0.48 (0.42), ML 0.62–0.79 (0.70), PrW 0.31–0.40 (0.35), PSL 0.07–0.10 (0.09), SDL 0.05–0.06 (0.05), PL 0.22–0.29 (0.26), PH 0.13–0.18 (0.15), PW 0.11–0.16 (0.14), PPL 0.13–0.18 (0.16), PPH 0.12–0.18 (0.15), PPW 0.13– 0.21 (0.16), MFL 0.40–0.54 (0.45), MTL 0.34–0.45 (0.37), CI 84–91 (87), SI 80–89 (81), REL 15–20 (16), FLI 25–29 (26), PSI 1.2–1.8 (1.6), MFI 103–117 (115), ACI1 69–72 (69), ACI2 93–103 (100).

Small-sized species; general body color dark brown to brown or orange-brown, with appendages brown or orange-brown to yellow-brown, becoming lighter toward extremities; setae golden brown; mandible with 6 teeth, inner teeth sometimes worn; basal margin of mandible sinuous, usually with a distinct basal depression, but without tooth; mandible mostly smooth and shining, with scattered piligerous punctae and some basal striae; anterior clypeal margin when viewed from anterodorsal angle undulating, usually forming 2 blunt teeth bordering midline; median lobe of clypeus with a pair of faint longitudinal carinulae that diverge anteriorly, apex of lobe with short transverse carinula; area in between median lobe and anterior clypeal margin forming a shallow concavity where mandibles insert; remaining surface of clypeus mostly smooth; posterior extension of clypeus between antennal insertions somewhat narrow (PCW 0.02–0.04), sides subparallel; frontal lobes of moderate width (FLD 0.12–0.16, FLI 25–29), not greatly obscuring torular lobes in full-face view; head subrectangular to oval-shaped (CI 84–91), posterior margin slightly depressed medially; eye somewhat small (EL 0.07–0.11, REL 15–20), oval-shaped, with 4–5 ommatidia at greatest diameter; head completely sculptured, mostly rugoreticulate, with a few longitudinal carinulae along midline; scape relatively short (SI 80–89), not reaching posterior margin of head when laid back; scape surface mostly smooth, with scattered piligerous punctae; flagellum with distinct 4-segmented antennal club, last segment noticeably bulging; pronotal sculpture variable, dorsum usually longitudinally rugose, with a small smooth patch mesad (type population), side usually rugose on upper half and smooth on lower half (type population), sometimes pronotum completely smooth, or completely rugose, with only a small smooth patch on side; dorsum of mesonotum rugulose punctate; katepisternum and side of propodeum punctate, sometimes with a few rugulae; propodeal dorsum punctate, with a few transverse carinulae; propodeal declivity mostly smooth, with a few transverse carinulae on upper half; promesonotum in profile low-domed, and roughly symmetrical; propodeal spines tuberculate to short (PSL 0.07–0.10, PSI 1.2–1.8); petiole usually somewhat short and stocky (PL/HW 0.47–0.54); petiolar node somewhat small (PH/PL 0.56–0.57), roughly symmetrical, dorsum reaching a defined apex, which points nearly vertical; postpetiole in profile variable, usually small and similar in size to petiolar node (type population), but sometimes bulging (PPH/PH 0.91–1.07); petiole and postpetiole usually mostly punctate, with anterior faces of nodes smooth, and posterior faces of nodes with a few rugulae; most of body dorsum with short standing pilosity; pilosity on gastral dorsum usually distinctly bilayered, with a layer of suberect setae, and a slightly more dense layer of decumbent setae (type population), but sometimes setae more uniformly suberect to subdecumbent and less clearly bilayered; suberect layer of setae sometimes slightly thickened; setae on scapes decumbent to appressed; setae on legs mostly subdecumbent to appressed, with longer suberect setae on femoral venters and coxae.

Queen
(7 measured) HL 0.59–0.67 (0.59), HW 0.52–0.61 (0.52), FLD 0.15–0.17 (0.15), PCW 0.04–0.05 (0.04), SL 0.42–0.50 (0.44), EL 0.14–0.17 (0.14), ACL 0.42–0.48 (0.44), ML 0.80–0.95 (0.80), PrW 0.46–0.55 (0.46), PSL 0.10–0.13 (0.11), SDL 0.07–0.08 (0.07), PL 0.28–0.34 (0.30), PH 0.17–0.20 (0.17), PW 0.15–0.17 (0.15), PPL 0.16–0.20 (0.18), PPH 0.17–0.22 (0.17), PPW 0.19– 0.24 (0.19), MFL 0.47–0.57 (0.47), MTL 0.39–0.48 (0.41), CI 88–92 (88), SI 79–86 (84) REL 26–28 (28), FLI 26–30 (28), PSI 1.3–1.7 (1.6), MFI 102–113 (110), ACI1 68–70 (68), ACI2 90–100 (100).

Same as worker except for standard queen modifications and as follows: pronotum with transverse carinulae on humeri, becoming smooth mesad; mesoscutum mostly with longitudinal rugulae and foveolae, midline and mesolateral margin smooth; scutellum smooth along midline, and longitudinally carinulate laterad; propodeum with transverse carinulae/rugulae that wrap around surface; mesopleuron mostly smooth; lower layer of setae on gastral dorsum very dense, almost pubescent; wing venation as in specimen CASENT0605933.

Type Material
Holotype worker. MÉXICO, Chiapas: 2km SE Custepec, 15.72141°N, 92.93936°W, 1860m, 17 May 2008, oak-pine forest, ex sifted leaf litter (R. S. Anderson, collection RSA2008-013) USNM, specimen CASENT0604771 Paratypes: MÉXICO, Chiapas: 3km SE Custepec, 15.71485°N, 92.93823°W ±50m, 1700m, 17 May 2008 (LLAMA, Wa-A-02-2-50) 1dq, 1w, CASC, CASENT0623277, CASENT0623280, 1w, EAPZ, CASENT0623281, 1w, ECOSCE, CASENT0623282, 1w, FMNH, CASENT0623283, 1w, ICN, CASENT0623284, 1w, INBC, CASENT0623286, 1w, JTLC, CASENT0623536, 1w, LACM, CASENT0623288, 1w, MGBPC, CASENT0623537, 1dq, 1w, MCZC, CASENT0623278, CASENT0623289, 1w, MZSP, CASENT0623290, 1w, UCDC, CASENT0623291, 1w, UNAM, CASENT0623292, 1dq, 1w, USNM, CASENT0623279, CASENT0623293, 1w, UVGC, CASENT0623294.

Etymology
The specific epithet crypticum refers to the cryptic nature of this species.

References based on Global Ant Biodiversity Informatics

 * Branstetter M.G. 2013. Revision of the Middle American clade of the ant genus Stenamma Westwood (Hymenoptera, Formicidae, Myrmicinae). ZooKeys 295: 1277
 * Dattilo W. et al. 2019. MEXICO ANTS: incidence and abundance along the Nearctic-Neotropical interface. Ecology https://doi.org/10.1002/ecy.2944
 * Longino J. T. L., and M. G. Branstetter. 2018. The truncated bell: an enigmatic but pervasive elevational diversity pattern in Middle American ants. Ecography 41: 1-12.