Stenamma lobinodus

This species is known only from Winkler or Berlese extractions of sifted leaf litter taken from montane, mesophyll forests. Nests have never been collected. It is likely a mid-elevation specialist given its occurrence between 860 to 1620 m.

Identification
Branstetter (2013) - Integument black to brown-black and shining; medium-sized species (see HL, ML, PrW below); anterior clypeal margin with shallow median emargination; basal margin of mandible relatively straight, without notch or substantial depression; face mostly smooth and shiny; mesosoma with coarse, rugose to rugoreticulate sculpture on most surfaces; promesonotum in profile distinctly asymmetrical, with anterior face gently rounded, dorsal surface flattened, and posterior face short, straight and forming a relatively sharp angle with dorsum; petiolar node, generally robust, and distinctly angled posteriad; postpetiolar node in profile with a longitudinal dorsal lobe that projects out posteriad over postpetiole; eye of moderate size (EL 0.11–0.14; REL 18-22), oval-shaped, with 7–8 ommatidia at greatest diameter; propodeal spines ranging from short tubercles to long spines (PSL 0.10–0.22, PSI 1.3-2.6); setae on gastral tergites relatively sparse, long, and mostly suberect; frontal lobes narrow, not obscuring torular lobe in full-face view (FLD 0.13-0.16, FLI 21-25). Similar species: Stenamma diversum, Stenamma tico.

Stenamma lobinodus can be separated easily from S. diversum and S. tico by observing its asymmetrical promesonotum and dorsal postpetiolar lobe. Although it has never been collected in sympatry with either species, it may co-occur with S. diversum in Oaxaca, Mexico.

Stenamma lobinodus, Stenamma llama and Stenamma tiburon constitute the lobinodus species group. This group can be diagnosed by the following characters: postpetiole with a distinctive longitudinal dorsal lobe, which sometimes projects posteriad over postpetiole (lobe less discernable in S. llama); petiolar node well-developed, and pointing distinctly posteriad, node usually very large in S. llama and S. lobinodus. Stenamma lobinodus and S. tiburon also share the feature that the promesonotum in profile is asymmetrical, with a long, rounded anterior face, flat top, and short posterior face. Sister to the lobinodus group is Stenamma pelophilum. This species does not have the characteristic lobe on the postpetiole that defines this group.

Stenamma lobinodus varies significantly across its range. Generally specimens at lower elevations (800–1100 m) are smaller and have shorter propodeal spines than specimens from higher sites. This gradation of sizes is very clear at the El Cielo reserve in Tamaulipas, Mexico, where Branstetter (2013) collected a series of Winkler samples from 860 to over 1600 m. The specimens from 1300 m and higher have substantially longer spines and more robust petioles. These larger specimens more closely match the ones from the populations in Querétaro.

Distribution
Atlantic slope, Mexico.

Distribution based on Regional Taxon Lists
Neotropical Region: Mexico.

Biology
Branstetter (2013) - At the El Cielo reserve in northern Mexico, Branstetter found a lone worker that looked like S. lobinodus foraging on a mud/clay bank bordering the main road (specimen viewed with a hand lens). Unfortunately the worker escaped and no nest was located. The presence of the worker on a mud bank suggests that S. lobinodus may nest in banks like the similar-looking Stenamma diversum. Both species have almost the same sculpture, color and propodeal spines.

Castes
Males have been collected but have not been described.

Nomenclature

 *  lobinodus. Stenamma lobinodus Branstetter, 2013: 150, figs. 105-108 (w.q.m.) MEXICO.

Worker
(10 measured) HL 0.65–0.76 (0.73), HW 0.57–0.71 (0.67), FLD 0.13–0.16 (0.15), PCW 0.03–0.05 (0.04), SL 0.52–0.62 (0.61), EL 0.11–0.14 (0.14), ACL 0.50–0.58 (0.56), ML 0.84–1.03 (0.97), PrW 0.41–0.51 (0.49), PSL 0.10–0.22 (0.20), SDL 0.06–0.09 (0.08), PL 0.35–0.44 (0.42), PH 0.23–0.29 (0.27), PW 0.14–0.18 (0.18), PPL 0.16–0.22 (0.21), PPH 0.16–0.19 (0.18), PPW 0.16–0.21 (0.20), MFL 0.59–0.71 (0.70) MTL 0.47–0.57 (0.55), CI 89–95 (92), SI 85–92 (90), REL 18–22 (21), FLI 21–25 (23), PSI 1.3–2.6 (2.4), PI 58–65 (62) MFI 96–103 (96), ACI1 64–68 (66), ACI2 90–100 (92).

Medium-sized species; general body color black to brownor red-black, with mandibles and appendages lighter, usually dark brown to yellow-brown; setae golden; mandible with 6 teeth, consisting of 3 distinct apical teeth, a basal tooth, and 2 inner teeth, which are often worn and indistinct; basal margin of mandible relatively straight, without a notch or significant depression; dorsal surface of mandible mostly smooth and shiny, with scattered piligerous punctae and a few short basal striae; median lobe of clypeus with a pair of vestigial longitudinal carinulae that diverge toward the anterior margin, apex with a short transverse carinula, remainder of clypeus mostly smooth and shiny; posterior extension of clypeus between frontal lobes moderately broad (PCW 0.03-0.05), with subparallel to slightly diverging sides; frontal lobes narrow (FLD 0.13–0.16, FLI 21–25), not obscuring torular lobes in full-face view; head roughly oval-shaped (CI 89–95), but appearing somewhat tear drop-shaped because of the angled anterior margin of clypeus and position of eyes; posterior margin of head flat, never distinctly depressed medially; eye of moderate size (EL 0.11–0.14; REL 18-22), oval-shaped, with 7–8 ommatidia at greatest diameter; face almost completely smooth and shiny, with scattered piligerous punctae and few longitudinal carinulae on genae; scape moderately long (SI 85–92), nearly reaching posterior margin of head when laid back; scape surface largely smooth and shiny, with scattered piligerous punctae, and sometimes with a few faint striae; flagellum with distinct 4-segmented antennal club; mesosoma almost entirely with coarse rugae and rugoreticulae; rugae usually with a predominately longitudinal orientation, but in some specimens rugae on mesonotum have a transverse orientation; katepisternum, metapleuron, and lateral portion of pronotum, with a variably sized patch of smooth cuticle; propodeal dorsum and declivity with transverse carinulae; promesonotum in profile distinctly asymmetrical, with anterior face gently rounded, dorsal surface flattened, and posterior face short, straight and forming a relatively sharp angle with dorsum; promesonotal suture indistinct, but usually discernable, especially when the orientation of the pronotal and mesonotal rugae differ; metanotal grove distinct and of moderate depth; propodeal spines present and often relatively long, but sometimes (usually specimens from lower elevations) becoming reduced to short tubercles (PSL 0.10–0.22, PSI 1.3–2.6); petiole in profile relatively long (PH/HW 0.58–0.65) and very distinctive, having a long peduncle and robust node, which is markedly angled posteriad; petiolar node in profile with long sloping anterior face that begins at about middle of peduncle, posterior face shorter, but usually at a similar angle, dorsum of node broadly rounded to subquadrate; postpetiole in profile asymmetrical, with a long sloping anterior face, and a short nearly vertical posterior face, dorsum with a distinct longitudinal lobe that projects out slightly posteriad over postpetiole, lobe in dorsal view attenuating posteriad, giving it a pinched-in appearance; petiole and postpetiole with variable amount of rugae and punctae; anterior faces of nodes mostly smooth and shiny with remaining surfaces punctatorugose; punctae most visible on ventral surfaces; posterior slope of petiolar node sometimes with a median keel; in a few aberrant populations the petiole and postpetiole lack rugae and are almost completely punctate; gaster smooth and shiny, with scattered piligerous punctae; most of body with sparse layer of moderately long standing seate; setae on scape mostly subdecumbent; setae on remaining appendages suberect to decumbent, with longer suberect setae on femoral venters and coxae.

Queen
(5 measured) HL 0.71–0.77 (0.72), HW 0.67–0.73 (0.67), FLD 0.16–0.17 (0.16), PCW 0.03–0.05 (0.03), SL 0.58–0.63 (0.58), EL 0.17–0.19 (0.17), ACL 0.55–0.60 (0.56), ML 1.03–1.16 (1.03), PrW 0.60–0.65 (0.60), PSL 0.17–0.23 (0.17), SDL 0.08–0.11 (0.09), PL 0.41–0.49 (0.41), PH 0.26–0.31 (0.28), PW 0.19–0.22 (0.19), PPL 0.20–0.24 (0.22), PPH 0.20–0.24 (0.21), PPW 0.23– 0.26 (0.23), MFL 0.67–0.74 (0.67), MTL 0.53–0.60 (0.53), CI 93–97 (93), SI 84–89 (87), REL 26–27 (26), FLI 23–24 (24), PSI 1.9–2.5 (1.9), MFI 98–102 (100), ACI1 66–69 (66), ACI2 94–96 (95).

Same as worker except for standard queen modifications and as follows: lateral surfaces of pronotum transversely rugose to rugoreticulate, median area smooth and shiny; mesoscutum longitudinally carinate, with carinae fanning out from middle of anterior margin; mesopleuron mostly smooth and shiny; wing venation as in photo.

Type Material
Holotype worker. MÉXICO: Tamaulipas, El Cielo, 1km NE La Gloria, 23.05060°N, 99.24269°W ±30m, 1570m, 22 Aug 2009 (M. G. Branstetter, collection MGB1450) USNM, specimen CASENT0605658. Paratypes: same data as holotype 1w, CASC, CASENT0605656, 1w, EAPZ, CASENT0605657, 1w, ECOSCE, CASENT0623336, 1w, FMNH, CASENT0623338; same data but El Cielo, Joya de Manantiales, 23.00835°N, 99.28511°W ±20m, 1430m, 22 Aug 2009 (M. G. Bran- stetter, MGB1446) 1w, ICN, CASENT0605641, 1w, INBC, CASENT0605643, 1w, LACM, CASENT0622422, 1w, MGBPC, CASENT0605644, 1w, MCZ, CASENT0623339, 1w, MZSP, CASENT0623340, 1w, UCDC, CASENT0623341, 1w, UNAM, CASENT0623342, 1dq, 1w, USNM, CASENT0605638, CASENT0623337, 1w, UVGC, CASENT0623343.

Etymology
The specific epithet lobinodus refers to the distinctive lobe on the postpetiole 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