Stenamma expolitum Smith

Branstetter, Michael G., 2013, Revision of the Middle American clade of the ant genus Stenamma Westwood (Hymenoptera, Formicidae, Myrmicinae), ZooKeys 295, pp. 1-277 : 84-88

Stenamma expolitum Smith Worker: Figure 83; Queen: Figure 84 A–D; Male: Figure 84 E–G; Map: Figure 85

Stenamma expolitum Smith, M. R. 1957: 36. Holotype worker: COSTA RICA, Santa Clara [ Limón] Province: Colombiana Farm, [ca. 10.167°N, 83.583°W], March-April 1924 (W. M. Mann) (USNM, Type No. 65967) [examined]. Longino, 2005: biology. Branstetter, 2009: worker images, phylogeny. Branstetter, 2012: phylogeny.

Worker diagnosis.

Integument mostly black, with legs dark brown, changing to orange-brown only at extremities and joints; medium-sized species (see HL, ML, PrW below); petiole and postpetiole almost completely smooth and shiny, with only a few faint vestigial punctae sometimes present; postpetiole in profile bulging, globular, usually appearing more voluminous than petiolar node; face almost completely smooth and shiny, except for variable number of faint carinulae and punctae; promesonotum almost completely smooth and shiny; eye relatively large (EL 0.13-0.16, REL 18-21), oval-shaped, with 7-9 ommatidia at greatest diameter; anterior margin of clypeus with median emargination; basal margin of mandible straight; propodeal spines absent (PSL 0.12-0.14, PSI 0.9). Similar species: Stenamma atribellum , Stenamma alas , Stenamma expolitico .

Geographic range.

Nicaragua to Costa Rica.

Worker description.

(10 measured) HL 0.75-0.93, HW 0.66-0.83 (0.82), FLD 0.20-0.23 (0.23), PCW 0.05-0.08 (0.06), SL 0.63-0.78 (0.76), EL 0.13-0.16 (0.16), ACL 0.63-0.74 (0.73), ML 1.02-1.26 (1.26), PrW 0.51-0.62 (0.61), PSL 0.12-0.14 (0.13), SDL 013-0.15 (0.14), PL 0.39-0.47 (0.47), PH 0.21-0.27 (0.26), PW 0.15-0.19 (0.18), PPL 0.24-0.30 (0.27), PPH 0.20-0.25 (0.24), PPW 0.19-0.24 (0.23), MFL 0.74-0.95 (0.91), MTL 0.58-0.75 (0.73), CI 87-92 (92), SI 91-96 (92), REL 18-21 (19), FLI 27-30 (28), PSI 0.9 (0.9), MFI 85-91 (90), ACI1 60-65 (64), ACI2 92-101.

Medium-sized species; general body color black to dark red-brown, with patches of dark brown on gaster; appendages mostly dark brown with joints and extremities a lighter orange-brown; setae dark golden brown; mandible usually with 6-7 teeth, consisting of 4 distinct apical teeth, a distinct basal tooth, and 1-2 inner teeth/denticles, which are usually worn and indistinct; basal margin of mandible straight, without a basal notch or depression; mandible surface mostly smooth and shiny, with scattered piligerous punctae and a few faint striae; anterior clypeal margin with median emargination; median lobe of clypeus obliquely flattened, mostly smooth and shiny, except for transverse carinula near anterior margin, remainder of clypeus mostly smooth and shiny; posterior extension of clypeus between antennal insertions somewhat wide (PCW 0.05-0.08), with sides subparallel to slightly diverging posteriad; frontal lobes of moderate width (FLD 0.20-0.23, FLI 27-30), not covering torular lobes in full-face view; head roughly oval-shaped (CI 87-92), with posterior margin flat, not depressed medially; eye relatively large (EL 0.13-0.16, REL 18-21), oval-shaped, with 7-9 ommatidia at greatest diameter; face completely smooth and shiny, except for variable number of very faint carinulae extending from frontal lobes to about midpoint of head, a few carinulae on gena, and scattered piligerous punctae; scape of moderate length (SI 91-96), usually reaching posterior margin of head when laid back; scape surface mostly smooth and shiny, with scattered piligerous punctae; funiculus with distinct 4-segmented antennal club; mesosoma almost completely smooth and shiny, except for transverse carinulae on propodeal dorsum, carinae along metanotal grove, and a few scattered rugulae and piligerous punctae; metanotal grove somewhat shallow and wide; propodeal dorsum in profile flat to distinctly convex; propodeal spines absent, at most forming nearly imperceptible nubs (PSL 0.12-0.14, PSI 0.9); petiole of moderate length (PL/HW 0.53-60); petiolar node in profile robust (PH/PL 0.54-0.60), wedge-shaped, with the anterior face long and sloping and the posterior face shorter and nearly vertical; node dorsum rounded, pointing vertically or slightly posteriad; petiolar node in profile robust, more globular than petiolar node, but similarly asymmetrical with long anterior face and short vertical posterior face; petiole and postpetiole mostly smooth and shining, with a few vestigial punctae; gaster smooth and shiny, with only scattered piligerous punctae; most of body dorsum with long standing pilosity; scape setae suberect to subdecumbent; setae on gastral tergites long, sparse, and uniformly suberect; setae on legs suberect to subdecumbent, with longer suberect setae on coxae and femoral venters.

Queen description.

(5 measured) HL 0.87-0.96 (0.90), HW 0.78-0.87 (0.81), FLD 0.24-0.27 (0.25), PCW 0.07-0.08 (0.07), SL 0.72-0.79 (0.76), EL 0.21-0.23 (0.23), ACL 0.70-0.77 (0.71), ML 1.23-1.43 (1.32), PrW 0.67-0.78 (0.74), PSL 0.11-0.15 (0.15), SDL 0.13-0.15 (0.15), PL 0.48-0.53 (0.48), PH 0.27-0.31 (0.29), PW 0.19-0.23 (0.21), PPL 0.30-0.33 (0.30), PPH 0.26-0.30 (0.29), PPW 0.26-0.30 (0.27), MFL 0.87-1.00 (0.93), MTL 0.67-0.79 (0.74), CI 90-92 (90), SI 91-93 (93), REL 26-28 (28), FLI 30-31 (30), PSI 0.9-1.0 (1.0), MFI 86-90 (87), ACI1 60-61 (60), ACI2 94-103 (94).

Same as worker except for standard queen modifications and the following: facial sculpture more developed, with distinct carinulae extending from frontal lobes to ocelli, and some carinulae on gena; pronotum with transverse striae near posterior margin; posterior third of mesoscutum with variable amount of striae/costae, orientation variable, most often longitudinal, but sometimes transverse, or obliquely angled mesad toward posterior margin; scutellum with variable number of longitudinal costae; propodeum with more distinct transverse carinae that extend across the dorsum to upper half of side; wing venation as in Figure 84D.

Male.

See Figure 84 E–G.

Biology.

Stenamma expolitum is a specialist inhabitant of clay banks. It occurs from 50-1300 m elevation in mature wet forest environments. Nests are generally found in vertical clay banks along streams or cuts along trails. The biology of Stenamma expolitum is reviewed in detail in Longino (2005) and in the overview of natural history section above. Also, see also the biology section for Stenamma alas above. These two species have nearly identical behaviors with subtle modifications, which I describe here. Stenamma expolitum constructs its nests with a vertical turret, rather than a horizontal one. Each nest usually has two chambers instead of one. The main chamber connects to the turret, but there is often a secondary chamber behind the turret. The queen and brood always occur in the main chamber. Colony size is probably similar to Stenamma alas , but so far, censused colonies tend to be smaller, with around 100 individuals. In Costa Rica, I have noticed that Stenamma expolitum can be abundant at very low elevations, whereas Stenamma alas is more common around 300 m and higher. It also seems to be easier to find colonies of Stenamma expolitum away from streams in trail cuts or in steep clay slopes, suggesting that the species may be more tolerant of drier substrates.

Comments.

Stenamma expolitum , along with Stenamma alas and Stenamma expolitico , belongs to the expolitum species group. This group is defined by the following: propodeal spines absent; petiole and postpetiole almost completely smooth and shiny, with only faint punctae sometimes present laterally; postpetiole in profile bulging, globular, appearing more voluminous than petiolar node; anterior clypeal margin with a median emargination; basal margin of mandible straight. Molecular phylogenetic data firmly show that Stenamma zelum is sister to the expolitum group ( Branstetter 2012), but this species is morphologically divergent and shares none of the expolitum group’s diagnostic character states.

Stenamma expolitum can be separated from other expolitum group species by its nearly completely smooth and shiny face. In the field, Stenamma expolitum can be separated from Stenamma alas by its nest structure, described above. Although they do not co-occur, Stenamma expolitum looks superficially like Stenamma atribellum , which is restricted to Cusuco, Honduras. Both species have completely smooth sculpturing, but the latter species has the anterior constriction of the gaster distinctly elongate.

Material examined.

COSTA RICA: Alajuela: Casa Eladio, Río Peñas Blancas, 10.3167°N, 84.7167°W, 800m, 2 Feb 1994 (J. Longino); 2km N Volcan Arenal, 10.483°N, 84.700°W, 600m, 3 Apr 2005 (J. Longino); Heredia: La Selva Biol. Sta., 10.4333°N, 84.0167°W, 50m, 8 Aug 2004 (J. Longino); P.N. Braulio Carrillo, 10.3378°N, 84.0500°W, 300m, 19 Sep 2006 (TEAM); 11km ESE la Virgen, 10.35°N, 84.05°W, 300m, 5 Nov 2003 (J. Longino); 10km SE La Virgen, 10.3333°N, 84.0833°W, 500m, 16 Feb 2003 (J. Longino); Limón: Colombiana Farm, [ca. 10.167°N, 83.583°W], Apr 1924 (W. M. Mann); NICARAGUA: Jinotega: PN Cerro Saslaya, 13.76650°N, 85.02485°W, 1040m, 12 May 2011 (LLAMA);Matagalpa: RN El Musún, 4.9km NNW Río Blanco, 12.97471°N, 85.23318°W, 1205m, 11 Oct 2008 (M. G. Branstetter).