Stenamma alas

Stenamma alas is a specialized inhabitant of clay bank environments. Nests are found in nearly vertical clay banks along streams or vertical cuts along trails. S. alas occurs in relatively pristine wet forest habitats from 50 to approximately 1800 m (note the holotype form of S. alas only reaches about 1600 m). I have noticed that Stenamma alas is most abundant at mid-elevations between 300–800 m elevation. At lower elevations, Stenamma expolitum, which is a closely related and often sympatric species, seems to be more dominant.

Identification
Branstetter (2013) - Integument mostly black to dark red-brown, with appendages uniformly orange-brown, or mostly dark brown changing to orange-brown at extremities; medium- to large-sized species (see HL, ML, PrW below); anterior clypeal margin with a median emargination; basal margin of mandible straight; propodeal spines absent (PSL 0.10–0.15, PSI 0.7–1.0); petiole and postpetiole almost completely smooth and shiny, with only faint vestigial punctae sometimes present; postpetiole in profile bulging, globular, appearing more voluminous than petiolar node; face with a fan of carinulae extending from frontal lobes to just past midpoint of head or further, sometimes reaching posterior and lateral margins, carinulae when completely covering face, often very dense (almost striate); promesonotum completely smooth and shiny, or with a variable number of transverse striae on dorsal surface; promesonotum in profile domed, symmetrical, and moderately to strongly bulging; eye relatively large (EL 0.14–0.18, REL 18–23), oval-shaped, with 8–10 ommatidia at greatest diameter; setae on gastral dorsum sparse, long, and mostly suberect; frontal lobes of moderate width (FLD 0.20–0.29, FLI 29–32), not obscuring torular lobes in full-face view. Similar species: Stenamma expolitico, Stenamma expolitum.

Stenamma alas, along with Stenamma expolitico and Stenamma expolitum, 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. Stenamma alas is easily separated from S. expolitico and S. expolitum by comparing the sculpturing on the face and promesonotal dorsum. In the field, the holotype form of S. alas (discussed below) can be separated from S. expolitum by the structure of the nest entrance. Stenamma alas nests always have a horizontal turret, whereas S. expolitum nests have vertical turrets.

Distribution
Costa Rica to Ecuador.

Distribution based on Regional Taxon Lists
Neotropical Region: Costa Rica, Ecuador, Panama.

Biology
Branstetter (2013) - Colonies include one to five closely spaced nests, but the queen, brood, and most of the workers only occupy one of them. Colonies seem to be continually building new nests and occasionally migrating from one to another. Each nest consists of a horizontally oriented ear-like turret that is sunk into a small alcove. The nest entrance is in the middle of the turret. Next to the entrance the workers always maintain a small clay “door pebble” and when the proper stimulus is applied to the nest entrance, such as an army ant or other predaceous ant, an S. alas worker quickly emerges from the nest and closes the entrance with the pebble. It is hypothesized that all of these complex nesting behaviors evolved to avoid predation by army ants.

Each Stenamma alas nest contains a single small chamber. Colonies are fairly large for Stenamma, with up to 250 individuals. All excavated colonies have contained only one egg-laying queen. Foragers are solitary, slow moving and freeze when disturbed. It is unknown what S. alas forages on primarily, but workers have been observed returning to the nest with cookie baits and small pieces of unidentified organic matter, suggesting that the species might be a generalist scavenger.

Castes
Males known but not described.

Nomenclature

 *  alas. Stenamma alas Longino, 2005: 672, figs. 2a-b (w.) COSTA RICA. Stanstetter, 2013: 45 (q.m.).

Branstetter (2013) - As I have circumscribed it here, S. alas represents a complex of species, whose boundaries are not clear. The type form of S. alas occurs only in Costa Rica. It is characterized by the following: facial carinulae extending to about midpoint of head, but not further; pronotum completely smooth and shiny; legs uniformly orange-brown. Variant 1 differs from the holotype form as follows: facial carinulae more extensive, sometimes very dense and extending all the way to the posterior margin of the head; legs dark brown to brown; dorsum of promesonotum with variably developed transverse striations. Variant 2 (Figure 48D–F) is the same as variant 1 except that the promesonotum in profile is strongly bulging upward, appearing high-domed. Variant 2 is known only from a few localities in the Bocas del Toro and Chiriquí provinces of Panama. It does not occur in sympatry with the other forms and some specimens appear intermediate, with the promesonotum less bulging. There is also variation in how dense and long the facial carinulae appear.

Variant 1 includes specimens from Costa Rica to Ecuador, but it does not appear to be a monophyletic entity. The specimens in Costa Rica occur at high elevation, above 1500 m and Longino (pers. comm.) reported finding nests in the ground, rather than in clay banks. One nest was found in a small clay hummock in the middle of a trail in forest. The other was in the ground under leaf litter in forest. There is some variation in how dense the facial carinulae are among sites, with a specimen from Las Alturas having very dense carinulae, similar to variant 2. The variant 1 specimens in Ecuador look nearly identical to those in Costa Rica, with some variation in facial sculpture. They are from lower elevation (800–900 m) and have nests in clay banks like the type form of S. alas (Donoso, pers. comm.). Molecular phylogenetic data show that variant 1 specimens from Costa Rica form a clade sister to Stenamma expolitum and Stenamma expolitico (Branstetter unpublished data). Specimens from Ecuador form a clade sister to S. alas. No specimens from Panama have been sampled yet. This result suggests that the variant 1 specimens in Costa Rica are distinct from the type form of S. alas and the specimens in Ecuador, but I cannot tell them apart based on worker or queen morphology. Thus, I treat S. alas as a para-phyletic species, but acknowledge that it could include multiple cryptic taxa. More morphological and molecular data will be needed to resolve this problematic species.

Worker
Branstetter (2013) - (17 measured, paratype JTLC000005880 in parentheses) HL 0.77–0.98 (0.85), HW 0.66–0.88 (0.76), FLD 0.20–0.29 (0.23), PCW 0.05– 0.09 (0.07), SL 0.65–0.77 (0.72), EL 0.14–0.18 (0.17), ACL 0.64–0.75 (0.70), ML 1.02–1.30 (1.16), PrW 0.51–0.66 (0.57), PSL 0.10–0.15 (0.13), SDL 0.11–0.15 (0.13), PL 0.37–0.51 (0.41), PH 0.22–0.28 (0.25), PW 0.16–0.22 (0.18), PPL 0.24– 0.32 (0.29), PPH 0.23–0.28 (0.25), PPW 0.20–0.28 (0.24), MFL 0.78–1.00 (0.88), MTL 0.64–0.78 (0.70), CI 86–93 (90), SI 83–101 (95), REL 18–23 (22), FLI 29–32 (30), PSI 0.7–1.0 (1.0), MFI 84–94 (87), ACI1 61–65 (62), ACI2 94–101 (98). Medium to large-sized species; general body color black to dark red-brown, with appendages uniformly orange-brown (type population), or mostly dark brown changing to orange-brown at joints and extremities; setae golden brown; mandible with 5–7 teeth, consisting of 4 distinct apical teeth, a basal tooth, and 1–2 worn teeth/denticles in be- tween; basal margin of mandible straight, without a basal notch or depression; mandible surface mostly smooth, with scattered piligerous punctae and a few striations on base and lateral surface; anterior clypeal margin with a median emargination; median lobe of clypeus obliquely flattened, mostly smooth and shiny, with a short transverse carinula near anterior margin, remainder of clyepeus mostly smooth and shiny; posterior extension of clypeus between antennal insertions somewhat wide (PCW 0.05–0.09), with sides subparallel to slightly diverging anteriorly; frontal lobes of moderate width (FLD 0.20–0.29, FLI 29–32), not greatly obscuring torular lobes in full-face view; head roughly oval- shaped (CI 86–93), with posterior margin flat, to genetly convex, not depressed medially; eye relatively large (EL 0.14–0.18, REL 18–23), oval-shaped, with 8–10 ommatidia at greatest diameter; face with a fan of carinulae extending from frontal lobes to just past midpoint of head (type population) or further, sometimes reaching posterior and lateral margins, carinulae when completely covering face, often very dense (almost striate); gena with some carinulae; posterolateral and ventral surfaces of head smooth and shiny; scape of moderate length (SI 83–101), reaching, but not distinctly surpassing posterior margin of head in full-face view; scape surface mostly smooth, with scattered piligerous punctae; flagellum with distinct 4-segmented antennal club; promesonotum completely smooth and shiny (type population), or with a variable number of transverse striae on dorsal surface, remainder of mesosoma mostly smooth, except for transverse carinulae on propodeal dorsum, and a few rugulae on side of propodeum and mesopleuron; promesonotum in profile domed, symmetrical, and moderately to strongly bulging; metanotal groove distinct, but often shallow; propodeal spines absent (PSL 0.10–0.15, PSI 0.7– 1.0); dorsum of propodeum in profile slightly to strongly convex, usually not flat; petiole and postpetiole almost completely smooth and shiny, with only faint vestigial punctae sometimes present, mostly on venters; postpetiole in profile bulging, globular, appearing more voluminous than petiolar node (PPH/PH 0.89–1.06, PW/PPW 0.73–0.84); petiole in profile appearing of moderate length (PL/HW 0.50–0.58); petiolar node in profile nearly symmetrical, dorsum of node broadly rounded, and pointed vertically to slightly posteriad; gaster smooth and shiny, with scattered piligerous punctae; face with short suberect to decumbent pilosity; setae on remainder of body dorsum sparse, long, and mostly suberect; setae on scapes subdecumbent; setae on legs mostly subdecumbent, with longer suberect setae on femoral venters and coxae.

Queen
Branstetter (2013) - (5 measured) HL 0.82–0.94 (0.82), HW 0.73–0.87 (0.73), FLD 0.23–0.27 (0.23), PCW 0.07–0.09 (0.07), SL 0.70–0.92 (0.70), EL 0.22–0.23 (0.22), ACL 0.71–0.78 (0.72), ML 1.18–1.42 (1.18), PrW 0.66–0.79 (0.66), PSL 0.13–0.15 (0.14), SDL 0.12–0.15 (0.13), PL 0.44–0.52 (0.44), PH 0.26–0.30, PW 0.19–0.23 (0.19), PPL 0.29–0.35 (0.29), PPH 0.27–0.31 (0.27), PPW 0.25–0.30 (0.25), MFL 0.86–0.99 (0.86), MTL 0.68–0.79 (0.68), CI 89–93 (89), SI 88–108 (96), REL 25–30 (30), FLI 30–32 (31), PSI 0.9–1.1 (1.0), MFI 85–91 (85), ACI1 62–63 (63), ACI2 79–103 (103).

Same as worker except for standard queen modifications and as follows: face sculpture usually slightly longer, and denser; pronotum with faint transverse striations; posterior half of mesoscutum with median patch of longitudinal carinulae; scutellum longitudinally carinulate; wing venation as shown by specimen CASENT0623104.

Type Material
Branstetter (2013) - Holotype worker: COSTA RICA, Prov. Heredia: 11km ESE La Virgen, 10°21'N 84°03'W [10.350°N, 84.050°W], 300m, 15 April 2004, (J. Longino, collection JTL5338) (INBio, specimen JTLC000005588) [examined].

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
 * 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.
 * Longino, J.T. 2005. Complex Nesting Behavior by Two Neotropical Species of the Ant Genus Stenamma (Hymenoptera: Formicidae). Biotropica 37(4):670-675