Pogonomyrmex inermis

A rather well studied seed harvesting ant that has relatively small colonies.

Identification
Johnson (2015) - Worker Within the P. bispinosus-group, the combination of: (1) superior propodeal spines absent or consisting of small denticles or tubercles uniquely characterize this species.

Queen This caste is diagnosed by: (1) caste-specific morphology of the mesosoma related to wing-bearing and presence of ocelli on head, (2) superior propodeal spines absent or consisting of small denticles, (3) inferior propodeal spines lacking, (4) in dorsal view, dorsum of postpetiole lacking transverse rugae, or rugae few and incomplete, and (5) body usually bicolored, with gaster noticeably darker than rest of body.

Male This caste is diagnosed by: (1) first gastral tergum lacking striae, (2) in full-face view, external margin of apical tooth curved inward, (3) dorsum of postpetiole strongly granulate, dull, to occasionally weakly rugoreticulate-vermiculate, interrugae dull, (4) rugae on cephalic dorsum prominent, subparallel, (5) pronotal sides and mesopleura with longitudinal rugae, (6) interrugae on cephalic dorsum, pronotal sides, and mesopleura weakly shining to shining, and (7) notauli present.

Pogonomyrmex inermis co-occurs with Pogonomyrmex uruguayensis, and it is distinguished from P. uruguayensis by absence of superior propodeal spines (reduced to denticles or small tubercles), which are present in P. uruguayensis. Additionally, workers of P. inermis are typically larger (HW = 1.59–1.86 mm) than those of P. uruguayensis (HW = 1.24–1.61 mm). Absence of superior propodeal spines and absence of striae on the first gastral tergum distinguish P. inermis from all congeners in Argentina.

Distribution based on Regional Taxon Lists
Neotropical Region: Argentina.

Habitat
Johnson (2015) - Pogonomyrmex inermis inhabits sites at elevations from 95–1370 m. This species is common in the Lower Monte Desert, southern Humid Chaco, southern Espinal, and southwestern Humid Pampas ecoregions as defined by Olson et al. (2001)

Biology
Johnson (2015) - Pogonomyrmex inermis is one of the more well-studied species of Pogonomyrmex in Argentina. This species harvests the seeds of various grass and nongrass species (Pirk & Lopez de Casenave, 2010, 2011; Pirk, Lopez de Casenave, & Marone, 2007; Pirk, Lopez de Casenave, Pol, Marone, & Milesi, 2009; Pol, Lopez de Casenave, & Pirk, 2011). Workers are solitary foragers, but additional observations show that foragers leave the nest to travel in few directions; they do not form a conspicuous trail, and they do not recruit nestmates to high density seed patches (Pol et al., 2015). Five colonies averaged 47–70 foragers, which consisted of about 15% of all workers in nests (Nobua-Behrmann, Lopez de Casenave, Milesi, & Pavan, 2013). Nests are typically distinguished by the large midden of seed chaff (15–20 cm in diameter) that surrounds or is adjacent to the nest entrance. Nests sometimes have 2–3 entrances with a tumulus that ranges up to 15 cm in diameter. Colonies of P. inermis are relatively small: two excavated colonies contained an average of 299 workers and 54 larvae and pupae (Nobua Behrmann et al., 2010).

Collection dates for sexuals range from 18 December to 23 January. Mating flights have not been observed, but they occur during the austral summer (December–January) based on finding two dealate queens on the ground on 21–22 January, and excavating queens from incipient nests on 25 December; all excavated queens (n = 7) were haplometrotic (R.A. Johnson, unpub. data). Queens of P. inermis are polyandrous with a mean ( ± SE) effective mating frequency of 6.52 ± 2.77 (range = 2–12; n = 20) (Pol et al., 2008).

Pol et al. (2015) - Field experiments studying foraging were conducted in the Monte desert, Argentina. This showed P. inermis employ a group-foraging strategy with limited recruitment. Much of their foraging activity is concentrated on smaller portions of the total area within ready foraging range of their nest.

Diet
Dietary preferences of these seed harvesting ants have been studied in the Monte Desert (Biosphere Reserve of Ñacuñán ), Argentina (see Pirk et. al 2011). In choice tests this species preferred seeds of grass species to those of forbs and shrub.

Nomenclature

 *  inermis. Pogonomyrmex inermis Forel, 1914d: 267 (w.) ARGENTINA. See also: Gallardo, 1932b: 128.

Worker
Johnson (2015) - Lectotype (n = 12). HL 1.74 (1.59–1.79); HW 1.86 (1.59–1.84); MOD 0.33 (0.30–0.39); OMD 0.37 (0.38–0.50); SL 1.26 (1.16–1.35); PNW 1.21 (1.04–1.19); HFL 1.78 (1.59–1.87); ML 1.85 (1.91–2.18); PW 0.50 (0.39–0.47); PPW 0.61 (0.53–0.63). Indices: SI 67.74 (66.67–76.10); CI 106.90 (98.18–106.98); OI 17.74 (17.34–22.16); HFI 95.70 (92.13–103.77).

Head quadrate to slightly wider than long (CI = 98.18– 106.98), widest just posterior to eyes; posterior margin flat to weakly concave in full-face view. Longitudinal rugae on cephalic dorsum prominent, wavy to irregular, often moderately rugoreticulate toward posterior margin; in full-face view, medial rugae diverging weakly toward posterior corners of head. In profile, rugae posterior to eyes converging near vertex. Cephalic interrugae strongly granulate, dull; vertex rugose. Anterior margin of clypeus concave; dorsal surface with several subparallel, longitudinal rugae. Mandible with six teeth; mandibular dorsum coarsely rugose. MOD ranging from 0.18–0.23x HL. In profile, eyes situated slightly anterior to middle of head, OMD = 1.10–1.54x MOD. Antennal scapes moderately long (SI = 66.67–76.10), failing to reach vertex by length of basal funicular segment; entire scape with longitudinal striae. Psammophore well-developed.

Mesosomal profile strongly convex; all mesosomal surfaces with prominent irregular rugae to rugoreticulate. Anterior margin of pronotal dorsum with irregular transverse rugae that traverse posteroventrally or become rugoreticulate on pronotal sides; dorsum of mesonotum with irregular longitudinal rugae, or rugoreticulate to vermiculate; rugae on mesopleura longitudinal or traversing posterodorsally; dorsum of propodeum with irregular transverse rugae that traverse ventrally or anteroventrally on propodeal sides. Interrugae on mesosoma weakly to strongly granulate, dull to weakly shining. Superior propodeal spines absent or consisting of small denticles or tubercles. Inferior propodeal spines lacking. Propodeal spiracles narrowly ovate facing posterad. Legs moderately to strongly coriarious, weakly shining.

Peduncle of petiole about 0.8x as long as petiolar node, anteroventral margin often with poorly-developed, broadly rounded process. In profile, petiolar node asymmetrical with anterior surface shorter than posterior surface; apex broadly rounded. In dorsal view, petiolar node longer than wide, widest near spatulate anterior margin. Sides and posterior surface of petiolar node with coarse, irregular, transverse rugae or rugoreticulate; interrugae weakly to moderately granulate, weakly shining to shining. Dorsum of postpetiole convex in profile; in dorsal view, widest near posterior margin, narrowing to anterior margin, maximal width about equal to length, moderately to strongly coriarious, dull to weakly shining. First gastral tergum moderately to strongly coriarious, weakly shining.

Erect whitish to yellowish pilosity moderately abundant on head, variable in length, longest hairs not exceeding MOD. Moderately abundant suberect to semidecumbent pilosity on scape; abundant decumbent hairs on funicular segments. Legs with moderately abundant suberect to decumbent white setae. Mesosoma, petiolar node, postpetiole, first gastral tergum with moderately dense, erect, white setae, often similar in length, longest hairs on mesosoma approaching MOD. Body tannish-brown to reddish-brown, posterior portion of petiolar node and postpetiole often darker brown, gaster dark brown to blackish.

Queen
Johnson (2015) - (n = 12). HL 1.74–2.02; HW 1.79–2.19; MOD 0.39–0.47; OMD 0.39–0.56; SL 1.23–1.39; PNW 1.28–1.51; HF 1.72–2.06; ML 2.29–2.65; PW 0.48–0.57; PPW 0.68–0.83. Indices: SI 60.98–73.33; CI 101.60–108.42; OI 18.26–23.50; HFI 86.76–103.52.

With caste-specific morphology of the mesosoma related to wing-bearing and presence of ocelli on head. In full-face view, head quadrate to wider than long (CI = 101.60–108.42), widest just posterior to eyes, posterior margin flat. Longitudinal rugae on cephalic dorsum prominent, wavy to irregular; in full-face view, medial rugae not diverging toward posterior corners of head, interrugae weakly to moderately granulate-punctate, weakly shining; vertex rugose, interrugae moderately granulate-punctate, dull to weakly shining. Mandible with six teeth, dorsal surface coarsely rugose. Psammophore well-developed.

All mesosomal surfaces with subparallel, regular, wavy, or irregular rugae; interrugae weakly to moderately granulate-punctate, weakly shining; propodeum unarmed or with small denticles; inferior propodeal spines absent. Peduncle of petiole about as long as petiolar node, anteroventral margin straight or with a weak process. In profile, petiolar node asymmetrical with anterior surface notably shorter than posterior surface, apex rounded to weakly angulate. Posterior surface of petiolar node with irregular transverse rugae; dorsum of postpetiole with very weak, usually incomplete transverse or oblique rugae, and/or mostly granulate-punctate; interrugae on posterior surface of petiolar node and dorsum of postpetiole weakly to moderately granulate-punctate, weakly shining. First gastral tergum weakly to moderately coriarious, weakly shining. Most body surfaces with moderately abundant suberect to erect, medium-length, white to yellowish setae; moderately abundant suberect to erect hairs on first gastral tergum, those on second and third terga restricted to posterior margin. Head, mesosoma, petiolar node, and postpetiole tannish-brown; gaster blackish-brown.

Male
(n = 12). HL 1.42–1.73; HW 1.43–1.70; MOD 0.46–0.59; OMD 0.20–0.31; SL 0.36–0.49; HFL 1.63–1.90; ML 2.30–2.78; PW 0.46–0.63; PPW 0.62–0.83. Indices: SI 24.16–31.33; CI 92.55–102.11; OI 30.87–38.00; HFI 109.40–127.70.

Type Material
Syntypes, 2 workers, 8 workers , 1 worker , ARGENTINA, San Luis: Alto Pencoso, #204 (C. Bruch leg.). MACN worker [CASENT0217257] designated LECTOTYPE by Johnson, 2015: 68.

Etymology
The specific epithet, inermis (from Latin, inermis = unarmed), refers to this species lacking superior propodeal spines.