Pogonomyrmex coarctatus

The only polymorphic Pogonomyrmex in South America.

Identification
Johnson (2015) - Worker Within the P. coarctatus-group, the combination of: (1) strongly polymorphic with supermajors, (2) very fine longitudinal striae cover most of the cephalic dorsum, areas lacking rugae weakly shining to shining, (3) in profile, lateral lobe of clypeus not enlarged with wide gap between clypeal lobe and frontal lobe (gap width similar to width of antennal scape), (4) interrugae on mesosoma weakly to moderately granulate, weakly shining, (5) weak to moderately coarse rugae on posterior surface of petiolar node, (6) rugae on dorsum of postpetiole lacking or with weak transverse rugae near posterior margin, (7) inferior propodeal spines lacking or very reduced in size and broadly rounded, and (8) body mostly concolorous reddish-orange to reddish-brown uniquely characterize this species.

Queen Diagnosis. This caste is diagnosed by: (1) caste-specific morphology of the mesosoma related to wing-bearing and presence of ocelli on head, (2) large size (HW > 3.65 mm), (3) striae mostly lacking on cephalic dorsum, (4) inferior propodeal spines poorly-developed, wider than high, broadly rounded, and (5) body mostly concolorous reddish-orange to reddish-brown.

Male This caste is diagnosed by: (1) first gastral tergum lacking striae, (2) large (HW > 1.75 mm; HL > 1.70 mm; ML > 3.20 mm), (3) hairs on head (especially posterior to eyes) and dorsum of mesosoma moderately dense, moderately long and flexuous, longest hairs rarely >0.5–0.8x MOD, (4) transverse rugae on dorsum of postpetiole weak, restricted to posterior margin, anterad portion strongly coriarious, dull, (5) rugae posterior to eyes and ocelli absent or weak, and (6) at least one and usually both mandibles with three teeth.

Pogonomyrmex coarctatus co-occurs with two other P. coarctatus-group species, Pogonomyrmex lobatus and Pogonomyrmex micans. Pogonomyrmex coarctatus can be distinguished from these two species based on the following characters: (1) P. coarctatus has fine, dense, longitudinal striae that are often indistinct and cover part to most of cephalic dorsum, (2) in profile, lateral lobe of clypeus not enlarged, with a wide gap between the clypeal lobe and frontal lobe (similar to width of antennal scape), (3) dorsum of postpetiole weakly to moderately granulate-punctate, occasionally with weak rugae near posterior margin, and (4) body mostly concolorous reddish-orange to reddish-brown. In P. lobatus, the cephalic dorsum is covered with very fine, dense striae, and the lateral lobe of clypeus is massively enlarged, nearly contacting the frontal lobe. In P. micans, the cephalic dorsum is covered with very fine, dense striae, the dorsum of postpetiole has prominent moderately coarse transverse rugae, and workers are bicolored (dark red and blackish). Pogonomyrmex marcusi is only known from mid- to higher elevations in central Bolivia, and thus is geographically isolated from P. coarctatus; these two species can be separated using characters in the key.

Distribution based on Regional Taxon Lists
Neotropical Region: Argentina, Uruguay.

Habitat
Pogonomyrmex coarctatus is a lowland species that occurs at elevations from 15–890 m. This species occurs throughout the Humid Pampas and Espinal ecoregions as well as the southern Uruguayan Savanna and southern portions of the Humid Chaco, Dry Chaco, and Low Monte Desert ecoregions as defined by Olson et al. (2001). Overall, P. coarctatus occurs in more mesic habitats than most congeners in Argentina, but it also reaches into the Low Monte Desert habitats in southern Buenos Aires Province. (Johnson 2015)

Biology
Johnson (2015) - Nests of P. coarctatus generally consist of a small hole in the ground, with a tumulus up to 10–15 cm in diameter. Nests are placed in open, exposed sites and they sometimes are surrounded by a small midden of seed chaff. Nests are most easily located by following baited workers to the nest. Foragers often form very loose columns with scattered individuals that harvest the seeds of various species. Foragers often climb vegetation to cut seeds directly from plants (pers. obs.). Colonies of P. coarctatus contain approximately 2000 workers (C. Smith, unpub. data).

Sexuals have been collected in nests from 18-20 December, mating flights were observed during late morning on 18 December, and foundresses have been collected from the ground on 21–23 January. Kusnezov (1951) observed a mating flight near mid-day on Jan 9, 1950, noting that it was a sunny, windless day and that isolated individuals flew from the nest. Queens of P. coarctatus are polyandrous with a mean ( ± SE) effective mating frequency of 8.5 ± 1.7 (range = 3.2–17.2, n = 10 colonies) (C. Smith, unpub. data). One queen of P. coarctatus had 27 ovarioles; queen dry mass averaged ( ± SE) 29.9 ± 1.8 mg (n = 3) (R.A. Johnson, unpub. data).

Castes
Johnson (2015) - Pogonomyrmex coarctatus is the only polymorphic species of Pogonomyrmex in South America. One feature of the [|P. coarctatus]-group species is that they exhibit a graded polymorphism that results from extending worker sizes, while maintaining the same allometry. These species range from the monomorphic P. marcusi and P. micans, to the moderately polymorphic P. lobatus that lacks supermajors, to the polymorphic P. coarctatus that has supermajors.

Nomenclature

 *  coarctatus. Pogonomyrmex coarctatus Mayr, 1868b: 170 (w.) ARGENTINA. Mayr, 1887: 614 (q.); Bruch, 1917a: 303 (m.). Senior synonym of striaticeps: Kusnezov, 1951a: 253. See also: Gallardo, 1932b: 150.
 * bruchi. Pogonomyrmex coarctatus r. bruchi Forel, 1913l: 217 (w.) ARGENTINA. Raised to species: Forel, 1914d: 268. Junior synonym of coarctatus: Johnson, 2015: 18.
 * striaticeps. Pogonomyrmex coarctatus var. striaticeps Emery, 1906c: 157 (footnote) (w.) ARGENTINA. Junior synonym of coarctatus: Kusnezov, 1951a: 253.

Worker
Johnson (2015) - Lectotype (n = 41). HL 3.37 (1.94–3.54); HW 3.66 (1.97–3.99); MOD 0.63(0.39–0.63); OMD 0.74 (0.42–0.94); SL 1.79 (1.28–2.10); PNW 1.97 (1.28–2.29); HFL 2.77 (1.75–2.90); ML 3.27 (2.20–3.54); PW 0.84 (0.43–0.95); PPW 0.98 (0.64–1.20). Indices: SI 48.91 (51.13–67.50); CI 108.61 (100.50–115.31); OI 17.21 (13.93–21.61); HFI 75.68 (71.57–91.88).

Highly polymorphic with supermajors. Head shape varies with worker size, quadrate in minors, increasingly wider than long in majors and supermajors (CI = 100.5–115.3, positively associated with head width, n = 42, R2 = 0.69, P < 0.0001; CI = 4.92*HW + 92.29); posterior margin concave medially in full-face view. Longitudinal cephalic striae very fine and dense, covering part to most of head but often indistinct, width of striae and interstriae similar; in full-face view, medial striae not diverging toward posterior corners of head. Cephalic interstriae weakly shining to shining. Vertex weakly striate, weakly shining to smooth and shining. Anterior margin of clypeus flat to weakly concave; dorsal surface with several moderately coarse, subparallel, longitudinal, oblique, or arcuate rugae. In profile, lateral lobe of clypeus not enlarged with wide gap between clypeal lobe and frontal lobe (gap width similar to width of antennal scape). Numerous long, curved, bristle-like, yellowish macrochaetae project from anterior margin of clypeus and ventral side of mandibles. Mandible with six teeth; mandibular dorsum coarsely rugose. MOD ranging from 0.15–0.23x HL. In profile, eyes situated anterior to middle of head, OMD = 1.00–1.58x MOD. Antennal scapes short (SI = 48.91–70.92), extending less than one-third the distance from posterior margin of eye to posterior corner of head. Base of antennal scapes smooth and shining, distal portion sometimes weakly granulate or weakly striate, weakly shining to shining; basal flange well-developed with carinate margin. Psammophore well-developed.

Mesosomal profile weakly to moderately convex; all mesosomal surfaces with prominent coarse, subparallel, weakly wavy to irregular, widely-spaced rugae. In dorsal view, humeral shoulders of pronotum weakly rounded to knoblike. Dorsum of promesonotum with longitudinal rugae that diverge anterad toward humeral shoulders of pronotum; transverse rugae on anterior surface of pronotum continue obliquely or longitudinally on pronotal sides; rugae on mesopleura angle posterodorsally. Superior propodeal spines moderately long, acuminate, connected by well-defined keel; spine length less than width between their bases; regular to weakly wavy transverse rugae on propodeal dorsum traverse ventrally or anteroventrally on sides. Inferior propodeal spines absent or very reduced in size, broadly rounded. Propodeal spiracles narrowly ovate facing posterad. Interrugae on mesosoma moderately granulate, weakly shining. Legs weakly granulate, weakly shining to smooth and strongly shining.

Peduncle of petiole about 0.7x as long as petiolar node, anteroventral margin with broadly rounded process. In profile, posterior surface of petiolar node weakly convex; node asymmetrical with anterior surface shorter than posterior surface, apex rounded. In dorsal view, petiolar node longer than wide, sides subparallel, slightly wider near middle, narrowing to spatulate to rounded anterior margin. Sides and posterior surface of petiolar node mostly smooth or with weak to moderately coarse, wavy to irregular, transverse, oblique, or longitudinal rugae, weakly shining. Dorsum of postpetiole convex in profile; in dorsal view, widest near posterior margin, narrowing to anterior margin, maximum width about equal to length; weakly to moderately granulate-punctate, occasionally with weak transverse rugae near posterior margin, weakly shining. First gastral tergum moderately coriarious, weakly shining to shining.

Erect white to yellowish pilosity sparse to moderately abundant on head, mostly similar in length, arising from foveae; longest hairs not exceeding MOD, few >0.5x MOD. Moderately abundant suberect pilosity on scape; abundant decumbent hairs on funicular segments. Legs with moderately abundant suberect to decumbent setae. Mesosoma with moderately dense erect setae that are mostly similar in length, longest approaching MOD; petiolar node, postpetiole, first gastral tergum with moderately dense erect setae that are mostly similar in length, longest notably shorter than MOD; long hairs on margins of posterior gastral terga often >MOD. Body mostly concolorous reddish-orange to reddish-brown.

Queen
Johnson (2015) - (n = 12). HL 3.35–3.61; HW 3.69–4.06; MOD 0.55–0.64; OMD 0.69–0.86; SL 1.79–2.09; PNW 2.21–2.49; HFL 2.42–2.89; ML 3.73–4.29; PW 1.01–1.20; PPW 1.37–1.49. Indices: SI 45.43–55.03; CI 107.93–114.87; OI 14.21–16.93; HFI 61.42–74.04.

Male
Johnson (2015) - (n = 12). HL 1.70–2.04; HW 1.79–2.05; MOD 0.58–0.73; OMD 0.19–0.36; SL 0.44–0.57; HFL 2.05–2.45; ML 3.21–3.78; PW 0.72–0.80; PPW 0.88–0.99. Indices: SI 23.23–29.38; CI 99.50–110.00; OI 32.22–36.87; HFI 103.02–131.11.

Etymology
Mayr (1868) did not discuss the naming of this species, and his reasoning for this name is unclear. The epithet coarctatus (from Latin, coarct = compressed, confined, or drawn close together, and –atus = suffix to denote provided with) apparently refers to an unknown structure that Mayr determined to be compressed or drawn close together.