Pogonomyrmex coarctatus

Robert A. Johnson, 2015, A taxonomic revision of South American species of the seed-harvester ant genus Pogonomyrmex (Hymenoptera: Formicidae). Part I, Zootaxa 4029 (1), pp. 1-142 : 18-27

Pogonomyrmex coarctatus

( Figures 2–5)

Pogonomyrmex coarctatus Mayr, 1868: 170 (worker). Syntypes examined: 2 workers [NMW], ARGENTINA, Buenos Aires: Bahía Blanca (Strobel leg.); Mayr, 1887: 614 (queen); Bruch, 1917: 303, figs. 1, 2 (male). See also Gallardo, 1932: 150, figs. 33–36. NMW worker here designated LECTOTYPE [CASENT0173362].

Pogonomyrmex coarctatus Mayr var. striaticeps Emery, 1906: 157 (worker, in footnote). Syntypes examined: 3 workers [MACN], ARGENTINA, Santa Fe: Rosario, #1609 (Hubrich leg.); Kusnezov, 1951: 253 (synonomy under coarctatus ; here confirmed). See also Gallardo, 1932: 156, figs. 37–38. MACN worker here designated LECTOTYPE [CASENT0249049].

Pogonomyrmex coarctatus race bruchi Forel, 1913: 217 (worker). Syntypes examined: 3 workers [MHNG], ARGENTINA, Buenos Aires: Bahía Blanca, Puerto Militar (Ingenieur Zelenka leg., 19 November 1913); 2 workers [MHNG], Buenos Aires: Estación Verónica, south of La Plata (Schuel leg.); 1 worker [MLPA], Buenos Aires. MHNG worker from Bahía Blanca here designated LECTOTYPE [CASENT0173345]. NEW SYNONOMY

Pogonomyrmex bruchi: Forel, 1914: 268 , raised to species. See also Gallardo, 1932: 157, fig. 39.

Worker. Diagnosis. 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 reddishbrown uniquely characterize this species ( Figures 2–3).

Measurements —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).

Redescription. 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 ( Figures 2–3).

Queen. Diagnosis. This caste is diagnosed by: (1) caste-specific morphology of the mesosoma related to wingbearing 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 ( Figure 4).

Measurements —(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. Diagnosis. 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 ( Figure 5).

Measurements —(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.

Additional material examined. ARGENTINA: Buenos Aires: Reserva Otamendi, 50’, Jan 21, 1999 & Nov 13, 2003 (CASC; CSC; RAJC); Tandil, Nov 20, 1959 (FML; LACM; MACN); Patagones, Jan 9, 1950 (FML); Bahía Blanca, Jan 3, 1964 & no date (MACN; MHNG; MZUSP; NMW); Medanos, Feb 4, 1938 (MLPA); San José Libertad, no date (MACN; MLPA); Azul, Apr 26, 1962 (USNM); Rosas, no date (USNM); Sierra Bayas, no date (LACM; MACN); Punta Picada, no date (MACN); Puerto Militar, no date (MACN); Punta Piedras, no date (MACN); Sierra de la Ventana, Nov 19, 2005 & no date (CSC; MACN; MCZ; MLPA); Coronel Suárez, Mar 1920 (MACN); Tres Arroyos, Mar 31, 1938 (MLPA); Monte Hermoso, no date (MACN; MLPA); Rt 33 at 14 km N Bahía Blanca, Nov 20, 2005 (CSC); 15 km SE Olavarría, Nov 18, 2005 (CSC); 32 km S Pehuajó, Nov 19, 2005 (CSC); Tornquist, Feb 7, 1947 (FML); no loc, no date (MACN; MLPA). Córdoba: Rt 9 at 6.5 mi E Bell Ville, 390’, Dec 21, 2005 (CSC; RAJC); Nono, 2940’, Jan 17, 2008 (RAJC); Rt 20 at 1.0 km N Nono, 2920’, Jan 23, 2006 (RAJC); Rt 9 at 3.5 km E Marcos Juárez, 330’, Jan 14, 2010 (RAJC); Alta Gracia La Granja, Sierras Córdoba, no date (MACN); Unguillo, no date (MLPA); no loc, no date (MACN). Entre Ríos: Parque Nacional El Palmar, Jan 6, 2006 (RGPC); Gualeguay, no date (MACN); Diamante, Mar 29, 1918 (MACN); San José de Feliciano, Dec 1972 (MACN); Rt 14 at 5.1 mi N Gualeguychú, 60’, Dec 17, 2005 (RAJC); Rt 39 at 36.1 km SE Basavilbaso, 160’, Feb 13, 2010 (RAJC); Estancia Sosa, no date (MACN; MLPA; USNM); La Picada, May 24, 1951 (FML; MCZ; MZUSP; USNM); 10.3 km S Jct Rts 14 & 39, 60’, Dec 17, 2005 (RAJC); Jct Rts 14 & 130, 70’, Dec 18, 2005 (CASC; MCZ; RAJC); Rt 16 at 6.1 km E Larroque, 190’, Jan 13, 2010 (RAJC); Rt 14 at 13.0 km N Ubajuay, 70’, Dec 18, 2005 (RAJC); Rt 18 at 7.3 mi SW San Salvador, 220’, Dec 19, 2005 (RAJC); Rt 18 at 20 km E Villaguay, 160’, Dec 19, 2005 (RAJC); Villaguay, 200’, Dec 19, 2005 (RAJC); Rt 18 at 3.3 mi NW Villaguay, 210’, Dec 19, 2005 (RAJC); Rt 18 at 12.8 mi W Villaguay, 150’, Dec 20, 2005 (RAJC); Rt 18 at 33.9 mi W Villaguay, 320’, Dec 20, 2005 (CSC; RAJC); Rt 127 at 2.0 km SW Federal, 230’, Jan 13, 2011 (RAJC); Polideportive Municipal General San Martín, Oct 28, 1954 (FML); Arroyo Tres Sauces, Jun 3, 1951 (FML); no loc, no date (USNM). La Pampa: Rt 188 at 28.1 km W Rancul, 950’, Jan 27, 2008 (RAJC); Monte Nievas, no date (MACN; USNM); Río Colorado, no date (MACN; MLPA); central La Pampa, no date (MACN). Río Negro: 35.6 km NW Jct Rts 251 & 2, 360’, Jan 22, 2011 (RAJC); Rt 22 at 15 km E Choele Choel, 400’, Jan 22, 2011 (RAJC); Rt 251 at 16.6 km N General Conesa, 390’, Jan 21, 2011 (RAJC); Rt 251 at 9.7 km S General Conesa, 230’, Jan 21, 2011 (RAJC); Sauce Blanco, no date (FML; LACM; MCZ; MZUSP); 51 km E Río Colorado, Nov 20, 2005 (CSC); no loc, no date (MACN; MLPA). San Luis: Cortaderas, Jan 29, 1958 (FML; LACM). Santa Fe: Rt 19 at 14.5 mi W Santa Clara de Buen Vista, 240’, Dec 20, 2005 (CSC; RAJC); Rt 34 at Cañada Rosquín, 230’, Dec 21, 2005 (CSC; RAJC); Rosario, no date (MACN); Fives Lille, no date (MCZ); Saladillo, no date (USNM). URUGUAY: no loc, no date (MCZ; MLPA; USNM). Florida: Florida, Jan 1952 (FML). Lavelleja: Villa Serrana, Jun 7, 1961 (USNM); Arequita, Jan 6, 1951 (FML). Maldonado: Maldonado, Feb 15, 1960 (FML). Montevideo: Montevideo, no date (MACN); near Montevideo, Feb 20, 1976 (LACM). Treinta y tres: Santa Clara de Olimar, Jan 9, 1960 (FML). Locations not found. ARGENTINA: Buenos Aires: Tuzumba, no date (MLPA); Punta dellnolio (?), Dec 1, 1957 (USNM). Río Negro: Menatia, Oct 2, 1914 (MLPA). URUGUAY: Canelones: Crizicon Falzon, no date (MLPA). Paysandú: Rt 30, km 476, Feb 25, 1961 (FML). Río Negro: Rt 24, Feb 25, 1961 (FML). Depto Unknown : Rt 2, km 214, Feb 26, 1961 (FML); C Mazangano, km 20, no date (FML); C de Amaro, Rt 8, Feb 18, 1961 (FML). Questionable locales (appear to be outside of geographic range): ARGENTINA: Catamarca: central Catamarca, no date (MACN). Mendoza: no loc, no date (MACN) ( Figure 6 A).

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.

Discussion. Pogonomyrmex coarctatus co-occurs with two other P. coarctatus -group species, P. lobatus and P. 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 reddishbrown. 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.

Pogonomyrmex coarctatus var. striaticeps was erected because its cephalic sculpturing was more fine and dense than that on the type specimen (see also Gallardo, 1932). Kusnezov (1951) synonomized P. coarctatus var. striaticeps without discussion. I concur with this synonomy because the degree of cephalic sculpturing in syntypes of P. coarctatus var. striaticeps occurs within colonies of P. coarctatus .

Kusnezov (1951) provided a thorough discussion on P. bruchi , indicating that it was closely related to P. coarctatus , and that it could be distinguished from the latter species by its smaller size, but more importantly by its much weaker degree of worker polymorphism. He also noted that the geographic range of P. bruchi occurred within that of P. coarctatus suggesting two possibilities: (1) P. bruchi was a distinct species given that its geographic range was entirely contained within that of P. coarctatus , and (2) P. bruchi was an insignificant variety of P. coarctatus in the event that the two taxa lacked morphological differences.

In his key to South American Pogonomyrmex, Taber (1998) noted that P. coarctatus and P. bruchi differed in the size of the trough at the base of the scape (conspicuous in P. coarctatus , absent to weak in P. bruchi ), the thickness of the femur on the foreleg (weakly incrassate in P. coarctatus , strongly incrassate in P. bruchi ), and in the keel between the superior propodeal spines (keel lacking in P. coarctatus , keel present in P. bruchi ). Cuezzo and Claver (2009) used these same characters to separate P. coarctatus and P. bruchi .

I have examined syntypes of P. coarctatus and P. bruchi , and there are no consistent differences between these two taxa. None of the characters discussed by Taber (1998) consistently differ between the two taxa, and variation that he noted occurs within colonies of P. coarctatus . Thus, I place P. bruchi as a junior synonym of P. coarctatus . Moreover, erecting P. bruchi as a species likely resulted from collecting few specimens and/or not examining nests to determine if workers were polymorphic.

Biology. 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).

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 ( Figure 7). These species range from the monomorphic P. m a rc u s i and P. m i c an s, to the moderately polymorphic P. lobatus that lacks supermajors, to the polymorphic P. coarctatus that has supermajors (also see below—life history and phenotypic diversity).

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).

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 ( Figure 6 A).