Pogonomyrmex mendozanus

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 : 87-93

Pogonomyrmex mendozanus

( Figures 43–46)

Pogonomyrmex mendozanus Cuezzo and Claver, 2009: 101, figs. 1, 2, 5–11 (worker, queen, male). Types examined: holotype worker (not examined) [IADIZA], 10 worker paratypes [FML], ARGENTINA, Mendoza: Departamento Santa Rosa, Ñancuñán (S. Claver leg., 12 February 1997).

Pogonomyrmex pronotalis Santschi : Claver & Fowler, 1993: 191; Pirk et al., 2004: 65; Pirk & Lopez de Casenave, 2006: 119; Pirk et al., 2007: 1092; Pol & Lopez de Casenave, 2004: 647; Pol et al., 2008: 92; Pirk et al., 2009: 908.

Worker. Diagnosis. Within the P. laticeps -group, the combination of: (1) head and mesosoma black, gaster dark ferruginous orange, and (2) head and mesosoma covered by fine, regular, incised rugae uniquely characterize this species ( Figure 43).

Measurements —(n = 16). HL 1.65–2.04; HW 1.87–2.28; MOD 0.37–0.49; OMD 0.47–0.67; SL 1.25–1.59; PNW 1.20–1.41; HF 1.79–2.28; ML 2.16–2.58; PW 0.49–0.67: PPW 0.65–0.82. Indices: SI 65.45–75.66; CI 102.50–113.33; OI 20.27–23.04; HFI 94.14–106.95. See also Cuezzo & Claver (2009).

Redescription. Head subquadrate to wider than long (CI = 102.50–113.33), widest just posterior to eyes; posterior margin flat to weakly concave in full-face view. Cephalic dorsum with fine, regular, incised, longitudinal rugae; in full-face view, medial rugae diverging weakly toward posterior corners of head. In profile, rugae posterior to eyes converging toward vertex. Cephalic interrugae appearing as furrows, weakly to moderately coriarious, weakly shining; vertex rugose. Anterior margin of clypeus weakly to moderately concave; dorsal surface with numerous subparallel, longitudinal rugae. Mandible with six teeth; mandibular dorsum coarsely rugose. Numerous long, curved, bristle-like, yellowish to brownish hairs project from anterior margin of clypeus and basolateral margin of mandibles. MOD ranging from 0.20–0.25x HL. In profile, eyes situated near middle of head, OMD = 1.24–1.43x MOD. Antennal scapes relatively long (SI = 65.45–75.66), reaching vertex or failing to reach vertex by less than length of basal funicular segment; scapes often with moderately coarse longitudinal striae. Basal flange of scape flattened with carinate margin. Psammophore well-developed.

Promesonotal profile flattened, propodeum descending; all mesosomal surfaces with prominent subparallel/ parallel rugae similar to that on cephalic dorsum. In profile and dorsal views, humeral shoulders of pronotum angulate, distinctly elevated above medial portion of pronotum. Dorsum of promesonotum with longitudinal rugae that diverge anterad toward humeral shoulders of pronotum; anterior margin of pronotum with transverse rugae that traverse posteroventrally or obliquely on pronotal sides. Mesopleura with transverse rugae, those near dorsal margin often traversing posterodorsally. Dorsum and sides of propodeum with transverse rugae. Superior propodeal spines long, acuminate; spines longer than distance between their bases. Inferior propodeal spines absent or reduced to indistinct broadly rounded processes. Propodeal spiracles narrowly ovate facing posterad. Interrugae on mesosoma weakly coriarious, weakly shining to shining. Legs moderately coriarious, weakly shining.

Peduncle of petiole slightly shorter than petiolar node, anteroventral margin usually flat, lacking tooth or lobe. In profile, petiolar node asymmetrical with anterior surface shorter than posterior surface; apex weakly angulate to rounded. In dorsal view, petiolar node longer than wide, widest near spatulate anterior margin; posterior surface with numerous fine, transverse to arcuate, wavy striae that curve posteroventrally on sides. Dorsum of postpetiole convex in profile; in dorsal view, widest near posterior margin, narrowing anterad; maximal width about equal to length; dorsum and sides with numerous transverse, wavy striae that are finer, denser than those on posterior surface of petiolar node. Interrugae on posterior surface of petiolar node and dorsum of postpetiole weakly to moderately punctate or coriarious, weakly shining to shining. First gastral tergum weakly to moderately coriarious, weakly shining to shining.

Erect white pilosity moderately abundant on head, variable in length, longest hairs not exceeding MOD. Moderately abundant suberect to semidecument pilosity on scape, abundant decumbent hairs on funicular segments. Legs with moderately abundant suberect white setae. Mesosoma, petiolar node, postpetiole with moderately dense, erect white setae, often similar in length, longest on pronotum, none>MOD; gastral terga with moderately dense pilosity, only those on posterior gastral terga approaching MOD. Head, mesosoma black; petiolar node dark orangish-brown to orangish-black; postpetiole, gaster lighter orangish-brown; mandibles, circumference of eyes, tarsi often dark reddish-brown ( Figure 43).

Brachyterous queen. Diagnosis. This caste is diagnosed by: (1) brachypterous with very small wings and small ocelli on head, (2) in dorsal view, mesoscutum poorly-developed, anterior margin barely surpassing humeral shoulders of pronotum, (3) pronotum well-developed, (4) in profile, the pronotum rises at an approximately 45o angle to meet the mesoscutum, (5) fine, regular, longitudinal rugae on head, mesoscutum, and mesoscutellum, and (6) head and mesosoma black; gaster dark ferruginous orange ( Figure 44).

Measurements —(n = 3). HL 2.07–2.53; HW 2.44–2.81; MOD 0.50–0.53; OMD 0.55–0.81; SL 1.58–1.74; PNW 1.54–1.68; HFL 2.28–2.48; ML 2.59–2.76; PW 0.70–0.77; PPW 1.00–1.05. Indices: SI 61.92–66.13; CI 111.07–119.81; OI 18.86–20.56; HFI 88.26–95.16. See also Cuezzo & Claver (2009).

Male. Diagnosis. This caste is diagnosed by: (1) first gastral tergum lacking striae, (2) weakly regular to regular, subparallel rugae prominent on sides of head, pronotal sides, and mesopleura, (3) in profile, rugae forming circumocular whorls posterior to eyes, and (4) notauli present ( Figure 45).

Measurements —(n = 12). HL 1.27–1.67; HW 1.44–1.68; MOD 0.46–0.56; OMD 0.23–0.40; SL 0.37–0.56; HFL 1.53–2.19; ML 2.47–2.91; PW 0.50–0.60; PPW 0.70–0.79. Indices: SI 23.42–34.15; CI 98.16–113.39; OI 29.34–33.94; HFI 106.25–133.54. See also Cuezzo & Claver (2009).

Additional material examined. ARGENTINA: Mendoza: Reserva Nancuñan, 1800’, Dec 12, 2003 & Oct 30, 2008 & Feb 14, 2010 (CASC; RAJC; RGPC); Fadina, Nancuñan, Feb 14, 2010 (RAJC); 115.0 km S Jct Rts 7 & 153, 1730’, Jan 23, 2008 (RAJC); 138.1 km S Jct Rts 7 & 153, 1690’, Jan 23, 2008 (RAJC); 22.1 km S Jct Rts 7 & 153, 1900’, Jan 23, 2008 (RAJC); 38.4 km S Jct Rts 7 & 153, 1870’, Jan 23, 2008 (RAJC); Rt 171 at 15.0 km SE Monte Coman, 1700’, Jan 27, 2008 (RAJC); Rt 7 at 10.3 km E La Paz, 1540’, Jan 23, 2008 (RAJC); Rt 7 at 9.8 km E La Dormida, 1720’, Jan 23, 2008 (RAJC); Rt 7 at 29.8 km E La Dormida, 1610’, Jan 23, 2008 (MCZ; RAJC); Rt 7 at 5.8 km W Desaguadero, 1530’, Dec 28, 2005 (RAJC); Rt 7 at 8.5 km W Desaguadero, 1520’, Mar 15, 2015 (RAJC); 20 mi SE Mendoza, Dec 26, 2006 (CSC); 8.5 km NW Real de Padre, Nov 22, 2005 (CSC); Rt 7 at 11.5 km W Santa Rosa, 2030’, Mar 15, 2015 (RAJC). San Juan: Nueva Castilla, Dec 1964 (MZUSP); Pie de Palo, Mar 11, 1920 (LACM; MCZ); Rt 20 near El Encón, no date (RGPC). San Luis: Rt 20 at 0.6 km SE border with San Juan, 1610’, Mar 16, 2015 (RAJC) ( Figure 42 B).

Etymology. The specific epithet, mendozanus (Latinization of Mendoza), is derived from the type locality occurring in Mendoza Province, Argentina.

Discussion. Pogonomyrmex mendozanus was misidentified as P. pronotalis in a list of ant species in Ñancuñan Biosphere Reserve (Claver & Fowler, 1993) and in subsequent publications (see above) until it was described by Cuezzo & Claver (2009). However, these two species are easily separated with coloration being one of the few characters in common. In describing P. mendozanus, Cuezzo & Claver (2009) listed several characters to distinguish it from P. pronotalis . Sculpturing on the head and mesosoma is the best character: rugae on the cephalic dorsum of P. mendozanus are fine, very regular, and incised, whereas they are coarse and irregular on P. pronotalis . Lack of striae on the first gastral tergum combined with the fine, regular, incised sculpturing on the head and mesosoma and coloration pattern separate P. mendozanus from all South American congeners.

Biology. Pogonomyrmex mendozanus is one of the more well-studied species of Pogonomyrmex in Argentina. This species harvests the seeds of various grass species and nongrass species to a lesser extent (Pirk & Lopez de Casenave, 2006, 2010, 2011; Pirk et al., 2007; Pirk et al., 2009; Pol & Lopez de Casenave, 2004; Pol et al., 2011), and it is a solitary forager that recruits nestmates to high-density seed patches (Pol et al., 2015). Five colonies averaged 108–186 foragers, which consisted of an estimated 10–13% of all workers in the nests (Nobua-Behrmann et al., 2013). Nests have a tumulus that ranges up to 40 cm in diameter; a midden of seed chaff sometimes surrounds the nest. Colonies of P. mendozanus are relatively small: two excavated colonies in Reserva Ñancuñan contained an average of 615 workers plus 292 larvae and pupae (Nobua Behrmann et al., 2010). Colony size was similar in another study, but varied by grazing intensity: colonies in lightly grazed areas contained 731 ± 249 (n = 12; mean ± SD) individuals including brood (535 ±105 of which were workers), while those in heavily grazed areas contained 557 ± 325 (n = 12) individuals including brood (382 ± 230 of which were workers). All colonies contained one reproductive queen (n = 19) (R.G. Pol, pers. comm.). Worker dry mass averaged 3.87 mg, while that of brachypterous queens averaged 9.16 mg; the mean queen to worker dry mass ratio was 2.37 (n = 2 colonies) (R.A. Johnson, unpub. data).

Collection dates for sexuals range from 28 December to 15 March, and mating flights have been observed on 21 January, 14–15 February, and 15 March. Flights occur during early to late afternoon on days following rain. Mating involves the brachypterous queens leaving their natal nest to mate at aggregations in low-growing vegetation at the top of or near their natal nest; aggregations are often small and contain approximately 20–30 individuals, but over 100 brachypterous queens have been observed on vegetation outside their nests (pers. obs.; R.G. Pol, pers. comm.) ( Figure 46). Queens mate with multiple males with an average (± SD) effective mating frequency of 8.75 ± 3.26 (range = 4–16; n = 24) (Pol et al., 2008). After mating, queens leave the aggregation to initiate a nest using independent colony founding (R.G. Pol, pers. comm.), which is an unusual behavior for brachypterous queens (see Johnson, 2010). No information is available, but these brachypterous queens are probably obligate foragers (see Peeters et al., 2012).

Pogonomyrmex mendozanus inhabits sites at elevations from 465–545 m. This species is common from southeastern San Juan to eastcentral Mendoza, and it appears to be restricted to the northern one-third of the Low Monte Desert ecoregion as defined by Olson et al. (2001) ( Figure 42 B). Interestingly, the distribution of P. mendozanus stops at the southern boundary of the High Monte Desert ecoregion. Field observations suggest that P. mendozanus is restricted to deep, loose, sandy soils (pers. obs.); nests are most common in open, disturbed areas such as roadsides (Pirk et al., 2004).