Pogonomyrmex laticeps

The two queen forms in this species, ergatoid and brachypterous queens, are both flightless.

Identification
Johnson (2015) - Worker Within the P. laticeps-group, the combination of: (1) head and mesosoma dark reddish-black, gaster black, (2) rugae on promesonotum transverse, oblique, or irregular, rarely longitudinal, and (3) medial rugae along posterior margin of head usually partly rugoreticulate uniquely characterize this species.

Ergatoid Queen This caste is diagnosed by: (1) ergatoid, with small ocelli on head and mesosoma lacking morphological structures related to wings, (2) larger species (HW > 1.40 mm), (3) first gastral tergum smooth and shining to moderately coriarious, (4) interrugae on cephalic dorsum smooth and shiny, and (5) head, mesosoma, petiolar node, postpetiole dark reddish-black; gaster blackish.

Brachypterous Queen 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 45° angle to meet the mesoscutum, and (5) head and mesosoma dark reddish-black, gaster black.

Male The combination of: (1) first gastral tergum lacking striae, (2) rugae on dorsum of postpetiole usually longitudinal, interrugae weakly shining to shining, (3) dorsum of mesoscutum and mesoscutellum with prominent wavy to weakly irregular longitudinal rugae, interrugae weakly shining to shining, (4) in dorsal view, dorsum of propodeum depressed, lacking rugae, weakly shining to shining; prominent rugae traverse posteroventrally lateral to depression, and (5) notauli present.

Pogonomyrmex laticeps is not known to co-occur with Pogonomyrmex mendozanus or Pogonomyrmex tinogasta. Pogonomyrmex laticeps can be distinguished from P. mendozanus by the coarse, irregular rugae on the head and mesosoma, whereas cephalic and mesosomal rugae on P. mendozanus are fine, incised, and very regular. Pogonomyrmex laticeps can be distinguished from P. tinogasta based on the following characters: (1) head and mesosoma dark reddish-black, gaster black, (2) rugae on promesonotum transverse, oblique, or irregular, rarely longitudinal, and (3) medial rugae along posterior margin of head usually partly rugoreticulate. In P. tinogasta: (1) the body is concolorous black except for a dark reddish band encircling the eye, (2) the promesonotal rugae are longitudinal, usually regular, and (3) the medial rugae along the posterior margin of the head are longitudinal, rarely rugoreticulate.

Distribution based on Regional Taxon Lists
Neotropical Region: Argentina.

Habitat
Pogonomyrmex laticeps inhabits sites at elevations from 240-2135 m. This species occurs from northwestern San Luis to southcentral Salta, and it is restricted to northern and central portions of the High Monte Desert and southwestern portions of the Dry Chaco ecoregions as defined by Olson et al. (2001)

Biology
Johnson (2015) - Pogonomyrmex laticeps is a solitary forager that harvests seeds. Nests usually have a tumulus that is up to 15 cm in diameter, sometimes with an external midden of seed chaff. Colonies of P. laticeps are relatively small: Kusnezov (1951) estimated colony size at 25–50 workers, but I have excavated colonies with up to 200 workers (Peeters et al., 2012), suggesting that colony size probably ranges up to 300–400 workers. Colonies in southern portions of the range sometimes contain up to 1000 workers, and very loose columns of scattered foragers have been observed.

This species is interesting morphologically because it has two non-flying queen phenotypes—ergatoid and brachypterous queens (Peeters et al., 2012). Only one queen phenotype is produced within a colony, and both queen phenotypes have similar reproductive potentials with a spermatheca and 12–15 ovarioles (Peeters et al., 2012). Colonies of both phenotypes can produce >50 queens, which Peeters et al. (2012) used to suggest that both queen phenotypes initiate nests using independent colony founding. Little is known about mating, but ergatoid queens have been collected from 6 February to 16 April. Brachypterous queens were larvae, pupae, or callows near the end of January and mature brachypterous queens have been collected from 20 February to 9 April. Mating has not been observed, but both ergatoid and brachypterous foundresses have been observed running on the ground (Mar 23 for ergatoid queens; Mar 24–25 for brachypterous queen), and both phenotypes exhibit independent colony founding; one haplometrotic ergatoid queen was excavated on March 22, and one haplometrotic brachypterous queen was excavated on March 24. Both ergatoid and brachypterous foundresses were observed to forage outside the nest (pers. obs.). Mating activities and colony founding appeared to have been triggered by late summer rains.

Castes
This species has dimorphic queens: ergatoid (permanently wingless) and brachypterous (short, non-functional wings). Surveys by Peeters et al. (2012) in western Argentina indicated that colonies near Chilecito, La Rioja Province, produced only ergatoid queens, while those near Punta Balasto, Catamarca Province (263 km away), produced only brachypterous queens. Brachypterous queens were significantly larger than ergatoid queens for 10 of 11 external characters, but both phenotypes had comparable reproductive potential, i.e., a spermatheca and a similar number of ovarioles. Using normal winged queens of the closely related P. uruguayensis for comparison, we determined that both queen phenotypes in P. laticeps had a full set of dorsal thoracic sclerites, albeit each sclerite was much reduced, whereas workers have a thorax without distinct dorsal sclerites. Sclerites were fused and immobile in ergatoid queens, while they were separable and fully articulated in brachypterous queens. Both phenotypes lacked the big indirect flight muscles, but brachypterous queens retained the tiny direct flight muscles. Overall, this dimorphism across populations indicates that there are alternative solutions to selective pressures against flying queens. We lack field data about colony founding strategy (independent or dependent) for either queen phenotype, but colonies at both sites produced numerous gynes, and we infer that all foundresses initiate colonies independently and are obligate foragers.

Nomenclature

 *  laticeps. Pogonomyrmex laticeps Santschi, 1922b: 350 (w.) ARGENTINA. Kusnezov, 1951a: 274 (ergatoid q.).

Description
Johnson (2015):

Worker
Lectotype (n = 52). HL 2.04 (1.56–2.07); HW 2.20 (1.65–2.25); MOD 0.37 (0.30–0.40); OMD 0.53 (0.39–0.64); SL 1.39 (1.08–1.59); PNW 1.38 (1.04–1.39); HFL 2.08 (1.45–2.24); ML 2.67 (1.85–2.62); PW 0.55 (0.0.39–0.59); PPW 0.79 (0.59–0.83). Indices: SI 63.18 (60.34–81.52); CI 107.84 (101.12–115.20); OI 16.82 (16.28–23.20); HFI 94.55 (81.22–110.56). See also Peeters et al. (2012).

Head subquadrate to wider than long (CI = 101.12–115.20), widest just posterior to eye; posterior margin flat in full-face view. Longitudinal rugae on cephalic dorsum prominent, weakly wavy to irregular, sometimes weakly rugoreticulate on medioposterior margin; in full-face view, medial rugae diverging weakly toward posterior corners of head. In profile, rugae posterior to eyes converging toward vertex. Cephalic interrugae weakly granulate, shining; vertex rugose. Anterior margin of clypeus flat to weakly concave; dorsal surface with numerous subparallel, longitudinal rugae. Mandible with six teeth; mandibular dorsum coarsely rugose. Numerous long, curved, bristle-like, yellowish hairs project from anterior margin of clypeus and basolateral margin of mandibles. MOD ranging from 0.18–0.23x HL. In profile, eyes situated near middle of head, OMD = 1.19–1.60x MOD. Antennal scapes relatively long (SI = 60.34–81.52), failing to reach vertex by up to length of basal funicular segment; scapes smooth and shining, distal portion often weakly striate. Basal flange of scape flattened, well-developed with carinate margin. Psammophore well-developed.

Mesosomal profile flat to weakly convex; all mesosomal surfaces with prominent rugae. In profile and dorsal views, humeral shoulders of pronotum sometimes angulate, weakly elevated above medial portion of pronotum. Dorsum of promesonotum and sides of pronotum with coarse, transverse, longitudinal or oblique, irregular rugae, rugoreticulate to vermiculate; promesonotal suture weakly impressed on occasional workers. Mesopleura with wavy to irregular rugae angling posterodorsally, rugae often more irregular to rugoreticulate near anterodorsal margin. Dorsum of propodeum with transverse to irregular rugae that traverse anteroventrally on sides. Propodeum with long, acuminate spines connected by well-defined keel; spine length similar to or slightly longer than distance between their bases. Inferior propodeal spines absent or reduced to indistinct rounded or triangular process. Propodeal spiracles narrowly ovate facing posterad. Interrugae on mesosoma smooth, strongly shining. Legs moderately coriarious, weakly shining.

Peduncle of petiole about 0.8x as long as petiolar node, anteroventral margin usually with rounded to triangular tooth-like process. In profile, petiolar node asymmetrical with anterior surface about one-half as long as posterior surface; apex weakly angulate to rounded. In dorsal view, petiolar node longer than wide, sides subparallel to slightly wider near spatulate anterior margin; posterior surface and sides with moderately strong, wavy to irregular, transverse to arcuate rugae. Dorsum of postpetiole convex in profile; in dorsal view, widest near posterior margin, narrowing to anterior margin; maximal width about equal to length; dorsum and sides with wavy to irregular transverse rugae that are finer, denser than on posterior surface of petiolar node. Interrugae on posterior surface of petiolar node smooth and strongly shining, weakly to moderately granulate, weakly shining on dorsum of postpetiole. First gastral tergum moderately coriarious, weakly shining to smooth, strongly shining.

Erect whitish 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 semidecumbent white setae. Mesosoma, petiolar node, postpetiole, and gastral terga with moderately dense, erect setae, variable in length, only those on posterior gastral terga approaching MOD. Head, mesosoma dark orangish-black to reddish-black; petiolar node, postpetiole slightly darker; legs, gaster dark brown to black.

Queen
Ergatoid. (n = 25). HL 1.85–2.18; HW 1.99–2.32; MOD 0.35–0.44; OMD 0.5–0.67; SL 1.25–1.65; PNW 1.28–1.45; HFL 1.83–2.24; ML 2.26–2.75; PW 0.49–0.64; PPW 0.78–0.96. Indices: SI 53.88–76.39; CI 104.19–113.00; OI 16.09–20.39; HFI 83.04–100.00. See also Peeters et al. (2012).

Brachypterous. (n = 30). HL 1.90–2.36; HW 2.09–2.49; MOD 0.40–0.46; OMD 0.55–0.69; SL 1.35–1.66; PNW 1.29–1.59; HFL 2.01–2.44; ML 2.26–2.87; PW 0.53–0.63; PPW 0.83–1.02. Indices: SI 57.26–70.09; CI 104.66–119.29; OI 16.46–19.66; HFI 84.10–102.14. See also Peeters et al. (2012).

Male
(n = 12). HL 1.20–1.43; HW 1.32–1.52; MOD 0.43–0.55; OMD 0.22–0.32; SL 0.34–0.42; HFL 1.50–1.96; ML 2.13–2.62; PW 0.44–0.58; PPW 0.65–0.81. Indices: SI 23.68–31.82; CI 103.50–122.58; OI 30.94–36.18; HFI 107.91–146.27.

Type Material
Syntypes: 2 workers [MACN], ARGENTINA, Catamarca: Masao, #1376 (Weiser leg., March 1921). worker [CASENT0217255] designated LECTOTYPE by Johnson, 2015: 80.

Several authors have misinterpreted the type locality for P. laticeps. In describing the species, Santschi (1922) listed the type locality as Masao, Catamarca Province, Argentina (see also Baldini & Scattolin, 1993). Alternatively, Gallardo (1932) listed the type locality as Masas, and Kempf (1972) apparently questioned the locality by indicating it as  (=Mazán?). Additionally, the label on an MCZ specimen was interpreted as Valle Masan.

Etymology
Johnson (2015) - The specific epithet, laticeps (from Latin, latus = wide, and the suffix -ceps = head), is derived from the wide head of this species; in his description, Santschi noted that the head was clearly wider than long.