Pogonomyrmex marcusi

Pogonomyrmex marcusi are solitary foragers that specialize on collecting grass seeds.

Identification
Johnson (2015) - Worker Diagnosis. Within the P. coarctatus-group, the combination of: (1) longitudinal striae cover part to most of cephalic dorsum, areas lacking striae shining to strongly shining, (2) in profile, lateral lobe of clypeus weakly enlarged, (3) interrugae on mesosoma smooth, strongly shining, (4) inferior propodeal spines wider than long, apex bluntly angulate to acuminate, and (5) head reddish-orange, first gastral tergum reddish-brown; rest of body reddish-orange to reddish-black uniquely characterize this species.

Queen 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 = 2.91 mm), (3) striae cover most of cephalic dorsum, and (4) inferior propodeal spines well-developed, triangular, apex bluntly angulate to acuminate.

Male This caste is diagnosed by: (1) first gastral tergum lacking striae, (2) large (HW > 1.65 mm; HL > 1.75 mm; ML > 2.95 mm), (3) hairs on head (especially posterior to eyes) and dorsum of mesosoma dense, long and flexuous, length of numerous hairs approach to exceed MOD, (4) mandibles with three teeth, and (5) interrugae on katepisternum weakly shining to shining.

Pogonomyrmex marcusi is not known to occur near any P. coarctatus-group species or any other species of Pogonomyrmex. Additionally, P. marcusi is the only P. coarctatus-group species that occurs at mid- to high elevations (2660 m), while the other three species are only known to occur below 1000 m. Pogonomyrmex marcusi is easily distinguished from other P. coarctatus-group species using characters given in the key.

Distribution based on Regional Taxon Lists
Neotropical Region: Bolivia, Peru.

Habitat
Pogonomyrmex marcusi is only known from arid habitats near Cochabamba, Bolivia, at elevations of approximately 2660 m (Marcus & Marcus, 1951). This species occurs in the Bolivian Montane Dry Forest ecoregion as defined by Olson et al. (2001). I also have an unverified record of P. marcusi (collection at Universidade Estadual de Santa Cruz, Brazil) from Valle Sagrado, near Cusco, Peru. Overall, P. marcusi is the only P. coarctatus-group species that occurs at mid- to high elevations. (Johnson 2015)

Biology
Pogonomyrmex marcusi is a solitary forager that travels up to 20 m from the nest to harvest the seeds of various grass species, but a file of foragers was observed returning to nests (Marcus & Marcus, 1951). Nests are cryptic and difficult to locate without finding a forager to follow back to the nest. Nests usually have several entrances that are placed under or at the edge of stones or under plants (Marcus & Marcus, 1951). Nest excavations indicate that P. marcusi colonies contain 600–700 workers plus 200 larvae and pupae (Marcus, 1953; Marcus & Marcus, 1951).

Sexuals of P. marcusi have been collected on 28 January, suggesting that mating flights occur during the austral summer. Marcus and Marcus (1951) also noted that several nests contained ergatogynes in addition to workers. It is unclear what they meant by ergatogynes, which they described only as having well-developed ovaries. By comparison, they indicated that workers had a smaller gaster and poorly-developed ovarioles with small eggs. I did not encounter any ergatogynes while examining individuals during this study.

Nomenclature

 *  marcusi. Pogonomyrmex marcusi Kusnezov, 1951a: 260 (w.) BOLIVIA (diagnosis in key). Kusnezov, 1954c: 82 (w.q.m.); Marcus, 1953: 46 (l.); Wheeler, G.C. & Wheeler, J. 1960b: 3 (l.).

Description
Johnson (2015):

Worker
lectotype (n = 5 plus 8 paralectotypes). HL 1.94 (1.88–2.24); HW 2.04 (1.91–2.33); MOD 0.41 (0.38–0.44); OMD 0.40 (0.39–0.48); SL 1.37 (1.10–1.55); PNW 1.30 (1.14–1.44); HFL 1.75 (1.69–2.15); ML 2.08 (2.20–2.75); PW 0.44 (0.44–0.53); PPW 0.59 (0.59–0.69). Indices: SI 67.16 (57.59–70.92); CI 105.15 (100.000–110.00); OI 20.10 (18.03–20.95); HFI 85.78 (83.18–95.13).

Monomorphic, head quadrate to wider than long (CI = 100.00–110.00), posterior margin flat to weakly concave in full-face view. Fine, dense, longitudinal striae cover only part of cephalic dorsum, width of striae and interstriae similar; in full-face view, medial striae not diverging toward posterior corners of head. Cephalic interstriae, most of cephalic dorsum, and vertex smooth, strongly 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 usually weakly enlarged. 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 0.18–0.22x HL. In profile, eyes situated anterior to middle of head, OMD = 0.88–1.18x MOD. Antennal scapes short (SI = 57.59–70.92), extending less than one-third the distance from posterior margin of eye to posterior corner of head. Antennal scapes smooth and 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 anteriorly toward humeral shoulders of pronotum; transverse rugae on anterior surface of pronotum continue obliquely or ventrally on pronotal sides; rugae on mesopleura angle posterad or posterodorsally. Superior propodeal spines long, acuminate, connected by well-defined keel; spine length slightly less than width between their bases; regular to weakly wavy, transverse rugae on propodeal dorsum traverse ventrally or anteroventrally on sides. Inferior propodeal spines wider than long, apex bluntly angulate to acuminate. Propodeal spiracles narrowly ovate facing posterad. Interrugae on mesosoma smooth, strongly 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 notably shorter than posterior surface, apex rounded. In dorsal view, petiolar node longer than wide, sides subparallel, slightly wider near middle, narrowing to spatulate anterior margin. Sides and posterior surface of petiolar node with coarse, wavy to irregular transverse, oblique, or longitudinal rugae. Dorsum of postpetiole convex in profile; in dorsal view, widest near posterior margin, narrowing to anterior margin, maximum width about equal to length. Regular to wavy, weak to moderately coarse, transverse rugae on dorsum of postpetiole, sometimes weakening or lacking near anterior margin; interrugae on posterior surface of petiolar node and dorsum of postpetiole smooth to moderately granulate, weakly shining to shining. First gastral tergum smooth and strongly shining.

Erect white to yellowish pilosity sparse to moderately abundant on head, mostly similar in length and 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; length of hairs on margins of posterior gastral terga often >MOD. Head reddish-orange, first gastral tergum reddish-brown; rest of body reddish-orange to reddish-black.

Queen
(n = 1). HL 2.74; HW 2.91; MOD 0.56; OMD 0.63; SL 1.61; PNW 2.06; HFL 2.43; ML 3.35; PW 0.87; PPW 1.17. Indices: SI 55.33; CI 106.20; OI 19.24; HFI 83.51.

Male
HL 1.69; HW 1.77; MOD 0.54; OMD 0.31; SL 0.45; HFL 2.09; ML 2.97; PW 0.63; PPW 0.78. Indices: SI 25.42; CI 104.73; OI 30.51; HFI 118.08.

Etymology
Kuznezov named this species after Dr. Harry Marcus, who collected the syntypes and studied this species.