Pogonomyrmex lobatus

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 : 27-32

Pogonomyrmex lobatus

( Figures 8–10)

Pogonomyrmex lobatus Santschi, 1921: 96 (worker). Syntypes examined: 6 workers [MACN], 1 worker [MCZ], 1 worker [MZUSP], ARGENTINA, Entre Ríos: Villaguay (Charles Bruch leg.). See also Gallardo, 1932: 160, fig. 41; Kusnezov, 1951: figs. 13b, 13c. MCZ worker here designated LECTOTYPE [CASENT0217260].

Worker. Diagnosis. Within the P. coarctatus -group, the combination of: (1) fine, dense, longitudinal striae cover cephalic dorsum, (2) in profile, lateral lobe of clypeus massively enlarged with narrow gap between lateral lobe and frontal lobe, the latter two structures nearly in contact, and (3) body mostly concolorous reddish-orange to reddishbrown uniquely characterize this species ( Figure 8).

Measurements —lectotype (n = 41). HL 2.31 (1.93–2.97); HW 2.39 (1.99–3.22); MOD 0.43 (0.38–0.60); OMD 0.43 (0.46–0.74); SL 1.42 (1.27–1.90); PNW 1.42 (1.26–1.93); HFL 1.94 (1.81–2.55); ML 2.66 (2.13–3.45); PW 0.52 (0.47–0.85); PPW 0.69 (0.66–1.13). Indices: SI 59.41 (55.21–70.00); CI 103.46 (101.00–113.33); OI 17.99 (14.91–21.37); HFI 81.17 (75.00–99.01).

Redescription. Moderately polymorphic, but lacking supermajors. Head shape varies with worker size, quadrate in minors and increasingly wider than long in majors (CI = 101.00–113.33, positively associated with head width, n = 42, R2 = 0.28, P <0.001); posterior margin weakly concave medially in full-face view. Fine, dense, longitudinal striae cover cephalic dorsum, width of striae and interstriae similar; in full-face view, medial striae not diverging toward posterior corners of head. Cephalic interstriae dull to weakly shining. Vertex with fine, dense striae, interstriae weakly shining to smooth and shining. In full-face view, entire cephalic dorsum with silky appearance. Anterior margin of clypeus concave; dorsal surface with numerous moderately coarse, subparallel, longitudinal to oblique rugae. In profile, lateral lobe of clypeus massively enlarged with narrow gap (much less than width of antennal scape) between clypeal lobe and frontal lobe, the latter two structures nearly in contact. Numerous long, curved, bristle-like, yellowish macrochaetae project from anterior margin of clypeus and ventral side of mandibles. Mandible with six teeth; mandibular dorsum strongly rugose. MOD ranging from 0.16–0.23x HL. In profile, eyes situated anterior to middle of head, OMD = 1.00–1.51x MOD. Antennal scapes short (SI = 55.21–70.00), extending less than one-third the distance from the posterior margin of the eye to the 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 irregular, widely-spaced rugae. In dorsal view, humeral portion of pronotum rounded to knoblike. Dorsum of promesonotum with longitudinal rugae that rarely diverge toward humeral shoulders of pronotum; transverse rugae on anterior surface of pronotum continue obliquely or longitudinally onto pronotal sides; rugae on mesopleura angle posterodorsally. Superior propodeal spines long, acuminate, connected by well-defined keel; spine length about equal to or 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 broadly rounded. Propodeal spiracles narrowly ovate facing posterad. Interrugae on mesosoma weakly to moderately granulate, weakly shining to shining. Legs weakly granulate, weakly shining.

Peduncle of petiole about 0.7x as long as petiolar node, anteroventral margin with broadly rounded to angulate 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, widest near center, narrowing to rounded or spatulate anterior margin. Sides and posterior surface of petiolar node with weak to moderately coarse, wavy to irregular transverse rugae. 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 punctate, rarely with weak irregular, transverse rugae posterad. Interrugae on posterior surface of petiolar node and dorsum of postpetiole weakly to moderately granulate, dull to weakly shining. First gastral tergum moderately coriarious, weakly shining to smooth and shining.

Erect white to yellowish pilosity moderately abundant on head, variable in length and arising from foveae; longest hairs not exceeding MOD. Moderately abundant suberect pilosity on scape; abundant decumbent hairs on funicular segments. Legs with moderately abundant suberect to decumbent setae. Mesosoma, petiolar node, postpetiole, first gastral tergum with moderately dense erect setae, longest about equal to MOD; abundant long hairs on posterior margins of gastral terga, longest>MOD. Body mostly concolorous reddish-orange to reddishbrown ( Figure 8).

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 = 3.41–3.55 mm), (3) striae cover cephalic dorsum, and (4) in profile, lateral lobe of clypeus massively enlarged with narrow gap (much less than width of antennal scape) between clypeal lobe and frontal lobe ( Figure 9).

Measurements —(n = 5). HL 3.09–3.24; HW 3.41–3.55; MOD 0.53–0.60; OMD 0.67–0.77; SL 1.87–1.93; PNW 2.37–2.51; HFL 2.61–2.77; ML 3.74–4.44; PW 1.02–1.16; PPW 1.25–1.33. Indices: SI 53.12–56.27; CI 105.25–114.89; OI 15.45–17.01; HFI 75.65–78.69.

Description. Large species (HW = 3.41–3.55 mm) with caste-specific morphology of the mesosoma related to wing-bearing and presence of ocelli on head. In full-face view, head wider than long (CI = 105.2–114.9), posterior margin weakly concave medially. Fine, dense, longitudinal striae cover cephalic dorsum; striae on vertex often less abundant and/or less prominent, interstriae weakly shining to shining. Mandible with six teeth, dorsal surface coarsely rugose. In profile, lateral lobe of clypeus massively enlarged with narrow gap (much less than width of antennal scape) between clypeal lobe and frontal lobe, the latter two structures nearly in contact. Psammophore well-developed.

All mesosomal surfaces with subparallel, weakly irregular rugae; superior propodeal spines short. Peduncle of petiole about as long as petiolar node, anteroventral margin often with blunt to angulate process. In profile, petiolar node asymmetrical with anterior surface notably shorter than posterior surface, apex rounded. Postpetiole slightly wider than long. Posterior surface of petiolar node and dorsum of postpetiole with weak to moderately strong, wavy to irregular, transverse rugae; interrugae weakly to moderately punctate, weakly shining. Gastral terga smooth, weakly shining to shining. Most body surfaces with moderately abundant short, suberect to erect, white to yellowish setae. Body mostly concolorous reddish-orange to reddish-brown, head often slightly lighter ( Figure 9).

Male. Diagnosis. This caste is diagnosed by: (1) first gastral tergum lacking striae, (2) large (HW> 1.75 mm; HL> 1.80 mm; ML> 3.30 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, present on posterior one-half of segment, often extending anterad (sometimes oblique or longitudinal more anterad), (5) rugae posterior to eyes and ocelli prominent, and (6) at least one and usually both mandibles with four teeth ( Figure 10).

Measurements —(n = 12). HL 1.83–2.06; HW 1.78–2.14; MOD 0.56–0.70; OMD 0.21–0.36; SL 0.45–0.61; HFL 2.18–2.61; ML 3.39–4.04; PW 0.74–0.93; PPW 0.90–1.12. Indices: SI 22.39–30.90; CI 93.47–105.94; OI 29.47–36.52; HFI 109.95–129.83.

Additional material examined. ARGENTINA: Córdoba: Rt 5 at 3.4 km S Alta Gracia, 1830’, Jan 23, 2006 (RAJC); Estancia Santo Domingo (Salsipuedes), Feb 24, 2009 (RGPC); Anisacate, no date (FML). Corrientes: Mercedes, no date (MACN). Entre Ríos: Rt 18 at 7.3 mi SW San Salvador, 220’, Dec 19, 2005 (CASC; CSC; MCZ; RAJC); La Picada, May 14, 1951 (FML; LACM); Rt 18 at 33.9 mi W Villaguay, 320’, Dec 20, 2005 (MCZ; RAJC); Villaguay, no date (MACN; MLPA). Santa Fe: 22 km S Reconquista, 45 m, Nov 15, 2003 (RAJC); 23.2 km NW Reconquista, Dec 5, 2005 (CSC); Villa Guillermina, no date (FML); Fives Lille, no date (MACN). BRAZIL: Río Grande do Sul: Porto Alegre, Mar 24, 1971 (LACM; MCZ; MZUSP); São Leopoldo, no date (MCZ). URUGUAY: Paysandú: Paysandú, Apr 25, 1961 & Feb 25, 1961 (FML; LACM). Questionable locales (appear to be outside of geographic range): ARGENTINA: Buenos Aires: Monte Hermoso, no date (MLPA) ( Figure 6 B).

Etymology. The specific epithet, lobatus (from Latin, lobat = lobed), refers to the massively enlarged lateral lobes of the clypeus.

Discussion. Pogonomyrmex lobatus co-occurs with P. coarctatus , and it is likely to co-occur with P. m i c an s. Pogonomyrmex lobatus can be distinguished from P. coarctatus using the following characters: (1) P. lobatus has a massively enlarged lateral lobe of the clypeus, with the lobe nearly contacting the frontal lobe, and (2) the cephalic dorsum is covered with very fine, dense, longitudinal striae. In P. coarctatus , the lateral lobe of the clypeus is not enlarged, with a wide gap between the clypeal lobe and frontal lobe, and the very fine, dense, longitudinal striae are often indistinct and cover only part to most of cephalic dorsum. In P. micans the lateral lobe of the clypeus is not enlarged, the dorsum of the postpetiole has prominent moderately coarse rugae, and workers are bicolored (dark red and blackish). In P. lobatus , the lateral lobe of the clypeus is massively enlarged, workers are a concolorous reddish-orange to reddish-brown, and the dorsum of the postpetiole is weakly to moderately punctate, rarely with weak rugae posterad. Pogonomyrmex marcusi is only known from mid- to higher elevations in central Bolivia and thus is geographically isolated from P. lobatus ; these two species can be separated using characters in the key.

Biology. Pogonomyrmex lobatus appears to be a relatively uncommon species that is a solitary forager, though it sometimes forms very loose foraging columns with few, scattered workers, similar to those seen in P. coarctatus . Nests range from a cryptic entrance to a 15 cm tumulus, and they sometimes have a small midden of seed chaff. Colonies of P. lobatus are probably similar in size to those of P. coarctatus , suggesting that they range up to about 2000 workers. Sexuals have been collected from nests and dealate queens from the ground on 19 December, indicating that mating flights occur during the austral summer.

Pogonomyrmex lobatus is a lowland species that occurs at elevations from 45–555 m. This species occurs in the Uruguayan Savanna, northern Espinal, southern Humid Chaco, and along the southeastern edge of the Dry Chaco ecoregions as defined by Olson et al. (2001) ( Figure 6 B).