Pogonomyrmex atratus, Santschi, 1922

Johnson, Robert A., 2021, A taxonomic revision of South American species of the seed-harvester ant genus Pogonomyrmex (Hymenoptera: Formicidae). Part II, Zootaxa 5033 (1), pp. 1-230 : 50-54

Pogonomyrmex atratus

( Figure 21 )

Distribution—22A

Pogonomyrmex vermiculatus var. atrata Santschi, 1922b: 347 (worker). Syntypes examined: 2 workers [MACN], 1 worker [MHNG], 11 workers [MLPA]. ARGENTINA, Mendoza Province: Tres Esquinas (Dr. Carette). See also Gallardo, 1932: 139, fig. 25. MHNG worker here designated LECTOTYPE [CASENT0173346].

Pogonomyrmex atratus Santschi : Kusnezov, 1951: 251, raised to species.

Worker diagnosis. Workers of this species are uniquely characterized by the following combination of features: (1) first gastral tergum smooth and shining, lacking striae, (2) body concolorous black, (3) longitudinal rugae on cephalic dorsum irregular, widely spaced, and (4) interrugae on cephalic dorsum strongly granulate, dull ( Figure 21 ).

Measurements — lectotype (n = 12). HL 1.78 (1.60–1.80); HW 1.73 (1.62–1.92); MOD 0.38 (0.34–0.43); OMD 0.42 (0.36–0.54); SL 1.25 (1.14–1.45); PNW 1.14 (1.04–1.30); HFL 1.71 (1.51–2.03); ML 1.96 (1.86–2.47); PW 0.41 (0.40–0.50); PPW 0.59 (0.54–0.71). Indices: SI 72.25 (69.51–80.49); CI 97.19 (97.59–106.67); OI 21.97 (20.61–24.57); HFI 98.84 (87.50–121.56).

Redescription. Head quadrate to subquadrate (CI = 97.19–106.67), widest just posterior to eyes; posterior margin flat to weakly convex in full-face view. Longitudinal rugae on cephalic dorsum prominent, rugae irregular and widely spaced; in full-face view, medial rugae weakly diverging toward posterior corners of head. Interrugae on cephalic dorsum strongly granulate, dull; posterior corners rugose, interrugae mostly smooth and shining. Anterior margin of clypeus strongly concave; dorsal surface with numerous subparallel, longitudinal rugae. Numerous long, curved, bristle-like, yellowish macrochaetae project from anterior margin of clypeus and basolateral margin of mandibles. Mandibles with six to eight teeth, most commonly seven (29% with six teeth, 66% with seven teeth, 5% with eight teeth, n = 38). Mandibular dorsum coarsely rugose. MOD ranging from 0.20–0.25× HL. In profile, eyes situated slightly anterior to middle of head, OMD = 0.95–1.29× MOD. In full-face view, eyes protruding slightly beyond lateral margins of head. Antennal scapes (SI = 69.51–80.49) failing to reach posterior corners of head by less than length of basal funicular segment; scapes shining, basal flange moderately well developed with carinate margin. Psammophore well developed.

Mesosomal profile weakly to moderately convex; all mesosomal surfaces with prominent, weakly to strongly irregular, subparallel rugae, occasionally rugoreticulate to vermiculate. In dorsal view, humeral shoulders of pronotum enlarged, dorsolateral margin weakly to strongly angulate. Dorsum of promesonotum with longitudinal rugae that diverge toward humeral shoulders of pronotum then traverse posteroventrally or longitudinally on pronotal sides; rugae on mesopleura angle posterodorsally. Superior propodeal spines well developed, slightly shorter than distance between their bases, spines connected by well defined keel; irregular transverse rugae on dorsum of propodeum traverse ventrally or anteroventrally on sides. Inferior propodeal spines, wider than high, tips rounded to weakly acuminate. Propodeal spiracles narrowly ovate facing posterad. Interrugae on mesosoma moderately to strongly granulate, dull to weakly shining. Legs weakly shining to shining.

Peduncle of petiole about 0.8× as long as petiolar node, anteroventral margin often with wide, rounded expansion, but lacking a triangular process. In profile, posterior surface of petiolar node weakly convex; node asymmetrical with anterior surface notably shorter than posterior surface, apex angulate to weakly rounded. In dorsal view, petiolar node longer than wide, widest near rounded to spatulate anterior margin. Sides and posterior surface of petiolar node with regular to weakly irregular transverse rugae. Dorsum of postpetiole convex in profile; in dorsal view, widest at or near posterior margin, narrowing anterad, maximum width about equal to length. Transverse rugae on dorsum of postpetiole finer, denser than those on posterior surface of petiolar node; interrugae on both surfaces moderately granulate-punctate, weakly shining. First gastral tergum smooth and shining.

Erect, white pilosity moderately abundant on head, most hairs uniformly short (<0.5× MOD) with one to few longer hairs approaching to exceeding MOD. Moderately abundant suberect to semidecumbent pilosity on scape; abundant decumbent hairs on funicular segments. Legs with moderately abundant suberect to semidecumbent white setae. Mesosoma, petiolar node, postpetiole, gastral terga with moderately dense, erect white setae, often similar in length, longest not exceeding MOD. Body concolorous black; mandibles, distal funicular segments, tarsi often slightly lighter brownish-black ( Figure 21 ).

Queen. Unknown.

Male. Unknown.

Additional material examined. ARGENTINA: Mendoza: Rt 220 at 40.0 km NW El Sosneado , 6580’, Jan 24, 2008 ( IFML; LACM; MACN; MCZC; RAJC; UCDC; USNM); Rt 220 at 41.7 km NW El Sosneado, 6650’, Jan 24, 2008 ( RAJC) ( Figure 22A ) .

Etymology. The specific epithet, atratus (Latin, atratus = clothed in black), refers to the deep black coloration that Santschi noted in the species description.

Discussion. Pogonomyrmex atratus likely co-occurs with an unidentified congener that was part of the syntype series. This worker lacked striae on the first gastral tergum, the superior propodeal spines consisted of denticles, and a coloration that consisted of head orangish-brown, dorsum of pronotum and mesonotum dark reddish to reddishblack, rest of body black. A label was added to this pin saying “Not a syntype of Pogonomyrmex vermiculatus var. atrata Santschi ”. Pogonomyrmex carbonarius and a P. brevibarbis -group species occur near areas inhabited by P. atratus . Pogonomyrmex atratus is distinguished from P. brevibarbis -group species by presence of transverse rugae on the posterior surface of the petiolar node and dorsum of postpetiole ( P. brevibarbis -group species usually lack rugae on both segments). Pogonomyrmex atratus is distinguished from P. carbonarius and all other concolorous black congeners by lacking striae on the first gastral tergum (striae present in P. carbonarius and all other concolorous black congeners).

Santschi (1922b) described P. atratus as a variety of P. vermiculatus , and Kusnezov (1951) raised P. atratus to species level saying only that sculpturing on the thorax was distinct from that of P. vermiculatus . He also wrote that this sculpturing was similar to that on P. carbonarius , from which it differed by lacking striae on the first gastral tergum. Based on this similarity, Kusnezov (1951) indicated that P. atratus might be a subspecies of P. carbonarius . The large series of workers collected during this study affirm specific status for P. atratus , which previously was known from only syntype workers collected> 90 years ago.

Biology. Workers are solitary, diurnal foragers that sometimes travel> 20 m to harvest seeds and related items. Nests are most easily located by baiting foragers with cookie crumbs, then following them to the nest. The two nests that I observed were in open habitat with tumuli that ranged from 15–20 cm in diameter; one nest was under a rock with the tumulus at the edge of the rock, the other was in a clump of Stipa sp. (Poaceae) . The site was dominated by sandy, gravelly, rocky soil. Partial nest excavations indicated that colonies contain approximately 500– 800 workers.

Pogonomyrmex atratus is only known from the type locality at Tres Esquinas, Mendoza, and one canyon in southwestern Mendoza Province. However, it seems doubtful that Tres Esquinas is the actual type locality given that this area is in the Low Monte Desert ecoregion ( Olson et al., 2001), where only desert species such as P. propinqua were collected. Based on habitats and elevations at which I collected P. atratus , the true type locality is probably west of Tres Esquinas in the foothills of the Andes. Overall, P. atratus inhabits elevations from 1990–2015 m suggesting that this species inhabits foothill and mid-elevation habitats along the western side of the Andes in Mendoza and possibly adjacent provinces. This species is only known from the Southern Andean steppe ecoregion, as defined by Olson et al. (2001) ( Figure 22A ).

Figures

FIGURE 21. Photographs of Pogonomyrmex atratus worker: (A) frontal view of head, (B) lateral view of body, and (C) dorsal view of body (CASENT0281101). Photographs by Shannon Hartman from www.AntWeb.org.

FIGURE 22. Geographic distribution of: (A) Pogonomyrmex atratus, (B) P. bolivianus, and (C) P. carbonarius. The large black circle in each panel denotes the type locality. See text regarding the type locality for P. atratus.