Linepithema oblongum

Little is known about this species. All collection records are montane, between 1300 and 3800 meters elevation, and from open habitats such as roadsides, pastures, and alpine grasslands with one record from an urban park/garden in La Paz, Bolivia. I observed several nests of this species along a roadside in grazed alpine grassland at 2600 meters elevation near Tafí del Valle, Argentina, in October 2002. Three colonies were found under stones and one in soil marked by two small entrances ringed with excavated earth. Two nests contained alate males and females. Foragers were active during the day and at night, sometimes forming dilute trails. (Wild 2007)

Identification
Wild (2007) – Worker Antennal scapes very long (SI > 120); mesopleura and metapleural bulla with dense pubescence; gastric pubescence usually fading to sparse on tergites 2–4.

Workers of the Argentine ant Linepithema humile, the sister species, are less gracile, usually bearing somewhat shorter scapes (SI < 127), lack standing setae on gastric tergites 1–2, and always have dense pubescence on all gastric tergites. Workers of Linepithema anathema have a more prominent propodeum, and have dense pubescence on all gastric tergites. Workers of the similarly elongate species Linepithema iniquum and Linepithema leucomelas have greater than 4 standing setae on the cephalic dorsum.

Male Forewing with 1 submarginal cell; propodeum with strongly concave posterior face, overhanging petiole; mesosoma not greatly swollen (MML < 1.3); head clearly longer than broad in full face view (CI 91–95); compound eyes small (OI 40–44); appendages somewhat elongate (FI > 56).

Distribution
Andean regions of Bolivia and northern Argentina.

Distribution based on Regional Taxon Lists
Neotropical Region: Argentina, Bolivia, Brazil, Chile.

Nomenclature

 *  oblongum. Iridomyrmex humilis var. oblongus Santschi, 1929d: 306 (w.) BRAZIL. Wild, 2007a: 98 (q.m.). Combination in Linepithema: Shattuck, 1992a: 16. Raised to species: Snelling & Hunt, 1976: 90.

Description
Wild (2007):

MISC Molecular evidence (Wild, unpublished) strongly supports a sister group relationship between L. oblongum and the morphologically similar pest species L. humile. This finding should elevate the importance of L. oblongum for comparative studies on the biology of the Argentine ant.

Specimens from Bolivia are usually somewhat lighter in color than those from Argentina.

Wilhelm Goetsch collected a Linepithema species from Copiapó, Chile, that was identified by Menozzi (1935: 321) as L. oblongum. This Chilean record is plausible, but given L. oblongum’s morphological similarity to the invasive L. humile which is abundant in Chile, the identity cannot be verified. Snelling and Hunt (1975) reached a similar conclusion. Goetsch himself identified his Copiapó ants as conspecific with the invasive Argentine ant, but his line drawings comparing the Copiapó ant with European L. humile show the slightly smaller eyes characteristic of L. oblongum (Goetsch 1957: 43). Further collections in northern Chile should clarify the distribution of this ant.

Worker
(n = 13) HL 0.61–0.75, HW 0.50–0.65, MFC 0.14–0.18, SL 0.65–0.83, FL 0.54–0.72, LHT 0.62–0.79, PW 0.35–0.44, ES 1.7–2.8, SI 120–139, CI 81–88, CDI 25–28, OI 28–38.

Head in full face view slender, longer than broad (CI 81–88), somewhat oval in shape, reaching widest point at or just posterior to compound eyes. Lateral margins broadly convex, posterior margin straight to weakly concave. Anterior clypeal margin with a broad, relatively shallow medial excision. Compound eyes of moderate size (ES 1.7–2.8), comprising 60–85 ommatidia. Antennal scapes long (SI 120–139), longer than HL and easily surpassing posterior margin of head in full face view. Frontal carinae moderately spaced (CDI 25–28). Maxillary palps relatively short, segments 4, 5 and 6 each noticeably shorter than segment 2.

Pronotum and mesonotum forming a single convexity. Posterior 2/3 of pronotal dorsum in lateral view straight or only slightly convex. Mesonotal dorsum nearly straight, not angular or strongly impressed, frequently with a slight medial impression. Metanotal groove slightly to moderately impressed. Propodeum in lateral view quadrate-forward.

Petiolar scale sharp and inclined anteriorly, in lateral view falling short of the propodeal spiracle.

Dorsum of head and mesosoma devoid of erect setae. Gastric tergites 1–2 ( = abdominal tergites 3 and 4) usually without erect setae, occasionally with 1–2 erect setae (mean = 0.1). Gastric tergite 3 with 2–5 erect setae (mean = 3.5). Venter of metasoma with scattered erect setae.

Head, mesosoma, and appendages covered in dense pubescence. Mesosoma and metapleural bulla with dense pubescence. Pubescence on gaster variable, even within a nest series, nearly always dense on gastric tergite 1 ( = abd. tergite 3), usually fading to sparse on tergites 2–4, surface relatively shining.

Body weakly to strongly bicolored, gaster always darker than head and mesosoma. Color varies from head and mesosoma testaceous and gaster brown to head and mesosoma brown and gaster dark brown.

Queen
(n = 2) HL 0.83–0.85, HW 0.75–0.78, SL 0.84–0.86, FL 0.81–0.82, LHT 0.91, EL 0.27–0.28, MML 1.55–1.58, WL 4.89–4.91, CI 90–91, SI 110–112, OI 33, WI 31–32, FI 52.

Moderately small species (MML 1.55–1.58). Head narrow in full face view (CI 90–91), posterior margin straight to slightly concave. Eyes moderately small (OI 33). Ocelli moderately small. Antennal scapes long (SI 110–112), in full face view scapes in repose surpassing posterior margin by a length greater than length of first funicular segment.

Forewings of moderate length relative to mesosomal length (WI 31–32). Forewings with Rs+M at least two times longer than M.f2. Legs long relative to mesosomal length (FI 52).

Dorsum of mesosoma and metasoma with scattered standing setae. Mesoscutum bearing 0–4 suberect setae. Body color medium brown to dark brown, gaster often somewhat darker than mesosoma. Antennal scapes, legs, and mandibles concolorous with body.

Male
(n = 4) HL 0.50–0.54, HW 0.47–0.49, SL 0.13–0.15, FL 0.55–0.57, LHT 0.51–0.52, EL 0.22–0.23, MML 0.98–1.01, WL 2.29–2.59, PH 0.23–0.24, CI 91–95, SI 24–29, OI 40–44, WI 23–26, FI 56–57.

Head longer than broad in full face view (CI 91–95). Eyes small (OI 40–44), occupying much of anterolateral surface of head anterior of midline and separated from posterolateral clypeal margin by a length less than width of antennal scape. Ocelli small and in full frontal view set above adjoining posterolateral margins. Antennal scape of moderately length (SI 24–29), about 2/3 length of 3rd antennal segment. Anterior clypeal margin straight to broadly convex. Mandibles variable, small to moderate in size, usually bearing a single apical tooth, sometimes a strong subapical tooth, and 8–12 denticles along masticatory margin. Masticatory margin relatively narrow to relatively broad, longer than or subequal in length to inner margin. Inner margin and exterior lateral margin converging, parallel, or diverging.

Mesosoma well developed, larger in bulk than metasoma. Mesoscutum enlarged, projecting forward in a convexity overhanging pronotum. Scutellum large, convex, nearly as tall as mesoscutum and projecting well above level of propodeum. Propodeum well developed and overhanging petiolar node, posterior propodeal face strongly concave. Forewings of moderate length relative to mesosomal length (WI 23–26) and bearing a single submarginal cell. Wing color smoky with dark brown veins and stigma. Legs of moderate length relative to mesosoma length (FI 56–57).

Petiolar scale taller than node length. Ventral process well developed and pointing posteriorly. Gaster oval in dorsal view, nearly twice as long as broad. Gonostylus produced as bluntly rounded pilose lobes. Volsella with cuspis present, digitus short and downturned distally.

Dorsal surfaces of body largely devoid of erect setae, mesoscutum lacking standing setae, posterior abdominal tergites with a few fine, short setae. Venter of gaster with scattered setae. Pubescence dense on body and appendages, becoming sparse only on medial propodeal dorsum.

Head, mesosoma and metasoma medium to dark brown. Legs, mandibles, and antennae medium brown, trochanters lighter in color.

Type Material
Holotype worker, Purmamarca, Jujuy, Argentina, J. Witte, examined.