Linepithema angulatum

Linepithema angulatum is considered a generalist ant (Calle et al. 2013) that usually inhabits anthropogenic modified ecosystems. It was collected in pine, bean, coffee, and peach crops, mature forest fragments, and home gardens. Interactions with other organisms are recorded from specimens labels, for example, attending aphids, and membracids; commensalism interactions with other insect families such as Curculionidae, Cercopidae, Forficulidae and Dryinidae have been also reported (Mera et al. 2010). Some records showed L. angulatum nesting in Cecropia sp.; this plant, native to the Neotropics, is usually associated with ants of the genus Azteca Forel (Longino 1991). (Escarraga & Guerrero, 2016)

Identification
Wild (2007) - Worker Metanotal groove strongly impressed; dorsal face of propodeum straight to slightly concave; mesopleuron and metapleuron lacking pubescence and strongly shining; cephalic dorsum posterior of clypeus usually lacking standing setae (occasionally with a pair of subdecumbent setae in Central America); head width > 0.53; color testaceous to medium brown.

Linepithema fuscum workers are smaller (HW > 0.53), usually darker in color, with relatively longer scapes (SI 102–108) and a much less impressed metanotal groove. Workers of Linepithema piliferum have standing setae on the cephalic dorsum (also present in some Central American L. angulatum) and longer antennal scapes (SI 99–120). Linepithema tsachila has standing setae on the cephalic dorsum (also present in some Central American L. angulatum) and a broader head with a deeply concave posterior margin. Linepithema pulex from the Atlantic forest region is usually smaller, with a more opaque head and sparse pubescence on gastric tergites 3–4.

Male Forewing with 2 submarginal cells; volsella with distal arm greater than 2/3 length of proximal arm; proximal arm of digitus broad at base and triangular in shape; eyes relatively large (EL > 0.30); legs relatively long for Fuscum-group species (FI > 70).

Distribution
Costa Rica south to the Brazilian Pantanal.

Distribution based on Regional Taxon Lists
Neotropical Region: Argentina, Bolivia, Brazil, Colombia, Costa Rica, Ecuador, Panama, Venezuela.

Biology
Wild (2007) - Collection records of L. angulatum range across a broad array of habitats from sea level to over 2800 meters. Two collections are from leaf litter in montane forest, one under a stone in a montane forest edge, one under bark in second-growth forest, one in rotting wood, and two in high elevation urban parks in Colombia and Ecuador. Wheeler (1942: 214) reports this species (as “pordescens”) nesting in a Tillandsia bromeliad in Costa Rica. This species has been observed recruiting to tuna baits placed 30cm underground in Peru (M. Frederickson, pers. com), to surface sausage baits in Venezuela, and to surface tuna baits in the Pantanal (Orr et al. 2003). One collection records L. angulatum tending pseudococcids on Croton gossypifolia in Colombia and another tending root-aphids in a nest in Ecuador. Male alates have been taken in June in Ecuador and in December in Colombia.

A series of studies by Orr and various coauthors (Orr and Seike 1998, Orr et al. 2001, Orr et al. 2003) documented the interactions between L. angulatum (as “L. humile” or “L. piliferum”) and the phorid parasitoid Pseudacteon lontrae Mattos and Orr 2002 in the Pantanal. The presence of phorid flies significantly alters the behavior of this species in the field, leading to changes in the ecological dominance hierarchy in the ant community.

Linepithema angulatum has been intercepted several times with orchids in various ports of entry into the U.S., suggesting that it has the opportunity to spread further with human commerce.

Nomenclature

 *  angulatum. Iridomyrmex humilis subsp. angulatus Emery, 1894c: 165, pl. 1, fig. 3 (w.q.) BOLIVIA. Wild, 2007a: 31 (m.). Combination in Linepithema: Shattuck, 1992a: 16. Raised to species and senior synonym of pordescens: Wild, 2007a: 29.
 * pordescens. Iridomyrmex pordescens Wheeler, W.M. 1942: 214 (w.) COSTA RICA. Combination in Linepithema: Shattuck, 1992a: 16. Junior synonym of angulatum: Wild, 2007a: 29.

Wild (2007) - Some worker specimens from Central America, including Wheeler’s types of pordescens, present particular difficulty for the diagnosis of this species as they occasionally have a small pair of subdecumbent setae on the cephalic dorsum. This setal character is easy to see in well-curated specimens and in South America its absence is the most reliable character for separating L. angulatum from the similar but more pilose Linepithema piliferum and Linepithema tsachila. Central American specimens are variable in size and in color, some specimens have large eyes with nearly 70 ommatidia (elsewhere < 60), but otherwise are similar in body proportion and in the diagnostic shape of the propodeum and metanotal groove. Pubescence varies within the species without much geographic structure from fine and appressed to longer and somewhat wooly. Specimens from parts of Colombia and Costa Rica are frequently darker in color than specimens from elsewhere.

The species boundaries of L. angulatum are still somewhat unclear. The broad sympatry of L. angulatum with the closely related species L. piliferum, L. tsachila and Linepithema fuscum generally supports the present scheme of recognized species, but the allopatric variation among specimens placed in L. angulatum is more problematic. Material collected south of Colombia is relatively uniform in spite of the probable paraphyly of South American L. angulatum with respect to the morphologically distinct Linepithema cryptobioticum (Wild, molecular data). However, variation among the relatively sparsely collected Central American populations, and between Central American and South American populations, suggests that further division of this group may become necessary. Male genital characters and DNA sequence data have proven useful in separating species in the fuscum-complex, but males of L. angulatum are only known from South America and freshly-collected Central American specimens were not available for molecular study. Given that most variation is allopatric, I feel it is preferable to treat these ants as a single species until more material becomes available.

Worker
Wild (2007) - (n = 33) HL 0.59–0.75, HW 0.54–0.72, MFC 0.15–0.20, SL 0.52–0.67, FL 0.46–0.60, LHT 0.47–0.62, PW 0.33–0.47, ES 1.21–2.08, SI 90–104, CI 87–96, CDI 26–30, OI 20–28.

Head in full face view somewhat longer than broad (CI 87–96), lateral margins convex, posterior margin slightly concave to slightly convex, often straight. Head normally reaches widest point posterior of compound eyes. Compound eyes of moderate size (OI 20–28), comprised of 45–70 ommatidia. Antennal scapes relatively short (SI 90–104), shorter than head length. In full face view, scapes in repose exceed posterior margin of head by a length less than or subequal to length of first funicular segment. Frontal carinae moderately spaced (CDI 26–30). Maxillary palps moderately short, approximately ½ HL or less, ultimate segment (segment 6) shorter than segment 2.

Pronotum and anterior mesonotum forming a continuous curve. Mesonotal dorsum relatively angular, anterior dorsal face often medially impressed or concave. Metanotal groove strongly impressed. Propodeum relatively high and globose, dorsal propodeal face usually straight to slightly concave in lateral view. Posterior propodeal face convex.

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

Cephalic dorsum (excluding clypeus) usually lacking erect setae, rarely with a single pair of subdecumbent setae near vertex (Central America). Pronotum with 1–3 erect setae (mean = 2.1). Mesonotum without erect setae. Gastric tergite 1 ( = abdominal tergite 3) bearing 0–2 erect setae (mean = 1.7) mesally, exclusive of a row of 4–5 subdecumbent setae along posterior margin of tergite, tergite 2 bearing 2–4 erect setae (mean = 3.0) exclusive of posterior row, tergite 3 bearing 2–6 erect setae (mean = 4.3) exclusive of posterior row. Venter of metasoma with scattered erect setae.

Sculpture on head and mesosomal dorsum shagreened and relatively opaque. Pubescence dense on head, mesosomal dorsum, anterior petiolar scale, and gastric tergites 1–4. Mesopleura and metapleural bulla without pubescence and strongly shining.

Color usually concolorous testaceous, occasionally medium brown.

Queen
Wild (2007) - (n = 4) HL 0.90–0.97, HW 0.83–0.90, SL 0.75–0.85, FL 0.82–0.93, LHT 0.93–1.05, EL 0.31–0.34, MML 1.98–2.09, WL 5.98–6.08, CI 92–95, SI 90–95, OI 34–36, WI 29, FI 41–47.

Relatively large species (MML > 1.9). Head longer than broad in full face view (CI 92–95), posterior margin slightly concave. Eyes of moderate size (OI 34–36). Ocelli of moderate size. Antennal scapes of moderate length (SI 90–95), in full face view scapes in repose surpassing posterior margin by a length less than length of first funicular segment.

Forewings moderately short relative to mesosomal length (WI 29). Forewings with Rs+M somewhat longer than M.f2. Legs moderate to short relative to mesosomal length (FI 41–47).

Dorsum of mesosoma and metasoma with scattered fine erect to subdecumbent setae, mesoscutum usually with more than 10 standing setae. Body color medium reddish-brown. Antennal scapes and femora concolorous with body, coxae and tibiae usually lighter in color.

Male
Wild (2007) - (n = 4) HL 0.73–0.77, HW 0.67–0.71, SL 0.20–0.21, FL 1.13–1.25, LHT 1.09–1.25, EL 0.32–0.35, MML 0.76–0.95, WL 4.1–4.75, PH 0.28–0.32, CI 92–95, SI 27–30, OI 43–47, WI 26–28, FI 70–73.

Head slightly longer than broad in full face view (CI 93–95). Eyes relatively large (OI 43–47), occupying much of anterolateral surface of head and separated from posterolateral clypeal margin by a length less than width of antennal scape. Ocelli large and in full frontal view set above adjoining posterolateral margins. Antennal scape of moderate length (SI 27–30), about 2/3 length of 3rd antennal segment. Anterior clypeal margin broadly convex medially. Mandibles large and worker-like, masticatory margin broad, much longer than inner margin, bearing 1–4 apical teeth followed by alternating series of teeth and denticles, similar to worker dentition. Inner margin and exterior lateral margin strongly diverging.

Mesosoma moderately developed, shorter in length than metasoma. Mesoscutum slightly enlarged, not projecting strongly forward or overhanging pronotum. Scutellum large, convex, nearly as tall as mesoscutum and projecting well above level of propodeum. Propodeum in lateral view low and not overhanging petiole, dorsal face rounding evenly into posterior face, posterior face straight to convex. Forewings long relative to mesosomal length (WI 26–28) and bearing two submarginal cells. Wing color clear to slightly smoky with darker brown veins and stigma. Legs long relative to mesosoma length (FI 70–73).

Petiolar node bearing a blunt, broadly-rounded scale, node height less than node length. Ventral profile of node straight to slightly convex and lacking a distinct process. Gaster elongate in dorsal view, about 3 times as long as broad. Gonostylus produced as a slender filament. Volsella with ventrodistal process absent or present as a small tooth. Cuspis absent. Digitus elongate, distal arm long, at least 2/3 length of proximal arm, and slightly longer than height of volsella in lateral view. Proximal arm wide at base, nearly as tall as adjoining volsella, triangular in shape, and narrowing to juncture with distal arm.

Dorsal surfaces of body with scattered erect setae, mesoscutum with more than 6 erect setae. Venter of gaster with scattered setae. Pubescence dense on body and appendages, becoming sparse only on medial propodeal dorsum. Head, body and appendages medium brown in color.

Type Material
Wild (2007) - Lectotype worker, by present designation, examined, 14 worker and 2 queen paralectotypes, Salinas sul Beni, Bolivia, Balzan MCSN, , examined; MCSN lectotype series contains additionally a single misidentified Paratrechina worker.