Linepithema piliferum

A montane forest species, colonies can be polydomous and polygynous.

Identification
Wild (2007) – Worker A large species (HW > 0.55); antennal scapes long (SI 99–120), in repose exceeding posterior margin of head by a length greater than or equal to length of first funicular segment; cephalic dorsum bearing at least 2, and often more, erect to subdecumbent setae; mesonotum without strong medial impression (sometimes weakly impressed); mesopleura and metapleura lacking pubescence and strongly shining.

This species can be difficult to diagnose owing to a large amount of intraspecific variation and the presence of two very similar sympatric species, Linepithema angulatum and Linepithema tsachila. All L. piliferum specimens have relatively long scapes and standing cephalic setae, but vary continuously in eye size, color, and degree of mesonotal impression. Specimens from Costa Rica, lower elevations in Ecuador, and parts of Colombia tend to have smaller eyes (< 60 ommatidia) and be lighter in color.

Workers of Linepithema angulatum usually have shorter antennal scapes (SI <104), a more deeply impressed metanotal groove, and in South America lack standing setae on the cephalic dorsum (present in some Central American populations). Workers of Linepithema tsachila have shorter antennal scapes (SI < 98), a more robust head in full-frontal view (CI usually > 94) that typically reaches its widest point near the level of the compound eyes, a deeply concave posterior margin, and often a faint bluish sheen to the integument.

Male Forewing with 2 submarginal cells; volsella with distal arm shorter than 1/3 length of proximal arm and shorter than cuspis; dorsal profile of volsella and proximal arm straight or only weakly concave; legs relatively short for Fuscum-group species (FI < 68).

Males of other Fuscum-group species either have a long distal arm of the digitus (L. angulatum, Linepithema fuscum, and Linepithema keiteli), or longer legs (FI > 70, L. tsachila).

Distribution
Mountains of northwestern South America to Costa Rica.

Distribution based on Regional Taxon Lists
Neotropical Region: Colombia, Costa Rica, Ecuador, Peru, Venezuela.

Biology
Wild (2007) - Linepithema piliferum is a montane species, with records running from 780 to 2340 meters. Where explicit habitat information has been recorded, three collections are from sifted litter in wet forest, five from cloud forest edge or roadside, one from 2nd growth forest edge, and one from a Psidium guayaba orchard. Four nest records are from under stones and one from soil. Two port-of-entry intercepts into the United States were nesting in Cattleya orchids. This species has been recorded tending root aphids, aleyrodids, and pseudococcids.

Colonies can be populous and are probably both polydomous and polygynous. I conducted three nest excavations in Ecuador in December 2002. Alate males and queens were present in all nests. Each of these nests were under series of several stones along roadsides in cloud forest. Nests contained dozens of separate brood chambers connected by tunnels, and at least one nest contained a colony of root aphids feeding on grass roots growing through the nest. In two of the colonies I found several dealate queens, each in a separate brood chamber.

Escarraga & Guerrero (2016) - Colombian specimens of Linepithema piliferum have been found in plantations of Eucalyptus, beans, coffee, pine, oak, and urapán (Fraxinus chinensis Roxburgh), as well as in mixed Andean forest, paddocks, forest fragments, forest edges, living fences, pastures, and glens.

Jack Longino: In Costa Rica this species is known from mid-elevation wet forest sites on the Atlantic slope (1100m on the Barva Transect in Braulio Carrillo National Park, and 800m in the Peñas Blancas Valley east of Monteverde). Workers have been collected in Winkler samples. It appears to be a very low density species. Project ALAS took 150 miniWinkler samples at the 1100m site, and L. piliferum occurred in only one of them. None have been collected at any other sites on the Barva Transect, where similar or greater sampling effort was carried out. In the Peñas Blancas Valley, L. piliferum occured in two of about two dozen Winkler samples, and a worker was found beneath a stone in a trail.

Nomenclature

 *  piliferum. Hypoclinea pilifera Mayr, 1870a: 393 (w.q.) COLOMBIA. Crozier, 1970: 113 (k.). Wild, 2007a: 103 (m.). Combination in Iridomyrmex: Dalla Torre, 1893: 169; in Linepithema: Shattuck, 1992a: 16.

Description
Wild (2007):

Worker
(n = 12) HL 0.65–0.83, HW 0.56–0.79, MFC 0.17–0.23, SL 0.61–0.79, FL 0.52–0.71, LHT 0.54–0.79, ES 1.42–2.93, PW 0.39–0.54, CI 86–95, SI 99–120, CDI 26–31, OI 21–36.

Head in full face view somewhat longer than broad (CI 86–95), lateral margins evenly convex, posterior margin straight to slightly concave. Head normally reaches widest point posterior of compound eyes. Compound eyes moderate to large in size (OI 21–36), comprised of 50–75 ommatidia. Antennal scapes relatively long (SI 99–120), normally slightly shorter than head length, rarely slightly longer than head length. In full face view, scapes in repose exceed posterior margin of head by a length greater than or equal to length of first funicular segment. Frontal carinae moderately spaced (CDI 26–31). Maxillary palps of moderate length, approximately ½ HL, ultimate segment (segment 6) variable, somewhat shorter to somewhat longer than segment 2.

Pronotum and anterior mesonotum forming a continuous curve. Mesonotal dorsum relatively straight to moderately angular, often with a slight medial impression. Metanotal groove not impressed to moderately impressed. Dorsal propodeal face straight to slightly convex in lateral view. Posterior propodeal face convex. Petiolar scale relatively sharp and inclined anteriorly, in dorsal view relatively narrow, in lateral view falling short of propodeal spiracle.

Cephalic dorsum (excluding clypeus) with numerous standing setae, 0–6 (mean = 3.8) erect to subdecumbent setae near antennal insertions and 0–3 (mean = 1.6) erect to subdecumbent setae near vertex. Pronotum with 0–4 erect to subdecumbent setae (mean = 2.3). Mesonotum without standing setae, or rarely with 1 small erect seta. Propodeal dorsum usually without standing setae, rarely with several erect setae. Gastric tergite 1 ( = abdominal tergite 3) bearing 0–4 erect to suberect setae (mean = 2.3) mesally, exclusive of a row of 5–10 subdecumbent setae along posterior margin of tergite, tergite 2 bearing 2–6 erect setae (mean = 4.3) exclusive of posterior row, tergite 3 bearing 3–7 erect setae (mean = 4.8) exclusive of posterior row. Venter of metasoma with scattered erect setae.

Sculpture on head and mesosomal dorsum shagreened, surface dull to moderately shining. Pubescence dense on head, mesosomal dorsum, anterior petiolar scale, and gastric tergites 1–4. Pubescence often long and somewhat wooly in appearance. Mesopleura and metapleural bulla without pubescence and strongly shining.

Body and appendages concolorous testaceous to dark reddish brown.

Queen
(n = 3) HL 0.96–1.05, HW 0.92–1.04, SL 0.88–0.94, FL 0.98–1.09, LHT 1.09–1.19, EL 0.34–0.40, MML 2.32–2.49, WL 8.57, CI 95–100, SI 90–96, OI 35–38, WI 34, FI 42–44.

Large species (MML 2.32–2.49). Head slightly longer than broad to about as long as broad in full face view (CI 95–100), posterior margin straight to slightly concave. Eyes of moderate size (OI 35–38). Ocelli of moderate size. Antennal scapes of moderate length (SI 90–96), in full face view scapes in repose surpassing posterior margin by a length approximately that of first funicular segment. Forewings long relative to mesosomal length (WI 34). Forewings with Rs+M subequal in length to M.f2. Legs moderately short relative to mesosomal length (FI 42–44).

Dorsum of mesosoma and metasoma with abundant fine erect to subdecumbent setae, mesoscutum with more than 20 fine suberect setae. Body and appendages concolorous medium brown.

Male
(n = 5) HL 0.66–0.71, HW 0.63–0.78, SL 0.23–0.24, FL 0.91–1.03, LHT 0.90–1.02, EL 0.34–0.37, MML 1.44–1.57, WL 4.4–4.9, PH 0.32–0.38, CI 90–97, SI 32–35, OI 51–53, WI 29–31, FI 61–66.

Head slightly longer than broad in full face view (CI 90–97). Eyes relatively large (OI 51–53), 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 moderately long (SI 32–35), 85–100% length of 3rd antennal segment. Anterior clypeal margin broadly convex medially. Mandibles large and nearly worker-like, masticatory margin broad, much longer than inner margin, bearing 1–4 apical teeth followed by 8–14 denticles. Inner margin and exterior lateral margin 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 not overhanging petiole, dorsal face rounding evenly into posterior face, posterior face straight to convex. Forewings long relative to mesosomal length (WI 29–31) and bearing two submarginal cells. Wing color clear to slightly smoky with darker brown veins and stigma. Legs long relative to mesosoma length (FI 61–66), although shorter than most Fuscum-group species.

Petiolar node bearing a blunt, broadly-rounded scale, node height taller than node length. Ventral profile of node strongly convex. Gaster elongate in dorsal view, 2.5–3 times as long as broad. Gonostylus produced as a slender filament. Volsella with ventrodistal process present as an elongate spine. Digitus elongate, distal arm short, shorter than 1/3 length of proximal arm and usually shorter than ventrodistal process. Cuspis absent. Proximal arm broad at base, greater than ½ height of adjoining volsella, and tapering distally.

Dorsal surfaces of body with scattered erect setae, mesoscutum with more than 10 erect setae. Venter of gaster with scattered setae. Pubescence dense on body and appendages, becoming sparse only on medial propodeal dorsum.

Head and body medium brown in color. Mandibles, antennae, and legs testaceous.

References based on Global Ant Biodiversity Informatics

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 * Crozier R. H. 1970. Karyotypes of twenty-one ant species (Hymenoptera: Formicidae), with reviews of the known ant karyotypes. Can. J. Genet. Cytol. 12: 109-128.
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 * Longino J. et al. ADMAC project. Accessed on March 24th 2017 at https://sites.google.com/site/admacsite/
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 * Ulloa Chacon P., M. L. Baena, J. Bustos, R. C. Aldana, J. A. Aldana, and M. A. Gamboa. 1996. Fauna de hormigas del departamento del Valle del Cauca (Colombia). Pp. 413-451. In Andrade-C M. G., G. Amat Garcia, and F. Fernandez. Insectoss de Colombia, estudios escogidos.
 * Wild A. L. 2007. Taxonomic revision of the ant genus Linepithema (Hymenoptera: Formicidae). University of California Publications in Entomology 126: 1-151