Linepithema fuscum

Little is known about the biology of L. fuscum. This species has been collected from 200 to nearly 3000 meters. One lone male was collected in a “shrubby pasture”, while a series from Madre de Dios was collected foraging inside a laboratory building. Crozier (1970) studied the karyotype of two populations of this species (called “Iridomyrmex pilifer” and “I. sp. nr. pilifer”, vouchers at MCZC, examined), and found the haploid chromosome number of both populations to be N=9 and otherwise indistinguishable from each other. (Wild 2007)

Identification
Wild (2007) - Worker Size small (HW < 0.52); cephalic dorsum posterior of compound eyes lacking standing hairs; mesopleura and metapleura lacking pubescence and strongly shining; eyes small (OI 17–23) with fewer than 50 ommatidia; body color medium brown to dark brown. Workers show remarkably little variation among localities.

Linepithema angulatum workers also lack standing setae on the cephalic dorsum but are larger (HW > 0.53) and normally testaceous to light brown in color, with relatively shorter scapes (SI 90–104, Fig. 80) and a more deeply impressed mesonotal groove. Linepithema piliferum and Linepithema tsachila workers have standing setae on the cephalic dorsum near the vertex. Workers of Linepithema keiteli from Hispaniola have a broader head (CI > 91) and a more shining integument. Linepithema neotropicum workers have larger eyes (> 50 ommatidia), a lower, more rounded propodeum, and at least some appressed pubescence on the metapleuron.

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

Distribution
Ecuador, Peru.

Distribution based on Regional Taxon Lists
Neotropical Region: Argentina, Ecuador, Peru, Venezuela.

Nomenclature

 *  fuscum. Linepithema fuscum Mayr, 1866a: 497, pl. fig. 7 (m.) PERU. Wild, 2007a: 51 (w.q.).

Wild (2007):

Linepithema fuscum is the type species for the genus and was described by Mayr (1866) from males collected in Lima, Peru. Males and workers have never been definitively associated, so the placement of the small, dark-colored workers described here from various Peruvian localities should be regarded as tentative. This association is based on the following line of evidence. Five distinct male forms are known from the fuscum species group, each diagnosable by their genitalia and each collected from more than one location. Four of these, L. angulatum, L. keiteli, L. piliferum and L. tsachila, have been associated with workers in at least one instance. Only L. fuscum itself remains without a worker association, and only one of the two remaining unassociated worker forms- the small dark form described above- is recorded from Peru. Both workers and males have been collected separately at a single location in Tingo Maria. Notably, the only other fuscum-group species collected at Tingo Maria is L. angulatum whose male is associated in a William P. MacKay collection from Cundinamarca, Colombia. Given the variation within the latter species, however, it remains a possibility that the males of L. fuscum may actually pertain to the workers of L. angulatum.

Worker
(n = 9) HL 0.53–0.58, HW 0.45–0.51, MFC 0.13–0.15, SL 0.48–0.54, FL 0.40–0.46, LHT 0.43–0.49, PW 0.30–0.32, ES 0.93–1.24, SI 102–108, CI 86–89, CDI 28–31, OI 17–23.

Head in full face view relatively narrow (CI 86–89), lateral margins convex, posterior margin straight to slightly convex. Compound eyes small (OI 17–23), comprised of 35–50 facets. Antennal scapes of moderate length (SI 102–108), shorter than head length. In full face view, scapes in repose surpass posterior margin of head by an amount subequal to or slightly greater than the length of the first funicular segment. Frontal carinae moderately to widely separated (CDI 28–31). Maxillary palps of moderate length, approximately ½ HL, ultimate segment (segment six) shorter than, or subequal in length to, segment 2.

Mesosoma in lateral view compact, with pronotum and mesonotum forming a single continuous convexity, often interrupted by a slight mesal mesonotal impression. Mesonotum in lateral view angular. Metanotal groove not impressed or only slightly impressed. Propodeum relatively high, only slightly depressed below level of mesonotum, dorsal face relatively flat on top, meeting posterior face at an angle. Posterior propodeal face convex.

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

Cephalic dorsum (excluding clypeus) lacking erect setae or rarely with 1–2 subdecumbent setae near antennal insertions. Pronotum with 2–4 (mean = 2.3) erect to suberect setae. Mesonotum without erect setae. Gastric tergite 1 ( = abdominal tergite 3) bearing 0–2 erect setae (mean = 1.6) exclusive of posterior row, tergite 2 with 2–5 (mean = 3.4) erect setae exclusive of posterior row, tergite 3 with 4–6 (mean = 4.4) erect setae. Venter of metasoma with scattered erect setae.

Surface of head and mesosomal dorsum shagreened and moderately shining. Pubescence dense and rather long on head and mesosomal dorsum. Mesopleura and metapleura lacking pubescence and strongly shining. Gastric tergites 1–4 with dense to moderate pubescence, surface moderately shining.

Body color medium brown to dark brown. Mandibles, antennae, trochanters and tarsi somewhat lighter.

Queen
(n = 2) HL 0.86–0.87, HW 0.80–0.84, SL 0.76–0.78, FL 0.80–0.81, LHT 0.89–0.91, EL 0.32–0.35, MML 1.87–1.91, WL n/a, CI 92–98, SI 92–95, OI 37–40, WI n/a, FI 43.

Moderately sized species (MML 1.87–1.91). Head longer than broad in full face view (CI 92–98), posterior margin slightly concave. Eyes large (OI 37–40). Ocelli moderately small. Antennal scapes of moderate length (SI 92–95), in full face view scapes in repose surpassing posterior margin by a length greater than or equal to length of first funicular segment.

Wings unknown (not present in examined material). Legs of moderate length relative to mesosomal length (FI 43).

Dorsum of mesosoma and metasoma with numerous standing setae. Mesoscutum with more than 10 standing setae. Body color medium brown. Antennal scapes, legs, and mandibles slightly lighter than body.

Male
(n = 5) HL 0.68 –0.74, HW 0.63–0.70, SL 0.19–0.21, EL 0.32–0.37, MML 1.48–1.67, WL 4.16–4.49, FL 1.01–1.07, LHT 1.01–1.07, CI 92–97, SI 27–30, OI 48–52, WI 26–28, FI 63–68.

Head slightly longer than broad in full face view (CI 92–97). Eyes relatively large (OI 47–52), 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 70–80% 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 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 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 62–68).

Petiolar node bearing a blunt, broadly-rounded scale, node height taller than node length. Ventral profile of node straight to slightly convex and lacking a distinct process. 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 a sharp spine. 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 narrow at base, less than ½ height of adjoining volsella, and filamentous in shape.

Dorsal surfaces of body with scattered erect setae, mesoscutum with more than 5 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
Lectotype male, by present designation, examined, and 8 male paralectotypes , NHMW, examined.