Linepithema

The genus has recently been revised by Wild (2007) and species determinations are fairly straightforward using available keys. Linepithema ants are a common but often overlooked element of the Neotropical myrmecofauna. These small, monomorphic dolichoderine ants are native to a variety of forest, grassland, and montane habitats in Central and South America and the Caribbean. The genus is widely recognized for the pestiferous Argentine ant Linepithema humile, an insect that has received considerable attention for its invasive behavior in Mediterranean climates worldwide (Roura-Pascual et al. 2004), but Linepithema also contains nearly twenty additional species, some of them locally abundant, about which little is known. (Wild 2007)

Identification
Wild (2007) - Worker (key characters in bold). Small dolichoderine ants (HW 0.42–0.80) with a monomorphic worker caste. Compound eyes comprising 17–110 ommatidia, centered anterior of midline of head in full face view, not touching lateral margins; '''mandible with dentition consisting of an elongate apical tooth and a smaller subapical tooth followed by a series of 3–4 small teeth alternating with denticles''', masticatory and inner margins meeting at a curve armed with 1–3 teeth or denticles; anteromedial clypeal margin with a broad, shallow concavity; palp formula 6:4; mesosoma lacking spines or teeth; propodeum in lateral view depressed below level of mesonotum; fourth gastric sternite keel-shaped posteriorly; pilosity moderate to reduced, head lacking standing setae along posterolateral corners and pronotum bearing fewer than 10 standing setae.

Queen. Mandible with dentition consisting of an elongate apical tooth and a smaller subapical tooth followed by a series of 3–5 small teeth alternating with denticles, masticatory and inner margins meeting at a curve armed with 1–3 teeth or denticles; anteromedial clypeal margin with a broad, shallow concavity; palp formula 6:4; axilla with a medial suture; mesoscutum covered with a dense, fine pubescence; venter of petiole with a slight lobe.

Male. (Excluding some worker-like males in populations of Linepithema dispertitum). Antennal scape shorter than third antennal segment; lateral ocelli emerging above posterior cephalic margin in full face view; anteromedial clypeal margin broadly convex; mandibles with a distinct masticatory margin bearing at least 4 teeth or denticles, sometimes approaching worker dentition; mesoscutum covered with dense, fine pubescence; petiolar scale not inclined anteriorly, instead with scale straight, inclined posteriorly, or present as a low node; forewings with 1–2 closed submarginal cells; hind wings with 2 closed cells; volsella with digitus narrow and sharply downturned distally.

Distribution
Ignoring the introduced range of Linepithema humile and Linepithema iniquum, Linepithema is found from the highlands of northern Mexico east into the Caribbean and south to northern Argentina. Common on Hispaniola and Puerto Rico. Relatively rare in the Amazon Basin, reaching their peak abundance and diversity instead between 20º and 30º degrees south latitude.

Biology
From the modern revision of the genus by Alex Wild (2007):

In subtropical South America, Linepithema ants are found near sea level in rainforests, scrub forests, and floodplains. In the Central Andes they ascend to 4,000 meters elevation. In northern South America, Central America, and the Caribbean Linepithema species are more typically montane, sometimes occurring locally at high densities to the apparent exclusion of other ant species.

Two species, Linepithema humile and the Argentine ant Linepithema iniquum, have been carried around the world with human commerce, although L. iniquum seems to establish only in greenhouses (Wheeler 1929, Creighton 1950).

Although Linepithema ants are often observed in undisturbed primary habitat, most species may also readily be found in pastures, lawns, roadsides and other disturbed habitats, suggesting that populations weather deforestation and habitat modification reasonably well. Some species, including the notorious Argentine ant, likely thrive with disturbance. Nonetheless, a few of the uncommon species have been recorded only from primary habitats and one, Linepithema flavescens, is known only from Haiti and has not been collected since 1934.

Linepithema ants show a stereotypical nesting and feeding behavior. Mature colonies are populous, often with more than 1,000 individuals, and the worker caste is monomorphic. The trophic habits of these ants are not unusual for dolichoderines, as they are generalist scavengers and predators with strong proclivities for tending nectaries and honeydew-producing insects. Honeydew feeding is ubiquitous throughout the genus and occurs both inside and outside the nest. Linepithema ants readily form chemical recruitment trails and can recruit in large numbers to food sources. They are commonly seen running in files on the ground and on low vegetation and are active both day and night. Some species are attacked by hostspecific parasitoid Pseudacteon phorid flies (Orr et al. 2001, Wild, pers. obs.).

Most species are polydomous, many are also polygynous, and at least one species, the Argentine ant L. humile, has both multicolonial and unicolonial populations (Krieger and Keller 2003, Tsutsui and Case 2001). However, colony life history information outside of L. humile is sparse. Most Linepithema ants construct superficial nests in soil, leaf litter, rotting wood, or under stones, but at least two species, Linepithema iniquum and Linepithema leucomelas, are predominantly arboreal. The morphology of two poorly known species, L. flavescens and Linepithema cryptobioticum, indicates a primarily subterranean existence although this has yet to be confirmed by field observation.

Mating behavior is unknown for most species, but there are hints of extensive variation within this genus. Queens of L. humile are flightless and mate in the nest (Krieger and Keller 2000), but the few other species for which observational data are known have flighted queens (Wild, pers. obs.) Interestingly, male morphology is more variable within Linepithema than it is among many other ant genera, suggesting that mating behavior in Linepithema may show extraordinary diversity. Males vary in size from much greater than the workers to somewhat smaller, with variation in the number of closed cells in the fore wing, the relative elongation of the body and wings, the degree of development of the terminalia, the degree of development of the mesosoma, and the extent to which the head and petiolar morphology approaches the female condition (Shattuck 1992c, Wild, pers. obs). Mating flights, where they have been observed, take place around dusk (Wild, pers. obs).

Association with other Organisms
An undescribed species of Linepithema from Dominican amber is a host for the nematode (Poinar et al., 2006).

Nomenclature

 *  LINEPITHEMA [Dolichoderinae: Leptomyrmecini]
 * Linepithema Mayr, 1866a: 496. Type-species: Linepithema fusca, by monotypy.

Description
Shattuck (1992) and Wild (2007)

Worker
HEAD. Vertex convex to weakly concave. Compound eyes present, approximately round; relatively anterior on head. Ocelli absent. Antennae 12 segmented. Scape short, at most surpassing the vertex by less than one-third its length. Anterolateral clypeal margin even with the mediolateral region. Anteromedial clypeal margin with a broad, shallow concavity. Anterior clypeal setae 8-12; short, less than twice the maximum scape diameter; straight. Posterior clypeal margin between the anterior and posterior surfaces of the antennal socket cavities. Anterior tentorial pit nearer the antennal socket than the mandibular insertion. Frontal carina present. Anterolateral hypostoma reduced to a thin sclerite. Medial hypostoma entire. Psammophore absent. MOUTHPARTS. Palp formula 6:4. Third maxillary palp segment subequal in length to about 1/3 longer than segment 4. Fifth maxillary palp segment at the apical extreme of segment 4. Mandible with 5-8 teeth and 5-13 denticles. Apical tooth elongate and much longer than the subapical tooth. Basal angle either distinct (with a well developed tooth or angle separating the masticatory and basal margins) or weakly defined by a denticle. Basal margin denticulate distally, smooth proximally. MESOSOMA. Posteroventral pronotum lateral, rounded. Mesopleural process absent. Anteromedial mesosternum even with the lateral regions. Declivitous and dorsal faces of propodeum convex; dorsal face subequal in length tothe declivitousface. Propodeal angle distinct or indistinct. Mesosomal spines and tooth absent. Erect pronotal hairs 0-8 (generally 0 or 2) elongate, much longer than the maximum scape width. Dorsal pro-mesonotal junction with the pronotum and mesonotum even. Metanotal groove forming a distinct angle between the mesonotum and propodeum. Metanotal spiracle either lateral and ventral ofthe dorsal surface, or dorsal and lying on the dorsal surface, when viewed in lateral profile. Propodeal spiracle lateral and usually ventral of the propodeal dorsum, rarely forming posterodorsal corners of the propodeum (Linepithema aztecoides). Hind tibial spur with well developed barbules along entire inner surface (except extreme base). PETIOLE. Scale present; rounded and forming an even arch dorsally, or ridged and with a distinct angle dorsally; varying from moderately inclined anteriorly (but with the anterior and posterior faces approximately the same length) to strongly inclined anteriorly (and with the anterior face much shorter than the posterior face). Venter with a slight to well developed lobe. GASTER. First tergite vertical and not concealing the petiole in dorsal view and with a groove or indentation forthe reception of either the basal portion orthe entire height of the petiole. Anteriortergosternal suture of the first segment extending laterally from the helcium in a dorsal arch which is approximately the same height as the helcial dorsum. Fifth tergite ventral, gaster with 4 apparent tergites. Gastral compression usually absent, rarely dorso-ventrally compressed (L. aztecoides). Fourth sternite keel-shaped posteriorly. GENERAL CHARACTERS. Worker caste monomorphic. Chromosome number 8 or 9 (n=8, 2n=16, L humile, Crozier 1968a; n=9, L piliferum, sp. nr. piliferum, Crozier 1970a). Integument thin and flexible, weakly sculptured. PROVENTRICULUS. Cupola much broader than bulb; round; with short pile; smooth, without sculpture; and with short, lateral phragma. Bulb completely hidden by cupola in lateral view. Occlusory tract absent.

Queen
HEAD. Vertex slightly convex to concave. Compound eyes relatively anterior on head. antennae 12 segmented. Scape short, surpassing vertex by less than 1/3 scape length. Anterolateral clypeal margin even with the mediolateral region. Anteromedial clypeal margin entire, without a central notch or concavity of any type. Anterior clypeal setae 6-15; short, less than twice the maximum scape diameter; straight. Posterior clypeal margin even with or posterior to the posterior surfaces of the antennal socket cavities. Anterior tentorial pit nearer the antennal socketthan the mandibular insertion. Anterolateral hypostoma reduced to a thin sclerite. Medial hypostoma entire. Psammophore absent. MOUTHPARTS. Palp formula 6:4. Third maxillary palp segment subequal in length to about 1/3 longer than segment 4. Fifth maxillary palp segment atthe apical extreme of segment 4. Mandible with 6-8 teeth and 7-11 denticles. Apical tooth elongate and much longer than the subapical tooth. Basal angle varying from distinct (with a well developed tooth or angle separating the masticatory and basal margins) to weakly defined by a denticle. Basal margin varying from denticulate distally. smooth proximally to denticulate along entire surface. MESOSOMA. Posteroventral pronotum lateral, rounded or angled. Episternal suture complete. Mesopleural process absent. Anteromedial mesosternum even with the lateral regions. Axilla parallel and with a suture medially. Anterior axillar suture straight. Declivitous and dorsal faces of propodeum convex; dorsal face subequal in length to the declivitous face. Propodeal angle distinct. Propodeal suture absent. Mesosomal spines and tooth absent. Erect mesoscutal hairs 2–35; short, less than twice the maximum scape diameter. Propodeal spiracle lateral and ventral of the propodeal dorsum. Hind tibial spur with well developed barbules along entire inner surface (except extreme base). WINGS. Radial cell closed. Forewing with 2 submarginal cells and one discoidal cell. Hind wing with 2 cells. PETIOLE. Scale present; ridged and with a distinct angle dorsally; vertical and not inclined anteriorly. Venter with a slight or weakly developed lobe. GASTER. First segment vertical and not concealing the petiole in dorsal view and with a groove or indentation for the reception of the entire height of the petiole. Fifth tergite ventral, gaster with 4 apparent tergites. Gastral compression absent (gaster circular in cross section). Fourth sternite flat across entire posterior border.

Male
HEAD. Inner margin of eye entire, flat (sometimes weakly concave). Scape length shorter than the length of funicular segments 2+3. First funicular segment barrel-shaped. Second funicular segment cylindrical, straight. Funicular segments 2 and 3 more than twice as long as broad. Third and fourth funicular segments straight. Anteromedial clypeal margin entire, without a central notch or concavity of any type. Anterior clypeal setae 8; short, about as long as the maximum diameter of the scape; straight. Posterior clypeal margin anterior to the posterior surfaces of the antennal socket cavities. Anterior tentorial pit nearer the antennal socket than the mandibular insertion. Anterolateral hypostoma reduced to a thin sclerite. Medial hypostoma entire. MOUTHPARTS. Palp formula 6:4. Third maxillary palp segment subequal in length to segment 4. Fifth maxillary palp at the apical extreme of segment 4. Mandible with 1 tooth (the apical and 6-14 denticles. Apical tooth of mandible varying from shorter to much longer than subapical tooth. Basal angle weakly defined by a denticle. Basal margin smooth and without teeth or denticles (or occasionally with a few small denticles near the basal angle). MESOSOMA. Posteroventral pronotum lateral, rounded. Episternal suture present, complete. Anteromedial mesosternum even with the lateral regions. Axilla parallel and with or without a suture medially. Anterior axillar suture straight. Declivitous face of propodeum convex or concave; dorsal face convex, longer than the declivitous face. Propodeal angle distinct or indistinct. WINGS. Radial cell closed. Forewing with 1–2 submarginal cells and one discoidal cell. Pterostigmal appendage absent. Hind wing with 2 cells. PETIOLE. Scale present; either rounded and forming an even arch dorsally, or forming a blunt angle dorsally; vertical and not inclined anteriorly. Venter with or without a well developed lobe. Attachmentto gaster narrow. GASTER. First segment elongated posteriorly, smooth and without a groove or indentation. GENITALIA. Pygostyles present, often reduced. Posterior margin of subgenital plate concave. Paramere triangular and very weakly divided, or with the gonostylus elongated into an articulating finger-like structure. Digitus with a down-turned tip. Cuspis absent or ventral of digitus. Ventral lobe of volsella present as either a weakly concave lobe or as a swelling. Aedeagus with ventral teeth.

Larva
Shape dolichoderoid. Protuberances present as a single boss located middorsally on abdominal tergite 1. Body hairs sparse; simple or bifid; short. 9 spiracular pairs. Antennae short.