Linepithema humile

Wild, A. L., 2004, Taxonomy and distribution of the Argentine ant, Linepithema humile (Hymenoptera: Formicidae)., Annals of the Entomological Society of America 97, pp. 1204-1215: 1207-1229

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Linepithema humile


Linepithema humile  HNS  (Mayr)

(Worker, Figs. 1 and 5; queen, Figs. 8, 9, and 10; male, Figs. 11, 12, and 13)

Hypoclinea humilis  HNS  Mayr 1868: 164. Worker description.

Iridomyrmex humilis  HNS  (Mayr); Emery 1888: 386-388. First combination in Iridomyrmex  HNS  .

Iridomyrmex humilis  HNS  (Mayr); Wheeler 1913: 27-29. Male and queen description, worker redescription.

Iridomyrmex humilis  HNS  (Mayr); Newell and Barber 1913: 38-39 (egg), 40-41 (larva), 42-45 (worker, male, queen pupae).

Iridomyrmex humilis variety arrogans Chopard  HNS  1921: 241-245. Syn. Nov. Junior synonym of I. humilis  HNS  by Bernard 1967: 251. Restored to subspecies of L. humile  HNS  by Shattuck 1992: 16.

Iridomyrmex riograndensis Borgmeier  HNS  1928: 64. Syn. Nov.

Iridomyrmex humilis  HNS  (Mayr); Wheeler and Wheeler 1951: 186-189. Summary of larval biology.

Linepithema riograndense (Borgmeier)  HNS  ; Shattuck 1992: 16.

Linepithema humile  HNS  (Mayr); Shattuck 1992: 16. First combination in Linepithema  HNS  .

Type Material Examined

Hypoclinea humilis  HNS  Mayr 1868. one worker HoLoTYPE. Argentina, Buenos Aires (1866, Stroebel coll.) [NHMW].

Iridomyrmex riograndensis Borgmeier  HNS  1928. Eight worker SYNTYPES. Brazil, Rio Grande do Sul, s.loc. (19.I.1918) [MZSP].

Additional Material Examined

South American material examined given in Table 1. Specimens collected outside of South America are listed below. Given the large quantity of Argentine ants collected in California, I only list here a representative subsample of California specimens that were given a relatively thorough examination under the microscope (e.g., setal counts and measurements).

AUSTRALIA. Sydney [MZSP]; Victoria (s. loc.) [BMNH]. BELGIUM. Bruxelles Capitale: Brussels [BMNH, NHMB], Brussels Botanical Garden [MHNG]. BERMUDA. Bermuda (s. loc.) [BMNH]. CAMEROON. Centre-Sud: Nkoemvom [bMNh]. FRANCE. Provence-Alpes-Co te d'Azur: Cannes [MCZC, NHMB], Castellane [BMNH], Hyeres [BMNH], Ste. Maxime [NHMB]; Midi-Pyrenees: Toulouse [IFML]. GERMANY. Berlin: Botanical Garden[MHNG]. ITALY. Campania: nr. Naples [BMNH]; Liguria: San Remo [NHMB]; Sicilia: Palermo[BMNH]; Toscana: Monte Argentario Giannella [BMNH], Orbetello [BMNH]; Varazze (Savona) [MZSP]. LESOTHO. Maseru: Maseru [BMNH]. MO- ROCCO. Tanger, Tangier [USNM]. MEXICO. Baja California: Ensenada, Cortera FR [AVSC]; Baja California Sur: Guerrero Negro [AVSC]; Distrito Federal: Mexico City [BMNH], Distrito Federal (s.loc.) [WPMC]. NAMIBIA. Erongo: Swakopmund [BMNH]. POLAND. Dolnoslaskie: Breslau [NMHB]. PORTUGAL. Faro: Algarve, Luz nr. Lagos [bMNh]; Lisboa: Cascais [USNM], Estoril [USNM], Lisbona [MCZC, NHMB], Mafra [USNM], Praia das Macas [USNM]; Madeira: Funchal [MCZC, NHMB], Porto Moniz [BMNH], RibeiraBrava [BMNH], Ilheu Chao [BMNH], Porto Santo [BMNH], Sao Vicente [BMNH], Vale de Paraiso [BMNH], Praia Formosa [BMNH], Porto da Cruz [BMNH], Feiteiras [BMNH], Caramujo [BMNH], Lower Levada [BMNH], Madeira Is. (s. loc.) [BMNH, MHNG, NHMB]; Porto: Leca [BMNH], Oporto [BMNH]. SOUTH AFRICA. Eastern Cape: Queenstown [BMNH], Somerset East [BMNH]; Mpumalanga: Nelspruit[BMNH]; Northern Cape: Colesberg [ALWC]; Western Cape: Capetown [BMNH], Table Mt. [BMNH], nr. George [BMNH]. SPAIN. Andalucia: Malaga [USNM]; Canarias: Arenara [BMNH], Cruz de Tejeda [BMNH], Gran Canaria, Las Palmas, Telde [BMNH, MCZC, USNM], Orotava [BMNH], Santa Brigida [BMNH], Tenerife, Agua Mansa [NHMB], Tenerife, Ladera de Guimar [BMNH], Tenerife, Volcan de Guimar [NHMB], Tenerife (s. loc.) [BMNH]; Cataluna: Playa de Aro [NHMB]; Galicia: Mte. Ferro b. Bayona [BMNH]; IslasBaleares: Minorca, CalaForcat[BMNH]. UNITED KINGDOM. Edinburgh: Edinburgh[BMNH]; Sussex: Lewes [BMNH]; Eastborne[BMNH]; Exeter [BMNH]; Windsor [BMNH]; Glasgow [BMNH]; W. Maidstone, Kent [bMNh]; Chillingham [BMNH]; Farnham House Lab, Imperial Bureau of Entomology [BMNH]. UNITED STATES. Alabama: Lowdnes Co., Ft. Deposit [USNM]; California: Alameda Co., Berkeley [UCDC, USNM]; Humboldt Co., Redway [ALWC]; Los Angeles Co., Pasadena[MZSP, USNM], Monterrey Co., Big Sur [ALWC]; Orange Co., Bolsa Chica Marsh [MZSP]; Riverside Co., Lake Skinner Camp [AVSC]; Sacramento Co., Sacramento [UCDC]; San Diego Co., UC Elliot Reserve [AVSC]; San Diego Co., San Diego [UCDC]; San Diego Co., E. San Diego [UCDC]; San Diego Co., Pacific Beach [UCDC]; San Diego Co., Mission Hills [UCDC]; San Diego Co., Kate Sessions Park [UCDC]; San Diego Co., Balboa Park [UCDC]; San Diego Co., Point Loma [UCDC]; San Joachin Co., Caswell State Park [PSWC]; San Luis Obispo Co., Oso Flaco Lake [LACM]; San Mateo Co., Colma [USNM]; San Mateo Co., San Bruno Mt. [PSWC]; Santa Clara Co., South Coyote [PSWC]; Sonoma Co., Russian R. 6k E. Healdsburg [UCDC]; Yolo Co., 6kW Capay [PSWC]; Yolo Co., Davis [PSWC, UCDC]; Yolo Co., Grasslands Regional Park, 8k SW Davis [PSWC, UCDC]; Florida: Escambia Co., Gonzalez [MCZC]; Louisiana: Plaquemines Co., Happy Jack [BMNH], Orleans Co., New Orleans [BMNH], Lousiana (s. loc.) [BMNH]; Mississippi: Coahoma Co., Clarkesdale [USNM], Copiah Co., Hazelhurst [MCZC], Oktibbeha Co., Starkville [BMNH]; South Carolina: York Co., York [BMNH].

Worker Measurements. HOLOTYPE: HL 0.74, HW 0.66, MFC 0.16, SL 0.76, FL 0.65, LHT 0.68, PW 0.45, ES 2.93, SI 115, CI 89.

Others (n = 81): HL 0.62-0.78, HW 0.53-0.72, MFC 0.14-0.18, SL 0.62-0.80, FL 0.52-0.68, LHT 0.57-0.76, PW 0.35-0.47, ES 1.98-3.82, SI 108-126, CI 84-93.

Worker Diagnosis. A large (HL> 0.62 mm) slender Linepithema  HNS  . Head in full-face view longer than broad (CI 84-93), narrowed anteriorly and reaching its widest point just posterior to the compound eyes. Lateral margins broadly convex, grading smoothly into posterior margin. Posterior margin of head straight in smaller workers to weakly concave in larger workers. Compound eyes large (ES 1.98-3.82), comprising 82- 110 ommatidia (normally around 100). Antennal scapes long (SI 108-126), as long or slightly longer than HL and easily surpassing posterior margin of the head in full-face view. Maxillary palpi relatively short, segments 4 and 5 both noticeably shorter than segment2.

Pronotum and mesonotum forming a continuous convexity in lateral view, mesonotal dorsum nearly straight, not angular or strongly impressed, although sometimes with a slight impression in the anterior half.

Metanotal groove moderately impressed. Propodeum in lateral view inclined anteriad. In lateral view, dorsal propodeal face meeting declivity in a distinct though obtuse angle, from which the declivity descends in a straight line to the level of the propodeal spiracle.

Dorsum of head, mesosoma, petiole, and abdominal tergites 3 and 4 (= gastric tergites 1 and 2) devoid of erect setae (very rarely with a pair of small setae on abdominal tergite 4). Clypeus bearing a pair of long, forward-projecting setae. Abdominal tergites 5 and 6 each bearing a pair of long, erect setae. Ventrum of metasoma with scattered erect setae. Gula with a pair of short setae. Body and appendages, including gula, the entire mesopleuron, and abdominal tergites, covered in dense pubescence.

Body and appendages concolorous, most commonly a medium reddish or yellowish brown but ranging in some populations from testaceous to dark brown, never yellow or piceous. Integument shagreened and lightly shining.

Worker Geographic Variation. Specimens from introduced populations outside of South America tend to fall toward the upper range of size variation in nearly all measurements, although there is considerable variation both in the native and the introduced ranges. The holotype worker from Buenos Aires is among the largest ants from either range. Some Paraguayan populations, particularly those farther than 10 km from the Paraguay River, have a slightly smaller eye size (<95 ommatidia) and tend to be smaller than ants in the southern Parana drainage and along the major riverways. In general, Paraguayan specimens vary more in color than specimens from elsewhere, from testaceous to dark brown. The diagnostically sparse pilosity is generally consistent across all specimens, but several workers from Campana, Buenos Aires, have small erect setae on abdominal tergite 4 (= gastric tergite 2). These Campana workers otherwise fall within the range ofvariation for L. humile  HNS  , and males from the same series clearly belong to L. humile  HNS  .

Queen Measurements. (n = 13) HL 0.83-0.92, HW 0.83-0.93, SL 0.81-0.89, WGL 4.42-4.51, WL 1.67- 2.09, FL 0.78-0.90, LHT 0.88-0.97, ES 7.3-9.4, SI 96- 102, CI 93-101.

Queen Diagnosis. A robust species, dificult to distinguish from queens ofrelated Linepithema  HNS  , with long antennal scapes and large eyes. Head in full face view normally somewhat longer than broad (CI 93-101), lateral margins convex and broadly curved into the posterior margin. Posterior margin ofhead straight to slightly concave, never deeply or conspicuously concave. Eyes large (ES 7.3-9.4). Antennal scapes long (SI 96 -102) and nearly equal to head length.

Entire body covered in a dense pubescence, a bit thicker and longer than that of the worker. Pilosity is also more developed than in the worker, with 2-11 (mean = 6) erect setae on the mesoscutum, 1-7 (mean = 4) erect setae on the scutellum, and 1-10 (mean = 3) erect setae on abdominal tergite 3, including the posterior row. Color as for the worker.

Queen Geographic Variation. Alate queens are much more common in collections from the native range than in collections from outside of South America. This observation is unlikely to be a sampling artifact given how heavily the introduced populations are represented in collections.

Male Measurements. (n = 25) HL 0.56-0.71, HW 0.56-0.74, SL 0.13-0.16, MML 1.40-1.96, MMW 0.76- 1.12, WGL 2.55-3.26, FL 0.60-0.77, LHT 0.51-0.66, SI 12.8-15.4, CI 98.2-106.0.

Male Diagnosis. A robust ant, larger than the worker, with an exceptionally well-developed mesosoma. Head about as broad as long in full face view (CI 98.2-106.0) and somewhat dorso-ventrally compressed in lateral view. Eyes large, occupying much of antero-lateral surface of head and forming the anterior margin ofthe head lateral to the clypeus and the lateral margin of the head anterior to midpoint. Ocelli large and in full frontal view set above the adjoining postero-lateral margins. Anterior clypeal margin straight to broadly convex. Mandibles small, having a single apical tooth and four to eight denticles along the masticatory margin and rounding into the inner margin. Masticatory margin relatively short, about the same length as the inner margin. Inner margin roughly parallel to, or even converging distally with, the exterior lateral margin.

Mesosoma well-developed, considerably wider than head width, and larger in bulk and in length than metasoma. Mesoscutum greatly enlarged, projecting forward in a convexity overhanging the pronotum. Scutellum large, convex, nearly as tall as mesoscutum and projecting well above the level ofthe propodeum. Propodeum overhanging petiolar node, and declivitous face strongly concave.

Wings short relative to mesosomal length (Fig. 17) and bearing a single submarginal cell. Wing color whitish or yellowish, with dark brown veins and stigma. Petiolar scale with a broad crest and taller than the length of the node. Ventral process well developed. Gaster oval in dorsal view, nearly twice as long as broad. Parameres terminating as rounded pilose lobes. Digitus short, with a sharp, downturned terminal ilament.

Dorsal surfaces of body largely devoid of erect setae, occasionally with a few ine, short setae scattered on mesoscutum, scutellum, and posterior abdominal tergites. Venter of gaster with scattered setae. Pubescence dense on body and appendages, becoming sparse only on the medial propodeal dorsum. Color as for the worker.

Male Geographic Variation. As in workers, specimens from introduced populations outside of South America tend to fall in the upper range of size variation.


Taxonomy. These taxonomic results support current nomenclatural use. The holotype worker of Mayr's Hypoclinea humilis  HNS  falls neatly within the range of variation present in the Argentine ant both in South America and in locations around the world where the ant is invasive (Figs. 15 and 16). The only older species-level name in the genus, Linepithema fuscum  HNS  Mayr 1866, pertains to a male ant whose slender build, elongate genitalia, and distinct queen-like wing venation indicate only a distant relation to L. humile  HNS  . Borgmeier's species riograndense  HNS  , described from Rio Grande do Sul, Brazil, is clearly conspecific with L. humile  HNS  and is synonymized here. Borgmeier's specimen identifications in MZSP reveal that he considered the name humile  HNS  to apply to a common, probably undescribed southern Brazilian Linepithema  HNS  with short antennal scapes and more extensive pilosity. The aptly named subspecies L. humile arrogans Chopard  HNS  , described from introduced L. humile  HNS  populations in southern France, was probably inadvertently resurrected by Shattuck (1992) from an earlier synonymy. Here, I return arrogans  HNS  to synonymy because there is no reason to view introduced Argentine ant populations as being heterospeciic.

Diagnosis. L. humile  HNS  diagnosis is straightforward in the male caste. The distinctive bulky males of humile  HNS  are not easily confused with males of any other species. Males of closely related forms share structural similarities with L. humile  HNS  (e.g., the undescribed species in Fig. 14) but are considerably smaller (Fig. 17) with a much less developed mesosoma. The lack of known intergrades strongly supports the speciic status of L. humile  HNS  . The only other congeneric males that share the size of humile  HNS  are montane Andean and Caribbean forms associated with L. fuscum  HNS  , but these are unlikely to be confused with L. humile  HNS  . Linepithema fuscum  HNS  -group males are structurally divergent (Shattuck 1992), with an unusually elongate habitus, a propodeum with a convex posterior face in lateral view, two submarginal cells in the forewing, and considerably longer wings relative to maximum mesosomal length (Fig. 17).

Diagnosis is somewhat more problematic in workers, as no single character serves to separate L. humile  HNS  from congeneric species. Table 2 provides a summary of the minimum combination of three character states that can diagnose nearly all L. humile  HNS  worker specimens over the full geographic distribution of Linepithema  HNS  . Figure 15 shows a consistent though not absolutely diagnostic difference in eye size versus head length between the large-eyed L. humile  HNS  and all other non-humile specimens. Figure 16 plots antennal scape length versus head length in L. humile  HNS  versus several other species, excluding the distinct long-scaped species L. oblongum  HNS  , L. leucomelas  HNS  , and ants of the L. iniquum  HNS  -complex. These species are readily recognizable with other characters. Specifically, iniquum-complex ants have a strongly impressed mesonotal dorsum (Fig. 3), pronotal setae, and smaller eyes (ES <2.0). L. leucomelas  HNS  has a distinct white/brown bicoloration reminiscent of the ant Tapinoma melanocephalum (F.)  HNS  1793, standing setae on gastric tergites 1 and 2, and smaller eyes (ES <2.0).

L. oblongum  HNS  (Fig. 6) is the species most similar to L. humile  HNS  . This poorly known ant seems to be conined to the high Andes in northern Argentina and Bolivia. Workers share the sparse pilosity and a similar mesosomal proile with L. humile  HNS  , but they are somewhat more elongate (CI 81-88, mean = 84 in L. oblongum  HNS  ; CI 84 -93, mean = 90 in L. humile  HNS  ) and have relatively smaller eyes (Fig. 18). Linepithema oblongum  HNS  workers also have a noticeably smoother and shinier integument on the gastric dorsum than L. humile  HNS  , and most workers have only sparse pubescence on gastric tergites2 and 3, although some of the larger specimens within a series may retain a dense pubescence. Males of L. oblongum  HNS  are much smaller than those of L. humile  HNS  (MML <1.1), and they lack the extraordinary mesosomal development of L. humile  HNS  males. This species may be the sister taxon of the Argentine ant, a possibility that is currently being pursued with molecular genetic data (unpublished data).

Distribution. The Argentine ant's native distribution seems to be limited to the Parana River drainage (Fig. 19), conirming the conclusion of Tsutsui et al. (2001). South American records of L. humile  HNS  outside the Paranaa drainage are invariably from urban areas, an observation that strongly supports the notion of recent introduction by human commerce. Paranaa drainage records are also more abundant than nonParanaa records (49 versus 8). Furthermore, most records fall within a few kilometers of the largest rivers: the Parana, the Paraguay, and the Uruguay. This is unlikely to be a sampling artifact, as evidenced from numerous records of other, non-humile species distant from major rivers (Fig. 20).

Records of L. humile  HNS  in South America show the following pattern: patchy local abundance in low areas of the Parana River drainage; common along major rivers (perhaps aided through frequent natural dispersal along the river); and very recent dispersal out of the Parana drainage with human activity. Interestingly, some of the more morphologically divergent L. humile  HNS  , including those with color variations and smaller compound eyes, are found>10 kilometers away from large rivers in the northern part of the native range. It is unlikely that this variation reflects the existence ofcryptic species, given that much ofthe variation is allopatric and that L. humile  HNS  males show remarkable consistency in diagnostic traits across populations. Specimens from the southern native range tend to look more like the common pest L. humile  HNS  , although there is still a fair amount of variation. Overall this pattern raises the hypothesis of a northern origin for the species with later dispersal along the rivers. This hypothesis could be tested with genetic data in a phylogeographic framework (Avise 2000).

The history and biology of the Argentine ant in its native range is liable to be complex. Argentine ants likely move along river channels during periods of natural disturbance, and some of the native range records probably correspond to recent local introductions through human commerce. It bears noting that L. humile  HNS  is present in many urban areas along the Paranaa and Paraguay rivers. The preponderance of Argentine ant records from lat, expansive lood plains suggests that records from fast-running, deeply channelized stretches of the Upper Parana such as Argentina's Foz do Iguacu also may not represent native populations.

The morphological diversity in native-range L. humile  HNS  raises the issue of intraspeciic diversity in other aspects of Argentine ant biology. Tsutsui and Case (2001) note variation in colony structure in the native range, and there also may be variation in mating systems and in colony life history. Studies that make use of contrasts between Argentine ant biology between native and introduced ranges would do well not to treat native range L. humile  HNS  as a monolithic entity.

Rather, care should be taken to chose L. humile  HNS  populations that are most likely to be close relatives of the introduced populations under study. Genetic work of Tsutsui et al. (2001) indicates that a southern Parana population represents the source population for California L. humile  HNS  . It also remains a possibility that some biological changes that contribute to the Argentine ants' invasive success occurred within the native range before introduction. Detailed studies ofArgentine ant biology mapped onto a population-level phylogeny over the whole of the native range could determine if this were the case, as well as shed light on the sequence of evolutionary events leading to invasiveness in Argentine ants.














Linepithema humile

Wild, A. L. 2004


Hypoclinea humilis

Mayr 1855