Solenopsis richteri

This native South American species was introduced into the United States sometime around 1918. It is closely related to the Red Imported Fire Ant (Solenopsis invicta) and they commonly hybridize in areas of overlap. However, S. richteri has largely been displaced by Solenopsis invicta in North America and is now restricted to southern Tennessee, northeast Mississippi and northwest Alabama as well as the tidewater area of Virginia. But even there, "pure" S. richteri seems rare and most populations appear to be intergrades between these two species.

Identification
Pitts et. al. (2018) - A member of the Solenopsis saevissima species-group. The gynes of the lighter varieties of S. richteri and Solenopsis interrupta are very similar in coloration due to the first tergite of the gaster sometimes having an orange tergal maculation with a distinct posterior margin. In many cases, the gynes of these two species are difficult to differentiate. However, they differ in the sculpturing of the postpetiole. Also, S. richteri gynes are normally darker in coloration and the OOI is much smaller than in S. interrupta. The workers of S. interrupta are larger than S. richteri and easily separated. Gynes of S. richteri in the United States are of the light form and are similar to those found in the northern range of the species in southern Brazil. This is probably the point of origin for the United States population.

The gynes of S. richteri from the westernmost areas of the native range tend to be more darkly colored than the eastern representatives, resembling somewhat the darker Solenopsis invicta gynes. However, S. richteri gynes are much darker in coloration and usually have a distinct tergal maculation on the gaster. Also, the lateral margins of the postpetiole are more concave than in S. invicta. The gynes of S. richteri have weakly concave to straight lateral margins. Males of S. richteri are similar in coloration to many of the species within the species-group. However, the OOI of S. richteri is much smaller than in S. interrupta. Also, the males of S. richteri sometimes have a very distinct tuberculate postpetiolar spiracle. Usually in populations in the United States and in the southern part of the native range near Buenos Aires, the tuberculate postpetiolar spiracle is smaller and not as distinct. Males of S. richteri tend to have stronger sculpturing in the United States than they do in South America.

The larvae differ from Solenopsis quinquecuspis by the presence of multi-branched setae and fine rugae on the head capsule, as well as being smaller in size.

Distribution
Pitts et. al. (2018) - In South America, the range of S. richteri extends from Parana State, Brazil south and west from Misiones Province to Mendoza Province in Argentina. This species also has been introduced into North America, where it is currently confined to the northwestern corner of Alabama, the northeastern corner of Mississippi, and south-central Tennessee. A broad S. richteri x invicta hybrid zone exists south and east of the S. richteri range, including much of Mississippi, northern Alabama, and the northwestern corner of Georgia (Shoemaker et al. 1996, Gardner et al. 2008, Oliver et al. 2009).

Distribution based on Regional Taxon Lists
Nearctic Region: United States. Neotropical Region: Argentina, Chile, Paraguay, Uruguay.

Biology
Pitts et. al. (2018) - Genetic studies of S. richteri in its native range have revealed pronounced nuclear and mtDNA differentiation between populations from southern Brazil and central Argentina (Ross and Shoemaker 2005), consistent with a long-term absence of gene flow between ants occupying these distant parts of its range.

Nomenclature

 *  richteri. Solenopsis pylades var. richteri Forel, 1909a: 267 (w.q.) ARGENTINA. Creighton, 1930b: 87 (m.); Wheeler, G.C. & Wheeler, J. 1977: 589 (l.). Subspecies of saevissima: Santschi, 1916e: 379; Creighton, 1950a: 232; Wilson, 1952b: 58. Raised to species: Buren, 1972: 4. Senior synonym of tricuspis: Creighton, 1930b: 87; of oblongiceps: Trager, 1991: 187. See also: Rhoades, 1977: 1.
 * tricuspis. Solenopsis pylades var. tricuspis Forel, 1912g: 4 (w.) ARGENTINA. Subspecies of saevissima: Santschi, 1916e: 379. Junior synonym of richteri: Creighton, 1930b: 87.
 * oblongiceps. Solenopsis saevissima st. oblongiceps Santschi, 1936d: 405, fig. 5 (w.) ARGENTINA. Junior synonym of saevissima: Wilson, 1952b: 55; of richteri: Trager, 1991: 187.

Worker
Buren (1972) - Head length .79 to 1.40 mm, usually about 1.3 to 1.4 mm in the majors; width, .69 to 1.33 mm, usually about 1.2 to 1.33 mm in the majors. Scape length, 1.05 to 1.12 mm in majors. Thoracic length about 1.66 to 1.75 mm in majors.

Head with broadly ellipitical sides, broadest near mid-length of head, about even with rear border of eyes; rear border with distinct, crease-like median cleft; occipital lobe peaks relatively close to the cleft, scapes in majors reaching, or nearly reaching, these peaks. In small and medium sized workers head broader anteriorly, occipital cleft nearly absent or weak, and scapes distinctly surpassing rear border of head. Distinct ocellar pit present in the majors, but a developed ocellus apparently never occurs in the worker.

Thorax in largest caste with strong pronotal shoulders and distinct promesonotal suture, median portion of pronotum immediately cephalad of this suture always shallowly but distinctly sunken; in profile base of propodeum straight, or nearly so, appearing longer than the declivity; promesonotum weakly convex in profile, usually not rising much above level of propodeum. In small and medium workers pronotal shoulders weak or not apparent, the promesonotal suture obliterated dorsally. In profile small workers with promesonotum flattened or weakly convex, of about the same height as propodeum.

Sculpture on mesopleura of majors appears as very fine punctostriae, fore part of metapleura with similar pattern; on rear portion of metapleura the striae lose the intercalated punctures, and become distinctly stronger and more widely spaced; nearly always a clear, smooth shining space between striated area and propodeal spiracle, striae immediately caudad of spiracle usually very weak or obliterated. In small and medium workers sculpturing proportionally coarser and less dense than in large workers. Mesonotal-propodeal suture very strongly impressed and distinct in all size workers.

Petiole with thick, blunt scale in all size classes, proportionately thicker in small workers, seen in profile, In majors, in anterodorsal view, pedicel anterior to spiracles relatively slender, and scale with rounded outline dorsally; petiole with ventral keel, without anteroventral tooth; postpetiole a little wider than petiole, in posterodorsal view rounded or convex anteriorly, sides converging behind except in very large majors where they may be parallel; in profile usually a break in outline due to transverse impression on rear dorsum near stridulative surface; in sculpture sides of pospetiole roughly and irregularly rugose and punctate; in dorsal view mid-frontal area usually smooth and shining or at least only weakly shagreened, a few transverse punctostriae on rear border.

Aside from sculpture described above, most surfaces smooth and shining, except for punctostriate areas in front of eyes. All parts with numerous erect hairs of various lengths; a few hairs on pronotum and mesonotum, and in double bilateral rows on the head much longer than others. Punctures from which this pilosity arises not very deep or large. Pubescent appressed hairs sparse or absent on nearly all surfaces.

Color distinctive due to large, often strikingly bright orange spot on first tergite of gaster; a similar large, bright spot present on first gastric sternite. These spots may not be present on all workers of anyone colony, but are usually present in a majority of the largest caste, about half of the medium sized workers, and a few of the minors. The color of the spots is not a characteristic of the integument, which appears to be colorless and transparent over these areas, but is due to a colored glandular or fatty mass lying just underneath the integument. Remainder of gaster very dark brownish black. Head, thorax, petiole, coxae, femora, tibiae, and scapes piceous brown. Mandibles, often lateral extensions of c1ypeus, cheeks, tips of scapes, funiculi, especially the clubs, and tarsi yellowish. In some colonies these areas concolorous with the rest of the head and thorax. Postpetiole sometimes concolorous with the dark colored surfaces, but often in majors bright orange except for an anteromedian V shaped mark. Variations on this basic color pattern often evident. Specimens from Uruguay and Argentina, for instance, often have the gastric spot brownish rather than orange, and have the yellow areas on the head more restricted or absent.

Pitts et. al. (2018) - Head ovate to weakly cordate. Head sculpture with small piligerous foveolae, <0.01 mm in diameter. Median frontal streak absent. Median ocellus in largest major workers absent. Mandibular costulae present, obsolescent medially. Humerus distinctly angulate, with distinct tuberculate boss. Mesonotum with 20–26 setae. Promesonotal suture curved medially, never projecting upward. Propodeum sculpture glabrous posteroventral to spiracle. Postpetiole shape as high as or higher than broad. Postpetiole with lower 0.50 or less transversely rugose to punctate-rugose, dorsum and face dorsal to sculpture nitid, glabrous. Color generally black with mandibles, lateral areas of clypeus, antennal fossae, mesosomal sutures, and T1 maculations dark brown to yellow brown.

Queen
Buren (1972) - Head length 1.25 to 1.30 mm, width 1.35 to 1.40 mm. Scape length 1.02 to 1.06 mm. Thoracic length 2.55 to 2.69 mm.

Head scapes slightly exceeding hind border. Eyes large, ocelli present; ocellar area somewhat raised and distorting the surface. Occiput with a median crease-like excision. Scale of petiole thinner than in worker, often produced into a blunt median point above when seen from behind. Postpetiole wider than long, seen from above; sides sub-parallel or weakly concave.

Nearly all sclerites of thorax smooth and shining. Metapleura with fine longitudinal striae, these somewhat stronger and more widely spaced toward the rear as in the worker; nearly always a clear shining space between the striated area and propodeal spiracle. Sides of petiole finely punctate and roughened. Sides of postpetiole with fine punctures and rough, irregular rugae or striae, dorsum nearly smooth or with weak shagreening anteriorly, with some weak transverse striae medially and posteriorly.

Anterior faces of petiole and pospetiole and declivity of propodeum with moderately dense, appressed pubsecence. Erect hairs numerous and present on all surfaces.

Colors similar to those of the worker in mature alates. Head, scapes, thorax, legs, and petiole piceous brown. Gaster more nearly black, but with a bright orange spot on the anterior portion of the first gastric tergite. Postpetiole or the rear portion of it usually the same color as the spot.

Pitts et. al. (2018) - Head. Slightly broader than long, quadrate, sides of head convex from eyes to occipital angles, straight to nearly straight ventral to eyes. Eye sometimes with 3–4 setae protruding from between ommatidia, setal length ≤ 3X width of ommatidium. Median ocellus large. Lateral ocelli small to moderate. Median ocellus circular, lateral ocelli slightly ovate. Clypeus projecting, carinal teeth stout and sharp, carinae sometimes indistinct, less so dorsally, slightly divergent ventrally. Paracarinal teeth small, usually well defined. Median clypeal tooth well developed. Approximately 0.50 of eye dorsal to midpoint of head.

Mesosoma. Parapsidal lines present on posterior 0.50 of disk. Mesonotum with indistinct, median furrow on posterior 0.25 or less. Bidentate median process present on metasternum. Wing venation as in Fig.

Metasoma. Lateral faces of postpetiole weakly to strongly concave. Petiolar and postpetiolar spiracles tuberculate in some cases.

Coloration, Sculpturing, and Pilosity. Piligerous foveolae small, sparse, width <0.01 mm in diameter, larger on head than on thorax and abdomen. Pubescence simple, pale brown to yellow and erect, longer and denser on head than elsewhere, longest on anterior edge of clypeus. Pubescence orange on T1 orange integumental maculation. Mesosoma with longest pubescence (length >0.30 mm) 2X longer than shortest pubescence. Mandible with several coarse, distinct costulae, obsolescent medially. Propodeum with fine striae posteriorly, anterior 0.25 polished. Posterior surface of petiolar node with 0.75 of lower surface with fine striae, dorsum polished. Posterior surface of postpetiolar node with 0.75 of lower surface finely striate, 12–14 striae present, median striae sometimes obsolescent laterally, dorsum is polished to slightly granulate. Remaining integument smooth and polished. Two color forms exist. Dark form is dark brown except antennal scape black, flagellum brown to orange, both T1 and S1 with medial orange maculation, other sternites brown with dark brown apices, and petiole sometimes orange with dark brown dorsum. Maculations on T1 and S1 are well defined laterally and apically. Light form is orange except brown on vertex, around compound eyes, apically on T1, and preapically on remaining tergites. Both forms have dark brown maculations anteriorly on pronotum, anteromedian area of mesonotum, area around parapsidal lines, sometimes on median area of axillae, anteromedian and triangular posteromedian area of scutellum, medially on anepisternum, and medially and laterally on propodeum. Both forms have dark brown finger-like projections of pigment from preapical tergal bands to apical setae. Internal margins of ocelli dark brown. Median frontal streak usually absent, not discernable from surrounding integument.

L ~7.5–8.5,HW1.4–1.5,VW1.0–1.2, HL 1.2–1.3, EL 0.3–0.5, OD 0.1–0.2, OOD 0.15–0.20, LOW 0.08–0.10, MOW 0.10–0.15, CD 0.15–0.20, MFC 0.20–0.25, EW 0.30–0.40, SL 0.92–1.23, PDL 0.18–0.23, LF1 0.01–0.16, LF2 0.07–0.1, LF3 0.08–0.11, WF1 0.07–0.10, FL 1.10–1.23, FW 0.21–0.32, MW 1.40–1.55, DLM 2.46–2.73, PRH 1.00–1.12, PL 0.72–0.91, PND 0.51–0.60, PH 0.60–0.72, PPL 0.39–0.53, DPW 0.59–0.69, PPW 0.59–0.71, PHB 0.28–0.51, N=10.

Male
Buren (1972) - Head length .76 to .84 mm, width 1.02 to 1.06 mm. Thorax 2.60 to 2.69 mm long, 1.45 to 1.47 mm wide.

Mandibles with two teeth. Clypeus without trace of carinae. Scapes very short. Eyes and ocelli large. Scale of petiole with singular wing-nut-like appearance; from behind, dorsal border weakly to moderately concave. Erect hairs numerous on all surfaces.

Concolorously black except for the very pale antennae.

Pitts et. al. (2018) - Head. Eye normally with 2–4 setae protruding from between ommatidia, setal length ≤3X width of ommatidium. Ocelli large and prominent, elliptical.

Mesosoma. Propodeum rounded, declivous face perpendicular, flat except with distinct to indistinct median longitudinal depression, basal face strongly convex transversely and longitudinally. Metapleuron broad, ~0.66 as wide as high. Wing venation as in Fig.

Metasoma. In cephalic view, dorsum of node transverse to having shallow median impression and weakly bilobate, sometimes lobes curve posteriorly. Petiolar spiracles distinctly tuberculate to not tuberculate. Postpetiolar spiracle distinctly tuberculate to greatly tuberculate, tubercles higher than width of base. Genitalia as in Figs.

Coloration, Sculpturing, and Pilosity. Pubescence short, yellow, erect to suberect and of uniform length over body (0.15–0.20 mm), longest on gena and vertex. Mesonotal pubescence dense. Propodeum with base striatogranulate, medially glabrous. Integument coarsely granulate in area between eye and insertion of antenna, area between ocelli, pronotum, posterior portion of metapleuron, and base of petiolar node. Gena granulate, sometimes weakly striato-granulate. Head otherwise granulate. Posterior surface of postpetiolar node with lower 0.50 of surface striato-granulate, remainder polished. Metapleuron sometimes with striae both dorsally and ventrally. Lateral faces of scutellum striate. Remaining integument smooth and polished. Color generally red brown to black. Antennae and legs yellow brown. Mandibles brown.

L ~6.1–6.5,HW1.0–1.1,VW0.4–0.5, HL 0.7–0.8, EL 0.4–0.5, OD 0.12–0.13, OOD 0.1–0.2, LOW 0.10–0.12, MOW 0.10–0.15, CD 0.17–0.23, MFC 0.10–0.15, EW 0.3–0.40, SL 0.15–0.20, SW 0.1–0.12, PDL 0.08–0.10, PEW 0.10–0.14, LF1 0.15–0.25, LF2 0.12–0.15, LF3 0.14–0.16, WF1 0.07–0.11, FL 1.1–1.2, FW 0.15–0.22, MW 1.39–1.52, DLM 2.40–2.62, PRH 0.89–1.12, PL 0.58–0.73, PND 0.51–0.60, PH 0.49–0.61, PPL 0.20–0.30, DPW 0.61–0.70, PPW 0.48–0.73, PHB 0.10–0.22, N=12.

Larva
Pitts et. al. (2018) - Fourth instar worker larva.—Head. Large, subpyriform in anterior view (height 0.52 mm, width 0.55 mm). Cranium slightly broader than long. Antenna with 2 or 3 sensilla, each bearing spinule. Integument of head with fine rugae. Occipital setal row with 6–8 simple setae, 0.08–0.14 mm, rarely some specimens with single bifid to denticulate seta, base ~0.8–0.95X total length of seta. First setal row on vertex with 2 simple setae, 0.09–0.11 mm long. Second setal row on vertex with 4 simple setae, inner 2 setae orientated farther from first setal row on vertex, appear out of alignment with row, 0.09–0.15 mm long. Setae ventral to antenna level simple, 0.11–0.18 mm long. Clypeus with transverse row of 4 setae, inner setae shorter than outer setae, 0.06–0.10 mm long. Labrum small, short (breadth 2.8X length), slightly narrowed medially. Labrum with 4 minute sensilla and 2 setae on dorsal surface of each half and apex with 5–7 sensilla on each half. Each half of epipharynx with 2–3 isolated sensilla. Straight medial portion of mandible with 2–5 teeth that decrease in size dorsally. Maxilla with apex conical, palpus peg-like with 5 sensilla, each bearing one spinule. Galea conical with 2 apical sensilla, each bearing one spinule. Labium with isolated patches of spinules dorsal to each palpus. Labial palpus slightly elevated with 5 sensilla, each bearing one spinule.

Body. Spiracles small, first spiracle larger than others. Body setae of 3 types. Simple setae, slightly curved (0.08–0.15 mm long), arranged in transverse row of 6–12 on ventral surface of each thoracic somite and on each of 3 anterior abdominal somites. Bifid setae (0.06–0.09 mm long) occur elsewhere, base ~0.5X length. Multibranched setae occur rarely in the area immediately posterior to head on thoracic dorsum.

Length. 2.7–3.1 mm.

Type Material
Pitts et. al. (2018) - Syntype workers, gynes, males. Argentina. Buenos Aires. Richter. .

Determination Clarifications
Pitts et. al. (2018) - Larval stage described by Wheeler and Wheeler (1977) was collected in Walker County, Alabama. At the time of collection, this area was likely part of the S. richteri x invicta hybrid zone (Vander Meer et al. 1985, Vander Meer and Lofgren 1988). These specimens probably represent the larvae of S. richteri x invicta, rather than pure S. richteri. The description differs from S. richteri larvae in several characters, the most conspicuous of which is the stated presence of bifid and denticulate setae on the head capsule. This is much more reminiscent of S. invicta. True S. richteri larvae lack bifid setae on the head capsule.