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.

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

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.

Description
Buren (1972):

Worker
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.

Queen
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.

Male
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.