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Genus Solenopsis - Fire Ants and Thief Ants

Fire ant with mites - Leptus? - Solenopsis invicta Southern Fire Ants? - Solenopsis xyloni unknown ant - Solenopsis xyloni Texas thief ants at a meat scrap - Solenopsis texana Ant - Solenopsis Solenopsis xyloni? - Solenopsis xyloni Southern Fire Ant?  - Solenopsis Solenopsis carolinensis? - Solenopsis molesta - female
Kingdom Animalia (Animals)
Phylum Arthropoda (Arthropods)
Subphylum Hexapoda (Hexapods)
Class Insecta (Insects)
Order Hymenoptera (Ants, Bees, Wasps and Sawflies)
No Taxon (Aculeata - Ants, Bees and Stinging Wasps)
Superfamily Formicoidea (Ants)
Family Formicidae (Ants)
Subfamily Myrmicinae
Tribe Solenopsidini
Genus Solenopsis (Fire Ants and Thief Ants)
Other Common Names
Larger usually dark brown to red spp. with variable worker size and morphology are known as fire ants, while the minute, usually yellowish (some darker) species are known as thief ants.
Synonyms and other taxonomic changes
Old subgenus (Diplorhoptrum) for thief ants is believed to be polyphyletic, and so is in synonymy, for now. Do not use subgenus names for this genus, please.
23 species listed at Nomina Insecta Nearctica.
Solenopsis can be identified using a microscope to verify these characteristics:
-- The pedicel (the waist between apparent thorax and abdomen -- technically, between mesosoma and gaster) is two-segmented.
-- Each antenna has ten segments with a two-segmented club.
-- The body surface is shining, with little evident sculpture, except some species (e.g., pergandei, tennesseensis) have conspicuous circular, punctate impressions at the base of the pilosity.
-- They lack propodeal spines or teeth, and usually are without any propodeal sculpture, though geminata has weak laterodorsal, longitudinal ridges.
Fire ants - "Sunbelt" states
Thief ants - Most speciose in Florida, but one to a few species occurring in all 48 states and southernmost Canada.
Most soil-nesting, but picta is twig-nesting, arboreal
Generalists, some thief ants nest close to or within interstices of nests of larger ants and forage in the chambers of the larger ants, reputedly eating some of their brood (but this is not well-documented).
Two fire ant species - invicta, richteri - introduced from South America, and now abundant in the Southeast, and sporadically in the Southwest (where irrigated). These two have hybridized extensively, and indeed, richteri may be heading for extinction in North America due to genetic swamping by invicta. Fire ants have a very painful sting, sometimes damage outdoor electrical equipment and certain crops and livestock, and are thus the target of extensive control programs and research. For Fire Ant bite remedies see this page.

There are also four native fire ant species, and an undetermined number of thief ant species, the latter in great need of taxonomic revision including into Mexico and beyond.
Internet References
Ant Web Arizona Solenopsis species collectively cover most known North American taxa.