Ants in the genus Solenopsis are distinctive and relatively common. The number of larger sized species in the genus (the fire ants) is much smaller than the minute typically subterranean species that make up what are called thief ants. The latter species were recently revised by Pacheco and Mackay (2013). The descriptions and summaries that follow are from their work.
The thief ants group of the genus Solenopsis is virtually cosmopolitan throughout the New World. The genus ranges from southern Canada to Argentina. The majority of the species can be found in tropical rain forests, especially in areas of thick leaf litter. Most colonies of the smaller species are found in the soil without an entrance hole, unless nuptial flights are occurring. Occasionally nests are discovered under stones, but are often found while one is excavating the nest of another ant species (Mackay and Mackay 2002). They are considered thief ants because they pilfer the nests of other ants, where they steal brood or food. Some species are considered pests, such as Solenopsis molesta, perhaps the most abundant of the "smaller species." It can be found in parks and residential areas and occasionally is a house pest. Many species can be collected with subterranean baited traps set at least 10 cm down in the soil column. The ecology of most species is virtually unknown.
Worker Total length about 1-5 mm (small to medium sized); monomorphic, dimorphic to polymorphic; allometry appears monophasic, marked polymorphism occurs where size range is large, where head increases allometrically, with changes in size and form of mandibles and with variation in the number of ommatidia; eyes ranging from 1 to over 60 ommatidia; antennae 10 segmented with 2-segmented club, ring segments variable in length, segments generally longer than broad in larger species and broader than long in smaller species (inconsistent); palpal formula usually 1,2; clypeus with median area sharply elevated above lateral areas and bounded by pair of slightly to strongly diverging clypeal carinae, usually well defined but weakly present in some species; carinae usually terminating in pair of weak to well-developed lateral teeth, often flanked by pair of extralateral teeth; a weak to well-developed median clypeal tooth present between lateral teeth; median area of clypeus with medial seta (always present and on medial tooth when present); promesonotal suture on pleurae entire only to height of spiracle in smaller species and minor workers of larger species, but may be complete in major workers of larger species; metanotal groove deeply impressed on dorsum; propodeum generally rounded, less commonly sharply angulate; declivity flat to slightly concave; spiracle generally round, ovoid or D-shaped in major workers of few large species, directed posteriorly; metapleural glands well developed; petiole always distinctly pedunculate, node high and rounded in small species and smaller workers of larger species; petiole and postpetiole nearly equal in size when viewed in profile in most cases; ventral surface of petiole below node distinctly to greatly swollen viewed in profile; subpeduncular process variable, with flange or tooth in many species; postpetiole not usually broadly attached to gaster; subpostpetiolar processes negligible to moderately well developed, with tooth present on few species; mostly pale yellow to reddish brown ants, few species dark brown to black, occasionally bicolored; exoskeleton not markedly sculptured, most species predominately smooth and shiny, with few species striated or with punctate or roughened sculpturing on head, propodeum, petiole or postpetiole; usually bearing sparse, long setae, often with appressed hair on scape and tibiae.
Queen Larger to much larger than worker, but with same general features; antennae 10 to 11 segmented, sometimes with both counts on same individual, with distinct 2-segmented club; palpi as in worker; clypeal carinae diverging anteriorly; propodeum generally rounded, occasionally angulate with ridges; petiole heavier, node broader, less markedly pedunculate than in worker, subpeduncular process negligible or small, with tooth or thin flange in some species; postpetiole more broadly attached to gaster; most surfaces smooth and shining, propodeum often striated with petiole and postpetiole occasionally sculptured.
Male Larger to much larger than worker (but nearly always smaller than gyne); antennae 12-13 segmented; scape short, barrel-shaped, about twice diameter of remaining funicular segments; clypeus rounded and swollen. Occasionally straight or slightly convex; clypeal setae not in clear patterns; mandibles weakly developed, with only 1 or 2 teeth, capable of being closed; palpi as in worker; petiole and postpetiole as in gyne, postpetiole more broadly attached to gaster; genitalia strongly retracted.
New World Species Complexes
Species complexes are used here for convenience. These complexes are morphologically based and constructed for convenience in identification of species. They do not necessarily reflect or imply monophyly within any particular group(s) of species.
There are keys, in English and in Spanish, to the species complexes and for the species within each complex.
- Pacheco, J.A. & Mackay, W.P. 2013. The systematics and biology of the New World thief ants of the genus Solenopsis (Hymenoptera: Formicidae). Edwin Mellen Press, Lewiston, New York. 501 pp. PDF