Fire ant is the common name for several species of ants in the genus Solenopsis. They are however only a minority in the genus, which includes over 200 species of Solenopsis worldwide. Solenopsis are stinging ants and most of their common names reflect this, for example, ginger ants and tropical fire ants. Many species also are called red ants because of their light brown colour, though species of ants in many other genera are similarly named for similar reasons.
The bodies of mature fire ants, like the bodies of all typical mature insects, are divided into three sections: the head, the thorax, and the abdomen, with three pairs of legs and a pair of antennae. Fire ants of those species invasive in the United States can be distinguished from other ants locally present, by their copper brown head and body with a darker abdomen. The worker ants are blackish to reddish, and their size varies from 2 to 6 mm (0.079 to 0.236 in). In an established nest these different sizes of ants all are present at the same time.
A typical fire ant colony produces large mounds in open areas, and feeds mostly on young plants and seeds. Fire ants often attack small animals and can kill them. Unlike many other ants, which bite and then spray acid on the wound, fire ants bite only to get a grip and then sting (from the abdomen) and inject a toxic alkaloid venom called Solenopsin, a compound from the class of piperidines. For humans, this is a painful sting, a sensation similar to what one feels when burned by fire (hence the name) and the after effects of the sting can be deadly to sensitive people. Fire ants are more aggressive than most native species and so have pushed many species away from their local habitat. These ants are renowned for their ability to survive extreme conditions. They do not hibernate, but can survive cold conditions, although this is costly to fire ant populations as observed during several winters in Tennessee, where 80 to 90% of colonies died due to several consecutive days of extremely low temperatures.
Fire ants nest in the soil, often near moist areas, such as river banks, pond shores, watered lawns, and highway shoulders. Usually, the nest will not be visible, as it will be built under objects such as timber, logs, rocks, or bricks. If there is no cover for nesting, dome-shaped mounds will be constructed, but these are usually only found in open spaces, such as fields, parks and lawns. These mounds can reach heights of 40 cm (16 in), but can be even higher on heavier soils, standing at 1.0m in height and 1.5m in diameter. Colonies are founded by small groups of queens or single queens. Even if only one queen survives, within a month or so, the colony can expand to thousands of individuals. Some colonies may be polygynous (having multiple queens per nest).