The first thing you notice about workers of the Southern fire ant, Solenopsis xyloni, is what fierce foragers they are.
You never see just one foraging worker. Instead, there's almost always a teeming mass.
Even when they are collecting sweets at extrafloral nectaries, Southern fire ants show up in greater numbers than most other species.
Southern fire ants are thought to be originally from throughout the southern and western United States. They have been displaced in many areas by the imported fire ant, Solenopsis invicta, but still occur widely in the dry areas of Arizona and California.
Solenopsis ants are relatively easy to tell from other ants because of the antennae have ten segments with a two-segmented swollen area or "club" at the end.
Solenopsis xyloni workers vary considerably in size and color, even within colonies. The larger workers tend to have lighter-colored heads and trunks than the smaller workers. In the area around Phoenix, Arizona, the Southern fire ants seem darker than those found elsewhere.
(These workers are feeding on their favorite meal of dried cat food.)
Where Solenopsis xyloni and S. invicta overlap it is difficult to distinguish the two species. Jacobson et. al. (2006) have developed a pcr technique and guidelines for identification (see references).
To make things even more confusing, it seems that Solenopsis xyloni hybridizes with Solenopsis geminata where the two overlap. Obviously this group is "interesting" from a taxonomic standpoint.
The foraging workers are often seen carrying bits of hard food or arthropod parts back to the nest. They also gather some seeds.
Wet food, like this watermelon, go straight to the crop.
Often the foraging trails around their nests are underground or partially covered, so you might not notice them until you dig into the soil or pull up a weed. Then they come boiling up seemingly out of nowhere.
When foragers cross a man-made structure, such as a walking trail, sidewalk, or tile floor, they form a dense foraging trail of numerous workers traveling in both directions.
You have to admire the ability of Southern fire ants to find, process and transport food very rapidly. Plus they seem to eat just about anything they encounter. It is no wonder the colonies can grow to a relatively large size.
Do you have fire ants where you live? Have you ever watched them gather food?
The Navajo Ant Project has a brief review of taxonomy
Ant Web shows some of the color variation within the species
Jacobson AL, Thompson DC, Murray L, Hanson SF. (2006). Establishing guidelines to improve identification of fire ants Solenopsis xyloni and Solenopsis invicta. J Econ Entomol. 99(2): 313-22.
Trager, J. C. (1991). A revision of the fire ants, Solenopsis geminata group (Hymenoptera: Formicidae, Myrmicinae). Journal of the New York Entomological Society. 99 :141-198.