Solenopsis molesta: they are tiny, they are secretive, they are thief ants.
Barely over a millimeter long, you really need a microscope to photograph them. This worker thief ant is running on a piece of cake frosting.
Now that's better. (Photograph by © AntWeb.org / CC-BY-SA-3.0 retrieved from Wikipedia.)
The workers are often golden yellow-brown in color and may be confused with pharoah's ants at first glance.
The characteristics of the thief ant worker that you notice under a microscope are the small size of the eyes and the unusual shape of the antennae. Solenopsis (Diplorhoptrum) molesta workers have antennae that are 10 segments long with a relatively large 2-segmented club at the tip. The segments between the scape and the club, the funiculus, are wider than they are long and show darker bands of color, making them distinct.
Some taxonomists have separated the thief ants into a separate genus or subgenus, Diplorhoptrum. Renthal, et al. have found a ring of antennal glands in the thief ants that are also found in other Solenopsis, which they say might be further evidence thief ants are indeed members of the genus Solenopsis.
Thief ants are named for their habit of living near other ant colonies, with tunnels that actually infiltrate their neighbors. The thief ants use the tunnels to go steal brood. They are not looking for free labor like some brood raiders, but use the stolen brood for food. They also feed on seeds and carrion of various sorts.
Solenopsis molesta ants are native to North America. William Morton Wheeler listed them amongst the first native ants to be found in homes.
Vinson and Rao discovered that plots with high levels of Solenopsis molesta colonies prevented new Solenopsis invicta colonies from becoming established. Solenopsis molesta is one of the few ant species that can coexist with Argentine ants, as well. They may be small, but evidently these little gals are tough!
Do you know of anyone working on thief ants? Do they occur where you live?
For more photographs, See:
Series of photographs of Solenopsis molesta by Benoit Guenard
Alex Wild also has captured the tiny thief ants. This one shows the antenna particularly well. Male thief ant with workers from Tucson, AZ.
Robert Renthal, Daniel Velasquez, David Olmos, and S. Bradleigh Vinson (2008), Occurrence of Antennal Glands in Ants. Microsc. Res. Tech. 71(11): 787–791.
S. B. Vinson and A. Rao. 2004. Inability of Incipient Solenopsis invicta (Hymenoptera: Formicidae) Colonies to Establish in a Plot with a High Density of Solenopsis (Diplorhoptrum) Colonies Environmental Entomology 33(6):1626-1631.