While investigating the Dorymyrmex bicolor ants I posted about on Monday, I spotted a few other ants that looked similar, mostly because of their red-brown and black coloration.
On closer observation, however, these ants were obviously another species. First of all, they were almost double the size.
The long hairs on the underside of their heads gave it away that these were harvester ants in the genus Pogonomyrmex. The 'beard" of hairs, called a psammophore, is characteristic of the genus. In fact, the name Pogonomymrex means “bearded one.” Psammo comes from the word for sand in Greek, so the psammophore acts like a basket or an extra pair of hands to help the ants move sand, dirt, and probably some types of food as well.
These turned out to be a color variant of Pogonomyrmex californicus. Cole (1968) describes the species as concolorous light ferrugineous red in coastal California, moving to concolorous black or brown to the eastern part of its range in southern Texas.
Ants of the Southwest has a good page about these harvester ants that shows some of the the color variations.
It seems that Dorymyrmex bicolor and Pogonomyrmex californicus are often found in the same open, arid environments. It's interesting that at this Maricopa, AZ site their coloration is so similar.