The "dark rover ants" (or simply "rover ants"), Brachymyrmex patagonicus, are tiny ants originally found in South America, interestingly from the same region as some of the other successful invaders: fire and Argentine ants. Dark rover ants were discovered in the United States in 1978, and have spread throughout the South and into Arizona. Not a lot of work has been published on their biology. Joe MacGown has articles about their taxonomy and distribution, and explains that dark rover ants were called B. musculus in the early literature. Alex Wild has a nice summary of their status and an identification guide.
Our back yard in Phoenix is home to numerous dark rover ants. They nest in the potted plants, and have kept all other ants - except the native fire ants - pretty much at bay.
I can not emphasize enough how very small they are. They are just about 2 mm in length. Rover ant is a good name, because you find single or a few ants roving everywhere. They trail up trees, but otherwise don't seem to form distinct trails.
In fact, I found their behavior to be rather ho hum until yesterday. That's when we discovered what looked like a meet and greet between two colonies on our patio. (The photos aren't very good. It was cloudy and of course my flash wouldn't cooperate).
It's hard to show the action in stills, but basically some ants were lined up having encounters with others. The ants would rapidly jerk back and forth when they met, possibly stridulating? (You would need an electron microscope to tell for sure, they are so small. :-)) The movement almost looked like they were getting a shock and jumping back.
Some of the encounters showed definite posturing.
Some of the ants "crouched" with their gasters held up.
Some other ants were moving rapidly, often circling around in a half circle and then rejoining the line.
Here's a couple I caught with their gasters up.
In this case the ant in front of the one with its gaster up seems to have lowered itself.
Running through this gauntlet of ants was an occasional worker carrying a pupa. See toward the right.
Unfortunately other duties called, so I couldn't follow the action very long. Watching what little I did made me think that these mini ants might just be worth investigating a bit more.