In mid May at about 6:00 in the evening in Ann Arbor, Michigan, USA, I was walking along the sidewalk and saw some small black ants swarming. I like to watch queens come out so I sat down to watch. I stayed about 45 minutes and I didn't see a single wing. What I did see was amazing. After a few minutes the ants started forming patterns. Two ants that were narrower than most would lock jaws. Then two normal ants would grab onto the heads of these two to form a plus sign or X. They would stay like that for 6-7 minutes then breakup. Sometimes 4 more of the normal ants would grab onto the abdomens of the 4 ants and form a plus sign of eight ants. In the time I was there there were always 15 to 30 of these symbols. I am not sure how many separate nests were participating. I left to get a camera and a collecting jar. When I came back the show was over. Is this common behavior? I have not seen it before.
That definitely sounds like an interesting behavior. If all of your ants were roughly the same size, dark brown/black, and on a sidewalk, chances are you're looking at Tetramorium caespitum, the so-called "pavement ant."
Most of the time when you see large groups of these ants congregating, they are fighting. These ants are often very aggressive to members of their own species, and neighboring colonies will often engage in territorial "wars." My guess is that the "plus signs" you observed were actually groups of ants biting on and attempting to subdue each other. Please see a fantastic photograph by Alex Wild of such a scene by clicking here. After such battles, it is not unusual to see wounded ants with the heads of their enemies still latched on to their legs or antennae.
I hope this helps!
Jesse Czekanski-Moir & the AntAsk Team