To see how this unfolds, take the perspective of an ant on the march. When it comes to a gap in its path, it slows down. The rest of the colony, still barreling along at 12 centimeters per second, comes trampling over its back. At this point, two simple rules kick in.

The first tells the ant that when it feels other ants walking on its back, it should freeze. “As long as someone walks over you, you stay put,” Garnier said.

This same process repeats in the other ants: They step over the first ant, but — uh-oh — the gap is still there, so the next ant in line slows, gets trampled and freezes in place. In this way, the ants build a bridge long enough to span whatever gap is in front of them. The trailing ants in the colony then walk over it.