Except, of course, individual ants have no idea how many of their colony-mates are holding fast over a gap. And this is where the second rule kicks in. As individual ants run the “bridging” algorithm, they have a sensitivity to being stampeded. When traffic over their backs is above a certain level, they hold in place, but when it dips below some threshold — perhaps because too many other ants are now occupied in bridge-building themselves — the ant unfreezes and rejoins the march.
This is just awesome. Every CS student should have to study a bit of biology and physics.