Army ant bridge-buliding algorithm

Army ants function without a leader and yet accomplish very organized-looking things, such as building bridges across gaps:

Researchers from the Swarm Lab believe they can break down the bridge-building process into a simple, two-rule system. Rule 1: If fellow ants are walking over you, stay put. Rule 2: If the number of ants walking over you isn’t higher than some rate, get moving again.

Full paper here (pdf).