Stigmergy

Observing swarms in nature, we will soon notice, that the single individuals don't communicate via messages. But showing some collective behavior, the members must exchange information in some way. This is done by a phenomena called stigmergy, which means communicating by changing the environment. 'The concept of stigmergy was introduced by Pierre-Paul? Grasse in the 1950's to describe the indirect communication taking place among individuals in social insect societies. Ants and termites coordinate the building up of their nests by using the nests themselves to exchange information. For finding shortest paths at foraging or other tasks, ants don't tell every single of their mates where to go, but change the environment by placing some pheromone on the way they chose. Following the most odorous path and leaving their pheromone on this way again, the other ants cooperate and after a short period, the shortest way stabilizes. It's not easy to use this method in artificial swarms. Creating artificial pheromone is nearly impossible at the moment and can't be considered as a serious way to create swarm behavior. Observing the behavior of the other members and acting after interpreting their actions or the changes appearing in the nearest environment is possible and can be a way to work with stigmergy.

Text initially taken from swarmrobot.org