Editor's introduction: stigmergy

The concept of stigmergy was introduced by French entomologist Pierre-Paul Grasse in the 1950s to describe the indirect communications taking place among individuals in a social insect colony. Stigmergy was originally defined by Grasse in his studies of the reconstruction of termite nests. Grasse showed that the regulation and coordination of the building activity do not depend on the workers themselves but are mainly achieved by the nest: A stimulating configuration triggers the response of a termite worker, transforming the configuration into another configuration that may trigger in turn another, possibly different, action performed by the same termite or any other worker in the colony. Although Grasse’s concept of stigmergy was attractive and stimulating, it has been overlooked by students of social insects because it left open the important operational issue of how stimuli should be organized in time and space to allow coordination. Despite the vagueness of Grasse’s formulation, stigmergy is a profound concept, the consequences of which are yet to be fully understood. Not only is stigmergy of potential importance for our understanding of the evolution and maintenance of sociality in animals, from communally breeding species to highly eusocial insects, but it may also turn out to be a crucial concept in other fields, such as artificial intelligence or the social, political, and economic sciences, to which its relevance is intuitively obvious. A special issue of Artificial Life on stigmergy makes a lot of sense because of the broad applicability of the concept to the biological realm and beyond. And indeed you will find in this special issue a unique blend of papers dealing with the history of stigmergy, stigmergy in social wasps, stigmergy in combinatorial optimization, stigmergy in communications networks, and stigmergy in robotics. Hopefully this special issue will generate enough interest to stimulate further work in other fields. The article by Guy Theraulaz and myself provides an historical survey of stigmergy. It shows that stigmergy, at the time it was introduced, offered a new framework to solve the “coordination paradox” in social insects—namely that the colony as a whole looks organized while each individual seems to be behaving erratically. But this paradox cannot be solved by stigmergy alone: In effect, stigmergy cannot, by itself, explain coordination. Two mechanisms that complement stigmergy are introduced: self-organization and self-assembly. Istvan Karsai’s article reviews how stigmergy has been applied to understanding nest building in social wasps. It also explains how Polistes wasps “use” stigmergy to build their nests. He shows, through the use of experiments and modeling, that these wasps respond to local configurations of cells. Such configurations are organized in space and time to ensure the emergence of a coherent adaptive structure. After the first two articles, clearly biologically oriented, the last two articles, one on optimization and the other on distributed robotics, show how powerful a concept stigmergy can be when applied to the design of artificial problem-solving systems. The article by Marco Dorigo, Gianni Di Caro, and Luca Gambardella is a survey of a set of recent and promising optimization techniques inspired by the stigmergic (or stigmergetic: There is no consensus on what word should be used!) foraging behavior of ants. Pheromone trails laid on the ground allow some species of ants to optimize