Self-organisation, as manifest, for example, by swarms, flocks, herds and other collectives, is a powerful natural force, capable of generating large and sustained structures. Yet the individuals who participate in these social groups may not even be aware of the structures that they are creating. Almost certainly, these structures emerge through the application of simple, local interactions. Improvised music is an uncertain activity, characterised by a lack of top-down organisation and busy, local activity between improvisers. Emerging structures may only be perceivable at a (temporal) distance. The development of higher-level musical structure arises from interactions at lower levels, and we propose here that the self-organisation of social animals provides a very suggestive analogy. This paper builds a model of interactivity based on stigmergy, the process by which social insects communicate indirectly by environment modification. The improvisational element of our model arises from the dynamics of a particle swarm. A process called interpretation extracts musical parameters from the aural environment, and uses these parameters to place attractors in the environment of the swarm, after which stigmergy can take place. The particle positions are reinterpreted as parameterised audio events. This paper describes this model and two applications, Swarm Music and Swarm Granulator.