Why "Stigmergic"?

Stigmergic communication happens in insects through the alteration of their environment. Some specific modification of the environment triggers some behaviour. For instance, starting the building of a nest induces other insects to complete it. The simle sight of an initiated nest is a stimulus to further work.
Typically stigmergic behaviour is a form of collaborative communication between members of a community. In the case of SPAM, it is used against opportunistic behaviour: the simple presence of an email address triggers unconditionate mass mailing, but in this way it can be detected.
On stigmergy, see: Wilson, E.O. Sociobiology: The New Synthesis. (Harvard University Press: 1975). Wilson prefers to call it sematectonic communication.