We focus here on "Open Collective Robotics", a research domain interested in studying concepts and techniques which could lead to true collectivities between humans and robots. We also present the MICRobES project, a long-term experiment intended to investigate how to design such collectivities. Therefore, the MICRobES robots have no prior specific functionality, they have to develop individual competences, a social organization, and sociability towards humans in order to "survive" in a dynamic environment.