Abstract. Current multi-agent simulations, which have many individual entities evolve and interact, often lead to the emergence of local groups of entities, but provide no means of manipulating them. To our mind, giving full a sense to multi-agent simulations would consist though in making use of such dynamically created potential groups, by granting them an existence of their own, and specific behaviours. Brought into operation, they would provide effective and new tools for modelling purposes : for instance, encapsulating physical laws which depend on scaling, thus giving means of apprehending micro-macro links in multi-agent simulations, or introducing the experimentater's viewpoints on the specific behaviours of such groups. We thus have to imagine how to give any set of agents means of becoming aware of their mutual interaction, and giving birth to new types of agents out of their collective activity. In other words we look for a computer equivalent to our own emergence recognition ability. We present here a conceptual reflexion on such matters in the light of our own experience in the development of the RIVAGE project at Orstom, which aims at simulating runoff and infiltration processes. Conversely, we believe that the development of our methods in such a novel and original field of research as the multi-agent simulation of pure physical processes will provide new ideas and tools useful for many multiagent architectures and modelling purposes.

Social organization is one of the fundamental aspects of animal behavior, and has received attention both from experimental and theoretical perspectives. Examples of social groups appear at every size scale from the microscopic aggregates of mammalian cells (such as fibroblasts) to macroscopic herds of wildbeast, flocks of birds, and fish schools. There are two general frameworks when modeling such problems: the Lagrangian viewpoint and the Eulerian one. In this paper, we use both the approaches in the study of fish alignment. An individual-based model (IBM) (Lagrangian) provides a virtual world where fish forming a fish school try to adopt a common angular position. Fish are assumed to lie in horizontal planes, an individual angular position is the angle made by the oriented axis associated with the individual (tail to head) with a fixed direction. Two main forces are acting, a force of alignment, whose strength is assumed to be fixed in a given experiment but may be modified, and a force of dispersion, accounting for all disturbances. A transition from dispersion-dominant to alignment-dominant can be observed in the IBM experiments. A related PDE model (Eulerian) is used to determine the transition with sufficient accuracy.

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The larval parasites of the pantropical lymphatic filariasis exhibit two types of circadian behaviour. Typically, they only appear in the human bloodstream at nighttime, synchronised with their mosquito vectors. In Polynesia and parts of Southeast Asia, free of nocturnal vectors, they are found at all hours, and each population biorhythm differs. Through a geometrical approach, we explain this circadian diversity by a single, dominant mutation: the clocks of individual parasites are set at midnight (ubiquitous) or at 2 p.m. Compared to other circadian genes, this mutation must be very old, as it is shared by four biologically remote genera of parasites. This seniority sheds new light on several theoretical and practical aspects of vector-parasite temporal relations. To cite this article: G.

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