INTRODUCTION Some 12,000 ant species are known by now, with colony sizes ranging from a few individuals to 20,000,000 individuals. What constraints does this vast range of colony sizes place on the systems of organisation that they use? Alternatively, how does this range of colony sizes reflect the different systems of organisation used? We shall examine these questions in relation to ant foraging strategy, which as well as being the most visible aspect of their activity illustrates most

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... the roles and limits of communication in their collective behavior. This paper aims to verify a prediction of the following hypothesis (Pasteels et al. 1985; Deneubourg et al. 1986 ). In theory, the organization of a small insect society can rely on most individuals at any moment "knowing", principally by learning, what it must do, where it must go, etc., and the workers' behavior has a strong determinist component. In a large insect society organization by individual learning is harder to achieve ). The workers' behavior is necessarily more random and their coordination becomes a major problem. To cope with this, a completely different organisational system is added to that already in place. This supplementary system is based on the complex collective structures, patterns and decisions that spontaneously emerge from simple autocatalytic interactions between numerous individuals and with the environment, mediated by essentially chemical communication (see, e.g., Goss and Deneubourg 1989; Beckers et al. in press; Deneubourg et al, 1989, in press; Goss et al. 1990 ). The prediction that follows from this hypothesis is that the larger the colony size, the less foraging is individually based and the more *Manuscript received by the editor April 5, 1989. 239 240 Psyche [Vol. 96 the individual foragers are coordinated by mass chemical communication. We shall use the following categories of foraging strategy that, as shall be discussed below, represent a crescendo of the integration of the individual forager into a network of communication: individual, recruitment, trunk trail and group hunting, their definitions being inspired by the work of different authors (e.g. Rosengren