Aggregation is one of the most basic social phenomena, and many activities of social insects are linked to it. For instance, the selection of a valuable site and the spatial organization of the population are very often by-products of amplifications based on the local density of nestmates. The patterns of aggregation are very diverse, ranging from the gathering of all animals in a unique site to their splitting between several ones. One might question how these multiple patterns emerge. Do ants actively initiate the formation of such patterns by modulating the emission of an attracting signal such as the trail pheromone? Alternatively, do patterns result from quantitative changes in the duration of interaction between animals once they have reached the gathering site, without any active modulation of the communications? To discuss these questions, we present two empirical studies: the gregarious behavior of cockroaches (Blatella) and self-assembly in the weaver ant (Oecophylla). Through experimental and theoretical studies, we show how a single behavior-the resting time-leads to a collective choice in both species. This behavior is a response to the density of conspecifics and can also be modulated by heterogeneities in the environment. In weaver ants, it allows the colony to focus the formation of chains in a given area among several potential sites. In cockroaches, it allows the gathering of individuals in particular shelters, depending on the proximity between strains. These results are discussed with emphasis on the role of aggregation processes in the emergence of cooperativity and task allocation.
The formation of droplets of ants Linepithema humile (Mayr) is observed under certain experimental conditions: a fluctuating aggregate forms at the end of a rod and a droplet containing up to 40 ants eventually falls down. When the flux of incoming ants is sufficient, this process can continue for several hours, leading to the formation and fall of tens of droplets. Previous work indicates that the time series of drop-to-drop intervals may result from a nonlinear low-dimensional dynamics, and the interdrop increments exhibit long-range anticorrelations. A model of aggregation and droplet formation, based on experimental observations, is introduced and shown to reproduce these properties.
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