HOW TRAIL LAYING AND TRAIL FOLLOWING CAN SOLVE FORAGING PROBLEMS FOR ANT COLONIES S. Goss, R. Beckers, J.L Deneubourg, S. Aron, J.M. Pasteels Unit of Behavioural Ecology, CP 231 Université Libre de Bruxelles 1050 Bruxelles Belgium Introduction One of the most striking features of an ant colony's behaviour is its capacity for the spatial organisation of foraging activity. The use of trail pheromone to guide fellow workers in the nest to a large food source or rich foraging zone has been extensively studied (e.g. Wilson 1971) and obviousiy contributes to foraging efficiency. We have recently, however, been able to show that trail laying and trail following behaviour are more than just a means of communicating a food source's location. When more than one trail is présent at a time, the interactions between foragers and the trails can lead to the collective sélection of the shortest path or the best food source, despite the fact that individual foragers have no means of making such choices. Furthermore, the interactions between the foragers and the food sources can lead to some remarkable foraging patterns, giving the illusion that the foraging force is obeying a complicated global blueprint. We shall illustrate thèse ideas by simplifying the ants' behaviour to the extrême. We ignore the recruiting power of a trail, considering only its function as a directional guide for recruits. We consider the individual ants to have no memory (though they know where their nest is) and no sight, being capable only of responding differentially to the pheromone concentation at each antenna. While of course ants are not so simple, this approach lets us see the extent to which "blind" individual mechanisms can lead to "far-sighted" collective choices. NATO ASI Séries, Vol. G 20 Behavioural Mechanisms of Food Sélection nJilcd h\ R N Hughes ^ Springer-Verlag Berlin Heidelberg 19W