In Småland in southern Sweden the ground is full of stones, carried down during the ice age. When the people of Småland began to farm the land they picked up the stones in an area, lay them in piles at the side and planted their crops. As the years went by they ploughed the fields and more stones came up and the piles turned into walls. As the walls expanded they joined each other, separating the land into a patchwork of separate fields. If you go to Småland today, you will find well-ploughed fields surrounded by thick walls with occasional piles of stones near the middle of the fields. Although some of the walls built in Småland must have involved planning, such a plan was not a requirement for their construction.
This story illustrates a central idea of this chapter: co-ordinated construction can be achieved by a group of workers without direct communication among them. The patchwork of fields separated by stone walls emerged from local interactions between the farmers and their environment. In this chapter we look at a number of spatial structures that can arise through local interactions between animals and their environment. Pillars and chambers in the nest of social insect colonies; rail, road, and supply networks in humans; and the pheromone trail systems of ants are all constructed in this way. Often the complexity of these patterns gives an impression of centralized design or planning. However, positive feedback operating in space can produce a rich variety of patterns without central organization.
Theraulaz et al. (2003) propose that local interactions between animals and their environment, a mechanism they call stigmergy, combined with interactions with large scale environmental features, which they call tem-
plates, provide two of the key mechanisms for nest construction by insect societies. Examples of templates include light, temperature, and humidity