Collective behaviour operates without central control, through interactions among individuals. The collective behaviour of ant colonies is based on simple olfactory interactions. Ant species differ enormously in the algorithms that regulate collective behaviour, reflecting diversity in ecology. I will contrast two species in very different ecological situations. Harvester ant colonies in the desert, where water is scarce but conditions are stable, regulate foraging to conserve water. Response to positive feedback from olfactory interactions depends on the risk of water loss, mediated by dopamine neurophysiology. For arboreal turtle ants in the tropical forest, life is easy but unpredictable, and a highly modular system uses negative feedback to sustain activity. In all natural systems, from ant colonies to brains, collective behaviour evolves in relation with changing conditions. Similar dynamics in environmental conditions may lead to the evolution of similar processes to regulate collective behaviour.