Why do ants walk in lines?

When a foraging ant finds food, it returns to the nest dragging its abdomen to leave a chemical trail of pheromones. Other ants detect the trail with their antennae and follow it, adding their own scent when they also find food. Shorter routes get travelled more quickly, so their scent builds up faster, while longer routes fade as the pheromone evaporates. This feedback loop lets a colony find efficient paths without any leader. A famous 1990 experiment with Argentine ants on a double bridge showed colonies tend to choose the shorter route.