Ants are social insects. They live in big colonies and have a very organised social life. Red ants make their nests inside the walls of our houses. They generally travel large distances in search for food. In absence of any maps or navigation aids, they have to rely on other mechanisms to find their way around.
As they walk the ants leave behind a scent that they use to trace their way to a food source or back to their home. The scent that the ants leave behind is called pheromone. The ants walk in a line because they follow the scent left behind by the leader. They are following a trail laid out by another ant that will take them to food, water or their home. As more ants travel the trail the scent gets stronger (easier to follow) and the random bumps are smoothed out making the trail straighter.
Once the food source is depleted,the returning ant will not leave any trail pheromone and hence the concentration of trail pheromone in that particular path gets faded. Thus the other worker ants stop following that path and look for a new source.