Ants use trail pheromones to recruit additional foragers to a food source. When a foraging ant discovers food, she lays a pheromone trail as she returns to the nest with the food. Other foragers can follow her trail to locate the food.
If another forager first encounters the trail in the middle, how does she know which direction is the nest and which direction is the food? Direction can be deduced from other cues. Ants can navigate by landmarks and skylines. The direction that leads away from the skyline over the nest is most likely the correct location of the food. This can be confirmed by encounters with other foragers. Foragers traveling the trail carrying food burdens are heading in the direction of the nest. Unburdened foragers are most likely going toward the food.