Social insects commonly use trail pheromones. For example, ants mark their paths with pheromones consisting of volatile hydrocarbons. Certain ants lay down an initial trail of pheromones as they return to the nest with food. This trail attracts other ants and serves as a guide. As long as the food source remains vailable, visiting ants will continuously renew the pheromone trail. The pheromone requies continuous renewal because it evaporates quickly. When the food supply begins to dwindle, the trail-making ceases. In at least one species of ant, trails that no longer lead to food are also marked with a repellent pheromone.