The organisation of collective foraging in the harvester termite Hodotermes mossambicus (Isoptera)

Summary The process by which the foragers build up a foraging system between the nest hole and the food source, starting from an initial random distribution, was investigated under conditions of both light and dark in the laboratory.The extent to which trail pheromone is used can be determined by observing the abdominal position of workers moving outside the nest and can be classified into three categories, namely no trail-laying, marking and directional trail-laying.When workers of this species emerge from the nest hole into light emitted from a certain direction they orientate photo-menotactically. First, they mark the area around the nest hole, moving in small loops; they then make larger exploratory loops without trail-laying. Once they find a source of food they return directly to the immediate vicinity of the nest hole. As the area around the hole has already been marked it is easy for the termites to locate the hole. When a piece of food has been deposited by the hole the termites display directional trail-laying behaviour by leaving a trail in the direction of the source of food, this trail being stronger nearer the nest and decreasing as it approaches its goal. Inexperienced explorers are biased in this direction with a statistically higher frequency. If successful they join in the foraging and repeatedly contribute to the trail, which then gradually brings together most of the foraging population.In darkness exploratory loops are made with continuous marking, always returning to the nest hole, whereby a scent gradient decreasing away from the hole is formed. Once a source of food has been discovered the termites are neither able to return directly to the nest hole nor to return along their outward marking trail. They leave marking trails making exploratory loops from the food source with successively increasing radii. Another scent gradient, decreasing outwards from the food, is created. If the termite reaches the increasing scent around the nest hole from the decreasing scent around the food source it will locate the hole chemotactically. As other food finders repeatedly cross from one field to the other the scents will combine and become concentrated, thus forming a foraging path.