ants ( Atta and Acromyrmex ) feed exclusively on a fungus that grows only within their colonies. They continually collect leaves which are taken to the colony, cut into tiny pieces and placed in fungal gardens. Workers specialise in tasks according to their sizes. The largest ants cut stalks, smaller workers chew the leaves and the smallest tend the fungus. Leafcutter ants are sensitive enough to recognise the reaction of the fungus to different plant material, apparently detecting chemical signals from the fungus. If a particular type of leaf is toxic to the fungus the colony will no longer collect it. The ants feed on structures produced by the fungi called gongylidia . Symbiotic bacteria on the exterior surface of the ants produce antibiotics that kill bacteria that may harm the fungi. Navigation Foraging ants travel distances of up to 200 metres (660 ft) from their nest and usually find their way back using scent trails. Some ants forage at night. Day foraging ants in hot and arid regions face death by desiccation, so the ability to find the shortest route back to the nest reduces that risk. Diurnal desert ants ( Cataglyphis fortis ) use visual landmarks in combination with other cues to navigate. In the absence of visual landmarks, the closely related Sahara desert ant ( Cataglyphis bicolor ) navigates by keeping track of direction as well as distance travelled, like an internal pedometer that counts how many steps they take in each direction. They integrate this information