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Leafcutter ants tending their fungus garden (photo by A. Wild)
The 'no entry' signal was identified by a simple but ingenious manipulation of ant trails. Elva Robinson a student in the social insect laboratory explains, 'I took material from the unrewarding branch of an ant colony's trail, in other words the branch that did not lead to food. This material was applied to a branched trail of a different colony where both branches led to food to see if it would repel the ants, which it did'. Elva has a hypothesis regarding the need for these 'no entry' signals. 'Because it is volatile the ants can detect the signal before getting to the junction and get ready to make the right turn, it speeds up the journey'. Currently the team is doing experiments to determine how the different pheromones work together to help the colony function better, such as making it easier to choose the correct branch where two trails divide.
Pharaoh's ants also use geometry to give their trails direction. Trails from the nests to food have repeating Y-shaped branches that fan out away from the nests with an angle of approximately 60° between branches. An experiment carried out by another student in the social insect laboratory, Duncan Jackson, found that if an ant laden with food was walking away from the nest they were unable to change route if the branching angles were 120° but they could if the angle was less. 'They make U-turns that redirect them towards the nest if they are carrying food and away from the nest if they have yet to load up', explains Duncan. 'At 120º the trail is completely symmetrical. It's impossible to know which way is which'.
Tropical (or Latin) American leafcutter ants are also under investigation at Sheffield. These ants handle waste hygienically to prevent contamination of the nest. A worker taking waste out of the nest does not enter the waste dump; instead they leave the waste in a connecting tunnel for ants that live in the dump to collect. The use of different chambers, different groups of ants living in the chambers, and the passing of waste from one ant to another, effectively prevent the nest being contaminated with waste.
'There is much we can learn from the simple solutions ants have found for complex problems', says Francis.
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