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Eureka! Meet the Antimedes

New Delhi, Feb. 20: No ant ever cried Eureka. But some ants appear to be applying the principle of Archimedes, the 3rd century BC Greek physicist-inventor, to protect themselves from floods using their own bodies to create raft-like structures, a new study has suggested.

Scientists in Switzerland, studying the behaviour of ants found in the Alps and the Pyrenees, have observed that the self-assembled rafts always have the most buoyant members of ant colonies at the base of the raft submerged in water.

Entomologists have for over 15 years documented the formation of rafts by some species of ants such as the red fire ant, native to South America. The rafts are among structures self-assembled by some species of insects using their own bodies to counter threats. Some species of honeybees surround and create an oven-like hotzone to kill predatory hornets, while ants, bees, and wasps form protective layers around vulnerable young larvae and pupae, or their queens.

Now, Jessica Purcell, an American entomologist at the University of Lausanne, and her colleagues have found that worker ants when threatened by rising water levels always placed the larvae and pupae — the younger stages of ants called the brood — at the base of the raft.

“This was a really big surprise,” Purcell, a post-doctoral researcher told The Telegraph. “We expected that the ants would place the most vulnerable and valuable members of the colony in the centre of the raft — they did place the queens at the centre, but not the brood,” she said.

The workers used their mandibles to bring together the members of the brood into a base and self-assembled into upper layers to create an igloo-shaped raft with the queen always in the centre of the raft, protected by workers on all sides. The study’s findings appeared yesterday in the journal PLOS One.

The researchers found that the members of the brood are more buoyant than workers and that they did not suffer any detectable decrease in survival because they were at the base of the raft. Both the brood members and workers could remain submerged in water for up to three hours and survive.

“Placing the brood at the base is a very efficient means of creating a highly buoyant raft, maximising the number of workers available to help the colony recover after the flood, and keeping the colony together with minimal loss of brood members,” Purcell said.

In using members of the brood to create a “floating platform”, the ants are exploiting buoyancy, the force that keeps objects afloat. Popular science history accounts suggest that Archimedes discovered the rule of buoyancy while taking a bath and ran out in joy on the streets naked, shouting Eureka.

“This is the first documentation of raft-formation and raft geometry in the laboratory,” said Himender Bharti, an assistant professor of zoology at Punjabi University, Patiala, who specialises in studying ants but was not associated with the research in Switzerland.

“But the raft-making behaviour is just another manifestation of attempts by ants to protect their future generations,” Bharti told this newspaper. “We also see this when we disturb ant colonies — the workers pick up larvae and pupae with their mandibles and take them to places that are perceived as safer zones.”

The Swiss team said the formation of rafts is a “progressive and co-ordinated process” that results in a collective structure with a well-defined geometry. “The ants take advantage of the properties of all members of their society when responding to an emergency situation,” Purcell said.