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Pseudo-eyes help ants find home faster

Ant nav The old saying 'two sets of eyes are better than one' is certainly true for a species of Australian desert ant.

Many insect species have two sets of eyes: large compound eyes that process visual information, and up to three smaller ocelli, or eyespots, usually located on the top of their head.

Flying insects, such as dragonflies, use their ocelli to help stabilise flight because they can detect changes in light intensity resulting from unplanned changes of direction or a sudden loss of altitude. But their function in terrestrial insects has been less well understood.

Now, researchers from Sydney's Macquarie University and Universite Paul Sabatier in France have found that information gathered by the ocelli provides a distinct compass mechanism. They found this mechanism, along with information gathered by the compound eyes, ensures ants always find the quickest route home.

Their research into (Melophorus bagoti) was published today in the Royal Society journal Biology Letters.

"Many ants use chemical trails to help them navigate, but the hot, dry desert would quickly evaporate any odours outside the nest," says lead author Dr Sebastian Schwarz of Macquarie University.

"Instead desert ants use landmarks such as trees and bushes and path integration, which keeps track of their distance and direction of travel and allows them to take the shortest route home."

It was assumed that the ocelli contributed to this path integration mechanism.

Two sets of eyes better than one

To test the role that the ocelli plays in ant navigation, Schwarz and colleagues enticed the ants to travel along a V-shaped two-leg journey to a feeding station and then return to their nest.

The control group of ants, with their compound eyes and ocelli uncovered, avoided having to retrace their steps by taking a short cut back to their nest.

But ants with either their eyes or ocelli covered had mixed results.

"Ants with their ocelli covered and compound eyes open were able to return to their nest with relatively little trouble by simply retracing their steps using compass cues from the sky," says Schwarz.

"But when their compound eyes were covered, they were only able to follow a fairly tortuous path back along the last leg of the journey. This suggests that the directional information from compound eyes is not available to the ocelli."

Schwarz believes the eyes and ocelli provide different, but complimentary information to assist the ant in navigation.

"The compound eyes provide directional information to the global path integrator, which keeps track of the nest position, whereas ocelli supply directional information to a distinct mechanism that buffers the most recent leg of travel."

The scientists speculate that because the ants inhabit a cluttered environment and frequently encounter aggressive ants from other colonies, they may be forced to leave their familiar route.

The ocelli-driven compass may allow them to make minor adjustments to their route and get back on their familiar foraging track, without having to abandon their journey and start again.