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Thursday, 20 January, 2000, 15:07 GMT
Ears found on butterflies

Ear A close-up view of the eardrum (centre)

Some butterflies that fly at night have ears and they need them to escape the jaws of bats, Canadian scientists report.

The ears are located on the forewings of the insects and are sensitive to ultrasound which bats use to navigate and locate their prey. The butterflies hear the bats coming and dart out of the way.

Such hearing is common in moths, known for their night-time flying, but until now it has never been demonstrated in butterflies.

And the discovery may have important implications for our understanding of butterfly evolution. Fossil records show that butterflies, which evolved from moths, became daytime flyers at roughly the same time bats developed large ears and the ability to use echoes to locate their prey.

It is possible that more than 50 million years ago butterflies took to daytime flight in order to survive the predations of bats, Dr Jayne Yack and Professor James Fullard say in the journal Nature.

"The butterfly, in effect, was ... 'invented' by the bat," they argue.

Rabbit ears

The researchers looked at one particular species in the family called Macrosoma heliconiaria which can be found on Barro Colorado Island, Panama.

The butterfly ears have the look of tiny rabbit ears, the team says. They have a very thin eardrum, stretched over an air-filled chamber.

The eardrums vibrate when there is a burst of ultrasonic sound. These vibrations are detected by nerve cells packed into three little organs on the inside of the chambers.

"It's quite an unusual ear for insects," Professor Fullard told BBC News Online. "It resembles the moth ear, but it is certainly more complicated."

The team witnessed how the butterflies escaped the clutches of bats by exposing the insects to intense bursts of ultrasonic noise.

When stimulated in this way, the insects accelerated and went into steep dives or climbs, upward or downward loops, spirals and horizontal sweeps.

Evolutionary pressure

Yack, of Carleton University, Ottawa, and Professor Fullard, from the University of Toronto, Ontario, say bats are very numerous and important night-time predators and must have exhibited intense evolutionary pressure on other creatures moving about at the same time.

They suggest that the hedylidae might even be the ancestors to all modern-day butterflies.

Professor Fullard says: "What we have here are a group of butterflies that moved into the night, or may be they are the remnants of the original insects - may be they had ears and the butterflies that didn't were pushed into the daytime by bats.

"I certainly think it is safe to say that bats played a role in the daytime preference of butterflies."

Ears that act as bat detectors are also found in lacewings, preying mantises and are suspected in some beetles.

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