WASHINGTON, Sept. 24 (Xinhua) -- Neurobiologists at the University of Massachusetts Medical School have now found a key mechanism that helps steer monarch butterflies to their ultimate destination during migration.

    This mechanism does not reside in the insects' brains, as previously thought, but in their antennae, a startling discovery that provides an entirely new insight into the organ's role in migration.

    The annual migration of monarch butterflies from across the eastern part of North America to a specific grove of fir trees in Mexico has long baffled scientists. They have tried to understand just how these delicate creatures can navigate up to 2,000 miles to a single location.

    "We've known that the insect antenna is a remarkable organ, responsible for sensing not only olfactory cues, but wind direction and even sound vibration," said Steven M. Reppert, professor and chair of neurobiology at the University of Massachusetts and senior author of the study.

    "But its role in precise orientation over the course of butterfly migration is an intriguing new discovery, one that may spark a new line of investigation into neural connections between the antennae and the sun compass, and navigation mechanisms in other insects," Reppert continued.

    In a paper to be published Friday in the journal Science, Reppert and his colleagues Christine Merlin and Robert J. Gegear demonstrate that the butterflies' antennae, formerly believed to be primarily odor detectors, were actually necessary for sun-related orientation, a critical function commonly thought to be lodged solely in the insect's brain.

    "Previous studies have shown that butterflies use their circadian clock, an internal timing device such as the one that controls our own sleep-wake cycles, to correct their flight orientation and maintain a southerly course even as the sun moves across the sky," Reppert said.

    The time correction factor of the sun compass mechanism was assumed to reside in the brain, where the sun compass itself is located, although this alleged role of brain clocks had never been tested directly.

    Reppert recalled an observation made 50 years ago, even before the discovery that millions of monarch butterflies fly to specific wintering grounds in Mexico: Migrating butterflies lost their sense of orientation when their antennae were removed. Now, Reppert and his colleagues have sought to unravel the migration mystery.

    In their studies, scientists removed the antennae of a number of butterflies and tested their ability to fly south in an outdoor flight simulator rigged to calculate the insects' flight direction.

    They found that the migratory butterflies without antennae could not orient themselves to the proper southerly direction, while butterflies with intact antennae could. Scientists also showed that the molecular cycles of the brain clocks were not altered by removing the antennae, and that the antennae actually contained circadian clocks that function independently of those in the brain.

    The scientists next covered the antennae with black paint, effectively blocking light sensing by the antennal clocks. Those butterflies homed in on an incorrectly fixed direction: The insect's brain could sense light but could not adjust the timing of the sun's movement across the sky in order to steer toward the proper destination.

    However, when the team used clear paint -- which did not alter antennal light input -- the butterflies accurately established the southerly flight orientation, indicating that the antenna's reading of light is key to navigation.