updated 5/4/2005 3:39:47 PM ET 2005-05-04T19:39:47

The monarch butterfly is known to use the angle of sunlight as a navigational guide on its annual fall migration from across North America to Mexico. But how it processes the information has been a mystery.

Now scientists have used a flight simulator and peeked inside the butterfly brain to learn that their light-detecting sensors are hard-wired to their circadian clocks, allowing the creatures to compensate for the time of day.

Monarchs are very adept at sensing ultraviolet light, a wavelength of sunlight that is invisible to the human eye and causes skin cancer. The butterflies can detect the sun's angle even on a cloudy day, allowing them to always head South. Scientists have known this for years.

Researchers led by Steven Reppert of University of Massachusetts Medical School found evidence last year that the butterflies rely on polarized ultraviolet light. Polarized light has been filtered to vibrate in one plane instead of all directions. In sunglasses, polarization reduced glare.

In the new work, Reppert and his colleagues discovered that ultraviolet photoreceptors dominate the part of the monarch eye that specializes in polarized light detection.

In flight tests, they blocked UV light and found the butterflies lost their way.

The new study also pinpointed the location of the butterfly's circadian clock, a processor in the brain that governs 24-hour activity and metabolic cycles of insects, humans and other animals. Key genes responsible for the clock's molecular "ticks" were expressed in a brain region called the dorsolateral protocerebrum. In there, tiny neural fibers connect the clock to polarization photoreceptors in the eye.

"This pathway has not been described in any other insect, and it may be a hallmark feature of butterflies that use a time-compensated sun compass," Reppert and his colleagues write in the journal Cell.

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