WASHINGTON -- The monarch butterfly depends on the sun rather than the earth's natural magnetic field to help it find its way from Canada to Mexico every year, researchers said on Monday.

The study, published in **** The Proceedings of the National Academy of Sciences ****, found monarch butterflies would naturally fly southwest toward their winter home in Mexico under normal sunny conditions. But if the butterflies' exposure to sunlight were altered, they would travel in a different direction.

The findings challenge earlier research that suggests the black and orange butterfly uses the earth's magnetic field to help it travel 2,175 miles (3,500 km) from eastern Canada to its winter home in the transverse neovolcanic mountains of central Mexico.

"We think it's the beginning of a big scientific story that is going to evolve because now we have this nice method of studying in the lab their migratory behavior with great precision," Barrie Frost, researcher at Queen's University in Kingston, Ontario, said in a telephone interview.

"We see this as the first of many, many studies" to understand the migratory patterns of monarch butterflies and other insects such as moths and dragonflies, Frost said.

The migration of the monarch has long fascinated scientists, who did not discover until the mid-1970s many of the butterflies' final destinations.

The monarch is struggling to recover from an intense, unexpected January freeze in Mexico that killed millions of the butterflies.

Frost and co-author Henrik Mouritsen of the University of Oldenburg in Germany collected 59 monarch butterflies near Lake Ontario, Canada, between Sept. 9 and Oct. 2, 2001, near their peak autumn migration.

At a test site in Canada, the monarchs were tethered to the top of four plastic cylinders and monitored with a tiny camera as a steady stream of air was blown beneath their wings.

---- Checking the Sun ---- To see if the sun played a role in the monarch's migration, the scientists divided the butterflies into three groups that were exposed to light at different 12-hour intervals. First, the butterflies lived indoors under these conditions for at least five days, then they were tested outside in natural sunlight.

The scientists found the first group of monarchs, which were exposed to sunlight from 7 a.m. to 7 p.m., flew in a southwesterly direction toward Mexico.

However, when the butterflies were tested between 1 a.m.

and 1 p.m., and exposed to fewer hours of sunlight, the monarchs flew toward the southeast.

The third group of butterflies, tested between 1 p.m. and 1 a.m., flew toward the northeast.

The researchers said the butterflies, which check where the sun is first thing in the morning, migrated off course when their "day" started six hours later than usual.

In a separate series of experiments to test whether monarchs use the earth's magnetic field to navigate, the butterflies were placed in a wooden boat house under simulated overcast skies.

During the first 20 minutes, the flying patterns of the 18 monarchs tested under simulated cloud cover were different from each other and "all over the map," Frost said.

Eleven of 18 butterflies continued to fly in the same direction 10 minutes after scientists simulated a change in the earth's magnetic field nearby.

Frost said there is evidence that butterflies and other insects can still migrate on cloudy days.

"I suspect those are not favorable conditions to migrate," Reuters quoted Frost as saying. "But if they do fly, there is some evidence ... that there are enough polarized rays of light that tell them where the sun is even though they can't see it."