A north-bound monarch, shot here in a field in Oklahoma. ((Billy Hefton/Associated Press))

Monarch butterflies have a 24-hour clock in their antennae that helps them navigate their 4,000-kilometre migration route, scientists say.

Researchers at the University of Massachusetts Medical School found that the clock, which is used in conjunction with a previously known "sun compass" to navigate, is in the insects' antennae, not in the brain.

"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, but its role in precise orientation over the course of butterfly migration is an intriguing new discovery," said Steven Reppert, senior author of the study, in a statement.

Monarch butterflies make an epic 4,000-kilometre journey from their wintering grounds in a particular Mexican forest to summers around the Great Lakes. (No one butterfly makes the complete round trip. It takes four or five generations of butterflies to complete.)

The butterflies have a compass in their brains that track the position of the sun, but because the sun moves across the sky through the day, scientists knew that a molecular clock was needed to compensate and keep the butterflies flying in the right direction.

It was assumed that the clock would also reside in the brain.

The researchers removed the antennae from a group of butterflies and compared their flight patterns to a group of control butterflies in an outdoor flight simulator.

The butterflies with intact antennae all flew toward the southwest. The butterflies without antennae flew in random directions.

The researchers also found that previously known molecular clocks in the butterflies' brains that regulate sleep cycles continued to function normally after the antennae were removed, showing that the clocks located in the antennae worked independently from those found in the brain.

Without their antennae, the butterflies lost their ability to navigate using the position of the sun in the sky, because they could no longer compensate for the time of day, the researchers wrote in their study published in this week's edition of Science.

To obtain further evidence that light-sensing biological clocks were located in the antennae, the researchers painted the antennae of one group of butterflies using black enamel paint and another group with transparent paint.

The butterflies with black painted antennae were just as lost as those without antennae, while those with the clear paint flew normally.

The observation that butterflies became lost in their flight after their antennae are removed is actually 50 years old, before it was known that butterflies navigate using the sun and spend the winter in specific areas in Mexico.