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It has confused scientists for decades - how how do Monarch butterflies manage to navigate 2,000 miles from breeding sites across the eastern United States to fir trees in central Mexico.

Researchers first found they used the sun - but then found they could still navigate on overcast days.

Now they believe they have finally solved the mystery - the insects have an internal magnetic compass.

A Monarch butterfly eats nectar from a swamp milkweed on the shore of Rock Lake in Pequot Lakes, Minnesota: Researchers say the insects use an internal magnetic compass to help navigate on their annual migrations from North America to central Mexico.

The new research in the journal Nature Communications was carried out by scientists at UMass Medical School and Worcester Polytechnic Institute.

'Taken as a whole, our study reveals another fascinating aspect of the monarch butterfly migratory behavior,' said author Steven Reppert.

'Greater knowledge of the mechanisms underlying the fall migration may well aid in its preservation, currently threatened by climate change and by the continuing loss of milkweed and overwintering habitats.

'A new vulnerability to now consider is the potential disruption of the magnetic compass in the monarchs by human-induced electromagnetic noise, which can also affect geomagnetic orientation in migratory birds.'

Co-author Robert Gegear, PhD, assistant professor of biology and biotechnology at WPI, said: 'Our study shows that monarchs use a sophisticated magnetic inclination compass system for navigation similar to that used by much larger-brained migratory vertebrates such as birds and sea turtles.'

Monarchs use a time-compensated sun compass in their antenna to help them make their 2,000 mile migratory journey to overwintering sites.

During the absence of daylight cues, such as under dense cloud cover, migrants have been, surprisingly, seen flying in the expected southerly direction.

It's been hypothesized that monarchs use geomagnetic cues to help navigate when day light cues are unavailable to them during migration.

The nsects also use the sun to get their route right

Using flight simulators equipped with artificial magnetic fields, Patrick Guerra, PhD, a postdoctoral fellow in the Reppert lab, examined monarch flight behavior under diffuse white light conditions.

He found that tethered monarchs in the simulators oriented themselves in a southerly direction.

Further tests in the simulator revealed that the butterflies used the inclination angle of Earth's magnetic field to guide their movement.

Reversing the direction of the inclination caused the monarchs to orient in the opposite direction, to the north instead of the south.

Monarch butterflies are collected in a net to be tested for the ophroyocystis elektroscirrha parasite that inhibits their flight, at El Capulin reserve, near Zitacuaro, Mexico.

Further work suggested the compass is in the antennae.

'For migratory monarchs, the inclination compass may serve as an important back up system when daylight cues are unavailable,' Guerra said.

'It may also augment hand-in-hand with the time-compensated sun compass to provide orientation and directionality throughout the migration process.'

The next step for Reppert and colleagues will be to evaluate the molecular and genetic mechanisms of magneto-reception in the monarch butterfly and the involvement of CRYs.