Every spring, Australia's capital comes under attack. The threat to Canberra is not from war or terrorism, but millions of large moths that descend on the city at night in the course of their annual migration. We've now learned the moths use magnetic fields to navigate their journey, something well established in birds and other migratory vertebrates but not previously found in insects.

Although the parliament house being swarmed by moths may be a lepidopterophobe's nightmare, it is not their target. Instead, their route is from breeding grounds on the plains to caves high in the Australian Alps that stay cool through the summer, a journey of more than 1,000 kilometers (620 miles). Canberra happens to be in the way, and its lights confuse the fluttering bugs.

The way the moths fly their way through improbably small gaps to swarm lights led to the popular assumption they use a lunar guidance system. Entomologists shared this assumption: "When we began this study, we were convinced that the Bogong moth would exclusively use celestial cues in the sky, such as the stars and the moon, for navigation during migration," said Professor Eric Warrant of the University of Lund, Sweden, in a statement.

Moths and butterflies, such as the famous monarch, are the only insects to undertake such great migrations over a well-defined route. Locusts, for example, can also travel immense distances but do so chasing the rain and food. Monarchs have been shown to rely primarily on the Sun, but Warrant wondered how bogong moths find their way at night.

It can't be memory since the moths are born at the breeding grounds and have never been to the mountains before. The caves are also much harder to find than the monarch's feeding grounds.

In Current Biology, Warrant reveals that the moths take up precious space in their tiny brains with the capacity to track magnetic fields. “We were very surprised when we discovered that these moths could sense the Earth's magnetic field just like night-migratory birds – and probably for the same reason,” he said. 

As their fluttering around lights reveals, the magnetic sense is not the whole story. Warrant and co-authors had the moths fly in a simulator equipped with a variable magnetic field. When the direction of landmarks like a painted mountain and the field were adjusted together, the moths smoothly changed course. When the researchers adjusted the magnetic field or the visual cues so that the two conflicted, the moths became disorientated, suggesting they rely on aligning the two.

The relative simplicity of the moth's nervous system may assist us in discovering how they detect magnetic fields, something we have so far failed to do in vertebrates.