Scientists have long been puzzled by the epic annual migration of Monarch butterflies from their summer habitat in Canada across North America to a specific spot where they converge in central Mexico 4,000 kilometres away.
The butterflies, which only live for a few months before laying eggs and dying, cannot possibly remember the way. It had long been thought they had an inbuilt compass, scientists could not explain how they knew where their ultimate destination lay.
Now, with his team at the University of Oldenburg, German scientist Henrik Mouritsen has solved the mystery, the Tagesspiegel newspaper reported on Friday.
Writing in the PNAS scientific journal, Mouritsen explained how the butterflies' navigation systems were actually quite flawed.
In early spring in Canada, the scientists caught some of the insects and drove them 2,500 kilometres to the west. They kept others in the east as a control group.
Then both groups were put in a flight simulator in which they could fly several hours in an artificial wind without actually getting anywhere.
The butterflies in the east aimed south west, but so did those who were in west Canada - suggesting they didn't have an inbuilt map in their tiny heads after all.
Mouritsen said the butterflies used natural landmarks such as the Rocky Mountains or the Gulf of Mexico to find their way. So for example if they started to the east of the Rockies, they would fly south west until sooner or later they hit the mountain barrier, wrote the paper.
"Then they just fly along the Rocky Mountains or the Gulf coast in such a way that they keep as close as possible to the direction of their inner compass," said Mouritsen.
The two landmarks act like great roads leading the butterflies in the right direction, which then converge on each other in Mexico.
The swarms of insects are then funnelled towards a range of volcanoes, where they are stopped in their tracks - and spend winter months, cool, but without frost.