A new study has revealed a secret of one of the world's most iconic species of moth.
Scientists have known for some time that the death's-head hawk moth, best known for its appearance in '90s horror classic The Silence of the Lambs, is able to maintain an almost perfectly straight flight path, even in strong crosswinds.
The moths use this skill every year to migrate thousands of kilometres between Europe and Africa, flying by night.
But scientists didn't quite know how, and tracking insects is hard, in part because of their small size. The kinds of radio tags used to follow birds can be too heavy for smaller fliers.
Transmitters have gotten tinier, though, and it helps that the death's-head hawk moth is huge compared to other insects, with a wingspan of up to 127 millimetres.
So, by fitting 14 moths with minuscule tracking devices weighing just 0.2 grams and following them by light plane, scientists were able to conduct the longest continuous tracking of an insect in the field — almost 90 kilometres.
What they found, published today in Science, was that the moths altered their flying strategy depending on the wind direction, according to study lead author Myles Menz from the Max Planck Institute and Townsville's James Cook University.
Their flight paths suggest the moths have some complex navigation skills, the authors said, challenging earlier ideas that insects are just wanderers.
The moths' flying strategies broadly fell into three clusters, Dr Menz said.
"If there were favourable tailwinds following [the moths], they flew up a bit higher and took advantage of these winds, which were going south-west.
"The other strategy was to fly partly downwind and offset a bit, and make an adjustment [along the journey].
"Then if the winds were unfavourable — say crosswinds or headwinds — the moths would fly down low and basically punch into it.
"Flying lower to the ground gives them more control and speed, because they're getting some buffering from the wind."
Combining those strategies with what the researchers suspect to be a sophisticated internal compass allowed the moths to keep a consistent direction while flying at speeds up to 67.9 kilometres per hour.
While it's known that birds employ a similar strategy of adjusting their flying height, how the death's-head hawkmoth managed in windy conditions was a mystery.
In the latter half of last century, some researchers thought that insect migration was mostly passive and relied on being pushed along by the wind.
But radar observations changed that thinking, providing empirical evidence for directional flight regardless of wind.
For this study, researchers released tagged moths in Germany in the hopes they’d start flying on their migration path toward the Alps.
The iconic species — dark coloured with yellow underwings and skull-like markings — was able to fly well with the tiny tracker glued to its back, said Martin Wikelski, a study co-author and migration researcher at Germany’s Max Planck Institute for Animal Behavior.
Professor Wikelski, the study’s pilot, took off in his plane, circling the area and waiting for any moths on the move. If he did pick up a signal from a tiny traveler, he would follow its radio blips for hours at a time.
"The little moth is guiding you," he said.
Though the number of moths tracked was fairly small, getting any close-up look at insect migration is significant, said Ryan Norris, an insect and bird migration researcher at the University of Guelph in Canada, who was not involved in the study.
"I was surprised at how far they could track them," he said.
"And it certainly is surprising that individual moths stay on this straight trajectory."
