The death-head's hawkmoth, the insect made famous by its appearance on the cover of cannibalistic 1991 horror movie The Silence of the Lambs, has given scientists a greater understanding of just how skilled the creatures can be.
Insects are the world’s smallest flying migrants and the joint Max Planck Institute of Animal Behaviour and University of Konstanz study radio-tagged and flew behind the hawkmoths for up to 80 kilometres as they navigated their way from Europe to north Africa — the longest distance over which any insect has been continuously monitored in the wild.
By closely following the hawkmoths during migration, the study unlocked a century-old mystery of what insects do over long-range journeys. And what they can do is accurately maintain a straight path for long distances in flight.
Scientists found the hawkmoths adopted strategies to buffer against prevailing winds, allowing them to hold their course throughout the night. When winds were favourable, they flew high and slow, allowing the air to carry them. But in harsh headwinds or cross winds, they flew low to the ground and increased speed to keep control of their path.
"This is important because it demonstrates that the insects are actively migrating and are maintaining control over their flight paths, even in unfavourable winds," study first author and lecturer at James Cook University in Australia, Dr Myles Menz, told The National.
"This allows them to select conditions in which to fly and to stay on course to reach their destination."
With trillions migrating every year, insects are some of the most common migrating animals on Earth. They include renowned species such as the monarch butterfly, locusts, mosquitoes and bees.
Migrating insects - in pictures
But even though insect migrants far outnumber better-known migrants, such as birds or mammals, their migration is the least understood form of long-range animal movement.
The problem, for the most part, has been methodological.
“Studying insects on the move is a formidable challenge,” said Dr Menz. “They’re usually too numerous to mark and find again, and too small to carry tracking devices.”
Much of what we know about insect migration has come from studies that sample insects at a single moment in time, such as through radar or direct observation, which has left vast gaps in our knowledge.
“Understanding what insects do during migration, and how they respond to weather, is a last frontier in migration science,” said Dr Menz.
How the migrating hawkmoths were tracked
The death’s-head hawkmoth is a large, nocturnal migrant that travels up to 4,000 kilometres between Europe and Africa every year. Like many insects, the species is multi-generational, which means that no individual knows the entire route.
At the MPI-AB in Konstanz, Germany, the team reared caterpillars until adulthood in the laboratory.
When moths emerged as adults, they were fixed with radio tags weighing 0.2 grams — less than 15 per cent of adults’ body weight.
“The moths would probably eat more weight than that in a night, so these tags are extremely light for the insects,” said Dr Menz.
The researchers released the tagged moths and waited for flight to begin, after which they chose one to follow at a time.
The team followed 14 moths each for up to 80 kilometres or four hours — long enough to be considered migratory flight — using antennae mounted on a Cessna plane to detect precise locations every five to 15 minutes. Insects were followed in the south-south-westerly direction from Konstanz into the Alps, which follows the route taken by hawkmoths towards the Mediterranean and north-west Africa.
The moths were tracked continuously until they stopped on route.
“When you’re in an airplane, it becomes extremely difficult to wait for the insects to begin migrating again because you would have to be in the air when this happens, which could be any time in the night,” said senior author Martin Wikelski, a movement ecologist from the MPI-AB and University of Konstanz, who piloted the plane during the study.
Dr Menz is hopeful his team's findings will "inspire more studies to answer many more big questions in this area”.
The next step is to establish how moths are able to maintain such straight lines.
“Based on past lab work, it’s possible that the insects are using internal compasses, both visual and magnetic, to chart their way around the world,” said Dr Menz.
