These mysterious moths fly at night in a straight line—but how?

Biologists used an airplane and tiny transmitters to track the death’s-head hawkmoth, setting a scientific record in the process.

By Jason Bittel
Published 15 Aug 2022, 14:42 BST
In their study, the researchers focused on the death’s-head hawkmoth—a large, nocturnal migrant that travels well ...
In their study, the researchers focused on the death’s-head hawkmoth—a large, nocturnal migrant that travels well over 2,000 miles between Europe and Africa every year.

Death’s-head hawkmoths can scream when provoked. Their backs feature a pattern that resembles a human skull, which landed them a deadly cameo in the film The Silence of the Lambs. And now, the infamous insects have helped scientists do something once thought impossible.

By fitting the moths with tiny temporary backpacks containing radio-transmitters and releasing them at night, scientists were able to follow along in a small airplane as the moths performed their annual southward migration.

The most striking flight path was a moth that flapped its way from an airport in Konstanz, Germany, more than 55 miles south into the Swiss Alps—the longest insect flight ever continuously tracked.

The newly recorded, one-night flight represents just a small portion of the death’s-head hawkmoth’s own 2,400-mile migration from northern Europe to the shores of the Mediterranean and beyond—perhaps even as far south as sub-Saharan Africa. One generation of the moths generally migrates out of Europe in the fall to breeding grounds in the south; the next generation flies back to Europe in the spring.

Researchers use lamps and sheets to create so-called “light traps” to attract and study insects. Here, a scientist inspects a death’s-head hawkmoth raised in a special enclosure. The insects were later tagged and released for study.

Compared to most other insects, death’s-head hawkmoths are fast fliers, with maximum flight speeds observed at 43 miles per hour. But to track them in an airplane traveling at much higher speed, the scientists kept pace by flying in overlapping circles while listening for a distinct woppp sound made when the aircraft’s antennae detected a moth more than a thousand feet below.

“With an antenna on each wing, it’s almost like you have two ears listening,” says Martin Wikelski, director of the Max Planck Institute of Animal Behaviour and senior author of a study published today in the journal Science.

As a long-time pilot, Wikelski was able to fly the Cessna 172 while his colleagues released one to two moths on the ground below. He’d know to start listening for pings when Myles Menz, the study’s lead author, would broadcast over the radio, “Moths away!”

Similar tracking studies have been carried out in birds, and Wikelski has also used the method successfully on bats and dragonflies. But the breakthrough with death’s-head hawkmoths comes from advances in the shrinking size of radio-transmitter technology as well as the insect’s larger size compared to most common moths, with wingspans wider than a soda can is tall.   

“They have been trying to do this with insects, and they finally got it,” says Gerard Talavera, an expert on butterfly migration and a National Geographic Explorer who was not involved in the new study. “It is fantastic to see people doing brave work like this.”

Flying ‘absolutely straight’

Not only is the study impressive as a proof-of-concept that may be valuable in the research of other insects, but it has also revealed some interesting aspects of moth migration.

For one, scientists have long suspected that the wind blows insects off course during migrations, a natural assumption considering that even large lepidopterans like death’s-head hawkmoths weigh less than an average shirt button.

So when Wikelski first took to the skies, he monitored wind direction and speed with his onboard instruments and plotted a course that assumed the moths would get whooshed in that direction. But in doing so, he quickly lost track of the moths.

“Then I realised, oh, the moths are still over there,” he says. “We realised they were going straight—absolutely straight—no matter what the wind was doing.”

To understand how the moths accomplished the feat, the scientists looked to their altitudes. When the wind was blowing in their faces, the moths increased their speed while flying low to the ground. And when the wind was at their backs, the animals soared up to around a thousand feet to make better use of the acceleration, but decreased their airspeeds in the process.

The moths are large for flying insects, weighing up to 3.5 grams, and were fixed with tiny radio tags weighing 0.2 grams—less than 15 percent of adults’ body weight.

In other words, the moths appeared to be carefully balancing their speed with their sense of direction. The scientists suspect that, like several other insects, the moth’s internal compass is calibrated using a combination of magnetism, vision, and possibly their sense of smell.

What’s more, the fact that the moths were able to show “complete compensation”, or maintaining a straight path even under assault from different windspeeds and directions, is another first for migratory insects.

“Apparently these insects have managed to find a system to keep perfectly on track on their navigational route,” says Wikelski. “And that is super exciting.”

Insect tracking’s next frontier

For scientists studying insect migration, the possibility of tracking individuals is groundbreaking, because it will allow them to answer questions they could only speculate about before.

“At what speed can they fly, and where do they stop, and what do they forage? These are things that were very much assumed, but this is the first time that you get real data for some of these questions,” says Talavera.

There are also real-world reasons for learning more about the world’s trillions of migrating insects.

“Desert locusts still affect one in ten people every year,” says Wikelski, referring to the way the migrating insects ravage crops, leading to famine and starvation. “And that creates major human conflict.”

The ability to track insects could one day help us stop the spread of invasive species, conserve endangered species such as the monarch butterfly, and curb the spread of insect-borne diseases, he says.

It may happen sooner than you think. A new pair of satellites scheduled to go into orbit in 2028 as part of a partnership between NASA and European space agencies will allow scientists to track large individual insects like moths and dragonflies not just over the course of one night, but as they fly all over the world.

Moths away, indeed.


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