"It's like a child's dream come true," says Martin Wikelski. Even as a boy he dreamed of watching migratory birds on their way south. And today, as a pilot, director of the Max Planck Institute for Animal Behavior in Radolfzell and professor at the University of Konstanz, he even flies after skull hawk moths in a Cessna 172, to which he has previously attached a 0.2 gram telemetry transmitter.
Martin Wikelski and his team have just described the results of this aircraft chase in Science magazine (volume 377, page 764): Apparently, the moths, which weigh at most 3.5 grams, use an internal navigation system that reliably brings them to their destination.
This in turn is sometimes even four thousand kilometers away. The squirrel hawk moth that hatched in Germany fly to the Mediterranean region in autumn and from there also reach regions south of the Sahara. The next spring, moths of this species fly back from the south to Central Europe and like to lay their eggs on potato plants, from whose leaves the yellow-green caterpillars that hatch feed.
After four moults, they are a good twelve centimeters long and weigh around 20 grams. After pupating, moths hatch that weigh an average of 2.6 grams, the record holder examined by Martin Wikelski even weighed 3.5 grams.
Skull hawk moths are therefore very large animals for the insect world, which means they can also carry one of the super-light telemetry transmitters, weighing just 0.2 grams, together with a small antenna.
Martin Wikelski and his team equipped 14 of these massive moths with such mini devices, whose signals are registered by antennas and a receiver. However, the signals do not go far. As a result, pedestrians or drivers often have very poor chances of tracking such tagged animals. So Martin Wikelski had good reasons to get his pilot's license and follow the flights of moths in a light Cessna aircraft.
"Fortunately, the air traffic controllers in the region know me well and are very supportive of my unusual flight plans," says the behavioral biologist happily. After all, night flights in which the pilot flies around six hundred meters directly over an insect every few minutes in order to gauge its position in the Alps are anything but routine maneuvers. But this is the only way to observe the flights of individual small birds or insects and thus not only fulfill the dreams of young people, but also the hopes of renowned behavioral biologists.
The behavior of the animals observed in this research is surprisingly similar. "Birds and bats, for example, seem to wait for evenings with favorable conditions before they fly off," explains Martin Wikelski. If the weather turns out to be unexpectedly bad at the altitude of the animals, they sometimes turn around and even fly back to their starting point. Commercial airplanes and even the skull hawk moths do it the same way today.
On the way, these large insects cover an average of 33.8 kilometers in an hour. And thus almost reach the pace of the human sprint world record at a distance of one hundred meters. A top speed of 69.7 kilometers per hour was measured. Even if the tailwind pushed the animals a little, this value clearly outclasses the top sprinters on two legs.
And the insects do not, as many biologists had expected, climb to the heights where the tailwind is strongest, but are often only 300 meters above the ground when the winds are weaker.
Very often, however, the skull hawk moths took off against a headwind and then made a beeline for their destination. From Konstanz this was often the 2067 meter high San Bernardino Pass, which has long been known for the huge flocks of migratory birds that fly south there in autumn. The moths were on the road for at least one, but sometimes more than three and a half hours. Start was usually a few minutes after sunset. Then the subsoil, which has been heated up during the day, often radiates heat that generates updrafts. Insects like to use this to save energy.
And they do it so successfully that Martin Wikelski, who followed them in his Cessna, also benefited from it: "I was obviously in similar wind conditions and my tank was still half full a few times after six hours of flight, although I actually already mean it should have reached maximum range.”
When the heat has dissipated after a few hours and the updrafts subside, the skull hawk moths usually land again. However, two of these moths flew 173.9 and 161.8 kilometers over the San Bernardino Pass in one night and landed behind it. In order to find the exact landing point for the animals, Martin Wikelski set off with the car complete with antennas and receivers and was actually able to locate the animals the following morning. They were sheltered in beehives. "There they camouflage themselves with a bee smell, so they are not attacked by the bees and can fortify themselves with their favorite food made of honey and nectar," explains Martin Wikelski.
The behavioral biologist does not know exactly how the moths navigate at night, but he has an urgent suspicion: "Similar to birds, the insects probably have a magnetic compass in their organism to determine their direction," the researcher suspects. "At the same time, their excellent night vision also helps them to make good use of the terrain."
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