Some species of moths cleverly trick the echolocation system of hunting bats, researchers have discovered: The acoustic structures of their wing tips optimally reflect the calls of the predators, so that they snap at these stubborn structures and miss the insect’s body. This is another example of the amazing acoustic concepts in the arms race between the hunters and the hunted of the night.
In order to escape the hungry looks of predators, a group of butterflies has shifted their activity in the course of evolution into the protection of darkness. But the moths did not stay safe there – they were followed by sophisticated hunters with highly developed technology: instead of their eyes, bats rely on their biosonar when hunting in the dark. To do this, they emit high-frequency screams, which are reflected by the insect’s body. Thanks to the fine echoes, they can precisely locate their prey and cleverly snap them out of the air. But the moths did not remain helpless victims. As with other predator-prey ratios in the animal kingdom, an arms race has developed in the course of evolution: Researchers have already demonstrated various strategies against bat attacks in different species of moths. Some react to the sounds of the bats with clever evasive maneuvers.
Wing structures in sight
But acoustic protection strategies are already known: the wing scales of some species of moths absorb ultrasound sounds in a sophisticated way, so that they cover the insects in a kind of acoustic camouflage. Others, on the other hand, specifically ensure strategic conspicuousness: some representatives of the species-rich group of silk moths have trail-like appendages at the bottom of the wings. These often winding structures generate particularly strong echoes. As a result, bats are more likely to snap at these appendages and not at the body, resulting in a survival advantage for the moths.
“However, there are many silkworms who don’t have this trolling lure,” says Thomas Neil of the University of Bristol. But apparently these moths do not do without acoustic protection either, as he and his colleagues are now making clear. “During our investigations, we first discovered that many species of silk moth without towing have corrugated or folded structures on the tips of their forewings. So we looked into the question of whether these structures could also serve as acoustic bait to thwart attacks by bats, ”says Neil. The researchers used an innovative acoustic tomography analysis method for this purpose. This enabled them to record the echoes of various species of moth from thousands of angles and analyze the acoustic characteristics.
Acoustic sophistication revealed
It was shown that when exposed to sound with bat frequencies, the structured wing tips generated significantly stronger echoes than the bodies of moths. This effect was even more intense than with the acoustic trolling lures of the other species, the researchers report. They also discovered that the fine structures specifically reflect the sound back to its place of origin. The folds and waves create the strongest echoes at the insect’s wing tips throughout the moth’s wing flapping cycle and from most possible attack directions by bats.
“We have shown that the structures can act as an acoustic illusion, which means that a bat is aiming at the wing tip rather than the more sensitive body of the butterfly,” says senior author Marc Holderied. As the researchers explain, it stands to reason that the wing tips are not good points of attack for the hunters, as they are difficult to snap and, due to their elasticity, lead to a ricochet that can save the butterfly’s life. “The idea that the fore wing reflector is an acoustic bait is also supported by our knowledge that these structures have only ever developed as an alternative to the acoustic hind wing baits: There is no species of moth that has produced both systems”, says Neil.
“The results of our study reveal another exciting aspect in the history of the acoustic arms race between bats and moths,” Holderied concludes. The scientists now also want to test experimentally how effective the process actually is: They plan to observe bats and moths with different wing characteristics in order to determine how great the survival advantage is that the acoustic concept offers.
Source: University of Bristol, Article: Current Biology, doi: 10.1016 / j.cub.2021.08.038