An acoustic battle between bats and their insect prey has raged in the night sky for more than 65 million years. Many different techniques are used and our new study reveals the fascinating strategy of the tiny, deaf ermine moth, which has developed a small wing structure that produces warning sounds. We hope this insight can inspire engineers to create new technology.
Bats rely on their secret weapon, echolocation, to find and capture their flying prey, and in response, nocturnal insects have evolved interesting defense mechanisms. For example, many silk moths rely on a kind of sound-absorbing stealth cloak that helps them “disappear” from bat sonar. Some large moth species have developed reflective decoys that lure bat attacks from their bodies to the tips of their wings.
The next level of defense is ears that allow insects, including many moths, to pick up the echolocation calls of bats and fly out of harm’s way. They can also use their sensory location awareness to blow off an attacking bat with ultrasonic sounds that scare or confuse their biosonar.
However, scientists have long been puzzled by the many earless moths that cannot detect their predators and are too small for decoys. How do they protect themselves?
We recently discovered that even earless moths, such as ermine moths (Yponomeuta), use acoustic signals as a defense against bat attacks. These moths have a small structure in their hindwings that creates a powerful ultrasonic signal that blocks bats’ echolocating sonar.
Because these moths have no hearing organs, they are unaware of and cannot control their unique defense mechanism. Instead, the sound production mechanism is linked to the flapping of their wings.
Protective wing beats
When we examined the ermine moth’s wing under a microscope, it became clear that one part of the wing stands out from the rest. Although most of it is covered in small hairs and scales, one patch of wing is clear and borders a corrugated structure of ridges and valleys. In our new study, we found that this structure produces sounds perfectly tuned to confuse bats.
Sound is a pressure wave that travels through a liquid or solid and requires a displacement of this medium, usually a vibration, to produce sound. Large vibrating surfaces above cavities are good for amplifying sound – a good example is a tymbal drum, where a tight head is stretched over a cavity. When the drum skin is struck by a drum stick, the skin vibrates at its natural frequencies and transmits these vibrations as sound to the surrounding air.
In ermine moths, the clear spot in the hindwing serves as a drum skin, while the corrugated structure of valleys and ridges act as drumsticks. During flight, the moth’s wing causes the ridges to snap one after the other in a specific order. Each click causes the clear patch, known as an aeroelastic tymbal, to vibrate and amplify the sound volume.
Recordings we made of ermine moths showed that their wings make clicking sounds during flight, which we were able to detect using a bat detector that converts ultrasound into sound audible to humans.
Using 3D X-rays and an advanced microscope technique called confocal microscopy, the lead author of our study, Hernaldo Mendoza Nava, mapped out the intricate properties of the materials that make up these moths’ aeroelastic tymbals. We then used computer simulations to test our hypothesis that the waves’ deformations stimulate the wing membrane in a way that produces sound. These simulations produced a sound that matched our recordings of the moths’ clicks in frequency, structure, amplitude, and direction.
Some eared moths can make similar warning sounds, but none have been shown (yet) to do so with an aeroelastic eardrum.
To our team of biologists and engineers, these wing structures are fascinating because they rely on a mechanism that we teach our engineering students to avoid. “Breakthrough” is an example of buckling instability – when a structure loses stability under load and suddenly snaps into a different state.
In case of buckling instability, the material does not break, but the structure usually loses its stiffness and can even collapse. This can have catastrophic consequences for any structure that carries loads, such as buildings, bridges and aircraft.
Inspired by nature
Historically, structures were made to be stiff enough to withstand external forces. Over the past decade, researchers and engineers have begun to question this default position and use instabilities to create structures with new capabilities.
An example of this is engineers designing morphing structures for future aircraft wings that autonomously adjust their shape to perform better as the environment changes. The aeroelastic eardrum of ermine moths embodies this concept and shows how nature can be a source of inspiration for new technology.
Our hope is that the aeroelastic tymbals of these deaf moths will encourage new developments in technical areas such as acoustic structural monitoring, where structures emit sound when overloaded. This is often used to check the security of infrastructure. It could also lead to innovations in soft robotics, where the robots are made from liquids and gels instead of metal and plastic.
This article is republished from The Conversation under a Creative Commons license. Read the original article.
Marc Holderied receives funding from the Biotechnology and Biological Sciences Research Council (grant no. BB/N009991/1) and the Engineering and Physical Sciences Research Council (grant no. EP/T002654/1). We thank Diamond Light Source for access to beamline I13 (proposal MT17616) and to Dr. Shashi Marathe and Kaz Wanelik for their assistance with the facility. We thank Daniel Robert for access and support with Laser Doppler vibrometry.
Alberto Pirrera has received funding for this research from the Engineering and Physical Sciences Research Council (grant no. EP/M013170/1).
Rainer Groh has received funding for this research from the Royal Academy of Engineering (grant no. RF/201718/17178). Hernaldo Mendoza Nava, a PhD candidate who worked on this project for his dissertation, was funded by the Science and Technology National Council (CONACYT-Mexico, CVU/studentship no. 530777/472285) and the Engineering and Physical Sciences Research Council through the EPSRC Center for Doctoral Training in Advanced Composites for Innovation and Science (Grant No. EP/L0160208/1).