The acoustic war between bats and the insects they hunt has been raging 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 small, deaf ermine moth, which has evolved a tiny wing structure that produces warning sounds. We hope this understanding will 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 defenses. For example, many silk moths use a type of sound-absorbing cloak that allows them to “disappear” from bat sonar. Some large moth species have evolved reflective traps that attract bat attacks away from their bodies and towards the tips of their wings.
The next level of defense is the ears, which allow insects, including many moths, to pick up bat echolocation calls and fly away unharmed. They can also use their sensory awareness of location to blast an attacking bat with ultrasonic sounds that will deter or confuse their biosonar.
But scientists have long been confused about the number of earless moths, which cannot detect their predators and are too small to be used as bait. How do they protect themselves?
Recently, even earless moths such as ermine moths (yponomeuta), use acoustic signals as a defense against bat attacks. These moths have a small structure on their hind wings that creates a strong ultrasonic signal that disrupts the bats’ echolocation sonar.
Since these moths do not have hearing organs, they are not aware of their unique defense mechanisms and cannot control them. Instead, the sound-producing mechanism is combined with the flapping of the wings.
Protective wing beats
When we examined the ermine moth’s wing under a microscope, it became clear that one part of the wing stood out from the others. Although most of it is covered with small hairs and scales, one part of the wing is clear and abuts a corrugated structure of ridges and valleys. In our new study, we found that this structure produces sound perfectly tuned to confuse bats.
Sound is a pressure wave that travels through a liquid or solid and requires displacement of that medium, usually vibration, to produce noise. Large vibrating surfaces over gaps are good for amplifying sound; A good example of this is a tymbal drum, which has skin stretched over a cavity. When the drumstick hits the drum head, the skin vibrates at its natural frequencies and transmits these vibrations as sound to the surrounding air.
In ermine moths, the transparent patch on the hind wing serves as drum skin, while the corrugated structure of the valleys and ridges serves as drumsticks. During flight, the moth’s wing allows the ridges to break one after another. Each click vibrates the transparent patch, known as the aeroelastic ringer, and increases the volume.
In the recordings we made of ermine moths, we found that their wings made clicking sounds during flight; We were able to detect this using a bat detector that converts ultrasound into sound that humans can hear.
Using an advanced microscopy technique called 3D X-ray and confocal microscopy, our study’s lead author, Hernaldo Mendoza Nava, mapped the complex properties of the materials that make up the aeroelastic bells of these moths. We then used computer simulations to test our hypothesis that deformations in the grooves stimulate the wing membrane to produce 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 (so far) been shown to do so with an aeroelastic bell.
To our team of biologists and engineers, these wing structures are fascinating because they rely on a mechanism we teach our engineering students to avoid. “Transition” is an example of flexural instability, which occurs when a structure loses stability when loaded and suddenly transitions to a different state.
In buckling instability, the material does not break, but the structure often loses its stiffness and may even collapse. This can have devastating consequences for any load-bearing structure such as buildings, bridges and aircraft.
We were inspired by nature
Historically, structures were built to be strong enough to withstand external forces. Over the last decade, researchers and engineers have begun to question this default position and use buckling instabilities to create structures with new capabilities.
One example of this is engineers designing transitional structures for future aircraft wings that autonomously adapt their shape to perform better when the environment changes. The aeroelastic bell of ermine moths embodies this concept and shows how nature can provide inspiration for new technology.
Our hope is that the aeroelastic bells of these deaf moths will spur new developments in engineering fields such as acoustic structural monitoring, where structures make sounds when overloaded. This is often used to control the security of the infrastructure. It could also lead to innovations in soft robotics, where robots are made of liquids and gels rather than 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). Diamond Light Source for access to beamline I13 (proposal MT17616) and Dr. for their assistance on site. Thanks to Shashi Marathe and Kaz Wanelik. We thank Daniel Robert for access and support with laser Doppler vibrometry.
Alberto Pirrera received funding for this research from the Engineering and Physical Sciences Research Council (grant no. EP/M013170/1).
Rainer Groh received funding for this research from the Royal Academy of Engineering (grant no. RF/201718/17178). PhD student Hernaldo Mendoza Nava, who worked on this project for his thesis, was funded by the National Council for Science and Technology (CONACYT-Mexico, CVU/student 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).