by A sonic battle between bats and their insect prey has been raging in the night skies for over 65 million years. Many different techniques are used, and our new study reveals the amazing strategy of the little deaf moth, which has evolved a tiny wing structure that produces warning sounds. We hope that this insight may inspire engineers to create new technology.
Bats rely on their secret weapon, echolocation, to locate and catch their flying prey, and in response, nocturnal insects have developed interesting defenses. Many silk moths, for example, rely on a type of sound-absorbing stealth cloak that makes them disappear from bat sonar. Some large moth species have evolved reflective buffers that draw bat attacks away from their bodies and towards their wingtips.
The next level of defense is ears that allow insects, including many moths, to pick up bat echo calls and fly out of harm’s way. They can also use their sensory awareness of location to blast an aggressive bat with ultrasonic sounds that deter or confuse their biodata.
However, scientists have long argued about the slow-moving moths that are unable to detect their predators and are too small for buffers. How do they protect themselves?
We recently discovered that even earless moths, such as ermine moths (Rape), use acoustic signals as a defense against bat attacks. These moths have a tiny structure in their hindwings, which creates a powerful ultrasonic signal that hunts the bat’s echo sonar.
Because these moths do not have hearing organs, they are unaware of their unique defense mechanism, and cannot control it. Instead, the sound production mechanism is accompanied by the flapping of its wings.
Defensive wing beats
When we studied the wing of the ermine moth under a microscope, it was clear that one part of the wing stands out from the rest. Although most of it is covered with hairs and small scales, one patch of wing is clear and located near a corrugated structure of ridges and valleys. In our new study, we found that this structure produces a sound that is perfectly tuned to confuse bats.
Sound is a pressure wave that travels through a fluid or solid and requires displacement of this medium, usually vibration, to produce noise. Large vibrating surfaces over a cavity are good for amplifying the sound – a good example is a tymbal drum, which has a taught skin stretched over a cavity. As a drumstick strikes the drum skin, the skin vibrates at its natural frequencies and transmits these vibrations into the surrounding air as sound.
In ermine moths, the clear patch in the hindwing acts as the skin of the drum, and the corrugated structure of the valleys and ridges act as backbones. During flight, the wing of the moth makes the ridges jump one after the other in sequence. Each jump makes the clear patch, called an air-elastic tymbal, vibrate and amplify the volume of the sound.
In our recordings of ermine moths it was found that their wings make a clicking noise during flight, which we could detect using a bat detector that converts the ultrasound into a sound that is audible to humans.
Using 3D X-rays and a sophisticated microscope technique called confocal microscopy, the lead author of our study, Hernaldo Mendoza Nava, mapped the complex properties of the materials that make up the air-elastic tymbals of these moths. We then used computer simulations to test our hypothesis that the deformations of the corrugations stimulate the wing membrane in a way that produces sound. These simulations produced a sound that matched our recordings of moth clicks in frequency, structure, amplitude and direction.
Some eared moths can make similar warning sounds, but none (so far) have been shown to do this with an aerodynamic tympanum.
For our team of biologists and engineers, the wing structures are interesting because they rely on a mechanism that we teach our engineering students to avoid. “Jump through” is an example of buckling instability – when a structure loses stability when loaded, and suddenly goes into a different state.
In buckling instability, the material does not break but usually the structure loses stiffness and can even collapse. This can have disastrous consequences for any load-bearing structure, such as buildings, bridges and airplanes.
Inspired by nature
Historically, structures were made to be rigid enough to withstand external forces. In the last decade, researchers and engineers have begun to question this default position, and have begun to use buckling instability to create structures with new capabilities.
One example is engineers designing morphing structures for future aircraft wings that autonomously adapt their shape to perform better when the environment changes. The aeroelastic print of ermine moths embodies this concept and shows how nature can be an inspiration for new technology.
Our hope is that the aeroelastic tympans of these deaf moths will inspire new developments in engineering fields such as structural acoustic monitoring, where structures produce sound when overloaded. This is often used to check the safety of the infrastructure. It could also lead to innovations in soft robotics, where the robots are made of fluids and gels instead of metals and plastics.
This article from The Conversation is republished 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 the I13 horizon (proposal MT17616) and Dr. Shashi Marathe and Kaz Wanelik for their assistance at the facility. 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 has received funding from the Royal Academy of Engineering (grant no. RF/201718/17178) for this research. Hernaldo Mendoza Nava, a PhD student who worked on this project for his thesis, was funded by the National Council of Science and Technology (CONACYT-Mexico, CVU/student number 530777/472285) and the Council for Engineering and Physical Sciences Research through the EPSRC Center for Doctoral Training in Advanced Materials for Innovation and Science (grant number EP/L0160208/1).
