The Mechanics of Insect Audition: Characterisation, Modelling and Application

The sense of hearing is one of the most widespread across the different species of animals in the world. Animals use hearing in communication, to listen for danger and to help find lunch. The frequencies of sound used can vary an enormous amount, from very low frequency detection (infrasound) in fish, to the extremely high frequencies used by bats to echolocate and hunt for prey (ultrasound). Of course humans also have a sense of hearing, ranging from low frequencies up to about 20 kHz, although as we get older, our ability to hear higher frequencies degrades. However, through our own ingenuity humans have learned to generate, detect and use ultrasound (frequencies above our frequency range). We use this in many different applications, including medical imaging, cleaning, material analysis and non-destructive testing. It was only by creating such ultrasound devices that people discovered that bats were using ultrasound to identify and chase insects, and that many insects had ears tuned to listen out for the hunting bats to try and escape becoming a meal. Recently, engineers have started to examine the way bats use ultrasound. This is because the bats can achieve far greater resolution and sensitivity than any human built ultrasound system. The engineers hope to be able to improve their artificial systems by working out what techniques the bats employ. Whilst we know a lot about the ultrasound signals the bats use, we know comparatively little about the hearing systems of the bat's prey; the insects. Many studies have shown us which insects are sensitive to ultrasound, for example by looking at the insect's behaviour when ultrasound is played back to it. And from that, eardrum-like structures in ultrasound sensitive insects were discovered. The performance of some insect ears has also been described using various techniques, including very hi-tech solutions such as laser interferometry (where a laser is used to measure the motion of the insect's eardrum in response to sound). However, the actual mechanical operation of the structures within the ears of these insects, and so our understanding of how they receive ultrasound and translate that to vibrations the nerve cells can detect is very poor. This new research will use a combination of engineering approaches to understand how the ultrasound sensitive ears of insects work. The mechanical motions of different structures in the ears will be measured, with their size, shape and material properties characterised. To do this several techniques will be used including laser interferometry and atomic force microscopy (AFM). An AFM images surfaces by touch, rather than light. It uses a very small, atomically sharp, tip that is dragged, or tapped, across the surface of an object. A record is made of how much this tip goes up and down allowing us to make a surface image. AFM's can be sensitive enough to map the atoms on the surface of a material. As well as imaging, an AFM tip can be pushed into a surface, allowing us to measure how soft or hard it is. Using this technique it's possible to map the stiffness of a material down to nanometre scales. Once all this new information is collected it will be used to help create computer models of the ear structures. We can compare the models with the actual motions we measure, helping us to understand what is happening in the ear. From this, the models provide us with a tool to explore the capabilities of other eardrums, and further our understanding of the different ear capabilities relating to their size, sensitivity and dynamic range. Finally, the new knowledge from this research has broader applications. Looking back to the engineers working on bat ultrasound signals, this research will show us how the ears that have evolved to detect the bat's calls operate. It may then help engineers striving to improve artificial ultrasound sensor systems across many different fields such as medicine, material science and engineering.

This project investigated one of the most common senses within the animal world: hearing. Whilst humans of course have a sense of hearing, the frequencies used are limited compared to those used by many other animals. We know that some animals evolved to use ultrasound, for example the bat to hunt insects, and in turn the insects listening to the bat in order to avoid becoming a tasty meal! By examining how animals use sound, engineers hope to be able to improve our own acoustic and ultrasonic devices.
Unfortunately, although we know a lot about the ultrasound calls the bats use, our knowledge of the various insect ears is relatively poor. Although we know that many insects can hear ultrasound, our understanding of the mechanical operation of their ears, and how they receive ultrasound and translate that to vibrations the nerve cells can detect is very poor.
The research project used a combination of engineering approaches in order to understand how certain insect ears work. Experimental measurements were made of how the structures in different ears move in response to sound. Other experiments were made to measure the ear structure’s size, shape and materials. The experimental data was then used to help us create three-dimensional computer models of the insect ears. This allowed us to analyse how the different ears respond to sounds, comparing our experimental measurements of the ear’s motion with the model’s, and also investigating what effects the surrounding body of the insect has on the sounds an insect hears.
The research project made several very interesting discoveries. The most significant of these was the discovery that the ear of the greater wax moth, a common moth found around the world, has the highest frequency sensitivity of any ear in the animal kingdom, ca. 300 kHz, and so also has the greatest acoustic bandwidth of any known ear. For engineers this is a very interesting discovery, as it points towards new and exciting ways of improving artificial acoustic sensors such as the tiny microphone you find in a mobile phone. The research also found that environmental, developmental and genetic effects can alter the mechanical function of insect ears in a variety of unexpected ways. It was discovered for example that an insect ear’s mechanical response can change with temperature, age, gender, and diet. In the case of the locust, it was also found that the two forms of desert locust, solitary or swarming, have different senses of hearing due to physical differences in their ears. This type of information is very useful for biologists as it allows them to understand what factors affect the acoustic response of an insect, and so how it behaves in different periods of its lifetime, or in different environments. Finally, the new knowledge from this research has broader applications. A new engineering project, led by the same head researcher, was funded, based on some of this work’s results on insect ears, seeking to create new biologically-inspired miniature microphones.