Great Cricket Ears
Katydid or crickets are the common name of certain large, singing, winged insects belonging to the long-horned grasshopper family. Katydids are green or, occasionally, pink and range in size from 11/4 to 5 inches long. Katydids are nocturnal and arboreal; they sing in the evening. Scientists studying a species of South American bush cricket with some of the smallest ears known have discovered it has hearing so sophisticated that it rivals our own. The study, published in Science, is the first to identify hearing organs in an insect that are evolutionary convergent to those of mammals. Led by the scientists at the University of Bristol, they show how the bush cricket’s (Copiphora Gorgonensis) auditory system has evolved over millions of years to develop auditory mechanisms strikingly similar to those of humans, but using an entirely different machinery.
Robert and his colleagues focused their study on the insect that can hear sound whose frequency ranges from 5,000 to 50,000 hertz. Humans, in comparison, can hear between about 20 and 20,000 hertz. These katydids sing at about 23,000 hertz, in ultrasound, or above the human range of hearing.
In mammals, hearing relies on three processing stages: an eardrum collecting sound, a middle ear impedance converter and a cochlear frequency analyzer. The bush cricket’s ears, which are found on its two front legs, can perform all three stages, using ears that, despite being much smaller, work like those of humans, but look very different.
After studying the bush cricket’s microscopic auditory system the researchers discovered how impedance conversion — the process of efficiently converting air-borne sounds into liquid-borne vibrations — takes place in these insects. The bush cricket’s miniature solution to the problem of impedance conversion relies on a system of mechanical levers, a sort of microscopic see-saw formed by its eardrum that makes the link to the inner ear.
Collaborating with archaeologist Professor Kate Robson-Brown at the University of Bristol’s Department of Archaeology and Anthropology, the research team also unveiled the complex internal anatomy of the cricket’s ears. They found a new organ for insects which allows the animal to separate out a wide range of frequencies. By measuring nanoscale vibration using laser Doppler technology, the team went on to show that this system works just like the cochlea of mammals, yet about sixty times smaller.
Daniel Robert, one of the study’s lead authors and Professor of Bionanoscience at the University of Bristol’s School of Biological Sciences, said: "Hearing is one of our most important senses as it enables us to perceive sounds with complex tonal structures. However, in other animals, hearing can often mean a matter of life or death, which explains why this insect has hearing that is so sophisticated. In the cacophony of their rain forest environment, it is crucial for these crickets to distinguish between a chorus of insect sounds and the ultrasounds of hunting bats."
"This discovery that some insects possess such complex biophysical mechanisms for auditory processing is a break through; it will help us develop bio-inspired hearing devices that are smaller and more accurate than ever before, and with much-improved functionality."
Dr Fernando Montealegre-Z, Senior Lecturer at the University of Lincoln and the study's other lead author, added: "We discovered a novel structure that constitutes the key element in hearing in these insects, which had not been considered in previous work. The organ is a fluid-filled vesicle, which we have named the Auditory Vesicle. This hearing organ mediates the process of conversion of acoustic energy (sound waves) to mechanical, hydraulic and electrochemical energy. The integration laser Doppler vibrometry, and micro-CT scanning from the labs of Professors Robert and Robson-Brown allowed to identify the auditory vesicle and to conclude that the process relies on a tympanal lever system analogous to the mammalian ossicles, which serves to transmit air-borne sound to the fluid (the auditory vesicle), and also on the mechanoreceptors."
The researchers are now investigating the ears of other insects, including a closely related katydid that sings at an amazingly high 150 kilohertz (150,000 hertz), the most ultrasonic singer of any known organism. The wavelength of such sounds is so short that the katydids must have ultrasensitive ear structures to catch it over distances, Montealegre-Zapata said. Ultimately, the researchers plan to engineer extremely tiny, extremely sensitive microphones and sound sensors.
For further information see Katydid.
Cricket image via University of Bristol.