Acoustic designs of orthopteran insects

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Crickets and katydids make sounds by rubbing their forewings together. Songs are generated by males to attract females for mating and in some species also used in male rivalry. Eardrums on their forelegs are backed by internal air tubes opening elsewhere on the body. Compared to humans, the signal system of these insects, both generators and receivers, is minaturized. We are trying to understand how such tiny and short structures function to broadcast sound wavelengths that are so long. Our research into generators asks how the shape and volume of body cavities below the wings affect the loudness of the broadcast sound. Some species have projecting sound-reflective body regions, while others completely enclose their sound-making forewings in a chamber: we are testing how these body modifications affect the sound field created around the insect. Being so small relative to wavelength, crickets are unable to fix the position of a distant caller by body 'sound shadow': no side-to-side differences in sound intensity are created by their body. Instead they conduct the sound across their body via internal air tubes and so sound waves reach the front and back of each eardrum at different times, after travelling different path lengths. As the cricket turns, the paths change relative to each other and cause sound interference at the eardrum giving usable binaural localization cues. We are comparing the internal air-tube arangements of the ears of many different cricket species. Finally, this research is a study of whether the difficulty of high-performance acoustic signalling, like the skill level needed by a concert pianist, can be the basis of honest assessment of a better mate by female katydids choosing among singing males. We change a male's signal quality and observe female preferences. The common theme of this work is gaining insight into how sound is processed by minaturized animal organs. One hopes to discover adaptations that will suggest improved design of similarly small devices such as the hearing aids used to counter human deafness.