Biomechanics of sound generators and receivers in crickets and bushcrickets Seminar

Male cricket and bushcrickets emit pure-tone mating calls by rubbing their wings together, a behaviour called wing stridulation. Crickets use low frequency signals (with high purity, at 5 kHz), but bushcrickets employ a broader range of frequencies (3-150 kHz), usually extending in to the ultrasonic range. In crickets the wings are bilaterally symmetrical, but each wing has its own individual resonant frequency (fo). The main sound radiating structures of the left wing display significantly lower resonance than those of the right wing (the overlapping wing). As these differences in wing fo are not observed in the call output, the exact mechanism enabling the production of tones with remarkable spectral purity remains unknown. I have studied the resonant properties of the wings of stridulating crickets using scanning laser Doppler vibrometry and cholinergic brain activation. These experiments show that the wings are not perfect resonators; even localized areas and cells in a single wing exhibit different resonances. Notably, as the wings join up during singing, all cells resonate at a frequency imposed by the fo of the left wing. These facts move the cricket-wing-stridulation system to the level of a complex network, where synchronization and/or coupling play key roles.

As insect species evolved to emit species-specific frequencies, their ears likewise became adapted to detect and localise species-specific acoustic signals. In the second part of my talk I will discuss the ears of bushcrickets, which are adapted to detect ultrasonic signals. In these insects, each ear consists of two adjacent tympanal membranes located on the forelegs (one pair per leg). Intriguingly, the mechanoreceptor cells are not in direct contact with the eardrums, but lay in between and on top of the acoustic trachea surface adjacent to the tympana, and are organized tonotopically. To date, it is unknown how sound-elicited vibrations are transmitted from the tympana to the mechanoreceptors. It is an important outstanding question as ultrasonic receivers are poorly understood in animals. I have discovered a novel organ - a fluid vesicle derived from the insect circulatory system. This organ connects the tympanal membranes with the mechanoreceptors, and acts as an impedance matching transformer. Travelling waves propagate inside the vesicle in a gradient, from the region containing the mechanoreceptors tuned to high frequencies, to the low-frequency tuned scolopidia, in a manner analogous to the mammalian cochlea.