Motor and mechanical bases of the courtship call of the male treehopper, Umbonia crassicornis

Abstract

This study is a physiological, anatomical, and biophysical analysis of how plant-borne vibrational signals are produced by the treehopper Umbonia crassicornis. During courtship, males and females engage in a vibrational duet, with each producing a characteristic call. For males, this consists of a frequency-modulated tonal signal which is accompanied by rhythmic broad-band clicks. While previous studies described these complex signals in detail, little is known about how they are produced. By combining video recordings, electromyograms, dissections and mechanical modelling, we describe the mechanism by which the male produces his courtship signal. High-speed videos show that the tonal portion of the call is produced by periodic dorso-ventral movements of the abdomen, with a relatively large amplitude oscillation alternating with a smaller oscillation. Electromyograms from the muscles we identified that produce this motion reveal that they fire at half the frequency of the abdominal oscillation, throughout the frequency-modulation of the tonal signal. Adding weight to the abdomen of a calling male reduces the frequency of motion, demonstrating that the abdominal motion is strongly influenced by its mechanical resonance. A mathematical model accounting for this resonance provides excellent qualitative agreement with measurements of both the muscle firing rate recorded electrophysiologically and the oscillatory motion of the abdomen as recorded in the high speed video. The model, electromyograms, and analysis of video recordings further suggest that the frequency modulation of the abdominal response is due to a simultaneous modulation in the muscle firing rate and a fluctuation in stiffness of the abdominal attachment.