Flight tones of insects are important for communication, anti-predator adaptation, and other behaviors. However, previous studies have mainly focused on flies and mosquitoes to elucidate the mechanisms behind these sounds. This study aimed to investigate the flight tone generation in wasps, a group for which these mechanisms remain unclear. We conducted a spatiotemporal analysis of wingbeats and flight tones in the Northern giant hornet (Vespa mandarinia). We used a high-speed camera to record the insect’s flight and analyzed the trajectory of the forewing’s leading edge during wingbeats. Simultaneously, we recorded the flight tones using a pair of microphones placed to the left and right, above and below, or ahead and behind the insect.
Temporal analysis of the wingbeat showed that the motion was not sinusoidal. Although there was no significant difference in speed between the forward and backward strokes. However, supination at the start of the stroke occurred rapidly, while pronation at the end took more time. Frequency analysis of the motion revealed peaks at several harmonics, with the fundamental frequency corresponding to the wingbeat frequency.
Next, we analyzed the flight tone temporally. Unlike the wingbeat, the sound waveform exhibited two or three oscillations per cycle. Frequency analysis showed that the sound pressure at the second and third harmonics equaled or exceeded the fundamental frequency. Spatial analysis revealed that sound waveforms recorded from the left and right were in phase, but waveforms from above and below or ahead and behind showed phase differences. Specifically, the pressure decreased during the forward stroke and increased during the backward stroke when recorded from above. Conversely, when recorded from below, the pressure increased during the forward stroke and decreased during the backward stroke, showing opposite phases. Additionally, when recorded from ahead and behind, the pressure increased during both the forward and backward strokes. The sound pressure decreased during supination and pronation when recorded from ahead, while it increased during supination and decreased during pronation when recorded from behind. Therefore, the forward and backward strokes, as well as pronation, were in phase, while supination occurred in the opposite phase.
These results strongly suggest that aerodynamic sound generation mechanisms, as observed in past studies on other insects, also apply to the flight tones of the Northern giant hornet.