Number of words: 6315 (from introduction to discussion) plus 1729 (two appendices).Elements: Main text file (including two tables and two appendices with three appendix tables), four figures, and an online supplement file.

Many species of grasshoppers in subfamilies Oedipodinae, Gomphocerinae and Acridinae make sounds when they escape by flying. We carried out four kinds of experiments with the Chinese grasshopper, Acrida cinerea (Orthoptera: Acrididae: Acridinae), whose males often make sounds while flying, to examine the mechanism of sound production during flight by grasshoppers. First, we recorded high-speed video and audio of the tethered flight of the males and found that they produced sounds when they clapped their hindwings together at the end of the upstroke. Second, we recorded the tethered flights of the males when we prevented them from clapping their hindwings to clarify whether the hindwing clapping produced the sounds, and we found that the obstruction of clapping hindwings prevented sound production by the males. Third, we recorded the free flight of the males and determined whether the sound produced by clapping hindwings was not specific to the tethered condition. The males clapped their hindwings during free flight, and sounds were produced when they clapped their hindwings. Finally, we observed the tethered and free flights of the females because the possibility of crepitation flight by the females existed, and we found that sound was produced by females clapping their hindwings during flight. These results showed that male A. cinerea make sounds during flight by clapping their hindwings, and females also have the ability to make sounds by using the same mechanism. This study provides the first experimental evidence on the mechanism of sound production during flight through observing the behaviors of grasshoppers.