To fly or to jump: Factors related to the mode of locomotion for escape by the Japanese rice grasshopper Oxya yezoensis

Various grasshopper species exhibit two modes of locomotion—flying and jumping—when escaping approaching predators. A field experiment on the Japanese rice grasshopper Oxya yezoensis was conducted to identify factors related to the decision of locomotion mode during escape attempt. A human investigator, acting as a predator model, approached each grasshopper three times consecutively. The relationship between locomotion mode and the following factors was examined: grass height, air temperature, and relative humidity as environmental factors; sex, body length, body weight, and wing length as prey traits; persistence in approaches as a predator trait; and flight initiation distance (the distance between predator and prey when the escape behavior is triggered) and distance fled (the distance moved by the prey during escape) as additional escape strategies. The results showed that females escaped by jumping more frequently than males. Moreover, the distances fled by grasshoppers escaping by flying were greater than those of grasshoppers escaping by jumping. Flight initiation distances of grasshoppers that jumped were either shorter than or comparable to those that flew. In the first escape attempt, escape by flying increased with wing length in females, increased with air temperature, and decreased with relative humidity in both sexes. In repeated escape attempts, grasshoppers consistently used the same locomotion mode more frequently than expected if decisions of locomotion modes were made independently. These results indicate that O. yezoensis uses two modes of locomotion depending on various ecological and biological factors in the field.