Micro-Air-Vehicles (MAVs) that mimic the flight mechanisms of insects have been attracting significant attention in recent years. These technologies are developed with the aim of lifesavings in the area with the risk of secondary disasters, maintenance works for constructions such as bridges, information collection on planet searches, monitoring of security risks for the purpose of security means. A number of researchers have attempted to develop small flap flying objects and MAV with various actuators and devices. However, these robots were not practical. One of the reasons for this is that the flying mechanism of insects has not yet been clarified sufficiently. We have clarified that a couple of large-scale vortex is formed over the wing. The purpose of the present study is to clarify the dynamic behavior and the detailed structure of the vortices of the flapping butterfly wing, and we carried out the PIV measurement around the flapping butterfly wing. The vortex ring develops over the wings when the wings flap downward to the bottom dead position and then passes through the butterfly completely and grows until reaching the wake at the bottom dead position. The vortex ring develops over the wing while growing from the leading edge toward the trailing edge. The maximum vorticity of the vortex ring over the wing moves from the leading edge to the trailing edge with the downward flapping. On the other hand, the vorticity of the LEV decays with downward flapping.

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