The aerodynamic force on flying insects result from the vortical flow structures that vary both spatially and temporally throughout flight. Due to these complexities and the inherent difficulties in studying flying insects in a natural setting, a complete picture of the vortical flow has been difficult to obtain experimentally. In this paper, Schlieren, a widely used technique for highspeed flow visualization, was adapted to capture the vortex structures around freely flying hawkmoth (Manduca). Flow features such as leading-edge vortex, trailing-edge vortex as well as the full vortex system in the wake was visualized directly. Quantification of the flow from the Schlieren images was then obtained by applying a physics-based optical flow method, extending the potential applications of the method to further studies of flying insects.