Abstract: Utilizing high-speed schlieren photography and particle-tracking-velocimetry, the wake flow of tethered houseflies is investigated. The high-speed schlieren photography is implemented on tethered houseflies inside an air container with a stable vertical temperature gradient to visualize the disturbed wake flow from the insects. The resulting photography images were then processed with the physics based optical flow method to derive the light-path averaged flow velocity. Additionally, the state of the art: Shake-the-Box system is implemented on a tethered housefly to measure the volumetric flow field in the wake of the insect, revealing interesting flow behavior and structures that can also be observed and correlated to the schlieren photography images. Comparing the dimensionless velocity magnitude of the wake flow from the two experiments, a good qualitative agreement is reached, suggesting the viability of high-speed schlieren photography in investigating the wake flow of small insects. Furthermore, the high-speed schlieren photography is successfully applied on a housefly that is taking off from the ground, visualizing the disturbed wake flow on the freely flying insect that is challenging to visualize with other methods.

Key words: Insect fight , · Schlieren photography , · Shake-the-Box , · Wake flow