<p>The aerodynamics of insect flight is not well understood despite it has been extensively investigated with various techniques and methods. Its complexities mainly have two folds: complex flow behavior and intricate wing morphology. The complex flow behavior in insect flight are resulted from flow unsteadiness and three-dimensional effects. However, most of the experimental studies on insect flight were performed with 2D flow measurement techniques whereas the 3D flow measurement techniques are still under developing. Even with the most advanced 3D flow measurement techniques, it is still impossible to measure the flow field closed to the wings and body. On the other hand, the intricate wing morphology complicates the experimental studies with mechanical flapping wings and make mechanical models difficult to mimic the flapping wing motion of insects. Therefore, to understand the authentic flow phenomena and associated aerodynamics of insect flight, it is inevitable to study the actual flying insects. </p> <p>In this thesis, a recently introduced technique of schlieren photography is first tested on free jet of match rockets with a physics based optical flow method to explore its potential of flow quantification of unsteady flow. Then the schlieren photography and optical flow method are adapted to tethered and feely flying houseflies to investigate the complex wake flow and structures. In the end, a particle tracking velocimetry system: Shake the Box system, is utilized to resolve the complex wake flow on a tethered house fly and to acquire some preliminary 3D flow field data</p>