The Kinematics and Flight Behaviors of Fruit Flies and Fungus Gnats in a Vertical Wind Tunnel

Insects utilize atmospheric flows for long range migration and dispersal. As these beneficial flows are located several kilometers above the ground, tiny insects must use convective upwellings as a transport mechanism to reach them. However, insect flight behaviors and mechanisms in these vertical upwellings are not well understood, prompting questions about how tiny insects control their flight in vertical flows. Here we investigate the flight behavior of two tiny insect species, the 3 mm fruit fly (Drosophila melanogaster) and the 2 mm fungus gnat (Lycoriella ingenua), while exposed to quiescent air and to a steady 0.5 m/s upwards flow (characterized via PIV) within a vertical wind tunnel. We use high-speed 3D photogrammetry, recording at 300 Hz (106 events) and 4700 Hz (43 events) at different magnifications, to capture flight trajectories and wingbeat kinematics, respectively. In the upwards flow as compared to quiescent air, both species exhibited an increased body pitching frequency, likely in an effort to maintain stability. With flow, flight trajectory sinuosity increased for the smaller, more weakly flying fungus gnats but decreased for the larger, stronger fruit flies. These results provide insight into how tiny insects may adapt their flight to a convective upwelling.