Small insects actively respond to updrafts by modulating their wing and body kinematics

Small insects utilize atmospheric flows to facilitate appetitive and migratory behaviors. In the convective and nocturnal boundary layers, these insects may experience high-speed updrafts which can present significant aerodynamic challenges. While it is known from field observations that small insects resist these updrafts, how they do so while maintaining flight stability is unknown. Here we investigate the flight behavior of two 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 0.433 m s-1 upward flow within a vertical wind tunnel. We used high-speed 3D photogrammetry to capture flight trajectories and wing and body kinematics. Both species maintained a constant vertical flight velocity in quiescent conditions and in the updraft by maintaining a consistent beat frequency while significantly reducing their stroke amplitude (and thus saving energy), though with some size-specific differences. The smaller fungus gnats kept the dorsal portion of their stroke more so than the larger fruit flies, thereby maintaining the lift-generating clap-and-fling mechanism which is generally required at the lower Reynolds numbers at which they operate. Both species also maintained a similar relative flow velocity, which combines wing tangential velocity and animal velocity with respect to the free stream. Finally, the updraft somewhat affected flight stability in the fungus gnat (as seen in its more sinuous flight trajectories) but not in the fruit fly, which did not change the sinuosity of its trajectories in the updraft. These findings provide a bridge between large- and small-scale studies of small insects in the atmosphere.