Weighted haltere and imposed haltere stroke reduction tethered flying Drosophila kinematics

In the true flies (Diptera) the hind wings have evolved into specialized mechanosensory organs known as halteres, which are sensitive to gyroscopic and other inertial forces. Together with the fly’s visual system, the halteres direct head and wing movements through a suite of equilibrium reflexes that are crucial to the fly’s ability to maintain stable flight. As in other animals (including humans), this presents challenges to the nervous system as equilibrium reflexes driven by the inertial sensory system must be integrated with those driven by the visual system in order to control an overlapping pool of motor outputs shared between the two of them. Here, we introduce an experimental paradigm for reproducibly altering haltere stroke kinematics and use it to quantify multisensory integration of wing and gaze equilibrium reflexes. We show that multisensory wing-steering responses reflect a linear superposition of individual haltere-driven and visually-driven responses, but that multisensory gaze responses follow a non-linear integration logic.

Methods
Video recordings of flying, tethered Drosophila were made using two synchronized high-speed cameras (TS3 or IL5, Fastec) at 1000 or 2000FPS. These captured the left and right aspects of each fly. Haltere and wing stroke joint-angle timeseries were produced using the DeepLabCut (https://github.com/DeepLabCut/) and/or DLTdv (http://biomech.web.unc.edu/dltdv/) software packages. Body axis length and haltere bulb cross-sectional area were estimated using custom MATLAB scripts.

Concurrent recordings were made with an industrial camera (Chameleon3, FLIR) at 100FPS. Head yaw and wing downstroke envelope timeseries were produced using the flyalyzer (https://github.com/michaelrauscher/flyalyzer) software package.