Halteres stabilize standing posture in crane flies.

All flies (Diptera) possess halteres, reduced hindwings that are active mechanosensory organs. Halteres are necessary for flight control in all winged flies, and many taxa also oscillate their halteres during non-flight behaviors. The functions of these oscillations and their resulting sensory input are unknown. We tracked the large halteres of crane flies (Tipulidae) during free, standing behaviors and found that halteres oscillate during postural adjustments, independent of wing movements. In the absence of body movements, haltere oscillations are driven by passive displacement of the legs. Displacement of the haltere itself is insufficient to initiate oscillations. Perturbation experiments showed that haltere sensory feedback informs stabilization behaviors by the legs, in which freely behaving flies with ablated halteres exhibited reduced postural stability against perturbations and required longer recovery times. The halteres provide inertial information across multiple kinds of stabilizing behaviors, whether the fly uses them to remain stationary on a moving substrate or to move its body into a more stable position after a perturbation. Taken together, these results show that halteres can inform leg-based behaviors, revealing a critical sensory role for halteres beyond their established function in flight.