Wings and halteres act as coupled dual-oscillators in flies

The mechanics of Dipteran thorax is dictated by a network of exoskeletal
linkages which, when deformed by the flight muscles, generate coordinated
wing movements. In Diptera, the forewings power flight, whereas the
hindwings have evolved into specialized structures called halteres which
provide rapid mechanosensory feedback for flight stabilization. Although
actuated by independent muscles, wing and haltere motion is precisely
phase-coordinated at high frequencies. Because wingbeat frequency is a
product of wing-thorax resonance, any wear-and-tear of wings or thorax
should impair flight ability. How robust is the Dipteran flight system
against such perturbations? Here, we show that wings and halteres are
independently-driven, coupled oscillators. We systematically reduced the
wing length in flies and observed how wing-haltere synchronization was
affected. The wing-wing system is a strongly-coupled oscillator, whereas
the wing-haltere system is weakly-coupled through mechanical linkages
which synchronize phase and frequency. Wing-haltere link acts in a
unidirectional manner; altering wingbeat frequency affects haltere
frequency, but not vice-versa. Exoskeletal linkages are thus key
morphological features of the Dipteran thorax which ensure wing-haltere
synchrony, despite severe wing damage.