![]() The relationship between body pitch angle and flight speed has been measured only under steady-state conditions, which precludes the identification of system dynamics and consequently the description of the time course of speed responses. While the body pitch rotational dynamics have previously been investigated, the pitch-to-speed transfer function has so far not been explored in detail. In the case of flight speed responses, these represent the dynamics of body pitch changes and how they consequently affect flight speed. Importantly, the identification of control laws requires a quantification of a system's plant properties. This method allows abstracting from the underlying complex physiological processes, while still capturing the fundamental high-level sensing and control principles. It is amenable to a behavioural system identification approach, in which the behaviour is modelled as a feedback controller and the inner dynamics are subsequently ‘reverse engineered’. The involvement of two sensory modalities for a stereotyped behavioural response offers a suitable model system to explore functional principles underlying multi-modal flight control mechanisms. The pitch angle θ is converted to v g via the pitch-to-speed plant. The sensor S, representing the halteres and the visual system, measures the rotational speed which is then integrated to get the measured pitch angle θ meas. ![]() P represents the plant for pitch rotation. In the inner pitch control loop (inset), the pitch controller C sets the pitch angle θ to the desired value θ ref by producing a torque M around the pitch axis. An outer visual loop sets the desired pitch angle ( θ ref) according to the error between the preferred speed v p and the measured ground speed v g. Hierarchical flight speed control scheme. It represents the set point pitch angle of an inner pitch control loop, which controls the body pitch angle-and hence flight speed-based on haltere signals (and possibly additional visual cues). An outer speed control loop compares a set point retinal slip speed (the ‘preferred’ retinal slip speed ) with current visual input and generates as output a body pitch angle. Taken together, the speed control system can be conceptualized as two nested feedback control loops ( figure 1). ![]() The important role of body pitch for the control of flight speed implies the involvement of a second sensory modality, namely mechanosensory feedback from the halteres, which sense the angular velocity of the body from the resulting Coriolis forces acting on them. Second, active control of the direction of the stroke-averaged aerodynamic force vector, as recently explored in free-flight studies, may play an additional role. First, the aerodynamic forces generated by the flapping wings depend on their motion relative to the surrounding air and therefore on flight speed. While tethered studies revealed a constant angle between the stroke-averaged flight force and the body, the free-flight situation manifests itself as somewhat more complex. These changes in pitch are brought about by subtle changes of the wing stroke pattern, suitable to modulate pitch torque. The principal mechanism by which a fly accelerates is to pitch its body nose down to point the flight force more forward, not unlike a helicopter. The underlying physiological mechanisms that lead up to these simple control laws are complex and remain only partly understood. These were shown to depend directly on the velocity (measured in metres per second) of the moving patterns according to an amazingly simple proportional control law (likewise for lift responses see ). By presenting flies with short bouts of regressive (back-to-front) moving patterns, the backward drift an insect might visually perceive when hit by a gust of wind was simulated to elicit compensatory forward acceleration responses. More recently, the visuomotor control mechanisms underlying flight speed responses were studied in fruitflies using a real-time controlled virtual reality (VR) environment termed TrackFly. Because these reflexive speed responses are highly stereotyped and easily elicited in freely flying insects, they have served as a powerful behavioural paradigm to study the underlying sensorimotor mechanisms for many decades (mosquitoes honey bees flies ). Fruitflies, like many other insects, control their ground speed from the visually perceived motion of objects passing by (optic flow) (for reviews, see ).
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