Dynamic properties of orientation to turbulent air current by walking carrion beetles
A beetle orients its walk with respect to the direction of even a brief air current lasting only a few seconds. In a discontinuous current, the degree of orientation increases with the proportion of time during which the air flows. In the absence of wind, a walking beetle generates its own opposing airstream, the velocity of which is rhythmically modulated. Sinusoidal modulation of the wind direction elicits approximately sinusoidal changes in walking direction. The amplitude of the following response at frequencies below 0.1 s-1 is almost as large as the amplitude of the change in wind direction. As the stimulus frequency increases, the response amplitude decreases. The Bode plot shows that at frequencies above 1 s-1 the phase of the following response lags by almost 360 &deg;. The dynamic temporal behaviour of the wind-orientation system is comparable with that of a control mechanism having low-pass characteristics of at least second order. It follows that rapid changes in wind direction do not immediately affect orientation behaviour. The wind-orientation system is thus well adapted to the air currents recorded in nature which, at the level of a walking beetle, can change very quickly and erratically in both direction and velocity.
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