The responses of trochanteral hair plate sensilla in the cockroach to periodic and random displacements

Two types of sensillum have previously been described in the trochanteral hair plate of the cockroach with differences in physical size, extracellularly recorded impulse amplitude and response to displacement. We have now examined the dynamic behaviour of the two types of sesillum in response to sinusoidal and random displacements. Type I sensilla have a frequency threshold of about 6 Hz, below which there is no response to sinusoidal displacement. Above the frequency threshold they behave as velocity sensors with a 90° phase lead of response over displacement and increasing response with increasing frequency. Type II sensilla have no detectable frequency threshold and behave as position sensors at low frequencies. With increasing frequency they display a phase lead over the stimulus but it nerver exceeds about 60°. Random stimulation followed by spectral analysis of the input-output relations give similar descriptions to the sinusoidal results except that low frequency position sensitivity is revealed in the Type I sensilla. The coherence function for both types of sensillum is low, indicating that the linear frequency response functions are poor approximations to the total behaviour of the sensilla. Earlier investigations of these receptors in a range of insects suggested that they are primarily involved in the control of joint positions during very slow movements. However, the present results, together with other recent work, indicates that they are also important in the control of rapid movements.