The aerodynamics and sensory physiology of range fractionation in the cereal filiform sensilla of the cricketGryllus bimaculatus

Summary

• 1. The deflection amplitude of cereal filiform hairs of different lengths was determined for various frequencies of air-current (Fig. 4). The angle threshold of the sensory neuron was then determined (Fig. 7). Both the deflection amplitude and angle threshold were length dependent.

• 2. In order to estimate the hair deflection, the spring stiffness of the hair supporting apparatus was measured. The stiffness varies by 10² depending on the hair length (Fig. 1).

• 3. Based on the mechanical properties measured, the deflection amplitude of hair in the sinusoidal air-current is estimated by means of a numerical solution of the equation of motion. The effect of the boundary layer due to the viscosity of air was taken into account. Long filiform hairs deflect more sensitively than short ones in the frequency range below 100 Hz (Fig. 4).

• 4. We compared a theoretical estimation of hair deflection with direct observation under relatively strong stimuli. The estimation and the observation are in good agreement (Fig. 5).

• 5. By using the estimated value of hair deflection and the sensory threshold (Shimozawa and Kanou 1984), we were able to determine the angle threshold of the sensory neuron. The angle threshold determined is 0.002° in long filiform hairs. In addition to this low angle threshold, sensory neurons with sensitivity only to fast deflection but not to slow deflection were revealed in association with the short filiform hairs (Fig. 7).

• 6. When oscillating, the filiform hairs show a self-damping property. The spring stiffness seems to be optimized in relation to the length and moment of inertia to give a critical-damping condition (Table 1).

• 7. The hair length, the spring stiffness, and the rate of relaxation of sensory neuron show a specific combination in a single sensillum. The specific combination underlies the range fractionation of the filiform sensilla.