Reception of medium vibration by thoracal hairs of caterpillars ofBarathra brassicae L. (Lepidoptera, Noctuidae)

Summary

• 1. A single sensory cell innervates each of the 8 filiform hairs on the thorax ofBarathra brassicae caterpillars.

• 2. It is not spontaneously active nor does it respond to a maintained deflection of the cuticular hair. It responds to changes in hair position away from or back to resting position in a purely phasic fashion (Fig. 1).

• 3. At a constant duration of hair motion for reaching the final deflection the number of occurring impulses (y) depends on hair-deviation-angle (x) according to the power functiony=6.76·x°^0.38.

• 4. The impulse rate (y) drops with time (t) after onset of step stimuli according toy = 1174.9·t^−0.67 (Fig. 3).

• 5. If one plots the first interpulse interval against angular velocity of hair motion a sigmoid shaped response curve results with its steepest slope in the range 50-100°/s (Fig. 6).

• 6. The directional characteristic of the sensory cell shows a cadioid-like profile of sensitivity (Fig. 15) with its sensitivity maximum for deflections in the direction where the dendrite inserts at the hair base.

• 7. The threshold of impulse initiation by sine stimulation drops from 2.5° hair deflection from resting position at 40 Hz to 0.5° deflection at 150 Hz and than stays at a nearly constant level up to 1000 Hz. The response breaks abruptly off around 1000 Hz (Fig. 9). Bristles on the thoracic segments show a minimum threshold curve with a sensitivity maximum of 0.3° at 150 Hz (Fig. 10).

• 8. At sine stimulation the sensory cell of the filiform hairs adapts only slowly. At a stimulus frequency of 100 Hz the response begins to decrease after 5 s and vanishes after ca. 20 s.

• 9. The impulses occur at a constant phase of the sine wave at a given stimulus frequency. At frequenciesf< 40 Hz every half sine wave is responded to, at frequenciesf≳= 40 Hz the response occurs only at hair motion to the ventral side. In the frequency range 40 Hz ≦f≦ 200 Hz every sine wave is responded to by one impulse.

• 10. Response to sine wave onset with different rise-time has an initial phasic or “quasi-phasic” peak of discharge rate (Fig. 13).

• 11. The medium vibration in the near-field of a flying wasp is an adequate stimulus for the filiform hair. A flying wasp can stimulate the sensory cell from a distance of maximally 70 cm.