Vibration sensitivity of pretarsal slit sensilla in the spider leg

Summary

• 1. The two single slit sensilla on the pretarsus of the spider leg (Cupiennius salei Keys.) are vibration sensitive. Due to their arrangement they are also stimulated effectively by active downward movement of the pretarsus which may result from muscular activity directly or from an increase of the hemolymph pressure (Figs. 1, 2, 3).

• 2. The slits are not tuned to a restricted frequency range. Their threshold displacement was determined electrophysiologically from 0.01 Hz to 1 kHz and found to be higher than in the slits of the metatarsal organ by roughly two orders of magnitude. The threshold decreases only slightly up to about 40 Hz but much more steeply at higher frequencies. The minimal threshold value for displacement within the tested frequency range was found at 1 kHz and measured 2 × 10⁻⁵ cm. The minimal threshold value for acceleration found at 0.01 Hz was 0.3 × 10⁻⁹ cm/s² (Fig. 4a). Thresholds vary with different preset tensions in the cuticle surrounding the slits (Fig. 5).

• 3. The shapes of the threshold curves are similar for dorsoventral and lateral displacement of the pretarsus (Fig. 4b). Values for lateral displacement are at most 3 dB higher than those for dorsoventral displacement.

• 4. As in other slit sensilla the frequency response of the pretarsal slits follows a power functiony(f) =f ^k (Fig. 7). The constantk varies between 0.39 and 0.44, reflecting receptor properties intermediate between those of a frequency-independent amplitude receptor (k = 0) and those of a velocity receptor (k = 1). Beyond 10 Hz the response of the receptor saturates. Phase locking between stimulus and response is observed up to about 100 Hz (stimuli 10 dB above threshold).