Dynamics of arthropod filiform hairs.: V.: The response of spider trichobothria to natural stimuli

Trichobothria are cuticular filiform hairs of arachnids, which respond to air movements. In the wandering spider, Cupiennius salei, about 900 trichobothria form characteristic groups on the pedipalps and legs and are used to detect and localize flying insect prey. We analyse the response of the receptor cells of metatarsal trichobothria of different lengths (200-800 mu m) to both natural and synthetic stimuli to understand what the single receptor can contribute to the detection of natural signals. (i) The receptor cells are not spontaneously active. Their response to hair deflection is strictly phasic. The airflow produced by a tethered flying fly (Calliphora sp.) elicits a physiological response up to a distance of 55 cm. The response strength, however, strongly depends on the orientation and position of the Ay. The receptor cells encode a wide range of airflow velocities (l mm s(-1) to l ms(-1)). The time-course and frequency content of the strongly fluctuating air movements characterizing a fly signal are well represented in the spiking pattern of the receptor cells. There is almost no adaptation to a broad-band noise stimulus but strong adaptation to monofrequency stimulation. The presence of a platform below the spider (corresponding to its dwelling plant) strongly enhances the response of the trichobothrium due to its 'amplifying' effect on flow velocity Without the platform, the sensory space of the trichobothrium decreases from 55 cm to 15 cm. (ii) With the hair directly coupled to the stimulating device, threshold curves do not depend on hair length (200-800 mu m). Physiologically and irrespective of their length, the trichobothria are all broadly tuned to a frequency range between about 50 and 100 Hz where threshold deflection angles are ca. 0.1 degrees and sometimes as small as 0.01 degrees. This contrasts the length dependence of the mechanical frequency tuning of trichobothria. Above threshold the bandpass characteristics of the cell response is much more pronounced than at threshold. (iii) Due to their low thresholds, their tuning and the highly phasic character of their response, the receptor cells of the trichobothria are well suited to detect and encode air particle movements produced by flying prey I,ow-frequency background air movements (wind) are filtered by both the mechanical and the physiological properties of the trichobothria.