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

Trichobothria are cuticular filiform hairs of arachnids, which respond to a ir movements. In the wandering spider, Cupiennius salei, about 900 trichobo thria form characteristic groups on the pedipalps and legs and are used to detect and localize flying insect prey. We analyse the response of the rece ptor cells of metatarsal trichobothria of different lengths (200-800 mu m) to both natural and synthetic stimuli to understand what the single recepto r can contribute to the detection of natural signals. (i) The receptor cell s are not spontaneously active. Their response to hair deflection is strict ly phasic. The airflow produced by a tethered flying fly (Calliphora sp.) e licits a physiological response up to a distance of 55 cm. The response str ength, 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) t o l ms(-1)). The time-course and frequency content of the strongly fluctuat ing air movements characterizing a fly signal are well represented in the s piking pattern of the receptor cells. There is almost no adaptation to a br oad-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 'am plifying' effect on flow velocity Without the platform, the sensory space o f the trichobothrium decreases from 55 cm to 15 cm. (ii) With the hair dire ctly coupled to the stimulating device, threshold curves do not depend on h air length (200-800 mu m). Physiologically and irrespective of their length , the trichobothria are all broadly tuned to a frequency range between abou t 50 and 100 Hz where threshold deflection angles are ca. 0.1 degrees and s ometimes as small as 0.01 degrees. This contrasts the length dependence of the mechanical frequency tuning of trichobothria. Above threshold the bandp ass characteristics of the cell response is much more pronounced than at th reshold. (iii) Due to their low thresholds, their tuning and the highly pha sic character of their response, the receptor cells of the trichobothria ar e well suited to detect and encode air particle movements produced by flyin g prey I,ow-frequency background air movements (wind) are filtered by both the mechanical and the physiological properties of the trichobothria.