Dynamics of arthropod filiform hairs. II. Mechanical properties of spider trichobothria ( Cupiennius salei Keys.)

Abstract

Adults of the wandering spider Cupiennius salei (Ctenidae) have 936 ( ± 31 s.d.) trichobothria or filiform hairs on their legs and pedipalps. This is the largest number of these air movement detectors recorded for a spider. The trichobothria are 100-1400 μm long and 5-15 μm wide (diameter at base). Many of them are bent distally pointing towards the spider body. Their feathery surface increases drag forces and thus mechanical sensitivity by enlarging the effective hair diameter. Typically, trichobothria are arranged in clusters of 2-30 hairs which increase in length towards the leg tip. The trichobothria’s mechanical directionality is either isotropic or it exhibits a preference for air flow parallel or perpendicular (from lateral) to the long leg axis. These differences are neither due to the distal bend of the hair nor to the bilateral symmetry of the cuticular cup at the hair base but to the spring supporting the hair. Different directional properties may be combined in the same cluster of hairs. Trichobothria are tuned to best frequency ranges between 40 and 600 Hz depending on hair length. Because, with increasing hair length, absolute mechanical sensitivity changes as well, the arrangement of hairs in a cluster provides for a fractionation of both the intensity and frequency range of a stimulus, in addition, in some cases, to that of stimulus direction. Boundary layer thickness above the spider leg in oscillating airflow varies between about 2600 μm at 10 Hz and 600 μm at 950 Hz. It is well within the range of hair lengths. In airflow perpendicular to the long leg axis particle velocity above the leg increases considerably as compared to the free field. The curved surface of the cuticular substrate has therefore to be taken into account when calculating hair motion. The experimentally measured properties of hair and air motion were also determined numerically using the theory developed in the companion paper (Humphrey et al. Phil. Trans. R. Soc. Lond . B 340, 423-444 (1993)). There is good agreement between the two. Short hairs are as good or better velocity sensors as long hairs but more sensitive acceleration sensors. In agreement with most of our measurements optimal hair length is not larger than boundary layer thickness at a hair’s best frequency. Best frequencies of hair deflection and of ratio a (maximum hair tip displacement:air particle displacement) differ from each other. The highest measured value for ratio a was 1.6. In only 22% of the cases hair tip displacement exceeded air particle displacement. Hair motion is insensitive to changes in hair mass as shown by the numerical comparison of a solid and a hollow hair.