DYNAMICS OF ARTHROPOD FILIFORM HAIRS .4. HAIR MOTION IN AIR AND WATER

Filiform cuticular hairs responding to movements of the surrounding me dium are widespread mechanoreceptors in both terrestrial and aquatic a rthropods. In this study we compare the motion response of such hairs to sinusoidal oscillations in air and in water, by applying a previous ly developed mathematical model (Humphrey et al. 1993). In addition to the physics underlying stimulus uptake in the two media, the effects of changing the values of various geometrical and mechanical parameter s characterizing a hair in motion are studied. The differences in the hair motion response for an identical hair in air and water are indeed considerable. They are mainly due to the difference in dynamic viscos ity of the two media, which entails an overriding contribution of the 'virtual (added) mass' to the effective inertia of the hair in water. The large effective moment of inertia for a hair in water enables it t o perform as a displacement sensor at low frequencies and as an accele ration sensor at high frequencies. Hairs in air function as displaceme nt, velocity and acceleration sensors over different frequency ranges. 'Boundary layer thickness' is smaller by a factor of 0.22 in water th an in air. As a consequence of this and of the proportionately larger drag force in water, we may expect shorter hairs among the receptors s erving the uptake of hydrodynamic stimuli than among those dealing wit h aerodynamic stimuli. This, in fact, seems to be the case. Whereas ha ir length greatly influences a hair's mechanical sensitivity both in a ir and water, hair diameter is of only minor importance in water. Our results point to several important differences between the hairs in th e two media regarding their 'tuning'. Mainly due to the importance of virtual mass, resonance frequencies for the same hair are much lower i n water than in air. Whereas hairs in air are more sensitive to change s in hair length regarding resonance frequency, hairs in water are the more sensitive regarding the amplitude of motion. Underlining the gen eral tendency found for the geometrical hair parameters, changing the spring stiffness and torsional damping influences a hair's tuning much more in air than in water. Our analysis suggests that the evolutionar y pressures on both the morphological and the mechanical properties ch aracterizing arthropod (or any other) filiform hair receptors must hav e been bigger in air than in water.