Dynamics of arthropod filiform hairs. III. Flow patterns related to air movement detection in a spider ( Cupiennius salei KEYS.)

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Abstract
In previous studies we related the mechanical properties of spider trichobothria to a generalized mathematical model of the movement of hair and air in filiform medium displacement receivers. We now present experiments aimed at understanding the complex stimulus fields the trichobothrial system is exposed to under natural conditions. Using the elicitation of prey capture as an indicator and a tethered humming fly as a stimulus source, it has been shown that the behaviourally effective range of the trichobothrial system in Cupiennius salei Keys, is approximately 20 cm in all horizontal directions. Additionally, the fly still elicits a suprathreshold deflection of trichobothria while distanced 50-70 cm from the spider prosoma. To gain insight into the fluid mechanics of the behaviourally effective situation we studied: first, undisturbed flow around the spider in a wind tunnel; second, background flow the spider is exposed to in the field; and third, flow produced by the tethered flying fly. 1. The motion of air around a complex geometrical structure like a spider is characterized by an uneven distribution of flow velocities over the spider body. With the flow approaching from the front, both the mean and r.m.s. values are higher above the legs than above the pro- and opisthosoma; the velocity in the wake behind the spider, however, is markedly decreased. The pattern of these gradients is more complicated when the spider’s horizontal orientation is changed with respect to the main flow direction. It introduces asymmetries, for exmple, increased vortical, unsteady flow on the leeward compared with the windward side. 2. Sitting on its dwelling plant and ambushing prey in its natural habitat, the background air flow around Cupiennius is characterized by low frequencies (< 10 Hz), a narrow frequency spectrum, and low velocities (typically below 0.1 m s -1 with less than 15% r.m.s. fluctuation). 3. The distinctive features of a biologically significant air flow (for example, that produced by the humming fly) seem to be a concentrated, i.e. directional unsteady, high speed flow of the order of 1 m s -1 , and a relatively broad frequency spectrum containing frequencies much higher than those of the background flow. For a spider, sitting on a solid substrate (a leaf of a bromeliad, for example), air speed just above the substrate increases and thus provides higher sensitivity when compared to a spider in a orb web, which is largely transparent to the airflow. The flow patterns stimulating the ensemble of the trichobothria contain directional cues in both the undisturbed flow and the flow due to prey cases.

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