Spider Joint Hair Sensilla: Adaptation to Proprioceptive Stimulation

The study focused on the mechanosensitive hair sensilla of the spider Cupiennius salei, particularly those at the tibia–metatarsus joint, which likely served a proprioreceptive function. It was found that these hairs, which number 20 on the tibia and 75 on the metatarsus, interlock during joint flexion and are highly sensitive to deflection in the natural direction of movement. The torque resisting this deflection was significantly lower in the natural direction compared to others. The torsional restoring constant was approximately 10^-10 Nm rad^-1 up to a deflection angle of 30°. The study recorded sensory action potentials during joint movements at natural step frequencies (0.3–3 Hz), showing that the rate of action potentials correlated with the velocity of hair deflection. These findings indicated that the joint hair sensilla were morphologically, mechanically, and physiologically adapted to their role in proprioreceptive stimulation during spider locomotion.