Communication by substratum vibration in the New Zealand tree weta, Hemideina femorata (Stenopelmatidae: Orthoptera)

Abstract

The propagation of vibrations along the trunk and branches of a manuka tree, generated in response to the impact of a steel ball-bearing on the trunk, was measured with an accelerometer. The impact generated bending waves which travelled along the trunk and into the branches. Close to the point of impact the waveform was dominated by a damped oscillation at 518 Hz; as the bending wave progressed away from the point of impact the frequency of the dominant waveform increased. Beyond 200cm the waveform became increasingly complex and a small-amplitude, high-frequency component progressively preceded the main wave. Branching points also induced complex waveforms, particularly where branches lay at a large angle to the trunk. Stridulating wetas also generated bending waves in the tree at a frequency close to that generated by the ball-bearing, as well as at a higher frequency of 7.5kHz. The acoustic frequency of stridulation peaked at 0.8 and 3.4kHz. Records from nerves serving the vibration-sensitive subgenual organs showed that wetas can detect oscillations at 1 kHz at 0.015 m s-2. A stridulating weta placed on the same log as a preparation in which the nerve from the subgenual organ was monitored generated oscillations well above the threshold for detection.