The swaying behavior of Extatosoma tiaratum: motion camouflage in a stick insect?

Several species of stick insects sway from side to side when blown by wind. Although anecdotal evidence suggests this is a camouflage strategy to resemble wind-blown vegetation, this behavior has never been experimentally investigated. We evaluated the responses of female Macleay’s Spectre (Extatosoma tiaratum) to wind cues and quantified the degree of conspicuousness of the insect against a background of natural wind-blown plants. Wind alone is sufficient to initiate but not maintain swaying behavior. Sway frequency declined over time in all trials, but the number of sways was significantly higher in variable wind conditions compared with constant wind conditions. This suggests the insects pay attention to environmental cues and adjust their behavior accordingly. Consistent with this view, in trials involving plants in which the insects did not sway, plant motion was significantly stronger than at times when insects were observed to sway. The movement of insects at these times was indeed consistent in the frequency domain with the movement of wind-blown plants. This study provides evidence that the swaying movement of E. tiaratum is quantitatively similar to that of wind-blown plants. Further studies will be required to identify functional benefits of this behavior and whether it represents a form of motion crypsis, motion masquerade, or a combination of the two.