Canopy parkour: movement ecology of post-hatch dispersal in a gliding nymphal stick insect (Extatosoma tiaratum)

Abstract

For flightless arboreal arthropods, to move from understory toward tree canopies is cognitively and energetically expensive, because various vegetational structures form complex threedimensional landscapes with air gaps. The exposure to predation and environmental perturbations in the canopy space further obstruct the movement process. How to effectively move through the canopy space thus requires effective navigation tactics and movement skills. In the Australian stick insect Extatosoma tiaratum, the first instar nymphs hatch on the forest floor and disperse toward tree canopies in the daytime. Here, we address the movement ecology of dispersal in the E. tiaratum nymphs under controlled laboratory conditions. We found the newly hatched nymphs ascend with high endurance, accomplishing 100+ m within 60 minutes.Their navigation toward open canopies is generally directed by negative gravitaxis, positive phototaxis and tactic response to vertically oriented contrast patterns. We also found nymphal E. tiaratum use directed jumping to cross air gaps and respond to tactile stimulations and potential threat with self-dropping reflexes, actions which both lead to aerial descent. In sum, the post-hatch dispersal in E. tiaratum consists of various visually mediated movements both on vegetational structures and in air, within which context gliding is an effective mechanism for height recovery after predator-and perturbation-induced descent. These results further support the fundamental role of a diurnal temporal niche, in addition to spatial niche, to the evolution of gliding in wingless arboreal invertebrates.