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

Abstract

For flightless arboreal arthropods, moving from the understory into tree canopies is cognitively and energetically challenging because vegetational structures present complex three-dimensional landscapes with substantial gaps. Predation risk and wind-induced perturbations in the canopy may further impede the movement process. In the Australian stick insect Extatosoma tiaratum, first-instar nymphs hatch on the forest floor and disperse toward tree canopies in the daytime. Here, we address such vertical movements and associated sensory cues in E. tiaratum nymphs. Newly hatched nymphs ascend with high endurance, travelling >100 m within 60 minutes. Navigation toward open canopies is underpinned by negative gravitaxis, positive phototaxis, and visual responses to vertically oriented contrast patterns. Nymphal E. tiaratum also use directed jumping to cross air gaps, and respond to tactile stimulation and potential threat with a self-dropping reflex, resulting aerial descent. Post-hatch dispersal in E. tiaratum thus consists of visually mediated displacement both on vegetational structures and in the air; within the latter context, gliding is then an effective mechanism enabling recovery after predator- and perturbation-induced descent. These results further support the importance of a diurnal niche, in addition to the arboreal spatial niche, in the evolution of gliding in wingless arboreal invertebrates. Summary statementTo effectively disperse into canopies, ground-hatched stick insects use gravity and visual cues to navigate during midday, jump to cross air gaps and respond to threat or perturbation with self-dropping.