How the velvet worm squirts slime

TL;DR

This study uncovers the elastohydrodynamic mechanism behind the velvet worm's rapid slime-squirting motion, revealing how passive elastic properties enable its unique defense and prey capture strategy.

Contribution

It provides the first quantitative analysis of the oscillatory motion of the velvet worm's slime jet and explains it through an elastohydrodynamic instability model.

Findings

Oscillatory motion occurs at 30-60 Hz frequency.

The motion results from an elastohydrodynamic instability.

Passive elastic properties are harnessed for rapid slime ejection.

Abstract

The rapid squirt of a proteinaceous slime jet endows the ancient velvet worms (Onychophora) with a unique mechanism for defense from predators and for capturing prey by entangling them in a disordered web that immobilizes their target. However, to date neither qualitative nor quantitative descriptions have been provided for this unique adaptation. Here we investigate the fast oscillatory motion of the oral papillae and the exiting liquid jet that oscillates with frequencies $f\sim 30-60$ Hz. Using anatomical images, high speed videography, theoretical analysis and a physical simulacrum we show that this fast oscillatory motion is the result of an elastohydrodynamic instability driven by the interplay between the elasticity of oral papillae and the fast unsteady flow during squirting. Our results demonstrate how passive strategies can be cleverly harnessed by organisms, while suggesting future oscillating micro-fluidic devices as well as novel ways for micro and nano fiber production using bioinspired strategies.