Terrestrial slug uses a vertical bridge of mucus to descend rapidly from heights

Abstract

Among the terrestrial slugs, mucus secretions are used for a variety of purposes that typically require the mucus to be excreted from the slug’s foot along surfaces or excreted onto epithelial linings. However, terrestrial slugs are also capable of producing mucus threads that are anchored to surfaces but extended into the open air. These threads are akin to spider silk and may serve as an alternative form of locomotion, allowing for rapid movement between surfaces of different heights. Herein, we report on the production of mucus threads by a terrestrial slug for the first time in Australia, and its possible use as a tethered vertical bridge for locomotion. Field observations revealed a striped field slug, Lehmannia nyctelia, descended from a single thread of mucus that was not only tethered to the top of a fence line but extended in front of the slug to the ground where it was tethered also to gravel. Our observations suggest that slugs produce tethered bridge lines of mucus, over which they can traverse in order to descend from heights. This is in contrast to previous reports of mucus thread production among this and other terrestrial slugs, in which individuals only hang freely from one end of a thread that is attached to a surface above. We suggest that producing a mucus thread that is tethered on both ends may be adaptive by stabilising individuals and preventing excessive sideways movements during descent, particularly when it is windy, which could lead to premature thread breakage.