The foot of a banana slug generates a propulsive force against surfaces via mucus secretions that act as both a lubricant and an adhesive.

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Snails and slugs only have one body part in contact with the ground at any one time, which raises the question: how do they develop a net force to produce propulsion? Most animals have multiple legs with some functioning as stationary supports while others move forward to act as future stationary supports. Initially, it may seem that using only one structure to both hold onto the ground and move over it is a paradox; however, snails and slugs evolved the answer long ago.

Though the precise physical mechanism has yet to be revealed, snail and slug mucus is apparently used to both lubricate the snail's movement over surfaces and to facilitate the transfer of adhesive force to the surface. Only a "viscoelastic" fluid could have both properties since ordinary fluids are either viscous or elastic.

Snails and slugs move by producing a pedal wave that travels from one end of the body to the other (depending on the species of snail). The interwave regions are stationary (relative to the ground) and exert force against the mucus layer which is then transmitted to the ground; for the interwave region, the mucus acts as an elastic or adhesive material capable of transmitting mechanical force. On the other hand, wave regions travel over the ground and do not exert force on it; in this case, the mucus acts as a viscous lubricant. There is mounting evidence that the mucus is of great importance when the snail/slug is crawling across vertical or inverted surfaces, but that it may not be entirely necessary for motility on flat, level surfaces. Edit Summary


"The locomotion of terrestrial gastropods is driven by a train of periodic muscle contractions (pedal waves) and relaxations (interwaves) that propagate from their tails to their heads. These ventral waves interact with a thin layer of mucus secreted by the animal that transmits propulsive forces to the ground." (Lai et al. 2010:3920)

"The pedal waves, along with the adhesive properties of the mucus layer, create the stable thrust needed for the animal to crawl on substrates at various inclinations. In most terrestrial gastropods...two types of mucus are generated during locomotion: a thick pedal mucus secreted through the glands located underneath the animal’s mouth and a much thinner mucus secreted through multiple glands located on the sole of the animal. It is clear that the mucus plays a key role in the ability of gastropods to lubricate their sliding motion while adhering firmly to the substrate at various inclinations...the viscoelastic nature of the mucus provides a mechanism for the generation of the net friction needed to propel the animal forward using direct pedal waves...the non-Newtonian nature of pedal mucus can result in the generation of a net propulsive force using direct pedal waves." (Lai et al. 2010:3921)

"Strong experimental evidence supports the hypothesis that the viscoelastic nature of this mucus leads to the generation of asymmetric shear forces under the foot with a net forward component that propels the animal forwards...the non-Newtonian nature of the pedal mucus can result in the generation of a net propulsive force when the waves are not lifted from the substrate...a mechanism of stick-and-release, similar to the one used in caterpillars, is consistent with the yield-stress rheology of the mucus." (Lai et al. 2010:3931)

Journal article
The mechanics of the adhesive locomotion of terrestrial gastropods

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California Banana SlugAriolimax californicusSpecies

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