If the snail's foot never lifted off the ground, then the animal would need the special mucus to achieve enough force to push itself across a horizontal surface. But if, however, a part of the snail's foot lifted up as the waves traveled through it then the animal could produce enough thrust to push itself forward even without the special physical properties of the mucus. Lifting part of its foot reduces the amount of friction the snail has to overcome to move.
This would be similar to a caterpillar, which lifts the middle part of its body up and stretches forward as it moves.
Lai tracked the movement of snails on a horizontal glass surface, using a high-resolution camera placed underneath. She measured the back-and-forth waves of muscle contractions by tracking the movement of distinctive speckles on the foot.
She also used a laser to measure the distance that the foot waves moved up-and-down off the glass surface. To measure forces, she placed the snails on a gel and measured how the gel deformed as the snails moved across it. She already knew how much force it took to deform the gel, so she was able to calculate how much force it took to produce the observed deformation. Based on their new measurements, the researchers found that the snails didn't require the special mucus to travel horizontally.
The lifting of the snail's foot as the waves travelled through it produced enough force to propel the animal even without the slime. The slime's adhesive ability still plays a crucial role, however, in allowing the animal to crawl upside down and up vertical surfaces. Apart from changing how we view snail locomotion, the work has practical applications as well. A number of other research groups have been making robots that move like snails.
Lai is currently working on a more detailed mathematical model of snail movement that could help refine these robots. A group at MIT has already built snail robots that can move on vertical walls and upside down, and researchers from Tohoku University in Japan are building an endoscope, a tool that doctors use to look inside the body, that would move like a snail.
Snail-like robots are less complicated to build as "there are no legs sticking out," said Lai, and their crawling motion allows them to traverse a wide variety of surfaces. Lai, a mechanical engineer, was the first author of the study published last October in the Journal of Experimental Biology. In November, she presented an updated mathematical model of snail locomotion at the annual conference of the American Physical Society.
Lai started working on snail locomotion in as an undergraduate, when she spent a summer doing research with Lasheras, the paper's senior author. Snail slime is secreted by glands located all over the body, though the largest, and that responsible for the silvery trails, is at the front of the foot.
When resting, snails produce enough mucus to glue themselves to a substrate and create a membranous seal called an epiphragm covering the opening of the shell.
The seal dries gradually and can become quite crispy, while the snail inside stays moist. They take big strides when they walk but nobody can see those little, secret footsies and tiny legs because they hide them inside the shell. Just like landing gear on a plane, they can retract. To answer this question — or ask a new one — email lastword newscientist.
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Martin Gorst Why do snails sometimes leave dotted trails?
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