Banana slugs stick to the substrate, thanks to their slime. Photo: Franco Folini.
Watch a banana slug move across the leaf litter, or up a fencepost, and you’ll be surprised by the grace with which this shell-less snail can move its slimy little body. Banana slugs move via muscular waves, called pedal waves—contractions and relaxations of the body. The slugs’ occasional arboreal lifestyle (they can be found perched slimily in trees) is made possible by the mucus they secrete, which acts like an adhesive. This mucus helps them move, but limits the distance they can travel in a day; slugs can produce only so much slime.
A slug’s muscular wave starts at its rear end and propagates forward. Slugs are incapable of moving backwards. First the tail moves forward. The wave travels along the body, inching the body forward. Finally, the head moves forward. At any given time, part of the body is contracted and possibly raised above the ground (the wave), and part of the body is relaxed and in contact with the ground (the interwave). The outside rim of the body is always in contact with the ground. The parts of the body in contact with the ground adhere to the ground, thanks to mucus.
Slugs produce two kinds of mucus: a thick mucus, secreted by a gland underneath the mouth, and a thin mucus, secreted along the length of the body. The mucus is made primarily of water, mixed with salts and a glycoprotein, which forms a gel network and gives the mucus its elastic properties. There is some debate about the role of mucus in locomotion. Mucus definitely provides adhesion, which allows slugs to climb trees. Mucus may also help to generate thrust, as it stiffens and deforms.
Mucus is beneficial to slugs in other ways: it prevents slugs from drying out, and its taste and texture deter predators. However, making all this mucus is energetically expensive for slugs. They cannot produce unlimited slime, and the amount they produce may limit the distance they can travel.