The uses of mucus

The shine on a path after rain is mostly water. The continuous silver line behind a snail is mucus, laid down on purpose. Pedal mucus, from glands in the foot, does several jobs at once. It lubricates the sole so the animal can cross a rough brick. It is adhesive enough to let a snail climb vertical glass and stay there. It fills small gaps, so a sharp edge becomes a smoother road. And it is a trail that other snails can read.

The trail costs the animal. Mucus is mostly water, but the rest is protein and carbohydrate, and a small body that lays a long line is spending food to do it. Some species follow a fresh trail, their own or another snail’s, because the second trip costs less. Others follow trails to a mate, a roost, or a place that has already proved damp. A trail is a slow message left in the open, useful for an hour or a night, then dried to a faint mark.

There is more than one recipe. The mucus under the foot is not the same as the thinner coat on the skin, or the defensive secretion some species produce when pinched, which can be sticky, bitter, or colored. The epiphragm is mucus too, sometimes stiffened with calcium, pulled across the shell opening like a storm door. None of these is a single commodity called “snail slime” in the way a label implies. They are secretions for particular jobs.

People do collect and filter some of these secretions for cosmetics. That trade is separate from the animal crossing your wall. The trail in front of you is a piece of locomotion that happens to be a liquid. It will dry on its own, and the next rain will take what remains. There is no need to touch it to confirm that it is wet.