Donovan knows his invertebrates

Dear Uncle Matthew and Aunt Alice:

We all know that our garden variety of snails leave a slimy trail on leaves when they move around. Buy why, when they traverse a cement sidewalk, is the track not a continuous one? It's almost as if the snail, upon reaching the sidewalk, is shocked and jumps, landing a short distance away, leaving slime, then jumps again, leaving a track that is slime, no slime, slime, no slime, etc. What mechanism leaves the no-slime segment of their track?

-- Ned in Point Loma

As that musty troubadour and scholar of invertebrate science Donovan once said, "First there is a mountain, then there is no mountain, then there is." He was talking about the snail upon his garden gate-- or maybe a caterpillar; the lyrics are a little hazy. But the snail upon your footpath moves the same way. Snails and slugs secrete a protective mucus from their undersides. Slugs move in continuous contact with whatever they're crawling on and leave an unbroken slime trail. Garden snails move like some caterpillars, hunching up their midsections and drawing their back ends forward. Thus the dotted line.