They are the zebra butterfly caterpillars. They eat a lot of leaves, but they don't harm the tree. They really stink if you touch them too. Cheryl

Oops! My mistake, not the zebra, it's the black and yellow swallowtail. Cheryl

I went back and found that one had gotten back on the leaves of the lemon tree. Actually, I'm not so sure it's not a slug rather than a caterpillar. It's about 2 inches or so long and the coloring is actually sort of mottled, with dark gray, light gray and some light brown. The only places I've seen them are on lemon leaves.

Giant swallowtail caterpillars look different at various times. At one stage they are brown/gray and with their shape they do look sort of slug like. They also look like a bird dropping to fool predaters.

Here's a picture I took of one on my Satsuma citrus. If it looks anything like this Cheryl is right.

If so -- lucky you! They make beautiful large butterflies.

Here is a link that might be useful: Giant swallowtail life cycle ...

That is one ugly caterpillar!