That would be the very characteristic mines of a very small insect called a leaf miner. The adult leaf miner lays an egg just under the epidermis of the leaf. When it hatches, the tiny larvae feeds on the inner tissue of the leaf in a pattern that look like serpentine patterns. If you hold a leaf up to the light, you can probably see the larvae, as well as dark specks of frass.

Leaf miners can belong to the same families as flies, moths, wasps, or beetles. It really doesn't matter who the adult is; the signature symptoms are the same.