Leaf Blotch Miner Moth

A leaf miner is the larva of an insect that lives in and eats the leaf tissue of plants. The vast majority of leaf-mining insects are moths (Lepidoptera), sawflies (Symphyta) and flies (Diptera), though beetles and wasps also exhibit this behavior. (Wikipedia).
Last photo of this series shows a leaf that was attacked by a larva of a moth like this one. Note how it is thin at the starting (in the right side of the photo), and widens as the larva makes its way through the leaf.