This is the trail of a Leafminer. With that said, the term Leafminer refers to a lifestyle, instead of to a specific creature. Certain beetles, flies, moths, and even some wasps fit into this category. Leafminers lay their eggs inside a leaf, in the tasty part in between the two outer layers. The eggs hatch and the larvae eat their way through the leaf. As the larva grows, the mine gets wider, until finally the larva eats its way through the leaf, and moves into its adult stage.
While the larva is inside the leaf, it is protected from most predators, although there are certain wasps that specialize in hunting leafmining larvae.
This particular Leafminer mine is from a fly in the genus Liriomyza. Leafminers are identified by the size and shape of the mine that they leave.