Bob Gore has reminded me that now is a good time to look for evidence of leafminers. Leafminers tunnel between the tough outer layers of a leaf, eating their way through the tender inner portions and can be the immature stage of any variey of insects--flies, moths, beetles, etc . There are different "mining" techniques and it looks like this is evidence of a serpentine leaf mine. As Bob says,
Notice that the little tracks tell the story of the larvae from egg to the next life stage, often pupae. The narrow ends of the tracks show where the eggs were inserted and later, as the tracks progress, when the larvae were small. As they grew you can see the increases in size in the width of the tracks as they ate more and grew larger. Finally, on the opposite end the larvae emerged.