These larvae are known as 'leaf miners' and they leave distinctive marks on the leaves they inhabit. Some leaf miners take long, winding paths through their leaves. The resulting 'serpentine' trails are often filled with the frass (droppings) of the larvae, which turns the trails black behind the leaf miners. To me, this seems like a big drawback of living inside your food.
|The trail left in a rose leaf by a leaf miner, possibly a larva of the moth Stigmella rosaefoliella.|
|A leaf miner is visible through a 'window' in a common morning glory (Ipomoea purpurea) leaf.|
|A morning-glory leaf miner (Bedellia somnulentella) in a common morning glory (Ipomoea purpurea) leaf.|