Have you ever noticed these white lines on leaves? What insect could be making these? It’s a tiny fly: the leaf-miner fly, or in Dutch: de mineervlieg.
The adult female fly makes a tiny hole in the leaf tissue with her ovipositor, used for drinking the plant’s sap, but also for laying its eggs inside the hole. (Because the male flies don’t have this ovipositor, they drink sap from the holes made by the females.) Once the egg is hatched, the larva begins eating its way through the leaf. This can be very damaging to the plant, as it obstructs the photosynthesis of the leaves.
Before becoming an adult fly, the larva evolves three times and then becomes a pupa, attaching itself underneath the leaf or letting itself fall and crawling under the ground to pupate. Not long after, the adult fly emerges.
Worldwide there are about 2500 species of the leaf-miner fly known. The larvae are about 2 to 3 mm long, the adult at most 6 mm.
I found this very interesting video of its life cycle online.