Leaf Miners

Leaf miner is a term used to describe the larvae of many different species of insect which live in and eat the leaf tissue of plants. The vast majority of leaf-mining insects are moths (Lepidoptera) and flies (Diptera), though some beetles and wasps also exhibit this behavior. Like Wood boring beetles, leaf miners are protected from many predators and plant defenses by feeding within the tissues of the leaves themselves, selectively eating only the layers that have the least amount of cellulose. In attacking Quercus robur they also selectively feed on tissues containing low tannin levels, which the tree has produced in greater abundance as a defense. Source: Wikipedia.com