insect that burrows between the leaf lamina.

Merriam-Webster Online Dictionary
leaf miner (noun)
any of various small insects (as moths or dipteran flies) that in the larval stages burrow in and eat the parenchyma of leaves
leaf miner (Wikipedia)
Leaf miner damage to a horse chestnut tree
Leaf with minor miner damage
Tomato with leaf miner damage
Leaf mines by the moth Phyllocnistis hyperpersea on a Persea borbonia leaf
Leaf miner trail on a fallen leaf in a Gondwana cool temperate rainforest. Note the initial thin width of the insect trail, becoming wider as the insect grows while it navigates around the leaf. Cryptocarya foveolata from Cobark Park, Barrington Tops, Australia

A leaf miner is any one of a large number of species of insects in which the larval stage lives in, and eats, the leaf tissue of plants. The vast majority of leaf-mining insects are moths (Lepidoptera), sawflies (Symphyta, close relatives of wasps), and flies (Diptera), though some beetles also exhibit this behavior.

Like woodboring beetles, leaf miners are protected from many predators and plant defenses by feeding within the tissues of the leaves, selectively eating only the layers that have the least amount of cellulose. When attacking Quercus robur (English oak), they also selectively feed on tissues containing lower levels of tannin, a deterrent chemical produced in great abundance by the tree.

The pattern of the feeding tunnel and the layer of the leaf being mined is often diagnostic of the insect responsible, sometimes even to species level. The mine often contains frass, or droppings, and the pattern of frass deposition, mine shape, and host plant identity are useful to determine the species and instar of the leaf miner. A few mining insects feed in other parts of a plant, such as the surface of a fruit.

It has been suggested that some patterns of leaf variegation may be part of a defensive strategy employed by plants to deceive adult leaf miners into thinking that a leaf has already been preyed upon.

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