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A leaf miner is a type of insect whose larva lives inside a leaf, consuming plant tissue within, but leaving the surfaces intact, so that part or all of the leaf is hollowed out. This leaves a pale, semi-transparent area on the leaf. There are a great many different types of leaf miner, and the term describes a form of insect behavior rather than a taxonomic group, or family, of insects. Leaf mining is found in several different families of insect, including moths, sawflies, flies and beetles. This mode of feeding could be a strategy for avoiding predators by remaining concealed within the leaves of the food plant, rather than exposed on the surface.
Adult leaf mining insects lay their eggs either on the surface of the leaf — in which case the larvae, when they hatch, bore into the leaf — or inside the leaf, so that the larvae hatch inside the leaf. Mines produced by insect larvae can be divided into serpentine types — which are long, often winding, tunnels that become wider as the larva grows — and blotches — which are irregularly shaped patches of hollowed out leaf. Usually, mining begins on mature leaves, but larvae boring into newly developing leaves can lead to the formation of galls: prominent areas of swollen leaf tissue formed around the larva by the multiplication or enlargement of undifferentiated cells from the still-growing leaf. Depending on the species, the larva will either bore its way out of the leaf to pupate or do so inside the leaf, the adult cutting its way out when it emerges.
A wide variety of plants are affected by leaf mining insects, including many of economic importance, but it is relatively rare for extensive crop damage to occur as overall plant health is not usually severely affected. Ornamental plants, however, can be rendered less attractive by the activities of these insects. In some cases, they can carry plant diseases.
Leaf miner damage can be minimized by the use of insecticides or, on a smaller scale, removal and destruction of damaged leaves. At least one plant, Caladium steudneriifolium, native to Ecuador, appears to have evolved its own defense: light colored patterns, known as variegation, on the leaves resemble leaf miner damage. Leaves that lack these patterns are attacked much more frequently by leaf miners than those that have either natural variegation or markings painted-on for experimental purposes. It seems that these markings discourage the leaf miner moths from laying their eggs on leaves which have them, and it has been speculated that variegation, which is found in many plants, might have evolved for this purpose.