Living plants are to food of an endless variety of organisms. When this concerns vertebrate animals this can be called macro-herbivory, and there usually hardly exists a recognisable relation between plant and grazer. This is different for the innumerable small organisms (insects, mites, nematodes, fungi etc.) the feed and damages living plants. Between plants and the “micro-herbivores” there generally exists a mutual relationship. Almost invariably the herbivore is specialised on a small group of related plant species, and often te plants react in a specific way on the herbivory. Traces of the activity of mico-herbivores therefore often are quite characteristic, certainly when seen in connection with the plant host.
Leaf mines are galleries or cavities that are eaten by insect larvae in the tissue of leaves. Generally the traces of the activity of these miniature mine workers are constant in shape. Remarkably, the plant hardly shows a recognisably reaction to the feeding activity of the miner. Sometimes some discolouration may occur, or callus, wound tissue is formed, but formation of new tissue does not occur. In many galls this is very different
Plants often react on the activity of phytophages by abnormal growth, often also discolouration. Often these changes are not determined only by the biochemical mechanism of the plant, but also by substances that are given off by the herbivore. When the herbivore induces the formation of novel, protective of nutritive tissue, one can speak of a gall.
However, most authors use a much wider definition of a gall: a recognisable modification in the aspect of a plant, derived from the interaction between the plant and a phytophagous organism. Many more definitions exist (Williams,