Arthropods that feed on specific structures within a plant such as the phloem are collectively termed selective tissue feeders in contrast to defoliators. Examination of gas exchange processes provides a common biological basis for comparison of herbivore impact on different plants or plant parts. Various abiotic interactions between spider mite feeding damage and gas exchange rates have also been examined. Nonselective tissue removal of either partial or entire leaves by arthropods with grinding mandibular surfaces are included under defoliation studies. Arthropods that feed on the mesophyll and palisade layers of leaves include a wide variety of taxonomic groups including Acari, Diptera, Homoptera, Hemiptera, Lepidoptera, and Hymenoptera as some of the more important examples. The effects of defoliation appear to be quite different from those caused by selective tissue feeder damage. Feeding by mesophyll feeders including spider mites, leaf miners, leafhoppers, and other miscellaneous taxa has consistently reduced photosynthetic rates per unit area.