The significance of the wide range of phototoxins, occurring in many plant species, with respect to their role in insect herbivory and on insects, is not fully understood. The types of compounds include polyacetylenes, quinones, furanocoumarins, tryptophan- and tyrosine-derived alkaloids and are distributed throughout some of the major families of flowering plants. Efforts are being made to determine the mechanisms of cellular toxicity of these compounds at the cellular and the organismal levels.