ABSTRACT Autofluorescent daily growth zones in locust resilin provide a detailed record of cross-linking of the polypeptide chains, since the fluorescence is caused by the tertiary cross-links, dityrosine and trityrosine. Environmental temperature changes are the main cause of autofluorescent daily growth layers in resilin, fluorescence increasing with temperature. Microdensitometric scans show that this rhythmicity is superficial, in harmony with the optical and mechanical isotropy of resilin. The molecular structure and properties of resilin appear to be homeo-statically controlled by the cells. With suitable rearing conditions, resilin ligaments were produced with half or twice as many fluorescent growth layers as usual, or with them almost absent. Evidence for a weak circadian rhythm influencing cross-linking is presented. A marked drop in fluorescence followed by a sharp rise occurs at adult emergence and is metabolically triggered. A metabolic control also operates to make the resilin grown before the moult less fluorescent than resilin grown after it. Further evidence for the very rapid cross-linking of resilin during its formation is discussed.