Sclerotized oothecae from the praying mantids, Stagmomantis carolina and Tenodera sinensis, and cocoon silks from the moths, Antheraea mylitta, A. polyphemus, Hyalophora cecropia, Bombyx mori, Plodia interpunctella and Ephestia cautella, were subjected to solid state 13C nuclear magnetic resonance and chemical analysis. The oothecae are composed of protein (83% of the wet wt), water (7–8%), diphenolic compounds (6%) and inorganic salts (2–3%). The major diphenols extracted in cold dilute acid are N- acylated derivatives of dopamine and norepinephrine, including N- malonyldopamine , N- acetyldopamine and N- acetylnorepinephrine . The acid-extractable diphenols account for <1% of the total diphenols estimated by solid state NMR analysis, indicating that nearly all of the diphenols in mantid egg cases are involved in covalent cross-links or adducts with oothecal proteins. The silks are lower than the oothecae in diphenolic compounds (0.1–1%), slightly higher in protein (88–90%) and about the same in water (7–8%) and inorganic salt content (3–4%). A higher proportion of the total diphenols is acid extractable from the silk (4–56%) than from the oothecae, and they include 3,4-dihydroxyphenylacetic acid, 3,4-dihydroxybenzoic acid, 2,5-dihydroxybenzoic acid, 3,4-dihydroxyphenylalanine, 3,4-dihydroxybenzaldehyde and N- acetylnorepinephrine . Subsequent metabolism of diphenols in these structures probably involves oxidation to quinonoid derivatives, followed by formation of covalent bonds between diphenol ring or side chain carbons and nucleophilic groups in the structural proteins.