The basis of structural colour variation in two species of Papilio

An account is given of the ultrastructure and optical properties of the structural colour scales of Papilio karna and P.palinurus . The scales are Urania‐type multilayers of 10 lamellae separated by air‐spaces. A theoretical model is elaborated to account for the results and it is concluded that colour variation between species is achieved through control of film thickness. It is proposed that the surface structure functions as an anti‐reflection coating. A generalised functional basis for the structural difference between Urania‐type and Morpho‐type scales is suggested.