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Summary 1. A review is given of the optical and architectural analogies between cholesteric liquid crystals and certain insect cuticles (Coleoptera: Scarabaeidae). Earlier observations on the optical properties (reflexion of circularly polarized light and high form optical rotation) are confirmed and extended. Both cholesteric liquid crystals and lamellate cuticle have helicoidal structure (Fig. i). Even though their chemistry and physical states are very different, we are justified in making the analogy, since their optical properties depend primarily on the pitch of their helicoidal architecture. 2. The unusual optical properties were located for the first time in the outer 5 to 20 μ of the exocuticle. This layer is transparent and has regular spacings in the range required for interference colours according to Bragg's law. Among Scarabaeid beetles which show interference colours, we distinguish two types of outer exocuticle. (i) Optically active cuticles which reflect circularly polarized interference colours; show high angles of form optical rotation in transmitted light; and anomalous form birefringence perpendicular to the cuticle surface (reversible by deproteinization). (2) Optically inactive cuticles which show none of the above properties and in which the form birefringence is parallel to the cuticle surface. In the electron microscope the ultrastructure of these two types of outer exocuticle is clearly different. 3. All of the optically active species reflect left hand circularly polarized light, irrespective of the wavelength of the reflected colour. They therefore appear dark when viewed through a right hand circular analyser. The sense of reflected circularly polarized light does not reverse at higher wavelengths as recorded by previous workers. (A simple treatment is given for combinations of various wavelengths with retardation plates of varying values, as used in circular analysers.) We confirm earlier reports that the sense of reflected circularly polarized light is of the opposite sense to the transmitted light. 4. Using monochromatic light we have measured the anomalous dispersion with wavelength of the magnitude of optical rotation for various optically active cuticles. The dispersion curves change from negative values at lower wavelengths to positive values at higher wavelengths, and cross the zero optical rotation axis at a wavelength (AQ) corresponding to the interference colour of each sample. There is reasonable agreement between A0 and the interference colour calculated from ultrastructural evidence and by comparison with interference filters of known wavelength. A dispersion curve measured for a combined sample of two cuticles with different dispersion curves showed that the resultant is an algebraic summation of the two component curves. 5. We present the first experimental verification of existing mathematical treatments of anomalous form optical rotatory dispersion curves. Although these treatments were derived for cholesteric liquid crystals, they give a reasonable fit to our measu...

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