Colours and metallic sheen in beetle shells

Torben Lenau and Michael Barfoed
Department of Manufacturing Engineering and Management
Technical University of Denmark
Building 424 DK - 2800 Lyngby, Denmark
Phone: +45 4525 4811 Fax: +45 4525 4803

The metallic and lucid colours of beetle shells can be explained by at least two different internal shell structures with different light reflecting properties. The multilayer stack is composed of layered pairs with different refractive indices and reflects all light types. The Bouligand structure resembling a liquid cholesteric crystal with a twist of layer directions reflects circular polarized light. The colours of the transparent structures are structural colours. In the multilayer structure they are caused by interference and in the Bouligand structures the colours come from diffraction. The shells are mostly made of cross linked chitin (a polysaccharid) and proteins. The products of the structure are released by living epidermal cells in a fluid deposition zone where after the products self assembly to a layered mesh. That the structure is formed by self assembly gives the hope that similar structures with the same optical properties also can be formed artificially without templates or other specific processes only mastered by nature. Key-words: optical aesthetics, interference colour, liquid crystal, chitin, self assembly.