Possible uses of the layered structure found in the scales of Hoplia coerulea (Coleoptera) Jean-Pol Vigneron, Marie Rassart, Priscilla Simonis, Jean-Francois Colomer and Annick Bay Research Center in Physics of Matter and Radiation (PMR), University of Namur (FUNDP), 61 rue de Bruxelles, B-5000 Namur Belgium; ABSTRACT The male of the beetle Hoplia coerulea is known for its spectacular blue-violet iridescence. The blue coloration is caused by the presence of an interesting photonic structure inside the scales which cover the dorsal parts of the insect’s body. This structure can be described as the stacking of chitin plates supporting arrays of parallel rods. The change of colour of this structure with humidity is investigated, as well as its response to some other external conditions, such as mechanical strain. Keywords: Coleoptera, Hoplia coerulea, caerulea, structural color, hygrometry, hygrochrome 1. INTRODUCTION From the optical point of view, living species have access to only a limited choice of “materials”. Not that the chemistry is limited: the variety of the organic biochemical compound is virtually infinite, but the refractive index of these compounds tend to converge into a narrow interval. One of the highest refractive index values of a transparent material involved in structural coloration of any living organism is that of guanine, used in many iridophores, including scales of fishes. A primary consequence of this limitation is that living organisms have evolved structures that tend to be complex, in order to fine-tune the visual effects, in spite of the limited choice of material properties at their disposal. In Hoplia coerulea, an intermediate refractive index is reached by an intimate mix of air and chitin, in an appropriate proportion, and this compound material is used together with solid chitin to produce a weakly iridescent Bragg mirror. A secondary consequence is that these structures are able to fulfil other functions than producing a static visual appearance. It so happen that the beetle, at the contact of water, assumes a passive emerald green coloration, water being able to infiltrate the hydrophillic structure of the scales. The mechanical deformation of the structure is also possible, and this can give rise to some visual response, at least in hypothetical biomimetic copies using mechanically softer materials. The objective of the present work is to analyze the structural coloration of the beetle in order to appreciate the advantage of the complex structure for adapting to influences, such as humidity. In section 2, a word is said of the beetle biology. Section 3 describe the insect’s scales ultrastructure and provides a theoretical analysis to explain the blue color. Section 4 reports on the observation that the insect changes color with humidity and section 5 describes several possibilities of deformations of the structure that could lead to some color change. Further author information: (Send correspondence to J.P. Vigneron) J.P.V.: E-mail: jean-pol.vigneron@fundp.ac.be, Telephone: +32 (0)81 724711 M.R.: E-mail: marie.rassart@fundp.ac.be, Telephone: +32 (0)81 724711 P.S.: E-mail: priscilla.simonis@fundp.ac.be, Telephone: +32 (0)81 724711 J.F.C.: E-mail: jean-francois.colomer@fundp.ac.be, Telephone: +32 (0)81 724710 A. B.: E-mail: annick.bay@student.fundp.ac.be, Telephone: +32 (0)81 724710 Biomimetics and Bioinspiration, edited by Raúl J. Martín-Palma, Akhlesh Lakhtakia, Proc. of SPIE Vol. 7401, 74010B · © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.825465 Proc. of SPIE Vol. 7401 74010B-1