title: Structural and physical evidence for an endocuticular gold reflector in the tortoise beetle, Charidotella ambita.
abstract: Charidotella ambita offers a unique opportunity for unambiguously locating its gold reflector by comparing the structure of reflecting and non-reflecting cuticle of the elytron and pronotum. Using light microscopy and TEM, the reflector was located underneath the macrofiber endocuticle just above the epidermis. The reflector is a multilayer comprising up to 50 bilayers alternating high and low density layers parallel to the surface of the cuticle. It is chirped, i.e., showing a progressive decrease in layer thickness from approximately 150 nm-100 nm across its depth. The high density layers in contact with the endocuticle fuse to the last macrofiber when the reflector is interrupted by a trabecula, demonstrating their cuticular nature. Simulated reflectance spectra from models of the multilayer matched the reflection spectra measured on the major gold patch of the elytron of living specimens. Previous reports in adult insects exhibiting metallic colors located their reflector in the upper strata and structures of the cuticle, i.e., epicuticle, exocuticle, scales and hairs. Thus, the endocuticular location of the reflector in C. ambita (and other tortoise beetles) appears unique for adult insects. Gold reflection appears in C. ambita only when the synthesis of the macrolayer endocuticle is complete, which may take up to 2 weeks. The development of the gold reflector coincides with the start of mating behavior, possibly suggesting a signaling function in conspecific recognition once sexual maturity has been reached.
authors.affiliation: Université Libre de Bruxelles, Evolutionary Biology and Ecology, C.P. 160/12, Avenue F.D. Roosevelt 50, B-1050 Brussels, Belgium. Electronic address: jmpastee@ulb.ac.be.
authors.forename: Jacques M
authors.initials: JM
authors.lastname: Pasteels
authors.affiliation: University of Namur (UNamur), Department of Physics, Physics of Matter and Radiation (PMR), Rue de Bruxelles 61, B-5000 Namur, Belgium. Electronic address: olivier.deparis@unamur.be.
authors.forename: Olivier
authors.initials: O
authors.lastname: Deparis
authors.affiliation: University of Namur (UNamur), Department of Physics, Physics of Matter and Radiation (PMR), Rue de Bruxelles 61, B-5000 Namur, Belgium; University of Exeter, College of Engineering, Mathematics and Physical Sciences, Stocker Road, Exeter EX4 4QL, United Kingdom. Electronic address: sebastien.mouchet@unamur.be.
authors.forename: Sébastien R
authors.initials: SR
authors.lastname: Mouchet
authors.affiliation: Smithsonian Tropical Research Institute, Apartado 0843-03092, Balboa, Ancon, Panama City, Panama. Electronic address: windsordm@gmail.com.
authors.forename: Donald M
authors.initials: DM
authors.lastname: Windsor
authors.affiliation: KU Leuven, Zoological Institute, Naamsestraat 59, Box 2466, B-3000 Leuven, Belgium. Electronic address: johan.billen@kuleuven.be.
authors.forename: Johan
authors.initials: J
authors.lastname: Billen
mesh_terms: D060105:Animal Shells / Q000648:ultrastructure; D000818:Animals; D001517:Coleoptera / Q000648:ultrastructure*; D003116:Color; D046529:Microscopy, Electron, Transmission
keywords: Chrysomelidae; Elytra; Gold reflector; Structural color; Tortoise beetle; Ultrastructure