Cuticle reflectivity and optical activity in scarab beetles: the role of uric acid

Abstract

The iridescent cuticle of certain Rutelino scarab beetles, which is a form optically active and selectively reflects circularly polarized light, incorporates an NH 4 OH -extractable component The ultraviolet absorption spectrum of this component, together with its chromatographic and refractive properties, identify it as uric acid (2,6,8-trihydroxypurine). All species of Plusiotis examined have uric acid in their reflecting layers, as do several species of Anoplognathus. Plusiotis resplendens has a reflecting layer with a uric acid volume fraction of 0.7, P . optima a volume fraction of 0.6. The reflecting layer of P . resplenden s has an anticlockwise helicoidal architecture, the optical thickness of the helicoidal p itch being such that it constructively interferes with visible light wavelengths. An anticlockwise helicoid constructively interferes with only the left circularly polarized component of incident light, right circularly polarized light being transmitted without attenuation. P. resplendens has a 1.8 /xm thick unidirectional layer embedded within the helicoid which functions as a perfect halfwave retardation plate for wavelength 590 nm . This halfwave plate enables the helicoidal reflector in this species to reflect both left and right circularly polarized components of incident light. After passing through the halfwave plate, transmitted right circularly polarized light becomes left circularly polarized ; this light is now reflected and emerges from the cuticle right circularly polarized, after passing back through the halfwave plate. Consequently the total reflectivity of circularly polarized incident light is greater in P. resplendens than in any other species examined; the plate also reduces multiple internal reflexions. Interferometric analysis of the refractive properties of the helicoidal reflectors in species of Plusiotis showed that the ordered incorporation of uric acid increases the birefringence of the system by a factor of five times, e.g. the in tact birefringence of the unidirectional layer of P . resplendens is 0.166 at wavelength 560 nm ; after uric acid extraction the birefringence is reduced to 0.034. As the coefficient of reflexion of a helicoidal reflector is directly proportional to the birefringence of the individual planes comprising the helicoid, beetles incorporating uric acid into their reflecting surfaces reflect circularly polarized light far more efficiently than beetles lacking uric acid. Refractive index values for a single multicomponent plane of the helicoid have been summarized as a biaxial indicatrix, with the Z axis tilte dat 45° to the plane of the epicuticle. Beetle reflecting layers which incorporate uric acid have twenty times greater optical rotatory power compared with reflecting layers lacking this component. Mathematical treatments dealing with helicoidal reflectors predict the form optical rotatory power to be a function of the square of the birefringence, which is in agreement with the experimental observations. To enable uric acid to have the optical effects mentioned above, an epitaxial incorporation into the helicoidal framework is necessary. Although uric acid is a common cytoplasmic reflecting material in arthropods, this is the first record of its presence in an extracellular (cuticular) reflector.

Polarized iridescence of the multilayered elytra of the Japanese jewel beetle,Chrysochroa fulgidissima

“…The measured reflectance spectra were modelled with refractive index values that varied between 1.60 and 1.70 (figures 12 and 13), where the latter value is similar to the maximal refractive index values derived for scarab beetle cuticle by Caveney [18] and for C. raja by Noyes et al [10]. The modelling showed that the peak wavelength and amplitude of the reflectance spectra are quite sensitive for the range of the refractive index.…”

“…[17]). The jewel beetle's multilayers are presumably homogeneous, causing only linear polarization, whereas circular polarization is a dominant feature in scarab beetles [18,20,23,24]. For scarab beetles, Caveney [18] concluded that the reflective layers had a high concentration of uric acid, causing a high refractive index value of about 1.70.…”

“…The jewel beetle's multilayers are presumably homogeneous, causing only linear polarization, whereas circular polarization is a dominant feature in scarab beetles [18,20,23,24]. For scarab beetles, Caveney [18] concluded that the reflective layers had a high concentration of uric acid, causing a high refractive index value of about 1.70. Durrer & Villiger [5] noticed for the elytra of the buprestid E. gigantea that the distribution of the high refractive material was patchy, but without further evidence they stated that the material would be melanin, with a refractive index of 2.0.…”

“…Reflecting multilayers in insect cuticle have been extensively studied in various beetle species [18][19][20][21][22]. In transmission electron micrographs, the multilayers are recognized as a stack of layers with alternating high and low electron density.…”

Coexistence of both gyroid chiralities in individual butterfly wing scales of Callophrys rubi

“…45), photonic properties do not provide the need for differentiation between these two bicontinuous forms. In the context of the enantiomeric composition and the question if the molecular chirality of chitin contributes to the selection of the enantiomeric type, it is useful to consider a structurally unrelated chiral manifestation of chitin: the Bouligand structure observed in a variety of beetles (17,19,48); in these beetles, which are strongly circularly polarizing (19,49,50) and where the chirality is widely believed to result from a process analogous to a cholesteric phase formation, only the LH enantiomer is observed. Interestingly, in the analogous phase found in the cellulose structure of a particular plant, Pollia condensata, both enantiomers are observed (18).…”