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An alternative explanation is thin-film interference in an additional ca. 180 nm-thick surface layer covering the epicuticle and the observed photonic structure. Such a layer could plausibly correspond to a wax and cement secretion deposited via pore canals after cuticle formation, as reported in insects [ 81, 82 ]. Simulations incorporating this layer yield a broad reflectance peak near 420 nm at normal incidence( Fig. S4a), in close agreement with the measured peak position. The presence of such a thin film could also explain the negligible intensities of detectable co-CP light reflection in the visible range if an underlying Bouligand structure with sufficient in-plane birefringence were present, since the thin-film interference can mask this signal( Fig. S4b). Furthermore, no such distinct superficial layer was observed in SEM or TEM images.
4 Conclusions
Unlike butterflies and beetles, the visual appearances of species from orders such as dipterans have not attracted much attention despite their striking colourful iridescence. In this work, the structural colours of C. vicina and L. richardsi blow flies’ abdomens were investigated. The colourations displayed by C. vicina and L. richardsi areblueandgreen, respectively, with their reflectance spectra at normal incidence peaking at 437 ± 8 nm and 537 ± 6 nm. They are primarily due to photonic structures in the cuticles of these insects, as revealed by electron microscopy, consisting of one-dimensional periodic multilayers that produce metallic iridescence. In C. vicina, we cannot entirely exclude the possibility of thin-film interference from a superficial epicuticular layer over a likely Bouligand structure imaged by TEM. This interference could contribute to the observed reflectance properties and help explaining the negligible intensities detected in co-CP reflectance spectra. However, no such thin-film layer was directly observed in our SEM or TEM analyses. Both species appear bright and chromatic with respect to the visual sensitivities of blow flies and three selected predators. These sensations are mainly due to the high absorption by the SWS and VSWS photoreceptors, inthecaseofC. vicina and of the LWS, MWS and SWS photoreceptors, in the case of L. richardsi. These results suggest that structural colouration in blow flies may play roles beyond visual signalling to conspecifics, potentially influencing predator-prey dynamics.
Acknowledgments
The authors thank Serge Berthier( Institut des NanoSciences de Paris), Nicolas Reckinger( UNamur), Corry Charlier( UNamur) and Caroline De Bona( UNamur) for technical help during measurements. This research used resources of the“ Plateforme Technologique de Calcul Intensif( PTCI)” 1 of UNamur, which is supported by the Belgian National Fund for Scientific Research( FRS-FNRS)-” Fonds de la Recherche Fondamentale Collective”( FRFC), the Walloon Region, and UNamur( Conventions No. 2.5020.11, GEQ U. G006.15, 1610468, RW / GEQ2016 and U.-G011.22). PTCI is a member of the“ Consortium des
Équipements de Calcul Intensif( CÉCI)” 2. This research also used the resources of the Electron Microscopy Service( SME) of UNamur 3 and the Bioimaging Centre of the University of Exeter 4. Funding
É Camus was supported by the ENS de Lyon and the Auvergne- Rhônes-Alpes region via an Explo’ RA SUP scholarship as well as the European Union through the Erasmus + program. SR Mouchet was supported by Wallonia-Brussels International( WBI) through a Postdoctoral Fellowship for Excellence program WBI. WORLD andbytheBelgianNationalFundforScientific Research( FRS- FNRS) as a Postdoctoral Researcher and as an Associate Researcher. This research was also supported by FRS-FNRS through the Researchers’ Credit CC 1.5075.11F. 1 https:// platforms. unamur. be / ptci.
Conflicts of interest The authors declare have no competing interests.
Data availability statement
Data available from the Zenodo Repository: https:// doi. org / 10.5281 / zenodo. 17857389.
Author contribution statement
SRM conceived the original project. ÉC conducted the species identification, measured the reflectance spectra, calculated the CIE coordinates and the quantum catches, as well as performed the morphological characterisation by optical microscopy and SEM. SRM performed the TEM observations. EDF and SRM conducted the co- and cross-CP reflectance spectrophotometry. SRM, ÉC, and EDF performed the simulations. ÉC, EDF, PV, OD and SRM discussed the results. SRM wrote the manuscript with input from ÉC. All authors commented on the manuscript and gave approval to its final version. Supplementary material
Supplementary material 1 Figure S1. Top view of the cuticle of Calliphora vicina’ sabdomen using SEM. In addition to long hairs( known as macrotrichia) observed by optical microscopy, shorter hairs( also called microtrichia) of about 20 lm can be observed.
Supplementary material 2 Co- and cross-circularly polarised reflectance spectrophotometry.
The dipteran cuticles were characterised using cross- and co-circularly polarised( CP) reflectance spectrophotometry. Both co-CP and cross-CP configurations were investigated, where co-CP corresponds to incident and reflected CP components with the same handedness( left( L) or right( R)) and cross-CP refers to different handedness of the incident and reflected CP components. The experimental set-up, detailed in [ 65 ], consisted of an Ocean Optics( Delray Beach, FL, USA) USB2000 + spectrometer and an Ocean Optics HPX-2000 xenon broadband fibre-coupled light source. CP incident light beam was generated by a rotatable polariser and an achromatic Fresnel rhomb quarter-wave retarder, orientated at 45 ° azimuth. The CP light handedness was selected by orientating the polariser azimuth at either 0 ° or 90 °.
2 http:// www. ceci-hpc. be. 3 https:// platforms. unamur. be / microscopie-electronique. 4 http:// biosciences. exeter. ac. uk / bioimaging /.