J. Eur. Opt. Society-Rapid Publ. 22, 12( 2026) 117
Figure
2. The photonic structures found in the cuticle of Calliphora vicina( a – c) and Lucilia richardsi( d – f) are multilayer reflectors, as observed by SEM( a, b, d, e) and TEM( c, f). One such single periodic multilayer is observed in C. vicina’ s cuticle( a – c). It is made of two kinds of layer with thickness equal to 98 ± 7 nm and 20 ± 6 nm. L. richardsi’ s cuticle comprises three multilayers: a top multilayer with 4 bilayers, the layers of which have a thickness equal to 78 ± 15 nm and 82 ± 12 nm; a middle multilayer containing about 25 layers, namely 13 58 ± 14 nm-thick dark contrasted layers and 12 60 ± 13 nm-thick light contrasted layers; and a bottom aperiodic multilayer composed of 3 light contrasted layers with thicknesses ranging from 175 nm to 370 nm and 3 dark contrasted layers.
decomposed in each layer into forward and backward waves propagating in the direction perpendicular to the layers.
The photonic band structures were calculated in the specific case of infinite one-dimensional photonic crystal based on the photonic structures observed in the dipteran cuticles using a Kronig – Penney model approach [ 62 ].
3 Results and discussion
Observations by optical microscopy revealed, in both cases, cuticles exhibiting quite homogeneous, shiny and metallic visual appearances( Figs. 1c, 1f). These cuticles are covered by black hairs, also called macrotrichia, the length of which ranges between ca. 200 and 300 lm. In addition, much shorter hairs, known as microtrichia, of about 20 lm can be observed by SEM( Fig. S1). Observations of cross-sections of the blow flies’ cuticles by SEM and TEM showed the photonic structures inside their integuments( Fig. 2). In the case of C. vicina, one single periodic planar multilayer was observed by SEM( Figs. 2a, 2b). It comprises 18 periods with the appearance of two layers, the thicknesses of which were measured to be 98 ± 7 nm and 20 ± 6 nm( Figs. 2a, 2b). Both layers have different contrasts( dark and light contrasted, respectively) with SEM techniques. In the case of L. richardsi, three different multilayers are observed( Figs. 2d – 2f). The top multilayer consists of 4 bilayers, the layers of which have thicknesses equal to 78 ± 15 nm and 82 ± 12 nm. The middle multilayer contains about 25 layers, namely 13 58 ± 14 nm-thick electron-dense layers and 12 60 ± 13 nm-thick electronlucent layers. The bottom aperiodic multilayer is composed of 3 electron-lucent layers with thicknesses ranging from 175 nm to 370 nm and 3 electron-dense layers with thicknesses ranging from 120 nm to 160 nm. These structures contrast to some extent with the multilayer observed in the exocuticle of the green-coloured L. sericata blow fly, which exhibits about 15 periods comprising two layers described as being 130 – 150-nm and 15 – 20-nm thick layers [ 11 ].
Under incident light normal to the samples, the specular spectral reflectance from the blow flies’ cuticles exhibits peaks located at 437 ± 8 nm and 537 ± 6 nm in the cases of C. vicina and L. richardsi, respectively( Fig. 3). These peaks were observed to blue-shift in combination with a decrease of their full width at half maximum( FWHM), when the incidence and detection angles increase( Figs. 3a, 3c, 4). Such a metallic iridescent optical response is typical of a multilayer reflector [ 61 ]. For a human perception point of view, this corresponds to colour changes following a curved path, from blue to violet and from green to violet, respectively, on the CIE chromaticity diagram( Figs. 3b, 3d).
Interestingly, TEM observations of the one-dimensional photonic structure in C. vicina’ s cuticle showed patterns that might be associated with a Bouligand structure( Fig. 2c). Such structures were found in the cuticles of