3D Tomographic Analysis of the Order‐Disorder Interplay in the Pachyrhynchus congestus mirabilis Weevil

Abstract

The bright colors of Pachyrhynchus weevils originate from complex dielectric nanostructures within their elytral scales. In contrast to previous work exhibiting highly ordered single‐network diamond‐type photonic crystals, here, it is shown by combining optical microscopy and spectroscopy measurements with 3D focused ion beam (FIB) tomography that the blue scales of P. congestus mirabilis differ from that of an ordered diamond structure. Through the use of FIB tomography on elytral scales filled with platinum (Pt) by electron beam‐assisted deposition, it is revealed that the red scales of this weevil possess a periodic diamond structure, while the network morphology of the blue scales exhibit diamond morphology only on the single scattering unit level with disorder on longer length scales. Full wave simulations performed on the reconstructed volumes indicate that this local order is sufficient to open a partial photonic bandgap even at low dielectric constant contrast between chitin and air in the absence of long‐range or translational order. The observation of disordered and ordered photonic crystals within a single organism opens up interesting questions on the cellular origin of coloration and studies on bio‐inspired replication of angle‐independent colors.

Pachyrhynchus Weevils Use 3D Photonic Crystals with Varying Degrees of Order to Create Diverse and Brilliant Displays

“…While the used procedure can approximate the ff , accurate 3D tomographic data for ordered and quasi‐ordered scales of P. congestus mirabilis weevils exhibited ff values of 0.44 and 0.32, respectively. [ 20 ] The study of other Pachyrhynchus specimens with ordered PCs reported ff ‐values of 0.4, but since they were also based on 2D data, these values are only estimates. [ 29,30 ]…”

“…Pachyrhynchus weevils are an ideal species to study variations in structural coloration, not only because of their vast color palette extending from deep‐red to the UV, but also because of their ability to create both ordered and quasi‐ordered 3D PCs. [ 20 ] Although ordered and quasi‐ordered 3D PCs within a single specimen have previously been reported, [ 21,22 ] ) these ordered and quasi‐ordered scales were differently colored and located in distinct sections of the elytron. Furthermore, these previously investigated species lack the vast range in colors found across Pachyrhynchus .…”

“…[ 20,23 ] Recently, Djeghdi et al. [ 20 ] investigated a focused ion beam/scanning electron microscopy (FIB‐SEM)‐based tomographic reconstruction of the quasi‐ordered structures within the elytral scales of a Pachyrhynchus congestus mirabilis weevil. This weevil possesses both blue‐colored scales based on a quasi‐ordered chitinous network as well as red‐colored scales incorporating an ordered diamond 3D PC in their wing scales.…”

“…These structures have garnered significant attention due to their ability to create angle‐independent, stable colors with natural materials, without the need for a complete PBG. These structures have been described in beetles such as Pachyrhynchus congestus mirabilis , [ 20 ] Eupholus magnificus , [ 21 ] and Sulawesiella rafaelae [ 22 ] . Unlike ordered PCs, quasi‐ordered PCs possess only short‐range order, that is, they possess a roughly constant nearest‐neighbor distance, but no long‐range order.…”

“…Previously, X‐ray and electron‐microscopy‐based techniques have been employed to describe these structures. [ 20,23 ] Recently, Djeghdi et al. [ 20 ] investigated a focused ion beam/scanning electron microscopy (FIB‐SEM)‐based tomographic reconstruction of the quasi‐ordered structures within the elytral scales of a Pachyrhynchus congestus mirabilis weevil.…”