Springtail coloration at a finer scale: mechanisms behind vibrant collembolan metallic colours

Abstract

The mechanisms and evolution of metallic structural colours are of both fundamental and applied interest, yet most work in arthropods has focused on derived butterflies and beetles with distinct hues. In particular, basal hexapods—groups with many scaled, metallic representatives—are currently poorly studied and controversial, with some recent studies suggesting either that thin-film (lamina thickness) or diffraction grating (longitudinal ridges, cross-ribs) elements produce these colours in early Lepidoptera and one springtail (Collembola) species. Especially the collembolan basal scale design, consisting of a single lamina and longitudinal ridges with smooth valleys lacking cross-ribs, makes them an interesting group to explore the mechanisms of metallic coloration. Using microspectroscopy, Raman spectroscopy, electron microscopy and finite-difference time-domain optical modelling, we investigated scale colour in seven springtail species that show clear metallic coloration. Reflectance spectra are largely uniform and exhibit a broadband metallic/golden coloration with peaks in the violet/blue region. Our simulations confirm the role of the longitudinal ridges, working in conjunction with thin-film effects to produce a broadband metallic coloration. Broadband coloration occurs through spatial colour mixing, which probably results from nanoscale variation in scale thickness and ridge height and distance. These results provide crucial insights into the colour production mechanisms in a basal scale design and highlight the need for further investigation of scaled, basal arthropods.

Antennapedia and optix regulate metallic silver wing scale development and cell shape in Bicyclus anynana butterflies

“…This means that a single silver scale, when observed at very high resolution, is made up of many small patches of distinct colors, such as blues, greens, oranges, and pinks. The broadband metallic reflectors seen in fossil moths (D'Alba et al, 2019;Qingqing et al, 2021), in extant moths (Kilchoer et al, 2019), and in springtails (Vanthournout et al, 2021), however, utilize thin-film interference from a single chitin layer, resulting from fused scales. Such a single thin film is also present as the lower lamina of archetypal butterfly scales, which are often also tuned to produce broadband colors (Thayer Article ll Wasik et al, 2014).…”

The Height of Chitinous Ridges Alone Produces the Entire Structural Color Palette

“…We kept the interridge distance constant because of the consistency of the horizontal interridge distances across butterfly species and colors. [10] We focused on the variation of the ridge height [11] inspired by recent evidence supporting the idea that the alteration of the vertical dimension (i.e., the thickness of the lower lamina) is a previously unconsidered evolutionary strategy to explore different colorations in butterflies. [12][13][14][15] The structures produced in the synthetic photocurable resin were transferred to a silicone elastomer through soft lithography and transformed into chitinous structures through a…”

Rapid and reversible humidity-dependent colour change by water film formation in a scaled springtail

“…Simulating even thicker water layers results in reflectance spectra with multiple peaks, consistent with the banded colours that are observed after excessive wetting (figure 6e). The metallic golden coloration of L. cyaneus is mainly determined by thin film effects originating from the thickness of the scale lamina [20], but ridge effects cannot be ruled out. This is consistent with our simulation results, which indicate that the lamina thickness is the main cause of the colour change.…”

“…We performed FDTD modelling using a commercial Maxwell equation solver (Ansys-Lumerical Solutions, Ansys Canada Ltd) with the lamina and ridges consisting of chitin and melanin [20], respectively. Refractive indices of chitin and melanin were taken from previous studies [24,25], we used the already available refractive index value of water (RI = 1.33) in the Ansys-Lumerical software.…”