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 elements (longitudinal ridges, crossribs) produce these colors in early Lepidoptera and one springtail (Collembola) species. Especially the Collembolan basal scale design, consisting of a singlelamina and longitudinal ridges with smooth valleys lacking crossribs, makes them an interesting group to explore the mechanisms of metallic colouration. Using microspectroscopy, Raman spectroscopy, electron microscopy and FDTD optical modelling we investigated scale colour in seven springtail species that show clear metallic colouration. Reflectance spectra are largely uniform and exhibit a broadband metallic/golden colouration 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 colouration. Broadband colouration occurs through spatial colour mixing which likely 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.

SEM and TEM pictures were collected using a FEG-SEM (FEI Quanta 200F, Netherlands) and a JEOL JEM 1010 (Jeol, Ltd, Tokyo, Japan) transmission electron microscope.

Reflectance spectra of scales and scales on body result from measurements on a CRAIC AX10 UV–Visible micro-spectrophotometer (CRAIC Technologies, Inc., USA).

Simulation spectra were obtained from Finite-Difference Time-Domain - (FDTD) modelling using a commercial Maxwell equation solver (Lumerical Solutions, Inc).