Structural colours, nature’s most pure and intense colours, originate when light is scattered via nanoscale modulations of the refractive index. Original colours in fossils illuminate the ecological interactions among extinct organisms and functional evolution of colours. Here we report multiple examples of vivid metallic colours in diverse insects from mid-Cretaceous amber. Scanning and transmission electron microscopy revealed a smooth outer surface and five alternating electron-dense and electron-lucent layers in the epicuticle of a fossil wasp, suggesting that multilayer reflectors, the most common biophotonic nanostructure in animals and even plants, are responsible for the exceptional preservation of colour in amber fossils. Based on theoretical modeling of the reflectance spectra, a reflective peak of wavelength of 514 nm was calculated, corresponding to the bluish green colour observed under white light. The green to blue structural colours in fossil wasps, beetles, and a fly most likely functioned as camouflage, although other functions such as thermoregulation cannot be ruled out. This discovery not only provides critical evidence of evolution of structural colours in arthropods, but also sheds light on the preservation potential of nanostructures of ancient animals through geological time.

For standard observation, fossils were polished using different grades of sandpaper and diatomite powder, to get as close to the inclusions as possible without damaging them. Where it aided observation, some pieces were polished into very thin slices, making the insect inclusions clearly observable and the surrounding amber matrix almost transparent in bright light. Photographs against various backgrounds were taken using a Canon EOS 5D Mark III digital camera, equipped with a Canon MP-E 65 mm macro lens (F2.8, 1–5X), and with an attached Canon MT-24EX twin flash. Focus stacking software (Zerene Stacker, Version 1.04) was used to increase the depth of field. The resultant images were edited in Photoshop only to correct brightness and contrast. Two amber-entombed wasps (metallic coloured and silvery) were selected for ultrastructural analysis of the cuticles. Ultrathin (70 nm thick) sections were cut with a Diatome 3 mm 45° diamond knife, mounted on formvar-coated copper grids, and air dried. Unstained sections were imaged using a JEM-2100 Plus transmission electron microscope (at Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences) at 80 keV. The sectioned samples were mounted on stubs with double-sided carbon tape. The sectioned surface was imaged with a LEO 1530 VP analytical scanning electron microscope controlled by JEOL InTouchScope version 1.05A software, using the variable-pressure secondary electron detector at low vacuum and at 18 keV.