THE FOSSIL RECORD OF INSECT COLOR ILLUMINATED BY MATURATION EXPERIMENTS (Invited Presentation)

Structural coloration underpins communication strategies in many insects but its evolution is poorly understood. This stems, in part, from limited evidence on how color alters during fossilization. Structural color is not preserved in some insect-rich biotas and, even where preserved, is not original; further, certain color-producing cuticular nanostructures, e.g., three dimensional photonic crystals, are unknown in fossils. We resolve these issues by using elevated pressures (up to 500 bar) and temperatures (up to 270°C) to simulate the effects of burial on structurally colored cuticles of modern beetles; conventional degradation experiments at room temperature and pressure simulated the effects of decay. Our experiments show that structural colors are altered by the effects of burial, not decay. The color generated by multilayer reflectors changes due to alteration of the refractive index and periodicity of the cuticle layers. Three dimensional photonic crystals are equally resistant to degradation and thus their absence in fossil insects is not a function of low preservation potential but implies that these color-producing nanostructures are a very recent evolutionary innovation. Structural colors alter directly to black above a threshold temperature in our experiments, identifiying burial temperature as the primary control on their preservation in fossils. Nonetheless color-producing nanostructures can survive in experimentally treated and fossil cuticles that now appear black. An extensive cryptic record is thus available in fossil insects to illuminate the evolution of structural color.