SPATIAL VARIATION OF CUTICULAR TRACE ELEMENTS IN PATTERNED INSECTS

Insects are the most diverse group of animals known and are adapted to live in almost all terrestrial, and some aquatic, biomes. The most striking ecological adaptations of insects include color patterns on their cuticle, which are produced primarily by pigments such as melanin. These pigments function predominantly in generating patterns used in intra- and interspecific signaling, e.g. camouflage and sexual display. Many fossil insects also exhibit striking color patterns on their cuticles, but the origins of these fossil patterns and their responsible pigments are unknown. Here, we resolve these issues using non-destructive synchrotron-X-ray fluorescence (XRF) to characterize the spatial distributions of trace elements in the cuticles of 30 species of modern insects with patterned cuticles, representing 13 insect families commonly preserved in the fossil record. We mapped the concentrations of 10 trace elements in cuticle regions of different colors that reflect spatial variations in the concentration of various pigments. Our results show that light atomic weight elements such as Ca and K are relatively concentrated in dark, melanin-rich regions of the cuticle. White, pterin-rich regions of the cuticle are rich in Fe and Zn to the contrary of red regions where S is in higher concentration. Principal Component Analyses (PCA) of the concentrations of each element in cuticle regions of different color reveal a strong taxonomical signal whereby representatives from individual insect families show similar concentrations of trace elements. Critically, this taxonomic chemical signal is overprinted by a strong color-specific signal: trace element concentrations in cuticle regions of similar color in different taxa resemble each other. In general, interspecific variation in dark, melanin-rich regions of the cuticle is explained primarily by variations in concentrations of K, Zn and to a lesser extent, Ca; these elements plus Mn are responsible for most variation in white cuticle regions, and S, Mn, and Fe, for red cuticle regions. Understanding pigment- and taxon-specific variation in trace element chemistry will greatly enhance our ability to interpret the original pigmentary colors of fossil insects, thus informing models of the evolution of color and its ecological functions in insects through deep time.