Cuticular interference reflectors in the golden pupae of danaine butterflies

Abstract

In pupae of Eploea core and Amauris ochlea most of the cuticle displays a strong metallic golden or silvery lustre. The reflection colours appear between 24 and 48 h after pupation, first as blue and greenish hues which later turn into gold or sliver, and vanish again about 24 h before emergence of the butterfly. If excised cuticle dries, the metallic sheen is lost and the cuticle becomes fully transparent. Absolute reflectance was measured by microspectrophotometry as over 70% from 550 to over 800 nm. At wavelengths below 550 nm, reflectance drops rapidly in golden cuticles and less so in silvery ones. Electron micrographs of reflecting cuticle show numerous alternating dense and clear layers (up to 250 pairs) in the endocuticle of reflecting regions (METAL-cuticle: Multiple Endocuticular Thin Alternating Layers). Proceeding from outside to inside, the clear C-layers first systematically grow thicker and then thinner again, whereas the thickness of the dense D-layers remains fairly constant throughout the stack. The refractive index of D- and C-layers was determined as 1.58 and about 1.37, respectively. Computed spectral reflectance curves optimally match the measured spectra, if the thickness values are corrected for shrinkage during specimen preparation (C-layers, + 20%; D-layers, + 5%). In conclusion the metallic-reflection colours can be quantitatively attributed to constructive interference of light at the layers of METAL-cuticle: no pigment is involved. The colour changes during pupal development can be fully explained as the successive formation or decomposition of these layers. The importance of statistical and systematic variations of layer thickness and of the high numbers of layers is discussed with regard to the formation of biological broadband reflectors. Compared with other biological interference reflectors, danaine cuticular gold and silver mirrors show extremely high reflectance and optical precision.