Structural origin of sexual dichromatic coloration and luster in the beetle Goliathus cacicus

In the beetle Goliathus cacicus, male elytra show silky luster with iridescence while female ones appear lusterless white without iridescence to our perception. We revealed by structural characterizations and optical measurements that elytral coloration and luster of both male and female beetles stem from the tubular scales on the elytra, whose interior is filled with a random structure. Distinct differences between male and female scales reside in the cortex thickness and the filling fraction of the random structure, which are important factors in the determination of the sexual dichromatic luster and iridescence. In additional to reflection peaks in the visible range, spectral measurements indicate that both male and female elytra display also strong iridescent ultraviolet (UV) reflections. Scattering spectra show that both male and female elytra display UV luster, although it is absent for female ones in the visible range.