Effects of cuticle structure and crystalline wax coverage on the coloration in young and old males of Calopteryx splendens and Calopteryx virgo

Male secondary sexual characters, such as color patterns, are often investigated at the macroscale level. However, micro- and nanoscale levels of morphological investigations may reveal functional features responsible for a particular coloration, thus providing more information, e.g., about the condition dependence of male sexual characters. The aim of this paper was to investigate cuticle color and its structure in males of two congeneric damselfly species, Calopteryx splendens and Calopteryx virgo, and reveal possible color changes with age. According to spectrometer measurements, C. splendens males were bluer and had a greater saturation of blue in their abdomen than C. virgo males, which were, in turn, greener and had more green saturation. Although the two species differed in the number of structural layers and the spacing of the layers, it seems that intactness of the wax crystals covering the epicuticle was most often the morphological trait which was related to the color parameters measured from males’ cuticles. The effect of the crystalline wax coverage on cuticle color was also confirmed by removing the wax using chloroform: after the treatment, the hue was bluer, the cuticle had a greater brightness and greater blue saturation, but less green saturation. Age differences influencing the color and structure of the cuticle were also observed: older males had more blue and green saturation and had more intact wax coverage than did younger males. Although multilayer reflection should be responsible for the iridescent color of males, our results suggest that wax coverage plays an important role in the color tuning of the male cuticle. This may have a considerable signal function, indicating the males’ viability to competing males or to females.