Uric acid deposition in larval integument of black swallowtails and speculation on its possible functions

From the first through third instar, larvae of the black swallowtail, Papilio polyxenes Fabr., display a distinctive color pattern characterized by an irregular circle of white pigment on the dorsum. This white spot is surrounded by brown pigmentation, creating the impression of a bird dropping. It has long been assumed that this color pattern evolved as a defense mechanism against avian predators. We report here that the source of the white color is accumulated uric acid. Although uric acid has traditionally been viewed as an excretory product, it also can act in biological systems as a powerful antioxidant. Thus, the possibility exists that the white spot serves a protective function not only against predators in a high fidelity mimicry system, but also against oxidative stress generated by the phototoxic allelochemicals that characterize most hostplants of Papilio species.