Ovipositing butterflies use a red receptor to see green

Swallowtail butterflies of the species Papilio aegeus oviposit on the leaves of Rutaceae plants in Australia. They possess receptor types with sensitivity peaks around 390 nm (violet receptor) and 610 nm (red receptor), in addition to the receptor types common in insects with sensitivity peaks at 360 nm (ultraviolet receptor), 440 nm (blue receptor) and 540 nm (green receptor). Multiple- and dual-choice experiments show that females of P. aegeus prefer to oviposit on substrata that look green to humans. A class of simple models is developed to describe this choice behaviour in terms of linear interactions between the different spectral types of photoreceptors. The green receptor has a positive influence, whereas the blue (and possibly the ultraviolet and violet) receptor and the red receptor have negative influences on the choice behaviour. Colour choice for oviposition is thus guided by a single chromatic mechanism. Caterpillars of P. aegeus grow faster on young leaves which, according to the model, should be preferred by females for oviposition. The importance of the red receptor for the discrimination between different green leaves is discussed in ecological and comparative contexts. Finally, in an evolutionary perspective, the possibility is discussed that colour vision systems like those of honeybees might have evolved as a combination of two or more such chromatic mechanisms.