Ovipositing butterflies use a red receptor to see green

Swallowtail butterflies of the species Papilio aegeus oviposit on the leave s of Rutaceae plants in Australia. They possess receptor types with sensiti vity peaks around 390 nm (violet receptor) and 610 nm (red receptor), in ad dition to the receptor types common in insects with sensitivity peaks at 36 0 nm (ultraviolet receptor), 440 nm (blue receptor) and 540 nm (green recep tor). Multiple- and dual-choice experiments show that females of P. aegeus prefer to oviposit on substrata that look green to humans. A class of simpl e models is developed to describe this choice behaviour in terms of linear interactions between the different spectral types of photoreceptors, The gr een receptor has a positive influence, whereas the blue (and possibly the u ltraviolet and violet) receptor and the red receptor have negative influenc es on the choice behaviour. Colour choice for oviposition is thus guided by a single chromatic mechanism. Caterpillars of P, aegeus grow faster an you ng leaves which, according to the model, should be preferred by females for oviposition, The importance of the red receptor for the discrimination bet ween different green leaves is discussed in ecological and comparative cont exts. 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.