Hidden in plain orange: aposematic coloration is cryptic to a colorblind insect predator

LAY SUMMARY Conspicuous red or yellow colors often act as a warning signal to predators, but conspicuous is in the eye of the beholder. Praying mantids, which are voracious insect predators, have monochromatic vision and are colorblind. To mantids, orange harlequin stinkbugs are cryptic against leaves, while individuals with iridescent patches are highly conspicuous. Future research should consider how predators with poor long-wavelength vision influence the evolution of warning coloration.In unpalatable prey, long-wavelength colors such as red or yellow are often thought to be aposematic (warning) signals, due to their high conspicuousness. However, conspicuousness depends on the visual physiology of the receivers. Tectocoris diophthalmus is a shieldback stinkbug with highly variable coloration; individuals may be all orange or have blue-green iridescent patches of variable size. Prior research has demonstrated the defenses of T. diophthalmus can induce avoidance learning in birds but not praying mantids, and that geographic patterns in variation may relate to the local density of arthropod predators. In this study, we use visual modeling and behavioral assays to test how praying mantids Hierodula majuscula may impose directional selection pressure on the coloration of T. diophthalmus. Mantids have monochromatic vision with peak sensitivity in the "green" region of the spectrum. Using a receptor excitation model, we show that orange bugs are much less conspicuous than iridescent conspecifics and may be inconspicuous against their typical green leaf background. In behavioral assays, mantids detected iridescent bugs from a greater distance on average. In binary choice experiments, mantids showed no color preference at short range, but approached iridescent bugs significantly more often when the choice had to be made at a greater distance, and could not distinguish between orange bugs and unoccupied leaves at distance. Together, this evidence suggests that H. majuscula should impose strong directional selection against iridescent bugs in nature, and that orange coloration may be performing dual roles of crypsis to mantids but aposematic signaling to birds.