Relative size matters: Eyespots on large insect prey deter small arthropod predators

Circular chromatic patterns that appear to resemble vertebrate eyes (“eyespots”) are commonplace in the animal kingdom and are widely believed to have evolved as an anti-predator defence. For example, experiments have shown that eyespots on caterpillar-like pastry baits can deter predation by domestic chicks. However, little is known about the extent to which eyespots deter (or promote) attack by arthropod predators. Here we describe two separate experiments in which salticid spiders (Salticus scenicus) and Chinese mantids (Tenodera sinensis) were presented with a choice of mealworm (Tenibrio molitor) larvae with or without eyespots. In a complementary experiment we observed the time taken for adult Chinese mantids to attack hawkmoth (Manduca quinquemaculata) larvae of two different sizes, with and without eyespots. All three experiments indicate that eyespots on insect larvae can deter predation, so long as the larvae with eyespots are sufficiently large compared to the size of the arthropod predator. In smaller caterpillars the selective benefit of eyespots may diminish and may even be selected against. Our results suggest that small arthropods can show an aversion to large prey with eyespots and help explain the presence of eyespots in medium-sized caterpillars, since these traits are unlikely to deter avian predators.