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 birds. 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 mealworms (Tenibrio molitor) 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 are sufficiently large compared with the size of the arthropod predator. However, when larvae are small relative to the arthropod predator, eyespots cease to be protective and may even promote attacks. 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, because these traits are unlikely to deter avian predators.