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.