The position of eyespots and thickened segments influence their protective value to caterpillars

It has long been believed that the paired circular markings ("eyespots") of Lepidoptera larvae, combined with their defensive postures (where the larvae swell their anterior body segments) protect them from potential predators. These traits could inhibit attacks by enhancing the similarity of the prey item to the predator’s own natural enemies (notably snakes), but alternatively, they may simply exploit the predator’s wariness of novel and/or conspicuous objects. To differentiate between these contrasting explanations, we evaluated the responses of naive chicks (Gallus gallus domesticus) to artificial caterpillar-like prey. In the first experiment, chicks were presented with model caterpillars either without eyespots or with eyespots positioned in 1 of 2 locations on their bodies: anteriorly or centrally. In the second experiment, chicks were presented with model caterpillars either lacking the thickening associated with the defensive posture, or with models in which a thickened section was placed centrally or anteriorly. In both cases, the chicks were significantly more wary of prey with anteriorly positioned defensive traits than either prey without these traits or prey with centrally positioned traits. Because prey with central eyespots and central thickening were equally novel and conspicuousness to those with these traits positioned in a more head-like anterior position, we infer that predator wariness was primarily influenced by the similarity of the prey to their potential natural enemies. These results support the idea that both caterpillar eyespots, and the defensive posture examined here, deter predators not simply because they are conspicuous, but because they enhance caterpillars’ resemblance to potentially dangerous vertebrates.