Caterpillar hair as a physical barrier against invertebrate predators

Predation has led to the evolution of defensive armor in prey species. The dense and long hairs of caterpillars (i.e., lepidopteran larvae) are generally believed to play an important role as a physical defence against predators. However, few studies have been undertaken to investigate how hairs protect caterpillars from a predator’s weapons. To determine the importance of caterpillar hairs as a defensive armor, we observed adults of Calosoma maximowiczi (Carabidae) attacking 5 caterpillar species with different hairiness under laboratory conditions. Carabids used their mandibles to catch caterpillars and thereafter fed on them. Almost all the larvae of 3 smooth species and a short-haired species were easily caught by carabids during their first attack. However, 53.2% of larvae in a long-haired species Lemyra imparilis (Erebidae: Arctiinae) were able to escape from carabid attacks. Even when Lemyra larvae were finally eaten, carabids required a larger number of attacks to catch Lemyra larvae. Dorsal hairs of Lemyra larvae were much longer than the mandible length of carabid adults for any body size, suggesting that the dorsal hairs can function as a physical barrier against carabid attacks. To test the hypothesis, we cut the dorsal hairs of Lemyra larvae shorter than the carabids’ mandibles. Cutting the dorsal hairs of Lemyra larvae resulted in fewer carabid attacks with higher success rates. Therefore, we conclude that long hairs can protect Lemyra larvae from carabid mandibles. This is the first study to clarify the adaptive significance of caterpillar hair length as a morphological defence.