Adaptive phenotypic plasticity of mandibles with respect to host plants

Polyphagous butterflies, i.e., those that feed on multiple host plants, need to evolve adaptations against the defenses of many plants. Studies have focused on chemical defenses of host plants and counter-adaptations of their herbivores. Physical defenses, despite being the first line of defense of many plants, have received little attention. Grass feeding butterflies are among the most speciose adaptive radiations among insects. Grasses rely primarily on physical rather than chemical defenses. Therefore, it is interesting to understand how butterflies have adapted to grass feeding. Mandibles are the principal biting and chewing organs in insect larvae, and mandible morphology should affect the ability of butterflies to effectively feed on their host plants. We here ask whether grass feeding butterflies have plasticity in mandibular morphology to cope with grass physical defenses, using the widespread grass feeding butterfly Mycalesis mineus as a model. We first show that physical defenses of grasses, such as leaf toughness, presence of silica deposits and trichomes, negatively affect body size of the butterfly. Our results further indicate that larvae of M. mineus show mandible plasticity, with increased mandibular length and relative mass investment, when feeding on tougher leaves and leaves with silica deposits. We suggest that the evolution of mandible plasticity may have been a key component of the coevolutionary arms race between grasses and herbivorous insects. This plasticity may have also allowed butterflies such as M. mineus to colonize a broad range of habitats and geographic regions.