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.

多食性蝴蝶，即以多种寄主植物为食的蝴蝶，需要进化出适应多种植物防御的能力。研究重点是寄主植物的化学防御和食草动物的反适应。尽管物理防御是许多植物的第一道防线，但很少受到关注。食草蝴蝶是昆虫中适应性辐射最特殊的昆虫之一。草主要依靠物理防御而不是化学防御。因此，了解蝴蝶如何适应草食是很有趣的。下颌骨是昆虫幼虫的主要咬合和咀嚼器官，下颌骨形态会影响蝴蝶有效取食寄主植物的能力。我们在这里以广泛分布的草食蝴蝶Mycalesis mineus作为模型，询问食草蝴蝶的下颌形态是否具有可塑性，以应对草的物理防御。我们首先表明草的物理防御，例如叶子的韧性、二氧化硅沉积物和毛状体的存在，会对蝴蝶的体型产生负面影响。我们的结果进一步表明，当吃较坚韧的叶子和含有二氧化硅沉积物的叶子时， M.mineus幼虫表现出下颌骨可塑性，下颌长度增加，相对质量投入增加。我们认为，下颌骨可塑性的进化可能是草类和草食性昆虫之间共同进化军备竞赛的关键组成部分。这种可塑性也可能使得蝴蝶（例如M.mineus）能够在广泛的栖息地和地理区域中定居。