Arms race of physical defences: hooked trichomes of ant-plants kill lycaenid caterpillars, but one specialist has a counter-defence.

The coevolution of insects and chemical plant defences has been described as an arms race, but it is unclear whether physical plant defences can produce similar outcomes. Here, we report a previously unknown interaction from the mutualism between ants and trees. Although trees are well protected against herbivory by aggressive ants, caterpillars of the genus (Lepidoptera: Lycaenidae) can feed on the leaves by appeasing the ants with nectar-like secretions. One ant-plant species, , bears hooked trichomes on its green surfaces. When placed on stems or petioles, caterpillars associated with other species ( and ) were quickly arrested by the sharp trichomes that pierced their cuticle, resulting in death by rapid blood loss and removal by ants. In striking contrast, caterpillars, which occur naturally on , could easily walk over the hooked trichomes without any injury. As hooked trichomes are a novel trait within , this interaction provides an example of de novo evolution of a physical plant defence, which in turn has been overcome by a specialist herbivore. Our study suggests that physical plant defences can lead to evolutionary arms races similar to those for chemical defences.