Feeding on small tree seeds at fruit falls is a specific adaptation of harpaline ground beetles and their larvae in tropical rain forests. Using mandibles as tools, they have to perforate the seed shell to reach the nutritious interior. The isometric growth of larval mandibles, known from predatory species, would result in a changing ratio between seed and mandible size during the course of development. The stable size of host tree seeds should select an optimum mandible size, similar for the three larval instars in spermophageous species. We found an increasing tendency to maintain the length of the apical mandible part (apex) in seed feeding species.The size increase is higher in the species from Australia, Africa and South East Asia than in the New World species of the genus Notiobia. Feeding experiments have revealed that larvae of species with a stronger increase in apex growth are also able to develop as predators of Drosophila larvae. Our results furthermore point to an influence of beetle size and shell hardness of the host tree seeds on the apex size and its growth rate.