Eclosion is a rapid process of morphological changes in insects, especially for the wings of butterflies. The orange oakleaf butterfly (Kallima inachus) transits from pupae to adults with a 9.3 fold instant increase in the surface area of their wings. To explore the mechanism for the rapid morphological changes in butterfly wings, we analyzed changes in microstructures in the wings of K. inachus. We found that there were lots of micron-sized foldable units in the wings at the pupal stage. The foldable units could provide as much as 31.35 times of increase in wing surface area. During eclosion, foldable units were flattened sequentially and resulted in a rapid increase in wing surface areas. The unfolding process was regulated by the structures and layouts of wing veins. Based on our observation, foldable units play important roles in both deformation and stretching of wings. The foldable units of microstructures may provide mimics for simulating entities of large-deformational bionic structures with practical application.