Fig. 7

Schematic diagram of 2D and 3D morphogenesis of insect adult structures via “folding and extension”. All structures essentially started from a sac-like monolayer structure, but the final structures differ (2D or 3D, monolayer or double layered). In a bi-layer 2D structure like an insect wing, internal space disappears during development by connecting the upper and lower layers. Then, flat bi-layer primordia form relatively simple parallel furrows which enable primordia to enlarge in a single direction. In a bi-layer 3D structure like the treehopper helmet, upper and lower layers stick together to form a bi-layer sheet. In contrast to the wing, this bi-layer structure has a 3D shape. The macro and micro furrows determine the expansion rate in multiple direction to form its 3D shape. In a monolayer 3D structure, the inner space of the primordium does not disappear during development. Furrows are formed in the surface of the primordium by bending a single layer of epithelium and are extended by hemolymphal pressure at molting