Butterfly eyespot color pattern formation requires physical contact of the pupal wing epithelium with extracellular materials for morphogenic signal propagation.

These results suggest that physical contact with an appropriate hydrophobic surface is required for morphogenic signals from organizers to propagate normally. The binding of the apical surface of the epithelium with an opposing surface may provide mechanical support for signal propagation. In addition to conventional molecular morphogens, there is a possibility that mechanical distortion of the epithelium that is propagated mechanically serves as a nonmolecular morphogen to induce subsequent molecular changes, in accordance with the distortion hypothesis for butterfly wing color pattern formation.