Thomas Lecuit1,2, L Mahadevan3. 1. Aix-Marseille Université, CNRS, IBDM - UMR7288, 13009 Marseille, France thomas.lecuit@univ-amu.fr lmahadev@g.harvard.edu. 2. Collège de France, 11 Place Marcelin Berthelot, 75005 Paris, France. 3. Departments of Physics, and Organismic and Evolutionary Biology, and School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, USA thomas.lecuit@univ-amu.fr lmahadev@g.harvard.edu.