The dynamics of femtosecond laser ablation from wide bandgap insulators (Al2O3, BaF2 and CaF2) at intensities below the single shot damage threshold (1011 - 1013 W/cm2) is characterized by efficient surface ionization, followed by the explosive emission of positive ions and small clusters, with a kinetic energy of about 100 eV (Coulomb explosion). The multiphoton coupling of the laser to the transparent material is strongly promoted by defect resonances within the bandgap, eventually generated during a considerable number of incubating pulses before a steady ablation regime is reached. At the bottom of the ablation crater, produced by an accumulation of several thousand laser pulses, periodic surface structures are developed, with a typical scaling in the nanometer range. Occasionally, these structures exhibit features like bifurcations or columns growing out of plane. The feature size and shape appears to be more sensitive to the applied laser intensity resp. irradiation dose than to wavelength or angle of incidence. The ripples cannot be explained as a result of an inhomogeneous energy input, e.g. due to interference. Instead, we suggest that the ripples are a consequence of the surface relaxation via self-organization.