  We show that the dominant mode of alignment plays an important role in dry
active nematics,leading to two dynamical sub-classes defined by the nature of
the instability of the nematic bands that characterize, in these systems, the
coexistence phase separating the isotropic and fluctuating nematic states. In
addition to the well-known instability inducing long undulations along the
band, another stronger instability leading to the break-up of the band in many
transversal segments may arise. We elucidate the origin of this strong
instability for a realistic model of self-propelled rods and determine the
high-order nonlinear terms responsible for it at the hydrodynamic level.
