  We study a continuum model of an extensile active nematic to show that
mesoscale turbulence develops in two stages: (i) ordered regions undergo an
intrinsic hydrodynamic instability generating walls, lines of stong bend
deformations, (ii) the walls relax by forming oppositely charged pairs of
defects. Both creation and annihilation of defect pairs reinstate nematic
regions which undergo further instabilities, leading to a dynamic steady state.
We compare this with the development of active turbulence in a contractile
active nematic.
