coupled to the order parameter. This coupling is analogue to the one present in polar fluids [@Toner1995; @Toner1998; @Toner2005] and is a generic mechanism for emergent structure in all active systems, as demonstrated in our recent work [@Gopinath2012].

The layout of the paper is as follows. First, we construct the hydrodynamic description of a self-propelled nematic using symmetry considerations highlighting the key features that distinguish the dynamics of the system from that of an active nematic. Then we examine the linear stability of the homogeneous nematic state. We show that there exists three dynamical mechanisms responsible for emergent structures in active fluids with nematic symmetry. The first is a model-independent instability that occurs in the vicinity of the mean field order disorder transition due to the coupling between order parameter and mass transport which renders the dynamics of the system self regulating. We argue that this instability is the basis for the emergence of bands and phase separation found ubiquitously active systems  [@Chate2006; @Yang2010; @Ginelli2010]. The second is the well known instability of director fluctuations that arises from nonequilibrium curvature-induced fluxes and is closely related to the giant number fluctuations observed in these systems [@Simha2002; @Ramaswamy2003]. These two instabilities are 