  In this paper, I characterize four particular classes of directed
multigraphs, or quivers, as images under left and right adjoints to the natural
vertex and edge functors. In particular, the following notions coincide: (1)
independent sets of vertices with a left adjoint functor to the vertex functor,
(2) independent sets of edges with a left adjoint functor to the edge functor,
(3) complete digraphs with a right adjoint functor to the vertex functor, (4)
bouquets with a right adjoint functor to the edge functor.
