We prove the local index theorem for the Rarita-Schwinger operator and higher Dirac operators using Gilkey's invariance theory. That is, we show that the supertrace of the heat kernel of a given geometric operator converges as time goes to zero, and identifies the limit as the Chern-Weil form of the Atiyah-Singer integrand.

We carry the index theory for manifolds with boundary of Bär and Ballmann over to first order differential operators on metric graphs. This results in an elegant proof for the index of such operators. Then the self-adjoint extensions and the spectrum of the Dirac operator on the complex line bundle are studied. We also introduce two types of boundary conditions for the Dirac operator, whose spectrum encodes information of the underlying topology of the graph.