I'm working on a "draw all possible Feynman diagrams up to order 2" problem for a scalar field that obeys the Klein-Gordon equation, and I'm wondering about a few things. When I did a course on particle physics and was first introduced to Feynman diagrams in the context of QED (but not QED itself in any detail), I was allowed to flip a line of propagation around, i.e. from propagating towards a vertex to propagating away from a vertex or vice versa, the particle would be an anti-particle and I'd still have a valid diagram. Can I do the same thing for my scalar particles? What is the anti-particle of such a particle? It seems the propagator doesn't distinguish between directions of propagation at all, so are they their own antiparticles? What kind of particles do I identify with my scalar field in the first place?