Hi all, I am learning Feynman diagrams, and I have a quick question: in diagrams such as the one for electron-positron annihilation (see http://en.wikipedia.org/wiki/Feynman_diagram#Electron-positron_annihilation_example) why is it that the line "in the middle" is that of an electron/positron? (Either or, as, in the link above, it is horizontal.) This seems to imply to me that there is an electron/positron acting as a mediator between the two, which doesn't seem to make sense. To me, it would make more sense if there was no middle line, and it looked like an X. Could someone explain if this is just a rule for how they're drawn, or if I'm missing something fundamental? Cheers.