How to draw a two (or even more) loop diagrams using Feynmf?

I tried
\documentclass{article}
\usepackage{feynmp}
\begin{document}
\unitlength = 1mm
\begin{fmffile}{twoloop}
\begin{fmfgraph}(100,60)
\fmfleft{i}
\fmfright{o}
\fmf{fermion}{i,v1,o}
\fmf{fermion}{v1,v2,v1}
\fmf{fermion}{v2,v2}
\end{fmfgraph}
\end{fmffile}
\end{document}
but it didn't work.
I plotted it using LaTeXit. The one loop diagram worked just fine so I guess it's the problem with my usage of Feynmf.