How to draw disconnected Feynman diagrams with the feynMF package?

I just discovered the feynMF package, a really neat tool for drawing Feynman diagrams.

I went ahead and drew a few diagrams which I'll include here just to set the right spirit for this question. I produced this

with this

\documentclass{article}
\usepackage{feynmf}

\begin{document}
$\begin{fmffile}{Diagram}
\begin{fmffile}{Diagram}
\begin{fmfgraph*}(40,15)
   \fmfpen{thin}
   \fmfleft{i}
   \fmfright{o}
   \fmf{plain}{i,o}
   \fmfdot{i,o}
   \fmfv{l.a=-90,l=$x$}{i}
   \fmfv{l.a=-90,l=$y$}{o}
\end{fmfgraph*}
\quad + \quad
\begin{fmfgraph*}(40,15)
   \fmfpen{thin}
   \fmfleft{i}
   \fmfright{o}
   \fmf{plain}{i,v,v,o}
   \fmfdot{i,o,v}
   \fmfv{l.a=-90,l=$x$}{i}
   \fmfv{l.a=-90,l=$y$}{o}
   \fmfv{l.a=-90,l=$z$}{v}
\end{fmfgraph*}
\quad + \quad
\begin{fmfgraph*}(40,15)
   \fmfpen{thin}
   \fmfleft{i}
   \fmfright{o}
   \fmf{plain}{i,v1,v2,o}
   \fmf{plain,left,tension=0.05}{v1,v2,v1}
   \fmfdot{i,o,v1,v2}
   \fmfv{l.a=-90,l=$x$}{i}
   \fmfv{l.a=-90,l=$y$}{o}
   \fmfv{l.a=-90}{v1,v2}
   \fmflabel{$z_{1}$}{v1}
   \fmflabel{$z_{2}$}{v2}
\end{fmfgraph*}
\quad + \enskip \dots
\end{fmffile}
\end{fmffile}$

\end{document}

Then I wanted to draw a few disconnected diagrams. This is where I got stuck. I'm trying to get this

Not very pretty I admit, but you get the idea, I hope. However, everything I tried so far did not compile. Is anybody sufficiently familiar with feynMF that could help me out here?