\usepackage {pstcol,pst-node,pst-tree}
\psset{angleB=-90,arrows=->,nrot=:U}
\newcommand{\molesworth}[1]{%
 \pstree[#1]{\Tdia{ }}
 {
  \Tp[arrows=->,edge={\ncbar[angleA=180]}]
  \nbput{Gabbitas}
  {\psset{linestyle=dashed,arrows=-} \Tp }
  \Tp[arrows=->,edge={\ncbar}]
  \naput{Thring}
 }
}
\psset{showbbox=true}
\begin{tabular}{l}
\molesworth{}\\[1cm]
\molesworth{xbbl=1cm,xbbr=1cm}
\end{tabular}