[Tex/LaTex] Arrow pointing at the middle of another arrow

Question

I am new to tikzp and need some help in performing this task(representing a moderator). Could somebody help me with my code? How can I extend the length of the arrow? And how to tell tikzp that it should draw this kind of arrow?

What I got:

What i want:

My Code:

\usepackage{tikz}
\usepackage{pgfplots}
\pgfplotsset{compat=1.8}
\usetikzlibrary{positioning}
\begin{document}


\begin{figure}[h]
\label{illustration}
\centering
\caption{The model}
\begin{tikzpicture}[
node distance=1cm and 1cm,
ar/.style={->,>=latex},
mynode/.style={
draw,
text width=4cm,
minimum height=1cm,
 align=center
 }
]
\node[mynode] (LE) {Level of expertise};
\node[mynode,right=of LE] (PS) {Problem solving time};
\node[mynode,below=of LE] (FI) {Fluid intelligence};


\draw[ar]
  (LE) -> node[above] {-}  (PS);
  \draw[ar]
  (FI) -> node[left] {-}  (PS);
\end{tikzpicture}
\end{figure}
\end{document}

Answer 1

Yet another answer, defining a coordinate on the fly when drawing the arrow from LE to PS.

\documentclass[border=1mm]{standalone}
\usepackage{tikz}
\usetikzlibrary{positioning}
\begin{document}

\begin{tikzpicture}%
  [node distance=1cm and 1cm,
   ar/.style={->,>=latex},
   mynode/.style=
    {draw,
     text width=4cm,
     minimum height=1cm,
     align=center
    }
  ]
  \node[mynode] (LE) {Level of expertise};
  \node[mynode,right=of LE] (PS) {Problem solving time};
  \draw[ar] (LE) -> node[above] {-} coordinate(LE-PS) (PS);
  \node[mynode,below=of LE-PS] (FI) {Fluid intelligence};
  \draw[ar] (FI) -> node[left] {-}  (LE-PS);
\end{tikzpicture}

\end{document}