\documentclass[10pt]{article}
\usepackage{tikz}
\usetikzlibrary{shapes,arrows}
\begin{document}
\pagestyle{empty}
\begin{tikzpicture} [scale=2.2]
\node (PS) at (1,1) {$\Pi\Sigma$};
\node (P)  at (0,1) {$\Pi$};
\node (S)  at (2,1) {$\Sigma$};
\node (SP) at (1,0) {$\Sigma\Pi$};

\draw[->] (S) -- node[above] {$\eta_\Pi\Sigma$} (PS);
\draw[->] (P) -- node[above] {$\Pi\eta_\Sigma$} (PS);

\draw[->] (P) -- node[left] {$\eta_\Sigma\Pi\ $} (SP);
\draw[->] (S) -- node[right] {$\ \Sigma\eta_\Pi$} (SP);

\draw[->] (PS) -- node[right] {$\lambda$} (SP);

\end{tikzpicture}
\end{document}