\begin{tikzpicture}
\tikzset{
arrowMe/.style={postaction=decorate,
decoration={markings, mark=at position .55 with {\arrow[very thick]{#1}}
} }}
\draw[arrowMe=stealth](0,0)--node[below]{$$a$$}(5,0);
\draw[arrowMe=stealth](0,3)--node[left]{$$b$$}(0,0);
\draw[arrowMe=stealth](0,3)--node[above]{$$a$$}(5,3);
\draw[arrowMe=stealth](5,0)--node[right]{$$b$$}(5,3);
\draw[dashed](0,1.25)--(5,1.25);
\draw[dashed] (0,1.75)--(5,1.75);
\begin{scope}[xshift=2.5cm,yshift=2cm]
\draw (0,0) rectangle (.4,.4);
\draw (.4,.05) to [bend left=20] (.2,.15);
\fill[color=black](.4,.3) to [bend left=40] (.3,.4) to (.4,.4) to (.4,.3);
\end{scope}
\begin{scope}[xshift=2cm,yshift=2cm]
\draw (0,0) rectangle (.4,.4);
\draw (0,.05) to [bend right=20] (.2,.15);
\fill[color=black](0,.3) to [bend right=40] (.1,.4) to (0,.4) to (0,.3);
\end{scope}
\begin{scope}[xshift=3.8cm,yshift=2.3cm]
\draw (0,0) rectangle (.4,.4);
\draw (.4,.05) to [bend left=20] (.2,.15);
\fill[color=black](.4,.3) to [bend left=40] (.3,.4) to (.4,.4) to (.4,.3);
\end{scope}
\draw (5,2.3)--(4.8,2.3)--(4.8,2.7)--(5,2.7);
\draw (0,.3)--(.2,.3)--(.2,.7)--(0,.7);
\fill[color=black](.1,.3) to [bend left=40] (.2,.4) to (.2,.3) to (.1,.3);
\draw (.2,.65) to [bend right=20] (0,.55);
\begin{scope}[rotate around={240:(1.2,1.2)}]
\begin{scope}[xshift=1cm,yshift=1cm]
\draw (0,0) rectangle (.4,.4);
\draw (0,.05) to [bend right=20] (.2,.15);
\fill[color=black](0,.3) to [bend right=40] (.1,.4) to (0,.4) to (0,.3);
\end{scope}
\end{scope}
\draw[thick,->] (.3,.5)--+(.6,.5);
\draw[thick,->] (1.5,1.4)--+(.4,.5);
\draw[thick,->] (3,2.2)--+(.6,.2);
\draw[thick,->] (4.3,2.5)--+(.4,0);
\end{tikzpicture}