digraph Morphisms {
    node [fontcolor=blue];
    edge [fontcolor=red];

    sigma1 [label="σ ~ σ1", shape=plaintext];
    sigma2 [label="σ ~ σ2", shape=plaintext];
    tau1 [label="τ ~ τ1", shape=plaintext];
    tau2 [label="τ ~ τ2", shape=plaintext];

    sigma1 -> tau1 [label="f_1"];
    sigma2 -> tau2 [label="f_2"];

    tau1 -> tau2 [label="m_τ"];
    sigma1 -> sigma2 [label="m_σ"];
}