Serine protease which, by converting prophenoloxidase 1 (PPO1) into its active form, plays an essential role in the melanization immune response to physical or septic wounding. May function in diverse PPO1-activating cascades that are negatively controlled by different serpin proteins; Spn27A and Spn28D in the hemolymph, and Spn28D and Spn77BA in the trachea. Also required in the systematic wound response by mediating the redox-dependent activation of the JNK cytoprotective cascade in neuronal tissues after integument wounding