Serine protease inhibitor which is required for pupal viability and plays an essential role in regulating the melanization reaction (PubMed:18801354, PubMed:22227521). Inhibits spontaneous melanization and appears to be involved in the melanization immune response to physical wounding in larvae and adults (PubMed:18801354, PubMed:22227521). Acts by negatively regulating the Hayan-phenoloxidase (PPO1) cascade in the hemolymph and possibly the trachea (PubMed:18801354, PubMed:22227521). May function by controlling the initial release of the activated form of PPO1, phenoloxidase (PO) and thus maintains PO availability for processes such as wound response and pigmentation (PubMed:18801354). Up-regulated by injury in larvae and adults. RNAi-mediated knockdown is often lethal at the pupal stage, with flies displaying small black or brown spots prior to death. A small percentage of adults survive when grown at 23 degrees Celsius to preclude pupal lethality. These adult escapers (then kept at 29 degrees Celsius) do not display melanotic areas until several hours after eclosion. These melanotic regions or spots are predominately in air-exposed parts of the body, such as those close to the cuticle, and the thoracic and abdominal spiracles. In the tracheae melanization occurs only in the trunk closest to the spiracle. The ptilinal suture and the surrounding area between the eyes also display various amounts of melanization. Around half display reduced pigmentation of their abdomen, and this often occurred in adults that had large melanotic areas. After septic injury, flies display impaired melanization at the wound site, and in their hemolymph there is an overall reduction in the protein level of PPO1 and no significant activity of PO. Belongs to the serpin family.