Serine protease inhibitor which plays an essential role in regulating the tracheal melanization immune response to bacterial and fungal infection (PubMed:18854145, PubMed:22227521). Acts by negatively regulating a protease cascade involving Mp1 and Sp7, that functions in the activation of the Hayan-phenoloxidase (PPO1) cascade (PubMed:18854145). Detected in the extracellular space at the apical surface of tracheal cells. In larvae, highly expressed in the tracheal system. Weak expression is also detected in other larval tissues including the gut and epidermis. RNAi-mediated knockdown results in spontaneous melanization of the tracheal system in larvae and pupae (PubMed:18854145, PubMed:22227521). Displays melanization at the 2nd or 3rd larval stage and affected larvae die a few days after it first occurs (PubMed:18854145). Melanization is first detected between tracheal cells and the overlying cuticle, then increases in intensity and spreads to the epithelial layer of the affected tracheal cells (PubMed:18854145). Increased expression of the antimicrobial gene Drs in the trachea and fat body (PubMed:18854145). Belongs to the serpin family.