Serine protease inhibitor with activity toward trypsin (PubMed:18956323). Negatively regulates the Toll signaling pathway and suppresses the expression of the antifungal peptide drosomycin (PubMed:19581577). Its negative regulation of the Toll signaling pathway also results in the inhibition of the melanization immune response via the phenoloxidase (PPO1) cascade (PubMed:19581577). Essential for unfolding and expansion of the wings after emergence from the pupal case (PubMed:18956323). May regulate the Toll pathway by blocking the proteolysis of the Toll ligand spz (PubMed:19581577). Expressed in nurse cells and oocytes. Expressed in wings. Expressed both maternally and zygotically. Expressed throughout development, with very weak expression between stages 10-16 of embryogenesis and weak expression in adults. In stage 5-6 blastoderm and gastrulating embryos, expression is uniform. In stage 16-17 embryos, expressed in the ventral and dorsal epidermis, posterior spiracles, foregut, hindgut, sensory nervous system primordium, pharynx and respiratory system. In larvae, expressed in the posterior spiracles and tracheal system. In pupae, expressed in the wing imaginal disks. Larval lethal (PubMed:19581577). Larvae develop melanotic spots and display an increase in the activity of the activated form of prophenoloxidase 1 (PPO1), phenoloxidase (PO) (PubMed:19581577). Larvae display enhanced activation of the Toll signaling pathway, with increased Tl, spz and Drs expression (PubMed:19581577). RNAi-mediated knockdown of maternal and zygotic expression results in adults that have a reduced lifespan (PubMed:19581577). Wing development appears to be unaffected, however after emergence from the pupal case adult wings fail to unfold and instead remain folded and hypotrophic until death (PubMed:18956323). Other aspects of development in embryos, larvae, pupae and adults appears to be unaffected. Belongs to the serpin family.