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Melanization is a key part of insect innate immunity, which is mainly induced in host after pathogen invasion. Phenoloxidase (PO) is a crucial component to catalyze melanization. As PO is not stable in insects, it is always stored as prophenoloxidase (PPO), thus the activation of PPO is an important process for melanization induction. PO is generated from its precursor proPO by prophenoloxidase activatingproteases (PAPs) while PO activity can’t be highly induced, at least in some insects. It has been demonstrated that proPO activation requires additional proteins to serve as a cofactor for PAPs Serine protease homolog (SPH) in Manduca sexta and some other insects. However, it is not fully known if PAPs and cofactors are required for PPO activation in mosquito and Drosophila melanogaster. Moreover, formerly demonstrated SPH1 (1a) in M. sexta seems having a series orthologs, which need to be explored.