The potato type I proteinase inhibitor family is a group of serine proteases that can alter physiological processes by regulating immune responses, wound healing, and tissue reorganization. In this study, we cloned the PmEglin cDNA from the salivary gland of Hirudinaria sp. and showed that it is a member of the potato type I proteinase inhibitor family. The PmEglin cDNA contains an open reading frame of 264 nucleotides that encode 87 amino acids. After removal of the signal peptide, the mature PmEglin protein has a predicted molecular weight of 8284.32 Da and an isoelectric point of 6.04, and appears to be a structurally stable hydrophilic protein. Structural analysis showed that PmEglin is similar to other Eglin C proteins, with a "wedge-shaped" conformation. Gly40-Asp50 is located on the thin edge of the wedge and forms the inhibitory loop, which binds to serine proteases to exert its inhibitory effects. PmEglin is predicted to inhibit neutrophil elastase and other proteins. Ingestion of a blood meal by Hirudinaria sp. significantly increased PmEglin mRNA transcript levels, which peaked at 3 d after ingestion. Once PmEglin protein levels returned to pre-feeding levels, PmEglin mRNA transcript levels gradually decreased. In conclusion, based on our analysis of the expression patterns of the PmEglin mRNA and the structural characteristics of the protein, we hypothesize that PmEglin functions as a serine protease inhibitor, which provides a theoretical foundation for the development and utilization of PmEglin protein and Hirudinaria sp. [ABSTRACT FROM AUTHOR]