Cysteine protease inhibitor which inhibits members of the peptidase C1 family (PubMed:12704112, PubMed:15664654). Does not inhibit asparaginyl endopeptidase (PubMed:15664654). May play a protective role against exogenous cysteine proteases derived from soil bacteria or fungi, or rotting fruits and vegetation (PubMed:24001183). Expressed in embryos, larvae and adults (at protein level). Expression transiently increases prior to the larval L2/L3, L3/L4 and L4/adult molts. Belongs to the cystatin family.