Cysteine protease which plays an essential role in the degradation of proteins in lysosomes (PubMed:15456850, PubMed:24829385). During early embryogenesis, maternally required for the proteolytic processing of yolk proteins in platelets, a lysosome-like structure where a slow and controlled degradation of yolk proteins occurs (PubMed:15456850, PubMed:24829385). In the gonad, required for the clearance of apoptotic germ cells in the engulfing cell phagolysosomes (PubMed:24829385). In embryos, required for the degradation of endocytic and autophagic cargos (PubMed:24829385). In embryos, may play a role in the degradation of lipid-containing droplets (PubMed:26773047). Required for larval development (PubMed:11707440, PubMed:15456850). Specificity close to that of papain. As compared to cathepsin B, cathepsin L exhibits higher activity toward protein substrates, but has little activity on Z-Arg-Arg-NHMec, and no peptidyl-dipeptidase activity. The zymogen form localizes to yolk platelets/granules in the developing oocyte and in the pseudocoelom (PubMed:15456850, PubMed:16857685). Following cell corpse phagocytosis, recruited to phagosomes during the late stages of phagolysosome formation (PubMed:24829385). Expressed in intestine, pharynx posterior bulb, hypodermis and cuticle (at protein level) (PubMed:11707440, PubMed:24829385). Expressed in germ cells, developing oocytes, sheath cells surrounding germ cells and oocytes, and in the eggshell (at protein level) (PubMed:11707440, PubMed:16857685). Expressed in embryos, larvae and adults (at protein level) (PubMed:11707440, PubMed:15456850, PubMed:16857685, PubMed:24829385). Expression transiently increases during early embryonic stages and prior to the larval L1/L2, L2/L3, L3/L4 and L4/adult molts (PubMed:11707440, PubMed:16857685). Highest expression in adults (PubMed:11707440). In embryos, expressed in gut cells (PubMed:11707440). In larvae, expressed in the hypodermis, intestine and pharyngeal lining (PubMed:11707440). In molting larvae, expressed in the old and new cuticle (PubMed:11707440). RNAi-mediated knockdown causes early embryonic lethality with embryos arrested at the 100-200 cell stage (PubMed:11707440). The few larvae which survive have incomplete gut development and die at the L1 stage (PubMed:11707440). RNAi-mediated knockdown at the L3 larval stage causes a growth delay, adults are shorter and thinner, and lay fewer eggs (PubMed:11707440). Belongs to the peptidase C1 family.