Cooperates with insulin-like peptides to stimulate the proliferation, polarization and motility of imaginal disk cells. May act by stabilizing the binding of insulin-like peptides to its receptor through a simultaneous interaction with both molecules to form a multiprotein signaling complex. Secreted in hemolymph. It is probably transported to target tissues via hemolymph. Primarily expressed in yolk cells and fat body. In larvae, it is expressed in the imaginal ring and weakly expressed in imaginal disks. More strongly expressed than Idgf1 and Idgf3. Expressed both maternally and zygotically. Expressed throughout development, with a much stronger expression during larval stages. Glycosylated. Lacks the typical Glu active site in position 152 that is replaced by a Gln residue, preventing the hydrolase activity. Its precise function remains unclear. Belongs to the glycosyl hydrolase 18 family. IDGF subfamily.