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 large salivary gland cells and weakly expressed in imaginal disks. Less expressed than Idgf2 and Idgf4. Expressed both maternally and zygotically. Expressed throughout development, with a much stronger expression during larval stages. Lacks the typical Glu active site in position 150 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.