The passage of macromolecules through biological membranes is an essential process for all multicellular organisms. Insects have developed a mechanism different from that known for all other eukaryotes investigated so far. This review discusses the function and evolution of this mechanism. Insect pupae do not feed during metamorphosis. Therefore they depend on material that has been accumulated during the larval life. At the end of this period, shortly before pupariation, a rise in titer of ecdysteroid hormones induces the incorporation of a large fraction of storage proteins (hexamerins) from the body fluid into the fat body cells. The transport of hexamerins across the cell-membrane is mediated by a specific ecdysteroid-controlled receptor. It is synthesized as a precursor protein that is subsequently processed into the active receptor. This receptor protein is very unusual because it is closely related to its own hexamerin ligand. Sequence comparison shows that the hexamerins and hexamerin receptors diverged early in insect evolution and derive from a common hemocyanin ancestor.