The activities of amylase, cysteine proteinase and trypsin decrease throughout the contents of the midgut. This is what one would expect when there is a flux of fluid from the posterior midgut to the anterior midgut in the ectoperitrophic space, as described for most insects (for reviews, see Terra and Ferreira, 1994 and Terra and Ferreira, 2005). This flux is thought to be caused by the secretion of fluid by posterior midgut cells and its absorption by anterior midgut cells. As the flux moves, it displaces forward enzymes and digestion products diffusing from the PM into the ectoperitrophic space. This counterflux prevents digestive enzymes from being lost to the feces and causes enzyme recycling.
Taking S. levis as a model, curculionid digestion differs from that of putative Coleoptera ancestors ( Terra and Ferreira, 2005) in that most terminal digestion of proteins takes place on the surface of midgut cells.