Feeding rate and the structure of protein digestion and absorption in lepidopteran midguts

The transformation of leaf protein into insect tissue can be distilled gros sly into four serial steps: consumption of protein, digestion of protein in to small peptides and free amino acids, absorption of these fragments acros s the gut epithelium, and construction of tissue from the absorbed fragment s. We examine these steps in caterpillars, with emphasis on determining whi ch, if any, process is limiting and how digestive events influence-and are influenced by-the rate of consumption. Five competing hypotheses are (1) th e steps are matched, (2) consumption is limiting, (3) digestion is limiting , (4) absorption is limiting and (5) post- absorptive processes are limitin g. For conditions appropriate to each of the five hypotheses, we use a chem ical reactor model to predict the concentration profiles of protein and pro tein breakdown products along the midgut,Wetest the predictions in two ways . First, based on midgut morphology and rates of digesta flow and protein b reakdown estimated from in vitro experiments, we calculate the axial concen tration profile of protein in the midgut of fifth instar Manduca sexta. Thi s calculation suggests that protein breakdown occurs primarily in the anter ior midgut, thus excluding the hypotheses that feeding and digestive proces ses are matched or that digestion is limiting. In addition, caterpillar fee ding responses to reduced concentrations of protein are not consistent with the hypothesis that consumption is limiting. Second, we review published s tudies that show midgut concentration profiles of protein and breakdown pro ducts, one supporting the hypothesis that absorption across the midgut epit helium is limiting and the other suggesting that post-absorptive processes are limiting Consequently, we interpret compensatory feeding by larval M. s exta as a mechanism for maintaining saturated rates of protein absorption o r of tissue construction. (C) 1999 Wiley-Liss, Inc.