Overview of Insect Midgut Function

Food may be acquired by biting with chewing parts, sucking by pierce-sucking mouthparts, or by ingesting a pre-digested or pre-dispersed meal. Digestion is a stepwise process by which the molecules of food are hydrolyzed into components able to be absorbed. The first step of digestion is the initial digestion, when food polymers result in oligomers, followed by the second step, intermediary digestion, corresponding to the conversion of oligomers into dimers and, finally, the third step, final digestion, in which the dimers are cleaved into monomers that are absorbed. The gut morphology varies among insect taxa from the basal plan formed by a capacious crop followed by a midgut with anteriorly placed ceca, ending in a hindgut. The midgut has inside an anatomical chitin-protein film, the peritrophic membrane (PM), that separates two luminal compartments: endoperitrophic (inside PM) and ectoperitrophic (outside PM) spaces. In polyneopterans and lower holometabolans, the enzymes of initial and intermediary digestion move freely inside the midgut, whereas in higher holometabolans, only the enzymes of initial digestion traverse PM into the endoperitrophic space. The enzymes of intermediary digestion are retained in the ectoperitrophic space because they are larger than the PM pores (7–9 nm dia). There are midgut countercurrent fluxes caused by the secretion of fluid in a posterior region and its absorption in an anterior region. These countercurrent fluxes decrease the loss of enzymes by excretion as part of the enhancement of digestive efficiency caused by the compartmentalization of digestion. The sites of water and nutrient absorption are identified with the use of a non-absorbable dye and nutrients. The concentration of dye indicates water removal, its dilution indicates water secretion, whereas a change in the ratio of nutrient and dye indicates nutrient absorption.