Publisher Summary This chapter presents recent progress in the study of the midgut, particularly with reference to ion and water transport. However, the midgut does not function in isolation, and so the attempt is to fit the role of the midgut into the context of the operation of the gut as a whole. The chapter considers the cockroach as an example of a classic generalist and looks at scavenging dipteran larvae, mainly aquatic. Midgut function is described in various types of insects such as solid/plant feeders (phytophagous insects), solid/animal feeders (carnivores), liquid/animal feeders (bloodsuckers), and liquid/plant feeders (sap and nectar feeders). The chapter also discusses the process of digestion of cellulose in insects. There are four categories of evidence that an insect can digest cellulose. It could survive on a diet of pure synthetic cellulose; it could be shown to incorporate and metabolize label from radioactive cellulose in the food; it could be shown to break down synthetic crystalline cellulose; or it could be shown that there is less cellulose in the excreta than was consumed in the food.

Insect Midgut Function

To study the molecular basis of differences in the insecticidal spectrum of Bacillus thuringienesis delta-endotoxins, we have performed binding studies with three delta-endotoxins on membrane preparations from larval insect mid-gut. Conditions for a standard binding assay were established through a detailed study of the binding of 125I-labeled Bt2 toxin, a recombinant B. thuringiensis delta-endotoxin, to brush border membrane vesicles of Manduca sexta. The toxins tested (Bt2, Bt3 and Bt73 toxins) are about equally toxic to M. sexta but differ in their toxicity against Heliothis virescens. Equilibrium binding studies revealed saturable, high-affinity binding sites on brush border membrane vesicles of M. sexta and H. virescens. While the affinity of the three toxins was not significantly different on H. virescens vesicles, marked differences in binding site concentration were measured which reflected the differences in in vivo toxicity. Competition experiments revealed heterogeneity in binding sites. For H. virescens, a three-site model was proposed. In M. sexta, one population of binding sites is shared by all three toxins, while another is only recognized by Bt3 toxin. Several other toxins, non-toxic or much less toxic to M. sexta than Bt2 toxin, did not or only marginally displace binding of 125I-labeled Bt2 toxin in this insect. No saturable binding of this toxin was observed to membrane preparations from tissues of several non-susceptible organisms. Together, these data provide new evidence that binding to a specific receptor on the membrane of gut epithelial cells is an important determinant with respect to differences in insecticidal spectrum of B. thuringiensis insecticidal crystal proteins.

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1432-1033.1989.tb15201.x

Specificity of Bacillus thuringiensis delta-endotoxins. Importance of specific receptors on the brush border membrane of the mid-gut of target insects.

Importance of specific receptors on the brush border membrane of the mid-gut of target insects

Immunocytochemical localization of Bacillus thuringiensis insecticidal crystal proteins in intoxicated insects

The δ-endotoxins are a superfamily of proteins that occur as crystalline inclusions in the spore-forming bacterium Bacillus thuringiensis ([35][1]). These toxins are of considerable interest as environmentally safe insecticides. Historically, B. thuringiensis toxins have been divided into two

S.T. Bishoff

Papers

Histopathological effects of Bacillus thuringiensis on the midgut of tobacco hornworm larvae (Manduca sexta): Low doses compared with fasting