epithelium.The rectum includes six to eight thick-walled rectal pads whose function is to absorb ions,water,and small organic molecules. Salivary glands Known by the names according to either the site(labial glands 腺，mandibular gland.上鄂腺)or function(silk glands,venom glands毒 腺) Saliva is watery,enzyme-containing fluid which serves to lubricate the food and initiate its digestion.Other substances occur in saliva, such as pectinase,enzyme,toxins(venoms),and are important in food acquisition. Gut physiology Digestion In some insects,especially fluid feeders,digestion may begin before the food is ingested (through the injection or regurgitation (of enzymes on to the food,but in general digestion occurs largely in the midgut,where most the enzymes are produced.These enzymes break down the complex substances in the food into more simple substance which can be absorbed and later assimilated. Carbohydrates are generally absorbed as monsaccharides so that,before they are absorbed,disaccharides (and polysaccharides must be broken down to their component monsaccharides.This may be a complex reaction tacking place partly in the gut wall and a variety enzymes is involved. Protein Proteins are broken down to the amino acids.The amino acids is used in the formation of new tissue proteins. Proteins are accumulated in large amounts by the developing oocytes.In the male of some species,proteins are accumulated by the accessory reproductive glands,probably for use in spermatophore production. Lipid(脂类)epithelium. The rectum includes six to eight thick-walled rectal pads whose function is to absorb ions, water, and small organic molecules. Salivary glands Known by the names according to either the site (labial glands下唇 腺, mandibular gland上鄂腺) or function (silk glands, venom glands毒 腺) Saliva is watery, enzyme-containing fluid which serves to lubricate the food and initiate its digestion. Other substances occur in saliva, such as pectinase, enzyme, toxins (venoms), and are important in food acquisition. Gut physiology Digestion In some insects, especially fluid feeders, digestion may begin before the food is ingested（摄食）through the injection or regurgitation （反刍）of enzymes on to the food, but in general digestion occurs largely in the midgut, where most the enzymes are produced. These enzymes break down the complex substances in the food into more simple substance which can be absorbed and later assimilated. Carbohydrates are generally absorbed as monsaccharides（单糖）so that, before they are absorbed, disaccharides（双糖）and polysaccharides（多糖）must be broken down to their component monsaccharides. This may be a complex reaction tacking place partly in the gut wall and a variety enzymes is involved. Protein Proteins are broken down to the amino acids. The amino acids is used in the formation of new tissue proteins. Proteins are accumulated in large amounts by the developing oocytes. In the male of some species, proteins are accumulated by the accessory reproductive glands, probably for use in spermatophore production. Lipid（脂类）