500 phytophagous species, in many omnivorous and carnivorous species the pH of the hindgut is greater than that of the midgut.

Many variables affect the pH of different regions of the gut. Generally the pH of the crop is the same as that of the food, though in some species it is consistently less than 7 because of the digestive activity of microorganisms or regurgitation of digestive juice from the midgut. The pH of the midgut differs among species but tends to be constant for a given species because of the presence in this region of buffering agents. In a few species there are local variations in pH within the midgut that can be related to changes in digestive function from one part to another. For example, in the cockroach Nauphoeta cinerea the pH of the anterior midgut is 6.0-7.2, which coincides with the pH optimum of the amylase found mainly in this region. In the posterior midgut, on the other hand, the pH is about 9, near the optimum for the proteinases that are active there (Elpidina et al., 2001). The hindgut typically has a pH slightly less than 7, presumably resulting from the presence of the nitrogenous waste product, uric acid (Chapter 18, Section 3.2). The hindgut contents of some phytophagous species may be quite acidic as a result of the formation of organic acids from cellulose by symbiotic microorganisms.

The relatively constant pH found in different regions of the gut results from the presence in the lumen of both inorganic and organic buffering agents. In some species, inorganic ions, especially phosphates, but including aluminum, ammonium, calcium, iron, magnesium, potassium, sodium, carbonate, chloride, and nitrate, seem to offer sufficient buffering capacity. In other species organic acids, including amino acids and proteins, tend to supplement or replace the buffering effect of the inorganic ions. For some of the inorganic ions and water, active secretory or resorption mechanisms are known to regulate their concentration in the midgut. These mechanisms are also capable of inducing fluid flow, especially in the ectoperitrophic space. Specifically, secretion of ions and water across the posterior midgut epithelium simultaneously with their resorption at the anterior end establishes a forward flow of digestive fluid. This is thought to conserve nutrients and recycle enzymes.

Redox potential, which measures ability to gain or lose electrons, that is, to be reduced or oxidized, respectively, is an important factor in digestion in some insects as it affects the structure of both dietary proteins and proteolytic enzymes. The gut redox potential, which is closely linked to pH, is normally positive, indicating oxidizing (aerobic) conditions. However, in species able to digest keratin the redox potential of the midgut fluid is strongly negative. It has been suggested that such an anaerobic (reducing) environment is necessary to enable the keratinase to split the disulfide bonds (House, 1974). Subsequently, normal proteases hydrolyze the polypeptides.

4.2.3. Control of Enzyme Synthesis and Secretion

Numerous studies have shown that enzyme activity in the midgut varies in relation to food intake, though it is not always clear whether it is synthesis and/or release of the enzymes that is being controlled. In many species, including Locusta migratoria and Tene-brio molitor, enzymes are not stored in the midgut cells but are liberated immediately into the gut lumen. In other insects, for example, Stomoxys calcitrans and some mosquitoes, enzymes are stored (possibly in an inactive form) and released when feeding occurs. The synthesis/release of enzyme in proportion to the amount of food ingested may be regulated by secretagogue, hormonal or neural mechanisms, though there is very little evidence for the latter. Unfortunately, for most species, the evidence presented in support of one mechanism or another is equivocal. In a secretagogue system enzymes are produced in response to food present in the midgut. Presumably the amount produced is directly influenced by the concentration of food in the lumen. The best evidence for secretagogue control of enzyme activity is for mosquitoes and other blood feeders. In Aedes a blood meal cannulated directly into the midgut stimulates production of a proportionate amount of trypsin (Briegel and Lea, 1975). These authors showed that a variety of components of blood could serve as secreta-gogues. However, it is noteworthy that the amount of enzyme produced by this means was reduced in insects whose median neurosecretory cells had been removed. Where hormonal control of enzyme production has been proposed, the amount of food passing along the foregut, measured as the degree of stretching of the gut wall, is believed to result in the release from the corpora cardiaca of a proportionate amount of neurosecretion which travels via the hemolymph to the midgut cells.

At present, there is no consensus as to whether a midgut epithelial cell produces a complete package of enzymes or whether the proportions of different enzymes can vary with changes in the diet. Certainly, a secretagogue method for regulating enzyme activity could more easily account for changes in the level of specific enzymes reported to occur with alterations to the diet of some species.

4.2.4. Digestion by Microorganisms

Microorganisms (bacteria, fungi, and protozoa) may be present in the gut, but for only a few species has there been a convincing demonstration of their importance in digestion. In many insects microorganisms appear to have no role, as the insects can be reared equally well in their absence. In other species microorganisms may be more important with respect to an insect's nutrition than digestion per se. Where a role for microorganisms in digestion has been demonstrated, the relationship between the microorganisms and insect host is not always obligate, but may be facultative or even accidental.

Bacteria are important cellulose-digesting agents in many phytophagous insects, especially wood-eating species whose hindgut may include a fermentation pouch in which the microorganisms are housed. In other species, for example, the wood-eating cockroach Panesthia, bacteria in the crop are essential for cellulose digestion. In larvae of the wax moth Galleria, bacteria normally present in the gut undoubtedly aid in the digestion of beeswax, yet bacteriologically sterile larvae produce an intrinsic lipase capable of degrading certain wax components. Finally, many insects feed on decaying vegetation and must, therefore, ingest a large number of saprophytic bacteria which, temporarily at least, would continue their degradative activity in the gut. In this sense, therefore, though the relationship is accidental, the microorganisms are assisting in digestion.

In lower termites and some primitive wood-eating cockroaches (Cryptocercus), flagellate and ciliate protozoa occur in enormous numbers in the hindgut. The relationship between the insects and protozoa is mutualistic; that is, in return for a suitable, anaerobic environment in which to live, the protozoa phagocytose particles of wood eaten by the insects, fermenting the cellulose and releasing large amounts of glucose (in Cryptocercus) or organic acids (in termites) for use by the insects. In higher termites (Termitidae) the hindgut contains bacteria, not protozoa, but there is no evidence that the bacteria produce cellulolytic enzymes (and see Section 4.2.1).

Fungi rarely play a direct role in the digestive process of insects, though it is reported that yeasts capable of hydrolyzing carbohydrates occur in the gut of some leafhoppers (Cicadellidae). However, a mutualistic relationship has evolved between many fungi and insects whereby the fungi convert wood into a more usable form, while the insects serve to

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