As noted above, digestion may be initiated by enzymes present in the saliva either mixed with the food as it enters the buccal cavity or secreted onto the food prior to ingestion. Most digestion is dependent, however, on enzymes secreted by the midgut epithelium. Digestion mostly occurs in the lumen of the midgut, though regurgitation of digestive fluid into the crop is important in some species. In wood-eating forms, much of the digestion is carried out by microorganisms in the hindgut (Section 4.2.4).

4.2.1. Digestive Enzymes

A wide variety and large number of digestive enzymes have been reported for insects. In many instances, however, enzymes have been characterized (and named) on the basis of their activity on unnatural substrates, that is, materials that do not occur in the normal diet of the insect. This is because many digestive enzymes, especially carbohydrases, are "group-specific"; that is, they hydrolyze any substrate that includes a particular bond between two parts of the molecule. For example, a-glucosidase splits all a-glucosides, including sucrose, maltose, furanose, trehalose, and melezitose. Further, in preparing enzyme extracts for analysis, either gut contents or midgut tissue homogenates are typically used. As House (1974) noted, the former may include enzymes derived from the food per se, while the latter contains endoenzymes (intracellular enzymes) that have no digestive function. Thus, reports on digestive enzyme activity must be examined cautiously.

As would be expected, the enzymes produced reflect both qualitatively and quantitatively the normal constituents of the diet. Omnivorous species produce enzymes for digesting proteins, fats, and carbohydrates. Carnivorous species produce mainly lipases and proteases; in some species these may be highly specific in action. Blow fly larvae (Lucilia cuprina), for example, produce large amounts of collagenase. The nature of the enzymes produced may change at different stages of the life history as the diet of an insect changes. For example, caterpillars feeding on plant tissue secrete a spectrum of enzymes, whereas nectar-feeding adult Lepidoptera produce only invertase. Interestingly, however, even in those endopterygotes in which the larvae and adults utilize the same food the properties of the enzymes change at metamorphosis. In Tenebrio, for example, the larval and adult trypsins and chymotrypsins differ in molecular size, substrate specificity, and kinetics, though why this should be is not clear.

Insects can digest a wide range of carbohydrates, even though only a few distinct enzymes may be produced. As noted earlier, a-glucosidase will hydrolyze all a-glucosides. Likewise, p-glucosidase facilitates splitting of cellobiose, gentiobiose, and phenylgluco-sides; p-galactosidase hydrolyzes p-galactosides such as lactose. In some species, however, there appear to be carbohydrate-digesting enzymes that exhibit absolute specificity. Thus, adult Lucilia cuprina produce an a-glucosidase, trehalase, that splits only trehalose. The normal polysaccharide-digesting enzyme produced is amylase for hydrolysis of starch, though particular species may produce enzymes for digestion of other polysaccharides. For

Beekeeping for Beginners

Beekeeping for Beginners

The information in this book is useful to anyone wanting to start beekeeping as a hobby or a business. It was written for beginners. Those who have never looked into beekeeping, may not understand the meaning of the terminology used by people in the industry. We have tried to overcome the problem by giving explanations. We want you to be able to use this book as a guide in to beekeeping.

Get My Free Ebook

Post a comment