| Popis: |
Within the digestive tract, the ingested food is broken down into simpler molecules by the action of enzymes and thus rendered absorbable through the gut wall. The midgut is composed of a long straight tube which often contains blind pouches (caeca). It is the main site of production of digestive enzymes and of absorption. In many insect species, it secretes a peritrophic membrane which is permeable to the digestive enzymes and to the products of digestion. In most insects, the structure of the midgut seems to be rather uniform, and thus the same cells are both secretory and absorptive. However, ultrastructural and cytochemical studies have shown a variety of differences which are related sometimes to cycles of cellular activities. In other cases, the midgut exhibits a clear division into functional segments or a mixture of different kinds of specialized cells may be present. Physiological experiments combined with ultrastructural and cytochemical investigations have sometimes clearly established the functions of these segments, or cells. It was studies of this type that showed the important role of the midgut epithelium in ionic regulation and mineral accumulation in insects belonging to several different orders. The transfer of fluid and ions through the wall of the midgut sometimes leads to curious arrangements that bring into contact a midgut segment with the Malpighian tubules, and concurrently certain specialized gut cells lose their digestive function in order to undertake an absorptive role. |
