INSECTS
pharynx immediately behind the mouth, followed by a narrower, tubular oesophagus (Oe), after which comes a sac-like enlargement, or crop (Cr), in which the food is temporarily stored, and finally an antecharnber to the stomach, named the proventriculus. The third part of the alimentary canal, connecting the stomach with the anal opening, is the intestine, usually composed of a narrow anterior part, and a wide posterior part, or rectum (Rect). Muscle layers surrounding the entire alimentary tube cause the food to be swallowed and to be passed along from one section to the next toward the rear exit.
With the taking of the food into the alimentary canal, the matter of nutrition is by no mans accomplished, for the animal is still confronted with the problem of getting the nutrient materials into the inside of its body, where alone they can be used. The alimentary tube has no openings anywhere along its course into the body cavity. Whatever food substances the tissues of the animal receive, therefore, must be taken through the walls of the tube in which they are inclosed, and this transposition is accomplished by dissolving them in a liquid. Most of the nutrient materials in the raw food matter, however, are not soluble in ordinary liquids; they must be changed chemically into a form that will dissolve. The process of getting the nutrient parts of the raw foodstuff into solution constitutes digestion.
The digestive liquids in insects are furnished mostly by the stomach walls or the walls of tubular glands that open into the stomach, but the secretion of a pair of large glands, called the salivary glands (Fig. 68, SlGl), which open between the mouth parts, perhaps has in some cases a digestive action on the food as it is taken into the mouth.
Digestion is a purely chemical process, but it must be a rapid one. Consequently the digestive juices contain not only substances that will transform the food materials into soluble compounds, but other substances that will
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