Page:Insects - Their Ways and Means of Living.djvu/142



pharynx irnrnediately behind the rnouth, followed by a narrower, tubular oesophagus (Oe), after which comes a sac- like enlargernent, or crop (Cr), in which the food is tern- porarily stored, and finally an antecharnber to the stornach, narned the proventriculus. The third part of the alimen- tary canal, connecting the stomach with the anal opening, is the intestine, usually cornposed of a narrow anterior part, and a wide posterior part, or rectum (Rect). Muscle layers surrounding the entire alirnentary 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 alirnentary canal, the/natter of nutrition is by no rneans accornplished, for the animal is still confronted with the problern of getting the nutrient rnaterials into the inside of its body, where alone they can be used. The alirnentary tube bas no openings anywhere along its course into the body cavity. Whatever food substances the tissues of the animal receive, therefore, rnust be taken through the walls of the tube in which they are inclosed, alad this transposition is accorn- plished by dissolving thern in a liquid. Most of the nutri- ent rnaterials in the raw food rnatter, however, are not soluble in ordinary liquids; they rnusr be changed chern- ically into a forrn that will dissolve. The process of get- ring the nutrient parts of the raw foodstuff into solution constitutes digestion. The digestive liquids in insects are furnished mostly by the stornach walls or the walls of tubular glands that open into the stornach, but the secretion of a pair of large glands, called the salivary glands (Fig. 68, SIGI), which open between the rnouth parts, perhaps bas m sorne 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 transforrn the food rnaterials into soluble cornpounds, but other substances that will

[ II0]

WAYS