An insect's mouth, located centrally at the base of the mouthparts, is a muscular valve (sphincter) that marks the "front" of the foregut. Food in the buccal cavity is sucked through the mouth opening and into the pharynx by contractile action of cibarial muscles. These muscles, located between the head capsule and the anterior wall of the pharynx, create suction by enlarging the volume of the pharynx (like opening a bellows). This "suction pump" mechanism is called the cibarial pump. It is especially well-developed in insects with piercing/sucking mouthparts.
From the pharynx, food passes into the esophagus by means of peristalsis (rhythmic muscular contractions of the gut wall). The esophagus is just a simple tube that connects the pharynx to the crop, a food-storage organ. Food remains in the crop until it can be processed through the remaining sections of the alimentary canal. While in the crop, some digestion may occur as a result of salivary enzymes that were added in the buccal cavity and/or other enzymes regurgitated from the midgut.
In some insects, the crop opens posteriorly into a muscular proventriculus. This organ contains tooth-like denticles that grind and pulverize food particles. The proventriculus serves much the same function as a gizzard in birds. The stomodeal valve, a sphincter muscle located just behind the proventriculus, regulates the flow of food from the stomodeum to the mesenteron.
In a developing embryo, the foregut arises as a simple invagination of the anterior body wall: this means that all of its tissues and organs are derived from embryonic ectoderm. In effect, the inside of the stomodeum is continuous with the outside of the insect's body. Since exoskeleton is secreted to protect the insect externally, it is not surprising to find that cells lining the foregut produce a similar structure (known as the intima) to protect themselves from abrasion by food particles. The hard denticles inside the proventriculus are made from this same material.
Next page: Mesenteron