Insects feed on almost every organic substance found in nature, and their digestive systems exhibit consider able variation_ The alimentary canal is a tube, usually somewhat coiled, which extends from the mouth to the anus (Figure 1). It is differentiated into three main regions: the foregut, or stomodaeum; the midgut, or mesenteron; and the hindgut, or proctodaeum. Both the foregut and hindgut are derived from ectodermal tissue and are lined internally by a thin layer of cuticle, the intima. This cuticle is shed at each molt along with the outer exoskeleton.

Fig1. Internal organs of a grasshopper, shown in longitudinal section (somewhat diagrammatic). ans, anus; ao, dorsal aorta; be, bursa copulatrix; ca, corpus allatum; cec, circumesophageal connective; cg, cerebral ganglion (part of the brain); em, gastric caeca; en, colon; cp, crop; eg, eggs; eso, esophagus; gn, ganglia of ventral nerve cord; hr, heart; hyp, hypopharynx; il, ileum; lbm, labium; lbr, labrum; mg, midgut or mesenteron; mt, Malpighian tubules; nc, ventral nerve cord; og, optic ganglion (part of the brain); ovd, oviduct; ovp, ovipositor; ovt, ovarian tubules; phx, pharynx; rec, rectum; segn, subesopha geal ganglion; slg, salivary gland; sld, salivary duct; spth, spermatheca; vag, vagina. (Redrawn from Robert Matheson: Entomology for Introductory Courses, Second Edition. Comstock Publishing Company, Inc.)

Most insects have a pair of glands lying below the anterior part of the alimentary canal (Figure 1, slg). The ducts from these glands extend forward and unite into a common duct that opens near the base of the labium or hypopharynx. These labial glands (so named because they open at the base of the labium) generally function as salivary glands. There is often an enlargement of the duct from each gland that serves as a reservoir for the salivary secretion. The labial glands in the larvae of Lepidoptera, Trichoptera, and Hymenoptera secrete silk, which is used in making cocoons and shelters and in food gathering by net spinning caddisflies.

The foregut is usually differentiated into a pharynx (phx, immediately beyond the mouth), esophagus (eso, a slender tube extending posteriorly from the pharynx), crop (cp, an enlargement of the posterior portion of the foregut), and proventriculus. At its posterior end is the stomodaeal valve, which regulates the passage of food between the foregut and midgut. In some groups, such as cockroaches and termites, the proventriculus may bear an armature of teeth internally; these are used to further crush the food before it enters the midgut. The intima is secreted by the foregut epithelium and is relatively impermeable. The intima and epithelium are often longitudinally folded. Outside of the epithelium is an inner layer of longitudinal muscles and an outer layer of circular muscles. The longitudinal muscles sometimes have insertions on the intima. The anterior part of the foregut is provided with dilator muscles, which have their origins on the walls and apodemes of the head and thorax and their insertions on the stomodaeal muscle layers, the epithelium, or intima. These are best developed in the pharyngeal region in sucking insects, where they make the pharynx into a sucking pump. The crop is specialized for the temporary storage of food. It may be a simple enlargement of the foregut, or, as in mosquitoes and Lepidoptera, it may be a lateral diverticulum off the digestive tract. Little or no digestion of food takes place in the foregut.

The midgut (mg) is usually an elongate tube of rather uniform diameter, sometimes differentiated into two or more parts. It often bears diverticula, the gastric caeca (em), near its anterior end. The midgut is not lined by cuticle. The epithelial layer of the midgut is in volved both with the secretion of digestive enzymes into the lumen and in the absorption of the products of digestion into the body of the insect. Individual midgut epithelial cells are generally rather short-lived and are constantly being replaced. These dividing cells may be scattered throughout the midgut, or may be concentrated as pockets of growth. Such areas are sometimes visible from the lumen of the gut as invaginated crypts and from the outer side as bulges (called nidi). The midgut is the primary site of digestion and absorption in the alimentary canal. In many species, the midgut epithelium and the food are separated by a peritrophic membrane-a nonliving, permeable network of chitin and protein that is secreted by the epithelium. The function of the peritrophic membrane is unclear. It may serve to limit abrasion of the epithelium, to inhibit the movement of pathogens from the food to the insect's tissues, or as a means of separating endo- and ectoperitrophic spaces within which digestive specialization can occur.

The hindgut extends from the pyloric valve, which lies between the midgut and hindgut, to the anus. Posteriorly it is supported by muscles extending to the abdominal walL The hindgut is generally differentiated into at least two regions, the anterior intestine and the posterior rectum (rec). The anterior intestine may be a simple tube, or it may be subdivided into an anterior ileum (il) and a posterior colon (en). The Malpighian tubules (mt), which are excretory organs (see later), arise at the anterior end of the hindgut, and their contents empty into it. The hindgut is the final site for resorption of water, salts, and any nutrients from the feces and urine. The rectum in several species has large, thick rectal pads that are important in re moving water from the feces.

The filter chamber is a modification of the alimentary canal in which two normally distant parts are held close together by connective tissue; it occurs in many of the Hemiptera and varies somewhat in form in different members of the order. The midgut in these insects is differentiated into three regions: the first, second, and third ventriculi. The first and second ventriculi are saclike structures immediately posterior to the esophagus, and the third ventriculus is a slender tube. Typically; the third ventriculus turns forward and comes to lie close to the first ventriculus, often coiling about it, where it is held in place by connective tissue. This complex-the first ventriculus, the coiled third ventriculus, and the connective tissue-forms the filter chamber (Figure 2). Beyond the filter chamber, the alimentary canal continues backward, usually as a slender tube, to enter the rectum. The Malpighian tubules emerge either from the filter chamber or just beyond it.

Fig2. The filter chamber of Hemiptera (diagrammatic). A, A simple type of filter chamber, in which the two ends of the midgut (the first and third ventriculi) are bound together; B, The filter chamber of a scale insect (Lecanium); C, A filter chamber in which the posterior part of the midgut (the third ventriculus) coils about the anterior part (the first ventriculus), with the hindgut emerging from the anterior end. In A and B the junction of the midgut and hindgut (where the Malpighian tubules, which are not shown, enter the alimentary tract) is in the filter chamber. eso, esophagus;Jch, filter chamber; hg, anterior portion of the hindgut; mt, Malpighian tubules; prct, proctodeum; rec, rectum; vnt, ventriculus (1, first; 2, second; 3, third).

Many Hemiptera live on plant juices, which they usually ingest in large quantities. Entomologists think the filter chamber is a device that allows water from ingested sap to pass directly from the anterior portion of the midgut to the hindgut, concentrating sap before di gestion in the posterior part of the midgut. This excess fluid passes from the anus as honeydew. However, because honeydew is often rich in nutrients such as car bohydrates and amino acids, there is some doubt about the exact function of the filter chamber.

Digestion is the process of changing food chemically and physically so that it can be absorbed and nourish various parts of the body. This process may be gin even before the food is ingested but usually occurs as the ingested materials pass through the digestive tract. Solid foods are broken down by various mechanical means (chiefly the mouthparts and proventricular teeth), and all foods are subjected to a battery of enzymes as they pass through the digestive tract.

Insects feed on a great variety of living, dead, and decomposing animals, plants, and fungi and on their products. In some cases, liquids such as blood or plant juices may constitute their entire food supply. The digestive system varies considerably with the different kinds of foods consumed. The food habits may even vary greatly in a single species. Larvae and adults usually have entirely different food habits and different types of digestive systems. Some adults do not feed at all.

Most insects take food into the body through the mouth. Some larvae that live endoparasitically in a host animal can absorb food through the surface of their bodies from host tissues. Many insects have chewing mandibles and maxillae that cut, crush, or macerate food materials and force them into the pharynx. In sucking insects, the pharynx functions as a pump that brings liquid food through the beak into the esophagus. Peristaltic action moves food is along the alimentary canal.

Saliva is usually added to the food, either as it enters the alimentary canal or before, as in the case of many sucking insects that inject it into the fluids they siphon up as foods. Saliva is generally produced by the labial glands, and the labial glands of many insects also produce amylase. In certain bees, these glands secrete invertase, which is later taken into the body with nec tar. In bloodsucking insects such as mosquitoes, the saliva generally contains no digestive enzymes but contains a substance that prevents coagulation of the blood and the consequent mechanical plugging of the food channel. This saliva causes the irritation produced by the bite of a bloodsucking insect.

Many insects eject digestive enzymes on food, and partial digestion may occur before the food is ingested. Flesh fly larvae discharge proteolytic enzymes onto their food, and aphids inject amylase into the plant tis sues and thus digest starch in the food plant. Such extraintestinal digestion is also found in the predaceous larvae of antlions and predaceous diving beetles, and bugs that feed on dry seeds.

Most chemical digestion of the food takes place within the midgut. Some of the midgut epithelial cells produce enzymes, and others absorb digested food. Sometimes the same cells carry out secretion and ab sorption. Enzymes may be released into the lumen of the midgut by the disintegration of the secretory cells (holocrine secretion) or by the release of small amounts of enzymes across the cell membrane (merocrine secretion).

Only a few species of insects produce enzymes that digest cellulose, but some can use cellulose as food as a result of symbiotic microorganisms present in their digestive tracts. These microorganisms, usually bacteria or flagellated protists, can digest the cellulose, and the insects absorb the products of this digestion. Such microorganisms are present in termites and many wood-boring beetles and are often housed in special organs connected to the gut.

The fat body is a large, often somewhat amorphous organ housed in the abdomen and thorax. In many ways its function is analogous to that of the liver among vertebrates. It serves as a food reservoir and is an important site of intermediate metabolism. In some species it is also important in storage excretion. The fat body is usually best developed in the late nymphal or larval instars. By the end of metamorphosis, it is often depleted. Some adult insects that do not feed retain their fat body in adult life.

اخر الاخبار

اخبار العتبة العباسية المقدسة

قسم صناعة الشبابيك يواصل أعمال الإدامة لأحد أبواب حرم مرقد أبي الفضل العباس (عليه السلام)

ضمن فعاليات ملتقى النورين.. المجمع العلمي يقيم ندوة فكرية في بابل

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مركز السيد جعفر الحلي الطبي يقدم الاستشارات والخدمات الصحية في ليالي القدر المباركة

المزيد

الآخبار الصحية

خمسة مؤشرات صحية مهملة قد تنقذ حياتك

متى تكون الحمى طبيعية؟

التدخين الإلكتروني يهدد قلبك.. دراسة جديدة تكشف مخاطر ارتفاع ضغط الدم

دراسة تكشف فائدة بعض أنواع الشاي في محاربة الاكتئاب

المزيد

الآخبار التكنلوجية

الجهاز المناعي يظهر دورا غير متوقع في صحة العظام

ابتكار من جامعة الشارقة منخفض التكلفة للحماية من الزلازل

اكتشاف تأثير غير متوقع للكاكاو على وظائف الدماغ

باحثون يرصدون أثر تغيّر المناخ في دم البشر

المزيد

مواضيع ذات صلة

Circulatory System of Insects

Excretory System of Insects

Epidemiology of Lyme disease

Protection and defence by the haemolymph in Insect

Hormones in insect

الانسلاخ Moulting or Ecdysis

Crawling, wriggling, swimming and walking

Cephalic sensory structures

الأمراض التي تنقلها الصراصير الحمراء

Colour production

رتبة البراغيث Siphonaptera

رتبة ثنائية الأجنحة (Diptera)