Structure of the gut

There are three main regions to the insect gut (or alimentary canal), with sphincters (valves) controlling food/fluid movement between regions (Fig. 3.13). The foregut (stomodeum) is concerned with ingestion, storage, grinding, and transport of food to the next region, the midgut (mesenteron). Here digestive enzymes are produced and secreted and absorption of the products of digestion occurs. The material remaining in the gut lumen together with urine from the Malpighian tubules then enters the hindgut (proc-todeum), where absorption of water, salts, and other valuable molecules occurs prior to elimination of the feces through the anus. The gut epithelium is one cell layer thick throughout the length of the canal and rests on a basement membrane surrounded by a variably developed muscle layer. Both the foregut and hindgut have a cuticular lining, whereas the midgut does not.

Each region of the gut displays several local specializations, which are variously developed in different insects, depending on diet. Typically the foregut is subdivided into a pharynx, an oesophagus (esophagus), and a crop (food storage area), and in insects that ingest solid food there is often a grinding organ, the

Fig. 3.12 The four major categories of insect feeding specialization. Many insects are typical of one category, but others cross two categories (or more, as in generalist cockroaches). (After Dow 1986.)

proventriculus (or gizzard). The proventriculus is especially well developed in orthopteroid insects, such as cockroaches, crickets, and termites, in which the epithelium is folded longitudinally to form ridges on which the cuticle is armed with spines or teeth. At the anterior end of the foregut the mouth opens into a preoral cavity bounded by the bases of the mouthparts and often divided into an upper area, or cibarium, and a lower part, or salivarium (Fig. 3.14a). The paired labial or salivary glands vary in size and arrangement from simple elongated tubes to complex branched or lobed structures.

Complicated glands occur in many Hemiptera that produce two types of saliva (see section 3.6.2). In

Lepidoptera, the labial glands produce silk, whereas mandibular glands secrete the saliva. Several types of secretory cell may occur in the salivary glands of one insect. The secretions from these cells are transported along cuticular ducts and emptied into the ventral part of the preoral cavity. In insects that store meals in their foregut, the crop may take up the greater portion of the food and often is capable of extreme distension, with a posterior sphincter controlling food retention. The crop may be an enlargement of part of the tubular gut (Fig. 3.7) or a lateral diverticulum.

The generalized midgut has two main areas: the tubular ventriculus and blind-ending lateral diverticula called caeca (ceca). Most cells of the midgut are

Box 3.3 The filter chamber of Hemiptera

Most Hemiptera have an unusual arrangement of the midgut which is related to their habit of feeding on plant fluids. An anterior and a posterior part of the gut (typically involving the midgut) are in intimate contact to allow concentration of the liquid food. This filter chamber allows excess water and relatively small molecules, such as simple sugars, to be passed quickly and directly from the anterior gut to the hindgut, thereby short-circuiting the main absorptive portion of the midgut. Thus, the digestive region is not diluted by water nor congested by superabundant food molecules. Well-developed filter chambers are characteristic of cicadas and spittle bugs, which feed on xylem (sap that is rich in ions, low in organic compounds, and with low osmotic pressure), and leafhoppers and coccoids, which feed on phloem (sap that is rich in nutrients, especially sugars, and with high osmotic pressure). The gut physiology of such sap-suckers has been rather poorly studied because accurate recording of gut fluid composition and osmotic pressure depends on the technically difficult task of taking readings from an intact gut.

Adult female coccoids of gall-inducing Apiomorpha species (Eriococcidae) (section 11.2.5) tap the vascular tissue of the gall wall to obtain phloem sap. Some species have a highly developed filter chamber formed from loops of the anterior midgut and anterior hindgut enclosed within the membranous rectum. Depicted here is the gut of an adult female of A. munita viewed from the ventral side of the body. The thread-like sucking mouthparts (Fig. 11.4c) in series with the cibarial pump connect to a short oesophagus, which can be seen here in both the main drawing and the enlarged lateral view of the filter chamber. The oesophagus terminates at the anterior midgut, which coils upon itself as three loops of the filter chamber. It emerges ventrally and forms a large midgut loop lying free in the hemolymph. Absorption of nutrients occurs in this free loop. The Malpighian tubules enter the gut at the commencement of the ileum, before the ileum enters the filter chamber where it is closely apposed to the much narrower anterior midgut. Within the irregular spiral of the filter chamber, the fluids in the two tubes move in opposite directions (as indicated by the arrows).

The filter chamber of these coccoids apparently transports sugar (perhaps by active pumps) and water (passively) from the anterior midgut to the ileum and then via the narrow colo-rectum to the rectum, from which it is eliminated as honeydew. In A. munita, other than water, the honeydew is mostly sugar (accounting for 80% of the total osmotic pressure of about 550 mOsm kg-1*). Remarkably, the osmotic pressure of the hemolymph (about 300 mOsm kg-1) is much lower than that within the filter chamber (about 450 mOsm kg-1) and rectum. Maintenance of this large osmotic difference may be facilitated by the impermeability of the rectal wall.

*Osmolarity values are from the unpublished data of P.D. Cooper & A.T. Marshall.

Fig. 3.13 Generalized insect alimentary canal showing division into three regions. The cuticular lining of the foregut and hindgut are indicated by thicker black lines. (After Dow 1986.)
Cicada Foregut Valve

Fig. 3.14 Preoral and anterior foregut morphology in (a) a generalized orthopteroid insect and (b) a xylem-feeding cicada. Musculature of the mouthparts and the (a) pharyngeal or (b) cibarial pump are indicated but not fully labeled. Contraction of the respective dilator muscles causes dilation of the pharynx or cibarium and fluid is drawn into the pump chamber. Relaxation of these muscles results in elastic return of the pharynx or cibarial walls and expels food upwards into the oesophagus. (After Snodgrass 1935.)

Fig. 3.14 Preoral and anterior foregut morphology in (a) a generalized orthopteroid insect and (b) a xylem-feeding cicada. Musculature of the mouthparts and the (a) pharyngeal or (b) cibarial pump are indicated but not fully labeled. Contraction of the respective dilator muscles causes dilation of the pharynx or cibarium and fluid is drawn into the pump chamber. Relaxation of these muscles results in elastic return of the pharynx or cibarial walls and expels food upwards into the oesophagus. (After Snodgrass 1935.)

structurally similar, being columnar with microvilli (finger-like protrusions) covering the inner surface. The distinction between the almost indiscernible foregut epithelium and the thickened epithelium of the midgut usually is visible in histological sections (Fig. 3.15). The midgut epithelium mostly is separated from the food by a thin sheath called the peritrophic membrane, consisting of a network of chitin fibrils in a protein-glycoprotein matrix. These proteins, called peritrophins, may have evolved from gastrointestinal mucus proteins by acquiring the ability to bind chitin. The peritrophic membrane either is delaminated from the whole midgut or produced by cells in the anterior region of the midgut. Exceptionally Hemiptera and Thysanoptera lack a peritrophic membrane, as do just the adults of several other orders.

Typically, the beginning of the hindgut is defined by the entry point of the Malpighian tubules, often into a distinct pylorus forming a muscular pyloric sphincter, followed by the ileum, colon, and rectum. The main functions of the hindgut are the absorption of water, salts, and other useful substances from the feces and urine; a detailed discussion of structure and function is presented in section 3.7.1.

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