Foregut

FIGURE 1 A diagrammatic representation of the insect gut.

Malpighian tubules, and the structures in the hindgut, namely the ileum and rectum (Fig. 1).

Midgut

The midgut is a tubular epithelium. Upon ingestion, food and fluids move through the esophagus and pass directly into the midgut. Because the cells in the midgut epithelium are derived from embryonic endoderm, the midgut is not lined with cuticle. Most of the cells in the midgut are involved in the secretion of digestive fluids and the absorption of nutrients from the midgut lumen. These secretory and absorptive cells have apical microvilli that greatly increase the surface area available for inward and outward transport. In insects that feed periodically (such as adult mosquitoes), the microvilli shorten during nonfeeding periods and lengthen following ingestion. Many insects have additional cell types termed goblet cells that are thought to be involved in the secretion of fluids that modify the acidity and alkalinity (pH) of the luminal fluid. These goblet cells have been intensively investigated in Lepidoptera, where they serve to produce a markedly alkaline pH.

The basal surface of the midgut cells possesses a network of longitudinal and circular muscles that upon contraction can produce peristaltic waves. These contractions serve to move the food along the gut and stir the midgut contents during digestion. Many insects possess globular outpocketings in the anterior region of the midgut, termed ceca. The cells types in the ceca are generally differentiated from those in the midgut proper.

Malpighian Tubules

The Malpighian tubules are the site of urine formation in all insects except the Collembola, Thysanura, and aphids. The Malpighian tubules are tubular epithelia that are diverticulae (outpocketed extensions) of the gut itself. The tubules open into the gut near the midgut-hindgut junction, and the lumina of these two tubular epithelia are continuous. The contents of the tubules flow into the gut lumen; the ends of the tubules distal to the gut are closed. Fluid is produced in the Malpighian tubules by secretion; and because the tubules are closed at the distal end, hydrostatic pressure builds up and fluid flows through the tubules into the gut.

The number of Malpighian tubules is quite variable depending on the insect species. Bloodsucking Hemiptera

FIGURE 2 A schematic diagram of the Malpighian tubules of E. hians. The Malpighian tubules are differentiated on the left and right side. The hindgut is composed, from anterior to posterior, of the ileum, colon, and rectum. [From Herbst, D. B., and Bradley, T. J. (1989). A Malpighian tubule lime gland in an insect inhabiting alkaline salt lakes. J. Exp. Biol. 145, 63-78.]

FIGURE 2 A schematic diagram of the Malpighian tubules of E. hians. The Malpighian tubules are differentiated on the left and right side. The hindgut is composed, from anterior to posterior, of the ileum, colon, and rectum. [From Herbst, D. B., and Bradley, T. J. (1989). A Malpighian tubule lime gland in an insect inhabiting alkaline salt lakes. J. Exp. Biol. 145, 63-78.]

(e.g., Rhodniusprolixus) and higher Diptera (e.g., Drosophila melanogaster) have as few as four tubules, whereas the desert locust (Schistocerca gregaria) has hundreds. Attached to the Malpighian tubules of many insects are longitudinal muscles. When these muscles contract, the tubules are waved about in the hemolymph, presumably for the purpose of stirring the fluid adjacent to the tubules and promoting fluid and solute transport. These tubules may also serve the more general function of promoting hemolymph circulation throughout the abdomen.

The Malpighian tubules of all species examined to date contain more than one cell type. In some cases, a single epithelial region contains two or more cell types (regions with heterologous cell types) reflecting, presumably, separate physiological roles for each cell type. In other species, the tubules are divided into distinct regions, each consisting of a single cell type (regions with homologous cell types). In these insects, each tubule region has a distinct function in transport. Finally, in many insects, the tubules show regional specialization as well as multiple cell types within a region. It is presumed that each cell type in these tubules performs a distinct function.

As an example of cell type heterogeneity, consider the Malpighian tubules of the larvae of the brine fly, Ephydra hians (Fig. 2). The tubules in this insect are differentiated on each side of the body, as well as along their length. On one side of the body is a pair of tubules called the lime gland tubules. The distal ends of these are secretory and contain two regions that can be differentiated on the basis of cell color: one white, one yellow. More proximal to the gut are expanded regions of the tubules that serve to store concentric concretions in the tubule lumen. Finally, two of these cells combine in a common ureter that empties into the gut. On the opposite side, the tubules have only the yellow and white regions of the tubules, with no storage section. This example illustrates the variety of cell types that can exist in a single tubule. The details of transport function in these and other highly complex tubules have not been fully elucidated.

FIGURE 3 Model of the transport processes occurring in the Malpighian tubules of adult mosquitoes, based on the work of Klaus Beyenbach using the species A. aegypti. Upon stimulation with mosquito natriuretic peptide (MNP), rates of transepithelial fluid secretion increase from an unstimulated rate of 0.4 nl min-1 to 2.8 nl min-1. In parallel, Na+ concentrations in the secreted fluid rise, and K+ concentrations fall. Electrophysiological studies reveal that MNP, working via cyclic AMP, induces an increase in basolateral membrane Na+ conductance, presumably through the actions of Na+ channels in the basolateral membrane of principal cells. The hyperpolarization of the transepithelial voltage and the decrease in transepithelial resistance are consistent with the activation of Na+ channels in the basolateral membrane of principal cells. (Figure and legend provided by Klaus Beyenbach.)

FIGURE 3 Model of the transport processes occurring in the Malpighian tubules of adult mosquitoes, based on the work of Klaus Beyenbach using the species A. aegypti. Upon stimulation with mosquito natriuretic peptide (MNP), rates of transepithelial fluid secretion increase from an unstimulated rate of 0.4 nl min-1 to 2.8 nl min-1. In parallel, Na+ concentrations in the secreted fluid rise, and K+ concentrations fall. Electrophysiological studies reveal that MNP, working via cyclic AMP, induces an increase in basolateral membrane Na+ conductance, presumably through the actions of Na+ channels in the basolateral membrane of principal cells. The hyperpolarization of the transepithelial voltage and the decrease in transepithelial resistance are consistent with the activation of Na+ channels in the basolateral membrane of principal cells. (Figure and legend provided by Klaus Beyenbach.)

In most Malpighian tubules, formation of the primary urine occurs in a cell type often referred to as the primary cell (Fig. 3). This cell type has extensive apical microvilli, often containing a central core of microfilaments. Frequently, these microvilli contain fingerlike extensions of the mitochondria and even of the endoplasmic reticulum. The basal surface of the cells exhibits deep infolds, often again closely associated with mitochondria by means of structures termed scalariform junctions. The intercellular spaces are occupied apically by septate or continuous junctions. More basally, the intercellular space contains gap junctions or undifferentiated basolateral membranes indistinguishable from the basal membrane infolds.

Numerous other cell types occur in the Malpighian tubules. It is presumed that each histologically distinguishable cell type performs a unique function within the tubules. In addition, distinct functions have been found in some cell types in the absence of histological or ultrastructural differentiation. A common cell type in Malpighian tubules is the stellate or secondary cell. This cell type possesses smaller microvilli than the primary cells, and these microvilli contain no mitochondria. As described later in the section on function, the secondary cells may be involved in modification of the primary urine produced by the primary cells.

Ileum

Posterior to the midgut, most insects possess a segment of gut referred to as the ileum. Because this region is part of the hindgut, it is covered on the apical surface by cuticle. The cells often show deep apical and basal membrane infoldings reflecting the role of these cells in fluid and solute transport. The basal surface of the cells is covered by layers of longitudinal and circular muscle that serve to generate peristaltic movements of the gut. These muscular contractions move the gut contents through the gut and may also serve an important role in reducing unstirred layers adjacent to both the apical and basal membranes of the epithelium.

Historically, the Malpighian tubules and rectum have been assumed to carry out most of the fluid transport in the posterior regions of the gut. The ileal epithelium is smaller in diameter, with less highly developed apical and basal membrane infolds, and a lower mitochondrial density than is observed in rectum. Nonetheless, when the transport properties of the ileum are investigated, this region of the gut is always found to be carrying out important transport functions.

Rectum

All insects possess an enlarged chamber called the rectum near the posterior end of the gut. The structure of the rectum can vary substantially from species to species. The rectal lumen is covered by a thick cuticle. Posterior to the rectum, insects possess an anal canal through which the feces and urine are eliminated. The strong bands of muscle surrounding the rectum contract during defecation, expelling the feces and urine into the external environment through the anus.

The recta of terrestrial insects are large and very active organs. Regions within the rectum are highly differentiated and contain cells with deep membrane folds. If (as in many orthopterans and lepidopterans) these cells are contained in thickened ridges that extend into the rectal lumen, they are referred to as rectal pads. If (as in many adult Diptera) the cells extend into the lumen as fingerlike structures, often on a thin stalk, they are referred to as rectal papillae. The rectal pads and the papillae are the major sites of fluid resorption and urine concentration. The cells in the rectal pads have a complicated array of intercellular junctions associated with the active sites of ion secretion and resorption. The evolution of a rectal structure permitting the formation of a urine hyperosmotic to the hemolymph was, along with the evolution of a waxy cuticle, a major adaptive event permitting insects to invade drier terrestrial habitats.

Both rectal pads and rectal papillae are covered with thick cuticle to protect the underlying epithelial cells from abrasion by the fecal material. Between the rectal pads, the rectum of terrestrial insects possesses a thin, cuticle-lined epithelium that, upon unfolding and stretching, permits rectal swelling during feces and fluid accumulation. In E. hians, the rectum is merely a thin, distensible epithelium that expands upon filling with feces prior to defecation. In this insect, the task of modifying the urine is carried out by the colon, which lies just anterior to the rectum. The presence of a rectum without transport capabilities is very unusual. In most insects the rectum is the major organ responsible for osmotic regulation of the urine.

Excretion outside the Gut

Storage excretion of concentric concretions containing calcium salts of urate and carbonate occurs in the fat body cells of most insects. Some insects contain nephrocytes, cells in the head that also store wastes and toxic elements. Finally, in collembolans and thysanurans, cephalic nephridial glands, sometimes termed labial glands, are responsible for excretory function, including the excretion of nitrogenous waste.

Bee Keeping

Bee Keeping

Make money with honey How to be a Beekeeper. Beekeeping can be a fascinating hobby or you can turn it into a lucrative business. The choice is yours. You need to know some basics to help you get started. The equipment needed to be a beekeeper. Where can you find the equipment you need? The best location for the hives. You can't just put bees in any spot. What needs to be considered when picking the location for your bees?

Get My Free Ebook


Post a comment