Waste materials and excess water pass from the haemolymph into the Malpighian tubules, by crossing the epithelial wall of these blind-ended tubes. Recent evidence shows that these cells contain pumps, proteins called proton-secreting V-ATPase.
These proteins use energy in the form of ATP (see respiration) to pump protons into the lumen of the Malpighian tubule. Protons are positively charged and to maintain charge balance the removal of protons from the epithelial cells, into the tubule lumen, is
balanced by the inward movement of potassium ions, which move from the haemolymph, into the epithelial cells and then out into the tubule lumen also. The diagram below shows a section through a
segment of a Malpighian tubule. The epithelial cells have microvilli ( finger like projections ) projecting into the tubule lumen and are rich in mitochondria ( green stripy rods ) which produce the ATP required by the pumps. A model of how ion transport across the epithelium is thought to take place is illustrated.
Fig:
The detailed structure of the cell at top right has been simplified to illustrate some of the transport mechanisms. The V-ATPase is shown as the orange circle pumping protons (H+) into the tubule lumen.
Removal of the protons from the epithelial cell makes the cytoplasm more negatively charged and also sets up a concentration gradient (that is an electrochemical gradient is established) and this attracts positive
ions, such as sodium (Na+) and potassium (K+) into the cell from the haemolymph. The influx of these positive ions drags in negative chloride ions to balance the charge. These ions move across the cytoplasm
of the cell, the so-called transcellular pathway. Note the potassium-chloride and sodium-chloride symporters, the proton-potassium and proton-sodium antiporters and the ion channels.The flux of ions
across the epithelial cell also draws across water, by osmosis. This probably takes place largely by the paracellular pathway, that is between the epithelial cells. Sugars and amino acids are swept along by the water into the tubule lumen. Since these materials are useful they will be reabsorbed later
downstream.Other small molecules (small enough to cross the basement membrane) will also move into the tubule through this pathway. The transport of a substance which depends directly on ATP, such as the
pumping of the protons in the Malpighian tubule, is called active transport. The transport of the other ions and water is passive (by facilitated diffusion) in of itself, but is dependent on proton transport and so
indirectly dependent on ATP. This mode of transport is called secondary active transport, e.g. the transport of potassium.In dry conditions many insects can produce a very concentrated urine, indeed one that is ‘bone-dry’. However, many insects ingest large quantities of water when feeding, such as blood sucking insects, and in this instance the rate of fluid-flow through the Malpighian tubules increases a thousandfold or more. Indeed, the rate of fluid transport in these tubules is said to be higher, gram for
gram, than any other tissue. Two hormones, released into the haemolymph, can stimulate Malpighian tubules to rapidly increase their rate of fluid transport: 5HT ( 5-hydroxytryptamine ) and a peptide hormone. Increased excretion is triggered by an increase in uric acid following a meal, which presumably triggers the release of the diuretic (urine-producing) hormones.Of course, not all the fluid transported through the tubules is excreted. The proximal (basal or lower or downstream) sections of the tubules, along with the hindgut ( especially the rectum) reabsorb some of the water, depending on need, and other useful
substances, such as certain ions, sugars and amino acids, so as to produce a final urine of the ‘desired’ concentration. It is in this proximal or lower part of the tubule that uric acid is transported into the tubule,
against a concentration gradient, and precipitates as crystals, e.g. of insoluble potassium urate as the urate combines with the high potassium content of the tubule lumen. In some insects these crystals can be seen filling the lumens of the proximal ends of the tubules. Presumably, peristalsis then moves these crystals along into the gut. Potassium and some of the chloride are recovered in this way, producing a urine high in sodium.
Digestive system of insect
All insects have a complete digestive system. This means that food processing occurs within a tube-like enclosure, the alimentary canal, running lengthwise through the body from mouth to anus. Ingested food usually travels in only one direction. This arrangement differs from an incomplete digestive system ( found in certain lower invertebrates like hydra and flatworms ) where a single opening to a
pouch-like cavity serves as both mouth and anus. Most biologists regard a complete digestive system as an evolutionary improvement over an incomplete digestive system because it permits functional
specialization — different parts of the system may be specially adapted for various functions of food digestion, nutrient absorption, and waste excretion. In most insects, the alimentary canal is subdivided
into three functional regions: foregut ( stomodeum ), midgut ( mesenteron ), and hindgut ( proctodeum ).
In addition to the alimentary canal, insects also have paired salivary glands and salivary reservoirs. These structures usually reside in the thorax ( adjacent to the foregut ). Salivary ducts lead from the glands to the reservoirs and then forward, through the head, to an opening ( the salivarium ) behind
the hypopharynx. Movements of the mouthparts help mix saliva with food in the buccal cavity.
Stomodaeum: 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 the action of cibarial muscles. These muscles are located between the head capsule and the anterior wall of the pharynx. When they contract, they 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.
Mesenteron: The midgut begins just past the stomodeal valve. Near its anterior end, finger-like projections (usually from 2 to 10) diverge from the walls of the midgut. These structures, the gastric caecae, provide extra surface area for secretion of enzymes or absorption of water (and other substances)
from the alimentary canal. The rest of the midgut is called the ventriculus — it is the primary site for enzymatic digestion of food and absorption of nutrients. Digestive cells lining the walls of the ventriculus have microscopic projections (microvilli) that increase surface area for nutrient absorption.The posterior end of the midgut is marked by another sphincter muscle, the pyloric valve. It regulates the flow of material from the mesenteron into the proctodaeum.
Proctodaeum: The pyloric valve serves as a point of origin for dozens to hundreds of Malpighian tubules. These long, spaghetti-like structures extend throughout most of the abdominal cavity where they serve
as excretory organs, removing nitrogenous wastes (principally ammonium ions, NH4+) from the hemolymph. The toxic NH4+ is quickly converted to urea and then to uric acid by a series of chemical reactions within the Malpighian tubules. The uric acid, a semi-solid, accumulates inside each tubule and is eventually emptied into the hindgut for elimination as part of the fecal pellet.
The rest of the hindgut plays a major role in homeostasis by regulating the absorption of water and salts from waste products in the alimentary canal. In some insects, the hindgut is visibly subdivided into an ileum, a colon, and a rectum. Efficient recovery of water is facilitated by six rectal pads that are
embedded in the walls of the rectum. These organs remove more than 90% of the water from a fecal pellet before it passes out of the body through the anus. Embryonically, the hindgut develops as an invagination of the body wall (from ectodermal tissue). Just like the foregut, it is lined with a thin, protective layer of cuticle (intima) that is secreted by the endothelial cells of the gut wall. When an insect molts, it sheds and replaces the intima in both the foregut and the hindgut.
