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Excretion is the process whereby an organism eliminates metabolic wastes and unwanted chemicals from its system. Metabolism is the sum total of all the chemical reactions occurring in the cells and body. Some products of these metabolic reactions are toxic and so must be processed or eliminated from the body. Others are simply materials that are present in excess and so must be eliminated as waste. The process of excretion is quite different to defecation, which is the removal of undigested food wastes from the gut. However, the gut of many animals also has a role in excretion as some materials may be excreted into the gut and eliminated with the faeces. In insects most excretory products are excreted into the gut lumen and eliminated along with faecal matter. Excretion is also important in eliminating excess water and other unwanted chemicals that may be ingested and enter the body fluids, such as plant poisons and excess salts.
One of the main functions of excretion is to remove excess nitrogen. Nitrogen enters the diet in the form of amino acids, nucleic acids and certain salts. One of the main products of excretion in aquatic organisms is ammonia. Ammonia contains nitrogen and is a small molecule which dissolves readily in water. This allows it to be easily excreted into the surrounding water. However, this becomes a problem for terrestrial organisms. Ammonia is toxic to cells and so must be quickly ejected from the body, however, being water-soluble it is typically ejected in solution, which requires water. The mammalian solution is to convert the ammonia into a less toxic substance called urea. This conversion takes place in the liver: the ammonia produced by cells enters the bloodstream where the liver removes it, converts it into urea which again enters the bloodstream to be excreted by the kidneys. Being less toxic, the urea can be temporarily stored and excreted in a concentrated solution, requiring less water.
Birds and reptiles have a better water-conserving system; they excrete uric acid (or urate salts). Uric acid is not readily soluble in water and is of low toxicity and so can be excreted with very little water. The dry excreta of birds is a mixture of faecal matter and uric acid crystals and when water is scarce birds can produce very dry excreta.
Arthropods, including insects, have adopted similar solutions. Woodlice, which are not insects but crustaceans, are only partially adapted to terrestrial conditions, preferring moist habitats, but they do excrete ammonia. Interestingly they can vent off ammonia gas, rather than relying on the wastage of water to remove the ammonia in solution. Insects are better adapted to dry conditions, although aquatic insects and some insect larvae excrete ammonia, most terrestrial forms excrete uric acid (or salts of uric acid
called urates, such as ammonium urate).
called urates, such as ammonium urate).
If one considers how small an insect is and how rapidly a small drop of water may evaporate, then one realises that insects have outstanding water-conserving systems. Bedbugs (Rhodnius) can survive for weeks without ingesting any water! Some insects can tolerate extremely dry conditions and may excrete uric acid as a dry crystalline powder, along with bone-dry faeces! Insects generally produce only trace amounts of urea.
The main excretory organ of the insect is the Malpighian tubule. Insects contain anything from 2 to 150 or more Malpighian tubules depending on the genus. Malpighian tubules are tubular outgrowths of the gut. They typically develop as pouches emerging from the junction between the midgut and the hindgut, though there actual final position varies – they may be attached to the midgut, hindgut or the midgut-hindgut junction as is the case with our ant above.
Each Malpighian tubule is a blind-ending tube whose lumen is continuous with the lumen of the gut. Each consists of a single layer of epithelial cells, forming the tubule wall, enclosed by an elastic membrane (basement membrane – a fibrous and porous protein mesh). In most insects there is a thin layer of striated muscle around this membrane. Typically muscle cells spiral around the distal end (the end furthest from the gut) of the tubule, causing it to twist and turn in gentle writhing movements as the muscles contract. The proximal end (near the gut) may be coated in circular and longitudinal muscle fibres, giving rise to peristalsis or squeezing movements which empty the contents of the tubule into the gut. In some cases, such as in caterpillars, the Malpighian tubules on each side (3 on each side in this case) empty into a small bladder, which then empties into the gut. In this case only the bladder may be muscular and its lumen is lined by cuticle (suggesting that the bladder is an extension of the hindgut).
The tubules do not just hang around in the air! The body cavity of the insect is filled with a fluid, usually colourless, called haemolymph. This fluid bathes the organs and tissues and is circulated around the insect body. The tubules are also typically loosely or firmly anchored in place by the tracheae which attach to them.
The twisting and turning of the Malpighain tubules presumably keeps them in contact with fresh haemolymph (perhaps by circulating the heamolymph around the tubule). Metabolic wastes and other unwanted chemicals that entered the insect system pass into the haemolymph, or are excreted into the haemolymph by the cells. These include nitrogenous waste and plant toxins such as alkaloids. It is the job of the Malpighian tubules to keep the haemolymph cleansed of these wastes – they remove wastes from the haemolymph and then excrete them into the gut lumen.
Outside the muscle layer is a ‘peritoneal covering’ of cells with embedded tracheoles, which carry oxygen to the Malpighian tubules which their mitochondria use to generate the needed ATP by aerobic respiration.
How do Malpighain tubules work?
Waste materials and excess water pass from the haemolymph into the Malpighain 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 (fingerlike 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.
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.
Some small organic molecules are also actively transported into the tubule lumen by the transcellular pathway, including alkaloids (plant compounds which may be toxic to the insect).
Uric acid, mostly in the form of negatively charged urate ions, is also actively transported by the transcellular pathway, though the exact mechanism is not well understood. This urate transport occurs in the proximal tubule and the urate combines with the potassium transported into the tubule to form insoluble potassium urate crystals. These crystals form roughly spherical concretions in the tubule lumen. The microvilli in the proximal tubule seem to undergo a cycle of elongation, as the urate concretions form, and retraction as the lumen fills up with urate waiting to be transported into the gut.
Once in the gut, remaining water may be reabsorbed as needed and the remaining urate excreted with the faeces, or separately. The midgut is divided from the hindgut by the pyloric sphincter and when this sphincter is closed the hindgut receives only the contents of the Malpighian tubules.
The mechanism of excretion demonstrated by the Malpighian tubule is one largely dependant on ‘secretion’ of unwanted materials, such as urate and excess sodium. This contrasts with the mammalian kidney which relies on ultrafiltration (filtration through microscopic pores), which removes most materials from the blood except large proteins and cells, followed by reabsorption of what the body needs to keep, such as sugars and amino acids. However, there is some filtration in the Malpighian tubule, namely the influx of materials through the paracellular pathway, having filtered across the basement membrane. Sugars and amino acids filtered in this way are then reabsorbed, as in the mammalian case. Similarly, there is some secretion in the mammalian kidney, for example the secretion of protons and ammonium in acid-base balance and the secretion of some drugs such as penicillin. However, the emphasis is different with the Malpighian tubule relying more on secretion, the mammalian kidney on filtration.
Other mechanisms of excretion
Some insects, such as the silverfish, springtails and aphids have no Malpighian tubules. Stick insects may have three types of Malpighian tubules. Clearly much remains to be learnt about excretion in insects. In addition to excretion by Malpighian tubules, insects often exhibit storage excretion in which waste materials are sequestered safely and kept inside special storage cells. For example, the fat body may contain urate cells which accumulate urate crystals throughout the life of the insect.
pH regulation and other functions of Malpighian tubules
The main function of Malpighian tubules may be the elimination of nitrogenous waste, but hand in hand with this comes the task of water conservation (eliminating waste whilst conserving water when necessary) or osmoregulation – regulating water content of the insect body and also regulation of ion balance. Considering their involvement in cleansing body fluids of unwanted materials it is not surprising that excretory organs typically have major roles also in regulating acid-base balance. Enzymes only work within a narrow range of acidity or pH and so an organism has to excrete excess acid or excess base to maintain the correct pH of its body fluids. Malpighian tubules also have a role in acid-base balance. The V-ATPase actively excretes protons and hence excess acid (an acid is a chemical which generates protons in solution as the protons are the true source of acidity).
Calcium is also excreted in large quantities by the Malpighian tubules of some insects. Generally, some of the Malpighian tubules, or one specific segment of the tubules, takes on this function. These tubules often become distended as they fill with calcium salt crystals. Some insects make use of this calcium in the construction of their burrows or larval cases, such as the helical calcium carbonate shells of some spittlebug (Ptyelus) larvae.
Finally, the Malpighian tubules of some insects may assume a glandular function in the secretion of silk.