Insects - Excretory Systems
Above: a cutaway of an ant. The Malpighian tubules are a key insect excretory organ.
Excretion

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).

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.

Malpighian tubules

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.

  • To learn more about the insect gut and insect nutrition click here.

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.

  • To learn more about the insect circulatory system click here.

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. (see transport across
membranes).

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
further 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.

  • For more on mechanisms of transport across cell membranes, click here.

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.
malpighian tubule showing ion transport
ant cutaway
transport pathways
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.