mechanism of excretion demonstrated by the Malpighian tubule is
one largely dependent 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,
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.
mechanisms of excretion
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 learned about
excretion in insects. In addition to excretion by Malpighian
insects often exhibit storage
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.
regulation and other functions of Malpighian tubules
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
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
or one specific segment of the tubules, takes on this function.
These tubules often become distended as they fill with calcium
crystals. Some insects make use of this calcium in the
construction of their burrows or larval cases, such as the helical
carbonate shells of some spittlebug (Ptyelus) larvae.
Finally, the Malpighian tubules of some insects may assume a
glandular function in the secretion of silk.
can survive for weeks