The
Malpighian tubule system is a type of excretory and
osmoregulatoryOsmoregulation is the active regulation of the osmotic pressure of an organism's fluids to maintain the homeostasis of the organism's water content; that is it keeps the organism's fluids from becoming too dilute or too concentrated. Osmotic pressure is a measure of the tendency of water to move...
system found in some
UniramiaS. M. Manton's polyphyletic Arthropod hypothesis with Uniramia as one of three Phyla.Uniramia is a group within the arthropods. In the past this group included the Onychophora, which are now considered a separate category...
(
InsectInsects are arthropods, having a hard exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae. They are the most diverse group of animals on the planet and include more than a million species that are already described. Insects represent more than half of all...
s and
MyriapodaMyriapoda is a subphylum of arthropods containing millipedes, centipedes, and others. The group contains 13,000 species, all of which are terrestrial ...
),
arachnidArachnids are a class of joint-legged invertebrate animals in the subphylum Chelicerata. All arachnids have eight legs, although in some species the front pair may convert to a sensory function. The term is derived from the Greek word , meaning "spider".Most arachnids are terrestrial...
s and
tardigradeTardigrades form the phylum Tardigrada, part of the superphylum Ecdysozoa. They are microscopic, water-dwelling, segmented animals with eight legs. Tardigrades were first described by Johann August Ephraim Goeze in 1773...
s.
The system consists of branching
tubuleA tubule is a very small tube or fistular structure.A system of surface-connected membranes in muscle that enables a nerve impulse to travel to the interior of the muscle fibre.In anatomy, a tubule is a minute tube lined with glandular epithelium....
s extending from the alimentary canal that absorbs solutes, water, and wastes from the surrounding
hemolymphHemolymph or haemolymph is the blood analogue used by all arthropods and most mollusks that have an open circulatory system.In these animals there is no distinction between blood and interstitial fluid. The liquid fills all of the interior of the body and surrounds all cells...
. The wastes then are released from the organism in the form of solid nitrogenous compounds. The system is named after
Marcello MalpighiMarcello Malpighi was an Italian doctor, who gave his name to several physiological features.-Early years:...
, a seventeenth century anatomist.
It is unclear as to whether the Malpighian tubules of arachnids and those of the Uniramia are
homologousIn evolutionary biology, homology refers to any similarity between characteristics of organisms that is due to their shared ancestry. The word homologous derives from the ancient Greek ομολογειν, 'to agree'. There are examples in different branches of biology...
or the result of
convergent evolutionConvergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, birds and bats do, and are capable of powered flight. The wings are similar in...
.
Malpighian tubules are slender tubes normally found in the posterior regions of arthropod alimentary canals.
The
Malpighian tubule system is a type of excretory and
osmoregulatoryOsmoregulation is the active regulation of the osmotic pressure of an organism's fluids to maintain the homeostasis of the organism's water content; that is it keeps the organism's fluids from becoming too dilute or too concentrated. Osmotic pressure is a measure of the tendency of water to move...
system found in some
UniramiaS. M. Manton's polyphyletic Arthropod hypothesis with Uniramia as one of three Phyla.Uniramia is a group within the arthropods. In the past this group included the Onychophora, which are now considered a separate category...
(
InsectInsects are arthropods, having a hard exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae. They are the most diverse group of animals on the planet and include more than a million species that are already described. Insects represent more than half of all...
s and
MyriapodaMyriapoda is a subphylum of arthropods containing millipedes, centipedes, and others. The group contains 13,000 species, all of which are terrestrial ...
),
arachnidArachnids are a class of joint-legged invertebrate animals in the subphylum Chelicerata. All arachnids have eight legs, although in some species the front pair may convert to a sensory function. The term is derived from the Greek word , meaning "spider".Most arachnids are terrestrial...
s and
tardigradeTardigrades form the phylum Tardigrada, part of the superphylum Ecdysozoa. They are microscopic, water-dwelling, segmented animals with eight legs. Tardigrades were first described by Johann August Ephraim Goeze in 1773...
s.
The system consists of branching
tubuleA tubule is a very small tube or fistular structure.A system of surface-connected membranes in muscle that enables a nerve impulse to travel to the interior of the muscle fibre.In anatomy, a tubule is a minute tube lined with glandular epithelium....
s extending from the alimentary canal that absorbs solutes, water, and wastes from the surrounding
hemolymphHemolymph or haemolymph is the blood analogue used by all arthropods and most mollusks that have an open circulatory system.In these animals there is no distinction between blood and interstitial fluid. The liquid fills all of the interior of the body and surrounds all cells...
. The wastes then are released from the organism in the form of solid nitrogenous compounds. The system is named after
Marcello MalpighiMarcello Malpighi was an Italian doctor, who gave his name to several physiological features.-Early years:...
, a seventeenth century anatomist.
It is unclear as to whether the Malpighian tubules of arachnids and those of the Uniramia are
homologousIn evolutionary biology, homology refers to any similarity between characteristics of organisms that is due to their shared ancestry. The word homologous derives from the ancient Greek ομολογειν, 'to agree'. There are examples in different branches of biology...
or the result of
convergent evolutionConvergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, birds and bats do, and are capable of powered flight. The wings are similar in...
.
Structure
Malpighian tubules are slender tubes normally found in the posterior regions of arthropod alimentary canals. Each tubule consists of a single layer of cells that is closed off at the distal end with the proximal end joining the alimentary canal at the junction between the midgut and hindgut. Most tubules are normally highly convoluted. The number of tubules varies between species although most occur in multiples of two. Tubules are usually bathed in
hemolymphHemolymph or haemolymph is the blood analogue used by all arthropods and most mollusks that have an open circulatory system.In these animals there is no distinction between blood and interstitial fluid. The liquid fills all of the interior of the body and surrounds all cells...
and are in proximity to fat body tissue. They contain
actinActin is a globular, roughly 42-kDa protein found in all eukaryotic cells where it may be present at concentrations of over 100 μM. It is also one of the most highly-conserved proteins, differing by no more than 20% in species as diverse as algae and humans...
for structural support and microvilli for propulsion of substances along the tubules. Malpighian tubules in most insects also contain accessory musculature associated with the tubules which may function to mix the contents of the tubules or expose the tubules to more hemolymph. The insect orders,
ThysanuraThe insects of the order Thysanura, usually referred to as silverfish, from their three long caudal filaments.The families Machilidae and Meinertellidae of the jumping bristletails were once included with Thysanura....
, Dermaptera and Thysanoptera do not possess these muscles and Collembola and
HemipteraHemiptera is an order of insects, comprising around 80,000 species of cicadas, aphids, planthoppers, leafhoppers, shield bugs, and others. They range in size from 1 mm to around 15 cm, and share a common arrangement of sucking mouthparts ....
:
AphididaeAphididae is a very large insect family in the aphid superfamily , of the order Hemiptera. There are several thousand species in this family, many of which are well-known for being serious plant pests...
completely lack a Malpighian tubule system.
General Mode of Action
Pre-urine is formed in the tubules, when nitrogenous waste and electrolytes are transported through the tubule walls. Wastes such as
UreaUrea or carbamide is an organic compound with the chemical formula
2CO. The molecule has two amine residues joined by a carbonyl functional group....
and amino acids are thought to diffuse through the walls, while ions such as
sodiumSodium is a metallic element with a symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals within "group 1"...
and
potassiumPotassium is the chemical element with the symbol K , atomic number 19, and atomic mass 39.0983. Potassium was first isolated from potash...
are transported by active pump mechanisms. Water follows thereafter. The pre-urine, along with digested food, merge in the hindgut. At this time, uric acid precipitates out, and sodium and potassium ions are actively absorbed by the rectum, along with water via
osmosisOsmosis is the diffusion of water through a semi-permeable membrane. More specifically, it is the movement of water across a semi-permeable membrane from an area of high water potential to an area of low water potential...
. Uric acid is left to mix with feces, which are then
excretedExcretion is the process of eliminating waste products of metabolism and other non-useful materials. It is an essential process in all forms of life. It contrasts secretion, where the substance may have specific tasks after leaving the cell....
.
Alternative modes of action
Complex cycling systems of malpighian tubules have been described in other insect orders.
HemipteranHemiptera is an order of insects, comprising around 80,000 species of cicadas, aphids, planthoppers, leafhoppers, shield bugs, and others. They range in size from 1 mm to around 15 cm, and share a common arrangement of sucking mouthparts ....
insects use tubules that permit movement of solutes into the distal portion of the tubules while reabsorption of water and essential ions directly to the hemolymph occurs in the proximal portion and the rectum. Both Coleoptera and
LepidopteraLepidoptera is an order of insects that includes moths and butterflies. It is one of the most speciose orders in the class Insecta, encompassing moths and the three superfamilies of butterflies, skipper butterflies, and moth-butterflies...
use a cryptonephridial arrangement where the distal end of the tubules are embedded in fat tissue surrounding the rectum. Such an arrangement may serve to increase the efficiency of solute processing in the Malpighian tubules.
Other uses
Although primarily involved in excretion and osmoregulation, Malpighian tubules have been modified in some insects to serve accessory functions. Larvae of the
New ZealandNew Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses , and numerous smaller islands, most notably Stewart Island/Rakiura and the Chatham Islands. The indigenous Māori named New Zealand Aotearoa, commonly translated as The Land of the Long White Cloud...
glowwormArachnocampa is a genus of four fungus gnat species which are, in their larval stage, glowworms. They are found mostly in New Zealand and Australia in caves and grottos, or sheltered places in forests....
,
Arachnocampa luminosa (Diptera:
MycetophilidaeMycetophilidae is a family of very small flies, forming the bulk of those species known as fungus gnats. There are approximately 3000 described species in 150 genera but the true number of species is undoubtedly much higher...
) use modified and swollen Malpighian tubules to produce a blue-green light attracting prey towards mucus-coated trap lines. In insects which feed on plant material containing noxious
allelochemicalsAllelopathy is a biological phenomenon that is characteristic of some plants, algae, bacteria, coral and fungi by which they produce certain biochemicals that influence the growth and development of other organisms. The biochemicals, called allelochemicals can have a beneficial or detrimental...
, malpighian tubules also serve to rapidly excrete such compounds from the hemolymph.