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Myriapods () are the members of
subphylum In zoological nomenclature, a subphylum is a taxonomic rank below the rank of phylum. The taxonomic rank of " subdivision" in fungi and plant taxonomy is equivalent to "subphylum" in zoological taxonomy. Some plant taxonomists have also used ...
Myriapoda, containing
arthropod Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chitin, ...
s such as
millipede Millipedes are a group of arthropods that are characterised by having two pairs of jointed legs on most body segments; they are known scientifically as the class Diplopoda, the name derived from this feature. Each double-legged segment is a re ...
s and
centipede Centipedes (from New Latin , "hundred", and Latin , "foot") are predatory arthropods belonging to the class Chilopoda (Ancient Greek , ''kheilos'', lip, and New Latin suffix , "foot", describing the forcipules) of the subphylum Myriapoda, an a ...
s. The group contains about 13,000 species, all of them
terrestrial Terrestrial refers to things related to land or the planet Earth. Terrestrial may also refer to: * Terrestrial animal, an animal that lives on land opposed to living in water, or sometimes an animal that lives on or near the ground, as opposed to ...
. The
fossil record A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved in ...
of myriapods reaches back into the late
Silurian The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the shortest period of the Paleozo ...
, although molecular evidence suggests a diversification in the Cambrian Period, and Cambrian fossils exist which resemble myriapods. The oldest unequivocal myriapod fossil is of the millipede '' Pneumodesmus newmani'', from the late Silurian (428 million years ago). ''P. newmani'' is also important as the earliest known terrestrial animal. The phylogenetic classification of myriapods is still debated. The scientific study of myriapods is myriapodology, and those who study myriapods are myriapodologists.


Anatomy

Myriapods have a single pair of antennae and, in most cases, simple eyes. Exceptions are the two classes symphylans and pauropods, and the millipede order
Polydesmida Polydesmida (from the Greek ''poly'' "many" and ''desmos'' "bond") is the largest order of millipedes, containing approximately 3,500 species, including all the millipedes reported to produce hydrogen cyanide (HCN). Description Members of the o ...
and the centipede order Geophilomorpha, which are all eyeless. The house centipedes ( Scutigera) on the other hand, have large and well-developed
compound eye A compound eye is a visual organ found in arthropods such as insects and crustaceans. It may consist of thousands of ommatidia, which are tiny independent photoreception units that consist of a cornea, lens, and photoreceptor cells which disti ...
s. The mouthparts lie on the underside of the head, with an "epistome" and labrum forming the upper lip, and a pair of
maxillae The maxilla (plural: ''maxillae'' ) in vertebrates is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. The tw ...
forming the lower lip. A pair of mandibles lie inside the mouth. Myriapods breathe through spiracles that connect to a tracheal system similar to that of insects. There is a long tubular heart that extends through much of the body, but usually few, if any, blood vessels. Malpighian tubules excrete nitrogenous waste into the digestive system, which typically consists of a simple tube. Although the ventral nerve cord has a
ganglion A ganglion is a group of neuron cell bodies in the peripheral nervous system. In the somatic nervous system this includes dorsal root ganglia and trigeminal ganglia among a few others. In the autonomic nervous system there are both sympathe ...
in each segment, the brain is relatively poorly developed. During mating, male myriapods produce a packet of sperm, or
spermatophore A spermatophore or sperm ampulla is a capsule or mass containing spermatozoa created by males of various animal species, especially salamanders and arthropods, and transferred in entirety to the female's ovipore during reproduction. Spermatophores ...
, which they must transfer to the female externally; this process is often complex and highly developed. The female lays eggs which hatch as much-shortened versions of the adults, with only a few segments and as few as three pairs of legs. With the exception of the two centipede orders Scolopendromorpha and Geophilomorpha, which have epimorphic development (all body segments are formed segments embryonically), the young add additional segments and limbs as they repeatedly
moult In biology, moulting (British English), or molting (American English), also known as sloughing, shedding, or in many invertebrates, ecdysis, is the manner in which an animal routinely casts off a part of its body (often, but not always, an outer ...
to reach the adult form. The process of adding new segments during postembryonic growth is known as
anamorphosis Anamorphosis is a distorted projection requiring the viewer to occupy a specific vantage point, use special devices, or both to view a recognizable image. It is used in painting, photography, sculpture and installation, toys, and film special e ...
, of which there are three types: euanamorphosis, emianamorphosis, and teloanamorphosis. In euanamorphosis, every moult is followed by addition of new segments, even after reaching sexual maturity; in emianamorphosis, new segments are added until a certain stage, and further moults happen without addition of segments; and in teloanamorphosis, where the addition of new segments stops after the adult form is reached, after no further moults occur.


Ecology

Myriapods are most abundant in moist forests, where they fulfill an important role in breaking down decaying plant material, although a few live in
grassland A grassland is an area where the vegetation is dominated by grasses (Poaceae). However, sedge ( Cyperaceae) and rush (Juncaceae) can also be found along with variable proportions of legumes, like clover, and other herbs. Grasslands occur natu ...
s,
semi-arid A semi-arid climate, semi-desert climate, or steppe climate is a dry climate sub-type. It is located on regions that receive precipitation below potential evapotranspiration, but not as low as a desert climate. There are different kinds of semi-ar ...
habitats or even deserts. A very small percentage of species are
littoral The littoral zone or nearshore is the part of a sea, lake, or river that is close to the shore. In coastal ecology, the littoral zone includes the intertidal zone extending from the high water mark (which is rarely inundated), to coastal areas ...
(found along the sea shore). The majority are detritivorous, with the exception of
centipede Centipedes (from New Latin , "hundred", and Latin , "foot") are predatory arthropods belonging to the class Chilopoda (Ancient Greek , ''kheilos'', lip, and New Latin suffix , "foot", describing the forcipules) of the subphylum Myriapoda, an a ...
s, which are chiefly nocturnal
predator Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill t ...
s. A few species of centipedes and millipedes are able to produce light and are therefore bioluminescent Pauropodans and
symphylan Symphylans, also known as garden centipedes or pseudocentipedes, are soil-dwelling arthropods of the class Symphyla in the subphylum Myriapoda. Symphylans resemble centipedes, but are very small, non-venomous, and only distantly related to bot ...
s are small, sometimes microscopic animals that resemble centipedes superficially and live in
soil Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former ...
s. Millipedes differ from the other groups in having their body segments fused into pairs, giving the appearance that each segment bears two pairs of
legs A leg is a weight-bearing and locomotive anatomical structure, usually having a columnar shape. During locomotion, legs function as "extensible struts". The combination of movements at all joints can be modeled as a single, linear element c ...
, while the other three groups have a single pair of legs on each body segment. Although not generally considered dangerous to humans, many millipedes produce noxious
secretion 440px Secretion is the movement of material from one point to another, such as a secreted chemical substance from a cell or gland. In contrast, excretion is the removal of certain substances or waste products from a cell or organism. The classica ...
s (often containing
benzoquinones Benzoquinone (C6H4O2) is a quinone with a single benzene ring. There are 2 (out of 3 hypothetical) benzoquinones: * 1,4-Benzoquinone, most commonly, right image (also ''para''-benzoquinone, ''p''-benzoquinone, ''para''-quinone, or just quinone) * 1 ...
) which in rare cases can cause temporary
blister A blister is a small pocket of body fluid (lymph, serum, plasma, blood, or pus) within the upper layers of the skin, usually caused by forceful rubbing (friction), burning, freezing, chemical exposure or infection. Most blisters are filled w ...
ing and discolouration of the skin. Large centipedes, however, can bite humans, and although the bite may cause intense pain and discomfort, fatalities are extremely rare.


Classification

There has been much debate as to which arthropod group is most closely related to the Myriapoda. Under the
Mandibulata Mandibulata, termed "mandibulates", is a clade of arthropods that comprises the extant subphyla Myriapoda (millipedes and others), Crustacea and Hexapoda (insects and others). Mandibulata is currently believed to be the sister group of the clad ...
hypothesis, Myriapoda is the
sister taxon In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree. Definition The expression is most easily illustrated by a cladogram: Taxon A and t ...
to
Pancrustacea Pancrustacea is the clade that comprises all crustaceans and hexapods. This grouping is contrary to the Atelocerata hypothesis, in which Myriapoda and Hexapoda are sister taxa, and Crustacea are only more distantly related. As of 2010, the P ...
, a group comprising the
Crustacea Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean group ...
and
Hexapoda The subphylum Hexapoda (from Greek for 'six legs') comprises most species of arthropods and includes the insects as well as three much smaller groups of wingless arthropods: Collembola, Protura, and Diplura (all of these were once considered in ...
(insects and their close relatives). Under the
Atelocerata Atelocerata is a proposed clade of arthropods that includes Hexapoda (insects and a few related taxa) and Myriapoda (millipedes, centipedes, and similar taxa), but excludes Crustacea (such as shrimp and lobsters) and Chelicerata (such as spiders a ...
hypothesis,
Hexapoda The subphylum Hexapoda (from Greek for 'six legs') comprises most species of arthropods and includes the insects as well as three much smaller groups of wingless arthropods: Collembola, Protura, and Diplura (all of these were once considered in ...
is the closest, whereas under the Paradoxopoda hypothesis,
Chelicerata The subphylum Chelicerata (from New Latin, , ) constitutes one of the major subdivisions of the phylum Arthropoda. It contains the sea spiders, horseshoe crabs, and arachnids (including harvestmen, scorpions, spiders, solifuges, ticks, and mit ...
is the closest. This last hypothesis, although supported by few, if any, morphological characters, is supported by a number of molecular studies. A 2020 study found numerous characters of the eye and preoral region suggesting that the closest relatives to
crown A crown is a traditional form of head adornment, or hat, worn by monarchs as a symbol of their power and dignity. A crown is often, by extension, a symbol of the monarch's government or items endorsed by it. The word itself is used, partic ...
myriapods are the extinct Euthycarcinoids. There are four classes of extant myriapods, Chilopoda (centipedes),
Diplopoda Millipedes are a group of arthropods that are characterised by having two pairs of jointed legs on most body segments; they are known scientifically as the class Diplopoda, the name derived from this feature. Each double-legged segment is a res ...
,
Pauropoda Pauropods are small, pale, millipede-like arthropods. Around 830 species in twelve families are found worldwide, living in soil and leaf mold. They look rather like centipedes, or millipedes, and may be a sister group of the latter. However, thi ...
and
Symphyla Symphylans, also known as garden centipedes or pseudocentipedes, are soil-dwelling arthropods of the class Symphyla in the subphylum Myriapoda. Symphylans resemble centipedes, but are very small, non-venomous, and only distantly related to both ...
, containing a total of around 12,000
species In biology, a species is the basic unit of Taxonomy (biology), classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of ...
. While each of these groups of myriapods is believed to be
monophyletic In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic grou ...
, relationships among them are less certain.


Centipedes

Centipede Centipedes (from New Latin , "hundred", and Latin , "foot") are predatory arthropods belonging to the class Chilopoda (Ancient Greek , ''kheilos'', lip, and New Latin suffix , "foot", describing the forcipules) of the subphylum Myriapoda, an a ...
s make up the class Chilopoda. They are fast,
predator Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill t ...
y and
venom Venom or zootoxin is a type of toxin produced by an animal that is actively delivered through a wound by means of a bite, sting, or similar action. The toxin is delivered through a specially evolved ''venom apparatus'', such as fangs or a st ...
ous, hunting mostly at night. There are around 3,300 species, ranging from the diminutive '' Nannarrup hoffmani'' (less than 12 mm or  in in length) to the giant '' Scolopendra gigantea'', which may exceed .


Millipedes

Millipede Millipedes are a group of arthropods that are characterised by having two pairs of jointed legs on most body segments; they are known scientifically as the class Diplopoda, the name derived from this feature. Each double-legged segment is a re ...
s form the class Diplopoda. Most millipedes are slower than centipedes, and feed on
leaf litter Plant litter (also leaf litter, tree litter, soil litter, litterfall or duff) is dead plant material (such as leaves, bark, needles, twigs, and cladodes) that have fallen to the ground. This detritus or dead organic material and its constituen ...
and
detritus In biology, detritus () is dead particulate organic material, as distinguished from dissolved organic material. Detritus typically includes the bodies or fragments of bodies of dead organisms, and fecal material. Detritus typically hosts commu ...
. Except for the first segment called collum, which don't have any appendages, and the next three segments with a single pair of legs each, they are distinguished by the fusion of each pair of body segments into a single unit, giving the appearance of having two pairs of legs per segment. It is also common for the sternites, pleurites and tergites to fuse into rigid armour rings. The males produce aflagellate sperm cells, unlike the rest of the myriapods which produce flagellated sperm. Around 12,000 species have been described, which may represent less than a tenth of the true global millipede diversity. Although the name "millipede" is a
compound word In linguistics, a compound is a lexeme (less precisely, a word or sign) that consists of more than one stem. Compounding, composition or nominal composition is the process of word formation that creates compound lexemes. Compounding occurs when t ...
formed from the
Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through ...
roots ''millia'' ("thousand") and ''pes'' (gen. ''pedis'') ("foot"), millipedes typically have between 36 and 400 legs. On 2021, however, was described ''
Eumillipes persephone ''Eumillipes'' is a genus of millipede in the family Siphonotidae. It contains a single species, ''E. persephone'', known from the Eastern Goldfields of Western Australia Western Australia (commonly abbreviated as WA) is a state of Australia ...
,'' the first species known to have 1,000 or more legs, possessing 1,306 of them.
Pill millipede Pill millipedes are any members of two living (and one extinct) orders of millipedes, often grouped together into a single superorder, Oniscomorpha. The name Oniscomorpha refers to the millipedes' resemblance to certain woodlice (Oniscidea), als ...
s are much shorter, and are capable of rolling up into a ball, like pillbugs.


Symphyla

Symphylans, or garden centipedes, are closely related to centipedes and millipedes. They are 3 to 6 cm long, and have 6 to 12 pairs of legs, depending on their life stage. Their eggs, which are white and spherical and covered with small hexagonal ridges, are laid in batches of 4 to 25 at a time, and usually take up to 40 days to hatch. There are about 200 species worldwide.


Pauropoda

Pauropoda Pauropods are small, pale, millipede-like arthropods. Around 830 species in twelve families are found worldwide, living in soil and leaf mold. They look rather like centipedes, or millipedes, and may be a sister group of the latter. However, thi ...
is another small group of small myriapods. They are typically 0.5–2.0 mm long and live in the soil on all continents except
Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest cont ...
. Over 700 species have been described. They are believed to be the
sister group In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree. Definition The expression is most easily illustrated by a cladogram: Taxon A and ...
to millipedes, and have the dorsal
tergite A ''tergum'' (Latin for "the back"; plural ''terga'', associated adjective tergal) is the dorsal ('upper') portion of an arthropod segment other than the head. The anterior edge is called the 'base' and posterior edge is called the 'apex' or 'mar ...
s fused across pairs of segments, similar to the more complete fusion of segments seen in millipedes.


Arthropleuridea

Arthropleuridea Arthropleuridea is an extinct subclass of myriapod arthropods that flourished during the Carboniferous period, having first arose during the Silurian, and perishing in the Early Permian. Members are characterized by possessing diplosegement (fuse ...
ns were ancient myriapods that are now extinct, known from the late
Silurian The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the shortest period of the Paleozo ...
to the
Permian The Permian ( ) is a geologic period and stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last period of the Paleoz ...
. The most famous members are from the
genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial no ...
''
Arthropleura ''Arthropleura'' () is a genus of extinct millipede arthropods that lived in what is now North America and Europe around 345 to 290 million years ago, from the Viséan stage of the lower Carboniferous Period to the Sakmarian stage of the low ...
'', which was a giant, probably
herbivorous A herbivore is an animal anatomically and physiologically adapted to eating plant material, for example foliage or marine algae, for the main component of its diet. As a result of their plant diet, herbivorous animals typically have mouthpar ...
, animal that could be up to long, but the group also includes species less than . Arthropleuridea was historically considered a distinct class of myriapods, but since 2000 scientific consensus has viewed the group as a subset of millipedes, although the relationship of arthropleurideans to other millipedes and to each other is debated.


Myriapod relationships

A variety of groupings (
clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...
s) of the myriapod classes have been proposed, some of which are mutually exclusive, and all of which represent hypotheses of evolutionary relationships. Traditional relationships supported by morphological similarities (anatomical or developmental similarities) are challenged by newer relationships supported by molecular evidence (including
DNA sequence DNA sequencing is the process of determining the nucleic acid sequence – the order of nucleotides in DNA. It includes any method or technology that is used to determine the order of the four bases: adenine, guanine, cytosine, and thymine. The ...
and
amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha a ...
similarities). * Dignatha (also called Collifera) is a clade consisting of millipedes and pauropods, and is supported by morphological similarities including the presence of a gnathochilarium (a modified jaw and plate apparatus) and a collum, a legless segment behind the head. * Trignatha (also called Atelopoda) is a grouping of centipedes and symphylans, united by similarities of mouthparts. * Edafopoda is a grouping of symphylans and pauropodans that is supported by shared genetic sequences, yet conflicts with Dignatha and Trignatha. * Progoneata is a group encompassing millipedes, pauropods and symphylans while excluding centipedes. Shared features include reproductive openings (
gonopore A gonopore, sometimes called a gonadopore, is a genital pore in many invertebrates. Hexapods, including insects have a single common gonopore, except mayflies, which have a pair of gonopores. More specifically, in the unmodified female it is th ...
s) behind the second body segment, and sensory hairs (
trichobothria Trichobothria (singular trichobothrium) are elongate setae ("hairs") present in arachnids, various orders of insects, and myriapods that function in the detection of airborne vibrations and currents, and electrical charge. In 1883, Friedrich Dahl ...
) with a bulb-like swelling. It is compatible with either Dignatha or Edafopoda.


See also

*
Euthycarcinoidea Euthycarcinoidea are an enigmatic group of extinct possibly amphibious arthropods that ranged from Cambrian to Triassic times. Fossils are known from Europe, North America, Argentina, Australia and Antarctica. Description The euthycarcinoid body ...
, a group of enigmatic arthropods that may be ancestral to myriapods * Colonization of land, major evolutionary stages leading to terrestrial organisms * Metamerism, the condition of multiple linearly repeated body segments


References


External links


Myriapod Fossil Record - University of Bristol

International Journal of Myriapodology

International Society of Myriapodology

British Myriapod and Isopod Group


{{Authority control Animal subphyla Extant Silurian first appearances Taxa named by Pierre André Latreille