Myriapoda is a
subphylumIn life, a subphylum is a taxonomic rank intermediate between phylum and superclass. The rank of subdivision in plants and fungi is equivalent to subphylum.Not all phyla are divided into subphyla...
of
arthropodAn arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
s containing
millipedeMillipedes are arthropods that have two pairs of legs per segment . Each segment that has two pairs of legs is a result of two single segments fused together as one...
s,
centipedeCentipedes are arthropods belonging to the class Chilopoda of the subphylum Myriapoda. They are elongated metameric animals with one pair of legs per body segment. Despite the name, centipedes can have a varying number of legs from under 20 to over 300. Centipedes have an odd number of pairs of...
s, and others. The group contains 13,000 species, all of which are
terrestrialTerrestrial animals are animals that live predominantly or entirely on land , as compared with aquatic animals, which live predominantly or entirely in the water , or amphibians, which rely on a combination of aquatic and terrestrial habitats...
. Although their name suggests they have
myriadMyriad , "numberlesscountless, infinite", is a classical Greek word for the number 10,000. In modern English, the word refers to an unspecified large quantity.-History and usage:...
(10,000) legs, myriapods range from having over 750 legs (
Illacme plenipesIllacme plenipes is a millipede found in the central region of the U.S. state of California. It has 750 legs, more than any other millipede. It was first seen in 1926, but was not rediscovered until 2005.-Legs:...
) to having fewer than ten legs.
The fossil record of myriapods reaches back into the late
SilurianThe Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...
, although molecular evidence suggests a diversification in the Cambrian Period, and Cambrian fossils exist which resemble myriapods.
The scientific study of myriapods is
myriapodology, and those who study myriapods are
myriapodologists.
Anatomy
Myriapods have a single pair of
antennaeAntennae in biology have historically been paired appendages used for sensing in arthropods. More recently, the term has also been applied to cilium structures present in most cell types of eukaryotes....
and, in most cases, simple eyes. The mouthparts lie on the underside of the head, with an "epistome" and labrum forming the upper lip, and a pair of
maxillaeIn arthropods, the maxillae are paired structures present on the head as mouthparts in members of the clade Mandibulata, used for tasting and manipulating food. Embryologically, the maxillae are derived from the 4th and 5th segment of the head and the maxillary palps; segmented appendages extending...
forming the lower lip. A pair of
mandiblesthumb|250px|The mandibles of a [[Bull ant]]The mandible of an arthropod is either of a pair of mouthparts used for biting, cutting and holding food. Mandibles are often simply referred to as jaws. Mandibles are present in the extant subphyla Myriapoda , Crustacea and Hexapoda...
lie inside the mouth. Myriapods breathe through
spiracleSpiracles are openings on the surface of some animals that usually lead to respiratory systems.-Vertebrates:The spiracle is a small hole behind each eye that opens to the mouth in some fishes. In the primitive jawless fish the first gill opening immediately behind the mouth is essentially similar...
s that connect to a
tracheal systemThe invertebrate trachea refers to the open respiratory system composed of spiracles, tracheae, and tracheoles that terrestrial arthropods have to transport metabolic gases to and from tissues....
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
ganglionIn anatomy, a ganglion is a biological tissue mass, most commonly a mass of nerve cell bodies. Cells found in a ganglion are called ganglion cells, though this term is also sometimes used to refer specifically to retinal ganglion cells....
in each segment, the brain is relatively poorly developed.
During mating, male myriapods produce a packet of sperm, or
spermatophoreA spermatophore or sperm ampulla is a capsule or mass created by males of various animal species, containing spermatozoa and transferred in entirety to the female's ovipore during copulation...
, 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. The young add additional segments and limbs as they repeatedly
moultEcdysis is the moulting of the cuticula in many invertebrates. This process of moulting is the defining feature of the clade Ecdysozoa, comprising the arthropods, nematodes, velvet worms, horsehair worms, rotifers, tardigrades and Cephalorhyncha...
to reach the adult form.
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
grasslandGrasslands are areas where the vegetation is dominated by grasses and other herbaceous plants . However, sedge and rush families can also be found. Grasslands occur naturally on all continents except Antarctica...
s,
semi-aridA semi-arid climate or steppe climate describes climatic regions that receive precipitation below potential evapotranspiration, but not extremely...
habitats or even deserts. The majority are
detritivorousDetritivores, also known as detritophages or detritus feeders or detritus eaters or saprophages, are heterotrophs that obtain nutrients by consuming detritus . By doing so, they contribute to decomposition and the nutrient cycles...
, with the exception of
centipedeCentipedes are arthropods belonging to the class Chilopoda of the subphylum Myriapoda. They are elongated metameric animals with one pair of legs per body segment. Despite the name, centipedes can have a varying number of legs from under 20 to over 300. Centipedes have an odd number of pairs of...
s, which are chiefly nocturnal predators. Pauropodans and symphylans are small, sometimes microscopic animals that resemble centipedes superficially and live in
soilSoil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...
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
legsThe arthropod leg is a form of jointed appendage of arthropods, usually used for walking. Many of the terms used for arthropod leg segments are of Latin origin, and may be confused with terms for bones: coxa , trochanter , femur, tibia, tarsus, ischium, metatarsus, carpus, dactylus ,...
, while the other three groups have a single pair of legs on each body segment.
Although not generally considered dangerous to humans, many myriapods produce noxious
secretionSecretion is the process of elaborating, releasing, and oozing chemicals, or a secreted chemical substance from a cell or gland. In contrast to excretion, the substance may have a certain function, rather than being a waste product...
s (often containing
benzoquinonesBenzoquinone is a quinone with a single benzene ring, of which there are only two:* 1,4-Benzoquinone, most commonly...
) which can cause temporary
blisterA blister is a small pocket of fluid within the upper layers of the skin, typically caused by forceful rubbing , burning, freezing, chemical exposure or infection. Most blisters are filled with a clear fluid called serum or plasma...
ing and discolouration of the skin.
Classification
There has been much debate as to which arthropod group is most closely related to the Myriapoda. Under the
MandibulataThe Mandibulata or mandibulates is a clade of arthropods that comprises the extant subphyla Myriapoda , Crustacea and Hexapoda . Mandibulata is currently believed to be the sister group of the clade Arachnomorpha, which comprises the rest of arthropods...
hypothesis, Myriapoda is the sister taxon to
PancrustaceaPancrustacea is a clade, comprising 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 Pancrustacea taxon is considered well accepted. The clade has...
, a group comprising the Crustacea and
HexapodaThe subphylum Hexapoda constitutes the largest grouping of arthropods and includes the insects as well as three much smaller groups of wingless arthropods: Collembola, Protura, and Diplura . The Collembola are very abundant in terrestrial environments...
. Under the
AtelocerataAtelocerata Heymons, 1901 is a proposed clade of arthropods that includes Hexapoda and Myriapoda , but excludes Crustacea and Chelicerata...
hypothesis,
HexapodaThe subphylum Hexapoda constitutes the largest grouping of arthropods and includes the insects as well as three much smaller groups of wingless arthropods: Collembola, Protura, and Diplura . The Collembola are very abundant in terrestrial environments...
is the closest, whereas under the Paradoxopoda hypothesis,
ChelicerataThe subphylum Chelicerata constitutes one of the major subdivisions of the phylum Arthropoda, and includes horseshoe crabs, scorpions, spiders and mites...
is the closest. This last hypothesis, although supported by few, if any, morphological characters, is supported by a number of
molecular studiesMolecular biology is the branch of biology that deals with the molecular basis of biological activity. This field overlaps with other areas of biology and chemistry, particularly genetics and biochemistry...
.
There are four classes of extant myriapods,
ChilopodaCentipedes are arthropods belonging to the class Chilopoda of the subphylum Myriapoda. They are elongated metameric animals with one pair of legs per body segment. Despite the name, centipedes can have a varying number of legs from under 20 to over 300. Centipedes have an odd number of pairs of...
(centipedes),
DiplopodaMillipedes are arthropods that have two pairs of legs per segment . Each segment that has two pairs of legs is a result of two single segments fused together as one...
,
PauropodaPauropods are small, pale, centipede-like arthropods. They form the order Pauropodina, belonging to the monotypic class Pauropoda. About 500 species in four families are found worldwide, living in soil and leaf mold...
and
SymphylaSymphylans, also known as garden centipedes or glasshouse symphylans, are soil-dwelling arthropods of the class Symphyla in the subphylum Myriapoda. Symphylans resemble centipedes, but are smaller and translucent. They can move rapidly through the pores between soil particles, and are typically...
, containing a total of around 12,000
speciesIn biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
. While each of these groups of myriapods is believed to be
monophyleticIn common cladistic usage, a monophyletic group is a taxon which forms a clade, meaning that it contains all the descendants of the possibly hypothetical closest common ancestor of the members of the group. The term is synonymous with the uncommon term holophyly...
, relationships among them are less certain.
Centipedes
CentipedeCentipedes are arthropods belonging to the class Chilopoda of the subphylum Myriapoda. They are elongated metameric animals with one pair of legs per body segment. Despite the name, centipedes can have a varying number of legs from under 20 to over 300. Centipedes have an odd number of pairs of...
s make up the order Chilopoda. They are fast, predatory and
venomVenom is the general term referring to any variety of toxins used by certain types of animals that inject it into their victims by the means of a bite or a sting...
ous, hunting mostly at night. There are around 3,300 species, ranging from the diminutive
Nannarup hoffmani (less than half an inch in length, mm) to the giant
Scolopendra giganteaScolopendra gigantea is the largest representative of the genus Scolopendra, regularly reaching lengths of and can exceed . It inhabits the northern and western regions of South America and the islands of Trinidad, Puerto Rico, Saint Thomas, U.S...
, which may exceed 30 centimetres (12 in).
Millipedes
Most
millipedeMillipedes are arthropods that have two pairs of legs per segment . Each segment that has two pairs of legs is a result of two single segments fused together as one...
s are slower than centipedes, and feed on leaf litter and
detritusDetritus is a biological term used to describe dead or waste organic material.Detritus may also refer to:* Detritus , a geological term used to describe the particles of rock produced by weathering...
. 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. Around 8,000 species have been described, which may represent less than a tenth of the true global millipede diversity. One species,
Illacme plenipesIllacme plenipes is a millipede found in the central region of the U.S. state of California. It has 750 legs, more than any other millipede. It was first seen in 1926, but was not rediscovered until 2005.-Legs:...
has the greatest number of legs of any animal, with 750. The name "millipede" is a compound word formed from the
LatinLatin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...
roots milli ("thousand") and ped ("foot"), although millipedes typically have between 36 and 400 legs.
Pill millipedePill millipedes make up two orders of millipedes, often grouped together into a single superorder, Oniscomorpha. The name Oniscomorpha refers to the resemblance of pill millipedes to certain woodlice, namely the pillbugs of the family Armadillidiidae, also called "potato bugs", "doodlebugs", or...
s are much shorter, and are capable of rolling up into a ball, like
pillbugsArmadillidiidae is a family of woodlice, a terrestrial crustacean group in the order Isopoda. Unlike members of other woodlouse families, members of this family can roll into a ball, an ability they share with the outwardly similar but unrelated pill millipedes and other animals...
.
Symphyla
About 200 species of
symphylaSymphylans, also known as garden centipedes or glasshouse symphylans, are soil-dwelling arthropods of the class Symphyla in the subphylum Myriapoda. Symphylans resemble centipedes, but are smaller and translucent. They can move rapidly through the pores between soil particles, and are typically...
ns are known worldwide. They resemble centipedes but are smaller and translucent. Many spend their lives as soil infauna, but some live arboreally. Juveniles have six pairs of legs, but, over a lifetime of several years, add an additional pair at each
moultEcdysis is the moulting of the cuticula in many invertebrates. This process of moulting is the defining feature of the clade Ecdysozoa, comprising the arthropods, nematodes, velvet worms, horsehair worms, rotifers, tardigrades and Cephalorhyncha...
so that the adult
instarAn instar is a developmental stage of arthropods, such as insects, between each molt , until sexual maturity is reached. Arthropods must shed the exoskeleton in order to grow or assume a new form. Differences between instars can often be seen in altered body proportions, colors, patterns, or...
has twelve pairs of legs.
Pauropoda
PauropodaPauropods are small, pale, centipede-like arthropods. They form the order Pauropodina, belonging to the monotypic class Pauropoda. About 500 species in four families are found worldwide, living in soil and leaf mold...
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. Over 700 species have been described. They are believed to be the sister group to millipedes, and have the dorsal tergites fused across pairs of segments, similar to the more complete fusion of segments seen in millipedes.
Arthropleuridea
ArthropleurideaArthropleuridea was a subclass of myriapod arthropods that flourished during the Carboniferous period, having first arose during the Silurian, and perishing due to climate change during the Early Permian. Members are defined by diplosomy, paranotal tergal lobes separated from the axis by a suture,...
were ancient myriapods that are now extinct. The most famous members are from the
genusIn biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
ArthropleuraArthropleura was a 0.3–2.6 metre long relative of centipedes and millipedes, native to the Upper Carboniferous of what is now northeastern North America and Scotland...
, which was a giant, probably
herbivorousHerbivores are organisms that are anatomically and physiologically adapted to eat plant-based foods. Herbivory is a form of consumption in which an organism principally eats autotrophs such as plants, algae and photosynthesizing bacteria. More generally, organisms that feed on autotrophs in...
, animal that could be up to 3 metres (9.8 ft) long. Arthropleuridea may be a division of the
millipedeMillipedes are arthropods that have two pairs of legs per segment . Each segment that has two pairs of legs is a result of two single segments fused together as one...
s.