Myriapods have a single pair ofantennae and, in most cases, simple eyes. Exceptions are the two classes ofsymphylans andpauropods, the millipede orderPolydesmida and the centipede orderGeophilomorpha, which are all eyeless.[10] The house centipedes (Scutigera) on the other hand, have large and well-developedcompound eyes.[11] Themouthparts lie on the underside of the head, with an "epistome" and labrum forming the upper lip, and a pair ofmaxillae forming the lower lip. A pair ofmandibles lie inside the mouth. Myriapods breathe throughspiracles that connect to atracheal 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.[12]
Malpighian tubules excrete nitrogenous waste into the digestive system, which typically consists of a simple tube. Although the ventral nerve cord has aganglion in each segment, the brain is relatively poorly developed.[12]
During mating, male myriapods produce a packet of sperm, orspermatophore, 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 ordersScolopendromorpha andGeophilomorpha, which have epimorphic development (all body segments are formed segments embryonically), the young add additional segments and limbs as they repeatedlymoult to reach the adult form.[12]
The process of adding new segments during postembryonic growth is known asanamorphosis, 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.[13]
Myriapods are most abundant in moist forests, where they fulfill an important role in breaking down decaying plant material,[2] although a few live ingrasslands,semi-arid habitats or even deserts.[14] A very small percentage of species arelittoral (found along the sea shore).[15][16] The majority aredetritivorous, with the exception ofcentipedes, which are chiefly nocturnalpredators.
A few species of centipedes and millipedes are able to produce light and are thereforebioluminescent.[17]Pauropodans andsymphylans are small, sometimes microscopic animals that resemble centipedes superficially and live insoils. Millipedes differ from the other groups in having theirbody segments fused into pairs, giving the appearance that each segment bears two pairs oflegs, 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 noxioussecretions (often containingbenzoquinones) which in rare cases can cause temporaryblistering and discolouration of the skin.[18] Large centipedes, however, can bite humans, and although the bite may cause intense pain and discomfort, fatalities are extremely rare.[19]
There has been much debate as to which arthropod group is most closely related to the Myriapoda.[20] Under theMandibulata hypothesis, Myriapoda is thesister taxon toPancrustacea, a group comprising theCrustacea andHexapoda (insects and their close relatives). Under theAtelocerata hypothesis,Hexapoda is the closest, whereas under theParadoxopoda hypothesis,Chelicerata is the closest. This last hypothesis, although supported by few, if any, morphological characters, is supported by a number ofmolecular studies.[21]
A 2020 study found numerous characters of the eye and preoral region suggesting that the closest relatives tocrown myriapods are the extinctEuthycarcinoids.[22] There are four classes of extant myriapods,Chilopoda (centipedes),Diplopoda,Pauropoda andSymphyla, containing a total of around 12,000species.[23] While each of these groups of myriapods is believed to bemonophyletic, relationships among them are less certain.[24]
Centipedes make up the class Chilopoda. They are fast,predatory andvenomous, hunting mostly at night. There are around 3,300 species,[23] ranging from the diminutiveNannarrup hoffmani (less than 12 mm or1⁄2 in in length)[25] to the giantScolopendra gigantea, which may exceed 30 centimetres (12 in).
Millipedes form the class Diplopoda. Most millipedes are slower than centipedes, and feed onleaf litter anddetritus. 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 ofbody 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.[26] The males produce aflagellate sperm cells, unlike the rest of the myriapods which produce flagellated sperm.[27] Around 12,000 species have been described, which may represent less than a tenth of the true global millipede diversity.[23] Although the name "millipede" is acompound word formed from theLatin rootsmillia ("thousand") andpes (gen.pedis) ("foot"), millipedes typically have between 36 and 400 legs. In 2021, however, was describedEumillipes persephone, the first species known to have 1,000 or more legs, possessing 1,306 of them.[28]Pill millipedes are much shorter, and are capable of rolling up into a ball, likepillbugs.
Symphylans, or garden centipedes, are closely related to centipedes and millipedes.[29][30] They are 3 to 6 cm long, and have 6 to 12 pairs of legs, depending on their life stage.[29][30][31] 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.[29][30][31] There are about 200 species worldwide.[32]
Pauropoda is another small group of small myriapods. They are typically 0.5–2.0 mm long and live in the soil on all continents exceptAntarctica.[citation needed] Over 700 species have been described.[23] They are believed to be thesister group to millipedes, and have the dorsaltergites fused across pairs of segments, similar to the more complete fusion of segments seen in millipedes.[33]
Arthropleurideans were ancient myriapods that are now extinct, known from the lateSilurian to thePermian. The most famous members are from thegenusArthropleura, which was a giant, probablyherbivorous, animal that could be up to 2.63 metres (8 ft 8 in) long,[34] but the group also includes species less than 1 cm (0.39 in). 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.[35][36]
Some of the various hypotheses of myriapod phylogeny. Morphological studies (trees a and b) support a sister grouping of Diplopoda and Pauropoda, while studies of DNA or amino acid similarities suggest a variety of different relationships, including the relationship of Pauropoda and Symphyla in tree c.
A variety of groupings (clades) 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 (includingDNA sequence andamino acid similarities).[37][38]
Dignatha (also called Collifera) is a clade consisting of millipedes and pauropods, and is supported by morphological similarities including the presence of agnathochilarium (a modified jaw and plate apparatus) and acollum, 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.[39]
Pectinopoda consist of millipedes and centipedes, a classification that also supports Edafopoda.[40][41]
Progoneata is a group encompassing millipedes, pauropods and symphylans while excluding centipedes. Shared features include reproductive openings (gonopores) behind the second body segment, and sensory hairs (trichobothria) with a bulb-like swelling. It is compatible with either Dignatha or Edafopoda.[38]
^Müller, C.H.G.; Rosenberg, J.; Richter, S.; Meyer-Rochow, V.B. (2003). "The compound eye ofScutigera coleoptrata (Linnaeus, 1758) (Chilopoda; Notostigmophora): an ultrastructural re-investigation that adds support to the Mandibulata concept".Zoomorphology.122 (4):191–209.doi:10.1007/s00435-003-0085-0.S2CID6466405.
^abcBarnes, Robert D. (1982).Invertebrate Zoology. Philadelphia, PA: Holt-Saunders International. pp. 810–827.ISBN978-0-03-056747-6.
^Rosenberg, Joerg; Meyer-Rochow, Victor Benno (2009). Meyer-Rochow V.B. (ed.).Bioluminescence in Focus - a collection of illuminating essays. Research Signpost; Trivandrum, Kerala, India. pp. 139–147.
^Wilson, Heather M.; Shear, William A. (2000). "Microdecemplicida, a new order of minute arthropleurideans (Arthropoda: Myriapoda) from the Devonian of New York State, U.S.A.".Transactions of the Royal Society of Edinburgh: Earth Sciences.90 (4):351–375.doi:10.1017/S0263593300002674.S2CID129597005.
^Edgecombe GD; Giribet G (2002)."Myriapod phylogeny and the relationships of Chilopoda"(PDF). In J Llorente Bousquets; JJ Morrone; HP Ulloa (eds.).Biodiversidad, Taxonomia y Biogeografia de Artropodos de Mexico: Hacia una S´ıntesis de su Conocimiento. Univ. Nac. Aut´on Mexico: Prensas Ciencias. pp. 143–168.
^Benavides, Ligia R.; Edgecombe, Gregory D.; Giribet, Gonzalo (2023). "Re-evaluating and dating myriapod diversification with phylotranscriptomics under a regime of dense taxon sampling".Molecular Phylogenetics and Evolution.178.doi:10.1016/j.ympev.2022.107621. Article No. 107621.
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