Myriapoda

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Scolopendra cingulata, a centipede

Myriapoda is a subphylum of arthropods containing millipedes, centipedes, and others. The group contains 13,000 species, all of which are terrestrial.[1] Although their name suggests they have many legs, myriapods range from having over 750 legs (Illacme plenipes) to having fewer than ten legs.[2]

The fossil record of myriapods reaches back into the late Silurian, although molecular evidence suggests a diversification in the Cambrian period,[3] and Cambrian fossils exist which resemble myriapods.[1]

Structure[change | change source]

Myriapods have a single pair of antennae 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 maxillae 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.

Classification[change | change source]

There has been much debate as to which arthropod group is most closely related to the Myriapoda.[4] Under the Mandibulata hypothesis, Myriapoda is the sister taxon to Pancrustacea, a group comprising the Crustacea and Hexapoda. Under the Atelocerata hypothesis, Hexapoda is the closest, whereas under the Paradoxopoda hypothesis, Chelicerata is the closest. This last hypothesis, although supported by few, if any, morphological characters, is supported by a number of molecular studies.[5]

There are four classes of extant myriapods, Chilopoda (centipedes), Diplopoda, Pauropoda and Symphyla, containing a total of around 12,000 species.[6] While each of these groups of myriapods is believed to be monophyletic, relationships among them are less certain.[7]

Centipedes[change | change source]

Scolopendra cingulata, a centipede

Centipedes make up the order Chilopoda. They are fast, predatory and venomous, hunting mostly at night. There are around 3,300 species,[6] ranging from the diminutive Nannarup hoffmani (less than half an inch in length, c. 12 mm)[8] to the giant Scolopendra gigantea, which may exceed 30 centimetres (12 in).

Millipedes[change | change source]

Tachypodoiulus niger, a millipede

Millipedes are the class Diplopoda, where most segments appear to have two pairs of legs. This happened because each pair of body segments fused (joined) into a single unit, which gives them the appearance of having two pairs of legs per segment.

They are slower than centipedes, and feed on leaf litter and detritus. Around 8,000 species have been described, which may represent less than a tenth of the true global millipede diversity.[6] One species, Illacme plenipes has the greatest number of legs of any animal, with 750.[2] Millipedes typically have between 36 and 400 legs. Pill millipedes are much shorter, and are capable of rolling up into a ball, like pillbugs.

Symphyla[change | change source]

Scutigerella immaculata, a symphylan

About 200 species of symphylans are known worldwide.[6] 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 moult so that the adult instar has twelve pairs of legs.[9]

Pauropoda[change | change source]

Pauropus huyxleyi, a pauropodan

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 except Antarctica.[10] Over 700 species have been described.[6] 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.[11]

Arthropleuridea[change | change source]

Arthropleuridea were ancient myriapods that are now extinct. The most famous members are from the genus Arthropleura, which was a giant, probably herbivorous, animal that could be up to 3 metres (9.8 ft) long. Arthropleuridea may be a division of the millipedes.

References[change | change source]

  1. 1.0 1.1 Ben Waggoner (1996). "Introduction to the Myriapoda". University of California, Berkeley. http://www.ucmp.berkeley.edu/arthropoda/uniramia/myriapoda.html.
  2. 2.0 2.1 Paul E. Marek & Jason E. Bond (2006). "Biodiversity hotspots: rediscovery of the world's leggiest animal". Nature 441 (7094): 707. doi:10.1038/441707a. PMID 16760967. http://www.nature.com/nature/journal/v441/n7094/abs/441707a.html.
  3. Markus Friedrich & Diethard Tautz (2002). "Ribosomal DNA phylogeny of the major extant arthropod classes and the evolution of myriapods". Nature 376 (6536): 165–167. doi:10.1038/376165a0. PMID 7603566.
  4. Gregory D. Edgecombe (2004). "Morphological data, extant Myriapoda, and the myriapod stem-group". Contributions to Zoology 73 (3): 207–252. http://dpc.uba.uva.nl/ctz/vol73/nr03/art02.
  5. Alexandre Hassanin (2006). "Phylogeny of Arthropoda inferred from mitochondrial sequences: strategies for limiting the misleading effects of multiple changes in pattern and rates of substitution". Molecular Phylogenetics and Evolution 38 (1): 100–116. doi:10.1016/j.ympev.2005.09.012. PMID 16290034.
  6. 6.0 6.1 6.2 6.3 6.4 A.D. Chapman (2005). Numbers of living species in Australia and the World. Department of the Environment and Heritage. p. 23. ISBN 0-642-56850-2. Archived from the original on 25 October 2007. http://web.archive.org/web/20071025220345/http://www.environment.gov.au/biodiversity/abrs/publications/other/species-numbers/pubs/number-living-species-report.pdf. Retrieved 01 January 2011.
  7. Jerome C. Regier, Heather M. Wilson & Jeffrey W. Shultz (2005). "Phylogenetic analysis of Myriapoda using three nuclear protein-coding genes". Molecular Phylogenetics and Evolution 34 (1): 147–158. doi:10.1016/j.ympev.2004.09.005. PMID 15579388.
  8. "Central Park survey finds new centipede". American Museum of Natural History. 2003. http://cbc.amnh.org/center/atthecbc/centi/centi.html.
  9. "Garden Symphylans". Integrated Pest Management on Peppermint-IPMP3.0. Oregon State University. http://mint.ippc.orst.edu/symphid.htm. Retrieved July 2, 2007.
  10. "Pauropods: Pauropoda". Insects and Spiders Scientific Reference. http://animals.jrank.org/pages/2563/Pauropods-Pauropoda.html. Retrieved July 2, 2007.
  11. David Kendall (2005). "Pauropods & Symphylids". Kendall Bioresearch. http://www.kendall-bioresearch.co.uk/myriapod.htm.