Myriapoda

Myriapoda Fossil range: Silurian - Recent
Lithobius forficatus, a centipede
Scientific classification
Kingdom: Animalia Phylum: Arthropoda Subphylum: Myriapoda Latreille, 1802
Classes [1]
Chilopoda Diplopoda Pauropoda Symphyla Arthropleuridea (extinct)

Myriapoda is a subphylum of arthropods containing millipedes, centipedes, and others. The group contains 13,000 species, all of which are terrestrial ^[2]. Although their name suggests they have myriad (10,000) legs, myriapods range from having over 750 legs (Illacme plenipes) ^[3] to having fewer than ten legs. They have a single pair of antennae and simple eyes.

Myriapods are most abundant in moist forests, where they fulfill an important role in breaking down decaying plant material ^[2], although a few live in grasslands, semi-arid habitats or even deserts ^[4]. The majority are herbivorous, with the exception of centipedes, which are chiefly nocturnal predators. Pauropodans and symphylans are small, sometimes microscopic animals that resemble centipedes superficially and live in soils. 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, 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 secretions (often containing benzoquinones) which can cause temporary blistering and discolouration of the skin ^[5].

The fossil record of myriapods reaches back into the late Silurian, although molecular evidence suggests a diversification in the Cambrian Period ^[6], and Cambrian fossils exist which resemble myriapods ^[2].

Classification

There has been much debate as to which arthropod group is most closely related to the Myriapoda. 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 ^[7].

There are four classes of extant myriapods, Chilopoda (centipedes), Diplopoda, Pauropoda and Symphyla, containing a total of around 12,000 species ^[8]. While each of these groups of myriapods is believed to be monophyletic, relationships among them are less certain ^[9].

Centipedes

Scolopendra cingulata, a centipede

Main article: centipede

Centipedes make up the order Chilopoda. They are fast, predatory and venomous, hunting mostly at night. There are around 3,300 described species ^[8], ranging from the diminutive Nannarup hoffmani (less than half an inch in length, c. 12 mm) ^[10] to the giant Scolopendra gigantea, which may exceed 30 cm (12 in).

Millipedes

Tachypodoiulus niger, a millipede

Main article: millipede

Most millipedes are slower than centipedes, and feed on leaf litter and detritus. 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 ^[8]. One species, Illacme plenipes has the greatest number of legs of any animal, with 750 ^[3]. Pill millipedes are much shorter, and are capable of rolling up into a ball, like pillbugs.

Symphyla

Scutigerella immaculata, a symphylan

Main article: Symphyla

About 200 species of symphylans are known worldwide ^[8]. 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 ^[11].

Pauropoda

Pauropus huyxleyi, a pauropodan

Main article: Pauropoda

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 ^[12]. Over 700 species have been described ^[8]. 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 ^[13].

Arthropleuridea

Main article: Arthropleuridea

Arthropleurids 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 meters (about ten feet) long. The Arthropleurids may be a division of the millipedes.

References