The book of earth: millipedes

Desmoxytes, whose species are commonly known as the dragon millipedes, is a genus of millipede of the family Paradoxosomatidae found in southeast Asia. The genus was described by Ralph Vary Chamberlin in 1923 and reviewed by Sergei Golovatch and Henrik Enghoff in 1994. At least twenty-nine species are known from southeastern China to Myanmar, Thailand, and Vietnam. One species, D. planata, has also been observed in Sri Lanka, the Andaman Islands, Seychelles, Java, Great Coco Island, and Fiji; however, this species has expanded its range by being transported through human activity. Several species have only recently been discovered and some have yet to be officially described.

Generally, species in this genus are striking in coloration and spiny, although several species are cave-dwelling, and show troglomorphic characters such as pale browinish coloration and long, narrow spines. This genus is unique for its sculptured, ornamented dorsum, and elaborate paranota. The maximum length for a species in this genus is around 3 centimeters (1.2 inches). Species in this genus also have the ability to produce hydrogen cyanide to ward off predators, which can give the dragon millipedes an almondlike smell.

Species

Other than D. planata, each species of dragon millipede is known primarily from only one or a few locations in their country of origin. Additionally, there is one species from Australia, Desmoxytoides hasenpuschorum, which is extremely similar to Desmoxytes but has been split into its own monotypic genus; there is speculation, even by the genus authority for Desmoxytoides, that this split is not necessary. Five species of Desmoxytes, including D. aspera, D. cervaria, D. draco, D. pilosa, and D. spectabilis, were originally placed in the separate genus Hylomus before it was lumped into Desmoxytes.

Desmoxytes eupterygota, a troglomorphic species from Hunan Province, China

Desmoxytes purpurosea, the "shocking pink dragon millipede," from Thailand
Binomial names Authority Year Range
Desmoxytes acantherpestes Golovatch & Enghoff 1994 Thailand
Desmoxytes aspera Attems 1937 Vietnam
Desmoxytes cattienensis Nguyen Duc Anh et al. 2005 Vietnam
Desmoxytes cervaria Attems 1953 Vietnam
Desmoxytes cervina Pocock 1895 Myanmar
Desmoxytes cornutus Zhang & Li 1982 People's Republic of China
Desmoxytes delfae Jeekel 1964 Thailand
Desmoxytes draco Cook & Loomis 1924 People's Republic of China
Desmoxytes enghoffi Nguyen Duc Anh et al. 2005 Vietnam
Desmoxytes eupterygota Golovatch et al. 2012 People's Republic of China
Desmoxytes getuhensis Liu, Golovatch & Tian 2014 People's Republic of China
Desmoxytes gigas Golovatch & Enghoff 1994 Thailand
Desmoxytes hostilis Golovatch & Enghoff 1994 Vietnam
Desmoxytes jeekeli Golovatch & Enghoff 1994 Thailand
Desmoxytes lingulata Liu, Golovatch & Tian 2014 People's Republic of China
Desmoxytes longispina Loksa 1960 People's Republic of China
Desmoxytes lui Golovatch et al. 2012 People's Republic of China
Desmoxytes minutuberculata Zhang 1986 People's Republic of China
Desmoxytes nodulosa Liu, Golovatch & Tian 2014 People's Republic of China
Desmoxytes parvula Liu, Golovatch & Tian 2014 People's Republic of China
Desmoxytes pilosa Attems 1937 Vietnam
Desmoxytes planata Pocock 1895 widespread
Desmoxytes proxima Nguyen Duc Anh et al. 2005 Vietnam
Desmoxytes pterygota Golovatch & Enghoff 1994 Thailand
Desmoxytes purpurosea Enghoff, Sutcharit & Panha 2007 Thailand
Desmoxytes rhinoceros Likhitrakarn, Golovatch & Panha 2015 Laos
Desmoxytes rhinoparva Likhitrakarn, Golovatch & Panha 2015 Laos
Desmoxytes rubra Golovatch & Enghoff 1994 Thailand
Desmoxytes scolopendroides Golovatch et al. 2010 People's Republic of China
Desmoxytes scutigeroides Golovatch et al. 2010 People's Republic of China
Desmoxytes specialis Nguyen Duc Anh et al. 2005 Vietnam
Desmoxytes spectabilis Attems 1937 Vietnam
Desmoxytes spinissima Golovatch et al. 2012 People's Republic of China
Desmoxytes taurina Pocock 1895 Myanmar
Desmoxytes terae Jeekel 1964 Malaysia, Thailand

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Parastemmiulus is an extinct genus of millipede in the family Stemmiulidae known from a fossil found in Mexico. There is one described species in the genus, Parastemmiulus elektron. The species is one of three millipedes described from Mexican amber, and the oldest Stemmiulidae fossil species as of 2013.

History and classification

Parastemmiulus elektron was described from a solitary fossil, which is preserved as an inclusion in a transparent chunk of Mexican amber. At the time of description, the amber specimen was housed in the fossil collection of the Instituto Nacional de Antropología e Historia in San Cristóbal de las Casas, Simojovel. The holotype fossil is composed of a very complete adult female recovered from the La Guadalupe Quarry. Mexican amber is recovered from fossil bearing rocks in the Simojovel region of Chiapas, Mexico. The amber dates from between 23 million years old at the oldest and 15 million years at the youngest. The La Guadalupe Quarry site is an outcrop of amber bearing strata belonging to the Mazantic shale. The deposits preserve a transitional river or stream environments near the coast and preserves fossils of a mangrove forest ecosystem.

The holotype was first studied by a team of researchers headed by Francisco Riquelme of the Universidad Autónoma del Estado de Morelos with their 2013 type description of the genus and species being published in the natural sciences journal Historical Biology. The genus name is a combination of the genus Stemmiulus and "para" meaning along side, a reference to the similarities between the two genera. The specific epithet elektron was is based from the Greek word meaning amber.

P. elektron is one of three millipede species described from Mexican amber, the others being Anbarrhacus adamantis and Maatidesmus paachtun, while a number of other species have been described from the similarly aged Dominican amber. One possible Stemmulid fossil was described from Dominican amber, however it was not complete, and has been regarded as doubtful. P. elektron is the first Stemmulid genus described from a fossil.

Description

The P. elektron female is reddish-brown in overall coloration with the legs a brownish tone that trends to hyaline. The body is composed of forty six segments and has a length of approximately 21 mm (0.83 in) . The ocellar field is shallow and triangular in shape with three ocelli, and is positioned just to the rear of the antennae. Two grooved possible sensory canals are present in the field, one connecting the rear and front ocelli, the other connecting the middle ocelli to the antenna socket. Present is a defined, pear-shaped Tömösváry organ located below the antenna socket and reaching the ocellar field. The antennae have a total of eight antennomeres that vary in length. The largest segment is antennomere 2, while the smallest is antennomere 7. The low ocelli numbers, differences in ocelli sizes, and varied antennomere lengths is a combination of characters only found in Stemmiulida. However, in contrast to P. elektron living species of the family do not have a Tömösváry organ.

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Pauropods are small, pale, millipede-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 mould. They look rather like centipedes, but are probably the sister group to millipedes. The name is derived from the Greek roots pauro "few" and podo "foot".

Anatomy and ecology

Ventral and dorsal views of Pauropus amicus from New South Wales, Australia.
Pauropods are soft, cylindrical animals with bodies 0.5 to 2 millimetres (0.02 to 0.08 in) long. The first instar has three pairs of legs, but that number increases with each moult so that adult species may have nine to eleven pairs of legs. They have neither eyes nor hearts. The body segments have ventral tracheal/spiracular pouches forming apodemes similar to those in millipedes and Symphyla, although the trachea usually connected to these structures are absent in most species. The antennae are branching, which is distinctive for the group.

Pauropods live in the soil, usually at densities of less than 100 per square metre (9/sq ft).

Evolution and systematics

Though no fossil pauropods have been found from before the time of the Baltic amber (40 to 35 million years ago), they seem to be an old group closely related to the millipedes (Diplopoda). Their head capsules show great similarities to millipedes: both have three pairs of mouthparts and the genital openings occur in the anterior part of the body. Moreover, both groups have a pupoid phase at the end of the embryonic development. The two groups probably have a common origin.

There are two orders: Hexamerocerata and Tetramerocerata; Hexamerocerata has a purely tropical range, while in Tetramerocerata, most genera are subcosmopolitan. Hexamerocerata has a 6-segmented and strongly telescopic antennal stalk and a 12-segmented trunk with 12 tergites and 11 pairs of legs. The representatives are white and proportionately long and large. The one family in this order, Millotauropodidae, has one genus and a few species. Tetramerocerata has a 4-segmented and scarcely telescopic antennal stalk, 6 tergites, and 8–10 pairs of legs. Representatives of this order are often small (sometimes very small), and white or brownish. Most species have nine pairs of legs as adults. The four families include Pauropodidae, Afrauropodidae, Brachypauropodidae, and Eurypauropodidae. Most genera and species belong to the family Pauropodidae.

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Anbarrhacus is an extinct genus of millipede in the family Platyrhacidae known from a fossil found in North America. There is one described species in the genus, Anbarrhacus adamantis, which is one of three millipedes described from Mexican amber.

History and classification

Anbarrhacus adamantis was described from a solitary fossil, which is preserved as an inclusion in a transparent chunk of Mexican amber. At the time of description, the amber specimen was housed in the fossil collection of the Instituto Nacional de Antropología e Historia in San Cristóbal de las Casas, Simojovel. The holotype fossil is composed of a very complete immature male recovered from the Guadalupe Victoria site. Mexican amber is recovered from fossil bearing rocks in the Simojovel region of Chiapas, Mexico. The amber dates from between 23 million years old at the oldest and 15 million years at the youngest. The Guadalupe Victoria site is an outcrop of amber bearing strata belonging to both the Mazantic Shale and Balumtum Sandstone. The deposits preserve a transitional river or stream environments near the coast and preserves fossils of a mangrove forest ecosystem.

The holotype was first studied by a team of researchers headed by Francisco Riquelme of the Universidad Autónoma del Estado de Morelos with their 2014 type description of the genus and species being published in the natural sciences journal PLOS One. The genus name is a derived from a combination of the Arabic voice word ánbar for "amber" and rhacus, which is used frequently as a genus suffix in the family Platyrhacidae. The specific epithet adamantis was coined from the neo Latin word adamantus meaning diamond, a reference to the patterning on the tops of the collum and metatergites.

A. adamantis is one of three millipede species described from Mexican amber, the others being Maatidesmus paachtun and Parastemmiulus elektron, while a number of other species have been described from the similarly aged Dominican amber.

Description

The A. adamantis male is mostly a creamy white in coloration with a yellowish tone to the labrum and areas of the legs. The sterna grade between yellowish and brown. The total body length is approximately 19.8 mm (0.78 in) and has a total of 17 body segments plus head. The head is a little wider than the next body segment, called the collum. The antennae have a total of seven antennomeres that vary in length and form a club-shaped tip. The longest segment is antennomere 5 followed by antennomere 6, and the shortest are antennomeres 1 and 7. Both antennomeres 5 and 6 have long setae near their apical ends, and the antennae have four sensory cones preserved. The upper plates of the body segments after the collum are heavily lobed forming metatergites and paranota. The surface of the paranota, side lobes of the metatergites, have a granular texturing, and the metatergites have a distinct diamond shaped patterning in the texturing. The segments gradually get wider from the head to about two thirds of the way down the body after which the segments start to narrow progressively to segment 17. The male is mature enough to show developing bulb shaped gonopods on segment 7.

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Arthropleuridea is an extinct subclass of myriapod arthropods that flourished during the Carboniferous period, having first arose during the Silurian, and perishing due to climate change just before the Early Permian. Members are characterized by possessing diplosegement (fused "double segments", as in modern-day millipedes) paranotal tergal lobes separated from the body axis by a suture, and by sclerotized plates buttressing the leg insertions. Despite their unique features, recent phylogenetic research suggests Arthropleuridea be included among millipedes in the class Diplopoda. The subclass contains three recognized orders, each with a single genus.

Arthropleuridea is most famous for Arthropleura (order Arthropleurida). Reaching over 2 meters in length, arthropleurids are among the largest arthropods ever to have lived. The lack of large terrestrial vertebrate predators and the highly oxygenic atmosphere at that time probably enabled them to grow so large. Arthropleurids lived in the moist coal swamps that were common at the time and may have burrowed in the undergrowth. They were either herbivores or detritivores. Besides their size, their most distinguishing features were their legs with eight segments (as many as 30 pairs) and extremely tough exoskeletons. There is no evidence of spiracles, so the animals must have used lungs or gills for respiration. Arthropleura became extinct as the climate became drier and the coal swamps dried out. Tracks from Arthropleura up to 50 cm wide have been found at Joggins, Nova Scotia.

Most arthropleurideans are thought to have been terrestrial, although, without any known respiratory structure, terrestriality is assumed only by analogy to modern arthropods. Early forms, however, including Eoarthropleura (order Eoarthropleurida), appear to have been aquatic.[citation needed] For this reason, some question Arthropleuridea's inclusion among millipedes because no modern aquatic myriapods are known. Eoarthropleura has been found from the Upper Silurian through the Upper Devonian of Europe and North America.

Microdecemplex, of the order Microdecemplicida, was smaller in comparison to the other arthropleurideans, at just a few millimeters long. The genus is known from the Middle through Upper Devonian of New York state, USA.

Classification and placement

After several decades of uncertainty, Arthropleuridea was placed within the Diplopoda in the year 2000. However, there is still controversy regarding the relationships of the three orders to living millipede groups. Some authors place Arthropleuridea within the Chilognatha, as a sister group to all living Chilognathan millipedes (Pentazonia + Helminthomorpha). An alternate hypothesis breaks up the subclass: placing the orders Arthropleurida and Eoarthropleurida within the basal Penicillata (as sister to the living Polyxenida), and leaving only Microdecemplicida as a sister group to the living Chilognatha. Under this hypothesis, Arthropleuridea would be paraphyletic.

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Myriapoda is a subphylum of arthropods containing millipedes, centipedes, and others. The group contains over 13,000 species, most of which are terrestrial. Although their name suggests they have myriad (10,000) legs, myriapods range from having over 750 legs (the millipede Illacme plenipes) to having fewer than ten legs.

The fossil record of myriapods reaches back into the late Silurian, 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

The head of Scutigera coleoptrata, showing antennae, compound eyes and mouthparts
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. 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 in each segment, the brain is relatively poorly developed.

During mating, male miryapodas produce a packet of sperm, or spermatophore, 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 moult 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 grasslands, semi-arid habitats or even deserts. A very small percentage of species are littoral (found along the sea shore). The majority are detritivorous, 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 millipedes produce noxious secretions (often containing benzoquinones) which in rare cases can cause temporary blistering 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 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. There are four classes of extant myriapods, Chilopoda (centipedes), Diplopoda, Pauropoda and Symphyla, containing a total of around 12,000 species. While each of these groups of myriapods is believed to be monophyletic, relationships among them are less certain.

Scolopendra cingulata, a centipede
Main article: Centipede
Centipedes make up the class Chilopoda. They are fast, predatory and venomous, hunting mostly at night. There are around 3,300 species, ranging from the diminutive Nannarrup hoffmani (less than 12 mm or 1⁄2 in in length) to the giant Scolopendra gigantea, which may exceed 30 centimetres (12 in).

Tachypodoiulus niger, a millipede
Main article: Millipede
Millipedes form the class Diplopoda. 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. The name "millipede" is a compound word formed from the Latin roots millia ("thousand") and pes (gen. pedis) ("foot"), although millipedes typically have between 36 and 400 legs. One species, Illacme plenipes, has the greatest number of legs of any animal, with 750. Pill millipedes are much shorter, and are capable of rolling up into a ball, like pillbugs.

Scutigerella immaculata, a symphylan
Main article: Symphyla
About 200 species of them 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 moult so that the adult instar has twelve pairs of legs.

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. 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.

Main article: Arthropleuridea
Arthropleurideans were ancient myriapods that are now extinct, known from the late Silurian to the Permian. The most famous members are from the genus Arthropleura, which was a giant, probably herbivorous, animal that could be up to 3 metres (10 ft) long, 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.

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 (including DNA sequence and amino acid 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.
Progoneata is a group encompassing Dignatha 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.
Edafopoda is a grouping of symphylans and pauropodans that is supported by shared genetic sequences, yet conflicts with Dignatha and Trignatha.

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.

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Not to be confused with Platydesmida or Polyzoniida.
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 order Polydesmida are also known as "flat-backed millipedes", because on most species, each body segment has wide lateral keels known as paranota. These keels are produced by the posterior half (metazonite) of each body ring behind the collum. Polydesmids have no eyes, and vary in length from 3 to 130 mm (0.12 to 5.12 in). Including the telson, adults have 19 or 20 rings, while juveniles may have from 7 to 19 rings. Mature males have a single pair of gonopods consisting of the modified forward leg pair of the 7th segment. Many of the larger species show bright coloration patterns which warn predators of their toxic secretions.

Ecology

Polydesmids are very common in leaf litter, where they burrow by levering with the anterior end of the body.

Classification

The c. 3500 species of Polydesmida are variously classified into four suborders (names ending in "-idea"), and 28 families, the largest (numerically) including Paradoxosomatidae, Xystodesmidae, and Chelodesmidae.

Dalodesmidea Hoffman, 1980. 2 families
Dalodesmidae Cook, 1896
Vaalogonopodidae Verhoeff, 1940
Leptodesmidea Brölemann, 1916. 13 families
Chelodesmoidea Cook, 1895
Chelodesmidae Cook, 1895
Platyrhacoidea Pocock, 1895
Aphelidesmidae Brölemann, 1916
Platyrhacidae Pocock, 1895
Rhachodesmoidea Carl, 1903
Rhachodesmidae Carl, 1903
Tridontomidae Loomis & Hoffman, 1962
Sphaeriodesmoidea Humbert & de Saussure, 1869
Campodesmidae Cook, 1896
Holistophallidae Silvestri, 1909
Sphaeriodesmidae Humbert & de Saussure, 1869
Xystodesmoidea Cook, 1895
Eurymerodesmidae Causey, 1951
Euryuridae Pocock, 1909
Gomphodesmidae Cook, 1896
Oxydesmidae Cook, 1895
Xystodesmidae Cook, 1895
Paradoxosomatidea Daday, 1889.[a] 1 family
Paradoxosomatidae Daday, 1889
Polydesmidea Pocock, 1887. 12 families
Oniscodesmoidea Simonsen, 1990
Dorsoporidae Loomis, 1958
Oniscodesmidae DeSaussure, 1860
Pyrgodesmoidea Silvestri, 1896
Ammodesmidae Cook, 1896
Cyrtodesmidae Cook, 1896
Pyrgodesmidae Silvestri, 1896
Haplodesmoidea Cook, 1895
Haplodesmidae Cook, 1895
Opisotretoidea Hoffman, 1980
Opisotretidae Hoffman, 1980
Polydesmoidea Leach, 1815
Cryptodesmidae Karsch, 1880
Polydesmidae Leach, 1815
Trichopolydesmoidea Verhoeff, 1910
Fuhrmannodesmidae Brölemann, 1916
Macrosternodesmidae Brölemann, 1916
Nearctodesmidae Chamberlin & Hoffman, 1958
Trichopolydesmidae Verhoeff, 1910
Representative diversity of Polydesmida

Asiomorpha coarctata (Paradoxosomatidae), an Asian species widely introduced by humans

Tasmaniosoma armatum (Dalodesmidae) from Tasmania

Eutrichodesmus aster (Haplodesmidae) from Vietnam, with unusual mid-dorsal crests

A species of Xystodesmidae from the United States showing aposematic coloration

An paradoxosomatid from China, with reduced paranota

Polydesmus collaris (Polydesmidae) from Europe

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Not to be confused with Polyxenidas.
Polyxenida is an order of millipedes readily distinguished by a unique body plan consisting of a soft, non-calcified body ornamented with tufts of bristles – traits that have inspired the common names "bristly millipedes" or "pincushion millipedes". There are at least 86 species in four families worldwide, and are the only living members of the subclass Penicillata.

Description

Polyxenida differ from other millipedes in having a soft, non-calcified exoskeleton, unique tufts of bristles or setae, fewer legs (no more than 17 pairs), and an absence of copulatory appendages in males. Individuals are small, not exceeding 7 millimeters (0.28 inches).

Defense

Bristly millipedes lack the chemical defenses and hard exoskeleton of other millipedes, and instead employ a unique defense mechanism: the distinctive barbed bristles can easily detach and become entangled in the limbs and mouth-parts of predatory insects, effectively immobilizing them.

Reproduction

Male Polyxenidans lack the modified sperm-transferring appendages (gonopods) found in most other millipede groups. As a result, sperm transfer is indirect: males deposit a spermatophore that is subsequently picked up by females.

Many species reproduce asexually by way of parthenogenesis, wherein females lay eggs without mating and males are absent or rare.

Classification

Phryssonotus brevicapensis (Synxenidae), a species from South Africa described in 2011
Polyxenida is the only living order of the subclass Penicillata, the basal subclass of millipedes. Penicillata is the sister group of all other living millipedes: the subclasses Pentazonia and Helminthomorpha).

In 2003 the Polyxenida contained 159 valid species and/or subspecies, although at least eight new species have been described since 2010.

Superfamily Polyxenoidea Lucas, 1840
Hypogexenidae Schubart, 1947
Lophoproctidae Silvestri, 1897
Polyxenidae Lucas, 1840
Superfamily Synxenoidea Silvestri, 1923
Synxenidae Silvestri, 1923
^ a b Nguyen Duy - Jacquemin, Monique; Uys, Charmaine; Geoffroy, Jean-Jacques (2011). "Two remarkable new species of Penicillata (Diplopoda, Polyxenida) from Table Mountain National Park (Cape Town, South Africa)" (PDF). ZooKeys. 156 (0): 85–103. doi:10.3897/zookeys.156.2211. PMC 3253573free to read. PMID 22303097.
^ Sierwald, Petra; Bond, Jason E. (2007). "Current Status of the Myriapod Class Diplopoda (Millipedes): Taxonomic Diversity and Phylogeny". Annual Review of Entomology. 52 (1): 401–420. doi:10.1146/annurev.ento.52.111805.090210. PMID 17163800.
^ Shear, W (2011). "Class Diplopoda de Blainville in Gervais, 1844. In: Zhang, Z.-Q.(ed.) Animal biodiversity : an outline of higher-level classification and survey of taxonomic richness" (PDF). Zootaxa. 3148: 159–164.
^ Nguyen Duy-Jacquemin, M.; J.-J. Geoffroy (2003). "A revised comprehensive checklist, relational database, and taxonomic system of reference for the bristly millipedes of the world (Diplopoda, Polyxenida)". African Invertebrates. 44 (1): 89–101.
^ Short, Megan; Huynh, Cuong (2011). "The genus Unixenus Jones, 1944 (Diplopoda, Penicillata, Polyxenida) in Australia". ZooKeys. 156: 105–122. doi:10.3897/zookeys.156.2168.
^ Short, Megan; Huynh, Cuong (2013). "Four new species of Unixenus Jones, 1944 (Diplopoda, Penicillata, Polyxenida) from Australia". ZooKeys. 278: 75–90. doi:10.3897/zookeys.278.4765.
Fossil history

The earliest representatives of Polyxenida are found in Lebanese amber from the Early Cretaceous period.

Some authors place the extinct orders Arthropleurida and Eoarthropleurida (each represented by a single genus) within the Penicillata as a sister group to Polyxenida.

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Sphaerotheriida (from the Latin sphaerium = ball) is an order of millipedes in the infraclass Pentazonia, sometimes known as giant pill millipedes. They inhabit Southern Africa, Madagascar, South and Southeast Asia, Australia and New Zealand. Like the Northern Hemisphere pill millipedes of the order Glomerida, these millipedes can roll into a ball when disturbed. When they are rolled-up, most sphaerotheriidans reach a maximum size of a cherry or golf ball, but some species from Madagascar can even reach the size of an orange (an example of island gigantism). When rolled-up, predators are unable to unravel giant pill millipedes since the margins of their second and last dorsal plates fit perfectly into one another, creating a sealed ball. A few giant pill millipede species are able to produce sound, the only millipedes known to do this. This order of millipedes is also unique in that some African species are used for medicinal purposes.

Morphology

Sphaerotheriidans are characterized by a relatively conservative body morphology; superficially all species and genera look the same. Dorsally, their head is followed by twelve body tergites (collum, thoracic shield, and ten normal tergites) and the anal shield. Ventrally, females possess twenty-one leg pairs (forty-two legs in total), while males carry two additional modified leg pairs, the anterior and posterior telopods under their anal shield. The telopods resemble chelae and/or clamp-like structures, which are probably used in holding the female during mating. In the position of the male telopods, the females instead have a sclerotized subanal plate, which in some species such as those belonging to the family Arthrosphaeridae, is enlarged and is used to produce vibrations (stridulation). Furthermore, unlike other large-bodied millipede orders, Sphaerotheriida do not have glands that excrete poisonous or ill-smelling substances. Instead they depend entirely on their rolling-up behavior for protection.

A pill millipede from the Western Ghats, India
Sphaerotheriida somewhat resemble the North American and Eurasian pill millipedes of the order Glomerida, but are generally larger in size (20–80 millimetres or 0.8–3.1 inches body length). Until the end of the 20th century only the largest known species in the order Glomerida rivalled the size of even the smallest known Sphaerotheriida, but in the early 21st century a much smaller Sphaerotheriid was described from Madagascar: full-grown specimens of Microsphaerotherium ivohibiensis are just the size of a pea. Also on Madagascar, some giant pill millipede species exhibit island gigantism, reaching more than 9.5 cm (3.7 in) in outstretched length and a size comparable to an orange when rolled up.

The orders differ in the number of tergites (10 or 11 in Glomerida, 12 in Sphaerotheriida) and legs (17 or 19 in Glomerida, 21 or 23 in Sphaerotheriida), and show great differences in their head morphology and genital openings, among other traits.

Both orders have the ability to roll into a perfect ball, protecting the head, antennae, and the vulnerable underside. However, this rolled-up position (volvation) is achieved differently. In Glomerida, the enlarged second body ring (thoracic shield) has a more or less visible gap within which fit the tips of tergites 3–11, whereas in Sphaerotheriida the tips of tergites 3–12 fit perfectly into a groove on the thoracic shield. Juvenile sphaerotheriidans show the same gap as the Glomerida. Many giant pill millipede species have special ledges ('locking carinae') on the underside of the tergite tips and the anal shield which can be moved above a brim on the thoracic shield. These millipedes remain passively locked-up since they need not continued muscle contraction to remain in the rolled-up position.

Distribution

Arthrosphaera bicolor (Arthrosphaeridae), named and painted by Reginald Innes Pocock
In general, Sphaerotheriida have a Gondwanan distribution (the exception is Zephroniidae from southeast Asia and adjacent regions). Gondwana was the large southern continent that formed after the breakup of the supercontinent Pangaea. It included the modern-day landmasses of South America (which however lacks sphaerotheriidans), New Zealand, Australia, India, and Madagascar. It is believed that the Sphaerotheriida originated in Gondwana and then diverged into its various forms.

Sphaerotheriida is divided into four families whose distributions do not overlap: Procyliosomatidae, Zephroniidae, Sphaerotheriidae and Arthrosphaeridae. The most basal family, Procyliosomatidae, lives in Australia and New Zealand. The Zephroniidae (synonym Sphaeropoeidae) occurs in southeast Asia from the Himalayas and China south and east to Sulawesi and inhabits some Philippines islands. The family Sphaerotheriidae only occurs in South Africa with isolated populations in Zimbabwe and Malawi (probably introduced). The Arthrosphaeridae are distributed in southern India and Madagascar.

A few giant pill millipede species have been dispersed by humans, probably inadvertently. Examples include the Sri Lankan Arthrosphaeridae species Arthrosphaera brandtii which has established a population in the Usambara Mountains, Tanzania, as well as some South African Sphaerotherium species which have isolated populations in Malawi. Another likely candidate is Sechelliosoma forcipatum, a small species of the southeast Asian family Zephroniidae, currently only known from a single island in the Seychelles.

Ecology

Little is known about the ecology, development and life history of Sphaerotheriida, but apparently all species are detritivores, feeding on dead organic matter such as leaves and wood on the forest floor. Like earthworms, they play an important role in decomposition; by breaking down decaying organic matter they release locked up nutrients back into the soil. Such recycling is essential for plant nutrition and accordingly for the entire ecology. It is possible that giant pill millipedes rely on special bacteria in their gut, much as termites do, to exploit the nutritional value of otherwise indigestible material such as lignin.

Like most millipedes, Sphaerotheriida inhabit mainly the leaf litter of humid forests. Some species, however, show an arboreal (tree-living) lifestyle, and in these the rolling-up reflex has been suppressed.

Defense against predation

Giant pill millipede in rolled-up position
The rolling-up ability and tough skeletal armor of the Sphaerotheriida offer protection against some predators, but a wide variety of predators feed on them, or even specialise in them as a source of food. Species that specialize in feeding on giant pill millipedes necessarily have evolved special structures or behaviors to overcome their defences. Examples include the South African snail family Chlamydephoridae which almost exclusively feeds on giant pill millipedes Another example is the meerkat (Suricata suricata) which has been reported (at least in captivity) to throw rolled-up sphaerotheriids against rocks in order to break them. This behaviour however, is their way of breaking open many refractory food items, such as snails and hard-shelled eggs.

Apart from the rolling-up behavior of giant pill millipedes, camouflage may be an important defense mechanism against predators that hunt by sight, such as birds.

Sphaerotheriida also are subject to internal parasitism, and several species of nematodes are obligate parasites of particular species of giant pill millipedes.

Taxonomy

Various sphaerotheriidans (and a beetle) illustrated in 1859
By 2014, over 326 species in around 34 genera in approximately 20 genera had been described.

Family: Arthrosphaeridae
Arthrosphaera Pocock, 1895 – 40 species, India, Sri Lanka
Zoosphaerium Pocock, 1895 – 55 species, Madagascar
Sphaeromimus de Saussure & Zehntner, 1902 – 10 species, southeast Madagascar
Microsphaerotherium Wesener & van den Spiegel, 2007 – 2 species, Madagascar
Family: Cyliosomatidae
Cyliosoma Pocock, 1895 – 15 species, Australia
Cynotelopus Jeekel, 1986 – monotypic, southwest Australia
Family: Procyliosomatidae
Procyliosoma Silvestri, 1917 – 11 species, Eastern Australia, Tasmania, New Zealand
Family: Sphaerotheriidae
Sphaerotherium Brandt, 1833 – 54 species, South Africa, Zimbabwe
Kylindotherium Attems, 1926 – monotypic, South Africa
Family: Zephroniidae
Bothrobelum Verhoeff, 1924 – monotypic, Borneo
Cryxus Leach, 1814 – monotypic, Asia
Indosphaera Attems, 1935 – 2 species N. India, Myanmar
Kophosphaera Attems, 1935 – 5 species, N. India, Nepal
Leptotelopus Silvestri, 1897 – monotypic, Myanmar
Prionobelum Verhoeff, 1924 – 8 species, Vietnam, SW China
Sphaerobelum Verhoeff, 1924 – 4 species, Vietnam
Zephronia Gray, 1832 – 37 species N. India, Myanmar, Malayan Peninsula, Java, Sumatra, Borneo
Sphaeropoeus Brandt, 1833 – 22 species, N. India, Myanmar, Malayan Peninsula, Java, Sumatra, Borneo
Tigridosphaera Jeekel, 2000 – 4 species, Malayan Peninsula
Castanotherium Pocock, 1895 – 50 species, Indonesian Islands, Philippines
Castanotheroides Chamberlin, 1921 – 3 species, Philippines
Sechelliosoma Mauriès, 1980 – monotypic, Seychelles
Rajasphaera Attems, 1935 – monotypic, Borneo
^ T. Wesener, I. Bespalova & P. Sierwald (2010). "Madagascar's living giants: discovery of five new species of endemic giant pill millipedes from Madagascar (Diplopoda: Sphaerotheriida: Arthrosphaeridae: Zoosphaerium)". African Invertebrates. 51 (1): 133–161. doi:10.5733/afin.051.0102.
^
^ Wesener, Thomas (2009). "Unexplored richness: discovery of 31 new species of giant pill millipedes endemic to Madagascar, with a special emphasis on microendemism (Diplopoda, Sphaerotheriida)" (PDF). Zootaxa. 2097: 1–134.
^ a b Wesener, Thomas (2014). "A new phylogenetic analysis of the Sphaerotheriida (Diplopoda) with a revision of the Australian giant pill-millipedes". Invertebrate Systematics. 28 (2): 196. doi:10.1071/IS13048.
Phylogeny

The first modern phylogenetic study of Sphaerotheriida (simplified below) was conducted by Wesener and VandenSpiegel in 2009, using morphological data from 36 species in 10 genera. The South African family Sphaerotheriidae was found to be sister to the Madagascar family Arthrosphaeridae. The Australian genus Procyliosoma was found to be distinct from all other genera and placed in its own family, Procyliosomatidae.

Pentazonia
Glomeridesmida
Glomeridesmus
Glomerida
Glomeroides
Sphaerotheriida
Procyliosomatidae
Zephroniidae
Sechelliosoma
Sphaeropoeus
Zephronia
Castanotherium
Sphaerotheriidae
Arthrosphaeridae
Sphaeromimus
Arthrosphaera
Microsphaerotherium
Zoosphaerium

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