Polyxenida Temporal range: | |
---|---|
![]() | |
Polyxenus lagurus | |
Scientific classification ![]() | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Arthropoda |
Subphylum: | Myriapoda |
Class: | Diplopoda |
Subclass: | Penicillata Latrielle, 1831 |
Order: | Polyxenida Verhoeff, 1934 |
Families | |
Synonyms | |
Subclass Contents |
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. These features have inspired the common names bristly millipedes or pincushion millipedes. [1] [2] This order includes about 148 species in four families worldwide, which represent the only living members of the subclass Penicillata. [3] [4]
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. [5] [3] Individuals are small, usually 1.5 to 4 millimeters (0.06 to 0.16 inches) long and not exceeding 7 millimeters (0.28 inches) long. [3] [4] Adults in most species have 13 pairs of legs, but in one species ( Lophoturus madecassus ), they have only 11 pairs of legs, and in one genus ( Phryssonotus ), they have 17 pairs of legs, except for one species (Phryssonotus brevicapensis) in which they (along with those in one other species, Condexenus biramipalpus ) have 15 pairs of legs. [3] [6]
Bristly millipedes lack the chemical defenses and hard exoskeleton of other millipedes. Instead, these millipedes employ a unique defense mechanism: Bristles at the rear end of these millipedes feature hooks and barbs. These distinctive barbed bristles can easily detach and become entangled in the limbs and mouth-parts of predatory insects, effectively immobilizing them. [7] [8]
Male Polyxenidans lack the modified sperm-transferring appendages (gonopods) found in most other millipede groups. Instead, sperm transfer is indirect: Males deposit spermatophores into webs that they construct. Females then find these spermatophores by following threads spun by the males and pick up the spermatophores with their genitalia. [9] [5]
Many species reproduce asexually by way of parthenogenesis, wherein females lay eggs without mating and males are absent or rare. [10] For example, studies of the common species Polyxenus lagurus have found males scarce or absent in parts of northeastern Europe. Authorities suspect that in these populations, this species reproduces by thelytoky, that is, parthenogenisis in which unfertilized females produce female offspring. [11]
Millipedes in this order grow and develop through a series of molts, adding segments and legs until they reach a fixed number in the adult stage, which is the same in a given species. Adults continue to molt, but they do not add segments or legs. This mode of development is known as hemianamorphosis. [12]
The typical pattern in this order is observed in the common species Polyxenus lagurus. In this species, millipedes hatch with only 3 pairs of legs and 4 tergites, then develop through a series of seven molts and emerge as adults with 13 leg pairs and 10 tergites in the eighth stage. In this process, this millipede goes through stages with 3, 4, 5, 6, 8, 10, 12, and 13 leg pairs. Species in which adults have a different number of legs deviate from this common pattern. [12]
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, which form the subclass Chilognatha. The subclass Chilognatha contains the infraclasses Pentazonia and Helminthomorpha. [13] [14]
In 2003, the order Polyxenida contained 159 valid species and/or subspecies. [4] At least eight new species have been described since 2010. [6] [15] [16]
The earliest representatives of Polyxenida are found in Lebanese amber from the Early Cretaceous period. The fossil records also include representatives found in Burmese amber, Baltic amber, and the Cretaceous amber of Haute-Provence in France. These fossils indicate that the families Polyxenidae and Synxenidae already existed in the Cretaceous perod. [17]
Some authors place the extinct orders Arthropleurida and Eoarthropleurida (each represented by a single genus) within the Penicillata as a sister group to Polyxenida. [18] Others consider these extinct orders to be a sister group to Penicillata. Still others deem these extinct orders to be a sister group to the subclass Chilognatha instead. [19]
Millipedes are a group of arthropods that are characterised by having two pairs of jointed legs on most body segments; they are known scientifically as the class Diplopoda, the name derived from this feature. Each double-legged segment is a result of two single segments fused together. Most millipedes have very elongated cylindrical or flattened bodies with more than 20 segments, while pill millipedes are shorter and can roll into a tight ball. Although the name "millipede" derives from Latin for "thousand feet", no species was known to have 1,000 or more until the discovery in 2020 of Eumillipes persephone, which can have over 1,300 legs. There are approximately 12,000 named species classified into 16 orders and around 140 families, making Diplopoda the largest class of myriapods, an arthropod group which also includes centipedes and other multi-legged creatures.
Arthropleuridea is an extinct subclass of myriapod arthropods that flourished during the Carboniferous period, having first arisen during the Silurian, and perishing in the Early Permian. Members are characterized by possessing diplosegement 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.
Glomerida is an order of pill-millipedes found primarily in the Northern Hemisphere. Also known as northern pill millipedes, they superficially resemble pill-bugs or woodlice, and can enroll into a protective ball. They have twelve body segments, 17 to 19 pairs of legs, and males have enlarged rear legs involved in mating. The order includes about 30 genera and at least 280 species, including Glomeris marginata, the common European pill-millipede. The order contains members in Europe, South-east Asia and the Americas from California to Guatemala. Although historically considered closely related with the similar sphaerotheriidans that also enroll, some DNA evidence suggest they may be more closely related to glomeridesmidans, a poorly known order that does not enroll.
Polydesmida is the largest order of millipedes, with more than 5,000 species, including all the millipedes reported to produce hydrogen cyanide (HCN). This order is also the most diverse of the millipede orders in terms of morphology. Millipedes in this order are found in all regions of the world other than Antarctica.
Polyxenidae is a family of millipedes in the order Polyxenida containing approximately 47 species in 19 genera. All adults in this family have 13 pairs of legs.
Polyxenus lagurus, known as the bristly millipede is a species of millipede found in many areas of Europe and North America. It is covered with detachable bristles that have the ability to entangle ants and spiders that attack the animal.
Polyxenus is a genus of millipede in the order Polyxenida, containing at least 30 valid species as of 2012.
Chordeumatida is a large order of millipedes containing more than 1,400 species. Also known as sausage millipedes, they are found nearly worldwide. Chordeumatida is the largest order in the superorder Nematophora, a group also known as spinning millipedes because their telsons feature spinnerets used to build nests of silk. These millipedes produce this silk to create chambers in which to molt or to lay their eggs.
Siphoniulus is a poorly known genus of millipede containing only two living species: S. alba from Indonesia, and S. neotropicus from Mexico and Guatemala. An additional two fossil species are known from Cretaceous amber. Siphoniulus species are the only members of the family Siphoniulidae and order Siphoniulida, making Siphoniulida the smallest millipede order. Few specimens are known, and their classification is contentious, although most recent studies place them as basal members of the Helminthomorpha.
Ammodesmidae is a family of small millipedes endemic to Africa, containing seven species in two genera. Ammodesmids range from 1.4 to 5.0 mm long with 18 or 19 body segments in both sexes, and are capable of rolling into a tight sphere.
Synxenidae is a family of bristle millipedes (Polyxenida). Three genera and around 10 species are known. Adult synxenids possess 15 or 17 pairs of legs, with the last two pair modified for small jumps. Adults in most species in this family have 17 pairs of legs, but in two species, they have only 15 pairs of legs.
Condexenus is a genus of bristle millipede containing the sole species Condexenus biramipalpus known from Namibia. Individuals are up to 3 mm long, and adults possess 11 body segments and 15 pairs of legs.
Phryssonotus is a genus of bristle millipedes containing around nine extant species. Species are characterized by the possession of dark, rear-projecting scale-shaped bristles (trichomes) on the tergites; all other bristles are long and hairlike. Adults in this genus have 17 pairs of legs, except for the species Phryssonotus brevicapensis, in which they have only 15 pairs of legs.
Lophoproctidae is a family of millipedes in the order Polyxenida containing approximately 65 species in 6 genera.
Lophoturus is a genus of millipedes belonging to the family Lophoproctidae. Species described after year 2000 include two from Queensland, Australia, three from Christmas Island, Australia, and three from the Caribbean and northern South America.
Metopidiotrichidae is a family of millipedes in the order Chordeumatida. These millipedes range from 4 mm to 17 mm in length. Adult millipedes in this family have 32 segments, not the 30 segments usually found in this order. Adult males in this family often feature a reduced or vestigial leg pair 10 as part of the gonopod complex, in addition to the two leg pairs typically modified into gonopods in this order. There are about 9 genera and at least 70 described species in Metopidiotrichidae.
Opisthocheiridae is a family of millipedes belonging to the order Chordeumatida. These millipedes range from 5 mm to 16 mm in length and are found from Belgium to Morocco. Adult millipedes in this family have 26 or 30 segments. This family includes the cave-dwelling species Opisthocheiron canayerensis, notable as one of only a few chordeumatidan species with only 26 segments in adults, four fewer segments than typically found in adults in this order. The adult female of this species has only 42 pairs of legs, and the adult male has only 40 pairs of walking legs, excluding two pairs of gonopods.
Termitodesmidae is a family of millipedes belonging to the order Glomeridesmida. These millipedes are found in India, Sri Lanka, and Vietnam. The five known species in this family are notable in that they are found only in termite mounds. Termitodesmidae is the only myriapod family known to depend on insects in such a commensal relationship.
Monographis is a terrestrial genus of millipede in the family Polyxenidae comprising 10 species.
Chamaesoma is a monotypic genus of millipede in the family Chamaesomatidae, and Chamaesoma broelemanni is the only species in this genus. This millipede is notable as one of only a few species in the order Chordeumatida with only 26 segments in adults, four fewer segments than typically found in adults this order. This species lives on the surface of the soil and is distributed across most of France and Luxembourg. The genus Chamaesoma and its only species were first described by the German zoologist Karl W. Verhoeff in 1913 based on type material found by the French zoologists Henri Brölemann and Henri Ribaut under wet leaf litter near St. Béat in the Pyrenees in France.