Polyxenida is an

order Order, ORDER or Orders may refer to: * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of ...

millipedes 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 resu ...

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 An exoskeleton (from Greek ''éxō'' "outer" and ''skeletós'' "skeleton") is an external skeleton that supports and protects an animal's body, in contrast to an internal skeleton ( endoskeleton) in for example, a human. In usage, some of the ...

, unique tufts of bristles or

setae In biology, setae (singular seta ; from the Latin word for "bristle") are any of a number of different bristle- or hair-like structures on living organisms. Animal setae Protostomes Annelid setae are stiff bristles present on the body. ...

, 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). 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.

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 (

gonopod Gonopods are specialized appendages of various arthropods used in reproduction or egg-laying. In males, they facilitate the transfer of sperm from male to female during mating, and thus are a type of intromittent organ. In crustaceans and millipe ...

s) found in most other millipede groups. As a result, sperm transfer is indirect: males deposit a

spermatophore A spermatophore or sperm ampulla is a capsule or mass containing spermatozoa created by males of various animal species, especially salamanders and arthropods, and transferred in entirety to the female's ovipore during reproduction. Spermatophore ...

that is subsequently picked up by females. Many species reproduce asexually by way of

parthenogenesis Parthenogenesis (; from the Greek grc, παρθένος, translit=parthénos, lit=virgin, label=none + grc, γένεσις, translit=génesis, lit=creation, label=none) is a natural form of asexual reproduction in which growth and developmen ...

, wherein females lay eggs without mating and males are absent or rare.

Classification

Polyxenida is the only

living Living or The Living may refer to: Common meanings *Life, a condition that distinguishes organisms from inorganic objects and dead organisms ** Living species, one that is not extinct *Personal life, the course of an individual human's life * H ...

order of the subclass Penicillata, the basal subclass of millipedes. Penicillata is the

sister group In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree. Definition The expression is most easily illustrated by a cladogram: Taxon A and ...

of all other living millipedes: the infraclasses

Pentazonia Pentazonia is a taxonomic infraclass of millipedes containing the pill-millipedes (Oniscomorpha) which can roll into a ball and the order Glomeridesmida which cannot. Defining traits (apomorphies) include divided sternites, a labrum with single ...

and

Helminthomorpha Chilognatha is a subclass of the class Diplopoda, which includes the vast majority of extant millipedes, about 12,000 species. Taxonomy The classification of Chilognatha presented below is based on Shear, 2011, and Shear & Edgecombe, 2010 (exti ...

. 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

Fossil history

The earliest representatives of Polyxenida are found in

Lebanese amber Lebanese amber is fossilized resin found in Lebanon and southwest Syria. It dates back approximately 130-125 million years to the Barremian of the Early Cretaceous. It formed on what was then the northern coast of Gondwana, believed to be a tropica ...

from the

Early Cretaceous The Early Cretaceous (geochronology, geochronological name) or the Lower Cretaceous (chronostratigraphy, chronostratigraphic name), is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 145& ...

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.

References

External links

* * *
Photos of Polyxenida
{{Taxonbar, from=Q6449719 Millipede orders Taxa named by Karl Wilhelm Verhoeff