''Xystrosoma'' is a

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

millipede Millipedes (originating from the Latin , "thousand", and , "foot") 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 derive ...

in the

family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...

Chamaesomatidae. The French

zoologist Zoology ( , ) is the scientific study of animals. Its studies include the structure, embryology, classification, habits, and distribution of all animals, both living and extinct, and how they interact with their ecosystems. Zoology is one ...

Henri Ribaut first described this genus in 1927 to contain five species newly discovered in

France France, officially the French Republic, is a country located primarily in Western Europe. Overseas France, Its overseas regions and territories include French Guiana in South America, Saint Pierre and Miquelon in the Atlantic Ocean#North Atlan ...

, including the

type species In International_Code_of_Zoological_Nomenclature, zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the spe ...

'' X. tectosagum''. This genus now includes ten

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

Distribution

Most species in this genus are found in the French

Pyrenees The Pyrenees are a mountain range straddling the border of France and Spain. They extend nearly from their union with the Cantabrian Mountains to Cap de Creus on the Mediterranean coast, reaching a maximum elevation of at the peak of Aneto. ...

. Three species in this genus are found in northern

Spain Spain, or the Kingdom of Spain, is a country in Southern Europe, Southern and Western Europe with territories in North Africa. Featuring the Punta de Tarifa, southernmost point of continental Europe, it is the largest country in Southern Eur ...

: two species found in the

autonomous community The autonomous communities () are the first-level administrative divisions of Spain, created in accordance with the Spanish Constitution of 1978, with the aim of guaranteeing limited autonomy to the nationalities and regions that make up Sp ...

Catalonia Catalonia is an autonomous community of Spain, designated as a ''nationalities and regions of Spain, nationality'' by its Statute of Autonomy of Catalonia of 2006, Statute of Autonomy. Most of its territory (except the Val d'Aran) is situate ...

('' X. coiffati'' and '' X. santllorence'') and one species found in the

Basque Basque may refer to: * Basques, an ethnic group of Spain and France * Basque language, their language Places * Basque Country (greater region), the homeland of the Basque people with parts in both Spain and France * Basque Country (autonomous co ...

autonomous community ('' X. vasconicum''). One species in this genus is found in northern

Portugal Portugal, officially the Portuguese Republic, is a country on the Iberian Peninsula in Southwestern Europe. Featuring Cabo da Roca, the westernmost point in continental Europe, Portugal borders Spain to its north and east, with which it share ...

('' X. lusitanicum'').

Description

Adult millipedes in this genus can have 26, 28, or 30 segments (counting the collum as the first segment and the telson as the last). This genus includes three species notable for featuring an unusual number of segments: The Spanish species ''X. santllorence'' is one of only a few species in the order Chordeumatida with only 26 segments in adults, four fewer than the 30 segments typically found in adults this order. The French species '' X. beatense'' and the Spanish species ''X. vasconicum'' are notable for featuring

sexual dimorphism Sexual dimorphism is the condition where sexes of the same species exhibit different Morphology (biology), morphological characteristics, including characteristics not directly involved in reproduction. The condition occurs in most dioecy, di ...

in segment number, with the usual 30 segments in adult females but only 28 segments in adult males. Adults in the other seven species in this genus feature the usual 30 segments in each sex. In males in the order Chordeumatida, the eighth leg pair become anterior gonopods, and the ninth leg pair become posterior gonopods. In this genus, the anterior gonopods are deeply divided transversely into anterior and posterior parts: A pair of elongated angiocoxites form the anterior part, and the colpocoxite forms the center of the posterior part. In most species in this genus, a pair of telopodites frame the colpocoxite, with these telopodites taking the form of robust stems. The genus ''Xystrosoma'' shares an extensive set of traits with '' Chamaesoma'', a closely related genus in the family Chamaesomatidae. These shared traits place these two genera in the same subfamily (Chamaesomatinae). For example, the head and dorsal surface of the trunk in both genera are covered with small

lamellae Lamella (: lamellae) means a small plate or flake in Latin, and in English may refer to: Biology * Lamella (mycology), a papery rib beneath a mushroom cap * Lamella (botany) * Lamella (surface anatomy), a plate-like structure in an animal * Lame ...

arranged as vertical plates oriented longitudinally. Furthermore, species in both genera are small and feature paranota located in a more ventral position than observed in other genera in the same family. The males in these two genera also develop similar modifications to their legs. For example, the anterior gonopods in both genera feature angiocoxites in front that are separate rather than fused, a colpocoxite toward the rear that is undivided rather than divided in the middle, and no flagella. Furthermore, in both genera, the posterior gonopods feature a coxa with a medial process and a telepodite with two segments, including a minute distal segment. Moreover, the eleventh leg pair in males of both genera feature coxal hooks. The millipedes in the genus ''Xystrosoma'' can be distinguished from their close relatives in the genus ''Chamaesoma'' based on other traits. For example, the dorsal lamellae are rounded and shaped like semicircles in ''Xystrosoma'', but these lamellae are pointed and shaped like triangles in ''Chamaesoma''. Furthermore, the fifth segment of each antenna is as wide as long in ''Chamaesoma'', but this segment is longer than wide in ''Xystrosoma''.

Species

This genus includes the following species: * '' Xystrosoma beatense'' Ribaut, 1927 * '' Xystrosoma cassagnaui'' Mauriès, 1965 * '' Xystrosoma catalonicum'' Ribaut, 1927 * '' Xystrosoma coiffaiti'' Mauriès, 1964 * '' Xystrosoma lusitanicum'' Mauriès, 2015 * '' Xystrosoma murinum'' Ribaut, 1927 * '' Xystrosoma pyrenaicum'' Ribaut, 1927 * ''

Xystrosoma santllorence ''Xystrosoma santllorence'' is a species of millipede in the family Chamaesomatidae. This millipede is notable as one of only a few species in the order Chordeumatida with only 26 segments in adults (counting the collum as the first segment and ...

'' Serra & Mauriès, 2018 * '' Xystrosoma tectosagum'' Ribaut, 1927 * '' Xystrosoma vasconicum'' Mauriès & Barraqueta, 1985

References

{{taxonbar, from1=Q4296606 Chordeumatida Millipedes of Europe Millipede genera Taxa named by Henri Ribaut