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Protura
The Protura, or proturans, and sometimes nicknamed coneheads,〔(【引用サイトリンク】url=http://www.cals.ncsu.edu/course/ent425/compendium/protura.html )〕〔(【引用サイトリンク】url=http://bugguide.net/node/view/99913/bgpage )〕 are very small (<2 mm long), soil-dwelling animals, so inconspicuous they were not noticed until the 20th century. The Protura constitute an order of hexapods that were previously regarded as insects, and sometimes treated as a class in their own right. Some evidence indicates the Protura are basal to all other hexapods,〔(【引用サイトリンク】title=Evidence from embryology for reconstructing the relationships of hexapod basal clades )〕 although not all researchers consider them Hexapoda, rendering the monophyly of Hexapoda unsettled. Uniquely among hexapods, proturans show anamorphic development, whereby body segments are added during moults. Szeptycki (2007) lists a total of 731 described species worldwide, in seven families, nearly 300 of which are contained in a single genus, ''Eosentomon''.〔(【引用サイトリンク】title=Catalogue of the World Protura )〕
==Morphology==
Proturans have no eyes, wings, or antennae, and lack pigmentation: they are usually white or pale brown. The sensory function of the antennae is fulfilled by the first of three pairs of five-segmented legs, which are held pointing forwards and have many tarsal sensilla and sensory hairs. The head is conical, and bears two pseudoculi with unknown function. The body is elongated and cylindrical,〔(【引用サイトリンク】title=Protura )〕 with a postanal telson at the end. The mouthparts are entognathous (enclosed within the head capsule) and consist of thin mandibles and maxillae.〔 There are no cerci at the end of the abdomen, which gives the group their name, from the Greek ''proto-'' (meaning "first", in this case implying primitive), and ''ura'', meaning "tail". The first three abdominal segments bear limb-like appendages〔 called "styli".〔 The genitalia are internal and the genital opening lies between the eleventh segment and the telson of the adult.〔 Members of Eosentomoidea possess spiracles and a simple tracheal system, while those in the Acerentomoidea lack these structures and perform gas exchange by diffusion.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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