Proboscis beetle - Proboscis beetle

A general term for insects in the order Protura of the class Insecta. They are also called Protoura. They are one of the insects that make up the so-called soil fauna, and live in fallen leaves and in the soil. They are tiny insects with a body length of about 1 mm, and are pale yellow or white in color, making them difficult to find with the naked eye. They are recognized as insects because they have three pairs of legs, but they have some unusual characteristics. Their body is slightly flattened and cylindrical, with a bean-shaped head that protrudes from the front end like a beak. They have no compound eyes, but in their place are pseudo-eyes of unknown function. They lack antennae, but instead their forelimbs, which are equipped with special sensory organs, are waved above and to the sides of their heads, acting as antennae. They were named protura because their forelimbs look like sickles. Their middle and hind limbs are walking limbs. Unlike normal insects, they are wingless and have 12 abdominal segments. Immediately after hatching, the abdomen has only 9 segments, but after several molts, the abdominal segments increase in number to 12 segments, as they undergo metamorphosis. Sensory hairs are regularly arranged on the surface of the body. They are an interesting group of insects in terms of phylogenetic taxonomy, but have no particular connection to human life. Numerous species and genera belonging to 4 families are known from all over the world, and more than 30 species in 9 genera are known from Japan, with the reed beetle Nipponentomon nippon being a representative species. This species is distributed in Honshu and Shikoku.

[Yamazaki Egane]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

昆虫綱カマアシムシ目Proturaの昆虫の総称。原尾類（目）ともいう。いわゆる土壌動物を構成する昆虫の一つで、落ち葉のなかや土壌中にすむ。体長1ミリメートル前後の微小虫で、しかも淡黄色ないし白色の半透明体をしているので、肉眼ではみつけにくい。3対の肢(あし)があることで昆虫と認められるが、いくつかの異例な特徴をもつ。体はやや扁平(へんぺい)な円筒形で、頭部は豆形、その前端は嘴(くちばし)状に突出する。複眼はなく、その位置に機能不明の擬眼がある。触角は欠いているが、そのかわり特殊な感覚器官を備えた前肢(ぜんし)を頭部上側方に振りかざしたようにして、触角の役割をさせている。この前肢があたかも鎌(かま)形にみえるので、カマアシムシと名づけられた。中肢と後肢は歩行肢である。通常の昆虫と異なり、無翅(むし)で、腹部は12節。孵化(ふか)直後は腹部は9節であるが、数回の脱皮で増節していき12節となる増節変態をする。体表には感覚毛が規則的に配列している。系統分類学上興味深い昆虫群であるが、人間生活とはとくにかかわり合いはない。世界各地から4科に属する多数の種属が知られ、日本からは9属30種以上が知られており、ヨシイムシNipponentomon nipponが代表種。この種は本州、四国に分布する。

［山崎柄根］

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