Protura

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Acerentomata

Eosentomata

Sinentomata

The Protura, or proturans, and sometimes nicknamed coneheads[2][3] are an order of hexapods previously regarded as insects, and sometimes treated as a class in their own right.[4] There is some evidence that the Protura are basal to all other hexapods[5], although hexapods are increasingly thought to be polyphyletic.[6] They are very small (<2 mm long) soil-dwelling animals and are unique among hexapods for showing anamorphic development, whereby body segments are added during moults.[7] Szeptycki (2007) lists a total of 731 described species worldwide, in seven families, nearly 300 of which are contained in a single genus, Eosentomon.[1]

Contents

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.[8] The head is conical, and bears two pseudoculi which are used for sensing light and may be the remnants of eyes.[9] The body is elongate and cylindrical[10], with a post-anal telson at the end. The mouthparts are entognathous (enclosed within the head capsule) and consist of thin mandibles and maxillae.[7] 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".[9] The first three abdominal segments bear limb-like appendages[8] called "styli".[9] The genitalia are internal and the genital opening lies between the eleventh segment and the telson of the adult.[8] Members of Eosentomoidea possess spiracles and a simple tracheal system while those in the Acerentomoidea lack these structures and perform gas exchange by diffusion.[8]

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