Acerentomata
| Protura | |
|---|---|
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Subphylum: | Hexapoda |
| Class: | Entognatha |
| Order: |
Protura Silvestri, 1907 |
| Families | |
The Protura, or proturans, and sometimes nicknamed coneheads, 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, 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. There are close to 800 species, described in seven families. Nearly 300 species are contained in a single genus, Eosentomon.
Proturans have no eyes, wings, or antennae, and, lacking pigmentation, 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 up, pointing forward and have many tarsal sensilla and sensory hairs. They walk with only four legs. The head is conical, and bears two pseudoculi with unknown function. The body is elongated and cylindrical, 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. The genitalia are everted from a chamber in both sexes. Members of Eosentomoidea possess spiracles and a simple tracheal system, while those in the Acerentomoidea lack these structures and perform gas exchange by diffusion.
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