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Protura

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Protura

The Protura, or proturans, and sometimes nicknamed coneheads,[2][3] 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 (biology) in their own right.[4] Some evidence indicates the Protura are basal to all other hexapods,[5] although not all researchers consider them Hexapoda, rendering the monophyly of Hexapoda unsettled.[6] Uniquely among hexapods, proturans show 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]

Morphology

Acerentomon species under stereo microscope

Proturans have no eyes, wings, or antennae, and lack 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]

Ecology

Proturans live chiefly in soil, mosses, and leaf litter[7] of moist temperate forests[10] that are not too acidic;[11] they have also been found beneath rocks or under the bark of trees,[9] as well as in animal burrows.[8] They are generally restricted to the uppermost 0.1 m (3.9 in),[11] but have been found as deep as 0.25 m (9.8 in).[12] Although they are sometimes considered uncommon,[10] they are probably often overlooked because of their small size:[9] densities of over 90,000 individuals per square metre have been measured.[13]

The diet of proturans is not yet sufficiently observed, but they feed on mycorrhizal fungi, dead Acari, and mushroom powder in culture,[8] and are thought to feed on decaying vegetable matter and fungi in the wild.[10][9] The styliform mouthparts suggest the Protura are fluid feeders, with evidence that some species suck out the contents of fungal hyphae.[11]

Proturans which live near the soil surface generally have one generation per year and have longer legs, while those that live deeper have shorter legs and reproduce less seasonally, although some migratory species move to deeper layers for the winter and shallower layers for the summer.[11]

Development

The larva has 9 abdominal segments, but the number increases through moulting until the full adult number of 12 is reached. Further moults may occur, but do not add any more body segments,[10] and it is not known whether the adults continue to moult through their lives.[8] Eggs have only been observed in a few species.[8] Five developmental stages follow: the prelarva hatches from the egg and has only weakly developed mouthparts and 9 abdominal segments; larva I follows and has fully developed mouthparts; larva II has ten abdominal segments; maturus junior has 12 abdominal segments and is followed by the adult.[8] The family Acerentomidae differs in having an extra preimago stage, with partially developed genitalia, between the maturus junior and the adult.[8]

History

Proturans were first discovered in the early 20th century, when Filippo Silvestri and Antonio Berlese discovered the animals independently.[11] The first species to be described was Acerentomon doderoi, published in 1907 by Silvestri,[8] based on material from near Syracuse, New York.[10]

References

  1. ^ a b Andrzej Szeptycki (2007). "Catalogue of the World Protura". 
  2. ^ "Proturans / Coneheads". North Carolina State University College of Agriculture and Life Sciences. Retrieved July 30, 2008. 
  3. ^ "Order Protura - Coneheads". http://bugguide.net bugguide.net, hosted by Iowa State University Department of Entomology. Retrieved July 30, 2008. 
  4. ^ Charles S. Henry (2005). "Insect phylogeny".  
  5. ^ Ryuichiro Machida (2006). "Evidence from embryology for reconstructing the relationships of hexapod basal clades". 
  6. ^ Charles E Cook, Qiaoyun Yue & Michael Akam (2005). "Mitochondrial genomes suggest that hexapods and crustaceans are mutually paraphyletic".  
  7. ^ a b c P. J. Gullan & P. S. Cranston (1994). The insects: an outline of entomology.  
  8. ^ a b c d e f g h i j k Christopher Tipping (2004). "Proturans".  
  9. ^ a b c d "Protura".  
  10. ^ a b c d e f g John R. Meyer (March 5, 2005). "Protura".  
  11. ^ a b c d e "Gordon's Protura Page". November 11, 2005. 
  12. ^ "Protura". Tree of Life Web Project. January 1, 2002. 
  13. ^ J. Krauß & W. Funke (1999). "Extraordinary high density of Protura in a windfall area of young spruce plants".  

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