| Protura | |
|---|---|
_micrograph.jpg) | |
| Acerentomon sp. under stereomicroscope | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Clade: | Pancrustacea |
| Subphylum: | Hexapoda |
| Order: | Protura Silvestri, 1907 |
| Families[1] | |
Acerentomata Eosentomata Sinentomata | |
TheProtura, orproturans, and sometimes nicknamedconeheads,[2][3] are very small (0.6–1.5mm long), soil-dwelling animals, so inconspicuous they were not noticed until the 20th century. The Protura constitute anorder ofhexapods that were previously regarded as insects, and sometimes treated as aclass in their own right.[1][4][5]
Some evidence indicates the Protura arebasal to all other hexapods,[6] although not all researchers consider themHexapoda, rendering themonophyly of Hexapoda unsettled.[7] Uniquely among hexapods, proturans showanamorphic development, wherebybody segments are added duringmoults.[8][page needed]
There are close to 800 species, described in sevenfamilies. Nearly 300 species are contained in a singlegenus,Eosentomon.[1][9]
Proturans have no eyes, wings, or antennae, and, lackingpigmentation, are usually whitish or pale brown. The sensory function of the absent antennae is fulfilled by the first pair of the three pairs of five-segmentedlegs, which are held up, pointing forward, and bearing numerous tarsalsensilla and sensory hairs. They ambulate using the four rear legs.[10] The head is conical, and bears twopseudoculi with unknown function. The body is elongated and cylindrical,[11] with a post-analtelson at the end. Themouthparts areentognathous (enclosed within the head capsule) and consist of narrowmandibles and maxillae.[8] There are nocerci at the end of the abdomen, which gives the group their name, from theGreekproto- (meaning "first", in this case implying primitive), andura, meaning "tail".[12] The first three abdominal segments bear short limb-likeappendages,[10] called "styli".[12] The first pair of styli is two-segmented, while the second and third pair are either two-segmented or unsegmented.[5] Thegenitalia are internal and the genital opening lies between the eleventh segment and the telson of the adult.[10] During mating, the genitalia of both sexes are everted from an abdominal chamber.[13] Only the two familiesEosentomidae andSinentomidae possess a simpletracheal system with a pair ofspiracles on both themesothorax and themetathorax, while proturans in the remaining families lack these structures and performgas exchange bydiffusion.[10][14][15]
Proturans live chiefly insoil,mosses, andleaf litter[8] of moist temperate forests[12] that are not too acidic.[16] They have also been found beneath rocks or under thebark of trees,[11] as well as in animal burrows.[10] They are generally restricted to the uppermost 0.1 m (3.9 in),[16] but have been found as deep as 0.25 m (9.8 in).[17] Although they are sometimes regarded as uncommon,[12] proturans are most likely overlooked because of their small size,[11] as densities of over 90,000 individuals per square metre have been recorded.[18]
The diet of proturans has not yet been sufficiently observed to be characterised. In laboratory culture, they may be fedmycorrhizal fungi, deadmites and pulverized, dried mushrooms;[10] they are believed to feed on decaying vegetable matter and fungi in the wild.[11][12] The styliform mouthparts suggest the Protura may be fluid feeders, based on evidence that some species suck out the liquid contents offungal hyphae.[16]
Proturan species which spend their lives near the soil surface generally produce one new generation of offspring each year; they also possess longer legs. Species living at deeper soil levels have shorter legs and tend to reproduce less seasonally. Somemigratory proturan species move to deeper soil layers for the winter and ascend to shallower soil layers for the summer.[16]
Proturans play a role insoil formation and composition by speeding decomposition, helping in the breakdown of leaf litter and recycling nutrients into the soil.[19]
Thenymph has 8 abdominal segments plus the telson; the number of abdominal segments increases throughmoulting until the full adult complement of 12 abdominal segments is achieved. Further moults may occur, but do not add additional body segments;[12] it is still not known whether the adults continue to moult throughout their lives.[10]Eggs have been observed in only a few species.[10]
In most proturan families, fivedevelopmental stages follow the egg stage: the prenymph hatches from the egg and has only weakly developed mouthparts and 8 abdominal segments; nymph I follows and has fully developed mouthparts; nymph II has 9 abdominal segments; "maturus junior" has 11 abdominal segments, and moults into the sexually mature adult.[10] Male individuals of the familyAcerentomidae differ from this five-stage scheme, having an additional developmental stage, thepreimago, which has partially developed genitalia and appears between the "maturus junior" and the adult stage.[10]
Proturans were first discovered in the early 20th century, whenFilippo Silvestri andAntonio Berlese independently described the animals.[16] The first species to be described wasAcerentomon doderoi, published in 1907 by Silvestri,[10] based on material found nearSyracuse, New York.[12]
Data related toProtura at Wikispecies
Media related toProtura at Wikimedia Commons
