| Protura | |
|---|---|
| Acerentomon doderoi | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Subphylum: | Hexapoda |
| Class: | Entognatha |
| Order: | Protura Silvestri, 1907 |
| Families [1] | |
|
Acerentomata Eosentomata |
|
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 the monophyly of hexapoda is unsettled.[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]
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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 with unknown function. The body is elongate and cylindrical,[9] 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".[10] The first three abdominal segments bear limb-like appendages[8] called "styli".[10] 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]
Proturans live chiefly in soil, moss and leaf litter[7] of moist temperate forests[10] which are not too acidic,[11] but 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 10 centimetres (3.9 in),[11] but have been found as deep as 10 inches (250 mm).[12] Although they are sometimes considered uncommon,[10] they are probably often overlooked because of their small size,[9] and densities of over 90,000 individuals per square metre have been measured.[13]
The diet of proturans is uncertain, 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 that Protura are fluid feeders and there is 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 there are also migratory species which move to deeper layers for the winter and shallower layers for the summer.[11]
The larva has nine abdominal segments, but the number increases through moulting until the full adult number of twelve is reached. Further moults may occur, but do not involve any additional body segments,[10] and it is not known whether the adults continue to moult throughout 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 nine abdominal segments; larva I follows and has fully developed mouthparts; larva II has ten adbominal segments; maturus junior has twelve abdominal segments and is followed by the adult.[8] The family Acerentomidae differs in having an extra pre-imago stage, with partially developed genitalia, between the maturus junior and the adult.[8]
Proturans were first discovered in the early twentieth 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]