Temporal range: Early Devonian–Recent
|Class:||Entognatha (but see text)|
Springtails (Collembola) form the largest of the three lineages of modern hexapods that are no longer considered insects (the other two are the Protura and Diplura). Although the three orders are sometimes grouped together in a class called Entognatha because they have internal mouthparts, they do not appear to be any more closely related to one another than they all are to insects, which have external mouthparts.
Some DNA sequence studies suggest that Collembola represent a separate evolutionary line from the other Hexapoda, but others disagree; this seems to be caused by widely divergent patterns of molecular evolution among the arthropods. The adjustments of traditional taxonomic rank for springtails reflects the occasional incompatibility of traditional groupings with modern cladistics: when they were included with the insects, they were ranked as an order; as part of the Entognatha, they are ranked as a subclass. If they are considered a basal lineage of Hexapoda, they are elevated to full class status.
Members of Collembola are normally less than 6 mm (0.24 in) long, have six or fewer abdominal segments and possess a tubular appendage (the collophore or ventral tube) with eversible vesicles, projecting ventrally from the first abdominal segment. The Poduromorpha and Entomobryomorpha have an elongated body, while the Symphypleona have a globular body. Collembola lack a tracheal respiration system, which forces them to respire through a porous cuticle, to the notable exception of Sminthuridae which exhibit a rudimentary, although fully functional, tracheal system.
Most species have an abdominal, tail-like appendage, the furcula, that is folded beneath the body to be used for jumping when the animal is threatened. It is held under tension by a small structure called the retinaculum and when released, snaps against the substrate, flinging the springtail into the air. All of this takes place in as little as 18 milliseconds.
Systematics and evolution 
Traditionally, the springtails were divided into the orders Arthropleona, Symphypleona and occasionally also Neelipleona. The Arthropleona were divided into two superfamilies, the Entomobryoidea and the Poduroidea. But actually, these two and the Symphypleona form three lineages, each of which is equally distant from the other two. Thus, the Arthropleona are abolished in modern classifications, and their superfamilies are raised in rank accordingly, being now the Entomobryomorpha and the Poduromorpha. Technically, the Arthropleona are thus a partial junior synonym of the Collembola. The term "Neopleona" is essentially synonymous with Symphypleona + Neelipleona.
The Neelipleona were originally seen as a particular advanced lineage of Symphypleona, based on the shared global body shape. But the global body of Neelipleona is realised in a completely different way than in Symphypleona. Subsequently, the Neelipleona were considered as being derived from the Entomobryomorpha. But analysis of 18S and 28S rRNA sequence data suggests that they form the most ancient lineage of springtails, which would explain their peculiar apomorphies.
Springtails are attested to since the Early Devonian. The fossil from , Rhyniella praecursor, is the oldest terrestrial arthropod, and was found in the famous Rhynie chert of Scotland. Given its morphology resembles extant species quite well, the radiation of the Hexapoda can be situated in the Silurian, or more.
Fossil collembola are rare. Most fossils are found in amber. Even these are rare and many amber deposits carry few or no collembola. The best deposits are from the early Eocene of Canada and Europe, Miocene of Central America, and the mid-Cretaceous of Burma and Canada. They display some unusual characteristics: first, all but one of the fossils from the Cretaceous belong to extinct genera, whereas none of the specimens from the Eocene or the Miocene are of extinct genera; second, the species from Burma are more similar to the modern fauna of Canada than are the Canadian Cretaceous specimens.
Springtails are cryptozoa frequently found in leaf litter and other decaying material, where they are primarily detritivores and microbivores, and one of the main biological agents responsible for the control and the dissemination of soil microorganisms.
In sheer numbers, they are reputed to be one of the most abundant of all macroscopic animals, with estimates of 100,000 individuals per cubic meter of topsoil, essentially everywhere on Earth where soil and related habitats (moss cushions, fallen wood, grass tufts, ant and termite nests) occur; only nematodes, crustaceans, and mites are likely to have global populations of similar magnitude, and each of those groups except mites is more inclusive: though taxonomic rank cannot be used for absolute comparisons, it is notable that nematodes are a phylum and crustaceans a subphylum. Most springtails are small and difficult to see by casual observation, but one springtail, the so-called snow flea (Hypogastrura nivicola), is readily observed on warm winter days when it is active and its dark color contrasts sharply with a background of snow.
In addition, a few species routinely climb trees and form a dominant component of canopy faunas, where they may be collected by beating or insecticide fogging. These tend to be the larger (>2 mm) species, mainly in the genera Entomobrya and Orchesella, though the densities on a per square meter basis are typically 1–2 orders of magnitude lower than soil populations of the same species. In temperate regions, a few species (e.g. Anurophorus spp., Entomobrya albocincta, Xenylla xavieri, Hypogastrura arborea) are almost exclusively arboreal. In tropical regions a single square meter of canopy habitat can support many species of Collembola.
The main ecological factor driving the local distribution of species is the vertical stratification of the environment: in woodland a continuous change in species assemblages can be observed from tree canopies to ground vegetation then to plant litter down to deeper soil horizons. This is a complex factor embracing both nutritional and physiological requirements, together with probable species interactions. Some species have been shown to exhibit negative or positive gravitropism, which adds a behavioural dimension to this still poorly understood vertical segregation.
As a group, springtails are highly sensitive to desiccation, because of their tegumentary respiration. The gregarious behaviour of Collembola, mostly driven by the attractive power of pheromones excreted by adults, gives more chance to every juvenile or adult individual to find suitable, better protected places, where desiccation could be avoided and reproduction rate could be kept at an optimum. Sensitivity to drought varies from species to species and increases during ecdysis. Given that springtails are moulting repeatedly during their entire life (an ancestral character in Hexapoda) they spend much time in concealed micro-sites where they can find protection against desiccation and predation during ecdysis. The high humidity environment of many caves also favours springtails and there are numerous cave adapted species.
The horizontal distribution of springtail species is affected by environmental factors which act at the landscape scale, such as soil acidity, moisture and light. Requirements for pH can be reconstructed experimentally. Altitudinal changes in species distribution can be at least partly explained by increased acidity at higher elevation. Moisture requirements explain why some species cannot live aboveground (vertical stratification), but also why some epigeal springtails are always found in the vicinity of ponds and lakes, such as the hygrophilous Isotomurus palustris. Adaptive features, such as the presence of a fan-like wettable mucro, allow some species to move at the surface of water (Sminthurides aquaticus, Sminthurides malmgreni). Podura aquatica, a unique representative of the family Poduridae (and one of the first springtails to have been described by Linnaeus), spends its entire life at the surface of water, its wettable eggs dropping in water until the non-wettable first instar hatches then surfaces.
In a variegated landscape, made of a patchwork of closed (woodland) and open (meadows, cereal crops) environments, most soil-dwelling species are not specialized and can be found everywhere, but most epigeal and litter-dwelling species are attracted to a particular environment, either forested or not. As a consequence of dispersal limitation, landuse change, when too rapid, may cause the local disappearance of slow-moving, specialist species.
Relationship with humans 
Springtails are well known as pests of some agricultural crops. Sminthurus viridis, the 'lucerne flea', has been shown to cause severe damage to agricultural crops, and is considered as a pest in Australia. Also Onychiuridae are known to feed on tubers and to damage them to some extent. However, by their capacity to carry spores of mycorrhizal fungi and mycorrhiza-helper bacteria on their tegument, soil springtails play a positive role in the establishment of plant-fungal symbioses and thus are beneficial to agriculture. They also contribute to controlling plant fungal diseases through their active consumption of mycelia and spores of damping-off and pathogenic fungi. It has been suggested that they could be reared to be used for the control of pathogenic fungi in greenhouses and other indoor cultures.
Various sources and publications have suggested that some springtails may parasitize humans, but this is entirely inconsistent with their biology, and no such phenomenon has ever been scientifically confirmed, though it has been documented that the scales or hairs from collembolans can cause irritation when rubbed onto the skin. They may sometimes be abundant indoors in damp places such as bathrooms and basements, and incidentally found on one's person.
More often, claims of persistent human skin infection by springtails may indicate a neurological problem, such as Morgellons Syndrome, or delusory parasitosis, a psychological rather than entomological problem. Researchers themselves may be subject to psychological phenomena. For example, a publication in 2004 claiming that springtails had been found in skin samples was later determined to be a case of pareidolia; that is, no springtail specimens were actually recovered, but the researchers had digitally enhanced photos of sample debris to create images resembling small arthropod heads, which then were claimed to be springtail remnants. However, Hopkin reports one instance of an entomologist aspirating an Isotoma species and in the process accidentally inhaling some of their eggs, which hatched in his nasal cavity and made him quite ill until they were flushed out.
Ecotoxicology laboratory animals 
Springtails are currently used in laboratory tests for the early detection of soil pollution. Acute and chronic toxicity tests have been performed by researchers, mostly using the parthenogenetic isotomid Folsomia candida. These tests have been standardized. More recently, avoidance tests have been also performed. They have been standardized, too. Avoidance tests are complementary to toxicity tests, but they also offer several advantages: they are more rapid (thus cheaper), more sensitive and they are environmentally more reliable, because in the real world Collembola may move far from pollution sources. It may be hypothesized that the soil could become locally depauperated in animals (and thus improper to normal use) while below thresholds of toxicity. Contrary to earthworms, and like many insects and molluscs, Collembola are very sensitive to herbicides and thus are threatened in no-tillage agriculture, which makes a more intense use of herbicides than conventional agriculture. The springtail Folsomia candida is also becoming a genomic model organism for soil toxicology. With microarray technology the expression of thousands of genes can be measured in parallel. The gene expression profiles of F. candida exposed to environmental toxicants allow fast and sensitive detection of pollution, and additionally clarifies molecular mechanisms causing toxicology.
Sexual reproduction occurs through the clustered or scattered deposition of spermatophores by male adults. Stimulation of spermatophore deposition by female pheromones has been demonstrated in Sinella curviseta. Mating behaviour can be observed in Symphypleona. Among Symphypleona, males of some Sminthuridae use a clasping organ located on their antenna. Many collembolan species, mostly those living in deeper soil horizons, are parthenogenetic, which favours reproduction to the detriment of genetic diversity and thereby to population tolerance of environmental hazards. Parthenogenesis (also called thelytoky) is under the control of symbiotic bacteria of the genus Wolbachia, which live, reproduce and are carried in female reproductive organs and eggs of Collembola. Feminizing Wolbachia species are widespread in arthropods and nematodes, where they co-evolved with most of their lineages.
See also 
- Texella reddelli, a predator of Collembola
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