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). The three orders are sometimes grouped together in a class called Entognatha because they have internal mouthparts, but they do not appear to be more closely related to one another than to insects, which have external mouthparts.
DNA sequence studies suggest that Collembola are a separate evolutionary line from the other Hexapoda. Consequently their taxonomic rank has changed according to need: 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.
The Neelipleona, which are monotypic, were originally seen as 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. Currently, the Neelipleona are considered as being derived from the Entomobryomorpha.
Springtails are attested to since the Early Devonian. The fossil from 400 million years ago, Rhyniella praecursor, is the 'oldest' terrestrial arthropod found in the famous Rhynie chert of Scotland. Given its morphology resembles current species quite well, the radiation of the Hexapoda can be situated in the Silurian, 420 million years ago or more.
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 (>2mm) species, mainly in the genera Entomobrya, Orchesella and Lepidocyrtus, though the densities on a per square metre basis are typically 1-2 orders of magnitude lower than soil populations of the same species. A very few species (e.g. Anurophorus spp., Entomobrya albocincta) are almost exclusively arboreal.
The main ecological factor driving locally the distribution of species is the vertical stratification of the environment: in woodlands 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 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: they lack trachea (exceptions exist, such as in Sminthuridae), which forces them to respire through a porous cuticle. 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 dryness 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.
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 of not. As a consequence of dispersal limitation, landuse change, when too rapid, may cause the local disappearance of slow-moving, specialized species.
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 can sometimes be abundant indoors in damp places such as bathrooms and basements, and under such circumstances may be found on one's person, but this is only accidental. Claims of persistent human skin infection by springtails may indicate a neurological problem, or else 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 they then 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.
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