Springtail
Springtails form the largest of the three lineages of modern hexapods that are no longer considered insects, i.e. Protura, Diplura and Collembola. Although the three lineages 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 are to insects, which have external mouthparts. There are more than 9000 species.
Springtails are omnivorous, free-living organisms that prefer moist conditions. They do not directly engage in the decomposition of organic matter, but contribute to it indirectly through the fragmentation of organic matter and the control of soil microbial communities. The word Collembola is from the Ancient Greek κόλλα meaning 'glue' and ἔμβολος meaning 'peg'; this name was given due to the existence of the collophore, which was previously thought to stick to surfaces to stabilize the creature.
Early DNA sequence studies suggested 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 reflect 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 within Hexapoda, they are elevated to full class status.
Morphology
Members of the Collembola are normally less than long, have six or fewer abdominal segments, and possess a tubular appendage with reversible, sticky vesicles, projecting ventrally from the first abdominal segment. It is believed to be associated with fluid uptake and balance, excretion, and orientation of the organism itself. Most species have an abdominal, tail-like appendage known as a furcula. It is located on the ventral side of the fourth abdominal segment and is folded beneath the body, held under tension by a small structure called the retinaculum. When released, it snaps against the substrate, flinging the springtail into the air and allowing for rapid evasion and travel. All of this takes place in as little as 18 milliseconds.Springtails also possess the ability to reduce their body size by as much as 30% through subsequent ecdyses if temperatures rise high enough. The shrinkage is genetically controlled. Since warmer conditions increase metabolic rates and energy requirements in organisms, the reduction in body size is advantageous to their survival.
The Poduromorpha and Entomobryomorpha have an elongated body, while the Symphypleona and Neelipleona have a globular body. Collembola lack a tracheal respiration system, which forces them to respire through a porous cuticle, except for the two families Sminthuridae and Actaletidae, which exhibit a single pair of spiracles between the head and the thorax, leading to a rudimentary, although fully functional, tracheal system. The anatomical variance present between different species partially depends on their vertical distribution across the various strata of terrestrial ecosystems. Surface-dwellers are generally larger, have darker pigments, longer antennae and a functioning furcula. Sub-surface-dwellers are usually unpigmented, have elongated bodies, and reduced furcula. They can be categorized into four main forms according to vertical distribution: atmobiotic, epedaphic, hemiedaphic, and euedaphic. Atmobiotic species inhabit macrophytes and litter surface. They are generally 8-10 millimeters in length, pigmented, have long limbs, and a full set of ocelli. Epedaphic species inhabit upper litter layers and fallen logs. They are slightly smaller and have less pronounced pigments, as well as less developed limbs and ocelli than the atmobiotic species. Hemiedaphic species inhabit the lower litter layers of decomposing organic material. They are 1-2 millimeters in length, have dispersed pigmentation, shortened limbs, and a reduced number of ocelli. Euedaphic species inhabit upper mineral layers known as the humus horizon. They are smaller than hemiedaphic species; have soft, elongated bodies; lack pigmentation and ocelli; and have reduced or absent furca.
Poduromorphs are characterized by their elongated bodies and conspicuous segmentation: they have three thoracic segments, six abdominal segments, including a well-developed prothorax with tergal chaetae, while the first thoracic segment in Entomobryomorpha is clearly reduced and bears no chaetae.
The digestive tract of springtails consists of three main components: foregut, midgut, and hindgut. The midgut is surrounded by a network of muscles and lined with a monolayer of columnar or cuboidal cells. Its function is to mix and transport food from the lumen into the hindgut through contraction. Many species of syntrophic bacteria, archaea, and fungi are present in the lumen. These different digestive regions have varying pH to support specific enzymatic activities and microbial populations. The anterior portion of the midgut and hindgut is slightly acidic while the posterior midgut portion is slightly alkaline. Between the midgut and hindgut is an alimentary canal called the pyloric region, which is a muscular sphincter. Malpighian tubules are absent.
Genomics
Given their small size, springtails have been neglected in terms of genome analysis. They are one of the few arthropod groups that do not have high-quality reference genomes. Even though some earlier genome sequences were produced, they do not meet modern standards. One of the first genomes was presented in 2025, that of Orchesella flavescens, which is 270 MB and was assembled into 6 chromosome-level scaffolds.Systematics and evolution
Traditionally, springtails were divided into the orders Arthropleona, Symphypleona, and occasionally also Neelipleona. The Arthropleona were divided into two superfamilies, the Entomobryoidea and the Poduroidea. However, recent phylogenetic studies show Arthropleona is paraphyletic. Thus, the Arthropleona are abolished in modern classifications, and their superfamilies are raised in rank accordingly, being now orders 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 was originally seen as a particularly advanced lineage of Symphypleona, based on the shared global body shape, but the global body of the Neelipleona is realized in a completely different way than in Symphypleona. Subsequently, the Neelipleona were considered as being derived from the Entomobryomorpha. Analysis of 18S and 28S rRNA sequence data, though, suggests that they form the most ancient lineage of springtails, which would explain their peculiar apomorphies. This phylogenetic relationship was also confirmed using a phylogeny based on mtDNA and whole-genome data.
The latest whole-genome phylogeny supporting four orders of Collembola:
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 closely, the radiation of the Hexapoda can be situated in the Silurian, or more. Additional research concerning the coprolites of ancient springtails allowed researchers to track their lineages back some 412 million years.
Fossil Collembola are rare. Instead, most 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 unexplained 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.
There are now about 8,000 described species of Collembola. However, to the light of ongoing developments in molecular methods and the rise of cryptic species unveiled by DNA barcoding within morphologically-described species, it has been speculated that the global species richness of Collembola could be at least an order of magnitude greater than a previous estimate of 50,000 species.