Collembola

The Springtails (Collembola) are a class of hexapod arthropods close to, and sometimes classified within, insects. They are tiny, ubiquitous animals, occupying all continents (even Antarctica). They are probably the most numerous hexapods on Earth: up to 15,320 springtails/m².

Its fossil record dates from the Devonian (Rhyniella, Protoisotome, about 400 million years ago), making it one of the oldest land animals.

Morphology

Their common name in English (springtail) comes from a retractable appendage (furcula or furca) with which they can propel themselves many times the length of their body, which usually does not exceed 5 or 6 mm.

Two segmented antennae rise above the head (sometimes due to subsegmentation they appear to have more segments). They have 6 abdominal segments or less. The antennae form four knuckles, which have their own musculature and can move independently of others. Unlike insects, behind the antennae is a post-antennal sensory organ (Tömösvary's Organ), absent only in springtails of epigeal life. Said organ has a variable shape and it is believed that it serves to perceive environmental chemical stimuli. The eyes, located behind the antennae and the post-antennal organ, are simple and vary in number, from one to eight on each side of the head. The oral apparatus is of the chewing type in most cases, although there are species of the Neanuridae family where there is an extension of the different mouthparts, which allows the individual to bite; in its ventral part, the head has a kind of channel that continues through the thorax until it reaches the first abdominal end.

In the thorax there is a pair of legs for each segment, three pairs. The abdomen has six segments. The genital opening is located on the fifth and is devoid of external structures related to reproduction. In the ventral section of the first segment there is a kind of viscous outgrowth called collophore, which helps the animal to adhere to very smooth substrates, even when it is in a vertical position, since at the beginning of the ventral tube, from which two vesicles emerge, is always irrigated by a mucous secretion. On the third abdominal segment on the ventral side there is a biramose formation called the tenaculum, folded under the body when the animal is not using it. Springtails also have a jumping organ called the furcula, which is retained by the tenaculum. The furcula is composed of the manubrium, teeth, and mucron; when activated it causes the specimen to jump.

Ecology

Orchesella cincta.

Springtails can be categorized by their diets, since they have a particular mouth morphology depending on what they eat. Depending on the species, we can observe sucking springtails, mycophagous, carnivorous, phytophagous, omnivorous, among other groups of specialists, Therefore, in a general way we can say that they feed on protozoa, nematodes, rotifers, tardigrades, bacteria, algae, fungi and decomposing leaf litter.

The way it reproduces is characteristic: the male does not have copulation organs suitable for transferring the germinal products to the female genital tract. On the other hand, the male produces a spermatophore where the spermatozoa are protected from drying out while deposition is carried out and when the female collects them. The spermatophore is situated at the apex of a short stalk that the male secures to the substrate. Frequently, before the deposition of the spermatophores, there is a long courtship with the female. At the end of courtship, the female places her genital opening on the spermatophore. Various cases of parthenogenesis have been reported in many species of springtails (Folsomia,Tullbergia, Neanura).

Microflora and microfauna feeders, especially in the rhizosphere; they disperse microorganisms, helminths and parasitic cestodes; they are parasite hosts; macrofauna prey; microecosystem engineers.

Soil springtails are generally subjected to strong predatory pressure due to their gregarious behavior, however they are found in high numbers in the soil and this raises the question of whether springtails have evolved defense strategies against predators. The most common strategy for springtails to escape from predators is the ability to jump; but eyeless ground-dwelling species of Onychiuridae, which have lost their ability to jump, release repellent fluids through small cunicular openings, the pseudocoels (they can occur on the back and are displayed in the face of attack) which can also secrete alarm signals. The aggregate behavior of springtails is closely related to communication and coordination between individuals. The maintenance of aggregation and its imitation depend on the presence and activity of signals. Additionally, through the production of sex and aggregation pheromones, springtails are able to synchronize molts and reproduction.

It has been shown that in springtails, as well as in other terrestrial arthropods, migratory behaviors occur as a reaction to changes in the quality of the environment and also to pollution.

In the summer of 2010, a Spanish-Russian expedition entered the Voronia Cave (Abkhazia) and found four new species of springtails, two of them being the arthropods found at the deepest depths (on land) on the planet. The specimen found at a greater depth, 1980 meters, was named Plutomurus orthobalaganensis and is blind, brown in color and measures 4 millimeters; the one found at 1600 meters, Schaefferia profundissima, measures one millimeter, is yellowish, and lacks eyes (or has a residual eye).

A species of Sminthurinae (Symphypleona: Sminthuridae)

Taxonomy

There is still no full consensus about the taxonomy of springtails, even if they are part of the class Insecta or not. Springtails are traditionally divided into four orders:

• Order Poduromorpha
• Order Entomobryomorpha
• Order Neelipleona
• Order Symphypleona

In some classifications, Poduromorpha and Entomobryomorpha are grouped together in the order Arthopleona, with the rank of superfamilies (named Pudoroidea and Entomobryoidea, respectively).

Phylogenetic tree

The most widely accepted phylogeny currently uses whole genome studiesː

There are more than 8,200 described species of worldwide distribution, in more than 670 genera, 31 families, 15 superfamilies and 4 orders.

Fossils are very rare. Most are found in amber. The best are from the Eocene of Canada and Europe, the Miocene of Central America, and the Middle Cretaceous of Burma and Canada. They have some characteristics that are difficult to explain.