Springtails are tiny, wingless arthropods of the class Collembola belonging to the subphylum Hexapoda. They form the largest of the three lineages of hexapods, having segmented bodies consisting of the head, thorax, and abdomen. However, unlike most insects, they cannot fly.
They are named for their ability to ‘spring’ into the air using a specialized fork-like structure called the furcula located on their abdomen. When threatened, the furcula releases like a spring, propelling the insect several inches away. The name of the class is due to the fact that the first abdominal segment contains a distinct tubular appendage called the collophore. It helps in water uptake, osmoregulation, excretion, and orientation.
These animals prefer living in dark, humid places and typically feed on fungi, bacteria, plant leaves, pollens, and carrion. Since they help break down debris, springtails improve soil fertility by replenishing its organic content.
Currently, around 3,600 species of springtails are found worldwide and are divided into 3 undisputed orders.
On average, these arthropods measure less than 0.24 in (6 mm) in length. Members of the order Symphypleona are about 1 to 3 mm long.
The females of one of the smallest springtail species, Mackenziella psocoides, measure within 0.3 mm in length. Following it are the members of the genera Megalothorax and Sphaeridia, which measure around 0.5 mm.
Some individuals of the largest known springtail species, Holocanthella duospinosa, have been recorded to reach around 0.66 in (17 mm).
Most springtails have slender, elongated bodies ranging from darker shades, like brown and black, to lighter shades, like white and gray. However, some species, like Thai red springtails and Florida orange springtails, are tinged in vibrant shades of red and orange.
Segmentation and Appendages
As hexapods, springtails are characterized by a three-part body composed of head, thorax, and abdomen, along with three pairs (a total of six) of thoracic legs. The abdomen consists of six or fewer segments and features a characteristic tubular appendage called the collophore, which projects ventrally from the first segment. This structure is believed to be associated with fluid uptake, osmoregulation, excretion, and orientation of the animal.
Another appendage called the furcula (also called furca) is found on the fourth abdominal segment. The furcula remains folded beneath the body, held by a thick band called the retinaculum. When the furcula is released, it snaps against a substrate, launching the springtail into the air, a mechanism that takes as little as 18 milliseconds.
They typically lack a distinct tracheal system and, hence, respire through gas exchange across the cuticle (cuticular respiration). However, members of the families Sminthuridae and Actaletidae possess a pair of spiracles between the head and the thorax through which respiration occurs.
The digestive tract is characterized by a foregut, midgut, and hindgut. A muscular pyloric region lies between the midgut and the hindgut.
The three regions have varying pH levels to accommodate different microbial populations and facilitate various enzymatic activities. While the anterior part of the midgut and the hindgut are slightly acidic with a pH of 6, the posterior midgut is mild alkaline with a pH of around 8.
Eyes
Though cave-dwelling springtails typically lack eyes, other species possess two dark spots, comprising clusters of up to eight ocelli or simple eyes. Of these, six ocelli help distinguish light and dark, while the other two detect polarized light. Additional eyes are located between and below the antennae on the face.
Skin
The bodies of springtails are covered in a cuticle, which repels dirt, water, and even oil (omniphobic). This covering is composed of microscaled bristles and mushroom-shaped granules arranged in a comb-like alignment with interconnecting ridges.
In some springtails, the cuticle is covered with tiny scales, similar to those on butterfly wings, which reflect light, giving them a shimmering appearance.
The name of the class was derived from the ancient Greek words kólla, meaning ‘glue,’ and émbolos meaning ‘peg.’ This taxonomy refers to the tube-like collophore, which is characteristic of the members of this class.
Springtails were traditionally classified under three orders: Arthropleona, Symphypleona, and Neelipleona. The order Arthropleona was further subdivided into two superfamilies named Entomobryoidea and Poduroidea. However, since phylogenetic studies have revealed that Arthropleona is paraphyletic, the order is no longer a part of the current taxonomy. Instead, both its superfamilies have been upgraded to orders: Entomobryomorpha and Poduromorpha.
The group Neelipleona was initially considered an advanced lineage of Symphypleona, but further analysis of 18S and 28S rRNA sequences revealed that this group represents the most ancient lineage of springtails. However, the exact taxonomic designation of Neelipleona (though currently classified as an order) remains uncertain.
Based on the most recent whole-genome phylogeny, the 3,600 species of springtails are grouped under 3 undisputed orders.
While members of the orders Poduromorpha and Entomobryomorpha have elongated bodies, those of Symphypleona and Neelipleona have globular bodies.
Springtails date back to the Early Devonian Period, approximately about 400 million years ago. The oldest terrestrial arthropod, Rhyniella praecursor, which also happens to be a springtail, was recovered from the Rhynie chert of Scotland. Since its morphology closely resembles modern-day springtails, the radiation of Hexapoda is believed to have occurred during the Silurian Period (over 420 million years ago). Additional evidence from coprolites (fossilized feces) of older forms has extended their lineage to around 412 million years ago.
Fossil records of springtails are rare, with most specimens preserved in amber. Among them, the best specimens have been unearthed from Canada (Early Eocene and Mid-Cretaceous), Europe (Early Eocene), Central America (Miocene), and Burma (Mid-Cretaceous).
Springtails thrive in dark, humid environments (cryptozoa) and are found virtually everywhere on the Earth where soil exists. They are among the most abundant macroscopic animals, with around 100,000 individuals per square meter of ground, showing greater diversity in the tropics compared to temperate regions.
Some species, like those in the genera Entomobrya and Orchesella, live in trees and form a dominant component of canopy fauna, while others prefer living in caves. The cave-dwelling species Plutomurus ortobalaganensis is found around 6,500 ft down the Krubera Cave. Springtails have also been spotted even at a height of 9,000 ft in the air. These animals also inhabit damp indoor spaces, such as bathrooms and basements.
These animals are generally omnivorous and prefer feeding on fungal hyphae, spores, molds, and mildews. They are also found to consume bacteria, plant leaves, pollens, decaying carrion, and dung.
Some carnivorous species, like Protaphorura armata, typically feed on small invertebrates, like rotifers, nematodes, and other springtails.
They have a lifespan of only about 1 or 2 years.
Male springtails deposit tiny sperm packets, known as spermatophores, either in clusters or scattered patterns on a substrate or directly at the female’s genital opening. In some species, like Sinella curviseta, it is the females that stimulate the deposition of these spermatophores by producing pheromones to attract males.
Courtship is usually observed in members of Symphypleona, especially in males of the family Sminthuridae, which grasp the females using a clasping organ located on their antennae.
The embryo hatches from fertilized eggs and undergoes a number of developmental (larval) stages by shedding its skin as it grows (molting). The number of molts varies widely among species, with some undergoing as few as 4 molts while others molting up to 50 times before growing into juveniles. These juveniles are typically smaller and lighter in color than the adults. The entire reproductive process in springtails is quite rapid, with the development from eggs to adults taking as little as 4 to 6 weeks.
Springtails living in deeper soil horizons are often parthenogenetic, where the females give birth to genetically identical offspring without the need for fertilization by males. This process is called thelytoky and is influenced by the presence of intracellular bacteria of the genus Wolbachia inside the female reproductive organs and eggs.
Their natural predators include small flies, beetles, bugs, pseudoscorpions, and larval fish. Cave-dwelling species are consumed by spiders and harvestmen.
However, they are mostly preyed upon by mites of the order Mesostigmata, belonging to the families Ascidae, Parasitidae, Laelapidae, Rhodacaridae, and Veigaiidae.
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