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Entognathans are hexapod arthropods related to insects but more primitive. They include three orders: Collembola, Diplura, and Protura. Entognathans have their mouthparts inside the head capsule and are small. Springtails are the most diverse and abundant of the Entognatans. Further studies are needed to determine their relationship to other insects.
Entognatans, members of the class Entognatha, are a group of hexapod (six-legged) arthropod animals closely related to insects, but generally more primitive and basal (separated earlier) than insects. The Entognatans include three orders: Collembola (springtails, with over 6000 species), Diplura (800 species), and Protura (sometimes called “coneheads”, with 731 species). Entognathans are thought to have evolved their six-legged form of locomotion independently of insects and of each other. Despite being placed in the same class, Entognatans are probably not monophyletic (descended from a common ancestor). Further updates may abolish the class entirely.
Entognathans are interesting and important due to their high abundance, small size, basal position among living arthropods, our poor understanding of their phylogeny, and the ability of some (spring) Entognatans to inhabit extreme environments such as the Antarctic Peninsula, where they are among the only native animals. Entognatans, whose name means “inner jaw”, have their mouthparts inside the head capsule, with only the tips sticking out. This is unlike all insects, which have external mouthparts. Entognathans were thought to be insects until closer analysis found that each had fundamental differences from that group.
As mentioned, entognatans are small. Eyeless dipluranes, which resemble transparent earwigs, are usually 2–5 mm long, although some members of the predatory genus Japyx can reach 5 cm (2 in) in size. Diplurans live in the leaf litter of temperate forests, using their cerci (tail appendages) to catch small bits of living and dead matter, including mites, other diplurans, and fungi. Predatory species have short circles.
Proturans, sometimes called coneheads due to their appearance, are another group of small (>2 mm) entognathans found in the ground, usually within 10 cm (4 in) of the surface. Proturans are probably the most basal of all hexapods and may appear to be some of the first arthropods to live on land and the first to branch off from marine crustaceans, although this is speculative. They are among the few hexapods that lack cerci, and are so small and easy to miss that they were only discovered in 1907.
The last order of the Entognatans are the springtails, which are the most diverse, numerous and famous of the Entognatans. They are considered to be among the most abundant macroscopic animals on Earth and can be found almost anywhere there is soil or soil-related habitat, and on all seven continents. Unlike other entognatans, springtails have eyes, although their vision is very poor. Springtails are more abundant in cold climates, a phenomenon diametrically opposed to most other arthropods. They get their name from the tails they use to launch themselves off the ground and away from predators. Springtails are usually less than 6 mm long, although some species reach 10 mm. Many species are difficult to see with the naked eye, being less than one mm long.
Further studies are needed to determine the exact relationship of entognathans to each other and to other insects. This information may help us understand how hexapods evolved from other groups and why entognatans are not as diverse as insects.
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