All about Diplura-
the two pronged bristletails

Diplurans are elusive and fast moving soil animals, seldom seen and when they are, can often be mistaken for symphylans or millipedes. They are small, eyeless and whitish, like most of the other soil mesofauna, with two long cerci, their tail appendages, giving them their common name. They have three lineages- the Campodeidae and the Japygidae with a rare third, the Projapygidae. These are discussed in more detail below.

Unlike insects, collembolans, proturans and diplurans continue moulting as adults, are wingless and entognathous, sharing similar retractable mouthparts, held inside a pouch. These differences were eventually considered important enough to remove all three from the Insecta class into their own- the Entognatha.

However, as mentioned on other pages, each of the three members of the almost definitely polyphyletic class of Entognatha (meaning probably not sharing a common ancestor, although having some similarities) are very unusual. Each has characteristics peculiar to itself, while sharing some others. Diplurans like the Campodeoidea are able to completely regenerate their bead-like antennae, legs and cerci, their long tail appendages, over a series of moults. This unique ability amongst the hexapods hints a little of their ancient pancrustacean ancestors. Modern crustaceans like lobsters are experts, able to start regenerating limbs within days, with moulting aiding the process.

Diplura keep growing and moulting throughout their short, usually one year lives, although it's possible that some species can live around three years, like Collembola. Some can moult at least thirty times.
Both diplurans and collembolans look like miniature adults as a newly hatched instar. However, unlike Collembola, some female diplurans are known to guard their eggs and young, like pseudoscorpions.
Unlike the first instars of Collembola, which able to begin feeding upon hatching, diplurans spend their first few days immobile, as a pre-larva, similar to pre-larval mites, before moulting and beginning to feed and explore their surroundings.

Campodeiodea consists of typically generalist feeders, although largely herbivorous, or detritivores. In comparison, most of Projapygoidea and the entirety of Japygoidea are predatory hunters, although still capable of detritivore behaviour.

Diplurans have articulated abdominal appendages on many of their segments, called styli, another feature they share with their order-mate, Protura as well as the order Archaeognatha, the jumping bristletails and some genera of Zygentoma- the silverfish and firebrats. Jumping bristletails, silverfish and firebrats can use their abdominal styli as extra legs in certain situations. It's been presumed that they are functionless in diplurans. This isn't correct, at least with the Campodeidae.
Usually, when traversing a flat surface, the dipluran's body is held out horizontally. But when climbing over uneven surfaces, the styli come into play, walking the abdomen over the surface. To the best of my knowledge, the photo below is the first documented evidence of this behaviour. 

The Japygoidea have adapted, pincer-shaped cerci, as shown in the photos below. The pincers are fully functional and are used for defence and to hold down prey, with the abdomen being curled over its back and head to feed on captured prey. When hunting, it’s common for Japygids to wait, buried, with their pincered tail exposed, ready to grab any prey that gets too close.

The photo below shows Teljapyx aotearoa, a newly described cave species from the South Island of New Zealand, which is so far, the only cave-dwelling dipluran in New Zealand.

Some members of Japygoida can reach up to 60mm, but most diplurans are between 2 and 5mm.

While diplurans all show adaptations to soil or cave life, being sightless and lacking pigment, some species do exhibit strong colouring, as can be seen with the yellowed dipluran below. This is due to a build-up of sub-epidermal soluble fats from their diet.