Among extant insect taxa they are some of the most evolutionarily primitive; they appeared in the Middle Devonian period at about the same time as the arachnids.

Their abdomen ends in three long tail-like structures, of which the lateral two are cerci, while the medial filament, which is longest, is an epiproct.

The mouthparts are partly retractable, with simple chewing mandibles and seven-segmented maxillary palps which are commonly longer than the legs.

[14] They have one or two pairs of eversible membranous vesicles on the underside of abdominal segments 1 to 7, which are used to absorb water and assisting with molting.

[17] Further unusual features are that the abdominal sternites are each composed of three sclerites, and they cement themselves to the substrate before molting, often using their own feces as glue.

[18] The body is covered with readily detached scales, that make the animals difficult to grip and also may protect the exoskeleton from abrasion.

The thin exoskeleton offers little protection against dehydration, and they accordingly must remain in moist air, such as in cool, damp situations under stones or bark.

[22][23] One hypothesis is that the external genitals of insects started as structures specialized for water-uptake, which could reach deeper crevices than the coxal vesicles, and over time the female would use it to take up sperm from the ground instead of water.

The young resemble the adults, and take up to two years to reach sexual maturity, depending on the species and conditions such as temperature and available food.