(20 species)
Entognatha are believed to be a primitive group in the evolutionary lineage of hexapods, whereas Ectognatha (i.e. insects = class Insecta) contains more advanced species. Entognatha have their mouthparts hidden inside their heads such that they are invisible from the outside, whereas the mouthparts of insects are prominently visible.
Entognatha contains three subclasses: Collembola (springtails), Diplura and Protura.
The subclass Collembola (springtails) contains small animals with body lengths usually in the range of a few millimeters, reaching 1.5 cm in some extreme cases. With 9000 described species worldwide springtails form the largest class in the Entognatha group, but they are far from the diversity of insects (> 1 million sp.)
A typical feature of springtails is the furcula, an appendage of the 4th abdominal segment, which is held beneath the body of the springtail. However if disturbed, springtails may release their spring-like ""tail"" which will result in them being lifted of the ground and thrown through the air. Evidently, this feature gave this order its name.
Springtails are virtually ubiquitous in terrestrial ecosystems, living in soil, on plants and even on surfaces of water, often very abundant.
Four Collembola orders are known, which can generally be distinguished based on their body shape.
Collembola do possess primitive compound eyes, those, however, contain only a few ocelli.
(Diplura ~500, Protura ~750)
