The Entognatha are a class of ametabolous arthropods, which, together with insects, make up the hexapods. They are wingless and so aren't considered to be insects, although they are related to them. The three orders that belong to this group are the Collembola (springtails), Diplura and Protura. It is agreed that the hexapodous condition of these animals has evolved independently from that of insects. Furthermore, this condition has evolved independently within each group. These three orders are not closely related however, and the Entognatha are therefore considered to be a paraphyletic group.

These three groups, if united with the order Thysanura, form the class Apterygota.

Morphology

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These minute arthropods are primitively wingless, unlike some other orders of insects that have lost their wings secondarily (but are derived from winged ancestors). Their name comes from their mouthparts that are withdrawn into the head. Other differences with insects are that each antennal segment is musculated; in insects, only the two basal segments are. Of the three orders, only collembolans possess eyes. Nevertheless, many collembolans are blind, and even when compound eyes are present, there are no more than eight ommatidia.

Arthropods

Entognatha