What features distinguish proturans from other primitive hexapods?

Proturans are distinguished from other primitive hexapods by several unique morphological and developmental features:

• Absence of Antennae and Eyes: Unlike most other hexapods, proturans lack both antennae and eyes. They compensate for the lack of antennae by carrying their front pair of legs forward to serve a sensory function.12
• Abdominal Segmentation: Proturans possess 12 abdominal segments, whereas most insects have 11. These segments are added postembryonically through successive molts, a process known as anamorphosis.1
• Mouthparts: While they are entognathous—meaning their mouthparts are withdrawn inside the head capsule—their mouthparts are specifically modified for sucking, though they retain mandibles.13
• Appendages: They lack cerci, which are sensory appendages found in many other hexapods. Additionally, they possess three pairs of ventral abdominal appendages and paired openings to large abdominal glands on the eighth abdominal segment.412

These minute, pale organisms are often classified within the class Entognatha alongside springtails and diplurans, though some authorities consider them distinct enough to be placed in the subclass Parainsecta.1