The most recognizable feature of the hexapods is the reduction in walking appendages to six, with three postcephalic, consolidated body segments forming the thorax, which provides much of the locomotory ability of the animals. The abdomen has 6 to 12 segments, with rudiments of limbs on the first 9 in primitive forms, and with gonopore near the posterior end of the body (opistogoneate). Trachea are used for respiration. During larval growth, there is no increase in the number of segments (epimorphic). Most of the other arthropods have more than three pairs of legs. Originally, the hexapods have antennae, trignatous mouthparts comprising unsegmented whole-limb mandibles, followed by the maxillae and labium.
The hexapods are terrestrial or secondarily aquatic arthropods, ruling the land, while the crustaceans dominate the marine environments.
The superclass Hexapoda includes the Collembola, Protura, Diplura, and Insecta. The first three taxa are soil or litter dwellers. Collembola (springtails) are probably the most abundant arthropods on earth. The Protura is a group of very small, pale arthropods that are rarely encountered. Diplurans are a few families of larger, pale arthropods that are frequently found in moist soils.
To be exact, the assemblage formed by the Hexapoda must be grouped as wingless Collembola, Protura, Diplura, and Apterygota (the primitively wingless, terrestrial insects) and Pterygota, the adults of which are winged or secondarily apterous.
The Pterygote insects have a varied metamorphosis, as the adults do not moult. Because they are unmodified, the mandibles articulate with the head-capsule at two points. Considering this metamorphosis, the Pterygota can be divided into two categories; the Exopterygotes and Endopterygota. The Exopterygotes normally have a simple, incomplete (hemimetabolous) metamorphosis. Their name is derived from the fact that their wings, usually without pupal instar, develop externally. The immature stages, called larvae or nymphs, usually resemble the adults in structure and habits. Likewise, the Endopterygota derive their name from the fact that their wings develop internally. They have a complete (holometabolous) metamorphosis, accompanied by a pupal instar. The larvae differ from the adults in structure and habits.
Some see the Thysanura (firebrats, silverfish), Collembola, Protura, and Diplura as the Apterygota, being primitively apterous forms with only a slight metamorphosis. They usually moult several times after attaining sexual maturity, and adults have one or more pairs of pregenital appendages. The mandibles usually articulate with head-capsule at a single point.
Traditionally, the division was based on the position of the mouthparts. The first three taxa, or Entognata, are entognatous, while the remaining (Ectognata), is ectognatous (see entognata). The entognaths all have the base of the mouthparts internalized, so that the mandible and maxilla are partly contained within the head capsule. The Collembola, Protura and Diplura also share reduced Malpighian tubules and compound eyes. Recent evidence suggest that the Diplura may instead be a sister group of the insects (see Diplura sistergroup).