Entognatha

Morphology

These minute arthropods are apterous, unlike some orders of insects that have lost their wings secondarily (but are derived from winged ancestors). Their mouthparts are enclosed within a pouch in the head capsule, called the gnathal pouch, so only the tips of the mandibles and maxillae are exposed beyond the cavity. This pouch is created in the embryo by a flap or lateral head sclerite near the mouth on each side of the head which fuses with the labium. Other differences from insects are that each antennal segment is musculated; in insects, only the two basal segments are. Sperm transfer is always indirect, and there is an ovipositor in the females. 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.

• Collembolans have a ventral tube termed a collophore on the first abdominal segment. The collophore is involved in moisture absorption. On the third abdominal segment is the retinaculum that holds the furcula. The furcula is the "spring" for which the Collembola are given the name springtails.
• Proturans, sometimes referred to as "coneheads", do not have eyes or antennae. They possess a telson and abdominal styli thought to be vestigial legs.
• Diplurans have a pair of caudal cerci, from which their name, meaning "two-tailed", is derived. They also possess abdominal styli.