protos - first , oura- tail.
- antennae absent
- tentorium absent
- fore legs enlarged, with many sensilla; front legs serve role of antennae
- very small, less than 2 mm long
- abdomen with 12 segments as adult
- Mouth parts entognathous
- cerci absent
- legs 5-segmented
- anamorphic development (segments added at moults) vs. epimorphic in all other hexapods
The larva has nine abdominal segments, but the number increases through moulting until the full adult number of twelve is reached. Further moults may occur, but do not involve any additional body segments (Meyer 2005), and it is not known whether the adults continue to moult throughout their lives. Eggs have only been observed in a few species (Tipping 2004). Five developmental stages follow: the prelarva hatches from the egg and has only weakly developed mouthparts and nine abdominal segments; larva I follows and has fully developed mouthparts; larva II has ten abdominal segments; maturus junior has twelve abdominal segments and is followed by the adult (Tipping 2004). The family Acerentomidae differs in having an extra pre-imago stage, with partially developed genitalia, between the maturus junior and the adult (Tipping 2004).