Q. When is an animal with six legs not an insect? A. When it’s an entognathous hexapod.
Until fairly recently it was thought that all animals with six legs were insects. It is now widely accepted that there are two classes of six-legged animals – the insects (class Insecta) and the entognathous hexapods (class Entognatha).
The Entognatha are six-legged animals characterized by mouthparts which are hidden away in a pouch in their head (entognathous). The insects (Insecta) have mouthparts which are not hidden (ectognathous).