How do entognathous mouthparts differ from ectognathous mouthparts?

The primary difference between entognathous and ectognathous mouthparts lies in their positioning relative to the head capsule.

• Entognathous mouthparts: These are withdrawn or sunk below the surface of the head, meaning they are not externally visible. This condition is a defining characteristic of the classes Protura, Collembola, and Diplura.1234
• Ectognathous mouthparts: These are exposed and located on the exterior of the head, similar to the arrangement found in pterygote (winged) insects. This condition is found in the Insecta subclass Apterygota, specifically within the orders Archaeognatha, Zygentoma, and the extinct Monura.234

Taxonomically, this distinction is a critical criterion used to separate these groups of hexapods. While the mouthparts are positioned differently, it is generally recognized that these structures evolved from ancestral walking limbs that became fused to the head.42