Which adaptation is unique to insects among all protostomes?

The evolution of wings and the ability to move by flying is a unique adaptation of insects among all protostomes. During the pre-adaptation, their wings might held the respiratory structures for greater exchange of gases or it function as a protective shields over their leg joints. 

Moreover, like dragonflies that are unable to tuck their wings when at rest, the next evolutionary burst is the foldable wings they have. These insects includes locusts and stoneflies.

igneous rocks which formed by the crystallization of magma at the surface of the earth called extrusive rocks. they are characterized by fine-grained textures because the rapid cooling at or near the surface did not provide enough time for large crystals to grow.