Which adaptation is unique to insects among all protostomes?

I think it is the ability to move by flying. Protostomes are one of the two groups of animals having a true body cavity (coelom) and are believed to share a common ancestor. They include the mollusks, annelids, and arthropods. In this animals the blastopore becomes the mouth. Other characteristics include, they have true tissues, they have bilateral symmetry, and that the mouth develop before the anus in during embryonic development.

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.