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The Insectoid Spirit Beasts were the exact same ones that had invaded the Realm of Genesis before, which Truhan and Celica's armies had fended off before being swallowed by the Spiritual Plane's dimensional hole.
Their appearances varied widely, there were heavy armored ones as tall as thirty meters, resembling giant scarabs or horned beetles, there were tall and skinny ones with sharp scythes, resembling mantises, there were smaller, fast ones with sharp proboscis, resembling mosquitoes, and there were small ones specialized on biting and stinging, resembling wasps.
Their colors varied a lot, each of their sections were usually of several colors, indicating that these bugs had more elemental affinities than normal Spiritian people. And indeed, it was because they were not naturally born in this world, but chimeric creatures born from one of the Spirit Generals serving the Ruler of this Plane.
Check "The Protagonist's Sister Is Actually the Strongest"!