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Insects were the first organisms on Earth to evolve active flight. It is likely that the ability to fly arose only once in insects, over 300 million years ago in the Carboniferous period. About 70 million years later active flight arose within vertebrates (pterosaurs), followed by separate origins in birds and bats.
Winged insects, or pterygotes, have radiated into over 100 times more species than all of the winged vertebrate lineages combined. They include all of the commonly encountered insects; the most common non-pterygote insect lineage are the Thysanura (silverfish and firebrats).
Many groups of insects have subsequently lost the ability to fly. Some of these (lice and fleas, for example) have lost all remnants of wings. It is only through their evident relationship with particular groups of winged insects (lice are closely related to psocids [book lice]; fleas are related to flies) that it is clear that their ancestors had wings.