Meganeura monyi was a prehistoric insect resembling and related to the present-day dragonfly of the Carboniferous period (300 million years ago), . With a wingspan of more than 75 cm (2.5 ft) wide, it was the largest known flying insect species ever to appear on Earth. (The Permian Meganeuropsis permiana is another contender). It was predatory, feeding on other insects and even small amphibians.
Controversy has prevailed as to how insects of the Carboniferous period were able to grow so large. The way oxygen is diffused through the insect's body via its tracheal breathing system puts an upper limit on body size, which prehistoric insects seem to have well exceeded. It was originally proposed (Harlé & Harlé, 1911) that Meganeura was only able to fly because the atmosphere at that time contained more oxygen than the present 20%. This theory was dismissed by fellow scientists, but has found approval more recently through further study into the relationship between gigantism and oxygen availability. If this theory is correct, these insect giants would have been perilously susceptible to falling oxygen levels and certainly could not survive in our modern atmosphere.
However, more recent research indicates that insects really do breathe, with "rapid cycles of tracheal compression and expansion". If correct, then there is no need to postulate an atmosphere with higher oxygen partial pressure.