Meganisoptera ia.k.a. Griffinflies and Protodonata) is a long-extinct order of extremely large flying insects, in existence around 500 - 250 million years ago.

Many of the species appear to be quite similar to some of today's insects such as damselflies and dragonflies - but, as the fossil record shows, some had wingspans approaching 70cm.

Soon after their discovery, it was questioned how they could reach such (comparatively) huge sizes. At that time, it was thought that the way in which insects 'breathe' would mean that they could not have been able to absorb enough oxygen to fly, hunt, and survive.

In 1911, a possible solution to the paradox came from engineer/naturalist Édouard Harlé in France, who suggested that perhaps the ancient atmosphere was much more oxygen-rich than today's 21%.

At the time the proposal was controversial, and is still not confirmed. The current general consensus is that the oxygen levels may have been somewhat higher and the efficiency of the 'breathing' mechanism of insects may have been underestimated.

Either way, it's agreed that a dragonfly with a 70cm wingspan would not be viable today.

See : Wikipedia