Explanation for why we don't see two-foot-long dragonflies anymore fails

Meganeuropsis permiana had a wingspan over 70 cm and weighed 100 g.

Late Palaeozoic skies were filled with giant insects.

An oxygen-constraint hypothesis held that such large dragonflies required highly oxygenated air.

Insects respire via spiracles, tracheae, and microscopic tracheoles; they can actively pump air into tracheae but pumping stops at the tracheoles.

Oxygen must cross into tissue by slow passive diffusion at the tracheoles, so larger insects would need wider or more numerous tracheoles to maintain oxygen supply.