That’s right: fossils from the Carboniferous and Permian times had insects with single wing lengths of more than 30 cm (that’s a foot for those of you still using Imperial measurements).
One hypothesis for why insects were able to be much bigger was that there was more oxygen in the atmosphere then. A new paper by Clapham and Karr suggests that’s true... up to a point.
Looking at the fossil record and that for oxygen, they find a good correlation between oxygen levels and insect sizes up until the end of the Jurassic. After that... the correlation falls apart. During the Cretaceous, the oxygen levels go up (though not as high as the Permian or Carboniferous), but the insects get smaller. And insects get even smaller after the K-T boundary, even though oxygen is pretty stable.
Clapham and Karr suggest that flying predators, notably birds. This struck me as counterintuitive at first, as larger bodies are explained as possible defences against predation. Clapham and Karr’s idea is the battle in the air is over manoeuvrability. Big animals can’t twist and turn and dodge so easily. Perhaps with the reduced oxygen levels of the Cretaceous, insect physiology wouldn’t support the re-evolution of large bodies as a defence against bird predators.
If you like your insects small, thank a bird.
Clapham ME, Karr JA. 2012. Environmental and biotic controls on the evolutionary history of insect body size. Proceedings of the National Academy of Sciences 109(27): 10927-10930. DOI: 10.1073/pnas.1204026109
Picture from here.