The biggest insect ever was a huge "dragonfly"

Most popular textbooks make mention of “giant dragonflies” that lived during the days before the dinosaurs. This is only partly true, for real dragonflies had still not evolved back then. Rather than being true dragonflies, they were the more primitive ‘griffinflies’ or Meganisopterans. Their fossil record is quite short. They lasted from the Late Carboniferous to the Late Permian, roughly 317 to 247 million years ago.

Meganisoptera is an extinct family of insects, all large and predatory and superficially like today’s odonatans, the dragonflies and damselflies. And the very largest of these was Meganeuropsis. It is known from two species, with the type species being the immense M.permiana. Meganeuropsis permiana, as its name suggests is from the Early Permian. Fossils of this huge insect were described by Frank Carpenter in 1939. The genus was scientifically described on the basis of a single incomplete wing fossil from Elmo, Kansas. Carpenter described a second species from Oklahoma named M.americana, a few years later. The wing fossil was, and still is, exhibited in the Museum of Comparative Zoology at Harvard University as part of the permanent exhibit.

From these two finds, and calculations of other griffinflies, it was estimated that Meganeuropsis had a body roughly 47 centimeters long, with a wingspan of 75 centimeters across. This is much wider than the South American damselfly Megaloprepus, the largest odonatan living today. Meganeuropsis was able to get this large possibly due to a far higher oxygen content in the atmosphere. During the Carboniferous, there was a massive influx in global oxygen levels and this allowed many land-living invertebrates to get to huge sizes.