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The largest insects that ever lived are members of the extinct dragonfly-like order Protodonata, which means “early dragonfly.” The name “griffinfly” has been proposed for members of this order because important differences distinguish them from modern-day dragonflies. Protodonata’s fossil record ranges from the Late Carboniferous to the Late Permian periods of the Paleozoic era. This was about 300 million years ago, before the dinosaurs, which evolved during the Mesozoic era.
The largest known species is the Late Permian Meganeuropsis permiana, with wingspans longer than 75 cm (30 in. or 2.5 ft) and an estimated weight of over 450 g (1 lb.), similar to a crow. This is larger than any insect that has ever lived, land- or air-based – the heaviest insect today is the larval stage of the Goliath Beetle, with a top weight of 115 g (4.1 oz). As the Late Carboniferous Meganeura is a species of similar size to Meganeuropsis and is much better known, the information in this article will primarily focus on Meganeura.
To get a better grasp of the size of Meganeura and its fellow Protodonata insects, consider that the largest dragonfly today, found in South America, has a wingspan of only seven inches, about a third the size of Meganeura. Meganeura was a predator, feeding on other insects and even small amphibians. Its name means “large-veined” after the network of veins on its wings. Most fossils of Meganeura are just fragments of wings, although a few full wings and even fewer full body impressions have been found. The few body impressions dug up so far show a globose head, large mandibles, a large thorax, strong spiny legs, and a long and slender abdomen, like that of a dragonfly. The only body impressions that have been found are of the family Meganeuridae. Other members of Protodonata are known just from wing fragments.
The tracheal breathing system of insects puts limits on their maximum size, which many prehistoric, especially Carboniferous insects well exceeded. However, it is thought that at the time of the Carboniferous, which contained huge numbers of trees and other oxygen-producing plants, the oxygen level of the Earth’s atmosphere was around 30-35% rather than today’s 20%. This would have made enough oxygen available that larger insects could have existed, and the fossil record indicates that indeed they did. However, some insects, like Meganeura, are still larger than the limit predicted by models of insect circulation, even taking the extra oxygen into account. Thus, the reasons for this broken limit have been the subject of much controversy in paleontology. It could take years or decades of debate and fossil hunting until we have a good answer.
The species Meganeura is so popular that a scientific periodical about fossil insects is named after it.