The largest insects ever were members of the extinct Protodonata order, with Meganeuropsis permiana having a wingspan of 75 cm and weighing over 450 g. They were predators, feeding on other insects and small amphibians. The high oxygen levels during the Carboniferous era allowed for larger insects to exist, but the reason for Meganeura’s size remains controversial.
The largest insects that ever lived are members of the extinct dragonfly-like order Protodonata, meaning “primitive dragonfly.” The name “griffon” was proposed for members of this order because important differences distinguish them from modern dragonflies. The fossil record of Protodonata ranges from the late Carboniferous to the late Permian 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 Meganeuropsis permiana from the Late Permian, with a longest wingspan of 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 ever lived, land or air: the heaviest insect today is the larval stage of the Goliath Beetle, weighing up to 115 g (4.1 oz). Because Late Carboniferous Meganeura is a species similar in size to Meganeuropsis and is much better known, the information in this article will focus primarily on Meganeura.
To better understand the size of Meganeura and its protodonate insect companions, consider that the largest dragonfly today, found in South America, has a wingspan of only seven inches, about one-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 Meganeura fossils are just fragments of wings, although a few complete wings and even fewer full-body impressions have been found. The few body prints found so far show a globular head, large mandibles, a large chest, strong spiny legs, and a long, slender abdomen, like that of a dragonfly. The only body prints that have been found are from the family Meganeuridae. Other members of Protodonata are known only from wing fragments.
The tracheal respiration system of insects places limits on their maximum size, which many prehistoric insects, especially Carboniferous ones, largely 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% instead of the current 20%. This would have made enough oxygen available for larger insects to exist, and the fossil record indicates that they did indeed exist. However, some insects, such as Meganeura, are still larger than the limit predicted by insect circulation models, even accounting for the extra oxygen. Therefore, the reasons for this broken limit have been the subject of much controversy in paleontology. It may take years or decades of debating and hunting for fossils before we have a good answer.
The Meganeura species is so popular that a scientific journal on fossil insects is named after it.
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