Meganeura: Giant Carboniferous Dragonfly

Meganeura was an insect that lived in the lush forests of the Carboniferous Period, and is most related to modern-day dragonflies. It is one of the largest true insects ever to have existed, with a wingspan of nearly two and a half feet. Fossil specimens of meganeura have been uncovered in France and England.

The reason for the giant size of meganeura as well as other insects living in the Carboniferous has to do with the oxygen content in the atmosphere. The Carboniferous represented an evolutionary explosion of terrestrial plants and very limited diversification of land-based fauna. Therefore, with few large herbivores to thin the forests in addition to the perfect warm and wet growing conditions, these forests covered the entire globe. When there are that many trees, they will gobble up copious amounts of carbon dioxide from the atmosphere to fuel photosynthesis and release a lot of oxygen as a byproduct. Since insects breathe through specialized tracheae in their skin, diffusing oxygen directly into their bodies, their size is restricted by the amount of oxygen present in the atmosphere. Thus, a higher content of oxygen (nearly 30%) during the Carboniferous Period would have allowed insects like meganeura to reach gigantic sizes.

In addition to its titanic size, meganeura differed from modern dragonflies in the number of appendages found at the end of its abdomen. It is unclear what these appendages were used for. They could have been used to help lock female and male partners into place during mating such as modern-day damselflies do, they could have been used in laying eggs, or even to help anchor the insect to its perches.

The size of meganeura might have been both an advantage and a liability. Modern-day dragonflies are among the best fliers in the insect world, able to fly at speeds of nearly 70 miles per hour with incredible agility. The size of meganeura makes this kind of speed seem doubtful, as well as maneuverability within the dense forests of the Carboniferous. Also, in order to maintain that body size and energy requirements for lifting it off the ground, meganeura would have needed to consume large amounts of food. Present-day dragonflies are insect-eaters, but only because their size restricts them to that prey. It seems very likely that meganeura would have been able to (and needed to) seek larger prey such as small lizards or other giant insects in order to survive.

Alas, meganeura could not survive the changes that Earth was undergoing. As the Carboniferous Period ended, climatic changes had caused the vast forests to shrink in size. With fewer trees to pump out large amounts of oxygen, those atmospheric oxygen levels dropped, and consequently so did the size of arthropods.