Insects, with almost a million species, represent the group of living beings with the greatest diversity on the planet and with a great variety of sizes. From tiny ants or wasps that measure less than a millimeter, to butterflies larger than a human hand, such as Ornithoptera alexandrae , or beetles with a larval stage weighing more than 100 grams, such as Hercules or Goliath beetles.
However, compared to other groups of animals, including invertebrates such as some molluscs, crustaceans or polychaetes, the size of the largest insects remains small. But it was not always like this.
Carboniferous and Permian time of giant insects
During the Carboniferous period, between 358 and 299 million years ago, insects reached much larger sizes, a trend that lasted until the Great Dying, the mass extinction event that happened at the end of the Permian, about 252 million years ago.
At this time large dragonflies such as Dunbaria could be found, with more than 30 centimeters; or animals as strange as Mazothairos , a genus of insects with six wings, with a wingspan of more than half a meter. But all of them seem small compared to the largest known insects, belonging to the family Meganeuridae .
Indeed, the gold, silver, and bronze go to three genera of prehistoric dragonflies in this family: Meganeura , Meganeuropsis , and Megatypus . All three genera had a wingspan that could exceed 70 centimeters. Meganeura is the oldest of the three: it lived during the Upper Pennsylvanian, at the end of the Carboniferous, between 307 and 299 million years ago. Meganeuropsis and Megatypus are more modern, they inhabited the earth at the beginning of the Permian, during the Cisularian, between 299 and 273 million years ago.
How did they get to be so big?
The truth is that since the disappearance of the Meganeuridae group at the end of the Permian, there have never been insects as huge as Meganeura and its relatives. Large meganeurids were apex predators. They dominated the food pyramid, flying over large open spaces and eating any animal smaller than themselves, which was the majority. They covered in the ecosystem the role that eagles and hawks play today.
Today’s insects, even the largest, are much smaller. And the answer to the size puzzle could lie in oxygen.
It is a fact that during the Carboniferous and early Permian periods, oxygen concentrations on earth were much higher than they are today.
Insects breathe through a system of tracheas that open to the outside in the abdomen, and run through the interior of the animal’s body, carrying oxygen directly to each cell. Hence, the increased presence of oxygen in the air during this time was proposed as a hypothesis for the gigantism of these insects.
It is well known that the availability of oxygen can act as a limiting factor in the size of animals, and especially arthropods; –part of the polar gigantism is explained by this fact–. Given the particular shape of the trachea of insects, it seems logical to think that a higher concentration of oxygen would allow a greater diffusion to the tissues, and with it, a greater size.
The highest density of the atmosphere
Also, of the two main components of air, oxygen is more dense than nitrogen. A hyper-oxygenated atmosphere is, therefore, denser, and certain studies relate this fact to a greater availability to adapt to large sizes, or even to acquire the ability to fly.
Recall Archimedes’ principle: every body immersed in a fluid experiences a push from bottom to top equal to the weight of the volume of fluid displaced by the body .
Following this principle, and since the atmosphere is still a fluid, a body immersed in a denser atmosphere experiences a greater upward thrust. Organisms that live in that atmosphere can afford a larger size, and are more easily able to fly.
But the debate is not settled.
What if oxygen wasn’t so limiting?
A study led by Professor Mark W. Westneat, from the Natural History Museum of Chicago, and published in 2003 in the prestigious journal Science showed that, contrary to what was believed, insects have a more complex respiratory system, with cycles rapid compression and expansion of the tracheas in the thorax and head, which generates a respiratory movement analogous to that of the lungs of vertebrates.
This increased efficiency in respiration would reduce the importance of oxygen as a limiting factor for size. The size limitation hypothesis based on the oxygen concentration of the air lost much of its explanatory power.
Following this discovery, a new explanation for the size of meganeurids was proposed.
Who hunts who?
This new hypothesis posits that these dragonflies were so large simply because they could be, entering a competitive evolutionary race for size between dragonflies and their prey, following a red queen dynamic similar to that which makes cheetahs and gazelles grow larger. running speed.
When the first Carboniferous dragonflies took flight, they had no natural predators. They would evolve to the maximum sizes physically allowed by their body plan, because there was nothing to stop them, limited only by restrictions on the construction of the exoskeleton and associated muscular apparatus.
With the Great Dying of the end of the Permian, the large animals became extinct, and among them, the giant dragonflies. The appearance of the first pterosaurs in the Triassic, the subsequent appearance of birds in the Jurassic, and bats in the Eocene, represented three successive new ways of filling that ecological niche vacated by giant insects. Never again did dragonflies and other insects get another chance to grow to such a massive size as meganeurids.
Bechly, G. 2004. Evolution and systematics. En AV Evans et al. (Eds.), Grzimek’s Animal Life Encyclopedia (2nd ed., Vols. 3-Insects, pp. 7-16).
Chapelle, G. et al. 1999. Polar gigantism dictated by oxygen availability. Nature, 399(6732), 114-115. DOI: 10.1038/20099
Dudley, R. 1998. Atmospheric oxygen, giant Paleozoic insects and the evolution of aerial locomotor performance. Journal of Experimental Biology, 201(8), 1043-1050. DOI: 10.1242/jeb.201.8.1043
Nel, A. et al. 2008. The Odonatoptera of the Late Permian Lodève Basin (Insecta). Journal of Iberian Geology: An International Publication of Earth Sciences, 34(1), 115-122.
Westneat, M. W. et al. 2003. Tracheal Respiration in Insects Visualized with Synchrotron X-ray Imaging. Science, 299(5606), 558-560. DOI: 10.1126/science.1078008