(Mar 31, 2026 01:21 AM)C C Wrote: https://arstechnica.com/science/2026/03/leading-explanation-for-ancient-giant-flying-insects-gets-squashed/

EXCERPTS: Three-hundred million years ago, the skies of the late Palaeozoic era were buzzing with giant insects. Meganeuropsis permiana, a predatory insect resembling a modern-day dragonfly, had a wingspan of over 70 centimeters and weighed 100 grams. Biologists looked at these ancient behemoths and asked why bugs aren’t this big anymore. Thirty years ago, they came up with an answer known as the “oxygen constraint hypothesis.”

For decades, we thought that any dragonflies the size of hawks needed highly oxygenated air to survive because insect breathing systems are less efficient than those of mammals, birds, or reptiles. As atmospheric oxygen levels dropped, there wasn’t enough to support giant bugs anymore. “It’s a simple, elegant explanation,” said Edward Snelling, a professor of veterinary science at the University of Pretoria. “But it’s wrong.”

Unlike mammals, insects don’t have a centralized pair of lungs and a closed circulatory system that delivers oxygen-rich blood to their tissues. “They breathe through internalized tubing called the tracheal system,” Snelling explained.

[...] Here, oxygen delivery relies on passive diffusion to cross the final barrier into the tissue. The problem with diffusion is that it’s notoriously slow. The oxygen constraint hypothesis argued that the larger the insect grows, the further the oxygen must travel to reach the deepest tissues. “As the insects get bigger and bigger, the challenge of diffusion becomes greater,” Snelling said.

[...] The late Palaeozoic was a time of hyperoxia, with atmospheric oxygen levels peaking around 30 percent, compared to the 21 percent we breathe today. Hyperoxia was supposed to let insects bypass the limitations of their breathing system and grow larger. But recently, Snelling led a team of researchers that tested this idea, as they describe in a recent Nature study. It just didn’t hold up.

[...] To put it simply, if a giant insect needed more oxygen, evolving a denser network of tracheoles would be a cheap and effective physiological upgrade. There was likely no anatomical roadblock stopping them from doing so, and they probably wouldn’t have to sacrifice flying power to achieve it.

But if the lack of oxygen didn’t kill the giant bugs, we’re still faced with an outstanding question: What’s stopping our present bugs from evolving to the size of a pigeon? “There are a few hypotheses that are out there,” Snelling said.

[...] One hypothesis is the rise of aerial vertebrate predators. The fossil record shows a decoupling between maximum insect wing length and atmospheric oxygen levels starting at around 135 million years ago, which roughly coincides with the evolution of birds and, later, bats. “This predatory pressure didn’t exist 300 million years ago,” Snelling said.

[...] Then there’s an issue of growing XL-sized exoskeletons. Insects must molt to grow. When they shed their hard outer shells, they are temporarily soft and squishy until the new exoskeleton hardens. Surface tension and basic structural mechanics can hold this soft body together in a tiny beetle, but they might struggle to do so if the bug is much larger... (MORE - missing details)

PAPER: https://doi.org/10.1038/s41586-026-10291-3