Explanation for why we don’t see two-foot-long dragonflies anymore fails - Ars Technica

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.

Air enters the insect’s body through specialized portholes on their exoskeleton called spiracles. From there, it travels down larger tubes, the tracheae, which gradually branch into microscopically thin, blind-ending tubes known as tracheoles. These tracheoles are embedded deep within the insect’s tissues, and mitochondria in neighboring cells cluster next to them.

Insects can actively pump air in and out of the larger tracheae by flexing their bodies, but this active pumping stops at the very end of the line, in the tiny tracheoles. 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.

To prevent the muscles from suffocating, a bigger insect would need significantly wider or far more numerous tracheoles to maintain the supply of oxygen, which implied there had to be a structural tipping point. If an insect gets too big, the volume of breathing tubes required to supply its muscles with oxygen would take up too much physical space. The tracheoles would crowd the very muscle fibers they were trying to fuel, leaving the insect with severely impaired flight performance.

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.

Snelling and his colleagues gathered 44 species of insects across ten distinct orders, representing nearly the entire body mass range of modern flying bugs. On the tiny end of the spectrum was the Trioza erytreae, weighing only 0.334 milligrams. On the heavy end was Goliathus albosignatus, the famous Goliath beetle that weighs 7.74 grams. “We were able to look at insects varying 10,000-fold in body size,” Snelling says.

Using transmission electron microscopes, the team took 1,320 high-resolution images of the insects’ flight muscles. They wanted to measure exactly what percentage of the muscle volume was being taken up by tracheoles, a metric known as tracheolar volume density. If the oxygen-constraint hypothesis was correct, the tracheolar volume density should have dramatically increased as the insects got larger, creeping close to a theoretical limit that would compromise the muscle’s mechanical power. “In our mind, it stands to reason that if very large insects are really challenged, then there should be evidence of this in the tracheoles,” Snelling said.

But his team found no such evidence.

It turned out that in the 0.5 milligram insects, tracheoles took up 0.47 percent of the flight muscle space. In the 5-gram insects, that number rose only to 0.83 percent. Over a 10,000-fold jump in body mass, the relative space occupied by these breathing tubes increased by a factor of just 1.8.