The Breathing Myth That Misled Insect Biology
- Wits Faculty of Health Sciences
A new study suggests that one of the most familiar explanations for insect size is wrong.
For years, scientists have argued that insects are small because they cannot breathe well enough to grow larger. Insects do not have lungs, nor do they transport oxygen in blood. Instead, they rely on a network of tiny air-filled tubes, called tracheae, which deliver oxygen directly to their tissues. Because oxygen moves through this system largely by diffusion, many biologists assumed it must impose a hard limit on insect size.
For decades, this idea, supported by fossil records, has shaped scientific understanding. Around 300 million years ago, the Earth’s atmosphere contained far more oxygen than it does today. At the same time, insects reached extraordinary sizes, including dragonfly-like predators with wingspans exceeding 70 centimetres. When oxygen levels later declined, these giant insects disappeared.
Researchers, including a postdoctoral fellow at the Wits Faculty of Health Sciences, Dr Ashleigh Donaldson, tested this idea by examining insect flight muscles directly. The results of the paper published in Nature show that insects were never close to running out of oxygen, even at very large body sizes. The findings challenge a long-standing assumption and reshape how scientists think about both ancient insect giants and modern insects living in a changing world.
Dr Donaldson says this is critical for understanding evolution, as it “points to other factors such as environment or predation as more important in shaping the size of living organisms today”.
Re-examining the insect evolution theory
Diffusion works well over short distances but becomes increasingly inefficient as distances grow.
This logic appeared to match the fossil record. Around 300 million years ago, when atmospheric oxygen levels were much higher than today, insects reached extraordinary sizes. When oxygen levels later declined, giant dragonfly-like insects disappeared.
The theory was widely accepted, but it rested on an assumption that had rarely been tested directly.
Dr Donaldson notes that the study suggests other factors are “likely responsible for keeping insect sizes in check”.
Looking where oxygen demand is highest
To test whether oxygen diffusion really limits insect size, the researchers focused on the most demanding tissue in an insect’s body: the flight muscles.
Flight muscles consume huge amounts of oxygen during sustained flight. If oxygen supply were a constraint, it should be most obvious here, especially in large insects.
The team analysed more than 1,300 high-resolution microscope images of flight muscle from 44 insect species across 10 orders. The dataset included insects weighing less than half a milligram and others spanning more than a 10,000-fold range in body mass.
They measured how much of each muscle was occupied by tracheoles, the finest oxygen-delivery tubes that sit directly next to muscle cells.
Bigger insects do not pack in more breathing tubes
The results showed that as insects increased in size, the proportion of flight muscle taken up by tracheoles increased only slightly.
This means that even large insects devote only a tiny fraction of their flight muscle to oxygen delivery. Most of the muscle remains free to generate power.
Estimates suggest that even the largest insects that ever lived would have used only about 1% of their flight muscle volume for oxygen supply.
For comparison, Dr Donaldson says birds and mammals often devote up to about 10% of their flight or heart muscle to blood capillaries, meaning “insects achieve effective oxygen delivery with a much smaller and only slightly changing investment”.
Why this matters now
At first glance, this might seem like an obscure question about ancient insects, but insects represent most terrestrial ecosystems.
“This helps scientists better understand how different animals, including humans, have evolved different ways to supply oxygen to their tissues,” explains Dr Donaldson.
The research shows that understanding how organisms function, how size evolves, and why insects struggle or thrive under different conditions offers insight into biological limits.
Dr Donaldson adds that this context helps explain why animals, including humans, evolved differently and the role that oxygen supply plays in their tissues.
“It also helps explain how ecosystems function, both today and in the past, by showing that factors other than oxygen, such as environment or predation, may be more important in shaping the size and roles of organisms within ecosystems.”
Rethinking an old story
The idea that oxygen limits insect size has long been repeated in textbooks, documentaries and popular science. This study shows that even widely accepted theories should be tested rather than simply assumed.
To put the matter to rest: oxygen supply isn’t what stops insect gigantism. They are shaped more by environmental and ecological factors than by breathing constraints.