Insects took to the skies about 350 million years ago, long before any birds. During the Carboniferous period, some of these flying insects grew to enormous sizes. For example, griffinflies had wingspans of around 70 cm—five times larger than today’s biggest dragonflies.
Back then, the Earth’s atmosphere was rich in oxygen, around 30% compared to today’s 21%. This higher oxygen level likely helped these gigantic insects fuel their energy-hungry flight.
Recent research published in Nature sheds new light on this idea. Scientists compared the muscles of various modern flying insects and found something surprising: griffinflies could potentially survive today despite the lower oxygen levels.
The Insect Breathing System
Flying is energy-intensive. Insects must fight against gravity continually. Their flight muscles need a lot of oxygen, which they receive through a unique system of air-filled tubes called the tracheal system. These tiny tubes transport oxygen directly to the muscles.
Each tracheole, the smallest part of this system, acts as a dead end. Oxygen travels via diffusion, moving from the tracheoles into the muscle tissue.
Challenging Old Beliefs
For years, scientists believed that the tracheal system limits the size of flying insects. That was the prevailing theory for over 30 years. When researchers looked closely at locust flight muscles years ago, they noted that the tracheoles only occupied about 1% of the muscle area’s cross-section.
To verify this, they analyzed 44 species of flying insects over five years. The results showed that tracheoles consistently occupied only about 1% of the cross-section, whereas the blood vessels in birds and mammals take up closer to 10%. This indicates insects could physically handle larger tracheoles without compromising muscle strength.
Interestingly, developing insects can grow more tracheoles in lower oxygen conditions, which they pass down to their offspring. This suggests that insect size has never been limited by the tracheal system.
Why No More Griffinflies?
Given today’s atmosphere, there’s no physiological reason why massive insects like griffinflies couldn’t fly. However, their absence might be attributed to more straightforward factors. Larger species are often more vulnerable to extinction, especially in changing conditions or with new predators.
Overall, the rich history of flying insects reveals a fascinating evolution—a mix of environmental influences and biological adaptation. Exploring these aspects helps us understand not just what has been, but also the potential of life on Earth today.
For further details, you can read more about the oxygen levels and insect evolution here.
