Three main factors limit the altitude that winged insects can reach: air density, temperature and oxygen availability. All three relate to the fact that Earth’s gravitational pull gets weaker the higher we rise above sea level, allowing air molecules to spread out. The fewer molecules a given volume of air contains, the ‘thinner’ – or less dense – it becomes.
Flying grows increasingly challenging as air density decreases because there are fewer molecules for an insect’s wings to push against. Insects need oxygen to survive just like we do, but by 6km up, oxygen levels fall below 50 per cent of sea level values, making it harder to maintain wing flapping.
Finally, fewer molecules mean less heat generated by molecules bumping into one another. Temperature varies in complex ways with altitude, and some layers of the atmosphere are warmer than others, but between Earth and about 10km up, the temperature steadily drops to less than -50°C.
Despite these hurdles, some insects have developed strategies that allow them to fly at high altitudes. In 2014, scientists found that alpine bumblebees living 3.25km above sea level use different flight mechanics at higher elevations, moving their wings in a wider arc to stay aloft in thin air.
In the lab, the bees could even fly in chambers that simulated air density and oxygen levels at 9km – higher than Mount Everest! In reality, the temperatures at such altitudes would shut down the flying muscles of bees.
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Ceri Perkins is a New York City-based writer and editor who covers the environment, science, nature and human behaviour. As a freelancer, she has lived around the world, from Madrid to the Scottish Highlands. Before going freelance, Ceri was based in Geneva, Switzerland, as a staff writer/editor at CERN, home of the Large Hadron Collider. Later, she was News Editor at NYC-based magazine Spectrum, where she edited news and opinion stories about the neuroscience and genetic underpinnings of autism. In her spare time, Ceri is typically either outdoors in nature or curled up inside with a stack of books and a pile of things to make or fix. She holds a Bachelor’s in Atmospheric Science, a Master's in Science Communication, and you can read her work in TED Ideas, BBC Earth, The Guardian, Physics World, New Scientist, and more.