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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