Three main factors limit the height that winged insects can reach: air density, temperature, and oxygen availability. All three relate to the fact that the higher we rise above sea level, the weaker the Earth’s gravitational pull, allowing air molecules to spread out. The fewer molecules a given volume of air contains, the “thinner” – or less dense – it becomes.
Flying becomes increasingly difficult when air density decreases because there are fewer molecules for an insect’s wings to press against. Insects need oxygen to survive, just like us, but at 4 miles (6 km) altitude, oxygen levels drop below 50 percent of sea level levels, making it harder to maintain flapping.
Finally, fewer molecules mean less heat generated by the collision of molecules. Temperature varies in complex ways with altitude, and some layers of the atmosphere are warmer than others, but between Earth and about 10 km altitude, the temperature steadily decreases to less than -50 °C.
Despite these hurdles, some insects have evolved strategies that allow them to fly at high altitudes. In 2014, scientists found that Alpine bumblebees, which live 2 miles (3.25 km) above sea level, use different flight mechanics at higher altitudes, moving their wings in a larger arc to stay aloft in thinner air.
In the lab, the bees could even fly in chambers that simulated air density and oxygen levels at 9 km altitude – higher than Mount Everest! In reality, temperatures at such altitudes would paralyze the bees’ flight muscles.
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