Sunday, May 13, 2012

Dragonfly Woman

So, could a diving insect take such a large bubble of air with it that it could, relying on oxygen diffusing into the bubble from the water, stay underwater indefinitely?

Well that's a good question. Let's ignore the problems involved with trapping and holding onto such a bubble for a moment an see whether it is possible.

Unfortunately not because we need to think about the nitrogen as well as the oxygen. But this topic did lead me to the Dragonfly Woman blog of Christine Goforth, a PhD student in Arizona so all is not lost.

Now, it may seem like a physical gill would allow an insect to remain underwater indefinitely, but this is unfortunately not the case.  As the oxygen is consumed by the insect, the concentration of oxygen in the bubble decreases and the gas mixture is thrown out of equilibrium – the ratio of oxygen to nitrogen is no longer the same.  Remember how the bubble wants to remain at equilibrium?  Well, it will do whatever it needs to so that equilibrium is restored.  There are two ways to do it: increase the oxygen in the bubble or decrease the nitrogen in the bubble.  As oxygen is consumed, nitrogen starts to flow out of the bubble to restore the equilibrium.  Oxygen flowing into the bubble from the water slows the loss of nitrogen, but there is so much less oxygen available in water compared to air that consumption of oxygen often outstrips the flow of oxygen into the bubble.  So, nitrogen slowly seeps out of the bubble, making the bubble smaller and smaller until the bug must go to the surface to replace the bubble altogether.

 This is from her Better Breathing Underwater in Aquatic Insects post which is actually a third part of a series of very interesting posts on insect respiration. Part one is Insect Respiration and part two is Aquatic Insect respiration.

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