Small insects have some of the highest metabolic rates on the planet, yet many routinely hold their breath for tens of minutes. So, why do they do that when creatures with lower metabolic rates have to breathe continually to meet their oxygen demands? Tim Bradley from the University of California, Irvine, explains that insects were thought to seal their breathing tubes to protect themselves from dehydration. However, in 2005 Bradley and Stefan Hetz suggested that insects may hold their breath to regulate the amount of damaging oxygen in their bodies. Since then Bradley has been trying to find out whether insects breathe discontinuously to protect themselves from dehydration or the harmful effects of oxygen. ‘A number of papers have attempted to examine the issue of water balance by looking at desert insects. The argument was that insects from a dry environment should show an exaggerated discontinuous breathing pattern because they have to conserve water in such an extreme manner. But, the results have been mixed,’ explains Bradley. So, he and Heidy Contreras decided to look at the problem from a different perspective. They monitored the breathing patterns of waterstriders that live in humid environments to find out whether they use a discontinuous breathing pattern (p. 1086). ‘One simple hypothesis would be that they shouldn't use discontinuous breathing at all because they don't face the problem of dehydration,’ says Bradley.
Collecting waterstriders from a mountain stream and bringing them back to the laboratory, Bradley and Contreras recorded the insect's breathing patterns in dry and humid conditions and at three different temperatures (to vary the insects' metabolic rates and oxygen demands) by measuring the amount of carbon dioxide exhaled. In humid conditions, the duo saw that the insects with high metabolic rates at 30°C breathed continuously to meet their metabolic demands. However, the insects with the lowest metabolic rates (at 10°C) breathed discontinuously. Even though the atmosphere was humid enough to protect the insects from dehydration, they still breathed discontinuously.
The duo also compared the breathing patterns of waterstriders with the same metabolic rates (at 20°C) in humid and dry environments and found that they were identical. The insects' breathing patterns would have been different if they were regulating them to conserve water, so the insects were not modifying their breathing patterns to protect themselves from dehydration. They must be regulating their breathing patterns to prevent oxidation damage when their metabolic rates are low.
Bradley explains that the insect respiratory system is so efficient at delivering oxygen that it can open for just a minute when the insect's oxygen demands are low and saturate the body with oxygen. Then the spiracles have to close as the insect consumes oxygen, returning it to a safe level. Instead of detecting humidity to protect the insect from dehydration, the insect's respiratory system detects oxygen and carbon dioxide levels and closes when oxygen levels are dangerously high.
‘I would hope that this paper puts an end to the notion that the insect is monitoring water vapour and changing its respiratory behaviour,’ says Bradley. He adds, ‘We understand the whole animal pattern of respiration but what we are now trying to do is to understand what is going on at the level of the spiracles: when do they open and close, how far do they open, what nerves control that, where is the location of the oxygen sensor and the carbon dioxide sensor; but those are tough questions to answer.’
