Sucking great lungfuls of air into the body is the main response of most mammals to physical exertion. As their metabolic rates rise, they inhale more deeply to maintain power. However, for insects, breathing is more complex. Equipped with a network of fine ventilation tubes that permeate every tissue in the body, insects have to tightly regulate the passage of oxygen into the body as their metabolic demands increase, by altering their breathing patterns. Timothy Bradley from the University of California, Irvine, USA, notes that respiratory patterns were thought to be principally determined by metabolic rate. But bugs don't just ramp up their metabolism in response to exercise; their metabolic rates increase as temperatures rise and during reproduction and digestion, so it wasn't clear whether other activities might result in different respiratory patterns. ‘There was some evidence that the respiratory control mechanism might be temperature sensitive’, says graduate student Erica Heinrich. She and Bradley began investigating the effects of feeding and temperature on the blood-sucking insect Rhodnius prolixus (p. 2752).
‘Rhodnius prolixus provides certain valuable features as a model insect for studying respiratory control’, says Heinrich, who explains that the insect's metabolic rate rockets after dining on blood. This allowed her to measure the metabolic rates and respiratory patterns of the insects as they exhaled CO2 after a satisfying rabbit blood meal, in addition to monitoring the effects of temperature on the unfed insects as she varied the temperature between 18 and 38°C.
Sure enough, as the temperature rose, the insects' metabolic rate increased 3.5 times above their resting metabolic rate and, as their metabolic demands increased, they initially opened and closed their spiracle valves (breathed discontinuously) until the metabolic rate became high enough and the spiracles remained open continuously.
Next, the duo measured the metabolic rates of the well-fed insects and they were amazed to see the bugs' metabolic rates hurtle to almost 14 times their resting metabolic rates. However, when they analysed the bugs' CO2 exhalation pattern, they found, surprisingly, that instead of holding the spiracles open and exhaling CO2 continuously, they continued closing the spiracles intermittently, even though the insects' metabolic rate was four times higher than that at the hottest temperature.
The duo also monitored the amount of CO2 released by the insects during each exhalation burst and found that they exhaled less CO2 per breath as the temperature rose. This was in contrast to the digesting insects, which exhaled more CO2 each time they opened their spiracles as their metabolic rates rose.
Having shown that the insects' respiratory pattern was different under the two situations, Bradley says, ‘It is overly simplistic to attribute respiratory pattern to metabolic rate alone’, and the duo suspects that the insects' respiratory system is temperature sensitive. Explaining that if the respiration system was not sensitive to temperature, insects should always exhale the same amount of CO2 each time that open their spiracles, Heinrich says, ‘However, we found that as the temperature increases, the volume of CO2 released in a burst decreases’. She and Bradley suspect that this temperature sensitivity may result from changes in the pH of water in the insect's body. ‘Increased temperature decreases the pH of water’, explains Heinrich. She adds, ‘If the CO2 threshold that triggers spiracle opening is sensed via pH, then exposure to high temperatures will trigger premature spiracle opening… and less CO2 would need to accumulate via metabolism to reach the pH threshold. This will result in reduced volumes of CO2 release during spiracle opening’.