When some insects stop rushing around and settle down, they switch to a discontinuous respiration mode where they cycle between closing, `fluttering'(rapidly opening and closing) and opening the spiracles that they breathe through. Why inactive insects switch to this form of interrupted breathing is hotly debated. Natalie Schimpf from The University of Queensland explains that there are several possible reasons: the insects could hold their breath to conserve water; discontinuous breathing could have evolved to improve respiration underground where oxygen is scarce and carbon dioxide levels high;or the animals could reduce oxygen levels in their tracheoles (breathing tubes) by holding their breath to prevent damage from the reactive atmospheric oxygen. And with evidence stacking up for and against each hypothesis, there was no sign of the debate dying down. However, when Craig White introduced Schimpf to the respiration phenomenon during her undergraduate lectures, the student noticed that no one had ever tested whether adult insects could adjust their discontinuous breathing patterns in response to long exposures to altered atmospheres. Only pupae had been tested. Together with Robbie Wilson,she and White quickly realised that testing whether adult insects can adjust their breathing patterns could help them unravel the reason behind insects using a discontinuous breathing pattern(p. 2773).
Deciding to work with cooperative cockroaches, the team obtained male Nauphoeta cinerea from a local pet food supplier; cockroaches are a lizard delicacy. Having overcome her distaste for the scuttling insects,Schimpf supplied them with dry and humid atmospheres; atmospheres with oxygen levels ranging from 5 to 40%; and atmospheres with carbon dioxide levels ranging from 0.3 to 6% for 5 weeks. Once the insects had acclimated to the new atmospheres, Schimpf settled individual adults in a darkened respirometry chamber and measured the gases exhaled by the inactive animals. Recording the cockroaches' responses to atmospheres with different oxygen, carbon dioxide and humidity levels, it was clear that the animals could alter their discontinuous gas exchange breathing pattern in response to different atmospheres. But would their new breathing patterns shed light on why the insects hold their breath?
After weeks of recording the cockroaches' gas exchange patterns, the team joined up with Philip Matthews to see whether any of the theories stood up to the tests. Reasoning that the insects would keep their spiracles closed for longer in a high carbon dioxide atmosphere if discontinuous breathing had evolved to help them cope underground, the team saw that the insects kept their spiracles closed for less time. So N. cinerea did not evolve discontinuous gas exchange to cope with a subterranean lifestyle. And when they analysed the breathing patterns of the insects from the low and high oxygen environments, instead of closing their spiracles for longer in high oxygen atmospheres to protect themselves from oxidative damage, the insects opened their spiracles for longer. So that ruled that theory out too.
That only left the water conservation theory. Would the cockroaches open their spiracles for longer in humid conditions and keep them closed in the dry to prevent respiratory water loss? They did; inactive N. cinerea use discontinuous gas exchange to conserve their body fluids. So discontinuous gas exchange seems to be essential for water conservation in N. cinerea,but the jury is still out on whether other insects hold their breath to conserve water.