Sprinting at top speed with your lungs burning, it can be hard to imagine working any harder, but – in comparison – the exertions of flying insects are truly heroic. Lutz Wasserthal from the University of Erlangen-Nuremberg, Germany, explains that blowflies can increase their metabolism by 100-fold during flight and routinely maintain this mindboggling exertion for up to half an hour. ‘It is of great interest to know how these insects manage the oxygen supply for these high metabolic demands’, says Wasserthal. Diffusion of air through the tracheal system that delivers oxygen to every cell in the body was thought to be sufficient to meet the metabolic demands of small flies, but Wasserthal wasn't sure if it was adequate to supply the souped-up flight muscles of beefier blowflies. Some large insects are known to use the muscular contractions that drive their wings to help pump air through their bodies, but it wasn't clear if the minute body deformations associated with flight in blowflies were sufficient to pump air through their bodies. Puzzled, Wasserthal decided to investigate how air flows through blowfly bodies as they fly.
Gently gluing flies by their backs to a tether so that he could analyse every detail of their flight, Wasserthal attached minute polyethylene tubes to the spiracles (breathing holes) on the fly's surface to measure the spiracle air pressure as they flew. Fortunately, the flies were unfazed by the restrictions of the tether and readily took to the wing. As Wasserthal recorded each wing beat, he could see the pressure at the spiracles at the front end of the insect's thorax decline in time with the wing's downbeat, pulling air into the fly's body. Then, as the fly raised its wings, the pressure at the spiracles in the fly's posterior thorax increased, drawing air through the body and expelling it out of the back end. Next, Wasserthal measured where CO2 was emitted and found that it was clearly emerging from the spiracles at the rear. The airflow through the insect's body was in one direction, pulling air from the front through the oxygen-hungry flight muscles and expelling it out of the rear. And when Wasserthal measured the oxygen concentrations inside the insect's body, he was amazed to see that the oxygen was undiluted by mixing with exhausted air and close to external atmospheric levels. ‘The unidirectional airflow guarantees fresh air and an inexhaustible oxygen content in the main trunks and air sacs for sustained flight: the flies never run short of oxygen’, says Wasserthal.
Knowing that reversals of the heartbeat also contribute to drawing air through the resting fly's body – by redistributing haemolymph (blood), which then adjusts the volume of air sacs to suck air through the body – Wasserthal also measured the flying insect's heartbeat and was pleased to see that these heartbeat reversals happened more frequently when the insect was flying than when it was at rest. ‘[Heartbeat reversals] support forced removal of dissolved CO2 by the haemolymph’, says Wasserthal.
Having shown that the minute deformations of the thorax produced by flight are sufficient to pump air through the blowfly's body, Wasserthal comments, ‘The gas exchange is an interdependent performance of the flight apparatus and is thus very economic’.
