Equipped with a delicate network of tubules that deliver oxygen directly to every tissue in the body, insects open and close the valves (spiracles) at the ends of the tubules (tracheae) to admit oxygen to the body and release carbon dioxide in a series of well-established patterns matched to their activity. One such pattern, known as the discontinuous gas exchange cycle (DGC), occurs when some insects are inactive. The spiracles are initially closed for extended periods as the insect consumes oxygen from the air in the closed tracheae. However, once the oxygen level has fallen sufficiently, the spiracles begin to open and close rapidly (flutter phase), to draw in more air. The carbon dioxide levels in the body then continue rising until they are high enough to trigger the final phase of the cycle, when the spiracles open for an extended period as the insect releases the accumulated carbon dioxide and takes in more oxygen ready to begin the cycle again. Yet, how this particular gas exchange pattern evolved is a mystery. Several competing theories had been proposed, but Eran Gefen says, ‘None of the existing hypotheses… is backed by unequivocal support’. Explaining that one theory suggests that DGC evolved to protect insects from dehydration, Shu-Ping Huang, Stav Talal, Amir Ayali and Gefen decided to test whether grasshoppers from three dramatically different environments saved water when using DGC.
‘We wanted to know whether xeric grasshoppers [that live in dry conditions] “do DGC better”’, says Gefen, so Huang and Talal headed out to three locations in Israel – ranging from the desiccated Negev desert to a hill overlooking the Sea of Galilee (where the rainfall is higher), to the wettest location, Mount Hermon on the Syrian border – to collect grasshoppers endemic to each environment. Back in the lab, Huang began the laborious process of measuring the insects’ gas exchange patterns and water losses over periods of several hours before scrutinising the carbon dioxide traces to identify examples of DGC and continuous breathing for analysis.
Noticing that 60% of the desert dwelling Tmethis pulchripennis grasshoppers used DGC while only 19% of the grasshoppers from the most humid environment – Ocneropsis lividipes – used DGC, Huang then compared the insects’ respiratory water loss rates when they were breathing discontinuously and continuously. If the grasshoppers were breathing discontinuously to protect themselves from desiccation, they would all reduce their respiratory water losses when they used DGC. However, the team was impressed to see that even though the desert species (T. pulchripennis) reduced its respiratory water losses, neither of the species that were adapted to more humid conditions (Ocneropsis bethlemita and O. lividipes) did. ‘DGC in the two Ocneropsis species did not result in any measureable water savings,’ says Gefen, adding ‘Our results suggest that DGC has not necessarily evolved as a water-saving mechanism’.
Instead, Gefen suggests that the ability to reduce water loss through DGC in some, but not all, species may be related to differences in tracheal dimensions. He explains that although T. pulchripennis may have developed a large tracheal system to conserve water during DGC, there is another possible explanation: that T. pulchripennis – which is the only insect in the study that can fly – evolved a larger tracheal system to deliver sufficient oxygen for the high metabolic demands of flight. ‘It would be extremely interesting to tease apart the two in the future’, says Gefen.
